Surface finish in machined plastics: the Ra you can hold

Ask for a mirror finish on a plastic part and an honest machinist will tell you it depends on the grade. A surface finish machined plastic part behaves differently from metal: the material is softer, it springs back under the tool, and it is far more sensitive to heat. Ra is still the number to quote, but the values you can hold are set by the polymer as much as by the insert.

This guide covers what Ra means on a polymer, which grades finish well, and how to move the number when a drawing demands it.

How Ra is defined and measured on polymers

Ra is the arithmetic average of the surface profile deviations from a mean line, expressed in micrometres. A stylus profilometer measures it, and the result depends on the cut-off length you use, so it should always be quoted alongside the value. On plastics the measurement is noisier than on metal: the material can compress under the stylus, and a soft grade may read better than it looks. Use a light stylus force and report the cut-off, because a surface finish machined plastic figure is only comparable when both parties measured with the same settings.

A quick shop-floor check is worth more than a single reading. Measure in two directions, across the feed marks and along them, and take three readings at different points. In our experience the across-feed direction is the one that tracks how the part feels and seals, so quote it explicitly.

Material effects: soft, filled and glassy grades

The grade sets the ceiling. Unfilled semi-crystalline materials such as acetal (POM) cut cleanly and hold a fine finish, because the chips shear away instead of smearing. Soft materials such as UHMW-PE and unfilled PTFE tear and smear, which lifts Ra. Glass-filled and carbon-filled grades give a good Ra on the matrix, but the exposed fibres stand proud of the surface and create a micro-texture that no amount of polishing removes. Amorphous grades such as PEI take a high polish because they soften and flow slightly at the tool edge, which is why amber and clear parts finish so well.

Tool geometry, edge radius and sharpness

A sharp, polished cutting edge with a small nose radius gives the best finish in most polymers. A large nose radius or a worn edge rubs the material instead of cutting it, raising both Ra and the cutting temperature. For finishing passes, use a positive rake and a generous clearance so the tool does not drag. Diamond-tipped and polycrystalline diamond tools are the usual choice for the finest work on abrasive filled grades, where carbide edges dull quickly.

Tool material follows the grade. High-speed steel is fine for short runs in soft polymers, carbide covers most unfilled grades, and polycrystalline diamond is the standard for sustained work in glass-filled material, where the abrasive filler can wear a carbide edge within a few hours. Whichever you choose, a fresh edge for the finishing pass is worth more than any change to the machine settings.

Feeds, speeds and the built-up edge problem

Too light a feed is a common mistake. When the chip is thinner than the tool edge radius, the tool rubs and the surface smears, so the finish gets worse rather than better. Keep the feed above the edge radius and raise the cutting speed to keep the material shearing cleanly; plastics cut best at high surface speed with a sharp edge. The other enemy is built-up edge, where melted polymer sticks to the tip and tears the surface behind it. A coolant mist and a clear chip path remove it.

Speed has a practical ceiling, too. Above a certain surface speed the heat input outruns the chip’s ability to carry it away, and the material starts to melt at the edge, which shows up as a smeared, glossy band on the part. When you see that band, ease the speed back and check the coolant rather than pushing harder.

Cooling, swarf clearance and re-cutting

Plastics carry heat away poorly, so the temperature at the edge climbs fast. Air or mist cooling keeps the part below its softening point and stops the chip from welding back onto the surface. Clear the swarf on every pass: a chip left in the cut is dragged across the finished face on the next revolution and re-cuts it. This is the single biggest cause of a surface finish machined plastic result that looks good on the first part and poor on the fiftieth. Our cut-to-size service and products pages set out the grades we finish most often.

Questions buyers ask about surface finish in machined plastics

What Ra is realistic in unfilled engineering plastics?

As a rule of thumb, unfilled grades such as POM, PEI and PEEK can be held between 0.2 and 0.8 µm with a sharp edge and a clean finishing pass. Below 0.2 µm you are polishing rather than turning, and the cost climbs quickly. Very soft grades cannot reach that band at all.

Why does a glass-filled grade look rougher than its Ra suggests?

Because the fibres break the surface and create a micro-texture the stylus averages away. The matrix between fibres can be smooth while the visible surface still looks matt and dry. If appearance matters, judge a glass-filled part by eye as well as by the gauge.

Can I polish a machined plastic part?

Yes, for unfilled amorphous grades, which respond well to a sequence of fine abrasives and a light buff. Semi-crystalline and filled grades improve less, because the fibres stay proud and the crystal structure resists flow. Polish can also round a sharp edge you meant to keep.

How should I specify finish on a drawing?

State the Ra value, the cut-off length and the measurement method, and add a note for any area where appearance matters more than the number. If a surface is functional rather than decorative, say so, because over-specifying finish is one of the fastest ways to raise the price of a part. Send the drawing to our team via the contact page and we will confirm what is achievable.