Polycarbonate (PC) is a high‑impact, transparent engineering plastic that Clarwe machines into tight‑tolerance parts for demanding optical and structural applications. With impact resistance significantly higher than ABS, service temperatures up to approximately 120–130 °C, and light transmission comparable to glass, CNC‑machined PC is well suited for machine guards, sensor windows, electronic enclosures, and functional prototypes that must withstand repeated mechanical and thermal stress.
Clarwe machines clear and black polycarbonate sheet and bar stock using optimized tooling and cutting parameters to manage heat generation, delivering consistent dimensional accuracy and surface finish options from as‑machined to vapor‑polished, near‑optical clarity for critical viewing areas.
At a Glance
| Standard Tolerance | ±0.15 mm (ISO 2768-c) |
| Achievable Tolerance | ±0.08–0.10 mm (controlled setup) |
| Min Wall Thickness | 1.0 mm (1.5 mm recommended) |
| Max Part Size | upto 1200 mm |
| Surface Finish (Ra) | 0.8 μm to 3.2 μm |
| Typical Lead Time | 5–10 business days |
| Available Grades | GP Clear/Black, Glass-Filled, FR, UV-Stabilised, Medical, PC/ABS Blend |
| Surface Finishes | As-machined, Fine pass, Bead blasting, Mechanical polishing, Vapor polishing, Flame polishing, Hard-coat |
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Upload polycarbonate drawings for a CNC machining quote Share your PC models and our engineers will confirm material grade, tolerances, and optical finish options before pricing. |
Material Properties of Polycarbonate
All values are typical reference ranges for general-purpose, unfilled, CNC-grade polycarbonate (clear sheet and bar stock). Properties vary by grade, UV stabiliser content, and glass-fibre loading. Confirm against the specific grade datasheet before finalising a design.
| Material | Density (g/cm³) | Tensile Strength (MPa) | Elongation at Break (%) | Hardness (Rockwell M) | HDT @ 1.8 MPa (°C) | Glass Transition Temp (°C) |
|---|---|---|---|---|---|---|
| Polycarbonate | 1.20 –1.22 | 60 –70 | 60 –100 | M70 / 75 –80 | 125 –140 | ~145 |
Notes on key values:
No melting point— PC is amorphous and softens progressively above Tg (~145 °C). Machining parameters must prevent localised surface temperatures approaching this threshold.
Elongation at break (60–100%)— unusually high for a rigid plastic; indicates PC will deform before fracture, which is why it does not shatter under impact.
Light transmission (88–90%)— applies to unfilled, uncoated clear PC only. Glass-filled, FR, and black grades are opaque.
Hardness (Rockwell M70 / Shore D 75–80)— softer than most metals; scratch resistance is a known limitation without hard-coat treatment.
Polycarbonate Grades for CNC Machining
Polycarbonate is available in several formulations, each tuned for specific performance requirements. The grade determines machinability, post-processing options, optical performance, and service behaviour. Selecting the correct grade at the design stage avoids material substitutions after quoting and ensures the part meets functional requirements from the outset.
| Grade | Stock Colours | Key Characteristics | CNC Applications |
|---|---|---|---|
| General-Purpose PC (Unfilled) | Clear, black | High impact strength, near-glass transparency (88-90%), good machinability; Tg ~145 °C | Machine guards, sensor windows, optical covers, functional prototypes |
| Glass-Filled PC (10-30% GF) | Natural, black | Higher stiffness and dimensional stability; reduced ductility; opaque; increased tool wear | Structural brackets, connectors, high-load housings |
| Flame-Retardant PC (PC-FR) | Black, dark grey | UL 94 V-0 rated at 1.5-3.0 mm; slightly more brittle than GP grade; opaque | Electrical enclosures, switchgear covers, transport interiors |
| UV-Stabilised PC | Clear, tinted | Resists yellowing and surface hazing under prolonged UV exposure; otherwise identical to GP in machinability | Outdoor panels, machine glazing, signage windows |
| Medical / Food-Contact PC | Clear, natural | FDA-compliant, low-extractable grades; ISO 10993 assessed variants; full material traceability required | Medical device housings, lab enclosures, fluid-contact components |
| PC/ABS Blend | Black, dark grey | HDT up to ~115 °C; better impact than neat ABS; harder to machine than GP PC; opaque | Automotive interior parts, higher-temperature consumer housings |
Tolerances for CNC-Machined Polycarbonate
Polycarbonate is more dimensionally sensitive than most metals and requires careful process control to hold tight tolerances reliably. Its high coefficient of thermal expansion (CTE ~65-70 µm/m·°C - approximately three times that of aluminium), low thermal conductivity, and residual stresses in extruded stock all limit achievable tolerances, particularly on larger parts or features machined at elevated cutting temperatures.
The default tolerance class for CNC-machined plastics is ISO 2768-c (coarse). Where tighter tolerances are required, they must be explicitly called out on the drawing and will require controlled fixturing, reduced finishing pass speeds, and temperature-stabilised measurement. Parts should be measured at 20 °C after sufficient thermal settling time - polycarbonate dimensions shift measurably with temperature variation.
Tolerance Reference Table
| Feature | ISO 2768-c Standard (plastics) | Achievable with Controlled Setup |
|---|---|---|
| Linear dimensions ≤ 30 mm | ±0.15 mm | ±0.08 - ±0.10 mm |
| Linear dimensions 30 - 120 mm | ±0.20 mm | ±0.10 - ±0.15 mm |
| Linear dimensions 120 - 400 mm | ±0.30 mm | ±0.18 - ±0.25 mm |
| Bored or reamed holes | ±0.10 mm | ±0.05 - ±0.08 mm |
| Flatness over 100 mm span | 0.25 mm | 0.12 - 0.18 mm |
| Angular dimensions | ±0.5° | ±0.25° |
PC's high CTE (~65–70 µm/m·°C — approximately 3× that of aluminium) means workpiece temperature must be stabilised between roughing and finishing passes on tight-tolerance features. Uniform wall thickness reduces differential cooling and flatness deviation after machining.
| Confirm if your tolerances are realistic for PC
Upload your drawing and we’ll review critical dimensions, wall thickness, and temperature conditions to advise achievable tolerances in polycarbonate. |
Surface Finish and Post-Processing Options for Polycarbonate
Polycarbonate accepts a wider range of post-processing treatments than most engineering plastics — including finishes that restore or achieve optical clarity. Choose based on your part's transparency, appearance, and functional requirements.
| Finish / Process | Surface Roughness (Ra) | Result on Clear PC | Best Used For | Limitations |
|---|---|---|---|---|
| As-Machined | 2.4–3.2 µm | Hazy, semi-translucent | Internal structural parts, enclosure bodies, fixtures & jigs | Not suitable where appearance or optical clarity is needed |
| Fine Machining Pass | 1.2–1.6 µm | Cleaner, slightly translucent | Functional enclosures, covers, panels where some texture is acceptable | Does not restore optical clarity |
| Bead Blasting | 1.6–4.0 µm | Makes clear PC fully opaque | Uniform matte finish on black PC stock | Avoid on walls < 1.0 mm; not suitable for transparent parts |
| Mechanical Polishing | ~0.8 µm | Restores semi-transparency | Flat or convex surfaces requiring improved clarity | Time-intensive; limited to accessible geometries |
| Vapor Polishing | < 0.4 µm | Near-optical / glass-like clarity | Optical components, light pipes, lenses, complex geometries | Anneal recommended after for heat or chemical service environments |
| Painting & Hard-Coat | — | Opaque / coated surface | Outdoor panels, glazing, scratch-resistant parts | Hard-coat cure temp < 145°C; water-based primers preferred |
Bonding and Assembly
Polycarbonate can be joined with methylene chloride-based solvent cements, two-part structural epoxies, or cyanoacrylates. Avoid ketone, ester, or aromatic solvent-based adhesives — they cause environmental stress cracking. For mechanical fastening: direct tapping suits low-cycle use (M3 minimum, coarse thread, 2× diameter engagement); for any repeat-assembly feature, specify heat-set or press-fit metal inserts to prevent thread strip-out over time.
Stress Relief Annealing for Dimensional Stability
Polycarbonate is a high-performance thermoplastic, but it is highly susceptible to internal residual stress during the CNC machining process. Rapid material removal and heat generation can lead to "crazing" (micro-cracking) or warping over time, especially when parts are exposed to chemicals or high temperatures.
At Clarwe, we offer optional post-machining annealing to ensure the longevity of your components. Our controlled heating and cooling cycle:
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Eliminates Internal Stress: Prevents spontaneous cracking and structural failure.
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Improves Dimensional Stability: Ensures tight tolerances are maintained throughout the part's lifecycle.
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Enhances Chemical Resistance: Reduces the risk of stress-corrosion cracking when cleaned or sterilized.
Engineer's Note:
We highly recommend annealing for polycarbonate parts with complex geometries, thick cross-sections, or those intended for medical and aerospace housings.
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Specify which PC surfaces must be optically clear Highlight transparency‑critical faces on your drawing and our team will recommend vapor polishing, mechanical polishing, or hard‑coat so the finish matches your application. |
Design Guidelines for CNC-Machined Polycarbonate Parts
The guidelines below reflect practical machining limits for polycarbonate parts in CNC milling and turning. Polycarbonate's combination of high ductility, low thermal conductivity, and sensitivity to stress concentrators means design decisions that are acceptable in metal - sharp internal corners, abrupt section changes, thin unsupported walls - carry higher risk in PC and should be resolved at the drawing stage.
Wall Thickness and Structural Features
Uniform wall thickness is important in CNC-machined polycarbonate. Abrupt transitions between thick and thin sections create differential cooling gradients after machining, which introduce residual stress and flatness deviation. Very thin sections are prone to vibration during cutting, which increases surface roughness, dimensional deviation, and the risk of stress cracking at the feature.
| Feature | Minimum Practical | Recommended Range | Notes |
|---|---|---|---|
| Structural walls | 1.0 mm | 1.5 - 4.0 mm | Below 1.0 mm increases vibration, chatter, and deflection risk |
| Unsupported thin webs | 0.75 mm | 1.2 - 2.5 mm | Supported on both ends; freestanding webs require more |
| Boss walls (for inserts) | 2× insert OD | 3.0 - 6.0 mm outer diameter | Undersized bosses crack under insert installation load - anneal after installation |
| Ribs | 0.5× adjacent wall thickness | 0.6× wall thickness | Use ribs rather than solid thick sections for stiffness |
Holes, Threads, and Inserts
| Feature | Minimum Size | Recommended Practice |
|---|---|---|
| Drilled holes | 1.0 mm diameter | Standard drill geometry; max depth-to-diameter ratio 10:1; clear chips frequently |
| Milled slots / pockets - end mill | 0.8 mm diameter | Below 0.8 mm, tool deflection and breakage risk increases |
| Pocket depth-to-width ratio | - | Keep below 4:1 for standard setups; deeper pockets need longer tooling and increased chip clearance |
| Tapped holes (direct, PC) | M3 minimum | Coarse thread; minimum 2× diameter thread engagement; limited to low-cycle use |
| Threaded inserts (heat-set / press) | M2 minimum | Preferred over direct tapping for any repeat-assembly feature; anneal after installation |
Internal Radii and Corner Geometry
Internal corner radii in pockets and slots are constrained by the tool diameter in use — a sharp internal corner (0 mm radius) is not machinable. In polycarbonate, sharp internal corners carry additional risk beyond the machining constraint: they are stress concentrators and a common initiation point for cracking both during cutting and in service.
Practical design rule:
Set internal radii to at least 1/3 of the pocket depth, and specify a radius slightly larger than the nearest standard tool size to avoid custom tooling.
For example, a 6 mm deep pocket should carry internal radii of at least 2 mm — specifying R2.5 or R3 matches standard 5 mm or 6 mm end mills. For floor-to-wall transitions, specify a small radius (0.5–1.0 mm) rather than a sharp corner to reduce stress concentration and improve surface finish at the transition.
Optical Surface Requirements
Clear polycarbonate surfaces requiring optical clarity must be identified explicitly on the drawing — as-machined PC is hazy and transparency is not restored by machining alone.
Specify the finish method and Ra value directly on transparency-critical faces (e.g., *"vapor polish to Ra ≤ 0.4 µm"*); a generic "clear finish" callout will default to as-machined.
See the Surface Finish and Post-Processing section above for a full breakdown of achievable clarity levels and recommended methods by application.
Polycarbonate Compared with Other CNC Plastic Materials
The table below compares polycarbonate against the most common alternative plastics in CNC machining. PC occupies a specific position: highest impact strength among the options listed, the only material with meaningful optical transparency, and mid-range on machinability and cost. Where those properties are not required, alternatives deliver better value or easier processing.
| Property | Polycarbonate (PC) | ABS | Acrylic (PMMA) | Nylon (PA6 / PA66) | POM (Delrin) | PEEK |
|---|---|---|---|---|---|---|
| Tensile Strength | 60 - 70 MPa | 40 - 55 MPa | 70 - 80 MPa | 70 - 85 MPa | 65 - 75 MPa | 100 - 110 MPa |
| Impact Strength (Izod, notched) | 50 - 90 kJ/m² | 15 - 35 kJ/m² | 2 - 5 kJ/m² | 5 - 10 kJ/m² | 5 - 10 kJ/m² | 55 kJ/m² |
| Heat Deflection Temp (@ 1.8 MPa) | 125 - 140 °C | 85 - 100 °C | 70 - 80 °C | 65 - 75 °C | 100 - 110 °C | 160 °C+ |
| Optical Transparency | High (88 - 90%) | Opaque | Very High (92%) | Opaque | Opaque | Opaque |
| Machinability | Moderate | Excellent | Easy | Moderate | Excellent | Difficult |
| Scratch Resistance | Poor | Moderate | Good | Good | Good | Excellent |
| Chemical Resistance | Moderate | Moderate | Good | Good | Excellent | Excellent |
| UV Resistance (untreated) | Poor | Poor | Good | Poor | Moderate | Good |
| Moisture Absorption | Low (0.1 - 0.4%) | Low (0.1 - 0.3%) | Low (0.1 - 0.4%) | High (2 - 8%) | Very Low (< 0.2%) | Very Low (< 0.1%) |
| Relative Material Cost | Medium | Low | Low - Medium | Low - Medium | Medium | High |
| CNC Applications | Guards, windows, enclosures, prototypes | Housings, fixtures, prototypes | Displays, lenses, signage | Gears, wear components | Precision bushings, gears | High-temp structural parts |
When to choose PC over the alternatives:
Over ABS- when impact strength above 35 kJ/m², service temperature above 100 °C, or optical transparency is required
Over Acrylic (PMMA)- when the part must withstand impact without shattering; acrylic is more transparent but brittle
Over Nylon- when dimensional stability in humid environments is critical; nylon absorbs moisture and changes dimension
Over POM (Delrin)- when optical clarity or higher impact performance is needed; POM machines more easily but is opaque and lower impact
Over PEEK- when budget is a constraint and service temperature stays below 130 °C; PEEK offers superior thermal and chemical resistance at significantly higher cost
Frequently Asked Questions
Can CNC-machined polycarbonate be made optically clear?
As-machined polycarbonate is hazy — tool marks scatter light and the surface is semi-translucent, not optically transparent. Optical clarity requires a secondary finishing operation. Vapor polishing is the most effective method: brief exposure to chlorinated solvent vapour micro-reflows the surface layer, achieving Ra < 0.4 µm with transmission values close to injection-moulded PC. Mechanical polishing is an alternative for flat or convex surfaces. Specify the finish method and Ra value explicitly on your drawing — a generic "clear finish" callout defaults to as-machined.
What tolerances can CNC machining hold on polycarbonate?
The default tolerance class for CNC-machined polycarbonate is ISO 2768-c (coarse), giving ±0.15 mm on features up to 30 mm. With controlled fixturing, reduced finishing pass speeds, and temperature-stabilised measurement at 20 °C, tolerances of ±0.08-0.10 mm are achievable on shorter features. Polycarbonate has a high coefficient of thermal expansion (~65-70 µm/m·°C - approximately three times that of aluminium), so dimensional variation increases with part size and temperature. Always call tight tolerances explicitly on your drawing; they cannot be assumed.
What is the difference between CNC machining polycarbonate and acrylic?
Both are transparent CNC-machinable plastics, but they serve different use cases. Acrylic (PMMA) offers higher raw light transmission (92% vs 88-90% for PC) and better scratch resistance, but is brittle - it shatters under impact and is prone to cracking at stress concentrators. Polycarbonate absorbs impact without fracturing (Izod impact strength 50-90 kJ/m² vs 2-5 kJ/m² for acrylic) and has a higher service temperature (HDT 125-140 °C vs 70-80 °C). Choose acrylic for display panels, lenses, and signage where scratch resistance and optical quality are the priority; choose polycarbonate where the part must survive impact, vibration, or elevated temperature without shattering.
Can polycarbonate be used for outdoor CNC-machined parts?
Standard unfilled polycarbonate has poor UV resistance - prolonged outdoor exposure causes yellowing, surface hazing, and eventual embrittlement. For outdoor applications, specify UV-stabilised PC grade, which incorporates UV absorber additives or a co-extruded UV cap layer to resist degradation. For parts requiring both outdoor UV resistance and optical clarity, a hard-coat applied after vapor polishing provides the most durable result. Standard GP-grade PC without UV stabilisation is not suitable for long-term outdoor exposure.
What is the typical lead time for CNC-machined polycarbonate parts?
Standard polycarbonate CNC machining orders at Clarwe are typically completed in 5–10 business days from drawing approval, depending on part complexity, required finishing operations, and order volume. Parts requiring vapor polishing, hard-coat application, or tight-tolerance controlled setups may add 1–3 days to the schedule. Upload your drawing for a confirmed lead time with your quote.
| Match PC grade to your application before ordering
Tell us whether you need GP, UV‑stabilised, FR, or medical‑grade polycarbonate so we can propose the right stock, finishing, and inspection plan for your parts. |
Where Polycarbonate Is Used - Industries and Applications
Polycarbonate is specified wherever a CNC-machined part must combine structural toughness, dimensional stability, and — in many cases — optical transparency.
| Industry | Typical Applications |
|---|---|
| Industrial Equipment & Machine Guards | Safety shields, transparent access panels, protective windows on machines and automated cells. UV-stabilised grades for near-UV lighting environments. |
| Electronics & Electrical Enclosures | Sensor windows, indicator lenses, terminal covers, light pipes. Flame-retardant grades (UL 94 V-0) for switchgear, power distribution panels, and transport interiors. |
| Automotive & Transportation | Lighting lenses, instrument panel covers, indicator windows. PC/ABS blend grades for structural interior parts. |
| Medical & Laboratory Equipment | Device housings, fluid-handling components, diagnostic windows, optical elements in imaging equipment. Medical-grade PC with full material traceability. |
| Optical Components & Light Management | Light pipes, diffusers, and sensor covers requiring near-glass optical clarity. Clear GP-grade PC, machined and vapor-polished. |
| Functional Prototypes & Bridge Production | Form, fit, and function validation from one-off through small-batch quantities. Standard CNC machining choice where optical and impact properties must match production intent. |
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Machining polycarbonate parts for your project? |