The complete guide · injection molding
How to specify an injection-molded part, step by step
Everything a molder needs to quote your part and cut a tool once — resin, walls, draft, tolerances, texture, and volume — in the order a molding engineer pins them down, with the rules of thumb shown.
The short version
- Name the resin by standard grade — “ABS, UL94 V-0” or “PA66-GF30” — “plastic” is not a material.
- Keep walls uniform — 1.2–3 mm nominal; hold variation under ~25% or sink marks and voids follow.
- Draft every vertical face — ≥1° per side smooth; add more for texture — no draft, no part ejection.
- Every undercut is a mechanism — each one adds a side action to the tool; question each.
- Tolerance by class, tight only where needed — a general class (e.g. DIN 16742) + a short critical list with datums.
- Volume drives the tool — annual quantity sets cavity count and tool class — pricing is meaningless without it.
01Specify the resin by standard grade — it drives everything
The material line is the single most load-bearing line of a molding spec. Shrinkage (and therefore the tool's dimensions), strength, temperature rating, flammability, and cost all follow from the grade, not the family.
Write resin family + grade + any certification: “ABS, UL94 V-0” · “PA66, 30% glass-filled” · “PP homopolymer, FDA-contact”. If you only know requirements (outdoor UV, 90 °C, food contact), state those and mark the grade “vendor to propose” — that is a quotable line; “plastic” is not.
02Design to one nominal wall — and say what it is
Molten plastic wants to flow through, fill, and cool at one rate. Uniform walls fill predictably; thick sections sink and void; abrupt thin sections short-shot. This is the #1 physics flag molders raise.
Nominal wall 1.2–3 mm for most resins; keep local variation within ~25% of nominal; core out thick bosses and transition gradually (3:1 taper). State the nominal wall in the spec — it anchors the whole flow analysis.
03Draft every face in the pull direction
The part must slide off steel. Vertical faces without draft gall, drag, and eject scarred — or lock the part in the tool. Draft is not optional; it is the geometry's rent.
Minimum 1° per side on smooth faces (0.5° only for shallow features), and add roughly 1.5° per 0.025 mm of texture depth on textured faces. Call the pull direction on the drawing so the molder can check your draft against it.
04Question every undercut
Any feature the tool cannot pull straight off — snap hooks, side holes, internal clips — needs a side action, lifter, or collapsible core. Each mechanism adds tooling cost and maintenance for the tool's whole life.
List undercuts explicitly with their purpose. A snap fit that can be redesigned as a pass-through slot saves a side action; a hole moved to the parting line is free. Where the undercut earns its keep (a living hinge, a clip), keep it and accept the cost knowingly — living hinges only work in PP/PE, never glass-filled resins.
05Tolerance by class, with a short critical list
Molded parts are not machined parts: dimensions come from a steel cavity plus shrinkage. A general tolerance class covers everything; only the few dimensions that mate with other parts deserve tight callouts — each with a datum.
General: a class per DIN 16742 (or the SPI equivalent) — typically ±0.1–0.3% of dimension. Critical list: the handful of fits, each toleranced and datum-referenced. Tolerancing everything tightly is the classic way to double a tool quote.
06Specify surface finish by number, not adjective
“Nice surface” is unquotable. Mold finishes have standard scales: SPI (A-1 glossy through D-3 heavy texture) and VDI 3400 for spark-eroded textures. The finish also feeds back into draft (step 3).
Call cosmetic faces by grade — “SPI B-2, cosmetic faces per attached view; non-cosmetic faces SPI C-3” or a VDI number — and identify which faces are cosmetic. State where gate marks, ejector marks, and the parting line may and may not fall.
07Give quantity and lifetime volume — the tool follows it
The same part at 500/year and 500,000/year gets different tools: cavity count, steel vs. aluminum, tool class (SPI 101 for a million cycles down to 105 for prototypes). No molder can price a tool without the volume.
State: prototype quantity now, expected annual volume, and program life. Then defer the tooling decisions by name — cavity count, gate type and location, runner design, tool steel — “molder to propose against the volumes above.”
Worked example: a handheld enclosure half
The whole chain, numbers shown — the same derivations the SpecBuildr interview performs and prints in your spec's math appendix.
ResinABS, UL94 V-0 (indoor, drop-rated, self-extinguishing)
Nominal wall2.0 mm; bosses cored, ribs at 60% of wall (1.2 mm)
Draft1.5°/side external (textured), 1°/side internal (smooth)
Undercuts2 snap hooks (lifters accepted); side ports moved to parting line
TolerancesDIN 16742 TG5 general; 4 critical fits ±0.05 mm w/ datum A|B
FinishVDI 27 texture external; SPI C-1 internal; no gate marks on face A
Volume2,500 prototype year 1 → 40k/yr production, 5-year life
Deferred to moldercavity count, gate type/location, tool steel, runner
The mistakes that make quotes expensive
✗“Plastic” as the material
Shrinkage varies by grade — the tool is cut for one resin. Name the grade or state requirements for the molder to propose against.
✗Thick sections and chunky bosses
They sink on the outside and void on the inside. Core them out; keep the one nominal wall.
✗No draft on “straight” walls
The part has to eject. Zero-draft faces gall and scar — or need a costlier tool action to release.
✗Tight tolerances everywhere
Every needless tight callout is polishing and steel-tuning money. Class for the many, datums and numbers for the few.
✗Living hinge in the wrong resin
Hinges demand polypropylene or PE flow properties; in glass-filled or amorphous resins they snap in weeks.
Frequently asked questions
What does a molder actually need to quote an injection-molded part?
Seven things: resin by grade (or requirements for the molder to propose), CAD with one nominal wall and drafted faces, undercuts listed, a general tolerance class plus a short critical-dimension list with datums, surface-finish grades by face, quantity with annual volume, and the deferred tooling decisions named. With those a molder can quote part AND tool without a clarification round.
What wall thickness should an injection-molded part have?
One uniform nominal wall, typically 1.2–3 mm depending on resin and part size, with local variation kept within about 25%. Thick sections sink and void; core them out rather than thickening the wall.
How much draft angle do I need?
At least 1° per side on smooth faces, plus roughly 1.5° per 0.025 mm of texture depth on textured faces. Shallow features can get away with 0.5°; zero draft is a tooling problem, not a preference.
What tolerances are realistic for injection molding?
A general class per DIN 16742 (roughly ±0.1–0.3% of dimension for technical parts) with tight callouts reserved for the few mating dimensions — each with a datum. Tolerancing everything tightly is the fastest way to inflate a tool quote.
Can I write a molding spec without being a plastics engineer?
Yes — if something asks the right questions. The SpecBuildr interview walks through resin requirements, walls, draft, undercuts, tolerances, texture, and volume in plain language and flags the physics violations (thick walls, missing draft, living-hinge-vs-resin) as you answer.
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