Explained CNC plastic: machining and plating limits

• 1
  Key takeaways
• 2
  What explained CNC plastic actually covers
• 3
  How the cutter behaves in plastic
• 4
  Tolerance and the reality of creep
• 5
  Why plastic needs a conductive skin
• 6
  Which resins survive the bath
• 7
  Geometry rules that plating imposes
• 8
  Which plastic suits which finishing route

Two processes sit under one name. The first is cutting a plastic blank with a rotating tool: milling, turning, drilling, sometimes a five-axis toolpath that reaches a face a three-axis setup cannot. The second is putting a metal skin on the finished part. They are linked only by the surface the cutter leaves behind.

That link matters more than most people expect. Plating chemistry does not care how the part was made. It cares about surface energy, roughness and whether the etch can reach every face. A part that cuts beautifully can still fail in the plating bath.

So the question is not "can you machine this plastic." Almost any thermoplastic can be cut. The real question is whether the resin, the geometry and the finish you want can all survive in the same part. That is what this page is about.

We machine plastics daily in Dongguan and Singapore, usually alongside metal parts for the same assembly. The failure patterns are consistent enough to describe.

Metal cutting removes material as chips that break and slide. Plastic cutting is softer and stickier. The material tends to deform before it fractures, so the tool pushes a ridge ahead of the edge instead of shearing cleanly.

That is why cutting speed and feed are set differently. On ABS and PC we keep the surface speed low and the feed per tooth high enough that the chip is thick. A thin chip rubs, and rubbing generates heat. Heat is what ruins plastic parts, not the cutting force.

Cooling matters, but not the way it does on steel. Flood coolant is often wrong because most plastics do not conduct heat well and the coolant just makes chips stick. Compressed air or a mist works better on POM, PA and PEEK.

Sharp tooling is not optional. A dull end mill on PC will burnish the wall and leave a white stress mark that shows through any later finishing step.

Climb milling usually gives a better wall on plastics because the cutter enters at maximum chip thickness and exits thin. Conventional milling on a soft resin leaves a torn edge on the exit side.

Roughing and finishing should be separate passes. Take the bulk in two or three roughing passes, then a light finishing pass of 0.2–0.3 mm radial engagement.

A plastic part will not hold the same tolerance as an aluminium part, and quoting it as if it will is the most common mistake we see. Machined plastic typically lands in the ±0.05 mm to ±0.1 mm band on critical features. Our metal work reaches ±0.005 mm, but that number does not transfer to a resin.

The reason is thermal expansion plus moisture uptake. POM and PA absorb water from the air and grow. A part measured wet and measured dry can differ by more than the machining tolerance. If a drawing calls for a tight bore in nylon, the drawing is asking for something the material cannot deliver.

Annealing before the final cut helps. Stress-relieved stock moves less after machining, so the finished part stays closer to nominal. For thin walls and long parts, we rough, anneal, then finish.

Thin walls are a second limit. Below roughly 1 mm, plastic deflects under tool pressure and the wall thickness varies along the length. The cutter is not removing material unevenly. The part is bending away from it.

Electroplating works by moving metal ions to a surface with an electrical current. That requires a conductive surface. A polymer is an insulator, so the first step cannot be electroplating at all. It has to be chemical.

The sequence runs like this. Clean the part to remove mold release, oils and cutting residue. Etch the surface with chromic acid to open it at a microscopic level. Neutralize the etchant, then activate with a catalyst such as palladium. Finally deposit a thin electroless nickel or copper layer. That layer is the conductor.

Only after that does the part go into an electrolytic bath for the nickel, copper, chrome or gold that people actually see. The electroless layer is invisible in the finished part, but the whole result depends on it.

Adhesion is mechanical, not chemical. The metal locks into the etched pores. No etch, no grip.

ABS is the reference material for plating, especially the grades with butadiene dispersed through the matrix. The etch dissolves the rubber phase and leaves a porous skeleton the metal can key into. That is why plated ABS is everywhere in automotive trim and appliance housings.

PC and PC/ABS blends plate acceptably but need a longer or hotter etch. Pure PC is tougher and more dimensionally stable, so it is often chosen when the part sees load as well as finish.

POM, PP, PE and PEEK are a different story. Their surfaces resist etching, so adhesion is weak or inconsistent. You can plate them with extra pretreatment steps, but the cost climbs and the yield drops.

Glass-filled and carbon-filled grades are the hardest case. The fibers do not etch, and they sit proud of the surface after the resin around them is attacked. The result is a rough, uneven deposit.

If aesthetics drive the part, choose the resin for plating. If mechanical performance drives it, plate a metal insert instead of the plastic.

Current density is not uniform. It is highest at edges and outside corners, lowest in deep recesses. A part with sharp external edges will build a thicker, brighter deposit there and a thin, dull one inside.

Blind holes deeper than about two times their diameter are a problem. The chemistry cannot circulate and the current cannot reach the bottom. Either open the hole through or accept an unplated bottom.

Internal corners should carry a radius of at least 0.5 mm. A sharp internal corner creates a low-current zone that plates thin, and it is also a stress riser in the base plastic.

Draft and parting lines do not exist on a machined part, which is an advantage. But you should still avoid deep narrow slots and knife edges for the same current-density reason.

Design for plating means designing for even current. Round the outside edges, radius the inside corners, and keep recesses shallow.