Chapter 15

Processes Used to Form Plastic Materials

Objectives

• Closed molding processes: injection molding, compression molding, rotational molding, blow molding, and extrusion.

• Open molding processes: boat hulls and shower enclosures.

• Blow molding processes: high volume production of bottles and other containers.

• Laminations and calendaring: sheet products.

Introduction• Plastic stock can be purchased in many forms: sheets, pellets, powder,

granules, and rods.• With plastics there is normally no scrap generated.• Trimmed material is usually ground up and reused.• Eight basic forming processes: compression molding, injection molding,

rotational molding, blow molding, thermoforming, extrusion, hand layup, and casting.

• Plastics are used extensively to form composites.• Casting: a liquid resin is poured into a mold and solidifies.• Molding: Softened semi-solid material is formed in the desired shape in a

mold or die using pressure and sometimes heat.

Closed Molding

• Many molding processes use a two piece closed mold.• The closed mold halves are normally held in a press which

provides pressure on the part during cooling or curing.• The five major closed molding processes are : compression

molding, injection molding, rotational molding, blow molding and extrusion.

• Each type of stock has its advantages and weaknesses and must be properly selected for use with a particular manufacturing process.

Compression Molding• There are two types of compression molding – hot and cold.• In cold compression molding the unheated mold closes on the molding

compound to form it in the desired shape.• Curing is sometimes done by moving the still closed mold into an oven.• In hot compression molding the molding compound is placed between

die halves that are heated between 225 and 3250F.• Pressure (100 to 2000 psi) and heat improve the flow into small channels

in complex parts.• Compression molding is used to make car body parts, appliance

components, truck liner panels, tote boxes, dinnerware, etc.

Injection Molding

• Pellets flow from a funnel shaped feed hopper into a heated compression cylinder.

• The plunger forces a controlled quantity of material through the injection molding nozzle into a closed mold.

• Often the mold will be cooled by water running through cooling channels.

• After the part cools (seconds) the mold is opened and the part is ejected.

• Injection molding is used to manufacture electrical equipment housing, cell phone cases, appliance parts, automotive components, microwavable dishes , etc.

Rotational Molding

• Rotational molding (www.rotomolding.org) is primarily used to make seamless hollow products such as balls, containers, picnic coolers, floats, and toys.

• Fastest growing in the U.S. plastics industry.• Powder or liquid coats the mold’s inside surface where it

forms a thick skin or uniform layer.• It is then cooled.• Some machines are capable of producing tanks as large as

500 gallons (1893 liters) in size.• One innovative boat manufacturer, Triumph Boats of

Durham, NC is making boats that will not chip or crack.

Blow Molding

• Used to make household bottles and containers for cosmetics, pharmaceuticals, foods, and toiletries.

• Flexible containers for juice, milk, and other beverages are now made by most manufacturers.

• The process can produce 60,000 half liter water bottles per hour (Sidel, Inc.).

• Extrusion blow molding is the most common.

Injection Blow Molding

• One of the major advantages of injection blow molding is that parts are produced with no flash (rough edges).

• This is not the case with injection molding or blow molding – flash must ne trimmed off.

• Cost of injection blow molding machines range from $ 100,000 to $ 400,000.

• Today 60% of all plastic bottles up to 16 oz. capacity are produced by injection blow molding machines.

• A disadvantage is that the process is not practical for making bottles with handles.

Thermoforming

• Thermoforming involves heating a thermoplastic sheet to its softening point and forcing the material into or over the contours of a one piece mold.

• Vacuum forming and drape forming are variations of thermo forming.

Extrusion

• Extrusion is used to produce more plastic products than any other process except injection molding.

• Extrusion is similar to squeezing toothpaste out of a tube.• The extruder converts thermoplastic powder, pellets, or

granules into a continuous melt.• The melt is then forced through a die opening to produce long

shapes.• Typical products include auto trim, house siding, garden

hoses, and soft drink straws.

Pultrusion• Pultrusion consists of pulling continuous roving through three processing

stages.• Bundles are prepared by pulling continuous roving through a resin bath or

impregnator.• Next the bundles are pulled into preforming fixtures where they are

partially shaped and excess resin and air are removed.• Finally the bundles are pulled through heated dies for forming and then

into an oven for curing.• The manufacturing work cell used for pultrusion consists of a pultrusion

machine, the forming dies, a pulling system, and a cut off saw.• Fishing rods, hockey sticks, tent poles arrows and golf club shafts are

pultruded.

Extrusion

• Film is either produced in tubular (blown) form (most frequently used) or in cast form (extruded through a linear slot die).

• Wire coating: Extrusion is also used to coat wire. Liquid plastic surrounds the wire. It is then cooled and wound with a coil winding machine.

• Extrusion blow molding: Most extrusion blow molded containers have flash. After the product is molded and cooled, the flash (excess material) is trimmed.

Open Molding

• Open molding is an important process for constructing composite material.

• Glass fiber reinforcement roving is encapsulated in resin.

• Open molding is used to make prototypes, pools, tanks, boats, etc.

• Virtually any size product can be open molded.• The major drawback is that it is time consuming,

resulting in high per unit labor cost.

Casting

• The advantage of casting is that intricate shapes can be made using flexible molds.

• Casting is used to make skylights, greenhouses, bus shelters, and boat parts.

• Centrifugal casting: Pipes, tubing, and other round objects can be produced using the centrifugal casting process.

• Centrifugal casting can even be used to manufacture long cylinders with external threads.

• It requires minimum labor and can be automated if high volumes are desired.

Summary• Plastic stock can be purchased in many forms: sheets, pellets, powder, granules, and rods.• With plastics there is normally no scrap generated.• Trimmed material is usually ground up and reused.• Eight basic forming processes: compression molding, injection molding, rotational molding,

blow molding, thermoforming, extrusion, hand layup, and casting.• Casting: a liquid resin is poured into a mold and solidifies.• Molding: Softened semi-solid material is formed in the desired shape in a mold or die using

pressure and sometimes heat.• The five major closed molding processes are : compression molding, injection molding,

rotational molding, blow molding and extrusion.• Rotational molding (www.rotomolding.org) is primarily used to make seamless hollow

products such as balls, containers, picnic coolers, floats, and toys.• Fastest growing in the U.S. plastics industry.• One of the major advantages of injection blow molding is that parts are produced with no

flash (rough edges).

Home Work

• 1. What is casting and what is molding?• 2. Where is rotational molding used?• 3. What is a major advantage of injection blow

molding?