Designing for Moldability: The Fundamentals That Decide Whether a Tool Gets Recut
Content in this Article
Moldability is easiest to control before tooling release. Align material, wall thickness, ejection, gating and acceptance criteria so appearance, dimensions and cycle time are considered together.
- Define function, environment, quantity and risk before selecting a material or process.
- Treat typical values as comparison points; confirm the exact grade and supplier capability before release.
- Use a first article or pilot lot to verify dimensions, function and records before scaling.
Define the decision
Document function, environment, quantity and risk so every option is compared against the same requirement.
Uniform Wall Thickness Is the One Rule That Subsumes the Others
Plastic shrinks as it cools, and it shrinks in proportion to local thickness. Put a 2 mm wall next to a 4 mm boss and the two zones cool at different rates, the thick zone pulls the thin zone toward it, and the part warps.
| Resin | Typical Wall (mm) | Max Local Thickness | Shrinkage % | Warp Sensitivity |
|---|---|---|---|---|
| ABS | 1.2 – 3.0 | 1.5× nominal | 0.4 – 0.7 | Low |
| PC | 1.0 – 3.5 | 1.5× nominal | 0.5 – 0.7 | Low |
| PC/ABS | 1.2 – 3.0 | 1.5× nominal | 0.5 – 0.7 | Low |
| PP | 1.0 – 3.5 | 1.5× nominal | 1.3 – 2.2 | Medium |
| Nylon 6/66 | 1.0 – 3.0 | 1.3× nominal | 1.0 – 2.0 | High |
Where thickness truly has to change, the transition must be gradual — a taper of at least 3:1 length-to-thickness-step — never abrupt. Cored-out thick sections, where a pocket is molded into the back of what would have been a solid slab, are how production parts keep the outside looking like a solid block while obeying the uniform-wall rule inside.
Draft Angles: Every Vertical Face Has to Climb Out of the Tool
A molded part with no draft grips the core like a ring on a finger. The ejector system can push it off, but it drags, scuffs, or stresses.
| Surface Type | Draft Minimum | Draft Recommended | Notes |
|---|---|---|---|
| Smooth polished (SPI A-2) | 0.5° | 1° | Lowest friction, smallest draft |
| Smooth machined (SPI B-1) | 1° | 1.5° | Default for cosmetic |
| Light texture MT-11010 | 2° | 3° | Roughly 0.025 mm texture depth |
| Medium texture MT-11020 | 3° | 4 – 5° | 0.05 mm texture depth |
| Heavy leather MT-11040 | 5° | 6 – 8° | 0.12 mm texture depth |
Moldability evaluation flow
Define requirements, narrow the material and process, then verify with a consistent acceptance method.
- 1Requirements
Function, environment, quantity and risk
- 2Options
Material, process, design controls and finishing
- 3Acceptance
Dimensions, functional tests and production records
Compare the practical options
Compare material, process, design controls and finishing together—not unit price alone.
Validate before scaling
Before scaling, verify dimensions, function and records with one controlled method across representative lots.
Radii Replace Sharp Corners Because Sharp Corners Crack
Sharp inside corners recommended practice two bad things at once: they concentrate stress so the part cracks early, and they force the flow front to make a 90° turn, which changes velocity, drops pressure, and locally heats the polymer. Both problems disappear with a radius.
Gate Location Is the One Decision the Mold Shop Wants Written Down First
The gate is the hole where molten plastic enters the cavity. Where it sits determines how the cavity fills, where the weld lines land, how the pressure decays across the part, and what the visible witness mark looks like.
| Gate Type | Witness Mark | Tool Cost | Best For |
|---|---|---|---|
| Edge gate | Visible stub | Lowest | Flat parts, non-cosmetic edge available |
| Sub / tunnel gate | Invisible, auto-trim | Medium | Cosmetic parts where gate can land on inside surface |
| Hot-tip / valve gate | Pin-point or invisible | Highest | High-volume cosmetic parts, multi-cavity |
| Fan gate | Wide flow spread | Medium | Wide thin parts, appearance panels |
| Diaphragm gate | Central ring | Medium | Round parts where concentric flow matters |
Gate into the thickest section so plastic fills from thick to thin; gating into thin and filling toward thick traps air and produces voids, sinks, or short shots. On cosmetic parts, insist on a sub-gate, valve gate, or edge gate landing on a non-cosmetic face, and mark the chosen location on the CAD file before quoting.
Pre-RFQ review: recommended practice and common mistakes
| Recommended practice | Common mistake |
|---|---|
| Document function, environment, quantity and risk | Select from a material name or machine specification alone |
| Review material, process, design controls and finishing together | Address manufacturing limits only after design freeze |
| Inspect critical dimensions and function on the first article | Scale production from visual approval alone |
| Keep material, revision and inspection records | Reuse old results after a material or process change |
- Function, environment, quantity and risk are documented
- Material, process, design controls and finishing are reviewed with the supplier
- Critical dimensions, appearance and functional acceptance are on the drawing or RFQ
- First-article or pilot-lot verification is planned
- Material, process and revision changes trigger a new review
FAQ, further reading and sources
What should be defined first for Moldability?
Start with function, environment, quantity and acceptance criteria. These inputs narrow the practical options faster than naming a machine or material first.
Can typical values in this article be released directly on a drawing?
No. Use them for early comparison, then confirm the exact grade, supplier capability and first-article result.
When is the process ready to scale?
Scale only after material, process, finishing and inspection are controlled and repeatable across representative lots.
Next, explore 3D printing services, CNC machining, materials, and the related reports linked on this page.
