Injection molding is the most widely used manufacturing process for producing plastic parts in volume — from automotive components and medical devices to consumer goods and industrial products.
This guide covers the injection molding process from start to finish, including process parameters, material selection, common defects, and how to evaluate an injection molding partner.
Injection molding works by melting plastic pellets and injecting the molten material into a closed mold under high pressure. The plastic cools and solidifies inside the mold cavity, then the mold opens and ejector pins push the finished part out.
The complete cycle (typical 15-60 seconds):
| Parameter | Typical range | Effect on quality |
|---|---|---|
| Barrel temperature | 180-320°C (varies by material) | Too low = poor filling; too high = material degradation |
| Nozzle temperature | 200-300°C | Affects gate freeze-off |
| Mold temperature | 40-120°C | Higher mold temp = better surface finish, longer cooling |
| Oil temperature | 40-60°C | Hydraulic system stability |
| Parameter | Typical range | Effect on quality |
|---|---|---|
| Injection pressure | 800-2,000 bar | Too low = short shots; too high = flash |
| Holding pressure | 500-1,500 bar (50-80% of injection) | Affects sink marks and dimensional stability |
| Injection speed | 10-200 mm/s | Fast = better filling for thin walls; slow = avoids burn marks |
| Back pressure | 5-20 bar | Affects material mixing and melt quality |
| Clamping force | 3-8 tons per cm² of projected area | Too low = flash; too high = mold damage |
Modern injection molding machines collect data on every cycle. The best molders use this data to: - Detect cycle deviations in real time - Trigger alarms when parameters drift outside control limits - Generate CP/CPK reports for quality documentation (required in automotive and medical) - Provide full traceability — every part can be traced to the exact cycle that produced it
Choosing the right plastic material is as important as selecting the right mold steel. Here's a quick reference:
| Material | Key properties | Typical applications | Shrinkage |
|---|---|---|---|
| PP (Polypropylene) | Chemical resistance, flexible | Automotive interiors, packaging, living hinges | 1.0-2.5% |
| PE (Polyethylene) | Tough, low cost | Bottles, containers, toys | 1.5-4.0% |
| ABS | Impact resistant, good surface | Consumer electronics, automotive trim | 0.4-0.9% |
| PS (Polystyrene) | Rigid, transparent, brittle | Disposable products, CD cases | 0.4-0.8% |
| PA6 (Nylon 6) | Strong, wear resistant | Gears, bearings, under-hood parts | 0.5-1.5% |
| PC (Polycarbonate) | Strong, transparent, heat resistant | Safety glasses, medical devices, light covers | 0.5-0.7% |
| POM (Acetal/Delrin) | Low friction, dimensionally stable | Gears, zippers, pump components | 1.8-2.5% |
| Material | Key properties | Typical applications | Shrinkage |
|---|---|---|---|
| PC/ABS blend | Balance of strength + processability | Automotive dashboards, electronics housings | 0.4-0.7% |
| PA6-GF30 | 30% glass-filled nylon, stiff | Structural parts, power tool housings | 0.2-0.8% |
| PBT-GF30 | Dimensional stability, electrical | Connectors, automotive electronics | 0.2-1.0% |
| PMMA (Acrylic) | Crystal clear, UV resistant | Light guides, lenses, displays | 0.4-0.7% |
| PEEK | High temperature (260°C continuous) | Medical implants, aerospace, oil & gas | 0.5-1.0% |
| LCP (Liquid Crystal) | Ultra-thin wall, high temp | Electrical connectors, SMT components | 0.1-0.5% |
| TPE/TPR | Rubber-like flexibility | Soft-touch grips, seals, gaskets | 0.3-2.0% |
| Defect | Cause | Solution |
|---|---|---|
| Short shot | Insufficient material, low injection speed | Increase shot size, raise injection pressure, or increase melt temperature |
| Flash | Clamping force too low, mold wear | Increase clamping force, check mold parting surface for damage |
| Sink marks | Insufficient packing, thick wall sections | Increase hold pressure and time, reduce wall thickness |
| Weld lines | Material flow fronts meeting at low temp | Increase melt/mold temp, relocate gate, or add overflow wells |
| Burn marks | Trapped air compression burn, shear heating | Improve venting, reduce injection speed at last fill |
| Warpage | Uneven cooling, molecular orientation | Balance mold temperature, reduce cooling time differences |
| Jetting | Material jetting through small gate into open cavity | Change gate design to avoid fountain flow, increase mold temp |
| Blisters | Gas trapped below surface | Dry material thoroughly, reduce melt temperature |
| Flow marks | Material cooling too fast at flow front | Increase injection speed, raise mold temperature |
| Production volume | Recommended mold type | Cavity count | Typical tool cost (USD) |
|---|---|---|---|
| 100-5,000 parts | Prototype mold (aluminum or 3D printed) | 1 | $2,000-8,000 |
| 5,000-100,000 | Standard production mold (P20 steel) | 1-4 | $8,000-30,000 |
| 100,000-1,000,000 | Multi-cavity production mold (H13 steel) | 4-8 | $30,000-80,000 |
| 1,000,000+ | High-cavity hot runner mold | 8-64 | $80,000-250,000+ |
Does the molder have machines in the right tonnage range? - Small parts (grams to 50g): 50-150 ton machines - Medium parts (50-500g): 150-400 ton machines - Large parts (500g-5kg): 400-1,000 ton machines - Very large (5kg+): 1,000-3,000+ ton machines
| Certification | What it means |
|---|---|
| ISO 9001:2015 | Basic quality management system |
| IATF 16949 | Automotive quality standard (required for Tier 1/2 suppliers) |
| ISO 13485 | Medical device quality management |
| UL recognition | Electrical safety testing |
Injection molded part cost = material cost + processing cost + tooling amortization + finishing/packaging
| Factor | Cost impact |
|---|---|
| Material choice | Engineering plastics (PA, PC) cost 2-5x commodity (PP, PE) |
| Part complexity | Higher complexity = longer cycle time + more expensive tool |
| Annual volume | Higher volumes amortize tooling cost over more parts |
| Tolerance requirements | Tight tolerances require more expensive molds and process control |
| Surface finish | Textured or high-gloss finishes add 10-30% to mold cost |
| Secondary operations | Painting, printing, assembly add 20-100%+ to unit cost |
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