PC Polycarbonate CNC Machining Services

PC CNC Machining Design Guidelines

Polycarbonate is one of the most forgiving engineering plastics to machine, but a few design decisions directly affect part cost, optical clarity, and dimensional stability. The guidelines below reflect how our engineers review PC drawings before quoting.

Wall Thickness and Section Design

Unlike injection molding, CNC machining does not require uniform walls, so PC parts can combine thick bosses with thin panels. For reliable machining we recommend keeping free-standing walls at 1.5 mm or above; walls under 1 mm can chatter, deflect under clamping pressure, or accumulate internal stress. For transparent parts, keep section changes gradual — abrupt thickness transitions show up as visible stress lines after polishing.

Corner Radii and Internal Features

Use an internal corner radius of at least 0.5 mm, ideally matching a standard end-mill radius. Sharp internal corners concentrate stress and are the most common origin of environmental stress cracking in PC. Where a sharp corner is functionally unavoidable, we add a relief radius on the non-functional side and anneal the finished part.

Holes, Threads and Fastening Features

• Drill with sharp, polished-flute drills; ream holes above Ø25 mm or when positional tolerance is tighter than ±0.05 mm.
• Machined threads work well from M2 upward; for repeated assembly and disassembly, specify heat-set or ultrasonic inserts instead — PC creeps under sustained clamp load.
• Avoid over-torquing fasteners in tapped PC; use torque-limiting drivers and document limits on the assembly instruction.
• For press fits against metal shafts, allow for PC’s relatively high thermal expansion (approximately 65–70 ×10⁻⁶/°C).

Tolerance and Fit Design

Our standard machining tolerance for PC is ±0.10 mm; critical features hold ±0.01 mm with in-process CMM verification. Because PC expands about six times more than steel, tell us the operating temperature range for precision fits — we compensate dimensions to a 20 °C reference and record it on the inspection report.

Recommended Machining Parameters for PC

Most PC machining defects — gumming, melted edges, stress cracks, cloudy surfaces — trace back to heat. Our process control keeps cutting heat out of the part.

Cutting Tools

We use sharp, polished-flute solid carbide tools for all PC work. Dull tools rub instead of shear, generating friction heat that melts the surface and locks stress into the part. For optical parts, tools are inspected and reground on a fixed schedule rather than run to failure.

Speeds and Feeds

Typical carbide surface speeds for PC run 300–500 m/min with a healthy chip load so the tool shears cleanly instead of rubbing. Light, multiple finishing passes remove stock without heating thin walls. Parameters are verified on first articles and locked into the traveler for repeat orders.

Cooling, Clamping and Stress Control

Preferred cooling is a directed air blast or mist, which avoids coolant residue on optical surfaces. When liquid coolant is needed, we use non-aromatic, water-soluble types — aromatic coolants and solvents attack PC and trigger stress cracking. Workholding uses soft jaws, low clamping force, and fixtures that spread pressure over large areas.

Annealing After Machining

Parts with tight tolerances or transparent requirements are annealed after roughing and again after finishing: heated to roughly 120–130 °C, soaked, then slow-cooled. Annealing relaxes machining stress, stabilizes dimensions, and dramatically reduces the risk of later environmental stress cracking.

Surface Finishing Options for Machined PC

As-Machined

With sharp tooling, as-machined PC typically reaches Ra 0.8–1.6 µm with a near-transparent cut surface. This is the default and most economical finish for structural and electrical parts.

Mechanical Polishing

Buffing with progressively finer compounds restores full optical clarity to machined faces and edges. Recommended for viewing windows, lens brackets, and display components.

Vapor Polishing

Solvent vapor smoothing melts a micro-layer of the surface, sealing tool marks and returning machined parts to glass-like clarity, including internal passages that cannot be buffed. Performed in a controlled environment with full ventilation and PPE.

Coating, Printing and Bonding

For outdoor use we offer UV-stable hard coatings that prevent yellowing and add scratch resistance. PC bonds well with PC-compatible adhesives (solvent and UV-cure types) and accepts screen printing and painting after corona or flame treatment.

PC vs Acrylic (PMMA): Which Transparent Plastic Should You Choose?

PC and acrylic are the two default choices for transparent machined parts. They look similar on the drawing but behave very differently in service.

Choose PC when the part must survive impact, vibration, or temperatures above 80 °C — machine guards, medical housings, protective windows. Choose acrylic when pure optical clarity, outdoor weathering, and lowest cost matter more than toughness — displays, lenses, signage, light guides.

CNC Machining vs Injection Molding for PC Parts

For prototypes, bridge production, and low-volume equipment parts, CNC machining is almost always cheaper and faster once tooling is accounted for. Molding becomes economical only after the per-part savings repay the mold — typically beyond 1,000 identical pieces. Many customers machine 200–500 PC parts while their mold is being built, then keep machining for spares and revisions.

What Affects the Cost of PC CNC Machining?

• Material grade: general-purpose PC is inexpensive; optical, UV-stabilized, flame-retardant (UL94 V-0), and medical grades cost more.
• Stock size: oversized blanks and thick transparent plate add material cost and machining time.
• Geometry and machining time: deep pockets, thin walls, and multiple setups drive hours on the machine.
• Tolerance and inspection: ±0.01 mm features with CMM reports add process-control time.
• Finishing: vapor polishing, hard coating, and printing are optional cost adders.
• Quantity: unit price drops sharply as setups and programming amortize across the run.

Send your STEP file and target quantity — we return a priced DFM review showing exactly which features drive cost and how to reduce them.

PC Material Standards and Certifications

• UL 94 flammability: standard PC typically rates V-2; flame-retardant grades achieve V-0 for electrical enclosures.
• FDA food contact: selected PC grades comply with FDA 21 CFR 177.1580 for food-contact components.
• Biocompatibility: medical-grade PC can meet ISO 10993 and USP Class VI for skin- and tissue-contact devices.
• RoHS / REACH: our PC stock is sourced from compliant mills with declarations on file.
• Quality system: parts are produced under ISO 9001:2015 with material certificates, COC, and FAI reports available.

PC Machining Case Examples

Three recent PC projects show how we apply the process controls described above.

Case 1: Transparent Flow Cell for an Analytical Instrument

Challenge: a clear PC flow cell with 0.8 mm internal channels and optical windows, requiring distortion-free transparency and leak-free sealing faces. Solution: carbide micro-milling with air-mist cooling, stress-relief annealing between operations, and vapor polishing of the optical path. Result: Ra 0.4 µm sealing faces, zero visible flow lines, and 100% leak testing at 3 bar. The customer now runs this part as a repeat production order.

Case 2: UL94 V-0 Connector Housing for Rail Electronics

Challenge: a flame-retardant PC housing with more than 40 tapped holes and ±0.05 mm positional tolerance across a 300 mm length. Solution: fixtured machining in two setups with in-process CMM checks, plus heat-set inserts instead of tapped threads at the assembly-critical locations. Result: first-article inspection passed 100% of characteristics, and the 800-piece batch delivered in 12 working days.

Case 3: Medical Sterilization Tray Prototype

Challenge: a medical-grade PC tray that survives repeated autoclave cycles without crazing. Solution: medical-grade resin with full traceability, generous corner radii per our DFM feedback, and a complete anneal cycle after machining. Result: 200 autoclave cycles at 121 °C with no stress cracking; the design was released to production with material certificates and COC.

PC Machining Tolerance and Surface Finish Reference

Reference values for solid machined PC; final capability depends on part geometry, grade, and feature location. Send your drawing and we will confirm tolerance and finish feasibility within 24 hours.