Plastic Processing Guide: CNC Machining Behavior

• 1
  What this guide covers
• 2
  What actually happens at the cutting edge
• 3
  Material behavior decides the cutting data
• 4
  Holding the part without distorting it
• 5
  Features that machine well and features that do not
• 6
  When CNC is the right route for a plastic part
• 7
  Common plastics and their machining behavior
• 8
  CNC machining versus injection molding for plastics

A CNC machine removes plastic the same way it removes metal: a rotating cutter sweeps along a controlled path and shears material off the blank. The difference is the workpiece. Polymers are soft, low-density and poor conductors of heat, so the energy that goes into the cut has nowhere to escape.

In aluminium, most of that energy leaves with the chip and the rest spreads into the body of the part. In POM or ABS, the chip carries less away and the polymer matrix holds the rest near the cutting zone. The material under the tool softens, then recovers as it cools. That cycle is why a plastic cut can look perfect off the machine and measure out of tolerance an hour later.

The second difference is stiffness. Plastics have a modulus roughly two orders of magnitude below aluminium. The tool pushes the wall away before it cuts, then the wall springs back after the pass. Cutting forces are small in absolute terms, but so is the resistance of the part.

So the whole process is a balance between three variables: how much heat you generate, how much force you push into the part, and how rigid the part is while the cut happens. Every parameter choice below is really a choice about one of those three.

Not all plastics cut alike, and the difference is not small. Semi-crystalline thermoplastics such as POM, PA and PEEK hold their shape as they soften, so they machine with clean chips and hold tight tolerances. Amorphous plastics such as PC, PMMA and ABS soften gradually and tend to smear instead of shear, especially at lower spindle speeds.

Glass- or carbon-filled grades behave like a different material. The fibers raise stiffness and wear resistance, but they are abrasive. Carbide tooling dulls faster, edge quality drops, and you need to think about dust extraction as well as chip evacuation.

Moisture is the third variable. PA and PEEK absorb water from the air. Machine a wet blank and it can shrink as it dries, taking a bore or a bearing seat with it. For close work on these grades, we ask for the blank to be dried and, where the drawing allows, stress-relieved before the finishing pass.

The practical rule: match the feed and speed to the material family, not to a single number from a chart. If a shop quotes one cutting recipe for every plastic, the tolerances on your drawing are probably not being read.

The fixture is where most plastic jobs fail. A metal vise that clamps a steel block at 2,000 N will flatten a plastic wall long before the cutter touches it. The part springs back after unclamping, and the machined face is no longer flat.

We use soft jaws machined to the part profile, vacuum chucks for flat plates, and low-pressure clamping with support underneath. Where a wall is thin, the answer is often to leave a sacrificial web or a tab that holds the part rigid during cutting and gets removed in a second operation.

Tool reach matters as much as clamp pressure. A long end mill that works in steel will deflect in plastic on a deep pocket pass. Short, stubby tools with the largest possible diameter for the corner radius reduce both deflection and chatter.

For parts machined from plate, the blank itself can move. Extruded sheet often carries internal stress from the original process, and removing one face releases it. Rough the part, let it sit, then take the finishing pass. That sequence costs one setup but saves a scrapped batch.

Flat plates, open pockets, through holes and stepped profiles are easy in plastic. The tool can reach them, the chips clear, and the part stays rigid. These are the jobs where CNC beats a mold on both cost and lead time.

Deep narrow pockets, sharp internal corners, and tall thin ribs are the opposite. A 2 mm end mill cutting 20 mm deep has a length-to-diameter ratio of 10:1, which means chatter and a tapered wall. If the design needs that geometry, a mold or a cast part is usually the better route.

Threads are another decision point. Cut threads in plastic strip easily if the mating screw is tightened hard. Molded-in inserts or heat-set inserts survive repeated assembly far better than a tapped hole in a soft wall.

Wall thickness drives everything else. A part with 1.5 mm walls will bend under cutting load no matter how it is clamped. Add ribs, add a flange, or accept that the tolerance has to open up. On the tight side, ±0.005 mm is achievable on stable geometries and thick sections, but it is not a number we can promise on a thin free-standing wall.

CNC plastic processing wins when the part is needed now, when the geometry is simple, or when the quantity is low enough that a mold cannot pay for itself. One prototype or 50 units, no tooling cost, and design changes applied by editing the program rather than cutting a new tool.

It also wins on tolerance and on features that molding cannot produce. Undercuts, internal channels and openings on multiple faces can be machined in one setup on a 5-axis machine. A molded part would need side actions or a second tool.

Injection molding wins on anything with volume. Once the tool exists, the per-part cost drops sharply, surface finish is consistent, and complex ribs and bosses come free. If your annual demand is in the thousands and the geometry is stable, molding is almost always cheaper per part.

The middle ground is a bridge. Machined parts validate the design and let the product ship while the mold is being cut. That is how most of our automotive and medical programs start.