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

Most of the expensive surprises in an injection molding program were visible in the CAD model weeks before anyone noticed them. A wall that’s too thick, an undercut nobody flagged, a cosmetic face with no draft—each one is cheap to fix on screen and costly to fix in steel. This checklist is a structured pass to catch those before you request quotes. It doesn’t replace your supplier’s engineering review, and it doesn’t replace the deeper DFM guide—it’s the run-through that makes both of those more productive.

Work through it category by category. Most items are a quick yes/no; the ones that aren’t are usually where the program’s risk is hiding.

How to Use This

Run the checklist against the part before the RFQ goes out. For anything you can’t answer cleanly, either resolve it or flag it as an open question for the supplier—an explicit question gets a useful answer, while a silent assumption tends to surface later as a revision. Each section links to a deeper guide if you need it.

Section	What it catches
1. Geometry & parting line	Moldability in two halves, seam placement, sharp corners
2. Draft	Faces that won’t release from the steel
3. Walls & thickness	Sink, voids, and warp from thick or uneven walls
4. Ribs, bosses & features	Sink-through and weak functional features
5. Undercuts & actions	Hidden tooling cost and complexity
6. Cosmetic surfaces & finish	Visible marks, finish ambiguity
7. Tolerances & critical dims	Over-tightened or unstated requirements
8. Material	Behavior, grade, and regulatory gaps

1. Geometry & Parting Line

• The basic shape can be formed by two mold halves opening in one direction (or the exceptions are identified).
• You know roughly where the parting line will fall, and whether it crosses a cosmetic or sealing surface.
• Sharp internal corners are radiused—sharp corners concentrate stress and impede flow.
• No unnecessary features that force tooling complexity without functional need.

2. Draft

• Every face in the draw direction has draft (a small taper) so the part releases.
• Textured faces carry extra draft—the coarser the texture, the more they need.
• Tall ribs, bosses, and deep features are drafted, not just the outer walls.
• Any “must stay vertical” face is flagged, because zero-draft usually costs tooling or a redesign.

3. Walls & Thickness

• Wall thickness is as uniform as the function allows—uniformity is the single biggest driver of a clean part.
• No thick, solid sections that will sink or form voids; heavy areas are cored out.
• Thickness transitions are gradual, not abrupt steps.
• Wall is within the typical range for the chosen resin (defer specifics to the datasheet).

4. Ribs, Bosses & Features

• Ribs and bosses follow the thickness-ratio guidance so they don’t sink the opposite (show) face.
• Screw bosses have appropriate wall and a relief so they don’t read through cosmetically.
• Snap-fits, living hinges, and similar functional features are designed deliberately, not added as afterthoughts.

5. Undercuts & Tooling Actions

• Undercuts are identified and sorted into “the part needs it” vs “it can be redesigned out.”
• Each remaining undercut has a plan—side action, lifter, or hand-load—and you understand it adds tooling cost.
• Threads, snaps, clips, and side holes are checked as potential undercuts.

6. Cosmetic Surfaces & Finish

• Cosmetic (“A”) surfaces are identified, and the finish is specified to a standard (SPI/VDI) rather than an adjective—see surface finish.
• Gate and ejector locations are kept off, or acceptable on, the show surface.
• You’ve decided where any visible parting line, weld line, or gate mark is allowed to land.

7. Tolerances & Critical Dimensions

• The few genuinely critical dimensions are identified; everything else carries a general tolerance.
• Tolerances are realistic for injection molding, not carried over from a machined-part drawing.
• A 2D drawing with GD&T and reference datums accompanies the CAD—CAD shows shape, the drawing says what’s controlled.

8. Material

• A specific resin and grade is selected, or approved alternatives are documented—see the material selection guide.
• The design accounts for that material’s behavior (shrinkage, warp tendency, whether it’s filled).
• Any regulatory, flammability, or environmental requirements on the material are stated.

Turning the Checklist Into an RFQ

A clean pass through this list is most of what a supplier needs to quote accurately. The remaining step is packaging it—CAD, the 2D drawing, material, volumes, cosmetic and tolerance callouts, and your open questions—into a structured request. The RFQ template shows where each of these belongs, and the DFM guide explains the reasoning behind the items above.

This is an independent buyer resource, not engineering review. A checklist catches the common, visible issues; it doesn’t replace a supplier’s moldflow and tooling analysis. Use it to raise the quality of your design and your questions before the quote, then let the supplier confirm moldability for your specific part.

Buyer FAQs

What should a plastic part design checklist cover before RFQ?

At minimum: overall geometry and parting-line direction, draft on all draw-direction faces, uniform wall thickness, ribs and bosses sized to avoid sink, undercuts identified and planned for, cosmetic surfaces and finish specified to a standard, realistic tolerances with critical dimensions called out, and a specific material. Catching these before RFQ produces more accurate quotes and fewer late, expensive revisions.

Does a DFM checklist replace supplier engineering review?

No. A buyer-side checklist catches the common, visible issues—missing draft, non-uniform walls, unflagged undercuts—so they’re resolved before quoting. It does not replace the supplier’s moldflow simulation, tooling analysis, and engineering review, which assess fill, warp, and manufacturability for your specific part and resin. The checklist makes that review more productive; it doesn’t substitute for it.

What’s the most common design issue caught in DFM review?

Non-uniform wall thickness. Thick and thin sections cool at different rates, which drives sink marks, warpage, and internal stress. Most other items on the list—ribs, bosses, transitions—come back to the same principle of keeping walls as uniform as the part’s function allows. It’s the first thing worth checking on any design.

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

Yes, for anything with critical dimensions or cosmetic requirements. The CAD model communicates shape, but a 2D drawing tells the supplier what must be controlled—critical dimensions, tolerances, GD&T, reference datums, cosmetic zones, and finish callouts. Without it, the supplier has to guess what matters, which leads to inconsistent quotes and acceptance disputes later.