Evaluation Guide for Selecting the Best FRP Composite Process

for Your Project

Liquid Composite Molding (LCM) vs. SMC

MOLDED FIBER GLASS COMPANIES

In this report, we compare and contrast the properties of the two majormaterial forms used in compression molding of modern FiberReinforced Plastic (FRP) products. Compression molding of FRP inmatched metal dies is the most cost effective method of producing highvalue composites in production volumes from thousands to millions ofpieces annually. This process has the best combination of cost, qualityand properties of all composite forming methods. The two majormaterial forms are Liquid Composite Molding (LCM) and Sheet MoldingCompound (SMC). Both forms take advantage of the benefits of FRPincluding; corrosion resistance, high strength to weight ratio, dimensionalstability, parts consolidation, dielectric strength, minimal finishing, highrepeatability, low tooling costs, and design flexibility.

The principal difference between LCM and SMC compression molding iswhether or not the fiber reinforcement moves, or flows, as the moldcloses to form the part. LCM uses a variety of preformed fiberreinforcements that are positively placed in the mold exactly where theyneed to be in the final molded part. A measured charge of liquidresin paste is placed on a portion of the reinforcement and a controlledclosure of the press flows the paste throughout the stationaryreinforcement to fill the mold. In contrast, SMC is a leather-like,pre-manufactured sheet combining fiber reinforcement and paste thatis cut and stacked to cover a portion of the mold. The controlled closureof the press flows the compound (fiber reinforcement as well as paste) tofill the mold cavity. In other words, the LCM process flows the resinthrough stationary reinforcing fibers to fill the mold cavity and the SMCprocess flows both resin and reinforcing fibers to fill the mold cavity.

Table 1 (page 2) summarizes the material and processing propertiesthat are similar in LCM and SMC.

Table 2 (page 3) summarizes the material and processing propertiesthat are significantly different in LCM compared to SMC.

ABSTRACT

The raw materials of LCM andSMC are primarily thermosettingresin, fiber reinforcement andinorganic fillers such as calciumcarbonate or clay. They typicallyaccount for 95% of the weight ofthe composite, the balance beingrelease agents, curatives, pigmentsand other functional additives. Therelative amounts of the ingredientscan be the same for SMC and LCMresulting in identical physical propertiessuch as density, hardness andflammability. A useful feature ofthese composites is the ability totailor the properties to specificuses. For example, flammabilityand smoke characteristics can beadjusted to meet strict codes formass transit interiors; electricalresistance can be adjusted from ohms(dissipative) to 1012 ohms (insulating);and mechanical stiffness is adjustablefrom flexible to rigid.

The thermosetting resin gives thesematerials good stiffness and hightemperature retention of mechanicalproperties. Thus, much of the growthin use of SMC and LCM is in metalsreplacement. A useful design analogyto metal forming is that LCM ismore like sheet metal and SMC ismore like a casting. One of theunique features of both SMC andLCM is low profile technology,which allows the molded part tohave exactly the same dimensionsas the room temperature steel molds.

This amazing property not onlyyields excellent dimensional control,it results in large panels as smoothas the best sheet metals.

Fiber content is easily measuredand indicative of strength andtoughness. Therefore it is commonlyspecified. Although usually reported as“weight percent”, it is the volumefraction of fiber that provides strength.The orientation of the fiber is alsocritical to strength. As compared to

a composite with isotropic fiberorientation, a composite with all itsfibers aligned in one direction willhave over three times the strengthin the direction of orientation butonly one third the strength at 90˚to that direction. From a designstandpoint, a common misapplicationis to assume isotropic laminates invectorless stress fields. Thissimplification is appropriate formetals design but leads to bothnon-optimized composite designsand to unexpected failures. Compositesare optimally used in designs thatorient reinforcement strengths tothe loads.

Regardless of the choice betweenLCM and SMC, both materialsprovide extreme design flexibility.The designer can specify combinationsof physical and chemical propertiesas well as part configurations thatare not possible with any othermaterial system.

Table 1. Properties that are similar in LCM and SMC

PROPERTY

Raw Materials Fiber Content (by weight)Strength (Average tensile and flexural)

Dimensional ControlHardness

Surface Quality•Orange Peel•Paintability

•Bond/IMC AdhesionPart Details

•ThicknessProcess

•Cycle Time•Net Shape

Tooling

LIQUID COMPOSITEMOLDING (LCM)

SHEET MOLDINGCOMPOUND (SMC)

•Tool Steel Core and Cavity•Stops and Heel Plates•Integral Heating•Telescopic Shear Edges•Ejectors

Resin, Reinforcing Fibers, Fillers, Additives

10-75%Varies the same with formulation

Excellent40 Barcol

Class AVery GoodExcellent

0.03 inch up to several inches

Nominal less than 3 minutes but depends on part geometry

Yes

YesYesYesYesYes

•Texture Yes

Fiber Rein forcement.LCM laminate fiber content can beroutinely varied from about 15% to50% by weight with random fibersand up to 75% by weight withoriented fibers. The fiber contentand orientation throughout theLCM part can vary as required bythe application because the fiberreinforcement is pre-placed in themold and does not move during themolding operation. Standard SMCwill contain between about 10%and 50% fiber by weight withspecial formulations containing upto 75% fiber by weight. Fibercontent throughout the SMC part isvery consistent as a result of thecompounding process and thecoupled flow of resin and fiberduring molding. For the same reason,fiber content throughout the SMCpart cannot be tailored or optimized tomeet application requirements.Fiber orientation in SMC is greatlyinfluenced by compound flow asthe mold closes and is therefore moredifficult to control.

Fiber Length.The most common fibers are E-glassrovings chopped to a length (orblend of lengths) from 1/8” to 4”.There is a relationship betweenfiber length and strength – longer isstronger. For polyester resins thestrength curve is asymptotic atabout one inch. Since longer fibersdo not flow as well, most SMCfibers are one inch long. LCMfibers are often longer because itimproves the integrity and thepermeability of the mat. Sometypes of impact performance andtoughness are improved by thelonger fibers. Both SMC and LCMcan incorporate continuous fibers ifrequired. Because the fibers do notflow, LCM can also incorporatefabrics and non-woven reinforcements.

Fiber Orienta tion.The orientation of reinforcing fiberin the LCM laminate is determinedby its positive placement in the moldcavity. Since the LCM reinforcementdoes not move during mold closure,the fiber orientation is highly tailoredand predictable. The fiber orientation

in SMC laminate is more difficult topredict and control. Because the fiberflows with the resin, the fibers tend toorient themselves in the direction offlow. This phenomenon can becontrolled to some extent by chargesize and location but this practicegenerally causes performance trade-offswith other laminate properties.

Ribs and Bosses.Because of the difficulty ofpre-forming glass fibers to detailssuch as ribs and bosses, LCM islimited to generally uniform crosssection shapes. The pre-combinedfibers in SMC on the other handflow easily to conform to manycomplex shapes.

Material Density.In order to properly flow during themolding process, SMC must have atleast a given proportion of mineralfiller. This restricts the density orspecific gravity of SMC on the lowend. LCM does not have such arestriction and therefore given identicalpart thickness, LCM can produce alower density part.

Table 2. Properties that are different in LCM and SMC

PROPERTY

Strength•Minimum Value•Standard Deviation

Local TailoringDensity

LIQUID COMPOSITEMOLDING (LCM)

SHEET MOLDINGCOMPOUND (SMC)

HigherLower

Can be Lower Low-side limited by minimumrequired filler content.

Yes Limited

Toughness (Impact and Hot Strength)Surface Quality (Waviness)Part Details

•Ribs•Bosses•Molded - in hardware•Molded - in studs•Molded - in cores•Thickness

Process (Required Molding Pressure)

Better GoodBetter Good

No YesLimited Yes

Yes LimitedNo YesYes Limited

Limited Yes100-500 psig 700-1,000 psig

Part Weight.Under the same design stressconditions, LCM can produce alighter weight part compared to SMC.This is the result of more consistentstrength, higher impact toughness andthe flexibility to design with lowerdensity laminates. See MaterialDensity and Mechanical Propertiessections for further discussion.

Mechanical P roper ties. LCM creates composite structuresthat provide higher design strengthallowables than similar laminatesmade from SMC. Since 1974, theMolded Fiber Glass Companies(MFG) has periodically comparedits modern LCM processes withstate-of-the-art SMC formulations.

Recently, MFG completed its latesttest runs on nearly 500 samplestaken from truck hoods that werecompression molded using the twoforms. The current samples weretaken from two current model truckhoods – one a three-piece hood ofSMC, the other a one-piece hoodof LCM. The LCM hood had anaverage glass content of about 20%by weight and the SMC hood wasslightly higher at about 25% glasscontent by weight.

Similar sample maps were devel-oped for each hood with specimensoriented in various importantdirections (longitudinal, cross-car,vertical and horizontal). ASTMD638 tensile and D790 flexuralstrength tests were performed onthe samples. Results for samplesfrom each area of the hoods were

averaged and plotted in boxplotsand frequency histograms asFigures 1 through 6.

•Flexural Strength.Figure 1 is a pair of boxplotscomparing the flexural strength ofLCM and SMC samples. FlexuralStrength describes the amount offorce required to bend and breakthe material when a specific thicknesstest piece is bent. A test piece issupported at both ends, a force isapplied to a small, concentratedarea in the center and the force andamount of bending is measured.Results are reported in pounds persquare inch (psi). See ASTM D790for specifics.

The horizontal line near the centerof each plot shows the samplemean, or average flexural strength.The box areas above and below themean line each represent onequartile or 25% of the datasamples. The whiskers above andbelow the boxed area each representthe remaining quartiles. The figureclearly shows that although theaverage flexural strength of the twoforms is about the same, the LCMform produces much less variationthan the SMC form. A closer lookshows that as many as 35% of theSMC samples have flexural strengthbelow the weakest LCM samples.

Figures 3 and 4 are the flexuralstrength frequency histograms forthe LCM and SMC samples. Thesehistograms show that although theaverage strength of both the LCMand SMC samples are about the

same, the variation in flexuralstrength is much greater in SMCthan in LCM. In fact, 6 of 14 SMCsample areas had average flexuralstrength below the lowest performingareas in the LCM part.

•Tensile Strength.Figures 2, 5 and 6 show results ofthe tensile strength testing on theLCM and SMC parts. Tensilestrength describes the amount offorce (tensile stress) required tobreak a sample of specific dimensionswhen the material is stretched to itsbreaking point. The tensile stress atfailure is divided by the cross-sectionalarea of the sample and results arereported in pounds per square inch(psi). See ASTM D638 for details.The tensile strength comparisonresults are similar to the flexuralstrength in that the average tensilestrength of the two forms is aboutthe same but the variation in theLCM parts is much lower than theSMC parts. One particularlyimportant result in the tensilestrength test is that the highestfrequency (or mode) of SMCstrength samples is at the lowestend of the scale whereas the modeof LCM strength samples is abovethe average strength.

Although the current testing showsthat some areas of SMC can havevery high strength compared toLCM, it is the low end of thevariation that causes parts to fail.In both the design process and enduse, when any area of laminate issubjected to stress, it is the weaker

locations that fail, similar to theweak links in a chain. The moreconsistent, controllable and predictableproperties of the resulting laminatemake LCM the process of choicewhen flexural and tensile strengthsare important.

•Impact Strength.Figure 7 is a comparison of impactstrength, or toughness, of LCM andSMC laminates as measured usingthe Dynatup testing method. Thismethod measures the reactive loadproduced when a weighted dart isdropped onto the laminate surfacefrom a controlled height. Theimportant performance measurementsare the energy absorbed (Max Load

x Drop Height) and the heightwhere penetration of the dart intothe material occurs.

The LCM laminate performs about25% better than SMC in the energyabsorbed in this test. Also, penetrationoccurs in the SMC at the 24 inchdrop height while the LCM requireda drop height of 45 inches forpenetration to start.

Molding Pressure.The molding pressure for LCM isusually a fraction of that requiredfor SMC. Therefore, the presstonnage required is less for LCMthan for SMC for a given part area.This is because the viscosity of the

LCM paste is very low comparedwith that of SMC and no reinforc-ing fibers are displaced in LCM, asthey are in SMC. In practice, thisallows compression molding oflarger parts in LCM than in SMCfor a given press tonnage and withinthe platen size limits of the press.

SMC Manufacturing Equipment Robotic Preform Equipment

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Liquid Composite Molding (LCM) and Sheet Molding Compound (SMC) produce laminateswith the same average flexural strength. However, the variation in strength at random orientationsthroughout the part is much less in LCM than in SMC. Glass content is 20% by weightfor LCM samples and 25% for SMC samples.

Liquid Composite Molding (LCM) and Sheet Molding Compound (SMC) produce laminateswith the same average tensile strength. However, the variation in strength at random orientationsthroughout the part is much less in LCM than in SMC. Glass content is 20% by weightfor LCM samples and 25% for SMC samples

Figure 2 Tensile Strength (TS) Comparison Boxplots

Figure 1 Flexural Strength (FS) Comparison Boxplots

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Distribution of Flexural Strength test results for samples taken from a truck hood moldedin the Liquid Composite Molding (LCM) form. Variation is low with a minimum sampleflexural strength of about 21,000 psi. Glass content is about 20% by weight.

Figure 3 Liquid Composite Molding (LCM) Flexural Strength (FS) Histogram

Figure 4 Sheet Molding Compound (SMC) Flexural Strength (FS) HistogramDistribution of Flexural Strength test results for samples taken from a truck hood moldedin the Sheet Molding Compound (SMC) form. Variation is higher with a minimum sample flexuralstrength down to about 15,000 psi. Glass content is about 25% by weight.

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Figure 5 Liquid Composite Molding (LCM) Tensile Strength (TS) HistogramDistribution of Tensile Strength test results for samples taken from a truck hood moldedin the Liquid Composite Molding (LCM) form. Variation is low with a minimum sampletensile strength of about 6000 psi. Glass content is about 20% by weight.

Distribution of Tensile Strength test results for samples taken from a truck hood moldedin the Sheet Molding Compound (SMC) form. Variation is higher with a minimumsample tensile strength of about 6000 psi. Glass content is about 25% by weight.

Figure 6 Sheet Molding Compound (SMC) Tensile Strength (TS) Histogram

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The current study compares andcontrasts the properties of FiberReinforced Plastic (FRP) compositesproduced from Liquid CompositeMolding (LCM) and Sheet MoldingCompound (SMC). Careful selectionand application of either form yields

Liquid Composite Molding (LCM) laminate demonstrates higher impact strength than sheetMolding Compound (SMC) laminate. Dart penetration begins at nearly 50% higher level forLCM compared to SMC.

Figure 7 Impact Strength Comparison of Liquid Composite Molding (LCM) and Sheet Molding Compound (SMC)

high quality products and providesextreme design flexibility. The designercan specify combinations of physicaland chemical properties as well as partconfigurations that are not possiblewith any other material systems.

Because of the versatility of FRP/Composites, the designer is encouragedto collaborate with a molder and/ormaterial supplier to optimizethe application.

MOLDED FIBER GLASS COMPANIES

MFG Is a Full Spectrum Composite Molding Supplier

Molding Process Capability• Open Molding (Hand Lay-up, Spray-up)

• Vacuum Infusion Processing (VIP) or RTM-lite• Resin Transfer Molding (RTM)

• DCPD RIM (Reaction Injection Molding)• MFG PRiME™ Process – Compression Molding with Preform Reinforcement

• Compression Molding • Direct Long Fiber Thermoplastic Molding (D-LFT)

• Filament Winding

Other Value-Added Processes• Robotic Gel Coating

• In-Mold Coating (IMC) • SMC/BMC Compounding

• Directed Fiber Preforming, APP• Bonding and Assembly

• Robotic Routing and Water Jet • Prime and Topcoat Paint Capabilities

• In Line Sequencing

The best value supplier is strategically located for your needs – perhaps near your operations, supply source, market or a speci�c labor market. MFG has established the largest full-service network of molding factories in North America. This factory network is supported by a shared foundation of engineering, design, R&D, procurement and program management that ensures consistent quality and service company-wide. This structure allows us to provide the most competitive cost structure possible.

10 Strategically Located Factories in North America Provides Nimble and Cost-E�cient Fabrication

Corporate/International HeadquartersMolded Fiber Glass Companies2925 MFG Place, P.O. Box 675

Ashtabula, OH 44005-0675

Toll-Free: (800) 860-0196 • Phone: (440) 997-5851www.molded�berglass.com