CNC plastic parts processing: how plastic behaves under a cutter
This page explains what happens when a rotating tool removes plastic, and why geometry that works in aluminum often fails in POM or PC. It is written for design engineers and buyers who need to know when CNC plastic parts processing is the right route and when it is not.
What the cutter actually does to plastic
A CNC tool removes plastic by shear, not by the clean chip formation you see in 6061. Plastic deforms ahead of the cutting edge, springs back after the edge passes, and recovers a fraction of the depth you set on the offset. That elastic recovery is why a finishing pass that hits nominal size in steel can leave a plastic bore a few thousandths undersize.
Heat is the second control variable. Plastic conducts heat roughly a thousand times worse than aluminum, so the energy that leaves a metal cut with the chip stays near the edge in plastic. Above the glass transition temperature the material softens, the edge rubs instead of shearing, and you get a smeared wall with pulled fibers.
Cutting speed and feed have to be read together. A two-flute carbide end mill at 12,000 rpm and 2,500 mm/min in POM leaves a clean wall; the same tool at metal feeds can weld chips back onto the surface. Operators watch chip color and sound, not just the readout.
Every plastic has its own window. The window is narrow for PEEK and wide for HDPE. That single fact drives tool choice, fixture design, and how much stock you leave for the finish pass.
Which plastics machine well, and which fight back
POM (Delrin) machines better than most metals. Chips break clean, dimensions hold, and you can hit ±0.005 mm on a stable fixture. It is the default for bushings, gears, and sliding parts where low friction matters more than temperature resistance.
ABS and PC are common for enclosures and brackets. ABS cuts easily but has low stiffness, so thin walls deflect under clamping. PC is tougher but gummy at high speed; keep the chipload up and the cutter sharp. Both need a light finishing pass because they scratch and haze.
PMMA (acrylic) and PA (nylon) are the problem children. Acrylic chips and crazes around holes; nylon absorbs moisture, moves after machining, and burrs at every edge. If a nylon part must hold tolerance, machine it, let it relax, then take a light second pass.
Reinforced grades change the rules. Carbon-fibre-filled and glass-filled resins are abrasive, so use diamond-coated or uncoated carbide and expect shorter tool life. They hold shape better than the unfilled grade, which often makes the tool cost worth it.
- 1EasyPOM, ABS, HDPE, PP
- 2ModeratePC, PMMA, PA, PEEK
- 3AbrasiveCarbon-fibre and glass-filled grades
Why plastic tolerances depend on wall thickness
A tolerance callout on a drawing says nothing about the environment the part will live in. Plastics expand roughly 5 to 10 times more than steel per degree, and moisture uptake adds another shift. A hole that measures 10.00 mm in the inspection room can be 9.94 mm on a humid factory floor.
Wall thickness drives the argument more than the material data sheet does. A 2 mm wall cools and stresses differently from a 12 mm wall in the same block. When a drawing puts ±0.02 mm on a 15 mm thick boss and ±0.05 mm on a 2 mm rib, the thick feature is usually the harder one to hold.
The practical route is to separate critical dimensions from cosmetic ones. Put tight tolerance only on the features that mate, seal, or locate. Everything else can run at general tolerances, and the part gets cheaper without losing function.
For parts that must hold size across seasons, we recommend a stress-relief cycle before the finishing pass. It adds time but removes the slow warp that shows up two weeks after shipment.
Design features that survive the cut
Sharp internal corners are the most common cause of cracked plastic parts. A cutter leaves a radius equal to its own corner radius, and that radius concentrates stress. Add a fillet, or at least accept the tool radius the shop can reach.
Deep pockets and tall thin ribs flex under cutting force. A 0.8 mm rib that machines fine in aluminum will chatter and snap in POM. Keep rib height under about 5 times the rib thickness, and let the shop use a support or a two-sided setup.
Threads in plastic work, but coarse is safer than fine. A fine thread in a soft resin strips with light torque, and the crest deforms during tapping. For repeated assembly, use a metal insert or a through-bolt.
Undercuts and internal grooves need either a 5-axis setup or a split design. On a 4,000 mm machine we can reach a lot of geometry, but every extra setup adds a chance for a locating error.
When to machine plastic, when to mold it
Use this as a first filter, not a final decision.
| Factor | CNC plastic parts processing | Injection molding |
|---|---|---|
| Volume | 1 to a few thousand parts | Tens of thousands and up |
| Tooling cost | None | Steel mold required |
| Geometry change | Edit the program | Cut a new mold |
| Lead time | Parts ship in 3–5 days | Weeks of tooling first |
| Material choice | Any machinable resin | Resin must flow |
| Tolerance | ±0.005 mm achievable | Draft and shrink limit it |
| Surface | Machined tool marks | Mold texture |
| Best fit | Prototypes, low volume, revisions | High volume, stable design |
The short version
If the design is still moving, the volume is under a few thousand, or you need a resin that will not flow in a mold, machine it. If the geometry is frozen and the annual volume is high, molding wins once you spread the tooling cost.
Questions engineers ask us
Can you hold ±0.005 mm on every plastic part?
±0.005 mm is the tolerance we work to on stable resins, thick enough walls, and features we can measure directly. It is not a blanket promise on thin ribs, unsupported bosses, or moisture-sensitive nylon.
Send the drawing and we will tell you which callouts we can hold, and which ones need a design change or a different material.
Which plastic is best for a sliding or wear part?
POM is the usual answer. It has low friction, good dimensional stability, and machines cleanly. PEEK is better when temperature or chemical resistance matters, but it costs far more and is harder to cut.
For load-bearing wear surfaces, a filled grade may hold up longer, though the abrasive filler shortens tool life.
How do you stop thin plastic parts from warping after machining?
We rough the part, let it relax, then take a light finishing pass. For thick sections we can add a stress-relief step before the final cut.
Clamping force matters too. Light vacuum or soft jaws spread the load, while a hard vise on a thin wall bends the part before the cutter touches it.
Do you machine carbon-fibre and glass-filled plastics?
Yes, with diamond-coated or uncoated carbide tooling and extra dust control. These grades are abrasive and wear edges quickly, so tool changes are more frequent.
The payoff is stiffness. A carbon-fibre part can replace a metal bracket at a fraction of the weight, which is why aerospace and EV projects use it.
What surface finish can I expect on a plastic part?
As-machined plastic typically lands between Ra 1.6 and 3.2 μm. A controlled finishing pass gets to Ra 0.8–1.6 μm, and polishing can reach Ra 0.2–0.8 μm on resins that take a polish.
Acrylic and PC polish well. Nylon and PP resist a fine finish because they tear rather than cut.
How fast can I get a quote and parts?
Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours after that, and parts ship in 3–5 days.
There is no minimum order quantity, so a single prototype and a 10,000 part run both go through the same process.
Send the drawing, get a DFM review
Upload your plastic part and we will come back with a quote, a manufacturability review, and the tolerance calls we can hold.
12-hour quote100% inspectionNo minimum order quantity
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
