Lesson 2.1ShapeChapter 2 · Molding plastics and rubber

Injection molding

Melt forced into a closed steel mold

Injection molding melts plastic pellets and forces the melt into a closed steel mold, where it cools into a finished part. Bottle caps, toy bricks, phone cases and car dashboards are all made this way. Once the mold exists, each extra part costs very little, which is why most plastic objects around you are injection molded.

Photo: The robotic engineer1, CC BY-SA 4.0, via Wikimedia Commons

Injection molding machine · cross-section, 6 stepsCircled numbers match the steps below. Panel B shows the same mold a moment later, opened to release the part.

Injection molding in a minute, by MecMade

How it works

Feed. Plastic pellets (raw plastic grains a few millimeters across) fall from the hopper (a funnel) into the barrel, a heated steel tube.
Melt. A turning screw drags the pellets forward. Friction from the screw and heat from the heater bands melt them, and the melt collects in front of the screw tip.
Clamp. The clamp unit closes the two mold halves and holds them shut with tens to thousands of tonnes of force. This stops melt leaking out at the parting line (the joint between the halves).
Inject and pack. The screw stops turning and rams forward like a plunger. Melt shoots through the nozzle and the sprue (the feed channel) into the cavity (the part-shaped hollow). The screw keeps pushing to pack in extra plastic as the part shrinks.
Cool. Water flowing through channels in the steel carries the heat away until the part is solid. This is usually the longest step. Meanwhile the screw turns again to prepare the next shot.
Open and eject. The moving half pulls back. Ejector pins push the part, with its sprue, off the core (the steel bump that shapes the inside). The mold closes and the cycle repeats.

Key facts

Materials: Thermoplastics (plastics that soften every time they are heated) such as polypropylene, ABS, nylon and polycarbonate; also some thermosets and liquid silicone rubber
Typical parts: Caps, toy bricks, electronics housings, car interior parts, syringes, combs
Part size: From under a gram to car bumpers weighing several kilograms
Precision: Typically about ±0.05 to ±0.1 mm; tighter on small precision parts
Surface finish: Copies the mold: anything from mirror gloss to a leather-like texture, with no finishing needed
Batch size it suits: Thousands to millions of identical parts
Tooling and machine cost: Molds from a few thousand dollars (simple, aluminum) to well over $100,000 (large, many cavities, hardened steel); machines from tens of thousands to over a million dollars
Speed: Typically 10–60 s per cycle; thin caps in a few seconds, often many parts per shot

Strengths and limits

Strengths

Very low cost per part once the mold is paid for
Fast, automated and highly repeatable
Complex shapes, snap-fit clips, threads and fine detail in one shot
Huge choice of plastics, colors and textures

Limits

The mold is costly and often takes weeks to make
A design change means reworking or remaking the steel mold
Walls must be fairly thin and even (often about 1–4 mm), or parts warp and show sink marks (small dents over thick spots)
Undercuts (features that would lock the part in the mold) need sliding mold parts, which add cost

Where it's used

PackagingBottle caps, closures, thin-walled food tubs
Consumer goodsToy bricks, kitchen tools, appliance housings
AutomotiveDashboards, door panels, bumpers, light housings, clips
MedicalSyringes, pipette tips, inhaler bodies
ElectronicsPhone and laptop casings, connectors, keys and buttons

How to recognize it

A fine line around the part where the two mold halves met (the parting line).
Small round marks, slightly raised or sunken, left by ejector pins on the hidden side.
A small nub or scar where the plastic entered (the gate mark).
Thin walls of even thickness backed by ribs (thin stiffening walls), and molded-in text such as a recycling symbol or date wheel.

Compare with

Overmolding and two-shot molding2.2the same machine, but a second material is molded onto or around the first.Blow molding2.3makes hollow parts such as bottles by inflating soft plastic against a mold, instead of filling a solid cavity.Die casting1.4the same principle with molten metal instead of plastic.

Watch it

tronicarts · The whole machine and one full cycle in 3D animation

Autodesk · A five-minute explainer with history and everyday examples

ARBURGofficial · A real press filling 72 cavities per shot at high speed

Ian Collier (How It's Made excerpt) · How the steel mold itself is machined and polished

See real photos

Wikipedia: Injection moulding — real molds, including a LEGO brick mold and a mold with side actions
Wikipedia: Injection molding machine — an open mold in a press and a large press with a robot
Wikimedia Commons photo search

Remember

Melt, inject under pressure, cool, eject: a part every few seconds to a minute.
The mold is the big cost and each part is cheap, so it pays off at high volume.
Good designs use thin, even walls, a slight draft (taper) and as few undercuts as possible.