Injection Molding
Also: plastic injection molding, injection moulding, custom injection molding, plastic molding, molded plastic parts, thermoplastic injection molding, rapid injection molding, prototype molding, bridge tooling, injection mold tooling
Injection molding shoots molten plastic into a steel or aluminum mold: identical parts in seconds at cents each, once a $3,000-100,000 mold is paid for.
768 injection molding companies in the Noramark directory
Plastic pellets are dried, melted by a rotating screw in a heated barrel, and injected at high pressure into a closed mold. The machine packs extra melt in as the part shrinks, holds it while it cools rigid, then opens the mold and ejects the part; a cycle takes 10-60 seconds for most parts. The mold is two blocks of steel or aluminum: the cavity (A) side forms the outside of the part and the core (B) side the inside, with ejector pins to push it out. Melt enters through a sprue, runners and gates, which are either trimmed off as scrap (cold runner) or kept molten in a heated manifold (hot runner). Slides and lifters move sideways to form undercuts, at extra tooling cost.
Once the mold exists, injection molding makes identical parts faster and cheaper than any other process: cents per part at volume, ±0.1 mm (±0.004") on typical features, with textures, logos, snap fits, living hinges and color molded in. Housings, enclosures, bezels, clips, gears, connectors, closures, medical devices and consumer products are molded by the million, in nearly every thermoplastic from polypropylene and ABS to glass-filled nylon, PEEK and Ultem.
The mold is the cost. A single-cavity aluminum prototype or bridge mold costs a few thousand dollars and runs thousands to tens of thousands of shots. Production molds in P20 pre-hardened steel, or hardened H13, S7 or 420 stainless for abrasive, corrosive and medical work, cost $15,000-100,000 and more and run hundreds of thousands to millions of cycles. The SPI (now PLASTICS Industry Association) mold classes name the expected life: Class 101 for a million cycles or more, 102 up to a million, 103 under 500,000, 104 under 100,000, and 105 prototype molds for under 500.
Molding rewards designing for the process: uniform walls, draft on every face parallel to the mold opening, ribs instead of thick sections, and gates where the flow can pack the part. Thick sections sink and void, uneven walls warp, and sharp corners concentrate stress. A mold is hard to change once cut, so molders run a design-for-manufacture review and often a mold-flow simulation first, and leave critical dimensions "steel safe": removing steel to tune a dimension is easy, adding it back is not.
At a glance
| Typical tolerances | Commercial tolerances about ±0.1 mm (±0.004") on features up to 25 mm (1"), plus roughly 0.1-0.2% of the dimension beyond that; fine tolerances of ±0.025-0.05 mm (±0.001-0.002") on small features in stable amorphous or glass-filled resins, at higher mold cost and with tighter process control. Semi-crystalline resins such as acetal, nylon, PP and PE need looser tolerances than ABS or polycarbonate. Flatness and warp depend on part design and gating more than on the machine. Tolerance guides by resin come from the PLASTICS Industry Association and the resin producers. |
|---|---|
| Size limits | Job-shop presses run from about 20 to 1,000 tons of clamp force, for parts from under 1 g to a few kg; bins, bumpers and pallets run on 1,500-4,000 ton presses at fewer shops. The clamp needed is roughly 0.3-0.7 tonnes per cm² (2-5 tons per in²) of projected area. Walls typically 0.8-4 mm (0.030-0.160") depending on resin and flow length; thin-wall packaging goes under 0.5 mm (0.020"), and walls over about 5 mm (0.200") sink and void unless gas-assist or foam molded. |
| Surface finish | The part copies the mold surface. SPI finish grades run from A-1 (diamond buffed, optically clear, about 0.012-0.025 µm Ra) through B (paper) and C (stone) to D (blasted, matte). Chemically etched textures (Mold-Tech and similar) and VDI 3400 EDM textures hide flow lines, sink and fingerprints but need extra draft. A polished A finish costs the most and shows every flaw; a light texture such as SPI D-1 or MT-11010 is the economical choice for housings. Parts can be painted, pad printed, laser marked, hot stamped, plated (ABS, PC/ABS) or decorated in the mold. |
| Lead time | Aluminum prototype and bridge molds 2-4 weeks to first shots (T1); production steel molds 6-12 weeks, and 12-20 weeks for multi-cavity, hot-runner or high-cavitation tools. Allow one to three rounds of sampling and mold adjustment and a first article inspection before production. Production runs from a qualified mold ship in 1-3 weeks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Amorphous resins (ABS, polycarbonate, PC/ABS, acrylic, Ultem PEI) shrink about 0.4-0.8% and hold the tightest tolerances and the best cosmetic surfaces. Semi-crystalline resins (polypropylene, HDPE, Delrin acetal, nylon 6/6, PEEK) shrink about 1-3%, warp more and need looser tolerances, but give better chemical, fatigue and wear resistance. Glass fiber stiffens and stabilizes nylon, PBT, PP and PC but wears molds and makes shrinkage directional. High-temperature resins (PEEK, Ultem, PPS) need mold temperatures around 140-200 °C (285-390 °F) and hardened steel. Rigid PVC is molded for fittings in stainless or plated molds because it releases corrosive gas when overheated. PTFE and UHMW cannot be injection molded (they do not flow): they are compression molded, ram extruded or machined, and PFA and FEP are the moldable fluoropolymers. G-10 and other thermoset laminates are machined, not molded.
What drives the cost
- Mold cost: size, number of cavities, slides and lifters for undercuts, hot runner, steel grade, polish and texture
- Cycle time, set mainly by the thickest wall: cooling time grows with roughly the square of the thickness
- Resin price and part weight: PEEK and Ultem cost an order of magnitude or more per kg than polypropylene or ABS
- Quantity: the mold is amortized over the program, and each run pays for setup and purging
- Tolerances and cosmetics: tight tolerances need steel molds, more sampling and process monitoring; Class A surfaces raise scrap
- Secondary operations: inserts, ultrasonic welding, pad printing, painting, assembly and packaging
- Documentation: first article inspection, capability studies, PPAP for automotive, validation for medical
When to use it
- Plastic parts from a few thousand to millions a year
- Complex shapes with snap fits, bosses, ribs, living hinges, and molded-in texture and color
- Repeatable parts where piece price matters more than tooling cost
- Bridge production from an aluminum mold while the steel production mold is built
- Engineering resins (glass-filled nylon, polycarbonate, PEEK) in shapes that machining cannot make economically
When not to
- Fewer than a few hundred parts: machine them or 3D print them (SLS, MJF, SLA)
- Designs still changing: mold changes are slow and expensive, and adding steel back is hard
- Very thick or solid parts, which sink, void and cycle slowly: core them out or machine them
- PTFE, UHMW and thermoset laminates such as G-10, which do not injection mold
- Very large, low-volume parts: thermoforming, rotational molding or fabrication cost far less in tooling
Design tips
- Keep walls uniform, typically 1.5-3 mm (0.060-0.120") for ABS and polycarbonate; blend unavoidable thickness changes over about three times the change.
- Draft every face parallel to the mold opening: 1° minimum, 2° for easy release, and about 1-1.5° more for each 0.025 mm (0.001") of texture depth.
- Make ribs 50-60% of the wall they join (40-50% for high-shrink resins) and no taller than about three times the wall, to avoid sink on the show side.
- Core out thick sections and bosses; a boss wall of about 60% of the nominal wall, supported by ribs or gussets, avoids sink and voids.
- Radius inside corners at least half the wall thickness and outside corners by the inside radius plus one wall, to cut stress and help flow.
- Avoid undercuts where you can: every slide or lifter adds cost and maintenance, and a pass-through shutoff or a small bump-off often replaces one.
- Tell the molder where gates, ejector marks and the parting line may and may not go on cosmetic surfaces, and which dimensions are critical.
- Ask for a DFM review and a mold-flow analysis before steel is cut, and keep critical dimensions steel safe so they can be tuned by removing metal.
Frequently asked questions
- How much does an injection mold cost?
- In the U.S., a single-cavity aluminum prototype mold for a small, simple part costs roughly $3,000-10,000; a production steel mold $15,000-60,000; and multi-cavity, hot-runner or large molds $75,000-250,000 and more. Undercuts, tight tolerances, polish and texture all add. Offshore molds cost less up front but add shipping time and slower changes.
- Aluminum vs steel injection molds: which should I use?
- Aluminum molds cost less, cut faster and cool faster, and suit prototypes, bridge tooling and runs of thousands to tens of thousands of parts, less with glass-filled resins. Steel molds (P20, H13, stainless) cost more and take longer but last hundreds of thousands to millions of cycles, hold tolerances longer and survive abrasive and high-temperature resins.
- What is the minimum order for injection molding?
- Most molders set a minimum run, often a few hundred to a few thousand parts or a minimum lot charge, because hanging the mold, heating it and purging the barrel takes hours whatever the quantity. Molding pays against machining or 3D printing somewhere between a few hundred and a few thousand parts, depending on size and complexity.
- What causes sink marks in injection molded parts?
- Thick sections that keep shrinking after the surface has frozen, pulling it inward: usually behind ribs, bosses and wall junctions. The design fixes are uniform walls, ribs at 50-60% of the wall and cored-out bosses; process fixes (more pack pressure and time, a gate nearer the thick area) help at the margin. Texture hides what is left.
- What are SPI mold classes?
- A classification from the SPI, now the PLASTICS Industry Association, that describes a mold by its expected life and build quality: Class 101 for a million cycles or more with hardened steel, 102 for up to a million, 103 for under 500,000, 104 for under 100,000 and 105 for prototype molds of under 500 cycles. Stating the class on the RFQ tells the toolmaker what to build and lets quotes be compared.
