Manufacturing
Injection Molding
High-volume plastic manufacturing by injecting melted polymer into a mold
Injection molding melts plastic pellets, injects molten polymer at high pressure into a precision steel mold, cools to solidify, and ejects the finished part. The dominant process for producing high-volume plastic parts: bottle caps, gears, automotive trim, consumer electronics housings. Cycle times of seconds. Tooling is expensive (tens of thousands of dollars) but per-part cost is low at scale. Quality depends on melt temperature, pressure, cooling rate, and mold design.
- Cycle time5 to 60 seconds typical
- Pressure30 to 150 MPa during injection
- Mold materialHardened tool steel
- Tooling cost$10,000 to $1M+
- Common polymersPP, ABS, PC, nylon, PE, POM
- Tolerances±0.05 mm typical
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Why injection molding matters
- High volume. Millions of identical parts at low unit cost.
- Consumer goods. Bottle caps, toys, electronics housings.
- Automotive. Trim, dashboards, lighting, fluid reservoirs.
- Medical. Syringes, IV connectors, device housings.
- Packaging. Containers, lids, closures, jars.
- Precision. Tight tolerances and fine surface finish.
- Complex shapes. Internal features, undercuts, multi-material.
Common misconceptions
- Cheap to start. Tooling is expensive — only economic at scale.
- Any geometry works. Draft, undercuts, parting lines constrain design.
- Wall thickness arbitrary. Uniform thin walls (1 to 4 mm) work best.
- Plastic is plastic. Hundreds of polymer grades with different processing windows.
- One shot fills. Pack-and-hold pressure compensates for cooling shrinkage.
- Cycle time is fixed. Cooling dominates and depends on wall thickness squared.
Frequently asked questions
How does injection molding work?
Plastic pellets feed into a heated barrel where a rotating screw melts them. The screw retracts, accumulating a shot of melt at the front. The screw then plunges forward, injecting melt at high pressure through a sprue and runners into the mold cavity. The part cools and solidifies, the mold opens, ejector pins push the part out, and the cycle repeats.
Why is tooling so expensive?
Molds are CNC-machined from hardened tool steel, polished to mirror finish, fitted with cooling channels, ejectors, gates, and possibly sliders for undercuts. They withstand thousands of pounds of clamping force and millions of cycles. Complex multi-cavity molds with hot runners and side actions can exceed a million dollars but produce parts for pennies each.
What polymers are used?
Thermoplastics that melt and resolidify reversibly. Polypropylene (PP) for caps and containers. ABS for housings and toys. Polycarbonate (PC) for clear strong parts. Nylon (PA) for gears and bearings. Polyethylene (PE) for bottles. POM (acetal) for precision mechanical parts. Glass-fiber filled grades add stiffness and dimensional stability.
What defects can occur?
Sink marks (depressions where thick sections cool and shrink). Warping (uneven cooling causing distortion). Short shots (mold not fully filled). Flash (excess plastic at parting line from low clamp force). Weld lines (where flow fronts meet, weak). Burns (trapped air ignites at high pressure). Each has fixable root causes in process or design.
How are draft and ejection designed?
Walls perpendicular to mold opening direction would stick. Draft angles of 0.5 to 3 degrees on vertical surfaces let the part release. Ejector pins push the cooled part free. Ribs, bosses, and hollows orient with mold opening. Designers consider parting line, draft, and ejector locations from the start to avoid expensive mold changes.
What is shrinkage?
Plastic shrinks 0.5 to 2.5% as it cools and solidifies. Crystalline polymers (PP, nylon, POM) shrink more than amorphous (ABS, PC). Mold cavities are oversized to compensate so finished parts hit nominal dimensions. Anisotropic shrinkage along flow direction differs from across. Glass-fiber fillers reduce shrinkage and improve dimensional consistency.
When isn't injection molding the right choice?
Low volumes. Below a few thousand parts, tooling cost rarely justifies. Use machining, 3D printing, or thermoforming. Very large parts (over 1 meter) push press capacity. Very thick walls cause sink and long cycles. Metals or thermoset plastics need different processes (die casting, compression molding). Geometry with severe undercuts may need expensive multi-action molds.