Custom Injection Molding Guide: Process, Cost & Tolerances

Injection molding is the process you choose when a plastic part has to be dimensionally repeatable, cosmetically clean, and produced in volumes where the tooling investment amortises. It is also the process that generates the most expensive mistakes, because almost every cost is locked in at the design stage — long before the first shot.

This guide is written for the person holding the purchase order, not the person holding the mould. It covers what the process does, what the tooling really costs, which tolerances are worth paying for, and how to tell a competent molder from a broker with a catalogue.

1. What custom injection molding is — and when it is the wrong choice

Thermoplastic pellets are melted, forced under pressure into a closed steel cavity, held while they cool, and ejected as a finished part. One cycle. No secondary shaping. The economics come from repetition: the first part can cost $12,000 and the hundred-thousandth can cost $0.42.

That is also the trap. Injection molding rewards volume and punishes uncertainty. It is the wrong choice when:

• You have not frozen the design. Every engineering change after steel is cut is a mould modification, not a file update. Budget 8–15% of the original tool cost per revision, plus two to four weeks of downtime.
• You need fewer than roughly 500–1,000 pieces. Below that, tooling dominates unit cost and vacuum forming or CNC-machined plastic usually wins.
• The part is large, shallow and simple. A 1.2 m × 0.8 m enclosure panel is a $3,000 vacuum forming tool or a $40,000 injection mould. There is no version of that maths where injection wins.
• You need it in three weeks. A production mould is 25–45 days from kickoff, plus T1 sampling and any correction loop.

2. The process, step by step

A single cycle runs 15–60 seconds and repeats perhaps 500,000 times. Every stage has a parameter that ends up in your part.

Cycle structure for a typical 2 mm ABS part on a 250-tonne machine.

3. Clamp force: how much tonnage your part actually needs

Buyers rarely ask this, and it is the first thing a molder checks. Clamp force must exceed the force trying to push the mould open — cavity pressure multiplied by projected area.

Worked example: a 200 × 150 mm flat housing with one cavity projects 300 cm². At 0.5 t/cm² that is 150 tonnes — so a 180-tonne machine, not a 250-tonne one. Put it in a 2-cavity tool and you need 300 tonnes. This is why cavity count and machine size are negotiated together, and why a molder quoting a 4-cavity tool on a small press is telling you something about their floor.

4. Mold types and what drives tooling cost

Tooling is the largest line item in your first order, and the easiest to under-specify. Three decisions dominate.

Steel grade

Mould steel selection. Service life assumes normal maintenance; abrasive fillers can halve it.

Runner system

A cold runner is cheaper to build and wastes material in the sprue and runners on every shot. A hot runner adds $3,000–$15,000 to the tool but eliminates that waste and shortens cycles — it pays back quickly on engineering resins, and is usually mandatory for multi-cavity tools where you cannot afford to regrind.

Cavitation

Two cavities cost roughly 1.6× a single cavity, four cost about 2.4×. But unit price does not fall proportionally, because machine time per part only improves if the press is large enough to carry the added projected area. Ask for the price at 1, 2 and 4 cavities and check whether the drop justifies the tooling delta at your actual annual volume.

5. Materials: shrinkage, drying and where each one belongs

Shrinkage is not a property you look up — it is a range, and the molder tunes it with packing pressure and mould temperature. What you need is the nominal value for the drawing and the tolerance band to expect.

Indicative processing windows. Always confirm against the specific grade's datasheet — flame retardants and glass fillers shift all four columns.

6. DFM rules that decide your part price

Design for manufacture is not a checklist you run at the end. These six items move part cost more than the resin choice.

Wall thickness

Keep it between 1.2 and 3.0 mm for commodity resins, and uniform wherever possible. Cycle time scales roughly with the square of wall thickness: going from 2 mm to 3 mm more than doubles cooling time. Thick sections also sink. If a design needs stiffness, add ribs, not wall.

Draft

1–2° on vertical walls, 0.5° minimum on textured surfaces plus 1° per 0.025 mm of texture depth. Zero draft means drag marks and, eventually, a scratched cavity.

Radien

Inside corner radius at least half the wall thickness, ideally equal to it. Sharp inside corners are stress concentrators and the first place a part cracks in service.

Ribs

Rib thickness 50–60% of the adjoining wall. A full-thickness rib creates a thick section at the base, which sinks on the cosmetic face opposite. Rib spacing at least 2× wall thickness.

Unterschneidet

Every undercut is either a side action (adds $800–$4,000 to the tool and 3–8 s to the cycle) or a lifter. Redesigning a snap-fit so it strips out of the cavity is usually the single highest-return change in a DFM review.

Gate location

The gate decides weld line position, flow orientation and where the part warps. Put it where a cosmetic mark is acceptable — typically a hidden internal face — and tell your molder which face is the A-surface before they cut steel, not after.

7. What injection molding really costs

Indicative ranges for a China-based supplier, 2026. Treat them as a sanity check, not a quote.

Indicative only. Freight, duty, mould trials and packaging are usually quoted separately — confirm what is included.

The number buyers consistently forget is total cost of ownership. A tool quoted at $9,000 with a 45-second cycle and 12% scrap is more expensive than a $13,000 tool running 32 seconds at 3% scrap, once you pass roughly 50,000 parts. Ask for cycle time and expected scrap rate in writing, and compare the two together.

8. Tolerances: what to specify, what to stop specifying

Over-tolerancing is the most common way buyers inflate their own price. Every dimension tighter than the process can hold reliably becomes an inspection operation, a rework loop, or a rejected lot.

Typical as-moulded capability. Anything tighter should be a machining operation, and priced as one.

9. Eight defects you will meet — and their fixes

Defect-to-cause map for common thermoplastic moulding. Persistent defects usually trace back to tooling, not settings.

10. Injection molding versus vacuum forming, blow molding and extrusion

These four processes overlap at the edges and are routinely quoted against each other. The honest comparison is about volume, geometry and tooling.

Process selection matrix. A part that fits two columns should be quoted in both.

If your part is a constant cross-section — a trim strip, a rail, a tube — extrusion is an order of magnitude cheaper per metre and should not be moulded at all. If it is a large shallow shell under 5,000 pieces, Vakuumverformung will usually beat injection on total cost. For a detailed head-to-head, see our vacuum forming vs injection molding comparison.

11. How to vet an injection molding supplier

Most sourcing failures are not technical. They are a supplier who accepted work outside their real capability. Six questions separate the two.

1. "What tonnage range is on your floor?" A shop with 80–250 tonne presses cannot make a 1.5 kg part, no matter what the website says. Ask for a machine list with tonnage and shot capacity.
2. "Who cuts the steel — you or a subcontractor?" In-house mould making means faster correction loops and someone accountable when T1 is wrong.
3. "Show me a first-article inspection report you issued last month." Not a certificate template. An actual report with actual measured values against a ballooned drawing.
4. "What is your scrap rate on a comparable part, and what happens to regrind?" Vague answers here usually mean regrind is going into your cosmetic parts.
5. "What is included in the tooling price, and who owns the tool?" Get tool ownership in writing. It should be you, transferable on request.
6. "Can I see the process parameter sheet for the part?" A molder who cannot produce a set-up sheet has no process control — they have operators turning knobs until the part looks right.

12. Frequently asked questions