Plastics CNC Miami: What to Check Before You Order

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  Key takeaways
• 2
  Which process route fits your plastic part
• 3
  Tolerance on plastic is a thermal problem
• 4
  Material behavior decides your cutter and your feed
• 5
  Fixturing is where plastic jobs fail quietly
• 6
  Surface finish and post-processing choices
• 7
  How to judge a supplier before you send the PO
• 8
  Six steps from CAD to a machined plastic part

A ±0.005 mm callout means nothing until you fix the temperature. POM and PA grow roughly 80–120 μm per meter per °C. A 300 mm part measured at 18 °C in the morning and 26 °C in the afternoon moves more than 0.1 mm. That is twenty times the tolerance you wrote on the drawing.

So the first judging question is not machine accuracy. It is whether the shop measures in a controlled room and reports the temperature. Ask for the inspection report and look for ambient conditions. If the number is missing, the ±0.005 mm claim is marketing.

Second, plastic relaxes after machining. Stress locked in the stock plate releases over hours. A feature cut to size at 10 a.m. can move by 20–40 μm by the next day. Good shops rough, stress-relieve, then finish. Cheap shops cut once and ship.

Third, wall thickness and tolerance trade off. Below 1.5 mm wall, deflection during cutting dominates. We hold ±0.005 mm on rigid sections and ±0.05 mm on thin free-standing walls, and we tell you which is which before quoting.

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  Ask for the measurement temperatureA report without ambient conditions is not a report.
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  Split your calloutsTight on datums and mating faces, loose on cosmetic surfaces.
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  Budget one settling dayFor parts above 200 mm, machine, wait, then finish.

ABS and PC machine like a soft brass. They cut clean with sharp carbide, 2-flute end mills, and air blast. The risk is heat. A dull tool smears the surface instead of cutting it, and you get a white, gummy finish that no bead blast will hide.

POM is the opposite. It is dimensionally stable and machines to a fine finish, but it chips at the edges and grabs the tool if the flute geometry is wrong. Use sharp, polished flutes and climb milling. Do not let the part sit in the chuck overnight.

PEEK and PA are hygroscopic. They absorb moisture from the air, swell, and then shrink when the shop dries them. Drying before machining is not optional for PEEK. If a supplier does not ask about your storage conditions, they have not run much PEEK.

Carbon fiber filled grades wear tools fast. Expect tool changes mid-run and a higher piece price. The reward is stiffness, which is exactly what you want on a thin bracket that would flex in unfilled PA.

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  ABS, PC, PMMASharp carbide, air blast, watch for smearing.
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  POM, PP, HDPEClimb mill, light finish pass, deburr by hand.
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  PEEK, PADry first, control humidity, expect movement.
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  Carbon fiber filledDiamond or coated tools, more tool changes.

Clamp pressure crushes plastic. A vise tightened like it is holding 4140 will oval a PC bushing before the first cut. Soft jaws, machined to the part profile, spread the load. Vacuum plates work well for flat panels down to about 1 mm thickness.

For long parts, support underneath. A 400 mm plastic rail held only at the ends will sag and chatter in the middle. We add sacrificial support blocks under the cut zone and remove them in the last operation.

Setup count drives cost more than spindle time on small plastic parts. Each re-fixture adds an alignment error and an hour of labor. A 5-axis machine that cuts five faces in one setup often costs less than a cheap 3-axis quote with four setups.

Ask how many setups the quote assumes, and where the datums are. If the answer is vague, the price will move once the shop sees the part.

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  Soft jaws or vacuumNever bare steel jaws on a finished plastic face.
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  Support the middleLong thin parts need support under the cut, not just at the ends.
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  Count the setupsEach re-fixture adds error and labor, not just time.

As-machined plastic lands around Ra 1.6–3.2 μm. That is fine for internal brackets and most prototype housings. For a sealing face or an optical surface, you need Ra 0.8–1.6 μm or finer, which means a separate finish pass at low feed and high spindle speed.

Bead blasting gives a uniform matte look and hides tool marks. It also removes material, typically 5–20 μm depending on pressure and time. If a bore is already at the low end of tolerance, blasting can push it out. Sequence matters: blast first, then ream critical bores.

Laser marking works on most plastics but needs contrast. Minimum character height is 1.5 mm for reliable readability. On dark POM, white marking reads well. On clear PC, expect low contrast unless you use a filled grade.

Painting and plating are rare on plastics for good reason. Adhesion is poor without a primer, and the coating changes dimensions. For color, anodizing does not apply. Use colored stock or a molded-in pigment instead.

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  Ra 1.6–3.2 μmAs-machined. Internal and non-cosmetic parts.
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  Ra 0.8–1.6 μmSealing and sliding surfaces. Needs a finish pass.
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  Ra 0.2–0.8 μmOptical or bearing surfaces. Polish after machining.

Ask three questions. What machines will run my part? Who programs it? Where is it inspected? A shop that answers all three in one sentence is a shop that owns the process. A shop that forwards the questions to a partner is a broker, and the tolerance you were promised belongs to somebody else.

Certifications are a filter, not a guarantee. ISO 9001:2015 covers process control. IATF 16949:2016 matters if the part feeds an automotive line. ISO 13485:2016 matters for medical devices. ISO 27001:2022 matters if you are sending unreleased CAD. Match the certificate to your industry, not to a logo wall.

Lead time is the other filter. Quotation and DFM feedback in 12 hours, production start within 24 hours, and parts shipping in 3–5 days is a normal cadence for machined plastic prototypes. If a quote promises the same speed on a 10,000-piece run, ask what is actually being machined.

Confidentiality is a real requirement for unreleased products. Uploads should be secure, and an NDA should be available on request, not after a negotiation.

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  Own the machinesAsk which spindle cuts your part, and who programs it.
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  Match the certificateISO 9001, IATF 16949, ISO 13485, ISO 27001 each cover different risk.
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  No MOQ means one part or ten thousandA shop that quotes both is set up for both.