Chap12.pdf

During the past 40–50 years, foamed polymershave found increasing importance in the world mar-ketplace due to the unique characteristics and prop-erties they provide when compared to solid plastics.While most thermosets and thermoplastics can bemade in a foamed or cellular structure under certainconditions, the materials known as polyurethaneshave become predominant for many applications inthis field. Through the proper selection of the start-ing materials, foamed polyurethanes can range incharacteristics from extremely soft, resilient cushion-ing products to very tough and rigid structural mem-bers. As varied as the products are, so too are themachines and processes used to produce them. Thischapter gives an overview of the many types ofequipment and production processes that are used forspecific products. Certain other systems, such aspolyureas, are also processed with the same equip-ment as polyurethanes.

The word polyurethane is somewhat misleadingsince, unlike most plastics, the final product is notmade by polymerizing a monomer. Instead, the prod-ucts contain a number of polyurethane groups in acomplex structure that is controlled by the choice ofstarting materials and the production conditions.Commercial products are manufactured by the reac-

tions of two liquids: isocyanate (NCO) compoundsand polyol (polyoxyalkalene) components, in thepresence of catalysts and processing aids.

Basic isocyanate chemistry has been available formore than 100 years, but it was not used commerciallyuntil the mid-1930s. Nearly simultaneously, theDuPont Co. in the United States and I.G. Farben inGermany began their developments. DuPont directedits efforts toward films and adhesives, while I.G.Farben concentrated on products to circumvent nylonpatents. Dr. Otto Bayer of Bayer AG (part of I.G.Farben at the time) is considered to be the “father” ofthe urethane industry, and work in the Bayer labs ledto commercial polyurethane products as well as to themachinery used to make them. During World War II,German polyurethane developments centered on prod-ucts to replace scarce materials. The major productswere rigid foams and cast elastomers. Following thewar, the technology was exploited by the UnitedStates, and rapid advances were made in the develop-ment of products and processes. Commercially impor-tant cushioning products were achieved by the end ofthe 1950s. While polyurethanes serve a worldwidemarket, the development centers in both chemistryand machinery have remained in Western Europe andthe United States.

CHAPTER 12

12-1

FOAM PROCESSINGCHAPTERCONTENTS:

INTRODUCTION12-1

BASICCHEMISTRY 12-1

FOAMPRODUCTIONMETHODS 12-2

FOAMEQUIPMENT 12-3

FOAMMACHINES 12-16

CONVEYINGSYSTEMS 12-21

FOAMMOLDS 12-23

RIGID FOAMLAMINATE BOARDLINES 12-26

FOAMFABRICATION

12-28

CARPETUNDERLAY 12-30

FOAM TYPES 12-33

FILLERUSAGE 12-33

TROUBLE-SHOOTING FORCONVENTIONALSLAB STOCKFOAM 12-34

The Contributor of this chapter is: Robert L. McBrayer, Consultant, Foamcon.The Reviewer of this chapter is: Oscar Grace, Technical Manager (retired), BASF Corporation.

INTRODUCTION

BASIC CHEMISTRYThe basic chemistry of flexible polyurethane

foams is not difficult to grasp. It is the reaction of analcohol or OH group with an isocyanate or NCOgroup. The alcohol is normally polyfunctional, rang-ing from 2–8 OH groups, which are referred to aspolyols. As the number of OH groups increases, thefoam structure becomes more rigid. Polyols are clas-sified as polyether or polyester, based on the starter(initiator) materials used in their manufacture. Theinitiator is reacted with propylene oxide, ethyleneoxide, or a combination of the two. The choice ofwhich initiator and oxide to use depends on the foamcharacteristics desired. A variety of polyols are avail-able to tailor the foam characteristics. The usual iso-cyanates for foam are toluene diisocyanate (TDI) anddiphenylmethane diisocyanate (MDI). Various formsare available, again tailored to specific applications.The foaming operation is complex because threebasic reactions are occurring concurrently and at dif-ferent rates. These reactions are chain extension, gasformation, and cross-linking.

CHAIN EXTENSIONThe primary reaction is that of the isocyanate

group of the alcohol to give a urethane linkage:H O (1)| ||

R1 – N = C = O + R– OH –––– R1–N–C–O–R +HEATIsocyanate Alcohol UrethaneThe urethane further reacts with additional iso-

cyanate to yield an allophanate:H O O (2)| || ||

R1– N = C – O + R1– N – C – O – R –––– R – O –C – N – R1

|O = C – N – R1

|H

Isocyanate Urethane Allophanate The primary catalysts used for this reaction are

organotin compounds; however, in very reactive sys-tems, no catalyst may be required.

Tool and Manufacturing Engineers Handbook Knowledge Base • Copyright © 1998 • Society of Manufacturing Engineers

CHAPTER 12

12-2

BASIC CHEMISTRY

GAS FORMATIONGas formation involves the reaction of the isocyanate with water

to form an aromatic amine compound plus carbon dioxide in a two-step reaction. This carbon dioxide causes the cell formation andfoaming. Until recently, the use of water in formulations for rigidfoam other than in minor quantities was a major differentiating fac-tor between flexible and rigid foams. A class of flexible foamsknown as integral skin foams was also formulated without water.

Rigid and flexible integral skin foams were produced byadding auxiliary blowing agents to the reacting mixture. Initially,the primary blowing agents were chlorofluorocarbons (CFCs),specifically CFC-11 and CFC-12. CFC-11, or methylene chloride,was also used in flexible foams to manufacture lower-densityand/or softer foams than were obtainable with water blowingalone. With the discovery that chlorinated compounds, such asCFCs and methylene chloride, destroyed the earth’s ozone layer,alternative materials were developed. These include completelychlorine-free hydrofluorocarbon compounds (HFCs), cyclopen-tane, and acetone. The alternatives have required modifications inprocessing equipment so they are used safely and efficiently. Newformulation techniques also allow production of both rigid andflexible integral skin foams with water as the only blowing agent.

H O (3)| ||

R1 – N = C = O + H – OH –––– R1 – N – C – OHIsocyanate Water Unstable carbamic acid

H O H O (4)| || | ||

R1 – N – C – OH –––– R1 – N – H + C = OAmine Carbon dioxide

The catalysts for this reaction are primarily tertiary amines; how-ever, some metal oxides have also been found effective. To stabilizethe foaming mixture, silicone or other specialty surfactants are used.

CROSS-LINKINGThe amine that is generated in the gas formation reaction

reacts with more isocyanate to form a disubstituted urea, whichcross-links the urethane polymer:

H H O H (5)| | || |

R1 – N – H + R1 – N = C = O –––– R1 – N – C – N – R1

Amine Isocyanate Disubstituted urea

Some of the disubstitute urea then reacts further with iso-cyanate to form highly cross-linked biuret structures:

H O H O H (6)| || | || |

R1 – N – C – N – R1 + R1 – N = C = O –––– R1 – N – C – N – R1

|O = C – N – R1

|H

Biuret

In addition to the previously mentioned components, foam for-mulations may contain one or a combination of additives to givespecific properties to the foam products. Among these additives arethose used to modify the burning characteristics of the foams: pig-ments; bacteriostats; inorganic fillers, such as glass fiber, silica, andbarium sulfate; organic fillers, such as melamine and phosphateester plasticizers; antistatic agents; UV stabilizers; cell openers; andinternal mold release agents.

FOAM PRODUCTION METHODSThere are two basic methods for commercial foam production:

spray and pour. The spray method is generally limited to rigid foams,while the pour method is used for all types of foams. The pourmethod is further subdivided into the categories of open pour, froth,and closed pour.

OPEN POURIn this method, the mixed foam reactants are dispensed either con-

tinuously or in timed shots into open cavities, where the reaction takesplace. Except for very fast systems (such as high-density foamedelastomers), all flexible and rigid foams can be made by this tech-nique. The cavity may be left open or closed after the pour. If the cav-ity is substantially open, the resulting material is called slab stockfoam. When the cavity is closed, the foam is termed molded. For flex-ible foams, the world production is approximately 50% slab and 50%molded. Little rigid foam is made today using the slab method due tooverall production inefficiencies.

Molds may be individual, such as those used for automotiveseats, or continuous, for products such as rigid foam laminateboard. Many specialized machines were developed over the yearsfor molding applications.

FROTHIn a specialized version of the open pour technique, highly cat-

alyzed systems are used or an auxiliary blowing agent is added to

produce a stable froth as the reacting mixture exits the mix head. Inrigid insulation foams, this results in low pressure generation andgood mold flow. The highly catalyzed flexible foams are used wherethe foam is poured into fabric. This is difficult to do with conven-tional foams due to fabric bleed through. Stable flexible froth, usingspecialized formulations and equipment, is also used to back car-pets for pile bonding and cushioning. Machines used to dispenserigid froth foam, such as the Auto-Froth® units supplied by BASFCorp., consist of pressurized component tanks, hose systems, andstatic mixer units. In addition to having pour capabilities, these unitscan be used with a spray gun. The spray foam in this case, however,is not of the same quality as that produced from a standard sprayfoam system.

CLOSED POURIn this method, the foam reactants are introduced into a closed

mold, and as the foam expands, it completely fills the mold. Themold is closed either for part design considerations or because thefoam reaction is too fast to permit closing the mold after the foamreactants are dispensed. For many applications, the foam mixtureis simply poured through a hole that is plugged after the shot.With faster chemical systems or for specific design considera-tions, the mix head is either fixed directly to the mold or it is heldagainst the mold until a pour gate is closed.


CHAPTER 12

12-3

FOAM EQUIPMENT

As a first consideration, foam equipment is classified as eitherlow- or high-pressure, depending on the pressure of the chemicalsas they enter the mix chamber or head. Typical low pressures are50–200 psi (345–1379 kPa), and high pressures are considered as1000–3500 psi (6.9–24.1 MPa).

Some machines, particularly those used for slab foam produc-tion, are hybrid types that use low-pressure pumps on somestreams and high pressure on others. The type of machine chosendepends on the foam type, versatility required, foam volume, andemployee skills. High-pressure machines require greater technicalskills for both operators and maintenance personnel.

Independent of the technique used, several common elementsare needed for effective foam production:

• Raw material supply.• Metering units.• Mix heads.• Temperature control system.• Process control system.

RAW MATERIAL SUPPLYRaw material supply includes the delivery containers from the

chemical suppliers, in-house storage tanks, blending tanks, and thesupply or day tanks for the foam machine. In some cases, a givencontainer may serve two or more purposes.

Supplier delivery is from rail cars, tank trucks, rubber bags intrailers, tote bins, or drums. When the delivery is made in largebulk containers, the materials are normally transferred to in-housebulk storage tanks. Handling of the bulk components depends onthe chemical requirements, such as the need to maintain tempera-ture or agitation. Isocyanates are stored in moisture-free condi-tions to prevent undesirable reactions. Some isocyanates alsorequire heating to prevent solidification.

For high-output slab stock machines, materials are normallydelivered directly from bulk storage to the metering pumps. Withother machines, the materials are delivered to intermediate condi-tioning tanks or day tanks at the foam machine for better mainte-nance of temperature and material conditioning. The specifics ofthe day tanks vary with the machine manufacturer and include sin-gle- and double-walled tanks with or without internal temperaturecontrol coils or plates, insulation, agitators, and means for recircu-lation. The tanks may also be pressurized or nonpressurized.Construction materials depend on the characteristics of the com-ponent, but the tanks are typically carbon steel with inner coatingsof phenolic or epoxy if the component is corrosive. Stainless steeltanks are required for specific applications. Automatic fill systemsare generally used to ensure proper component conditioning.

METERING UNITSMetering units may be high- or low-pressure units. Independent

of these classifications, the units must deliver and maintain a highdegree of accuracy, usually within ±1%. Low-pressure machines areavailable to handle as many individual components as necessary tomeet the formulation requirements, while high-pressure units aretypically limited to two or three components, unless special provi-sions are made. Output capabilities range from a few ounces or mil-liliters per second for applications such as pour-in-place gaskets, to1000 lb/min (454 kg/min) or more for large molded parts or slabstock production. With machines that are used for periodic shots,

recirculation capabilities ensure proper material conditioning andexact material delivery during the shot. With continuous pourmachines, recirculation capability may be a needless expense.

Low-pressure MachinesLow-pressure machines normally use high-precision gear

pumps. Specially designed pumps are required if abrasive fillersare used in the foam formulation. To eliminate problems that existwith seal leakage and environmental controls, isocyanate pumpsare supplied with magnetic drive coupling systems. Various pumpdrives include direct drive units with DC or variable-frequency ACmotors, gear motors, chain drives with replaceable sprockets foroutput adjustment, gear trains, and power pulley drives. High-out-put machines may be configured as hybrid machines (with high-and low-pressure pumps) for better accuracy in metering the spe-cific components. On slab stock machines, there is the capabilityfor many streams, but all of the streams may not be active concur-rently. They are installed to permit quick formulation changes toreduce change time and eliminate waste. With shot machines, thetrend is to minimize the number of streams and thereby eliminateproblems that occur with on/off operations.

High-pressure MachinesHigh-pressure machines are frequently called reaction injection

molding, reaction impingement mixing (RIM), or high-pressureimpingement mixing (HPIM) machines. This terminology, devel-oped during the early 1970s, refers to systems where the componentmixing occurs through impingement of the component streams athigh pressure without using mechanical stirrers. Another term, liq-uid injection mixing (LIM), was used for urethanes; however, theterm was later trademarked by General Electric for liquid siliconerubber systems. While they are most frequently considered shotmachines, RIM machines are also used for continuous pour applica-tions such as laminated rigid foam panel production. There are twotypes of high-pressure metering: pumps and cylinders.

Metering pump systems. This type of pump uses high-preci-sion axial, radial, or in-line piston pumps capable of delivering thepressures required. These are modified versions of pumps that wereoriginally designed for hydraulic oil or fuel service. Because of theclose tolerances and construction materials, these pumps are notused with particulate materials that are abrasive, such as glass ormineral fibers. Very high viscosity materials are also not handledsuccessfully, since the pumps have virtually no suction capabilityand clearances are very small. The nominal maximum viscositylimit of these pumps is 2000 centipoise (cP) at operating tempera-ture. Through the use of booster pumps, higher viscosities are han-dled. The metering pumps are either fixed or adjustable outputtypes. Various drive systems are used, and output can be variedmanually through DC or variable-frequency AC motors, or withservo systems. Figure 12-1 is a schematic of a high-pressuremachine shown with double-wall day tanks for temperature control.

Cylinder metering units. Designated reinforced RIM (RRIM)machines use lance- or plunger-type cylinders that are capable ofhandling abrasives and high-viscosity materials. With a lance cylin-der, shown in Fig. 12-2, the moving lance does not contact theinside surface of the cylinder; it only contacts the labyrinth seals sothat pressure is developed. The material is displaced, and the cylin-der does not empty completely after each lance stroke. Cylindersmay be driven independently or with hydraulic slave cylinders.

FOAM EQUIPMENT


A schematic of a lance cylinder-type machine is shown in Fig.12-3. To reduce the capital cost, a machine may have only a singlecylinder to meter the abrasive component while a metering pumphandles the nonabrasive component.

Although operating pressures for cylinder machines are gener-ally on the same order as metering pumps, special units candeliver impingement pressures up to 15,000 psi (103 MPa) for dif-ficult-to-mix systems. While cylinder units are usually made toaccommodate a fixed shot capability, special units, such as tan-dem cylinders that are carefully controlled to have overlappingstrokes and precise operation of switching valves, are made to per-mit continuous output. Gusmer-Admiral also offers a continuouscylinder metering unit, the CDC pumping system. This double-acting cylinder system, shown in Fig. 12-4, is not a lance cylinderdue to the internal piston seal requirement. Other true cylindermetering units are used for special purposes, such as color addi-tion or very low output, where it is difficult to achieve the desiredaccuracy with a rotating or reciprocating pump. Piston seal wearconsiderations limit these pumping units to nonabrasive service.

Spray foam machines are also cylinder-metering units. Theyare normally driven pneumatically, and the stroke of each cylinderis locked to ensure a constant ratio of materials. Ratios are variedby changing cylinders. Spray units usually have external-mix, self-cleaning, hand-held guns, and the impingement pressures canreach 3500 psi (24 MPa). Valving arrangements are used to ensureessentially continuous flow with these machines. Most spraymachines do not have recirculation capability, and componentsmay be warmed by heat-traced hoses to ensure uniform fast reac-tion of the spray foam; this provides the desired laydown for asmooth surface. A typical pneumatically operated spray unit is

shown in Fig. 12-5. Linden provides a hydraulically powered unitfor spray or pour that eliminates problems that can occur if thecompressed air supply pressure is inconsistent.

MIX HEADSMix heads are classified in two ways: recirculating or nonrecir-

culating, and low-pressure or high-pressure. In a recirculatinghead, the components are recirculated from the day tanks throughports in the mix head and back to the tank. The recirculating typewith proper temperature control is essential for good operationwhen foam dispensing is intermittent. While recirculating, backpressure is controlled at the mix head. Spool valves or ports openwhen the shot is called for. Some continuous pour mix heads alsoprovide some recirculation; however, most are nonrecirculating.With nonrecirculating heads, pumps are started or valving isopened in a sequence that ensures start-up is smooth and lossesare minimized. On start-up, the minor streams (catalysts, surfac-tants, and liquid additives) are turned on first, followed by thepolyol and then the isocyanate.

Low-pressure MixingLow-pressure machine mix heads are chambers with rotating

impellers that are driven by hydraulic or electric motors. Drivespeeds are usually variable from 2000 to 6000 rpm. The impellersmay be a low- or high-shear design, depending on the mixing effi-ciency required. Low-pressure mixers can give excellent mixing andare designed for a wide range of output, including very low output.Essentially all of the mixing is done by the mechanical action. Themixer imparts high shear energy due to the close tolerances betweenthe mixing element and the mixer barrel. Rotational speed and/or

CHAPTER 12

12-4

FOAM EQUIPMENT

Hydraulicunit for

mix head

Isocyanatecomponent

Filter

Mix head

Meteringpump

Resincomponent

Fig. 12-1 High-pressure (RIM) machine schematic.


back pressure on the mix chamber is varied to alter the foam cellstructure and mixing efficiency. One design of a high-shear mixeris shown in Fig. 12-6.

Hybrid machines require less efficient mechanical mixers,since some mixing is achieved by impingement of the streamseven though the impingement efficiency is low. The mixing ele-ment is simpler than the high-shear type and is usually a pin or“Christmas tree” type, as shown in Fig. 12-6.

Low-pressure mix heads, with the exception of the hybridmachine types, have valving arrangements for the componentsthat ensure simultaneous opening to minimize a condition knownas lead-lag. With lead-lag, a component enters the mix chamber outof sequence with the other(s), resulting in the wrong ratio of com-ponents and defects in the foam product. The valving arrangementsinclude individual component cone or ball valves that are opened bya single operator and spool valves that are coupled to an opening/closing cylinder. Depending on the manufacturer, low-pressure mixheads may have two to six component streams. Elastogran Poly-urethane GmbH developed a low-pressure mix head with up toseven ports that can be opened hydraulically in any combination.This offers the possibility of formulation changes between shots.With all low-pressure heads, separate valving is provided for solventflushing and air purge to evaporate any solvent residue.

A disadvantage of low-pressure mix heads is the need to purgeand flush the head periodically, sometimes after each shot. This isbecause cured foam residues can build up on both the stirrer andmixer barrel. This build-up can diminish mixing quality, causing

CHAPTER 12

12-5

FOAM EQUIPMENT

Hydraulicunit for

mix head

Mix head

Isocyanatecomponent

Transfercylinder

Hydraulicoil reservoir

Hydrauliccylinder

Meteringpump

Resincomponent

Fig. 12-3 High-pressure lance cylinder (RRIM) machine schematic.

Cylinder body

Plunger

Labyrinthhigh-pressure

seal��&2Fromdaytank

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head

Fig. 12-2 Lance cylinder concept.


CHAPTER 12

12-6

FOAM EQUIPMENT

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FOAM EQUIPMENT

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the mixer to overheat or jam. After the component streams arestopped, the head is first blown out with a blast of air. The cham-ber is then flushed with an appropriate solvent followed byanother air blast to dry the head. Methylene chloride is a commonsolvent, but cost and environmental concerns have led to a searchfor alternative solvents. Several proprietary agents are available.Hot-water flushing systems were introduced in the 1980s and worksatisfactorily for some applications. The flush water may be reusedafter decanting to remove foam solids. Low-pressure mix headstypically are partially disassembled each day following operationsfor thorough cleaning.

The use of particulate fillers to modify foam properties pre-sents a number of problems in processing. These include thestrong tendency of fillers to settle and cake; oil absorption ofsome fillers, which increases viscosity and mixing difficulty;metering pump wear; and filler agglomeration. Edge-Sweets (PTI)Co. has developed and patented a low-pressure mix head system,designated FFH, which overcomes these problems by meteringdry fillers into the mix chamber. Up to four different dry fillerscan be metered, either simultaneously or in sequence. The fillersare metered into an auger screw that is mounted concentrically tothe standard low-pressure mixer. The screw forces the fillers intothe mix chamber. The basic configuration, shown in Fig. 12-7, isdesigned for continuous operation.

High-pressure MixingAlthough used since the early days of the industry, high-pres-

sure impingement mixing has become increasingly popular since

the mid-1970s. The reasons are threefold. First, they can handlevery fast reacting systems. Second, flush solvent is eliminated.Finally, the high-pressure machines are less complicated andrequire less overall maintenance than low-pressure machines. Thehigh pressure in the mix head is dissipated in mixing and heatenergy. The discharge pressure from the mix head is the back pres-sure created by the material flow through the restriction of the dis-charge nozzle outlet. The heads are opened and closed hydrauli-cally, and no mechanical stirrers are required. For effectivemixing, the mix head chamber diameter is matched to the desiredmachine output. If the output is too low for the diameter, the mixquality is poor. If the output is too high, splashing is a problemwhen making open pours.

As the interest in high pressure grew, the machine manufactur-ers were particularly aggressive in patenting head designs, and inmany cases, aggressively protected those patent rights. This hasled to many commercialized concepts. Another reason for devel-oping new mix head designs was to improve the mixing efficiencyat high pressure to approach the quality of the best low-pressuremixers. A schematic of one early type of high-pressure head (cross-licensed between Elastogran Polyurethane GmbH and KraussMaffei) is shown schematically in Fig. 12-8. This head featuresrecirculation grooves that are cut into the single moving part, thecontrol and clean-out piston. The component streams enter thehead directly opposite from each other for direct impingement.When the head is closed, the components are pumped through therecirculation grooves under the same conditions they see when themix head is open. The impingement pressure is controlled byfixed orifices or needle nozzles that are mounted in the mix headbody. The control piston opens very rapidly as the head is shiftedfrom the recirculation state to the mixing state; this is on the orderof a few microseconds. Speed is essential to prevent operationalproblems, because the component streams are shut off momentar-ily during the switch. The mix head is sealed between the reactingcomponents when closed by the close tolerance of the parts and aself-renewing polyurethane seal. As the patents for the recircula-tion grooves expired, this simple concept was adopted by othermanufacturers in a variety of head designs.

The majority of high-pressure machines are equipped withtwo-component mix heads. While heads are available with morecomponent streams, these designs usually support specific require-ments. For example, Cannon introduced its TRIO mix head withcomponents entering at 120˚ angles to each other. This arrange-ment improves mixing, particularly with very fast reacting chemi-cal systems. Four-component mix heads are also available for spe-cific applications.

While the straight head design shown in Fig. 12-8 is satisfactoryfor many applications, special head designs are available to reducesplashing and/or improve mix quality. The most popular of these isthe L-head design, shown in Fig. 12-9. This design features bothmixing (or transverse) and cleanout pistons. Heads of this typeinclude the Cannon FPL, Elastogran SMA, Gusmer-Admiral RIMX, Krauss-Maffei UL, and Linden Industries MHL. The heads varyin mix chamber length, impingement nozzle arrangements, andother details to avoid patent infringement. Another Elastogran vari-ant is the B-head design, which has three-component injection andactuated pins below the mix zone to smooth the flow. The majorcomponent is normally split into two opposing streams and theminor component enters roughly perpendicular to the opposedstreams. The B-head schematic is shown in Fig. 12-10, and theoperating principle is shown in Fig. 12-11.

Hennecke Machinery has taken two other approaches toimproved mixing. These are the MQ and MX mix heads, shown in

CHAPTER 12

12-8

FOAM EQUIPMENT

High-shear type Pin type

Fig. 12-6 Low-pressure mix head stirre rs.


Fillerdrivemotor

Mixerdrivemotor

Dry fillerhopper

Fillermeteringauger

Mix chamber

Reactive filled mix into moldor onto a conveyer

Recirculation valvesfor chemicals

Fig. 12-7 FFH mix head system.

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FOAM EQUIPM

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Oil

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Figs. 12-12 and 12-13, respectively. All of these designs provideimproved mixing compared to the straight mix head shown inFig. 12-8.

Special heads were also developed for adding color to thereacting mixture. This application is particularly directed to inte-gral skin or high-density microcellular foams, which require very

uniform color. Cannon has the CCS color system, which directsthe liquid color pigment through a hole bored through the cleanoutpiston. Another successful approach for color addition is donewith a third stream added to the basic L-head configuration.

With some chemical systems, particularly high-density micro-cellular and integral skin foams, aftermixers are used in closedmold pours. These mixers are channels cut into the mold that dolittle actual mixing. They create back pressure on the mix head,which increases its efficiency.

TEMPERATURE CONTROLSince polyurethane foaming involves chemical reactions, good

temperature control is essential for maintaining foam productionconsistency. Particularly demanding are the requirements for high-pressure machines due to the heat generated by recirculating underhigh pressure. It is common to see a temperature increase of 10–15˚F (6–8˚ C) in a single pass of the material through the mix head.One way to avoid heat build-up is to have automatic switching forlow-pressure recirculation when there is sufficient time betweenfoam shots. Most machine systems are equipped for heating andcooling to cope with a range of ambient conditions. Production sys-tems typically achieve temperature control within ±2˚ F (±1˚ C).

The machine day tanks are part of the temperature control sys-tem if they are fitted with jackets, external coils, or internal coils.Heating is done by electrical units immersed in the component tankor fitted to the component line at an appropriate location. A morecommon heating method is to recirculate hot water that is generatedin a separate unit. Cooling water is supplied from a chiller unit orother local supply (water mains or wells) if the temperature ismaintained at 60˚ F (16˚ C) or less. Using sensors located in thecomponent streams, the temperature is controlled by either cyclingbetween the hot and cold water sources or by using temperedwater. When heat exchangers are used, the location is a matter ofdesign philosophy.

With all types of machines, separate temperature control loopswith dedicated lines and recirculation pumps are used. Some high-pressure machines use a recirculating system that also provideslow-pressure feed to the inlets of the high-pressure meteringpumps. The simplest system for high-pressure systems uses themetering pumps as recirculating pumps, with the heat exchangersplaced in the return lines to the day tank. Plate heat exchangers havebecome increasingly popular due to their efficiency, low pressuredrop, and ease of cleaning compared to tube-and-shell heatexchangers.

PROCESS CONTROL SYSTEMSSystems range from basic relay logic systems, which turn

equipment on and off in response to manual actuation andmechanical timers, to highly sophisticated computer-monitoredelectronic systems. Statistical process control data are generatedby various means including high-pressure flowmeters, pressuretransducers, and rapid-response temperature sensors. Flowmetersystems can provide flow control by using variable-speed drivesfor metering pumps or by servo adjustment of pump output.Programmable systems frequently feature CRT displays for easyaccess to control and process parameters.

The costs for modern process controls can exceed that for themechanical components of the machinery. In addition to control-ling the foam machinery, the systems can also control any convey-ing systems, temperature systems, mix head traversers, and moldsfor complete system integration.

CHAPTER 12

12-12

FOAM EQUIPMENT

IJKHydrauliccontrolblock

Mixing pinpiston

Control/clean-outpiston

Componentout

Componentin%1=I'3?K�� %&'123��Oil

Fig. 12-10 Elastogran B-type mix head.


CH

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12

12-13

FOAM EQUIPM

ENT

Foamingposition

1.

2.

3.

Recirculationwith mixing

pins inmix position

Closed andrecirculation

position

Mixing pin

Fig. 12-11 Operating principle of Elastogran B-type mix head.

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CHAPTER 12

12-14

FOAM EQUIPMENT

<=>?HIJKTUVWabclmnoxyz{ABCMNOYZ[efgqrs}~�Zfr~O� ��%11� ��)*56*6Clean out

piston

Sliding throttle valve

Throttle valvestroke

adjustment

Oil

Oil

Oil

Oil

Oil

Componentpressure

adjustment

Oil

Oil

Polyol Polyol

Oil

Mix chamber

Vortex chamber

Isocyanate Isocyanate

Outlet

Mix positionClosed/recirculation

position

Fig. 12-12 Operating principle of Hennecke MQ mix head.


CHAPTER 12

12-15

FOAM EQUIPMENT

&'23>?>?@JKLVWXbcdnopz{|Oil

Oil

Oil

Oil

Oil Oil

Isocyanate

Mix chamber

Throttling sleeve

Throttling sleevestroke

adjustment

Polyol

Pressure reducingchamber

coKW@*+67BCBCDNOPZ[\fghrst~��gsO[DControlpiston

Outlet

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Closed/recirculation positionMix position�� Fig. 12-13 Operating principle of Hennecke MX mix head.


There are three main types of foam machines: slab stock, blockfoam, and molded foam.

SLAB STOCK FOAM MACHINESSlab stock foam machines produce large continuous slabs or

blocks of foam, which are later fabricated for use in final prod-ucts. Fabricated parts include furniture cushions, mattress cores,carpet underlay, packaging, clothing innerliners, recreational vehi-cle seating, vehicle interior trim pads, and aircraft seating.

A conveying system is an integral part of a basic slab stock foammachine, and most lines are supplied as a single unit. The convey-ing system is a powered slat conveyer that is capable of varying thespeed and angle from horizontal. The conveyer also has moving sidewalls that match the speed of the slat conveyer. The side walls aremoved to adjust the width of the foam block buns. Bun widthchanges are made during production; this width must increase ifsuch changes are made. Many variations of slab foam machineryexist. Machinery suppliers have attempted to make the buns morerectangular to reduce trim losses when the foam is fabricated. Atleast 10% more prime foam is obtainable from a rectangular blockcompared to a conventional crowned block.

To describe a crowned block, Fig. 12-14 shows the profile of arising foam bun. The components are poured onto an angled con-veyer to permit higher buns than are possible on a flat conveyer.As the foam mixture leaves the mix head, it is clear or slightlycloudy (as long as the polyol is clear and contains no fillers). Themix head or a dispensing hose is traversed across the conveyer todistribute the material as evenly as possible. The conveyer andside support walls are lined with treated paper or plastic film thatmoves at the same speed as the conveyer. As soon as the reactionbecomes visible, the material is said to be creaming; the creamline is the distance from the mix head to the start of creaming.

As the foam begins to rise, there is frictional drag on the foamfrom the side wall paper/film. As a result, the foam at the sidewalls does not rise as much as that in the center of the bun,thereby giving a crowned shape. Generally the higher the bun, the

higher the crown. Careful control of the traversing action canreduce the crowning. The foam reaches its maximum height, atwhich time “health bubbles” appear; these indicate that the cellwalls are rupturing and the gas generated (which causes the foamto rise) is being released. A slight amount of sigh back occursshortly thereafter; this may slightly reduce the crown. A bun’scross-section shows where the trim loss occurs, as indicated inFig. 12-15. Obviously, a flatter bun reduces the trim loss.

Conventional slab stock machines have output capabilitiesapproaching 1100 lb/min (499 kg/min) and conveyer speeds up to33 ft/min (10 m/min). Conveyer lengths and widths vary dependingon the intended use of the foam but are generally 50-200 ft (15-61m) long and 6-8 ft (1.8-2.4 m) wide. Bun height is controlled bythe angle of the conveyer and the output of the machine. Therefore,good coordination is required between conveyer speed, conveyerangle, metering output, and the chemistry to maintain consistentproduction. Conventional conveyer lines (and the conversion ofthose lines to make flat-top buns) require the most floor space ofany flexible slab stock foam line.

Several mechanical methods are used to make buns that aremore rectangular. The machinery is often referred to as flat-topequipment. Numerous patents were issued for these methods.Royalties are frequently required when these special machines areused. Even with royalty payments considered, flat-topping nor-mally results in lower-cost foam production.

The earliest flat-top methods were modifications to the con-ventional conveyer system. A schematic of a conventional systemis shown in Fig. 12-16. Side wall paper is not shown in this figure.

Figure 12-17 shows the Draka/Petzetakis modification. In thisversion, an additional side wall film is used. The film is lifted at arate equal to the rise rate of the foam. Frictional drag is minimizedto give a relatively flat-topped bun. This system is difficult to con-trol and is labor intensive, since additional operators are requiredto handle the film.

Another modification of a conventional machine to give a flat-top bun is the Planiblock, Hennecke, or Econo Foam process.

CHAPTER 12

12-16

FOAM MACHINES

FOAM MACHINES

Sigh back

Mix head

Laydown

Cream line

Begin rise

Fig. 12-14 Slab foam rise profile.


Although the concept of these processes is the same, their detailsdiffer. The Planiblock and Hennecke systems were designed toretrofit existing lines, while the Econo Foam system was a com-plete line system. These systems use a top release paper to helpspread the foam uniformly across the conveyer. The rise is re-strained by special slats or pressure plates to give the flat-top sur-face. The top paper is automatically perforated to allow the foamto gas off, and the foam is essentially continuously molded. Aschematic of the Planiblock system is shown in Fig. 12-18.

The most popular current flat-top equipment is the Maxfoamapproach, developed by Unifoam Company and shown in Fig. 12-19. This approach requires completely new equipment. In this case,the components are dispensed through a fixed mix head at the bot-tom of a trough located at the end of a conveyer. The rising foamoverflows the trough onto an insulated fall plate. The fall plate angleand the conveyer speed are adjusted to keep the top surface flat. Inessence, the foam “rises” down. The original equipment had capa-bility for only one width at a time. To get varying widths, the troughwas exchanged. Newer versions, called Varimax, can change thewidth easily and quickly. The output of a Maxfoam line is lower

CHAPTER 12

12-17

FOAM MACHINES

Trim scrap

Fig. 12-15 Conventional slab foam cross-section.

Slat conveyer

Traversingmix head

Release paperfeed roll

Polyethylenefilm feed roll Film lifter

Fig. 12-17 Draka/Petzetakis modification for flat-top foam.

Slat conveyer

Traversing mix head


Fig. 12-16 Conventional slab foam conveyer.


than that of a conventional line; outputs range from 440–880 lb/min(200–400 kg/min), and conveyer speeds are 13–26 ft/min (4–8m/min). Bun heights are normally 40–50 in. (1–1.3 m).

All of the conveying systems described previously are designedfor high-capacity production of slab foams. The basic layout of theline requires a large floor area. A machine was developed to reducefloor space requirements while providing accurately shaped roundor rectangular buns. This is the Vertifoam® machine, developed byHyman PLC and Crain Industries, now licensed by VertifoamInternational, Ltd., which produces foam vertically instead of horizontally. The metering unit capacity is low compared to that ofhorizontal machines: 200 lb/min (91 kg/min). Maximum block di-mensions are 7 2 7 ft (2.1 2 2.1 m), and the block is cut to 4.5-ft(1.4-m) lengths at the end of the foaming conveyer. Block dimen-

sions can be varied quickly, and round blocks up to 5.5 ft (1.7 m)in diameter are possible. The machine is shown in Fig. 12-20.

Round blocks are also made on other machines with specialconveyer configurations and are desirable to reduce scrap losseswhen foams are peeled from a bun in the same manner as trees arepeeled for plywood. Peeling is used to make wide, continuoussheets for applications such as carpet underlay.

With the elimination of CFCs as auxiliary blowing agents, for-mulation adjustments were necessary to make various foam grades.Some of the chemical modifications consist of low-boiling-pointhydrofluorocarbons (HFCs) and normal gases, such as carbon diox-ide and cyclopentane. These materials present unique meteringrequirements for successful use. The products are used by addingthem batchwise to a proper mix tank. This requires higher-pressure-

Slat conveyer

Traversingmix head



Hold downslats

Fig. 12-18 Planiblock flat-top bun modification.

CHAPTER 12

12-18

FOAM MACHINES

Fixedmix head

Trough Belt conveyer

Fall plate

Bottom paperfeed roll

Side wall paper feed roll

Fig. 12-19 Maxfoam/Varimax slab foam equipment.


rated tanks than those normally used. Cyclopentane adds the com-plication of being a flammable and potentially explosive product.

Machinery manufacturers have responded to the challenge byintroducing in-line blending equipment. These units include theCellomat™ system from Hennecke and the EasyFroth™ systemfrom Cannon. Cannon has gone one step further, introducing acomplete slab foam production line, the CarDio® system, asshown in Fig. 12-21. Liquid carbon dioxide is blended into thepolyol and fed to a multicomponent low-pressure mixer. Themixed materials exiting the mix head are a rapidly expandingfroth. To control this froth, a special laydown device stabilizes thefroth and provides a smooth laydown with the foam pre-expandedto about 30% of its full height. Conveyer speeds are generallyslower and output is lower than those for Maxfoam machines, butthe pre-expansion gives a fully cured foam quicker. A typical out-put of 90–130 lb/min (41–59 kg/min) and a conveyer speed of 3.3ft/min (1 m/min) is used to produce full-size blocks. The CarDiosystem can be retrofitted to existing conventional or Maxfoamslab stock machines.

Since the carbon dioxide that is generated in the water-iso-cyanate reaction is the primary blowing agent in flexible foams,increasing the water level is a means of replacing CFCs or methyl-ene chloride if the objective is only to adjust density. Unfortunately,other foam properties are also affected (some adversely). While

other formulation changes can overcome physical property changes,higher water levels give higher exothermic heat, which can lead tofoam scorching (discoloration) or autoignition. To overcome thisproblem, rapid cooling systems were developed and patented forslab stock foams; these systems eliminate the need for auxiliaryblowing agents. Two commercial systems are Enviro-Cure®, devel-oped by Crain Industries, and RapidCure®, developed by GeneralFoam. Both of these systems can be retrofitted to existing convey-ing systems. They use cooling chambers to pull cool air through thefoam to reduce its temperature. A short delay between foaming andcooling is desirable to get the proper foam cure. Depending on theparticular foam machine operation, the foam block may requiretrimming, or barrier films may be applied to ensure that the cool airflows completely through the foam. The RapidCure system addi-tionally has a carbon absorption tower to prevent volatile materialemissions to the atmosphere.

For many years, foam formulations were modified to maintainthe density of the foams as the production plant altitude varied. Athigher altitudes, the foam has a lower density for a given formula-tion. Taking this into account, Foamex patented the variable-pres-sure foaming (VPF) slab stock process, which can produce a widerange of densities using water as the only source of blowing agent.This is done by adjusting the ambient pressure around the foam asit reacts and rises.

BLOCK FOAM MACHINESPreparation of foams in large discrete blocks was done for many

years, primarily in lesser-developed countries. Machinery developedfor this purpose is shown in Fig. 12-22. The foam is supplied tothe day tanks as two components: a polyol blended with water andcatalysts and an isocyanate. These components are fed automati-cally to the tanks. The cycle starts with the polyol component in themix tank with both weigh tanks filled. The mixer is started, and thedrain valve for the isocyanate tank is opened. After a predeterminedmix time, the hinged bottom of the mix tank opens, dropping themixed foam reactants into the mold box. The bottom then closes,and the next polyol component charge is fed to the mix tank to act

CHAPTER 12

12-19

FOAM MACHINES

Blockcut-off

Paper rewind

Paper feed

Foam mixture

Cut blockto storage

Proc

ess

dire

ctio

n

Fig. 12-20 Vertifoam® equipment.

CO2meteringpump

CO2 -polyolpremix

unit

CO2 Polyol

Laydown device

Low pressuremix head

Additionalcomponents

Fall plateConveyer

Isocyanate

Fig. 12-21 CarDio® slab foam schematic.


as a diluent to the residual foam and to nearly eliminate foam build-up in the mix tank. The mold box, which is on wheels, is movedaway from the machine. An end flap is raised and a floating lid isplaced on the top to achieve the desired rectangular configurationfor maximum foam utilization.

This equipment, commonly known as the “Golden Bucket”,presents several processing difficulties. The components requirecareful formulation to avoid foaming problems with the partialprepolymer that is left in the mix tank at the end of each cycle. Ifmix tank cleaning is required, the component in the tank is oftenscrapped. As the foam mixture is dumped from the mix tank, itsplashes and traps air, frequently leading to foam defects. It is dif-ficult to make low-density foams with this equipment.

The discrete block foaming system was updated by developingand patenting the Controlled Environment Foaming (CEF) systemby Foam One. In principle, it is similar to the VPF system previ-

ously described, where the foam is made under reduced-pressureconditions. This process can eliminate an auxiliary blowing agentin preparing low-density foams. Foams are made in any cross-sec-tion desired for final fabrication. The foam components are intro-duced into a closed mold. By ensuring full mold fill under vac-uum conditions, the foam yield is maximized, and larger blocksare possible than with the older block foam method. An additionaladvantage of the CEF system is that the foam is virtually skin free.A schematic of the CEF system (less the proprietary mixingequipment) is shown in Fig. 12-23.

MOLDED FOAM MACHINESMolded foam techniques are used when the final product can-

not be made by fabrication from slab foam because of design,cost, or chemical reaction constraints. Molded products includeitems such as automotive cushions with molded-in frames or sup-port wires, automotive instrument panels, automotive exteriorbody parts, refrigerators, building doors, and metal-faced con-struction panels.

Metering units for molded foam are either high- or low-pres-sure units with the output sized to most effectively produce therequired part. There are several types of conveying systems avail-able for molded foams such as rotary tables, hanging conveyers,drag chain conveyers, shuttle systems, and laminating lines. Thetype of conveyer chosen depends on factors such as the number ofparts required per unit time, foam cure rate, pour technique, andpart complexity, which may require multiple operations or moldmanipulation.

With the conveying system, some type of mix head support isrequired to manipulate the head over the mold. This can rangefrom a manually operated boom to an automatic multiple-axisrobot. Molds are poured while either open or closed.

FOAM CONFIGURATIONThe flexible molded foam industry grew with three basic tech-

nologies: cold cure, hot cure, and high resiliency. Cold-cure foamsare processed at ambient temperatures. Molds may be temperature

CHAPTER 12

12-20

FOAM MACHINES

Mold A

Mold handlingsystem

Foam

Round or rectangular

Door

Mold B

Pressure control valve

Containment vessel

ScrubberVacuumpump

Reactive mix

Fig. 12-23 Foam One CEF system.

Weightanks

Mixer

Mix tank

Mold box

Hinged bottom

Fig. 12-22 Block foam (Golden Bucket) machine schematic.


CHAPTER 12

12-21

CONVEYING SYSTEMS

controlled to maintain uniformity of processing and properties;however, acceptable foams are made without additional heat.Many products are made in this way, including automotive interiortrim parts, instrument panels, door panels, head restraints, andarmrests. Hot-cure foams for cushioning applications require cur-ing in the mold at elevated temperature to provide satisfactory

processing and products. High-resiliency (HR) foams were devel-oped in the 1970s to meet new requirements for automotive seat-ing. These foams require lower temperatures and shorter curetimes than hot-cure foams and are the predominant materials formolded cushioning today.

CONVEYING SYSTEMSHot-cure molded foams require conveying systems with high-

temperature oven capabilities. The typical conveyer configuration isa “racetrack” design, with a drag chain conveyer. Molds aremounted in carriers that ride on parallel drive chains. Mold openingand closing is usually done by using curved rails. When high-resiliency foams were introduced, production was started on exist-ing lines. Modifications were made to increase line speeds and toreduce oven requirements. Few changes were required to the meter-ing equipment. Racetrack lines are continuous movement lines, butit is difficult to provide special movement of individual molds. Aschematic of racetrack lines is shown in Fig. 12-24. Racetrack linesare most frequently used when large volumes of identical parts arerequired over extended periods of time. All mold temperature con-trol on these lines is done by hot-air recirculation. As technologyadvanced, the cure temperature for high-resiliency foams decreased,and the ovens on older lines often served only to conserve foamexothermic heat for cure enhancement. Mold carriers are switchedon these lines by using forklifts or overhead cranes.

A variant of the drag chain conveyer system is the hanging con-veyer. In this design, the molds are mounted in carriers suspendedfrom an overhead conveyer chain. Provisions are made for moldtemperature control by mounting hot-water heater units in the moldcarriers. A single mold heater may serve several molds. Power to theheaters is supplied through slip rails. Because the mold carriers areprecisely positioned, overhead conveying systems offer the possibil-ity for more mold actuation, such as for clamping airbags and forautomatic opening and closing devices. Mold carriers are changedby guide rails that are switched onto the overhead system.

Drag chain conveyers are also made with a single drive chain,with wheeled carts riding on a steel track. Various mold sizes canbe mounted on the carts to provide production flexibility. As withthe overhead chain type, power is supplied to the mold carriers fortemperature control or mold actuation.

Rotary table or carousel systems are used for many molded foamapplications, both flexible and rigid, and the lines may be custombuilt for specific part applications. The usual construction for heavymolds features the table supported by a central bearing point, withthe outer edge supported by heavy-duty nonpneumatic wheels. Thetable is electrically or hydraulically actuated by a caterpillar drive thatengages drive dogs on the outer edge of the table, a drive chainaround the circumference, or pneumatic tires driving along the outervertical edge. Light-duty tables are supported by only the centralbearing; they are driven by the same means as the heavy tables or bycam follower systems.

The tables are usually indexed between stations. Indexing pro-vides more working time for operators and simplifies the pourpatterns and mix head carriers. While mold carriers can bechanged automatically without interrupting production, cost con-siderations generally require that the molds are changed by forkliftor overhead cranes during break periods. Rotary tables are moreversatile than the racetrack systems, since it is easy to install a

variety of molds on the table and achieve proper control. Moldsare frequently temperature controlled with tempered water sys-tems, and provisions for vacuum are easily made when moldingparts into plastic or special fabric covers. A heavy-duty rotarytable system is shown in Fig. 12-25.

Many molded foam applications are best met by using station-ary molds. These applications include a variety of parts, such asautomotive trim parts and instrument panels, automotive exteriorbody panels, residential entrance doors, specialized seating prod-ucts, and refrigerated display cases. Multiple molds are servicedby using manually operated booms or by robots. Another approachis to use ring line systems. This system has multiple mix headsthat are supplied from a single metering unit. The mix heads areoperated in any sequence; however, only one head may pour at agiven time. While the mix heads are most likely fixed to themolds, the system is adaptable to boom or traverser operation. Thenumber of heads possible is determined by the line length andfoam reaction time requirements, with a typical maximum of 12heads. Figure 12-26 shows the concept.

Hot cure foam configuration

High resiliency foam configuration

18 9

4

67 10 2

3

5

67 10 2

18

1. Pour station 2. Mold closure 3. High-temperature oven 500 - 600¡ F (260 - 316¡ C) 4. Cure oven 250 - 300¡ F (121 - 149¡ C) 5. Cure oven 300 - 350¡ F (149 - 177¡ C) 6. Mold opening and part removal 7. Mold cleaning 8. Mold release application 9. Mold cooling10. Insert placement

Fig. 12-24 Molded foam lines—racetrack configuration.


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CONVEYING SYSTEMS

Stat

ion

7Sta

tion 8

Station 6

Stat

ion

3Station 2

Station 1

Statio

n 4

Station 5

Station 1Part removal and mold cleaning

Station 2Automatic mold release spray

Station 3Part substrate placement

Station 4Automatic foaming into mold

Stations 5 – 8Foam cure

1. Rotary table2. Ventilation system support3. Main control panel4. Safety fence

5. Mold release spray unit6. Mold carrier7. Mix head carrier8. Foam metering machine

9. Table drive unit10. Mold temperature control unit11. Chiller unit12. Vacuum system

8

4 7

6

5

4

9 1 2 3

10

11

12

Fig. 12-25 Molded foam line—rotary table configuration.

Tool and M


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Foam molds for cushion production are usually cast aluminum;however, some small-scale operations have used epoxy or glassfiber-reinforced polyester or epoxy molds. Molds for integral skinand microcellular foams used for automotive exterior parts arenickel-plated aluminum or steel since grain, texturing, or Class-Apaintable surfaces are important for the applications. Automotiveinstrument panel and door trim molds are typically cast or machinedaluminum or epoxy-surfaced cast aluminum. The epoxy surfaces onthe aluminum facilitate tuning molds for better fit. Metal molds areoften made with cast-in temperature control coils for optimum effi-ciency. Machined metal tools are gun drilled for water temperaturecontrol. Some molds for small parts, such as those for shoe soles,are electrically heated. Molds are either self-contained (all means ofactuation are part of a single unit) or they are mounted in mold car-riers. Mold carriers range from simple clamping frames to heavyhydraulic presses, depending on the total system requirements.

MOLD CARRIERSSimple clamping frames are used on the drag chain and hanging

conveyer systems. The back edge of the frame is hinged, and thefront edge is equipped with some type of toggle clamp. Openingand closing of the mold carrier is a function of the conveying sys-tem. The molds are adjusted in the frames to achieve the propersealing at the parting line. Small molds used on stationary systemsalso use this concept, since they are easily manipulated manually.

The simple mold carriers can be upgraded to carriers withhydraulic or pneumatic actuation for opening, closing, and locking,and all are controlled by programmable logic controllers (PLC).

These carriers are also provided with inflatable air cushions tomove the cavity or lid of the mold after the frame is clamped toensure full metal-to-metal contact across the entire parting line. Thiseliminates frequently adjusting clamps during prolonged productionor when molds are changed. The air cushions provide self-adjustment.

As molds get larger or the foaming pressure increases, the pre-viously discussed mold carriers may not be suitable. Mold carriersthat have their origins in presses used for woodworking, plasticcompression molding, and injection molding have been developedfor foam handling. Both pneumatically operated (Fig. 12-27) andhydraulically operated (Fig. 12-28) mold carriers are available.These carriers are used in producing door panels, steering wheels,computer housings, and automotive body panels and fascias.

FIXTURINGFour-post presses used for mold spotting or compression mold-

ing are also used in foam-part production for products such as res-idential doors or refrigeration units. Part fixtures with metal orplastic faces and other fittings are placed in the press. When thepress is closed, the foam is injected into the cavities through pourholes in the part.

For hot-molded flexible foams, the lids generally “float” underrestrained conditions, with the lid lifting from the foam rise andthen settling as sigh back occurs. Vents are drilled in the lid inappropriate areas to eliminate trapped air. Venting is also placedaround the parting or flash line, where the cavity and lid meet.High-resiliency foam molds are of higher quality than hot-curemolds. They are of heavier construction and have very tight flash

CHAPTER 12

12-23

FOAM MOLDS

Isocyanatemetering unit

Polyolmetering unit

Hyd

raul

ic

Fixedmix head

Blockingunit

Switch-overunits

2 3 41

Fig. 12-26 Molded foam line—ring line concept.

FOAM MOLDS


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FOAM M

OLDS

Upper moldsupport

Lower moldsupport

Air bagpressure units

Base frame

Controlpanel

Fig. 12-27 Pneumatically operated mold carrier.

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FOAM M

OLDS

8� �� *((�� **��*+��(4(4��'(��77C7�75¡

15¡15¡

E7

X1

X2

Upper edgestroke table

E8

Fig. 12-28 Hydraulically operated mold carrier.

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lines; the lid is not permitted to lift during the foam rise, becausethis can lead to foam collapse problems. Some molds are fittedwith extruded silicone rubber seals to ensure sealing. Venting isalso more carefully controlled with high-resiliency foams.

MOLD TEMPERATUREThe initial mold temperature is critical. With hot-molded foams,

the optimum is about 100˚ F (38˚ C). Much lower temperatures givea soft foam layer, a densified layer, and then the desired foam in thecore. As the temperature is increased, the densified layer becomesthinner and closer to the surface. When the mold gets too hot, theskin that forms is loose and flaky, creating a poor appearance, orareas of very closed cell foam can exist. High-resiliency foams canbe poured into cold molds with few problems other than higher den-sity and longer cure, since there is usually no skin and none of thedensification that occurs with hot foam. If the mold gets too highwith high-resiliency foam, outer surface defects and collapse be-come a problem.

Instrument panels can show a range of problems if the moldtemperature is not correct. These panels are a composite of an outerskin (ABS, vinyl, or polyolefin), semiflexible foam, and an insert orretainer (plastic, metal, or wood fiber). If the mold temperature islow, the foam density increases, and the part does not completelyfill out. If the mold temperature is too high, premature foam gel canoccur, resulting in collapse or voiding. Gas pressure may alsoincrease to the point that when the part is demolded, the skin isblown away from the foam. Shrinkage may also be a problem.

CRUSHINGCushion foams usually have a large number of closed cells on

demold. This requires crushing the foam after demolding, sincethe foam otherwise shrinks. Crushing is done by passing the foamthrough crushing rolls or by putting it into a vacuum chamber,where it is cycled through atmospheric pressure and a vacuum.Special techniques are available for high-resiliency foams to elim-inate the need for crushing. Among these is the patented timed-pressure release (TPR) method, in which the foam mold is openedand then reclamped during the closed mold cure cycle. The sameprinciple is also used in instrument panel production to prevent“gassing,” or blowing away of the cover on demolding.

Rigid foam molds for some applications are essentially the sameas the molds used for flexible foams. Wood simulation parts aremade with silicone rubber mold liners so that the exact surfacereplication is achieved. For molded products such as refrigerators,hot-water heaters, and doors, the outer metal or plastic shell of theproduct acts as the mold; however, supporting fixtures or jigs arerequired to prevent part distortion. These supporting structures takethe place of mold carriers; they are integrated into shuttle or transferline conveying systems or used in stationary systems specificallydesigned for the product.

MOLD RELEASESMold releases are used to facilitate demolding of foam parts

where the foam contacts the mold surface. Various combinationsof natural and synthetic waxes are used to obtain the best results.For high-resiliency foams, some mold releases add dimethylsili-cone fluids to cause cell breakdown at the foam surface, providinggreater breathability. Mold releases are characterized as water-based or solvent-based, depending on the carrier. Water-basedmold releases are more difficult to use with high-resiliency foams,because the molds are too cold to evaporate the water in a shorttime. Alcohol in the release assists evaporation. For cushionfoams, the mold release does not require removal from the partbefore it is used. Other parts may require removal of the releasebecause of postoperation requirements. In some cases, solvent orsoap washing is required. Some products, such as steering wheelsand wood simulation, use in-mold coatings that act as the moldrelease and form the base or top color for the f inished part.Depending on the character of the in-mold coating, a light film ofa conventional mold release is sprayed on the mold first. Excessmold release can build up on molds. Because part sticking canthen result, operators may incorrectly add more release. The moldrelease can act as a thermal barrier, and trapped solvent can causedefects. To reduce this problem for integral skin and high-densitymicrocellular foams where surface appearance is critical, internalmold releases were developed. These releases are generally propri-etary and covered by patents. The basic approach is to use a mater-ial such as zinc stearate as the internal release. Even with an internalmold release, a periodic light coat of a standard mold release is usu-ally necessary to ensure good part release. Mold release is mini-mized if the part is to be painted.

CHAPTER 12

12-26

FOAM MOLDS

RIGID FOAM LAMINATE BOARD LINES While a number of urethane foam products are manufactured

in essentially their final form, few products match the complexityof production sophistication for faced rigid foam panels used inbuilding wall, roof, and cold storage unit construction. These linesincorporate flexible or rigid facing layers, with rigid insulatingfoam continuously and automatically starting with rolls or sheetsof facing material and the urethane raw materials, and ending withcut-to-length banded panels ready for shipment. A complete pro-duction line is shown in Fig. 12-29. Such a line can produce pan-els in the ranges shown in Table 12-1.

The largest conveyer of this type for steel-faced panels has amaximum output rate of 65 ft/min (20 m/min).

Because of the foaming pressure of the rigid foam, the con-veyer is sturdily built to support the flat panels. Furthermore, theslat conveyer is properly guided to eliminate the pitch of the slatas it reverses direction at the end of the conveyer. This pitch, unless

controlled, results in a polygon effect, which marks the face of thepanel. Likewise, the upper and lower slat conveyers are absolutelyspeed-synchronized to prevent damage to the facings, particularly topainted finishes. Replaceable seal chains run at the edges of theconveyer slats to retain the reacting foam and give the desired edgeprofile of the finished panel.

PRODUCTION CYCLEThe production cycle takes the following steps:

1. The flexible facing material is fed to the roll forming machineto give the desired face appearance. If the face material is notflexible, the sheets are fed into the interface conveyer by avacuum lifter. Roll forming dies can be changed in 30–60minutes to meet varying requirements.


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RIGID FOAM LAM

INATE BOARD LINES

1 Coil unwinding station 3 Rollforming machine

2 Vacuum lifter

Infeed and profiling

4 Coating gantry 6 High pressure metering machine

5 Tank farm

Metering and coating

8 Upper and lower slat chain band

7 Double slat conveyer 9 Sealing chain structure

The double belt

11 Cross cutter (band saw)

10 Cross cutter (circular saw)12 Cooling conveyer

Cutting and cooling

14 Rack stacker13 Stacking system

Stacking and storing

16 Control15 Main control panel

Controlling

15

96

16

5

2

1

3

4

8

7

10 11

1213

14

Fig. 12-29 Double-belt rigid panel production line.

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As produced, many foamed polyurethanes are not immediatelyuseful as an end product. Various fabrication methods were devel-oped for specific types of foam, and some of these are discussed inthe following sections.

TRIMMING, SLITTING AND CUTTINGA variety of equipment is available for cutting and fabricating

slab stock foams into the sizes and shapes required for finishedproducts. Cutting blades for flexible foams are kerfless to avoid rip-ping the foam and producing fine dust. Some types of cuttingequipment are:

Bandknife SlittersThese slitters are positioned at any angle; however, the common

orientation is horizontal or vertical. Some saws are equipped withautomatic sharpeners. In some operations, buns up to 200 ft (61 m)long are slit by fixed-position horizontal slitters and reversible beltconveyers running at up to 200 ft/min (61 m/min). The foam is cutinto thin roll stock that is used for lamination. The loose roll fromthe cutting operation may be placed in another machine for diame-ter compression and wrapping for shipment. Smaller slitter versionsare used with automatic or manual tables for cutting small bun sec-tions. The table surfaces are movable.

Baumer SlittersThese special slitters use bun lengths of about 400 ft (122 m).

The ends of two 200 ft (61 m) buns are glued together to form anover-and-under loop that conforms to the conveyer. The slitter cutsfrom the inside of the loop. The slitter blade is automatically indexeddown to maintain foam thickness. Roll stock is the final product.

Carousel SlittersShort bun sections are placed on a special rotary table that is

equipped with a vacuum to hold the buns in place. The table indexesthe foam through a horizontal slitter blade to cut the foam to the

desired thickness. The carousel and slitter are programmed to givethe desired thick-cut slab foams, which are used for products suchas furniture cushions or mattresses.

Profile CuttersThese cutters are a modification of the bandknife slitters using

tables. The tables are fitted with dies and compression devices. Thefoam is compressed as it passes the slitter blade. When the foamrecovers, it has the desired shape, such as dimpled foam used for pack-aging, mattress pads, pillows, and balls. The concept is shown in Fig.12-30. Modern versions of these cutters use computer-controlled cir-cular knife blades that can cut extremely complicated configurations.

Die CuttersThese cutters are used to cut thin sheets of foam (roll stock) to

the desired contour for applications such as automotive trim coversor panels. Sharp die steel bands are fixed in plywood bases. Thebases are mounted on a stamping press, which may be pneumatic or

Movable platen

Foam

Saw bladepath

Fixed height bed�� Fig. 12-30 Profile cutter.

2. The facings are preheated to ensure proper adhesion of thefoam to the face. Heated air in a closed-loop system is used,since infrared heating is ineffective with bare metal surfaces.

3. Rigid foam is dispersed using high-pressure metering equip-ment. The mix head, mounted on a continuously variable-speedtraversing mechanism, is a special unit with a fan nozzle thatdisperses the foam uniformly across the lower facing material.Since the mix head does not have the self-cleaning ability of astandard high-pressure head, foam build-up in the head canoccur. To avoid production interruptions, two mix heads aremounted on the traversing mechanism, and foam componentflow can be switched between heads. The fouled head is auto-matically decoupled from the traverser for cleaning.

4. The conveyer is temperature controlled by heated air thatpasses through the upper and lower chords of the conveyer.

The side wall chains are equipped with electrical resistanceheaters.

5. Upon exiting the double-belt conveyer, the foam panelenters the cutoff section. Circular or band saw arrangementsare possible. Panels are cut in approximately 10 secondswith a tolerance of ±0.04 in. (±1 mm). For roof panels,which require an end-to-end overlap, saws are arranged toautomatically cut the overlap section.

6. With panels over 4 in. (101 mm) thick, cooling is required,and a cooling conveyer is used.

7. If the panels are used for cold storage units, they are addi-tionally milled to form a tight-fitting, tongue-and-grooveshape.

8. At the end of the line, the panels exit on automatic stackingconveyers and are transferred to banding and wrapping ma-chines as required.

The entire production line is maintained by a central controlsystem that is equipped with monitors. Each section can, however,be operated independently for set-up and maintenance. Full processdocumentation is maintained by integration of a compatible indus-trial computer.

CHAPTER 12

12-28

RIGID FOAM LAMINATE BOARD LINES

FOAM FABRICATION

TABLE 12-1Rigid Panel Sizes

Cut panel length 7–80 ft (2–24 m)Finished panel width 15–50 in. (381–1270 mm)Finished panel thickness 1.5–8.0 in. (38–203 mm)


hydraulic, to cut the foam sheet. When used with special foamsdesigned for flame lamination, the die-cutting operation featureselectrically heated platens/cutters to combine the operations of diecutting with the shaping of a flat foam piece and edge sealing in asingle operation. Lamination of the foam to a cover can also be inte-grated into the operation using appropriate adhesives.

RoutersHigh-speed special rotary blades are used to cut grooves in thin

sheet foam for the same applications as given for die cutters. Routersare used where it is not practical to fabricate the foam by other means.

Log PeelersWhere there is a requirement to have the foam slit wider than a

standard bun width, log peelers are used to cut circular buns in thesame fashion as plywood is cut from wooden logs.

Hot-wire CuttersFoam is also cut by electrically heated wires. Such systems have

limited commercial application because of slow cutting speed, prob-lems with foam melting or discoloration, and high maintenance. Ifsmall foam blocks are required, however, hot-wire cutting can pro-vide very accurately cut blocks.

LAMINATIONLamination is used to bond foam to fabrics or other cover and

substrate materials for use in automotive trim covers, headliners,sound-deadening pads, trunk liners, seating, and similar products.At one time, many automotive seat cushions were made using lami-nated parts; the cushions were referred to as “skived” seats, and theprocess is still used today to make aircraft and recreational vehicleseats. Two important factors in lamination are the choice of adhe-sive and the process used. Adhesive choice is based on economicsand application requirements. Independent of the adhesive andprocess, urethane foam that is laminated to most fabrics requiresgood web tension and alignment control.

ADHESIVE TYPES

Solvent-based Rubber or Urethane SystemsSolvent-based systems are among the highest quality adhesives.

The solvents chosen are generally volatile organic solvents, whichare evaporated by various means. Since many of the solvents are

flammable, excess air is used to dilute the vapors so that they do notsustain combustion or explode in dryer chambers. The solventvapors must be eliminated from atmospheric discharge, through useof a vapor incinerator, carbon absorption, or more modern equip-ment that condenses the solvent for recovery and reuse. Carbonabsorption has the disadvantage of the carbon pellets requiring peri-odic regeneration or replacement. If regeneration is done, the vaporsare recovered or incinerated. Incineration is costly unless the heat isrecovered for plant heating or other process requirements.

Latex AdhesivesThese adhesives are water based but frequently contain other

compounds, such as ammonia, that require special handling of thevapors from the dryer. Higher heat is required for latex adhesivesthan for the solvent types.

Hot-melt SystemsThese systems use a variety of conventional hot-melt glues,

thermoplastic films, and powdered adhesives. Powders have beenused in laminating fabrics for garments since about 1965. Poly-esters are the most frequently used powders. These polyesters arealso supplied in films or meltblown webs or in molten forms. Themost common polyesters are crystalline terephthalate copoly-esters. These types have melting points of 210–270˚ F (99–132˚C) and crystallize from the melt in 1–10 minutes, depending onthe grade. Adhesive crystallinity is important in fabric laminationto withstand laundering or dry cleaning. Melt viscosities cover awide range, and the choice depends on the equipment used forlamination. If the viscosity is too low, the adhesive will flow intothe cellular structure, and poor bonds result. Other solid thermo-plastic adhesives that are widely used include polyamide, EVA,thermoplastic urethane. Adhesives are also made with the copoly-mers of polyamide, EVA, and thermoplastic urethane.

PowderPowder is applied by dry powder spray units. The powder is

applied to one of the substrates, which then passes under heaters tomelt the adhesive. The second substrate is then applied, and thecomposite passes through nip rolls before it is taken up on awinder. Powder application has an advantage over molten hot meltsin that adhesive build-up in melt trays and doctor blades is elimi-nated. A schematic of a dry powder system is shown in Fig. 12-31.

CHAPTER 12

12-29

FOAM FABRICATION

Substrate B

Substrate A

Dry powderapplication

Open mesh conveyer belt Conveyer belt Winder

Nip rolls

Heater

Fig. 12-31 Dry powder laminating line.


CHAPTER 12

12-30

FOAM FABRICATION

CARPET UNDERLAYCarpet underlay has become a major market for urethane foams,

where it has replaced sponge rubber or jute fiber pads. Urethaneunderlay has greater durability and is a more uniform product.

PRIME FOAMUnderlay is produced from virgin foam slab stock that is cut to

the desired thickness. The highest-quality foams used for this appli-cation are made from graft polyols, which incorporate styrene and/oracrylonitrile polymer into the matrix. The slit foam is laminated to asuitable film that permits easy installation of the carpet. Without thefilm, the nonslip characteristics of the foam make it difficult tostretch the carpet and avoid distortion of the underlay. Special lami-nating films made from ethylene-acrylic acid copolymers, ethylene-vinyl acetate copolymer, polyethylene, or terpolymers are used toface the underlay. The film is applied using a machine that heats andapplies it under carefully controlled pressure. The use of film avoidsusing liquid adhesives, which are more difficult to apply.

BONDED FOAMRebond foam underlay is made from urethane scrap and/or slab

foam made especially for this purpose. While the most desirablescrap is from slab production, molded foam scrap is used if it doesnot contain excessive tough skin or mold release residue, whichwould interfere with bonding. Scrap may be a preconsumer or post-consumer product. Bonded foam underlay is characterized by highdensity, high load bearing capability, and relatively low strength

compared to prime foam. The high load bearing capability makesbonded foam a preferred product for commercial carpet underlayapplications. Bonded foam is also used for other products such asautomotive headrests, cushioning inserts for firmness and lateralstability, and energy absorption inserts in snowmobile seats.

To make bonded foam, the scrap is first ground into particlesusing a shredder. A low free-NCO prepolymer is sprayed onto theparticles in a blender/tumbler. If the adhesive is properly applied,the foam particles are not sticky to the touch. The mixture is pouredinto large block molds or processed continuously on special purposeconveyers. Curing is done by heat, water-catalyst blends, or steam,all under varying degrees of pressure that depend on the densityrequired. The blocks are round or rectangular, depending on thefinal fabrication process.

Direct BackingDirect backing of carpet requires that the foam applied to the

carpet does not penetrate the carpet facing. Bleed-through givesan unacceptable product. The viscosity of the foaming materials iskept high, and the carpet must have the proper backing.

In one process developed and patented by Textile Rubber andChemical Co., the foam components are poured on a continuousTeflon®-coated f iberglass belt that has been sprayed with anacrylic release film. The reacting foam components are spreadwith an air doctor blade to obtain a uniform thickness. The reverseside of the heated carpet is then brought into contact with the

Thermoplastic Urethane FilmsThermoplastic urethane films and thermoformable urethane slab

foams can be combined in equipment to both laminate and form thecomposite, providing a product such as insulation or sound-deaden-ing panels for automotive applications. The thermoformable foam ismade in slabs and cut to feed into the transport conveyer of the lam-inating machine. It is heated in a hot-air oven; it then moves into theforming section, and at the same time, the thermoplastic urethanefilm is unrolled onto the sheet and cut to the proper length. Thecomposite is then fed to the forming press, where the heated foam iscooled to set it to shape and fuse the film to the foam. The thermo-plastic film serves as the show and protective surface. A processline for this operation is shown in Fig. 12-32.

Flame BondingFlame bonding is melting a foam surface with an open flame to

produce a bond with a fabric. The foam is melted by a 2000˚ F(1093˚ C) flame before contacting the fabric. This process, used formany years, was originally based on specially formulated flame-laminatable foams. Polyester-based foams were preferred from aprocessing standpoint; however, they created problems in use be-cause of degradation in warm, humid environments. The originalpolyether-based foams did not usually provide the desired bondstrength. With the advent of the newer polyether polyols, a standardgrade of flexible polyether-based foam can now be successfullylaminated. To overcome problems with vapors from flaming thefoam, most makers of flame-laminating equipment have incorpo-rated scrubbers into their machines.

Composite ProductsComposite products containing plastic films (such as PVC or

ABS), nonporous fabrics, rigid plastics, metal shapes, or fiber-board are made using the natural adhesive characteristics of theurethane foam during the foaming process. Mixed liquid foamcomponents are injected or poured between the substrates, and theproducts are formed in suitable molds. Common applications ofthis approach are automotive instrument panels, head restraints,and door panels. The structural RIM (SRIM) process also uses theadhesive characteristics of the urethane foam to bond materialssuch as nonwoven or woven glass fiber into rigid shapes that areused for door panel substrates, sunroof covers, spare tire covers,and bumper beams. In some cases, a plastic film is combined in acomposite in the same operation. The glass fiber mat is placed in amold, the urethane foam is dispensed on the mat, and the mold lidis closed. The rigidity of the product is controlled by the type andamount of glass, as well as by the foam formulation used.

DIELECTRIC SEALING High frequency is used to seal or emboss suitable substrates.

This is commonly used for PVC, polyester, and nylon fabrics orfilms. Special formulations for flexible foams make them seal-able; however, even standard foams can be used in dielectricallysealed products if the foam thickness is small.

Radio frequency power is directed to the composite, and theseal is made by the heat-sealing die or electrode that is firmlypressed against the substrate. Time, energy input, and pressure areadjusted to obtain a satisfactory bond.

Embossing is used to simulate stitching or other decorativepatterns on the sealing film by using suitably engraved brass dies.


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CARPET UNDERLAY

Product dischargeconveyer

Automaticfoam sheet

transfer

Thermoformablefoam supply

palletAligning andfoam feedconveyer

Air heater

Foam sheetcontact heater

TPU film rollfeed and

cladding unit

Formingpress

Manualpart removal

Fig. 12-32 Lamination line for thermoformable foam.

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CHAPTER 12

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CARPET UNDERLAY

foam. After passing over heated platens while still on the continu-ous belt to cure the foam, the carpet is placed onto a finish roll.

Frothing techniques for urethanes that are similar to those forlatex and PVC are also used. Foam expansion is largely accom-plished by frothing, using a mix head as shown in Fig. 12-33. Thefroth is applied to the carpet by either knife coating to a releasepaper, with the carpet positioned on top, or by direct knife coat-ing, as shown in Fig. 12-34. Union Carbide and Dow patentedvariations of this process. Dow licenses the process to use itsEnhancer foam technology.

Spray foam technology was developed and patented by ICI inthe 1950s. A traversing spray distributes a very fast reacting flexible

foam system directly on the back of untreated carpet. The foam isformulated to give partial penetration of the foaming mixture intothe carpet to bond the tuft to the primary backing. After partial cur-ing under heat lamps, the carpet passes over a heated embossing rollto compact the foam skin surface for improved durability.

A signif icant amount of direct-backed carpet is made asmolded products for the automotive industry. Large complex partsare made in this manner, and molding provides the opportunity tovary the foam thickness as required to fit the car. In many cases,the carpet is preformed prior to backing. In addition to providingcushioning, carpet backing serves as a sound insulator.

Froth foam machine

Let-off Take-upCuring oven

250 - 300° F (121 - 149° C)

Cooling

Ambient

Deliveryhose

Knifecoater

Additivedispenser

Uncoated carpet Coated carpet�(Fig. 12-34 Schematic of froth carpet coating line.

Mixing head

Outlet

Rear stator

Rotor shaft

Annular cavity

Front statorRotor

Injection pointInlet

Fig. 12-33 Mix head for froth application.


FOAM TYPES

CHAPTER 12

12-33

FOAM TYPES

RETICULATED FOAMReticulated foam has an extremely open cell structure. These

foams are used for applications such as filters, sound absorbingpads, and speaker covers. Over the years, numerous patentedprocesses were developed. Among these are:

• Immersion of the foam in an aqueous solution of an alkalinehydroxide, a water soluble glycol, and an aliphatic alchohol.U.S. 3,423,337.

• As above but replacing the aliphatic alcohol with a mono-cyclic aromatic alcohol. U.S. 3,423,338.

• Rupturing the cells with a water jet. U.S. 3,862,282.• Rapid and transient heating by use of gas compression heat-

ing. U.S. 3,329,759.• Exposure to light pulses (strobe light). U.S. 3,175,030.• Contact with orthotoluidine. U.S. 3,753,756.• Hydrolysis with aqueous sodium hydroxide. U.S. 3,171,820.• Sealing the foam and combustible mixture in a chamber and

igniting the combustible mixture. U.S. 3,175,025.• Stretching and heat setting of the foam in the distorted state.

U.S. 3,425,890.• Passing a heated stream of gas through the foam to melt the

cell walls. U.S. 3,475,525.

Numerous formulation variations were also patented for theapplication. The preferred methods are those that do not use sol-vents or water due to the extra processing required to dry the foamafter treatment. Perhaps the most common industrial processes arethe light pulse and combustion methods.

RECYCLING FOAMThere is pressure on the plastics industry to increase recycling.

In 1990, the Polyurethanes Division of the Society of the PlasticsIndustry established the Polyurethane Recycle and RecoveryCouncil (PURRC) to address the issue of polyurethane productwaste disposal. The goal was to reduce the amount of poly-urethane that was landfilled by 25% by 1995. Similar activitieswere established in other parts of the world, particularly inEurope, where recycling is far ahead of that in the United States.

Rebond FoamFlexible urethane foams are recycled in many ways. The result-

ing product is rebond or bonded foam. There is a market for thisproduct, and the demand has outpaced the industry’s ability to sup-ply it. Many tons of foam scrap are imported to the United Statesevery year to cover the shortage of scrap for rebond. Scrap fromboth slab and molded foams is used. Rebond foams are character-ized by relatively high densities and low strength properties, butthe most important property for most applications is its high load-bearing capability. Carpet underlay is a major market, and manyspecifications are written for the product. Federal specificationshave limited the extenders that are used for bonding adhesives, andthe amount of polyester scrap that is combined with the polyetherfoam is restricted to prevent aging problems. Postconsumer wasteis no different from preconsumer waste when the product is used inrebond foams.

A major effort is under way to recover foams from automotiveseating. New seat assembly techniques and construction make iteasier to recover this foam. Data show that 97% of the foam from

pour-in-place seats can be recovered; it takes approximately eightminutes to disassemble a front seat. Ninety-two percent of thefoam from cut-and-sew seats is recoverable, and it takes approxi-mately 13 minutes to disassemble this type of front seat.

Recovered scrap foam is worth $500–$1000 per metric ton.

Filler UsageAnother potential application for the scrap from seats or from

high-density foamed elastomers is the Ecostream process devel-oped by Woodbridge. The foam is finely ground and used as afiller in new seats. One need for this technique is to find a moreeconomical way to grind the foam; cryogenic grinding is currentlyused. The Illinois Institute of Technology has developed a processfor solid-state shear extrusion (SSSE), which appears promising.

Ground scrap from reject or recovered high-density foamedelastomers used in bumper covers or other automotive body panelsis also usable as a filler in making new similar parts. When usedin this way, new machinery techniques were developed to over-come swelling of the filler when it was in contact with one of thereactive components. The problem was overcome by providingthird-component metering on RIM machinery. As with the Eco-stream process, the scrap is finely ground. Larger scrap particlesare compression molded to make nonshow automotive parts.

Automotive Shredder ResidueThere are foams in the automobile that are difficult to recover.

They include the foam in instrument panels, door panels, headlin-ers, and sun visors. In all likelihood, these foams would end up aspart of the automotive shredder residue (ASR) that is left from thefinal breakdown of a scrapped car. ASR, or fluff, remains after asmuch metal as possible is removed. It is a mixture of rubber, vari-ous plastics, textiles, rust, and fluids with an average density of 25lb/ft3 (40.5 kg/m3). It is a poor fuel because it contains an averageash content of nearly 58%. The National Research Council ofCanada reported in 1994 that there are several reasonable solu-tions to the ASR disposal problem:

• While there was concern that hazardous materials wouldleach from the ASR if it was landfilled, this was not a prob-lem. In fact, the ASR absorbs hazardous materials from otherproducts in the landfill. ASR also performs better than dirt asa day cover for landfills; it settles less and is not easilywashed away.

• ASR can be used in particleboard when combined with otherrecycled plastics. The resultant board is stronger than woodparticleboard.

• Tertiary recovery processes, such as pyrolysis, can recoveroil, gas, and carbon black.

A more selective mixed ground scrap containing semiflexiblefoam with textiles and other plastic scrap is made into compositesheets. The scrap is mixed with polymeric isocyanate and thenpressed into shape and cured with steam and heat. The product issuitable for glove box liners, package trays, and similar applications.

Rigid foam scrap and panel trimmings, including bits of metal,are processed in the same way as the semiflexible foam previouslymentioned. The resultant product is a very strong and durableboard stock that is used for building walls and floors. Gymnasiumfloors made from this board function very well.


CHAPTER 12

12-34

FOAM TYPES

Chemical RecyclingThere are a variety of chemical means for recovering foams.

Some were commercialized to make polyols that are used forrigid foams or as part of a system for sound-deadening viscoelas-tic foams. In 1992, the PURRC presented a summary of the pos-sibilities for chemical recycling of polyurethanes. This is shownin Table 12-2.

The use of urethanes as a fuel was also evaluated, and thePURRC has development projects underway to determine the bestcourse of developing this fuel.

TABLE 12-2Chemical Recycling of Polyurethanes

Method Patents Literature

Hydrolysis 41 46Glycolysis 57 24Aminolysis 10 10Partial chemical thermal 5 3Pyrolysis 8 63ISO hydrolysis 24 1Thermal processing 5 6Polyester hydrolysis 0 17Other recycle 46 5CFC recovery 3 2Miscellaneous 11 13

TROUBLESHOOTING FOR CONVENTIONAL SLABSTOCK FOAM

Many problems are encountered in producing slab stockpolyurethane foams. The machines often have 10 or more indepen-dent metering streams and chemical costs alone exceeding $1000per minute of operation. For this reason, it is important to quicklyidentify and resolve production problems.

The following list is designed to assist the operator of a conven-tional slab stock machine in solving these problems.

Within any defect, there may be several causes. These causesand the suggested remedies may at times appear contradictory. Thisoccurs because of the multiple factors, chemical and mechanical,that affect foam preparation and are often interrelated. Using thesesuggestions may, however, eliminate some of the trial and error instabilizing line operations. The problems and solutions are mostspecific to conventional foams and production lines; however, someare related to high-resiliency foam and flat-top machine problems.

Before discussing problems and solutions, a brief description ofthe slab formation is given to assist in understanding the problem

areas. Figure 12-35 shows the profile of a rising foam bun. Con-ventional foams are poured on an angled conveyer to permit pour-ing a higher bun than is possible on a flat conveyer. As the foamreactants are discharged from the mix head, they are deposited onthe conveyer (a). The mix head traverses perpendicularly to the con-veyer direction. Traverse speed is adjusted to give a uniform finalbun height. The reactants are clear or only slightly cloudy in the lay-down area when conventional polyols are used. Polymer polyolsresult in an opaque laydown. As the foaming reaction becomes visi-ble (b), the material creams; the cream line is the distance from thelaydown to the start of creaming. A gradual rise occurs throughoutthe foam-blowing reaction (c). When the foaming mass reaches itsmaximum height (d), “health bubbles,” the release of blowing gasesbreaking through the top skin surface, appear. In e, a slight amountof settling or “sigh back” normally occurs.

Tables 12-3, 12-4, and 12-5 describe defects, causes, and pos-sible remedies for producing slab stock foam.

(a) Foam reactants

(b) Opaque laydown

(c) Rise

(d) Maximum height

(e) Settling or “sigh back”

Fig. 12-35 Profile of a rising foam bun.


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TROUBLESHOOTING FOR CONVENTIONAL SLAB STOCK FOAMTABLE 12-3

Troubleshooting for Conventional Slab Stock Foam

Defect Description Possible Cause Suggested Remedy

A. Creeping cream line Cream line moves toward the laydown 1. Rapid foam reaction 1. a. Reduce blowing catalyst levelb. Lower component temperaturesc. Reduce air injection to mix headd. Increase mix head back pressure

2. Machine output, conveyer speed 2. a. Decrease metering unit outputor conveyer angle imbalance b. Increase conveyer speed

c. Increase conveyer angle

B. Undercutting Liquid reactants from The a flow under 1. Slow reaction initiation 1. a. Increase amine catalystthe materials reacting in b (see Fig. 12-35). b. Increase component temperaturesThis can cause streaks, splits, or densification lines c. Increase air injection to the mix head

d. Decrease mix head back pressure

2. Machine output, conveyer speed, or 2. a. Decrease metering unit outputconveyer angle imbalance b. Increase conveyer speed

c. Decrease conveyer angle

C. Boiling Foam not rising, with severe bubbling on the surface 1. Gas generation too fast 1. a. Decrease amine catalystb. Reduce component temperatures

2. Silicone surfactant level too low or 2. a. Increase surfactant levelinactive b. Check surfactant efficiency by

hand mix3. Metering problems 3 Check component stream outputs for

proper amount and consistent flow

D. Streaks Adjacent areas of different cell structure 1. Result of undercutting 1. See B above

2. Improper head traverser speed 2. Adjust as necessary

3. Traverse width too high, 3. Narrow traverse widthgiving streaks near edge

4. Mixer speed too low 4. Increase mixer speed

5. Leaking lines or head connections 5. Eliminate leaksdripping material into rising foam

6. Cold components causing 6. Increase component temperaturespoor mix

7. Splashing during laydown 7. a. Increase nozzle diameterb. Increase nozzle lengthc. Decrease metering outputd. Install diffuser screen

8. Air leaks through mixer seal 8. Replace seal

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D. Streaks Adjacent areas of different cell structure 9. Overheating of mixer 9. Replace or lubricate mixer seal(continued) 10. Improper incorporation of 10. a. Check additive compatibility

additives b. Increase mixer streamc. If appropriate, increase pressure on

additive stream

E. Collapse Foam rises and then dramatically falls back 1. Gas generation too fast 1. a. Reduce amine catalyst levelwith little final rise b. Reduce component temperatures

2. Loss of activity of gel catalyst (tin) 2. a. Check activity of catalystb. Increase tin catalyst level

3. Loss of surfactant activity 3. a. Check activity of surfactantb. Increase surfactant level

4. System contamination 4. Check individual component streams for contamination by hand mix

5. Metering problems, particularly if 5. Check component stream output for collapse areas are mixed with areas fluctuationsof good foam

6. Excessive vibration or jerky motion 6. Make necessary repairsof conveyer

F. Smoking Visible vapors rising from the foam, 1. Isocyanate level too high 1. Reduce isocyanate levelgenerally with a strong isocyanate odor 2. Metering inaccuracies 2. Check component output for

quantity and fluctuations

G. Moon craters, pimples Pock marks or pits on the top surface 1. Excessive air entrapment during 1. a. Eliminate air leakage into mix headof the cured foam laydown, a (see Fig. 12-35) b. Reduce output

c. Increase mixer nozzle diameterd. Increase nozzle distance to conveyere. Install diffuser screen

2. Inadequate dissolved gases in 2. a. Increase day tank pressuresthe major components b. Increase nucleation air

3. Excessive gas entrapment in polyol or 3. Allow gas to escape before using the isocyanate from unloading operations component

H. Splits Horizontal or vertical separations within the foam, 1. Gel rate exceeds gas generation 1. Restore proper catalyst balancesometimes breaking through the skin during the later stages of rise

2. Excessive movement of foam during 2. Reduce conveyer vibration or erratic the later stages of rise movement

3. Gas generation exceeds gel rate 3. Restore proper catalyst balance

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H. Splits Horizontal or vertical separations within the foam, 4. Surfactant level too low or has 4. a. Increase surfactant level(continued) sometimes breaking through the skin lost reactivity b. Check surfactant activity

5. Undercutting 5. See B above

6. Excessively fine call structure 6. a. Reduce amine catalyst levelb. Decrease mix chamber pressure

7. Excessively high isocyanate level 7. Reduce isocyanate level

8. Erratic metering 8. Check component outputs

9. Leaking hoses or fittings 9. Eliminate leaks

10. “Paper” splits 10. Ensure paper (or film) feed is correct and wrinkles are eliminated

11. Excessive reactivity 11. a. Reduce component temperatureb. Reduce all catalyst levels

I. Worm holes, buckshot, Spherical or tubular voids in the foam core 1. Associated with cratering 1. See G abovepeaholes, mouse holes, 2. High surfactant level 2. Decrease surfactant leveland blow holes

3. Gelation too fast 3. Reduce tin catalyst level

4. High auxiliary blowing agent 4. Reduce component temperaturetemperature

5. Leaking hoses or fittings 5. Eliminate leaks

6. Foam residue dropping into foam 6. Clean mixer chamber

J. Tacky skin Bun surface remains sticky for a prolonged time 1. Low catalyst levels 1. Increase catalyst levels

2. Low ambient temperature 2. a. Heat bun surfaceb. Spray bun surface with mist of water

containing amine catalyst

K. Crumbly or flaky skin Surface of foam has little integrity, tears easily 1. Excess isocyanate 1. a. Decrease isocyanate levelb. Check flow rate to ensure uniformity

2. Insufficient mixing 2. a. Increase mixer speedb. Lengthen mixer outlet nozzle

L. Thick or heavy skin Bun surface skin is thick and of high density 1. Low catalyst levels 1. Increase catalyst levels

2. Low ambient temperature 2. a. Warm conveyerb. Heat bun top surface

M. Gummy bottom, Bun bottom surface has closed cells, and separation 1. Tin catalyst too high 1. Lower tin catalyst levelbottom cavitation occurs between bun and bottom paper/film 2. Conveyer too hot 2. Cool conveyer

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N. Closed cells, glassy Cell walls remain intact, giving a shiny appearance 1. Gel rate too high 1. Reduce tin catalyst levelcells, shiners, windows, by light reflection; foam has pneumatic feel 2. Incorrect mixer speed 2. Increase mixer speed and/or decrease and mirrors when compressed hold-up time in mixer

3. Insufficient air nucleation 3. a. Increase air injection to mixerb. Increase day tank pad pressure

4. Mix head pressure too high 4. Decrease mix chamber pressure byreducing output or increasing the outlet nozzle diameter

5. Component temperatures too high 5. Reduce temperatures

6. TDI isomer ratio change 6. Determine if the isocyanate has partially frozen; thaw and mix if required

O. Shrinkage Contraction of foam on cooling, thereby 1. Excessive closed cells 1. See N abovecausing wrinkling and compacting

P. Scorching, discoloration Yellow to brown discoloration of 1. High water and/or high index 1. Reduce water and/or isocyanatefoam bun core; in extreme cases, formulationthis can lead to bun autoignition 2. Bun stacking in storage too close 2. Provide adequate free air space in storage

3. Color change from heat effect on 3. Determine effect of heat on additives; additives, particularly flame retardants choose alternatives as appropriate

4. Formation of “black nylon” in graft 4. Reduce water level(polymer) polyol formulations

5. Inadequate stabilizer in polyol 5. Contact polyol supplierfor higher-stabilized product

6. Delayed exotherm 6. Increase amine catalyst

7. Foam excessively open celled 7. Increase tin catalystfor water level used

8. Component contamination 8. Check for presence of soluble iron salts

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TABLE 12-4Troubleshooting for Flexible Molded Foams

The following table lists common problems encountered in producing molded foam parts such as seat cushions or head restraints. The causes and remedies are specified forboth low-pressure (LP) and high-pressure (HP) impingement mixing machines. While defects and suggested remedies are given as individual items, in practice multipledefects and/or causes may be present, complicating troubleshooting efforts. In considering the causes for specific defects, some causes may be contradictory. The guide belowcan, however, eliminate some trial and error in solving line problems.

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A. Splits Tears in the foam that are generally 1. Gel rate exceeds gas generation rate 1. Adjust catalyst levelsparallel to the surface 2. Excessive movement of foam during 2. a. Change pour pattern

latter stage of rise b. Reduce shot weight

3. Lid movement during critical stage 3. a. Close lid before gel pointof gel b. Avoid lid movement

4. Excessive mold pressure 4. a. Reduce shot weightb. Increase ventingc. Use floating lid (hot-cure foam only)

5. Gas generation rate exceeds gel rate 5. Adjust catalyst levels

B. Collapse Foam cell structure is destroyed and 1. Gas generation rate too fast 1. Adjust catalyst levelshigh-density residue is left 2. Loss of catalyst activity 2. Check catalyst activity and replace

if necessary

3. Surfactant deficiency 3. a. Check to ensure that proper type is used for application

b. Check activity and replace if necessaryc. Increase level

4. Pressure relief collapse 4. a. Clean vents(high-resiliency foam) b. Increase number of vents

c. Reduce vent size

C. Foam shrinkage with Foam surface is dimpled soon after demolding, or 1. Incorrect mixer speed (LP) 1. a. Increase mixer speedclosed cells (“shiny” or foam distorts upon cooling, as dimensions are b. Reduce hold-up time in mixer“glassy” cells) reduced by condensation of gases in closed cells 2. Impingement pressure too high (HP) 2. Reduce impingement pressure

3. Gel rate too high 3. a. Reduce gel catalystb. Reduce component temperatures

4. Mold too hot 4. a. Reduce mold temperatureb. Ensure uniform mold temperature

5. Excess air in components 5. a. Reduce padding pressure on day tankb. Reduce mix air (LP)

D. Coarse cells Large cell structure with harsh foam feel 1. Incorrect mixer speed (LP) 1. a. Increase mixer speedb. Increase hold-up time in mixer

2. Insufficient dissolved air in 2. a. Increase day tank padding pressurecomponents b. Increase mix air (LP)

3. Insufficient shear in mix 3. a. Increase hold-up timechamber (LP) b. Increase back pressure

c. Reduce mixer clearance

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D. Coarse cells Large cell structure with harsh foam feel 4. Surfactant deficiency 4. a. Check surfactant activity(continued) b. Increase tin catalyst (hot-cure foam)

E. Visible pour pattern Laydown pattern of foam visible on 1. Splashing, erratic laydown 1. a. Decrease distance between end of mix foam surface and possibly causing ridges head nozzle and mold

b. Decrease throughput of machinec. Increase outlet nozzle diameter (LP)

2. Dirty mold 2. Clean mold to remove excessmold release and foam build-up

F. Blow holes Large holes in foam from escaping gas 1. Air trapped during pour 1. See E1

2. Gelation too fast 2. Adjust catalyst levels

3. Component temperature too high 3. Reduce component temperatures

G. Loose skin Adhesion of thin foam skin is too low, causing it 1. Mold temperature low 1. Increase mold temperature(hot-cure foam) to separate from the foam 2. Amine catalyst level low 2. a. Increase amine catalyst

b. Increase isocyanate index

3. Dirty mold 3. Clean mold

4. Underfill 4. Increase shot weight

5. Lid movement during rise 5. Limit lid movement by clamps or alignment pins

6. Foam undercutting 6. a. Increase machine outputb. Change pour pattern

H. Flaky skin Crumbly surface or friable skin causes 1. Excess isocyanate 1. a. Check calibration(hot-cure foam) surface to rub off easily b. Reduce isocyanate index

2. Insufficient mixing 2. a. Increase mixer speedb. Decrease mixer clearancec. Increase hold-up

I. Thick skin (hot-cure Heavy, elastomeric skin surface 1. Mold temperature low 1. Increase mold temperaturefoam) 2. Surfactant level low 2. a. Increase surfactant level

b. Use more efficient surfactant

3. Dirty mold 3. Clean mold

J. Tight skin (high- Skin surface has a low porosity, giving a 1. Mold release 1. Switch to mold release with resiliency foam) pneumatic feel a cell opener added

2. Surfactant level high 2. a. Reduce surfactant levelb. Use less efficient surfactantc. Adjust catalyst levels

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K. Hard flash Material vented from mold is very hard 1. Overfill 1. Reduce shot timeand difficult to trim 2. Slow gel rate 2. Adjust catalysts

3. Excess mold pressure 3. a. Reduce shot timeb. Change vent locationsc. Change vent size

A. Air trap void Smooth and shiny bubbles that are near 1. Flow restricted in the mold 1. Revise flow pattern by regating the surface of the foam causing trapped air pockets or removing obstructions to flow

2. Air entrained in foam reactants 2. a. Reduce day tank agitator speedb. Reduce day tank padding pressurec. Maintain day tank at upper limit

3. Air drawn in through gate 3. Seal gate area outside edge

4. Underfill 4. See D below5. Machine output too high, causing 5. Reduce machine output

splashing or sputtering in mold

6. Improper or blocked vents 6. Check vent location, clean blocked vents

7. Improper mold orientation, with 7. Reorient moldnatural high spot too low

B. Foam wet spot Resin- or isocyanate-rich spots at beginning 1. Mix ratio incorrect 1. Recalibrate machineor end of pour, giving soft, hard, or sticky spot 2. Mix head opening slowly 2. Check hydraulic system, particularly

accumulator pressure3. Improper mix 3. Check and adjust impingement pressures4. Fouled component nozzle 4. Remove and clean nozzle; replace

if needed5. Pump malfunction 5. Check pump operation, particularly

pump relief valve

C. Voids Irregular voids with open cell structure 1. Hydraulic oil contamination 1. Check for hydraulic oil leaks at mix head or “ratty” appearance and from hoses

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TABLE 12-5Troubleshooting for Composites Such As Instrument and Door Panels

The following table lists common problems encountered in producing composite foam parts such as those having a plastic or metallic insert/substrate and a formed plasticfilm skin. The causes and remedies are specified for high-pressure impingement mixing machines; however, the suggestions given may apply to low-pressure machine oper-ations. While defects and suggested remedies are given as individual items, in practice, multiple defects and/or causes may be present, complicating troubleshooting efforts.


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C. Voids Irregular voids with open cell structure 2. Mold release contamination 2. Avoid overuse or overspray of mold (continued) or “ratty” appearance release

3. Mold not clamping properly 3. Check mold operation, adjust clamps

4. Turbulent foam flow 4. a. Reduce machine outputb. Eliminate obstructions

5. Mold overfill 5. Reduce shot size

6. Contamination on substrate or skin 6. Ensure that substrate and skin are free ofoil, mold release, or other contaminants

7. Improper component temperatures 7. Check and reset temperatures as needed

8. Movement of substrate during 8. Check substrate fixing points or devicesfoam injection

9. Mold sealing failure 9. Check mold operation for proper clamping; replace seals as needed

D. Underfill Areas of the part not filled with foam 1. Low shot weight 1. Check shot timer

2. Mold clamps open 2. Check mold for proper operation

3. Blocked vents 3. Check and clean vents

4. Mold cold 4. a. Check and adjust temperaturecontroller as needed

b. Start mold preheat before startof operations

5. Components cold 5. Check and adjust component temperatures

E. Splits Larger areas of foam with tears that are 1. Improper stabilization of the foam 1. Adjust catalysisgenerally parallel to the surface of the part 2. Movement of the upper and lower 2. Check mold for proper operation

mold halves before foam gels

3. Movement of the substrate before 3. Check substrate fixing points or devicesthe foam gels

F. Tears Foam separation due to stress occurring 1. Foam sticking to the mold 1. a. Clean moldduring demolding b. Check mold release application

2. Substrate sticking 2. Check substrate fixing points or devices for proper release

3. Negative draft angle 3. Modify mold

4. Vacuum holding skin not released 4. Check for proper vacuum controlbefore demold

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G. Gassing Large gas bubble that blows foam apart at 1. Mold too hot 1. Cool mold to approximately 105˚ F demolding, generally resulting in a split that (41˚ C)is parallel to the surface; may also blow skin away 2. Demold time too short 2. Extend demold timefrom the foam

3. Components too hot 3. Reduce component temperature

4. Vents blocked 4. Clean vents

H. Delamination Foam separates from skin or substrate 1. Basic incompatibility of materials 1. a. Add adhesion promoter to foam systemb. Surface treat substrate (such as with

corona discharge or charring with flame)c. Mold plastic substrate with slightly

rough surface

2. Contamination by mold release 2. Ensure that parts are clean and free of or oil contaminates

I. Waviness Unevenness of part surface not due 1. Foam shrinkage 1. a. Check calibration for proper ratioto mold configuration b. Check component temperature and

adjust if too high

c. Check mold temperature andadjust if too high

2. Overfill 2. Reduce fill time

J. Wrinkles Distortion of the skin 1. Improper forming of skin 1. Check vacuum-forming or skin-casting operation

2. Mold shape and skin forming mold 2. Tune moldsdo not match

3. Unexpected skin shrinkage 3. Check skin compound against standards;adjust as needed

4. Inadequate vacuum to hold skin to 4. Check vacuum pump and controlshape in mold cavity

5. Improper loading of skin into mold 5. Check operator attention to skin loading

K. Surface dent Depression in the skin surface after demolding 1. Foreign matter in the mold 1. Ensure that mold cavity is clean before loading skin

2. Mold defect 2. Check mold condition and repair as needed

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BIBLIOGRAPHY

Bibliography

Bender, R.J., Handbook of Foamed Plastics, Lake PublishingCorporation, Libertyville, IL, 1965.

Buist, J.M., Developments in Polyurethane, Applied Science Pub.,London, 1978.

Buist, J.M. and Gudgeon, H., Advances in PolyurethaneTechnology, Maclaren and Sons, London, 1968.

Doyle, E.N., The Development and Use of Polyurethane Products,McGraw Hill, New York, 1971.

Ortel, G., Polyurethane Handbook, Hanser, New York, 1985.

Saunders, J.H. and Frisch, K.C., Plastic Foams, Part I, MarcelDekker, New York, 1972.

Saunders, J.H. and Frisch, K.C., Polyurethanes, Chemistry andTechnology, Part I, Chemistry Interscience, New York, 1962.

Saunders, J.H. and Frisch, K.C., Polyurethanes, Chemistry andTechnology, Part II, Technology Interscience, New York, 1964.

Woods, G., The ICI Polyurethanes Book, Wiley and Sons, NewYork, 1987.

Woods, G.W., Flexible Polyurethane Foams, Chemistry andTechnology Applied Science, New Jersey, 1982.


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