Plastics get their behavior from polymer chains

A polymer is a substance made of very large molecules built from repeating units. Small starting molecules called monomers react to build those molecules. Many polymers have long chains; some also have branches or chemical links between chains.

A plastic combines a polymer with ingredients that may change its color, flexibility, processing behavior or resistance to weather. The polymer name is a starting point, not a complete material specification. Two grades of the same plastic can behave differently.

Plastics is one of the four material families in the material-family foundations topic. A polymer reinforced with glass or carbon fibers is a composite; the course places composites in Wood & Composites.

Thermoplastics soften again; cured thermosets do not remelt

Curing is a chemical change; cooling is a temperature change. A two-part epoxy becoming solid is not simply a hot plastic cooling down.

Supporting detail: Addendum 1 — Softening, melting and heat resistance are different questions.

Light weight and corrosion resistance come with limits

• Light weight: many unfilled plastics have much lower density than metals. Low weight does not automatically mean high stiffness.
• Chemical resistance: plastics do not rust like steel, but a chemical may soften, swell or crack a particular plastic. Check the material, chemical, temperature and exposure time together.
• Low softening or melting temperatures: many common thermoplastics are easy to form with heat but unsuitable for hot, loaded service.
• Creep: slow deformation under a sustained load. A plastic shelf can keep sagging even when its load stays constant.
• UV degradation: sunlight can discolor or embrittle susceptible grades. Outdoor service calls for a suitable resin and stabilization, not just a preferred color.
• Electrical insulation: many unfilled plastics resist current flow. Conductive fillers and special formulations can change that behavior.

Supporting detail: Addendum 2 — Why a short strength test does not settle a long service life.

Match common thermoplastics to their useful properties

These are typical uses and tradeoffs, not design ratings for every grade. All materials in this table are thermoplastics.

Supporting detail: Addendum 3 — Material references for checking a particular choice.

Epoxy and polyester resin cure into thermosets

“Polyester” alone is ambiguous. PET packaging is a thermoplastic polyester. The unsaturated polyester resin used in a conventional fiberglass layup cures into a thermoset. See the hand-layup card in Plastic Forming Processes.

Supporting detail: Addendum 4 — Where the resin comparison comes from.

Resin codes identify plastic; they do not promise recycling

The number on a container is a resin identification code (RIC). It can help sorting, but it does not say whether the local recycling program accepts that product.

For example, code 5 identifies PP; it does not prove that a PP lid is accepted with PP containers. Use the collector's instructions for that item. Do not assign PETG to code 1 just because its name contains PET.

Supporting detail: Addendum 5 — Verified code sources and the standard's exact distinction.

Practice resin codes Practice reheating behavior

Next: Plastic Forming Processes and Choosing a Plastic Forming Process.

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.

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.

1. Clamp: close the mold and hold it against the pressure of the incoming melt.
2. Inject: fill the cavity.
3. Hold/pack: maintain pressure to feed additional material as it shrinks, while the gate can still pass melt.
4. Cool: allow enough heat to leave for the part to keep its shape during removal.
5. 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.

Supporting detail: Addendum 6 — See the machine and cycle in a public explainer.

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

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.

Supporting detail: Addendum 7 — Three design clues to recognize.

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.

Supporting detail: Addendum 8 — Public process reference.

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.

Supporting detail: Addendum 9 — Public process reference.

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.

Supporting detail: Addendum 10 — Public process reference.

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.

Supporting detail: Addendum 11 — Public process reference.

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.

Supporting detail: Addendum 12 — Public process reference.

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.

Supporting detail: Addendum 13 — Public process reference.

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.

Supporting detail: Addendum 14 — Public process reference.

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.

Use part shape to build a short list

“Solid” does not mean “make every region thick.” An injection-molded housing is a discrete part, yet hollowing out bulky sections often improves it. Several processes can make a tank; size, quantity, resin and detail determine the better candidate.

Practice choosing a process

Spread tooling cost over the number of acceptable parts

A simple comparison is average cost per part = tooling cost / quantity + variable cost per part. Variable cost includes the costs that grow with production, such as material and processing time. Tooling cost is paid even if only a few parts are made.

Illustrative estimates, not supplier prices: suppose two processes can both meet a cover's specification. Route A costs $900 for tooling and $14 per part. Route B costs $18,000 for tooling and $2 per part.

Supporting detail: Addendum 15 — Find the quantity where the two estimates are equal.

Practice tooling cost per part Practice a cost crossover

Cycle time and cavity count set the ideal production rate

A cavity makes one part per cycle. With cycle time in seconds, ideal parts per hour = 3600 / cycle time × cavities.

For a 30-second cycle and two identical cavities: 3600 / 30 × 2 = 240 parts/h. That assumes uninterrupted operation and every part acceptable. Use the complete repeating cycle time, including opening and ejection; do not add overlapping cooling time twice.

Actual useful output is lower when setup, stoppages or rejects occur. More cavities may increase tooling cost and require a larger machine, so a higher ideal rate does not by itself settle the process choice.

Practice production rate

Explain the choice with both a benefit and a limitation

A useful selection sounds like this: “For 300 large hollow PE garden reservoirs, investigate rotational molding because the shape fits and tooling can be economical at modest quantities. Confirm wall control and cycle time before committing.”

The quantity alone does not prove that choice. A narrow neck, precision sealing face, delivery deadline or different resin could change it. State the assumptions, identify a credible alternative, and compare the features that matter.

Supporting detail: Addendum 16 — Further process-selection reading.