What Is Injection Molding?
Injection molding is the most common method for mass-producing plastic parts. Molten polymer is injected under high pressure into a steel or aluminum mold cavity, cooled, and ejected as a finished part. Cycle times range from a few seconds to under a minute, making it the go-to process for anything produced in the thousands or millions.
If you’ve held a plastic part today — a bottle cap, phone case, electrical connector, dashboard component — it was almost certainly injection molded.
How the Process Works
- Clamping. The two halves of the mold are clamped together under force (measured in tons). Clamp tonnage depends on part size and projected area — small parts may need 50 tons, automotive panels can require 3,000+.
- Injection. Plastic pellets are fed from a hopper into a heated barrel with a reciprocating screw. The screw melts the material and pushes it into the mold cavity at pressures typically between 10,000 and 30,000 PSI.
- Cooling. The part cools inside the mold. Cooling channels (waterlines) circulate fluid through the mold to control temperature and cycle time. Cooling is usually the longest phase — often 60–70% of total cycle time.
- Ejection. Ejector pins push the part out of the mold. The mold closes and the cycle repeats.
Common Materials
Nearly any thermoplastic can be injection molded. The most common:
- Polypropylene (PP) — Living hinges, packaging, automotive. Cheap and versatile.
- ABS — Consumer electronics, housings, LEGO bricks. Good impact strength and finish.
- Nylon (PA6, PA66) — Gears, structural components, under-hood automotive. Strong and wear-resistant.
- Polycarbonate (PC) — Lenses, safety equipment, transparent housings. High impact and optical clarity.
- Polyethylene (HDPE, LDPE) — Containers, caps, toys. Low cost, chemically resistant.
- PEEK, PPS, Ultem — High-performance engineering parts. Expensive material but exceptional thermal and chemical resistance.
What It Costs
Injection molding has high upfront tooling cost but very low per-part cost at volume.
| Cost Element | Typical Range |
|---|---|
| Simple single-cavity mold (aluminum) | $3,000 – $10,000 |
| Production steel mold (P20/H13) | $15,000 – $100,000+ |
| Multi-cavity or complex mold | $50,000 – $500,000+ |
| Part cost at volume (simple part) | $0.10 – $2.00 |
The crossover point where injection molding beats other processes is typically around 1,000–5,000 parts, depending on part complexity and size.
Design Considerations
- Draft angles. Walls need 1–2° of draft to release from the mold. Zero draft means stuck parts and damaged tooling.
- Wall thickness. Keep it uniform. Thick sections cool slower, causing sink marks, warping, and voids. Typical range: 1–4 mm depending on material.
- Undercuts. Features that prevent the part from pulling straight out of the mold require side actions or lifters — adding cost. Design them out when possible.
- Gate location. Where the plastic enters the cavity affects fill pattern, weld lines, and cosmetics. Gate vestige (the mark left behind) matters on visible surfaces.
- Ribs over thick walls. Need structural stiffness? Use ribs (typically 60% of wall thickness) instead of making the whole wall thicker.
- Radii. Sharp internal corners create stress concentrations and impede flow. Minimum 0.5 mm radius on all internal corners; 1 mm or more is better.
When to Use Injection Molding
- Production volumes above 1,000 units (sweet spot is 10,000+)
- Parts requiring tight tolerances (±0.005″ is achievable)
- Complex geometry with fine detail
- Consistent cosmetic finish requirements
- Multi-material or overmolded assemblies
When to Consider Alternatives
- Under 500 parts: Look at urethane casting or 3D printing
- Very large parts: Rotational molding or blow molding
- Hollow parts: Blow molding
- Continuous profiles: Extrusion
- Thermoset materials: Compression molding
Bottom Line
Injection molding is the workhorse of plastics manufacturing. The tooling investment pays for itself quickly at volume, and the process delivers consistency that’s hard to match. The key is designing parts for the process — uniform walls, proper draft, strategic gating — rather than designing in a vacuum and hoping the mold maker can figure it out.
Planning an injection molded product? PartSnap offers DFM-focused design, rapid prototyping, and production tooling management.