How-To · Materials
How to Cut Acrylic, HDPE and Other Plastics on a CNC Router
Plastics cut beautifully on a CNC router when you treat them differently from wood. The main enemy is heat: plastics soften at low temperatures, and a bit that rubs instead of cutting will melt the material, weld chips back into the slot, and leave gummy edges.
This guide covers the plastics hobbyists cut most often, the bits that suit them, how to keep chips cool, how to hold thin sheet flat, and how to finish edges. As with any material, start from your bit maker's recommendations and test in scrap.
Common plastics and how they behave
| Plastic | Machining character | Notes |
|---|---|---|
| Cast acrylic (PMMA) | Cuts cleanly with good chips; can give a good edge | Brittle; can chip or crack with dull bits or heavy cuts |
| Extruded acrylic | Softens and gums more easily | Usually cheaper; needs more attention to heat |
| HDPE | Soft, slippery, cuts easily, stringy chips | Hard to glue; fuzzy edges if feed is too low |
| Polycarbonate | Tough, less brittle than acrylic, machines well | Unlike lasering, routing is a common way to cut it |
| Acetal (POM, often sold as Delrin) | Machines very cleanly; good for precise parts | Popular for gears, bushings and jigs |
| ABS sheet | Machines reasonably; can melt with heat | Common in enclosures and panels |
| PVC sheet | Machines easily by router | Keep chips contained and dust controlled; never laser it |
Check the supplier's data for any sheet you buy, and ask whether acrylic is cast or extruded. The difference shows up at the machine.
Choosing bits for plastic
Single-flute and O-flute
Fewer flutes means more room for chips and a larger chip at a given feed and RPM, which carries heat away. Single-flute bits are the default for many plastics. O-flute bits are single-flute bits with a polished, rounded flute designed specifically for plastics. Our guide to single-flute end mills covers the general bit style.
Browse O-flute bits for acrylic on Amazon
Upcut, downcut or compression
Upcut evacuates chips best, which matters with plastics, but can lift thin sheet and fray the top edge. Downcut gives a clean top edge but packs chips into the slot, which builds heat. For many plastic jobs, an upcut single flute with good workholding is the starting point. See upcut vs downcut vs compression for details.
Sharpness
A dull bit rubs, and rubbing melts plastic. Keep a dedicated bit for plastics if you can, so wood resin and MDF glue don't dull it first.
Engraving plastics
V-bits work well for engraving acrylic signs. Engraving the back of clear acrylic and viewing it from the front is a popular technique for edge-lit signs. See V-bits for engraving.
Keeping heat down
- Keep the chip load up. If you see fine powder or melted strings, the chip is too small. Raise feed or lower RPM.
- Don't dwell. Avoid pausing in the cut and use ramps rather than slow straight plunges.
- Clear chips. Air blast or a dust shoe stops chips from being recut. Recut chips are a major heat source.
- Use sensible depth per pass. Very shallow passes at slow feed can rub; very deep passes on a flexible machine can chatter and crack acrylic. Find the middle.
- Consider a finishing pass. A light final pass with a good chip load can improve edges.
- Watch the slot. If chips start welding into the cut behind the bit, stop and adjust before it grabs the bit.
Coolant mists used for metals are not usually needed for plastics on a hobby router, and some fluids can attack certain plastics. Air is the simple, safe option. Check compatibility with the plastic supplier if you consider a liquid.
Browse air blast nozzles on Amazon
Workholding for thin sheet
Thin plastic sheet flexes and vibrates, and parts can fly when cut free. Hold it flat across its whole area, not just at the edges.
Tape and CA glue
Excellent for flat sheet; keeps small parts in place after cut-through.
Vacuum table
Fast and even hold on full sheets; less effective on small parts. See vacuum tables.
Many clamps
Use several around the perimeter and add tabs to cut parts. See low-profile clamps.
Screws through scrap
Pre-drill acrylic to avoid cracking; don't overtighten.
Leave the protective film or paper on the sheet while cutting where you can. It protects the surface from scratches and clamp marks. Peel it after the job.
Finishing edges
Even a good cut leaves tool marks. Common finishing options:
- Scrape: a deburring tool or cabinet scraper removes small burrs and tool marks. See deburring tools.
- Sand: progress through finer grits, wet-sanding for acrylic if you want a clear edge.
- Polish: plastic polishing compounds and a buffing wheel can bring acrylic edges close to clear.
- Flame polish: some fabricators use a torch on acrylic edges. It requires practice, can introduce stresses that later crack the part, especially near solvent-cemented joints, and is a fire hazard. Many hobbyists skip it.
Browse plastic polishing kits on Amazon
Buying and preparing plastic sheet
Good results start before the sheet reaches the machine.
- Know what you're buying. Ask whether acrylic is cast or extruded, and buy from a supplier that labels the material. Unlabeled scrap plastic can be anything.
- Check thickness. Plastic sheet thickness varies from nominal, sometimes noticeably. Measure with calipers and set the stock thickness in CAM to the measured value, especially for through-cuts and pockets.
- Store sheets flat and out of direct sun. Warped sheet is hard to hold flat and cuts unevenly.
- Keep the masking on until the part is finished to avoid scratches from chips, clamps and handling.
- Let it acclimate. Plastic expands and contracts with temperature more than many people expect. For precise fits, cut and measure at a similar temperature to where the part will be used.
- Clean the sheet before tape and glue so dust doesn't weaken the hold.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Melted, gummy edges | Heat from rubbing or recut chips | Larger chip load, single-flute bit, air blast |
| Chips welded in slot | Poor chip evacuation | Upcut bit, air, shallower passes, adaptive toolpaths |
| Cracks or chips in acrylic | Dull bit, heavy cut, vibration, clamp pressure | Sharp bit, lighter passes, better support |
| Fuzzy HDPE edges | Feed too low, dull bit | Raise feed, sharp single flute |
| Part lifted during cut | Upcut pulling thin sheet | More hold-down, tabs, or compression/downcut on final pass |
Safety notes
- Wear eye protection; plastic chips are sharp and can be flung.
- Keep dust collection running, and use a respirator rated for fine particles. See respirator vs dust mask.
- Melting plastic can release fumes. If you smell burning plastic, stop and fix the cause, and ventilate the space.
- Check the supplier's safety data sheet for any plastic you machine regularly.
FAQ
What is the best bit for cutting acrylic on a CNC?
A sharp single-flute or O-flute bit is the common starting point, because it clears chips well and keeps heat down. Use your bit maker's recommendations for feeds and speeds.
Why does my acrylic melt when I cut it?
The bit is generating more heat than the chips carry away, usually from too small a chip load, recut chips or a dull bit. Raise the feed or lower RPM, clear chips with air, and use a sharp single-flute bit.
Is cast or extruded acrylic better for CNC?
Cast acrylic generally machines more cleanly. Extruded acrylic tends to soften and gum more easily, though it can be cut successfully with careful heat control.
Can I cut polycarbonate on a CNC router?
Yes. Polycarbonate routes well and is tougher than acrylic. It's lasering polycarbonate that gives poor results, not routing it.
Bottom line
Cutting plastic is about managing heat. Use sharp single-flute or O-flute bits, keep the chip load healthy, clear chips with air, and hold thin sheet flat across its whole area. Know whether your acrylic is cast or extruded, start from the bit maker's guidance, and test in scrap. With those habits, acrylic signs, HDPE cutting boards and acetal jigs become some of the cleanest parts your router makes.