Plastic Part Design Checklist: A Pre-RFQ Self-Check for Buyers

Most expensive tooling revisions fix design flaws that were already visible in CAD. Non-uniform walls, unacknowledged undercuts, and zero-draft cosmetic faces are cheap to change in software and expensive to fix in steel. Run this checklist before you distribute an RFQ so quotes sit on a manufacturable baseline. It does not replace a supplier’s formal engineering review or moldflow analysis.

Flag unresolved items as open questions in the RFQ. Otherwise suppliers make silent, divergent assumptions.

1. Geometry and Parting Line

• The primary shape can be molded by two tool halves opening in a single direction (or exceptions are noted).
• The anticipated parting line location avoids critical sealing or cosmetic surfaces.
• Sharp internal corners are radiused to reduce stress concentrations and improve polymer flow.
• Unnecessary complex features have been removed to minimize tooling cost.

2. Draft

• All faces parallel to the draw direction possess draft (taper) to allow ejection.
• Textured surfaces incorporate additional draft proportional to the texture depth.
• Internal features (tall ribs, deep bosses) are drafted.
• Faces requiring zero draft are explicitly flagged for tooling assessment.

3. Walls and Thickness

• Wall thickness is as uniform as functionally possible to prevent differential cooling and warpage.
• Thick, solid sections are cored out to prevent sink marks and internal voids.
• Transitions between different wall thicknesses are gradual.
• The nominal wall thickness aligns with the selected resin’s flow characteristics.

4. Ribs, Bosses, and Features

• Ribs and bosses adhere to standard thickness ratios (typically 50-70% of the nominal wall) to prevent sink marks on the opposing face.
• Screw bosses feature appropriate wall thicknesses and base reliefs.
• Snap-fits and living hinges are engineered to specific strain limits, not approximated.

5. Undercuts and Tooling Actions

• All undercuts are evaluated for necessity; non-essential undercuts are redesigned out.
• Necessary undercuts are identified, acknowledging the requirement for side actions, lifters, or hand-loads, which increase tooling cost.
• Threads, side holes, and clips are reviewed as functional undercuts.

6. Cosmetic Surfaces and Finish

• Primary cosmetic (“A”) surfaces are identified in the drawing.
• Surface finish is specified using an industry standard (e.g., SPI or VDI) rather than subjective adjectives.
• Gate and ejector pin locations are restricted from critical cosmetic surfaces.
• Allowable locations for weld lines and parting line mismatch are defined.

7. Tolerances and Critical Dimensions

• Only functionally critical dimensions carry tight tolerances; the remainder utilize general block tolerances.
• Tolerances are calibrated for plastic injection molding, not inherited from machined metal standards.
• A 2D drawing with GD&T and reference datums accompanies the 3D CAD model. (CAD defines the shape; the drawing defines the control requirements).

8. Material

• A specific resin family and candidate grade are selected, or acceptable alternatives are documented.
• The CAD model accommodates the shrinkage and behavioral characteristics of the selected material (e.g., accounting for glass-fill anisotropy).
• Regulatory requirements (flammability, UV resistance, medical compliance) are explicitly stated.

Submitting the RFQ

A clean design supports accurate quoting. Package the CAD file, 2D drawing, material specification, estimated annual volumes, and cosmetic requirements into a structured request. Use the RFQ template to organize these elements, and consult the deeper DFM guide for the engineering principles behind this checklist.

Buyer FAQs

What should a plastic part design checklist cover before RFQ?

Confirm overall moldability, draft, uniform wall thickness, cored-out thick sections, identified undercuts, standardized cosmetic callouts, realistic plastic tolerances, and a named material. Anything still open goes in the RFQ as an explicit question, not a silent assumption.

Does a DFM checklist replace supplier engineering review?

No. Use this checklist to catch basic geometry and specification errors so quotes are comparable. Still require the supplier’s moldflow analysis, cooling layout, and structural tooling review before you release steel.

What is the most common design issue caught in DFM review?

Non-uniform wall thickness. Thick and thin sections cool at different rates and drive sink marks, internal voids, warpage, and residual stress. Fix wall uniformity in CAD before you chase process tweaks at T1.

Do I need a 2D drawing if I’m supplying CAD?

Yes. CAD defines geometry; the 2D drawing defines control. It carries critical tolerances, GD&T datums, surface finish standards, and material requirements. Without a drawing, the supplier must guess which dimensions are critical. Put that guess out of the quote.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide engineering services, or certify suppliers. Design and tooling decisions are part-specific. Confirm them through your supplier’s engineering review.