CourseWiki/Materials and Manufacturing Processes/Plastics

Plastic Forming Processes

How eight plastic and composite processes shape material, what they can make, and where tooling and geometry limit the choice.

Start with the shape and how the material becomes solid

These processes make a shape by moving softened material, stretching sheet or curing resin in tooling. A mold shapes a part against its surfaces; a die gives flowing material a cross-section. Tooling is the equipment made specifically to produce the part.

Use the process-category foundations topic for the five-category framework. Here, follow what happens to the material: thermoplastics usually set by cooling; thermosets set by curing. Review Plastics as Materials if that distinction is unclear.

Four process comparisons: blow molding inflates a hot tube into a bottle; rotational molding coats a rotating mold to make a hollow tank; vacuum forming stretches sheet into an open tray; extrusion pushes melt through a die to make a continuous hollow profile.

The starting form and the finished shape narrow the choice before cost enters the discussion. The diagrams show typical versions of each process.

For equipment operation and hazards, use the Safety collection's Equipment Operating Procedure, Machine Hazards and Housekeeping and Chemicals and Safety Data Sheets topics.

Injection molding fills a closed cavity, then repeats the cycle

In conventional thermoplastic injection molding, a heated barrel and screw prepare the melt. The screw advances to push a measured shot (the material delivered in one injection) through a nozzle into a clamped mold.

Five stages of a thermoplastic injection molding cycle: clamp the mold, inject melt, hold pressure to pack material as it shrinks, cool until the part is stiff enough, then open and eject before repeating. Cooling begins during filling and packing. — A thermoplastic cycle. Cooling overlaps filling and packing; the five labels describe the main jobs, not five completely separate thermal events.

Clamp: close the mold and hold it against the pressure of the incoming melt.
Inject: fill the cavity.
Hold/pack: maintain pressure to feed additional material as it shrinks, while the gate can still pass melt.
Cool: allow enough heat to leave for the part to keep its shape during removal.
Eject: open the mold and push the part free, commonly with ejector pins. Close for the next cycle.

Materials and products: thermoplastics such as ABS, PP and nylon become housings, caps, clips and gears. Specialized injection systems also mold thermosets; those require curing, so the cooling-based cycle above is not universal.

Tooling and quantity: detailed, pressure-resistant molds are a substantial initial investment. Repeated cycles and multiple cavities can make the investment worthwhile for many identical parts.

See the machine and cycle in a public explainer

The BPF injection-molding explainer describes the machine and process. No shop-specific settings or procedures are implied here.

Sprue, runner and gate carry melt into the part

In a conventional sprue-and-runner layout, the sprue receives melt from the nozzle, a runner distributes it, and a gate is the restricted entrance to a part cavity. One mold may contain several cavities, and one cavity may have several gates.

Draft is a slight taper on a surface in the mold-opening direction. It helps the part release instead of scraping along the tool. Injection molding can produce ribs, bosses (raised features around holes) and fine detail, but every feature must have a way out of the mold.

An undercut traps material behind a tool feature. It may require a moving side core, another special mechanism or a redesigned part. A simple two-part mold cannot produce every enclosed hollow shape.

Uniform walls reduce sink marks and uneven shrinkage; Practice

Uniform walls reduce sink marks and uneven shrinkage

Aim for a reasonably uniform wall thickness, with gradual transitions where thickness must change. A thick mass cools differently from an adjacent thin wall. Its shrinkage can pull the surface inward, leaving a sink mark, or contribute to distortion.

A solid block under a thin housing wall is therefore a warning sign. Hollow out the bulky region where possible and use appropriately sized ribs for support. Packing and cooling settings also matter; changing the drawing is not the only remedy.

Three design clues to recognize

A shallow depression opposite a heavy rib suggests sink.
Scraping during release suggests inadequate draft or another ejection problem.
A sudden thick-to-thin step suggests uneven cooling and difficult filling.

Protolabs' defect guide illustrates these relationships. Wall and draft requirements depend on resin, texture, depth and tooling; no single angle or thickness fits every part.

Practice mold design problems

Blow molding inflates a hot tube or preform into a hollow part

A mold closes around a hot parison (tube), or a previously molded preform. Air pressure expands the material against the mold, then it cools. Extrusion blow molding starts with a tube; injection-based versions start with a molded preform.

Materials and products: thermoplastics; HDPE detergent bottles and PET beverage bottles are familiar examples made by different blow-molding variants.

Tooling and quantity: dedicated molds and machinery favor repeated production, especially large packaging runs. Mold complexity and the chosen variant affect cost; lower forming pressure does not make the whole production line inexpensive.

Geometry: good for thin-walled hollow containers. Stretching affects wall thickness; external detail is controlled better than internal detail. It does not directly make a solid gear or complex internal ribs. Mold parting lines, and pinch-off seams in extrusion-blown parts, may remain visible.

Public process reference

BPF: extrusion blow molding shows how the tube becomes a container.

Rotational molding coats a mold to make a large hollow shell

A measured charge, usually thermoplastic powder, goes inside a mold. Heating and slow rotation about two axes distribute material over the inside surface as it melts. The mold keeps moving during cooling before the shell is removed.

Materials and products: predominantly polyethylene; tanks, playground shells and kayaks.

Tooling and quantity: molds do not withstand injection pressure, so tooling is often less costly for a large part. Heating and cooling the mold takes time, making low-to-medium quantities attractive compared with faster processes.

Geometry: large hollow parts with relatively even walls are a strength. Local thickness, corners and fine tolerances still need care. Tiny details, very thin walls and solid precision parts are poor fits. This is slow coating of the mold, not high-speed centrifugal casting.

Public process reference

BPF: rotational molding explains the sequence and the predominance of polyethylene.

Thermoforming stretches a sheet into a tray or shell

Heat thermoplastic sheet until it is pliable, form it against a tool, cool it and trim the edge. In vacuum forming, removing air between sheet and tool lets outside air pressure press the sheet against the tool. Pressure forming adds pressure on the other side.

Materials and products: thermoplastic sheets such as PETG, ABS and polystyrene; packaging trays, display covers and equipment liners.

Tooling and quantity: a simple one-sided tool can suit prototypes or short runs. Durable tools and automated equipment also support very large packaging volumes.

Geometry: best for open, sheet-like shapes with draft. Deep draws stretch and thin the sheet; the tool controls one surface most directly. It cannot create a solid boss or a closed bottle from one sheet in this basic process. Undercuts complicate release.

Public process reference

BPF: vacuum forming describes heating, forming, release and trimming.

Extrusion makes a continuous, constant cross-section

A screw moves and melts plastic, then pushes it continuously through a shaped die. Cooling and sizing equipment establish the final dimensions. The product is cut to length or wound.

Materials and products: commonly thermoplastics such as PVC and PE; pipe, tubing, sheet, film and window profiles.

Tooling and quantity: a profile die is often simpler than a complex injection mold, but the production line and setup still cost money. Long runs spread those costs over many meters of product.

Geometry: the die can make solid or hollow cross-sections. It cannot directly put a cap on the end of a tube or create isolated holes along one side; those require other operations. A long product with the same section everywhere is the main clue.

Public process reference

BPF: extrusion profiles and sheet explains the die and downstream sizing equipment.

Compression molding presses a charge between matched tools

Place a measured amount of material in an open mold. Closing the heated tool applies pressure and spreads the charge into shape. With a thermosetting compound, the part cures before removal; cooling alone is not what hardens it.

Materials and products: often thermosets or fiber-filled thermosetting compounds; electrical components and composite body panels. Thermoplastic sheet composites can also be compression molded, with a cooling-based solidification step.

Tooling and quantity: matched tools and a press require investment, often justified by repeated medium-to-high production. Tooling is not automatically cheap; size, surface quality and precision matter.

Geometry: both faces can be shaped, including ribs and shaped panels. Material must flow before it cures; extremely thin features, deep narrow details and trapped undercuts are difficult.

Public process reference

The thermoset section of BPF's plastics-process reference explains the reaction under heat and pressure.

Casting fills a mold with liquid resin for small quantities

Liquid resin fills a mold and hardens. In common small-batch resin casting, mixed components react and cure. A flexible mold made from a master pattern can reproduce surface detail.

Materials and products: often thermosets such as epoxy, polyurethane or unsaturated polyester; prototypes, decorative replicas and electrical encapsulation. Casting is broader than polyurethane casting; some cast plastics, such as cast acrylic, are thermoplastic after polymerization.

Tooling and quantity: flexible molds can have a low initial cost but limited life. Labor and cure time make this useful for short runs rather than a default for millions of pieces.

Geometry: good detail and some undercuts are possible with flexible tooling. Trapped air, cure shrinkage and heat from the reaction limit thin passages and thick pours. A closed hollow space still needs a removable or sacrificial core, or a different casting method.

Public process reference

The polymer-casting section of Formlabs' manufacturing guide explains flexible molds and short-run economics.

Hand layup builds a fiber-reinforced composite against a tool

In wet hand layup, reinforcing fabric or mat is placed against a mold, wetted with liquid resin and consolidated to reduce trapped air. Layers build thickness, then the resin cures.

Materials and products: usually a thermosetting polyester or epoxy matrix with glass or carbon fibers; boat shells and custom covers. These are composites in the Wood & Composites material family. Prepreg layup uses reinforcement already containing resin and is a related but different route.

Tooling and quantity: relatively simple one-sided molds support small runs and large custom parts. Manual work and cure time limit production rate.

Geometry: good for broad shells. The mold gives one face its finish; the other depends on the layup. Tight corners, inaccessible cavities and precise thickness on both faces are difficult. Fiber direction affects strength, so layer arrangement is part of the design.

Public process reference

ACMA's composite guide, Appendix E.1.1, explains wet hand layup and its dependence on workmanship.

Apply these comparisons in Choosing a Plastic Forming Process. Plastic separating, fabricating, 3D printing and finishing are separate topics; for the general milling process, see Milling Operations.

Appears in

DET 130 DET Plastics and Forming