Last updated: 2026-09-05 · 5 min read
What Files Does a Mold Maker Need from You — Which CAD Formats, Drawings and Specifications Ensure an Accurate Quote and a First-Time-Right Mold?
A mold maker needs three file types to quote and build: (1) 3D part model (STEP or IGES), (2) 2D drawing with GD&T and critical dimensions, and (3) resin and production specifications. Missing files cause misquoted molds, T1 failures and costly rework. Send all files before the deposit — incomplete submissions delay the project 2–6 weeks.
- The 3D part model (STEP format preferred, IGES acceptable) is the primary reference for cavity machining. The mold maker imports the STEP file into CAM software to generate toolpaths for CNC machining the cavity. The model must include: all part features (ribs, bosses, snap fits, threads), nominal dimensions (no shrinkage applied — the mold maker applies shrinkage based on the resin), and the parting line or parting surface if already determined. Accept formats: STEP AP203 or AP214 (preferred), IGES (acceptable), Parasolid (.x_t), SolidWorks (.sldprt). Avoid: STL (mesh, not precise), PDF 3D (not machinable).
- The 2D drawing (PDF format) defines what the 3D model does not: critical dimensions with tolerances (GD&T), surface finish requirements (SPI A1–D3), parting line location, gate location preference, ejector pin marks allowed, and cosmetic vs. non-cosmetic surfaces. The drawing is the legal document — if the 3D model and drawing conflict, the drawing governs. Include: title block (part name, material, drawing number, revision), datum reference frame, critical-to-function dimensions with ±tolerance, and notes on secondary operations (ultrasonic welding, pad printing, assembly).
- Production specifications tell the mold maker how many cavities, what mold base standard, what mold life and what press the mold must fit. Include: annual volume (determinates cavity count and steel selection), resin grade with data sheet (determinates shrinkage, mold temp, hardness), press specification (platen size, tie bar spacing, clamp tonnage, nozzle radius), and mold life requirement (SPI class 101–105). For multi-cavity molds: specify family mold (different parts) or multi-cavity (identical parts). Missing specs = the mold maker guesses, and guesses are often wrong.
What File Package Gets You an Accurate Mold Quote?
| File | Format | Required? | What It Defines |
|---|---|---|---|
| 3D part model | STEP (AP214) | Yes | Cavity geometry for CNC |
| 2D drawing | Yes | Tolerances, GD&T, surface finish | |
| Resin data sheet | Yes | Shrinkage, mold temp, processing | |
| Press specification | PDF or text | Yes | Platen size, tonnage, bolt pattern |
| Annual volume | Email or RFQ | Yes | Cavity count, steel selection |
| SPI mold class | RFQ text | Recommended | Mold life (101–105) |
| Surface finish spec | 2D drawing | If cosmetic | SPI A1–D3 |
| Assembly drawing | If mating parts | Fit and clearance requirements |
The more complete the file package, the more accurate the quote. A mold maker receiving only a 3D model (no drawing, no resin, no volume) must guess the tolerances, steel and cavity count — the quote will include contingency (10–20% extra) to cover the unknowns. A complete file package gets a precise quote with no contingency padding.
GD&T — Geometric Dimensioning and Tolerancing, a symbolic language on 2D drawings that defines not just dimension values but the shape, orientation and location tolerances of each feature. Symbols include flatness, parallelism, perpendicularity, position (true position), profile and runout. GD&T lets the mold maker know which dimensions are functional datums and which are reference-only. The injection tolerance guide covers what tolerances are achievable in production.
What Three Common File Mistakes Cause Mold Delays?
- Sending a mesh file (STL) instead of a solid model (STEP) — the mold maker cannot machine from a mesh. STL files are triangulated meshes — the surface is made of thousands of tiny triangles, not precise mathematical surfaces. CNC software cannot generate accurate toolpaths from STL. The mold maker must reverse-engineer the STL back to a solid model — this takes 1–3 days and introduces errors. Always send STEP (AP214) — it contains precise B-rep surfaces that CAM software can machine directly. If the part was designed in SolidWorks, export as STEP, not STL.
- Applying shrinkage to the 3D model before sending — the mold maker applies shrinkage based on the resin. If the part designer scales the model by 1.005 (0.5% shrinkage for ABS) and the mold maker also applies 1.005, the cavity is 1.010× nominal — the parts are 0.5% too large. The correct workflow: send the model at nominal dimensions (1.000×), and specify the resin grade on the drawing. The mold maker applies the correct shrinkage factor from the resin data sheet. If you have already applied shrinkage, note it in the RFQ so the mold maker can compensate.
- Omitting critical dimensions from the 2D drawing — the mold maker does not know which dimensions are tight. A 3D model shows nominal geometry but does not indicate which dimensions are critical. Without tolerances on the drawing, the mold maker assumes standard machining tolerance (±0.1mm) — if a critical dimension requires ±0.02mm, the mold must be machined differently (CNC grinding or EDM instead of milling). Mark every critical dimension with GD&T callouts on the 2D drawing. If a dimension is non-critical, leave it unmarked — the mold maker applies general tolerance.
B-rep — Boundary Representation, the mathematical definition of a solid model’s surfaces. STEP files use B-rep to describe precise curved surfaces (cylinders, cones, NURBS free-form). Unlike STL meshes (triangulated approximations), B-rep surfaces are exact — CNC software can generate toolpaths directly from them. The mold flow analysis guide also requires a precise solid model for mesh generation.
Mold maker needs: 3D STEP model (cavity geometry), 2D PDF drawing (tolerances, GD&T), resin data sheet (shrinkage, mold temp), press spec (platen, tonnage), annual volume (cavity count). Complete files = accurate quote. Missing files = 2–6 week delay.
- 3D model in STEP AP214 — not STL, not IGES (STEP is exact B-rep).
- 2D drawing with GD&T callouts — critical dims with ±tolerance marked.
- Resin data sheet included — shrinkage factor determines cavity size.
- Press spec provided — platen size and tonnage must match mold base.
- No pre-applied shrinkage on model — mold maker applies it from resin data.
Frequently Asked Questions
Can I send a 2D drawing only (no 3D model)?
Yes — but the mold maker must create a 3D model from the 2D drawing, which adds $500–$2,000 and 3–5 days to the project. The reconstructed model may not capture complex surfaces accurately (organic shapes, free-form curves). For simple parts (flat brackets, rectangular housings): 2D drawing only is acceptable. For complex parts (curved surfaces, snap fits, optical features): a 3D model is essential. The 3D model is the single source of truth for cavity machining.
What if I do not know which resin I want to use?
Specify the functional requirements instead: operating temperature, impact resistance, chemical exposure, color, regulatory requirements (flame rating, food contact). The mold maker or a material engineer can recommend a resin grade based on the requirements. However, the resin must be selected before the mold is built — the shrinkage factor determines the cavity dimensions. If the resin is unknown at quote stage: request a preliminary quote with a placeholder resin (e.g., ABS, 0.5% shrinkage), and finalize the resin before the mold order is placed.
How do I protect my intellectual property when sending files to a mold maker?
Sign a Non-Disclosure Agreement (NDA) before sending any files. The NDA should cover: the files are confidential, the mold maker cannot share the design with third parties, the mold maker cannot produce the part for other customers, and the mold design (cavity layout, electrode designs) belongs to you. Most reputable mold makers have standard NDAs. For high-value IP: send only the portion of the model relevant to the mold (not the full assembly), and watermark the drawings with “CONFIDENTIAL — [Your Company Name].”
Key Takeaways
- Must-send: 3D STEP model, 2D PDF drawing, resin data sheet.
- Also needed: press spec, annual volume, SPI mold class.
- Common mistakes: STL instead of STEP, pre-applied shrinkage, missing tolerances.
- Complete files = accurate quote. Missing files = 2–6 week delay.
Send the 3D STEP model, 2D drawing with GD&T, resin data sheet and press specification — the review confirms the file package is complete, identifies any missing information, and provides an accurate mold quote with defined delivery timeline.
Sources: Plastics Technology — File Preparation for Mold Making · Plastics Technology Online — CAD Data Exchange in Tooling · Society of Plastics Engineers (SPE) — CAD/CAM in Mold Design Technical Papers · STEP File Format Reference — AP203 vs AP214 · NIST — STEP Standard for CAD Data Exchange (ISO 10303)
— Arthur, Tooling Engineer at aCatMould