Injection Plastic Molding: Process, Tooling and Cost Guide

Injection plastic molding melts thermoplastic pellets and forces them under pressure into a steel or aluminum mold, where the part cools and is ejected in seconds. It is the cheapest way to make the same plastic part thousands of times — once you have paid for the tool. The whole decision is a trade between tooling cost up front and part cost forever after.

Tooling is the number that surprises people. Molds are classified by expected life (the SPI/ SPE classes below), and the class you buy should match the volume you actually expect in the next two years — not the volume in the pitch deck.

Cost drivers inside a class: number of cavities, side actions and lifters for undercuts, surface finish (a polished optical finish can add thousands), tolerance callouts, and how many engineering changes you make after the steel is cut. Cutting steel twice is the single most avoidable expense in plastic injection molding.

Piece price is roughly material + machine time + labor + scrap + margin. Material is part weight times resin price plus runner waste. Machine time is cycle time times the hourly press rate divided by the number of cavities. A 30-second cycle on a $60/hour press in a two-cavity tool is about $0.25 of machine time per part; the same part in a four-cavity tool is about $0.13.

Compare that against alternatives before committing. Below roughly 1,000-2,000 parts, 3D printing or urethane casting usually wins. Between there and full production, a bridge tool buys you time. See our economics of tooling breakdown for the full model.

If you are choosing between molding, casting and printing for a product still in development, our team runs the DFM review and the tooling quote together so the two decisions are made with the same numbers. See low-volume manufacturing and design for manufacturing, or talk to an engineer.

Cost splits in two. Tooling runs about $3,000-$12,000 for an aluminum bridge mold and $12,000-$80,000 for a production steel mold, depending on cavitation, side actions and finish. Piece price for a typical palm-sized part is usually $0.20-$2.00, driven by resin weight, cycle time and cavity count.

Break-even is usually somewhere between 500 and 2,000 parts against 3D printing or urethane casting. Divide the tooling cost by the per-part savings versus the alternative process; if the answer is more parts than you will sell in two years, use a bridge tool instead.

Aluminum bridge tools typically take 2-4 weeks to first shots. Production steel tools take 6-12 weeks, plus 1-3 weeks of tuning after T1 samples. Add time if the part still needs DFM changes when the tool is quoted.

Most thermoplastics mold well between 1.5 mm and 3.0 mm, held uniform across the part. Thicker sections increase cooling time (which scales with the square of thickness) and cause sink and voids; thinner sections risk short shots unless flow length is short.

If you paid for it, your contract should say you own it and can move it to another molder. Get tool ownership, storage responsibility, maintenance intervals and transfer terms in writing before the purchase order — retrieving an unowned tool later is slow and expensive.