Designing parts for plastic CNC machining is different from metal. Plastics are softer, more thermally sensitive, and behave differently under cutting forces. Here are 10 engineering tips to get better parts, faster quotes, and lower costs.

1. Specify realistic tolerances.
Plastic expands and contracts with temperature far more than metal. While we routinely achieve ±0.01mm on POM and PEEK, specifying tighter tolerances than necessary drives up cost. Use ISO 2768-m for general dimensions and reserve tight tolerances for critical features only.

2. Avoid sharp internal corners.
A sharp 90° internal corner requires EDM or special tooling. Design with a minimum internal radius equal to the tool radius — typically 1.5mm or larger. This also reduces stress concentration.

3. Mind the wall thickness.
Plastic walls thinner than 1mm can deflect during machining, vibrate, or even melt. Minimum recommended wall thickness is 1.5mm for most engineering plastics. For PEEK parts, 2mm minimum prevents chatter.

4. Threads in plastic need relief.
Internal threads in plastic benefit from a countersink at the entry and an undercut at the bottom. Use coarse threads (UNC/ISO metric coarse) rather than fine threads — they strip less easily in plastic.

5. Deep pockets require larger tools.
A pocket deeper than 4x the tool diameter requires extended-reach tooling and longer cycle times. If possible, design pockets with depth less than 3x diameter. Alternatively, split deep features across two parts.

6. Design for the material, not the drawing.
PEEK and PTFE behave completely differently during machining. PTFE deforms easily — avoid thin unsupported sections. POM machines beautifully but can chip at sharp edges — add a small chamfer to all edges.

7. Surface finish expectations.
As-machined plastic surfaces typically achieve Ra 0.8-1.6µm. Mirror finishes require polishing — design extra stock allowance (0.05-0.1mm) if polishing is specified. PMMA (acrylic) can achieve optical clarity with vapor polishing.

8. Avoid undercuts unless necessary.
Undercuts require special tooling or multi-axis setups. If an undercut is unavoidable, make it accessible from the part exterior and provide generous tool clearance.

9. Choose the right material early.
Material selection affects every aspect of machining: feeds, speeds, tool wear, and achievable tolerances. Involve your machining partner during the design phase. At PlasticPartsCustom, we offer free DFM (Design for Manufacturability) feedback with every quote.

10. Provide complete documentation.
A 3D STEP file plus a 2D drawing with critical dimensions, tolerances, and surface finish notes ensures your parts are made right the first time. Include material specifications, any regulatory requirements, and inspection criteria.

Get a Free DFM Review:
Upload your design to PlasticPartsCustom for a complimentary manufacturability review. Our engineers will flag potential issues and suggest cost-saving improvements before machining begins.