Plastics are excellent CNC materials — if you know their quirks. Each plastic has its own character: some melt easily, others produce long stringy chips, and a few machine as beautifully as metal. This guide shows you how to achieve clean results with the most common plastics.
The Most Important Plastics for CNC Milling
POM (Polyoxymethylene / Delrin)
The dream material for CNC machining:
- Machines almost like metal — clean chips, no stringing
- Dimensionally stable and low-warpage
- Excellent surfaces straight off the machine
- Ideal for functional parts, gears, bushings
Cutting data POM: 18,000–24,000 RPM, feed rate 2,000–4,000 mm/min, depth of cut up to 1×D
PMMA (Acrylic / Plexiglas)
Beautiful but tricky:
- Crystal-clear optics — perfect for displays and signs
- Danger: Melts! If the cutter stops or the feed rate is too slow
- Use single-flute cutters with polished flutes
- Edge polishing possible with flame or chemical treatment
Cutting data PMMA: 18,000–24,000 RPM, feed rate 2,500–4,000 mm/min. Never stop moving!
PA (Nylon / Polyamide)
Tough and elastic:
- Tends to produce long, stringy chips
- Absorbs moisture — dimensions can change
- Good for gears, bushings, and structural parts
- Use sharp tools and aggressive feed rates
Cutting data PA: 15,000–20,000 RPM, feed rate 1,500–3,000 mm/min
PE and PP (Polyethylene / Polypropylene)
Soft and flexible:
- Difficult to clamp — tends to deform under pressure
- Produces long, continuous chips
- Use sharp single-flute cutters
- Applications: cutting boards, chemical containers
Cutting data PE/PP: 15,000–20,000 RPM, moderate feed rates, low clamping pressure
PVC (Hard PVC / Foam PVC)
- Hard PVC: Mills well, but produces toxic fumes — extraction essential!
- Foam PVC (Forex): Light, easy to mill, popular in sign making
General Rules for Plastic Milling
The Golden Rules
- Keep it cool: Plastics melt from heat — ensure good chip evacuation
- Stay moving: Never let the cutter dwell in the material
- Sharp tools only: Dull tools generate heat instead of cutting
- Single-flute preferred: More space for chip evacuation
Clamping Plastics
- Thin sheets: Vacuum table or double-sided tape
- Blocks: Vise or clamps — but not too tight (deformation!)
- Acrylic: Leave protective film on during milling
Cutting Parameters Overview
| Material | Spindle Speed | Feed Rate | Depth of Cut | Special Notes |
|---|---|---|---|---|
| POM | 18,000–24,000 RPM | 2,000–4,000 mm/min | Up to 1×D | Dream material, everything works |
| PMMA | 18,000–24,000 RPM | 2,500–4,000 mm/min | Up to 1×D | Never stop! Polished single-flute |
| PA | 15,000–20,000 RPM | 1,500–3,000 mm/min | Up to 0.5×D | Sharp tools, aggressive feed |
| PE/PP | 15,000–20,000 RPM | 1,500–2,500 mm/min | Up to 0.5×D | Low clamping pressure |
| Hard PVC | 15,000–20,000 RPM | 2,000–3,500 mm/min | Up to 1×D | Extraction for fumes! |
| Foam PVC | 18,000–24,000 RPM | 3,000–5,000 mm/min | Full thickness possible | Easy to machine |
Common Problems and Solutions
| Problem | Cause | Solution |
|---|---|---|
| Melted edges | Too slow, dull tool | Increase feed rate, replace tool |
| Long stringy chips | Material property (PA, PE) | Sharper tool, more aggressive parameters |
| Milky/white surface (acrylic) | Wrong tool or parameters | Polished single-flute, higher feed rate |
| Workpiece warping | Clamping pressure too high | Reduce pressure, use vacuum table |
| Burrs on edges | Dull tool | Replace tool, consider finish pass |
FAQ — Frequently Asked Questions
Can I mill all plastics on my CNC?
Almost all thermoplastics, yes. Thermosets (epoxy, phenolic) are more difficult due to abrasive fibers. PVC requires good extraction (toxic fumes).
Do I need special tools for plastic?
Not strictly, but recommended. Single-flute cutters with polished flutes work best. The polished surface prevents material adhesion.
Can I achieve optical-quality edges on acrylic?
Not directly from the milling machine (although it comes close with polished single-flute cutters). For perfectly transparent edges, you need to flame polish or solvent polish afterwards.
Conclusion: Plastics Love Speed
The golden rule for plastic milling: keep it fast, keep it sharp, keep it moving. With the right tools and parameters, plastics are grateful materials that produce excellent results.
Need the right tools for plastic machining? Explore our milling cutters.
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