Learn Plastic Forming and Shaping Explained – Extrusion, Injection Molding (Equipment, Cycle, Shrinkage, Defects), Blow Molding (Extrusion, Injection, Stretch, Multilayer), Rotational Molding, Thermoforming, Casting, Compression/Transfer Molding and Foam Molding

Polymer Shaping Processes:

Polymer shaping processes transform raw plastics into the countless products used in daily life from bottles and containers to automotive components, medical devices and packaging. These processes rely on careful control of temperature, pressure, and flow, exploiting the unique viscoelastic behavior, thermal expansion and processing sensitivity of polymers. This article covers the major shaping methods: extrusion, injection molding, blow molding, rotational molding, thermoforming, casting, compression molding, transfer molding and foam molding. For each, we examine principles, equipment, applications and common defects.

Extrusion:

Extrusion is a compression process in which material is forced through a die orifice to produce a long continuous product with a cross section determined by the orifice shape. It is widely used for thermoplastics and elastomers rarely for thermosets to mass produce tubing, pipes, hose, structural shapes (window and door moldings), sheet, film, filaments and coated wire. The process is continuous, the extrudate is later cut to length.

In polymer extrusion, feedstock (pellets, granules or powder) is fed from a hopper into a barrel where it is heated, melted and forced through the die by a rotating screw. The two main components are the barrel and the screw, the die is a special tool made for each profile. Barrel internal diameters range from 25 to 150 mm (1.0–6.0 in). The barrel is long relative to its diameter with L/D ratios usually 10–30. Higher ratios suit thermoplastics, lower values suit elastomers. Electric heaters initially melt the pellets, subsequent mixing and mechanical working generate additional heat. In some cases, enough heat is generated by shearing that external heating is unnecessary, and sometimes the barrel must be externally cooled to prevent overheating. The screw rotates at about 60 rev/min.

The screw has three sections with distinct functions:
• Feed section: moves stock from the hopper and preheats it;
• Compression section: transforms the polymer into a liquid, extracts entrapped air, and compresses the material;
• Metering section: homogenizes the melt and builds sufficient pressure to pump it through the die.
Screw geometry determines operation. The channel depth (`dc`) is large in the feed section to admit granular polymer gradually reduced in the compression section to increase pressure and small in the metering section where pressure peaks. The flight diameter is smaller than the barrel diameter by about 0.05 mm clearance to limit melt leakage backward.

Optimal section lengths vary by polymer. For gradually melting polymers like low‑density polyethylene (LDPE), equal sections work. For crystalline polymers (e.g., nylon) melting is abrupt so a short compression section is appropriate. Amorphous polymers like PVC melt slowly requiring a compression zone almost the full screw length. General purpose screws are a compromise to avoid frequent costly changes.

Before reaching the die, the melt passes through a screen pack wire meshes on a breaker plate with small axial holes. The screen pack filters contaminants, builds pressure and straightens the flow to remove the polymer’s memory of circular motion preventing twisting and distortion of the extrudate.

Defects in Extrusion:

Melt fracture occurs when stresses before and during die flow are so high that the melt fails, producing a highly irregular surface. It can be caused by a sharp die entrance that induces turbulent flow, as opposed to streamlined, laminar flow in a gradually converging die.

Sharkskin is a more common defect: the product surface roughens upon exiting the die because friction at the die wall creates a velocity profile, with tensile stresses at the surface causing minor ruptures.

Bambooing is a severe defect, it happens if the velocity gradient is extreme, prominent marks appear, giving a bamboo‑pole appearance.

Injection Molding:

In injection molding, polymer is heated to a highly plastic state and forced under high pressure into a mold cavity where it solidifies. The part (molding) is then removed. The process produces discrete almost always net shape components. Cycle times typically range from 10 to 30 seconds though larger parts may take 1 minute or more. Molds can have multiple cavities producing several parts per cycle. Complex and intricate shapes are possible but the mold must allow part removal. Part sizes range from about 50 g to 25 kg with upper limits including refrigerator doors and automobile bumpers. The mold is the critical tooling for large complex parts it can cost hundreds of thousands of dollars. Thus injection molding is economical only for large production quantities.

Feedstock (pellets or granules) enters a heated cylinder and the melt is forced into the mold either by a hydraulic plunger or by a rotating screw. As in extrusion, the barrel is externally heated but a far greater portion of heat comes from frictional heating. Modern machines are of the reciprocating screw type. The screw rotates building pressure at the mold entrance then moves backward a predetermined distance controlling shot volume. The screw then stops rotating and is pushed forward hydraulically forcing molten plastic into the cavity at pressures typically 70–200 MPa.

For thermoplastics, molds are kept cool about 90°C. Thermoset molds are heated to about 200°C to promote cross linking. After cooling (thermoplastics) or curing (thermosets), the mold opens and ejectors remove the part, the cycle repeats automatically. Elastomers are also injection molded. Because the material is molten, complex shapes with good dimensional accuracy are obtained but uneven cooling leads to residual stresses. Molds with moving or unscrewing mandrels allow molding of parts with multiple cavities or internal/external threads.

Equipment:

An injection molding machine has two principal components:

1. The injection unit similar to an extruder, a barrel with a hopper and a reciprocating screw that both mixes and acts as a ram, a nonreturn valve near the screw tip prevents backflow. The injection unit melts and homogenizes the polymer and injects it into the mold.
2. The clamping unit holds the two mold halves in alignment keeps the mold closed during injection with sufficient clamping force and opens/closes the mold at the right times. It consists of fixed and movable platens operated by hydraulic pistons or mechanical toggle devices. Clamping forces of several thousand tons are available.

Cycle Sequence:

1. Mold is closed and clamped.
2. A shot of polymer melt at proper temperature and viscosity is injected under high pressure into the cavity. The plastic cools and solidifies on contact with the cold mold. Ram pressure is maintained to pack additional melt and compensate for contraction.
3. The screw rotates and retracts with the nonreturn valve open allowing fresh polymer into the forward barrel portion while the part in the mold fully solidifies.
4. The mold opens and the part is ejected.

Shrinkage and Defects:

Polymers have high thermal expansion coefficients so significant shrinkage occurs during cooling. Crystalline plastics shrink more than amorphous ones. Shrinkage is expressed as linear reduction per unit dimension (mm/mm). Fillers reduce shrinkage. Before making a mold, shrinkage values for the specific compound must be obtained from the producer. Mold cavity dimensions are made larger than the desired part dimensions to compensate. Because shrinkage varies among plastics, the same mold will produce different part sizes for different polymers.

Shrinkage is affected by injection pressure, compaction time, molding temperature and part thickness. Higher injection pressure forces more material into the cavity, reducing shrinkage. Longer compaction time if the gate hasn’t solidified, also reduces shrinkage. Higher molding temperature actually lowers shrinkage because the melt viscosity drops allowing more packing, this effect outweighs the larger temperature difference. Thicker parts show greater shrinkage because the outer skin solidifies first when the gate solidifies, the interior molten material accounts for most remaining shrinkage.

Common defects:

Short shots incomplete filling due to insufficient temperature/pressure or inadequate machine shot capacity. Correct by increasing temperature/pressure or using a larger machine.

Flashing melt squeezed into the parting surface or around ejection pins, caused by excessive vents/clearances, too‑high injection pressure relative to clamping force, too‑high melt temperature or excessive shot size.

Sink marks and voids in thick sections. A sink mark occurs when outer skin solidifies but internal contraction depresses the surface. A void occurs when surface retains shape but internal shrinkage creates a cavity due to tensile stresses. Increase packing pressure or design uniform, thinner sections.

Weld lines formed when melt flows around a core and meets from opposite directions. These boundaries have inferior mechanical properties. Address with higher melt temperature, higher injection pressure, alternative gating or better venting.

Blow Molding:

Blow molding uses air pressure to inflate soft plastic inside a mold cavity, producing one piece hollow thin walled parts like bottles. It is a high‑volume process for consumer beverages, competing with glass. Two steps:
1. Fabricating a starting tube of molten plastic (parison) by extrusion or injection molding and
2. Inflating it to the final shape. Blow molding and rotational molding both make hollow seamless parts from thermoplastics (rotational also works for thermosets) with sizes from 5 mL to 38,000‑L drums. Blow molding favors mass produced small containers, rotational molding favors large hollow shapes.

Extrusion Blow Molding:

A tube (parison) is extruded vertically, clamped into a mold with a cavity much larger than the tube and blown outward. The blow ratio may be as high as 7:1. Blowing uses hot air at 350–700 kPa. Drums up to 2000 L can be made. Die materials: steel, aluminum, beryllium copper. In continuous operations, molds move with the tubing, close around it, seal one end, break into individual sections and move away as air is injected. The part cools and is ejected. Corrugated pipe is made by continuous blow molding with horizontal extrusion and moving molds.

Injection Blow Molding:

The parison is injection molded into cool dies then transferred to a blow molding die by an indexing mechanism. Hot air expands it to the cavity walls. Typical products: plastic beverage bottles (polyethylene or PEEK) and small containers. Advantages over extrusion blow molding: higher production rate, better dimensional accuracy, lower scrap rates, less material waste. However, extrusion blow molding can produce larger containers because injection molds for large parisons are expensive. Also extrusion blow molding is more feasible for double layer bottles used for medicines, personal care products, and chemicals.

Stretch Blow Molding:

A variation where the blowing rod extends downward into the parison during inflation, stretching the plastic and inducing biaxial orientation. This yields greater rigidity, transparency and impact resistance. Most widely used material: polyethylene terephthalate (PET) which has low permeability and is strengthened by stretching ideal for carbonated beverage bottles (e.g. 2‑L soda bottles).

Multilayer Blow Molding:

Uses coextruded tubes or parisons to produce multilayer structures, e.g. food/beverage packaging with odor/permeation barriers, taste/aroma protection, scuff resistance, printability and hot fill capability. Also used in cosmetics and pharmaceutical containers.

Rotational Molding (Rotomolding):

Rotational molding forms large, hollow parts from most thermoplastics and some thermosets. It favors complex external geometries, larger parts, and lower production quantities than blow molding.Process:
1. A premeasured amount of polymer powder is loaded into a split mold.
2. The mold is heated usually in an oven and rotated simultaneously about two perpendicular axes, tumbling the powder against all internal surfaces to fuse into a uniform layer.
3. While still rotating, the mold is cooled to solidify the skin.
4. The mold opens and the part is removed. Speeds are slow, gravity not centrifugal force produces the coating.

For thermosets, a chemical agent is added, cross linking occurs after forming. Liquid polymers (plastisols, especially PVC) can also be used in a variant called slush molding or slush casting where the mold is heated and rotated forcing the liquid against walls to coat them, parts are thin walled products like boots and toys.

Molds in rotational molding are simple and inexpensive compared to injection or blow molds but cycle times are long around 10 minutes or more. Production often uses a multicavity indexing machine with three stations:
1. unload/load (finished part removed, powder loaded),
2. heating chamber (hot air convection at ~375°C/700°F), and
3. cooling station (forced cold air or water spray). All three molds work simultaneously.

Rotational molding can produce parts with complex hollow shapes, wall thicknesses as small as 0.4 mm, and sizes up to 1.8 m × 1.8 m × 3.6 m with volumes up to 80,000 L. Outer surface finish replicates the mold interior. Cycle times are longer, so quality control focuses on accurate powder weight, proper rotational speed and temperature time relationships.

Products include hollow toys (hobby horses, balls), boat/canoe hulls, sandboxes, swimming pools, buoys, flotation devices, truck body parts, automotive dashboards, fuel tanks, luggage, furniture, garbage cans, fashion mannequins, industrial barrels, containers, storage tanks, portable outhouses and septic tanks. Most popular material: polyethylene especially HDPE, others include polypropylene, ABS and high impact polystyrene. Metallic or plastic inserts can be molded integrally.

Thermoforming:

Thermoforming shapes thermoplastic sheets or films over a mold using heat and pressure. A sheet is clamped and heated usually by radiant heaters above the glass transition temperature to the sag point then forced against the mold by vacuum or air pressure. Sheets come as coiled strips or cut lengths and can be filled with additives for specific properties. The mold is at room temperature so the shape sets upon contact. Vacuum (negative pressure) suffices for most materials due to low strength but thicker/complex parts need air pressure from about 100 to 2000 kPa depending on thickness and material. Mechanical plugs may assist.

The process has two main steps: heating and forming. Radiant heaters are located about 125 mm (5 in) from the sheet; heating time depends on polymer type, thickness, and color.

Methods:

Vacuum thermoforming is the first method (1950s). Negative pressure draws the preheated sheet into a concave (negative) mold cavity. Vacuum holes are about 0.8 mm (0.031 in) diameter, leaving minor marks.

Pressure thermoforming uses positive pressure to force the sheet into the cavity. Higher pressures (3–4 atm) are possible versus the 1 atm limit of vacuum. The sheet is pressurized from above with vent holes in the mold to exhaust air.

Mechanical thermoforming uses matching positive and negative molds brought together against the heated sheet without air pressure. Advantages: better dimensional control and surface detailing on both sides. Disadvantage: two mold halves are required making molds more costly.

Positive vs. Negative Molds:

A negative mold has a concave cavity, a positive mold is convex. The choice affects which surface reproduces the mold contour: negative exterior surface matches the cavity, interior follows the sheet. Positive interior matches the convex mold, exterior approximates. Thinning patterns also differ: with a positive mold, the portion contacting the top (base of a tub) solidifies quickly and undergoes little stretching resulting in a thick base but thin walls. A negative mold gives more even stretching before contact with the cold surface.

Applications and Materials:

Only thermoplastics can be thermoformed (thermosets and elastomers are cross linked and cannot be reheated). Common plastics: polystyrene, cellulose acetate and butyrate, ABS, PVC, acrylic (PMMA), polyethylene and polypropylene. Mass production in packaging uses rapid feeding through a heating chamber and multiple cavity molds. Sometimes the extruder producing the sheet is directly upstream, eliminating reheating; filling of food items is often immediately downstream. Thin film packaging includes blister packs and skin packs for cosmetics, toiletries, tools and fasteners. Large parts from thicker sheet include business machine covers, boat hulls, shower stalls, light diffusers, advertising signs, bathtubs, toys, skylights (acrylic for transparency) and refrigerator door liners (ABS for formability and oil/fat resistance). Thermoforming combines drawing and stretching so the material must have high uniform elongation, thermoplastics have this due to high strain rate sensitivity. Molds are typically aluminum strength not required with small through holes (<0.5 mm) for vacuum. Defects: tearing, nonuniform wall thickness, improper filling, poor definition and lack of surface details.

Casting:

Casting involves pouring liquid resin into a mold using gravity to fill the cavity and allowing hardening. Both thermoplastics and thermosets are cast. Thermoplastic examples: acrylics, polystyrene, nylons, vinyls (PVC).
Conversion to solid can be by
1. heating to a fluid state, pouring, then cooling;
2. polymerizing a low molecular weight prepolymer or monomer in the mold; or
3. pouring a plastisol (fine PVC particles suspended in plasticizer) into a heated mold to gel and solidify.

Thermosets cast include polyurethane, unsaturated polyesters, phenolics and epoxies the liquid ingredients are poured then polymerization and cross linking occur with heat/catalysts. Reactions must be slow enough to allow pouring, fast reacting systems (e.g. some polyurethanes) need alternative processes like reaction injection molding.

Advantages over injection molding:

Simpler, cheaper molds; parts are relatively free of residual stresses and viscoelastic memory; suited to low production quantities. Acrylic sheets (Plexiglas, Lucite) are cast between polished glass plates achieving flatness and optical quality not possible by extrusion.

Disadvantage:

Significant shrinkage - acrylic sheets contract about 20% volumetrically, much more than in injection molding where high pressure packs the cavity.

Slush casting (shell casting):

A liquid plastisol is poured into a heated split mold, a skin forms on the surface. After the desired skin thickness excess liquid is poured out, the mold is opened for part removal. An important application: encapsulation of transformers, coils, connectors, and other electrical components in plastic.

Compression Molding:

Compression molding is widely used for thermosetting plastics, rubber tires and polymer matrix composites. A preshaped charge like powder, pellets, liquid or preform is placed into a heated mold cavity, typically ~200°C, but can be higher. The mold halves close compressing the charge under pressure (10–150 MPa) to flow and fill the cavity. Heat cures the thermoset. The part is removed after curing. Flash is formed and later trimmed. The charge amount must be precisely controlled. Preheating (infrared, convection oven or heated screw) shortens cycle time. Presses are vertical with upstroke or downstroke actuation (upstroke more common) powered by hydraulic cylinders with clamping capacities up to several hundred tons. Molds are simpler than injection molds, no sprue/runner, but must be heated (electric, steam, or hot oil). They are classified as hand molds (trials), semiautomatic (operator loads/unloads), or automatic (fully programmed).

Mold types: 

Flash (shallow/flat parts), positive (high‑density), semipositive (quality). Undercuts are not recommended, but side opening dies can be designed. Part complexity is less than injection molding but dimensional control is better. Surface areas up to ~2.5 m². Dies are typically tool steel, chrome plated or polished for improved finish, they are less costly than injection molds.

Materials: 

Phenolics, melamine, urea formaldehyde, epoxies, urethanes, elastomers. Typical products: electric plugs/sockets, pot handles, dinnerware plates, dishes, handles, container caps, fittings, electrical/electronic components, washing machine agitators, housings and fiber reinforced parts with chopped fibers.

Advantages: 

Simpler molds, less scrap, low residual stresses.

Disadvantage: 

Longer cycle times and lower production rates than injection molding.

Transfer Molding:

Transfer molding is an advancement of compression molding. The uncured thermosetting resin is placed in a heated pot or chamber, heated then injected into closed heated molds by a ram, plunger, or rotating screw feeder at pressures up to 300 MPa. Viscous flow generates heat that homogenizes the material. Curing occurs by cross linking. Because the resin is molten when entering the mold part complexity and dimensional control approach injection molding.

Two variants:

Pot transfer: Charge injected from a pot through a vertical sprue; Plunger transfer: Plunger injects from a heated well through lateral channels. In both scrap (cull) remains in the well and channels, the sprue in pot transfer is also scrap. Since polymers are thermosetting, scrap cannot be reused.

Transfer molding: It is used for the same materials (thermosets, elastomers). It can produce more intricate shapes than compression molding, but less than injection molding. It also accommodates inserts, metal or ceramic placed in the cavity before injection, bonding with the plastic during molding. Typical parts: electrical connectors, electronic components, rubber/silicone parts encapsulation of microelectronic devices. Suitable for intricate shapes with varying wall thicknesses. Molds are more expensive than compression molds and some excess material is left in channels.

Foam Molding:

Foam molding produces cellular structured products like styrofoam cups, food containers, insulating blocks and packaging. Raw material: expandable polystyrene beads. The structure may be open cell (low viscosity polymers) or closed cell (high viscosity). In the basic process, polystyrene beads are placed in a mold with a blowing agent (pentane or inert gas) and heated (usually by steam). Beads expand up to 50 times their original size, taking the shape of the cavity. Expansion is controlled by temperature and time. Additives like hollow glass beads or plastic spheres can modify properties.

Polystyrene beads come in three sizes: small for cups (finished density ~50 kg/m³), medium for molded shapes, and large for insulating blocks (~15–30 kg/m³, later cut to size). Bead size depends on minimum wall thickness, smaller beads for thinner parts. Beads can be pre‑colored. Both thermoplastics and thermosets can be foam molded (thermosets are liquid processed similar to reaction injection molding). A common method uses preexpanded beads, partially expanded by steam or hot air/water/oven, in an open chamber then stabilized in a storage bin for 3–12 hours before final molding.

Structural Foam Molding:

This produces products with a solid outer skin and cellular core. Thermoplastics are mixed with a blowing agent usually nitrogen gas and injection molded into cold molds. Rapid cooling against cold surfaces creates a rigid skin up to 2 mm thick and a cellular core. Overall density can be as low as 40% of the solid plastic, giving high stiffness to weight ratio. Typical products: furniture components, computer and business machine housings and moldings replacing wood.

Polyurethane Foam Processing:

Used for furniture cushions and insulating blocks. Two or more components are mixed, chemical reactions occur after the mixture is poured into molds or sprayed over surfaces. Low pressure and high pressure machines with computer controls ensure proper mixing. The solidified product has a cellular structure whose characteristics depend on component type and proportions.