Polycarbonate (PC) is the material of choice when a CNC-machined part must combine structural toughness with optical transparency - a combination no other common engineering plastic delivers. With impact strength of 50-90 kJ/m², a continuous service temperature up to 115 °C, and near-glass-level light transmission of 88-90%, PC is specified for machine guards, sensor windows, electronic enclosures, and functional prototypes where ABS lacks the thermal or impact margin and acrylic is too brittle.
In CNC machining, polycarbonate is supplied as extruded clear or black sheet and bar stock - available under trade names Lexan (SABIC) and Makrolon (Covestro). It machines with moderate difficulty: lower cutting forces than metals, but heat-sensitive enough to require controlled feeds, sharp tooling, and solvent-free coolants to prevent surface softening and stress cracking. Grade selection - from general-purpose unfilled PC to flame-retardant, glass-filled, UV-stabilized, and medical-contact variants - directly affects machinability, post-processing options, and service performance.
Material Properties of Polycarbonate
All values below are typical reference ranges for general-purpose, unfilled, CNC-grade polycarbonate (clear sheet and bar stock). Properties vary by grade, UV stabiliser content, glass-fibre loading, and test conditions. These figures are suitable for design reference only - always confirm against the specific grade datasheet before finalising a design.
| Material | Density (g/cm³) | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation at Break (%) | Hardness (Rockwell M) | Glass Transition Temp (°C) |
|---|---|---|---|---|---|---|
| Polycarbonate | 1.20-1.22 | 55-65 | 60 - 70 | 60 - 100 | M70 | ~145 |
Notes on key values:
No melting point- PC is amorphous and softens progressively above Tg (~145 °C). Machining parameters must be set to 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)- softer than most metals and harder to scratch than ABS; scratch resistance remains 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.
Glass-filled and flame-retardant grades behave differently during cutting compared to general-purpose PC. Glass-filled grades are abrasive and accelerate tool wear - carbide tooling is required and finish quality will differ from unfilled stock. FR grades may exhibit slightly reduced ductility, increasing edge chipping risk on thin features. Medical and food-contact grades require documented material traceability - confirm stock availability and certification requirements before specifying on a drawing.
| 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 |
Polycarbonate Machinability - Parameters, Tooling and Surface Finish
Polycarbonate machines with moderate difficulty compared to other engineering plastics. Cutting forces are low, but PC's amorphous structure and low thermal conductivity make heat management the primary challenge. Unlike semi-crystalline plastics such as POM or Nylon, PC does not chip cleanly at all speeds - insufficient heat removal causes surface softening, smearing at the cutting edge, and stress cracking at features under fixturing load.
Sharp tooling and controlled feed rates are the two most important variables. High feed rate moves heat away in the chip rather than allowing it to accumulate at the cut zone. Single or two-flute end mills are preferred for chip evacuation. Compressed air or water-based mist cooling is used in most setups - organic solvent-based coolants (ketones, esters, aromatics) must be avoided entirely, as they cause environmental stress cracking in polycarbonate.
Reference Machining Parameters
These values are indicative for general-purpose, unfilled CNC-grade polycarbonate using sharp carbide tools. Adjust based on machine rigidity, tool geometry, stock condition, and specific grade. Glass-filled grades require carbide tooling throughout and expect accelerated tool wear.
| Operation | Spindle Speed | Feed Rate | Depth of Cut | Tooling Note |
|---|---|---|---|---|
| CNC milling - roughing | 3,000 - 5,000 RPM | 500 - 1,500 mm/min | 1.0 - 2.5 mm | 1-2 flute carbide end mill; compressed air recommended |
| CNC milling - finishing | 4,000 - 8,000 RPM | 800 - 2,000 mm/min | 0.2 - 0.5 mm | Sharp tool; avoid dwelling - heat buildup causes surface haze |
| CNC turning | 500 - 1,500 RPM | 0.1 - 0.3 mm/rev | 0.5 - 2.0 mm | Positive rake angle; sharp uncoated carbide or HSS insert |
| Drilling | 1,000 - 2,500 RPM | 0.05 - 0.15 mm/rev | - | Clear chips frequently; standard drill geometry; avoid high pressure |
As-Machined Surface Roughness
| Condition | Ra Value |
|---|---|
| As-machined (standard finish) | 2.4-3.2 µm |
| As-machined (fine finishing pass) | 1.2 - 1.6 µm |
| Bead-blasted | 1.6 - 4.0 µm (uniform matte, non-directional) |
Standard as-machined PC produces Ra 2.4-3.2 µm - acceptable for functional parts, structural components, and enclosures where surface appearance is not a requirement. As-machined surfaces on clear PC will appear hazy and semi-translucent, not optically transparent. Where clarity is required, see Surface Finish options below.
Annealing
CNC-machined polycarbonate parts that will be used in elevated-temperature environments, tight-fit assemblies, or applications involving chemical exposure benefit from stress-relief annealing before use. Residual machining stresses - particularly from aggressive cuts, heavy fixturing, or deep pockets - can cause warping, micro-cracking, or premature failure in service if left unrelieved.
Process:Heat the part in a circulating-air oven to 121-129 °C. Hold for 1 hour per 3 mm of section thickness (minimum 1 hour, maximum 4 hours for thick sections). Allow to cool slowly inside the oven - rapid cooling reintroduces thermal stress. Parts should be supported on a flat, non-marring surface during the cycle to prevent sag.
Annealing is particularly recommended before: solvent bonding or adhesive assembly, painting or coating, press-fit insert installation, and any application where dimensional stability over time is critical.
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.
As-Machined Surface Roughness
| 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° |
Polycarbonate's high CTE means thermal stabilisation of the workpiece between roughing and finishing passes is important on tight-tolerance features. For parts with mixed wall thicknesses, differential cooling after machining can introduce flatness deviation beyond what the tolerance table above suggests - design for uniform wall thickness where dimensional stability is critical.
Surface Finish and Post-Processing Options for Polycarbonate
Polycarbonate accepts a wider range of post-processing treatments than most engineering plastics, including finishing options that restore or achieve optical clarity - something no other common CNC plastic supports. The correct finish depends on whether the part needs to be transparent, cosmetically clean, chemically protected, or structurally bonded.
As-Machined
The default condition after CNC machining. Clear PC surfaces will appear hazy and semi-translucent - tool paths scatter light and Ra 2.4-3.2 µm is typical. Black PC appears matte with visible directional tool marks. Suitable for internal structural parts, enclosure bodies, fixtures, and jigs where surface appearance and transparency are not requirements.
Fine Machining Pass
A dedicated light finishing pass using a sharp tool at reduced depth of cut (0.2-0.3 mm) and increased feed rate improves surface roughness to Ra 1.2-1.6 µm. This produces a cleaner, slightly translucent surface on clear PC and reduces visible tool marks on black stock. Appropriate as a standalone finish for functional enclosures and covers where some surface texture is acceptable.
Bead Blasting
Fine glass or ceramic beads propelled at the part surface under controlled pressure produce a uniform matte texture, eliminating directional tool marks across all exposed faces. The resulting finish is consistent and non-directional. Ra values after bead blasting typically fall between 1.6 and 4.0 µm depending on bead size and dwell time. Clear PC becomes fully opaque after bead blasting - use only on black stock or parts where transparency is not required.
Avoid high-pressure blasting on PC walls below 1.0 mm - distortion and surface damage risk increases significantly.
Mechanical Polishing
Progressive abrasive polishing (wet/dry paper from 400 to 2000 grit, followed by polishing compound) restores clarity to flat or convex surfaces on clear PC, achieving Ra ~0.8 µm and semi-transparent results. Time-intensive and limited to geometrically accessible surfaces. Suitable for flat windows, simple covers, and edges where vapor polishing is not specified.
Vapor Polishing
The most effective method for restoring near-optical clarity to CNC-machined polycarbonate. The part is briefly exposed (20-60 seconds) to chlorinated solvent vapor, which micro-reflows the surface layer without dimensional change, achieving Ra < 0.4 µm and transmission values close to moulded PC. Effective on complex geometries including internal faces, pockets, and curved surfaces that mechanical polishing cannot reach.
Vapor polishing introduces a thin layer of solvent-induced surface stress - parts intended for elevated-temperature service or chemical exposure should be annealed after vapor polishing.
Painting and Hard-Coat
ABS bonds well to paint; polycarbonate requires more careful surface preparation - solvent-based primers must be compatible with PC to avoid stress cracking. Water-based primers are the safer default. Hard-coat (silica or titanium dioxide-based) significantly improves scratch resistance and is the standard treatment for outdoor PC panels and machine glazing exposed to repeated contact or cleaning. Curing temperatures for hard-coat must remain below PC's Tg (~145 °C).
Bonding and Assembly
Polycarbonate can be joined using methylene chloride or ethylene dichloride-based solvent cements (which dissolve the PC surface to form a molecular bond), structural two-part epoxies, or cyanoacrylates. Avoid ketone, ester, or aromatic solvent-based adhesives - they cause environmental stress cracking. Solvent bonding produces clean, flush bond lines and is the standard approach for PC-to-PC enclosure and window assembly.
For mechanical fastening, direct tapping is possible for low-cycle use (M3 minimum, coarse thread, 2× diameter engagement). For repeated disassembly, heat-set or press-fit metal inserts are correct - they prevent thread strip-out over time. Boss diameter should be at least 2× insert outer diameter. Anneal the part after insert installation if the assembly will see elevated temperatures or chemical exposure.
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 - a radius equal to at least half the end mill diameter will always be present.
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 under load. Generous radii are particularly important in PC.
As a 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 and eliminates the need for a non-standard tool.
For floor-to-wall transitions, a small radius (0.5-1.0 mm) is preferred over a sharp corner - it reduces stress concentration in service and improves surface finish at the transition.
Transparency and Optical Surface Requirements
Clear polycarbonate surfaces requiring optical clarity must be identified explicitly on the drawing. As-machined PC is hazy - transparency is not restored by machining alone and requires a secondary finishing operation.
| Requirement | Recommended Finish | Expected Result |
|---|---|---|
| Functional transparency (light transmission, not cosmetic) | Fine machining pass | Ra 1.2-1.6 µm; semi-translucent |
| Cosmetic clarity (visible panels, covers) | Mechanical polishing | Ra ~0.8 µm; semi-transparent |
| Near-optical clarity (windows, light pipes, sensors) | Vapor polishing | Ra < 0.4 µm; near-moulded clarity |
| Scratch-resistant outdoor surface | Hard-coat after polishing | Improved abrasion resistance; maintains clarity |
Call out surface finish on transparency-critical faces using Ra values and finish method (e.g.,"vapor polish to Ra ≤ 0.4 µm") so that toolpath strategy and post-processing can be planned at quoting stage. Do not rely on a generic "clear finish" callout - it is ambiguous and will default to as-machined.
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
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ISO 9001:2015 · AS9100D · ISO 13485:2016
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 vapor micro-reflows the surface layer, achieving Ra < 0.4 µm and transmission values close to injection-moulded PC. Mechanical polishing (400-2000 grit progression) is an alternative for flat or convex surfaces. Always specify the required finish method and Ra value explicitly on your drawing - a generic "clear finish" callout will default 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.
Where Polycarbonate Is Used - Industries and Applications
Polycarbonate is specified wherever a CNC-machined part must combine structural toughness with dimensional stability, and - in many cases - optical transparency. It is the default choice when impact resistance above the ABS threshold is required and the service temperature stays within 115-130 °C.
Industrial Equipment and Machine Guards
The single largest application area for machined polycarbonate. Safety shields, transparent access doors, protective windows, and viewing panels on CNC machines, presses, and automated cells are routinely machined from clear GP-grade PC sheet. Impact resistance allows the guard to absorb accidental tool or workpiece contact without shattering, while transparency maintains visibility of moving components. UV-stabilised grades are specified where guards are exposed to direct lighting or near-UV industrial sources over extended service periods.
Electronics and Electrical Enclosures
Polycarbonate is widely used for electronic housings, terminal covers, sensor windows, indicator lenses, and light pipes. Volume resistivity of ≥10¹⁵ Ω·cm makes it a reliable electrical insulator. Flame-retardant grades (UL 94 V-0) are selected for components in switchgear, power distribution panels, and transport interiors where ignition resistance is a regulatory requirement. CNC machining is the correct process when enclosure geometry is too complex for off-the-shelf extrusions or when pre-tooling prototypes are needed before injection mould investment.
Automotive and Transportation
Interior structural components, lighting lenses, instrument panel covers, and indicator windows use CNC-machined PC where heat resistance above 100 °C and impact performance are required simultaneously - a combination ABS cannot reliably deliver. PC/ABS blend grades are commonly specified for automotive interior parts where processing ease and cost are balanced against performance. Pre-production validation parts and low-volume runs are machined from stock before injection mould tooling is committed.
Medical and Laboratory Equipment
Medical-grade and food-contact polycarbonate is machined for device housings, fluid-handling components, lab enclosures, diagnostic windows, and optical components in imaging equipment. FDA-compliant and ISO 10993-assessed grades are used where material traceability and biocompatibility documentation are required. CNC machining supports prototype and low-volume production without minimum order quantities, with full material certification available through the QA & QC process.
Optical Components and Light Management
Clear GP-grade PC machined and vapor-polished to Ra < 0.4 µm is used for light pipes, diffusers, sensor covers, indicator windows, and secondary optical elements where near-glass clarity is required in a tough, impact-resistant housing. Acrylic delivers higher raw transmission (92% vs 88-90%) but shatters under impact - machined PC with vapor polishing is the correct choice when both clarity and toughness are requirements.
Functional Prototypes and Bridge Production
Polycarbonate is a standard material for functional prototypes and pre-production bridge parts where the mechanical behaviour of an injection-moulded PC component needs to be validated before tooling investment. Identical material system means form, fit, and function testing - including snap-fit deflection, boss pull-out, and thermal cycling - is directly transferable. CNC machining from bar or sheet stock supports one-off through small-batch quantities with no tooling cost.
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ISO 9001:2015 · AS9100D · ISO 13485:2016