{"id":876,"date":"2026-11-02T16:30:56","date_gmt":"2026-11-02T08:30:56","guid":{"rendered":"https:\/\/cntopplastics.com\/surface-finish-machined-plastics\/"},"modified":"2026-11-02T16:30:56","modified_gmt":"2026-11-02T08:30:56","slug":"surface-finish-machined-plastics","status":"publish","type":"post","link":"https:\/\/cntopplastics.com\/de\/surface-finish-machined-plastics\/","title":{"rendered":"Surface finish in machined plastics: the Ra you can hold"},"content":{"rendered":"

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.<\/p>\n

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

How Ra is defined and measured on polymers<\/h2>\n

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.<\/p>\n

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.<\/p>\n

Material effects: soft, filled and glassy grades<\/h2>\n

The grade sets the ceiling. Unfilled semi-crystalline materials such as acetal (POM)<\/a> 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<\/a> take a high polish because they soften and flow slightly at the tool edge, which is why amber and clear parts finish so well.<\/p>\n

Tool geometry, edge radius and sharpness<\/h2>\n

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.<\/p>\n

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.<\/p>\n

\"ULTEM<\/figure>\n

Feeds, speeds and the built-up edge problem<\/h2>\n

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.<\/p>\n

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.<\/p>\n

Cooling, swarf clearance and re-cutting<\/h2>\n

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<\/a> and products pages<\/a> set out the grades we finish most often.<\/p>\n

\"POM-C<\/figure>\n\n\n\n\n\n\n\n\n\n\n
Grade<\/th>\nTypical achievable Ra<\/th>\nNotes<\/th>\n<\/tr>\n<\/thead>\n
POM (unfilled)<\/td>\n0.4 to 0.8 \u00b5m<\/td>\nCuts clean, holds finish<\/td>\n<\/tr>\n
PEI (ungef\u00fcllt)<\/td>\n0.2 to 0.6 \u00b5m<\/td>\nPolishes well<\/td>\n<\/tr>\n
PEEK (ungef\u00fcllt)<\/td>\n0.4 to 0.8 \u00b5m<\/td>\nTough, needs sharp edge<\/td>\n<\/tr>\n
Glass-filled PEEK or PPS<\/td>\n1.0 to 1.6 \u00b5m<\/td>\nFibres raise Ra<\/td>\n<\/tr>\n
UHMW-PE<\/td>\n1.5 to 3.0 \u00b5m<\/td>\nTears and smears<\/td>\n<\/tr>\n
Unfilled PTFE<\/td>\n1.0 to 2.0 \u00b5m<\/td>\nVery soft, hard to hold<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Questions buyers ask about surface finish in machined plastics<\/h2>\n

What Ra is realistic in unfilled engineering plastics?<\/h3>\n

As a rule of thumb, unfilled grades such as POM, PEI and PEEK can be held between 0.2 and 0.8 \u00b5m with a sharp edge and a clean finishing pass. Below 0.2 \u00b5m you are polishing rather than turning, and the cost climbs quickly. Very soft grades cannot reach that band at all.<\/p>\n

Why does a glass-filled grade look rougher than its Ra suggests?<\/h3>\n

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.<\/p>\n

Can I polish a machined plastic part?<\/h3>\n

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.<\/p>\n

How should I specify finish on a drawing?<\/h3>\n

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<\/a> and we will confirm what is achievable.<\/p>\n","protected":false},"excerpt":{"rendered":"

What Ra is achievable in engineering plastics, why it is harder than in metals, and how to move it. Tooling, feeds, coolant and grade all matter.<\/p>\n","protected":false},"author":193679612,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_coblocks_attr":"","_coblocks_dimensions":"","_coblocks_responsive_height":"","_coblocks_accordion_ie_support":"","advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_seo_schema_type":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_wpcom_ai_launchpad_first_post":false,"_tp_seo_title":"","_tp_seo_desc":"","_tp_summary":"","_tp_updated":"","_jetpack_feature_clip_id":0,"_jetpack_hide_featured_image":false,"_jetpack_featured_image_duplicate_dismissed":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[132593],"tags":[132591,132588],"class_list":["post-876","post","type-post","status-publish","format-standard","hentry","category-machining","tag-machining","tag-technical"],"jetpack_publicize_connections":[],"jetpack_likes_enabled":true,"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pcp8Cz-e8","jetpack-related-posts":[{"id":534,"url":"https:\/\/cntopplastics.com\/de\/semiconductor-wafer-handling-plastic-parts\/","url_meta":{"origin":876,"position":0},"title":"Plastic parts for wafer handling: what actually matters","author":"jessemei","date":"2026-10-20","format":false,"excerpt":"Wafer-handling parts are judged on particles, outgassing, ESD and stability, not datasheet strength. Choose the grade, then verify the process.","rel":"","context":"In "Industry Applications"","block_context":{"text":"Industry Applications","link":"https:\/\/cntopplastics.com\/de\/category\/industry-applications\/"},"img":{"alt_text":"Black ESD PEEK stock: three stacked black PEEK plates, a black PEEK rod disc and a machined PEEK bushing","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-peek-esd.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-peek-esd.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-peek-esd.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-peek-esd.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":561,"url":"https:\/\/cntopplastics.com\/de\/machining-esd-plastics-resistivity\/","url_meta":{"origin":876,"position":1},"title":"Machining ESD plastics without losing the resistivity","author":"jessemei","date":"2026-10-26","format":false,"excerpt":"Cutting, sanding or overheating a dissipative grade can shift surface resistivity by decades. How to machine ESD plastics and verify the result.","rel":"","context":"In "Machining"","block_context":{"text":"Machining","link":"https:\/\/cntopplastics.com\/de\/category\/machining\/"},"img":{"alt_text":"POM ESD Anti-static Sheet","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-esd.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-esd.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-esd.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-esd.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":318,"url":"https:\/\/cntopplastics.com\/de\/engineering-plastic-sheet-rod-order-spec\/","url_meta":{"origin":876,"position":2},"title":"Engineering plastic sheet and rod: how to specify an order","author":"jessemei","date":"2026-10-15","format":false,"excerpt":"Grade code, form, thickness tolerance, finish and certification: five fields that decide if your plastic order arrives usable. A buyer's checklist.","rel":"","context":"In "Delivery"","block_context":{"text":"Delivery","link":"https:\/\/cntopplastics.com\/de\/category\/delivery\/"},"img":{"alt_text":"POM-C Natural Sheet and Rod","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-c-nat.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-c-nat.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-c-nat.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pom-c-nat.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":538,"url":"https:\/\/cntopplastics.com\/de\/fda-21-cfr-177-food-contact-plastics\/","url_meta":{"origin":876,"position":3},"title":"FDA 21 CFR 177: plastic grades cleared for food contact","author":"jessemei","date":"2026-10-21","format":false,"excerpt":"21 CFR 177 lists polymers and their conditions of use. Which grades sit inside, what a compliant declaration says, and where it stops. A buyer's summary.","rel":"","context":"In "Compliance"","block_context":{"text":"Compliance","link":"https:\/\/cntopplastics.com\/de\/category\/compliance\/"},"img":{"alt_text":"UHMW-PE Sheet and Rod","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-uhmwpe-nat.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-uhmwpe-nat.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-uhmwpe-nat.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-uhmwpe-nat.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":881,"url":"https:\/\/cntopplastics.com\/de\/switchgear-busbar-insulation-plastics\/","url_meta":{"origin":876,"position":4},"title":"Insulation parts for switchgear and busbars by voltage class","author":"jessemei","date":"2026-11-03","format":false,"excerpt":"Insulation choice changes as voltage climbs. Compare dielectric strength, CTI and arc resistance. Match the material to your voltage class, wet or dry.","rel":"","context":"In "Industry Applications"","block_context":{"text":"Industry Applications","link":"https:\/\/cntopplastics.com\/de\/category\/industry-applications\/"},"img":{"alt_text":"PPS 40% Glass Fibre Sheet","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pps-gf40.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pps-gf40.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pps-gf40.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-pps-gf40.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":845,"url":"https:\/\/cntopplastics.com\/de\/hs-codes-duties-engineering-plastics\/","url_meta":{"origin":876,"position":5},"title":"HS codes and origin rules for engineering plastics","author":"jessemei","date":"2026-10-30","format":false,"excerpt":"Which HS heading covers plastic sheet, rod and tube, how duty is assessed, and how origin is set. A 2026 guide for importers of stock shapes.","rel":"","context":"In "Delivery"","block_context":{"text":"Delivery","link":"https:\/\/cntopplastics.com\/de\/category\/delivery\/"},"img":{"alt_text":"PTFE 25% Glass Filled","src":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-ptfe-gl25.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-ptfe-gl25.jpg?fit=1000%2C1000&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-ptfe-gl25.jpg?fit=1000%2C1000&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/cntopplastics.com\/wp-content\/uploads\/2026\/10\/tp-ptfe-gl25.jpg?fit=1000%2C1000&ssl=1&resize=700%2C400 2x"},"classes":[]}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/posts\/876","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/users\/193679612"}],"replies":[{"embeddable":true,"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/comments?post=876"}],"version-history":[{"count":0,"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/posts\/876\/revisions"}],"wp:attachment":[{"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/media?parent=876"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/categories?post=876"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cntopplastics.com\/de\/wp-json\/wp\/v2\/tags?post=876"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}