Overview
Polycarbonate is the engineering plastic engineers reach for when impact resistance is the primary design driver — especially when transparency matters too. The combination of optical clarity (~86% light transmittance), extreme toughness (notched Izod 12–16 ft·lb/in, multiples higher than any other unfilled engineering plastic), and decent heat resistance (~115°C continuous) is unique. Machine guards, safety face shields, bullet-resistant glazing, headlight lenses, and impact-rated electronics housings all rely on PC’s signature property mix.
The big design trap is environmental stress cracking (ESC). PC is chemically vulnerable to a long list of everyday agents — alcohols, cleaning chemicals, gasoline, ketones, esters, strong bases — that cause crazing and brittle failure under even mild stress. A PC machine guard that’s been wiped down with IPA during cleaning, then re-loaded under bolt preload, can craze and crack months later. This is not a manufacturing defect; it’s PC behaving normally under conditions the designer didn’t anticipate.
The second design trap is UV degradation. Unstabilized PC yellows in months outdoors and embrittles within a year or two. UV-stabilized grades exist and work well (10+ year outdoor lifetime) but must be specified — substituting standard PC for UV-stabilized PC in an outdoor application is a documented field-failure pattern.
When neither ESC nor UV is the limiting factor, PC is one of the easiest engineering plastics to design with: machines well, bonds well, thermoforms exceptionally, prints well on industrial FDM, and accepts paint and adhesives reliably.
Machining notes
PC machines as one of the more forgiving engineering plastics. Practical recipe:
- Sharp carbide tooling, polished edges
- Speed: 200–500 SFM
- Feed: 0.005–0.015 in/rev
- Coolant: recommended for production, air-blast OK for light cuts
The PC-specific machining concern is residual stress development. Machining (especially heavy cuts) introduces stresses in the surface layer that can later cause crazing or ESC failure when the part contacts solvents. For ESC-critical or optical applications:
- Use sharp tooling and moderate feeds. Avoid heavy chip loads.
- Anneal post-machining at 120–125°C for 1–2 hours to relieve residual stress. This dramatically improves solvent compatibility.
- Avoid sharp internal corners. Stress concentrations + ESC = guaranteed cracks.
Surface finishes of 16 Ra are readily achievable. Optical finishes (Ra <\1.4 μm) require diamond flycutting on specialized equipment — typical CNC machining cannot reach optical-quality directly.
PC bonds exceptionally well. Methylene chloride is the classic solvent cement for PC. Acrylic adhesives (Weld-On 16, Lord 403), cyanoacrylates, and silicones all work with no surface preparation beyond cleaning. The same chemistry that causes ESC also enables bonding — PC is happy to dissolve into adhesives.
The ESC story — most important PC fact
Environmental stress cracking is the single most common cause of PC field failures. The mechanism: certain chemicals plasticize the PC surface, allowing molecular chains to disentangle under stress. The result is crazing (small interconnected cracks) that propagate into brittle fracture.
The vulnerable chemistry list is broader than most designers expect:
| Category | Examples |
|---|---|
| Aromatic solvents | Toluene, xylene, benzene |
| Chlorinated solvents | Methylene chloride, chloroform, perchloroethylene |
| Ketones | Acetone, MEK, MIBK |
| Esters | Ethyl acetate, butyl acetate |
| Strong bases | NaOH, ammonia, alkaline cleaners |
| Alcohols (under stress) | IPA, methanol |
| Certain fuels | Gasoline (long-term), aviation fuels |
| Cleaning agents | Many industrial degreasers |
Critically, IPA wiping during assembly is a documented failure source. A PC part wiped with isopropyl alcohol — a near-universal assembly cleaner — then loaded under bolt preload can develop crazing and fail months later. For PC assemblies, specify cleaning protocols that avoid these chemistries.
The mitigations:
- Anneal machined or molded PC at 120–125°C to relieve residual stress
- Specify cleaning chemistry (water + mild detergent, not IPA)
- For aggressive chemical service, switch to PETG or acrylic (lower impact strength but no ESC issues with most common chemistries)
Variant selection guidance
- Unfilled clear (Lexan 9034 / Makrolon 3108) — the default. Machine guards, optical glazing, safety equipment.
- UV-stabilized (Lexan XL10 / Makrolon UV) — anything outdoor. Non-negotiable for skylights, signage, exterior glazing.
- GF20–40 — structural stiffness applications. Sacrifices impact; if impact matters, stay unfilled.
- PC-ABS (Cycoloy / Bayblend) — consumer electronics housings, automotive interior, power tool bodies. Lower cost, similar impact at room temperature, slightly lower max-service temp.
- FR grades — UL94 V-0 electrical housings. Halogen-free phosphate FR is standard.
- Medical grade (Lexan HP / Makrolon RX) — IV connectors, blood- contact components, gamma-sterilizable medical devices.
Failure modes worth designing around
ESC is the dominant failure mode (see section above). Design and spec around it.
UV yellowing for outdoor applications. Specify UV grades — the substitution risk during procurement is real.
Notch sensitivity — PC’s signature impact resistance plummets at sharp corners. A part with sharp internal radii is no more impact-rated than polystyrene. Radius everything generously on impact-critical PC.
Scratch susceptibility — PC’s hardness (Rockwell M70) is fine for non-visible parts but optical applications need hard-coat surfacing. Anti-scratch coatings are standard for safety eyewear and headlight lenses.
Alkaline hydrolysis in hot caustic environments. Not for hot dishwasher service over years, even though short-term it’s fine.
Applications by industry
- Industrial machinery — machine guards, safety enclosures, observation windows. The #1 use of clear PC sheet.
- Architecture and construction — skylights, sound walls, riot shields, hurricane glazing, greenhouse panels. UV-stabilized grades.
- Automotive — headlight lenses (the dominant material), interior trim, sunroofs. Hard-coated for scratch resistance.
- Safety equipment — face shields, safety glasses, riot shields, hockey/sports visors. PC’s impact resistance + clarity is unmatched.
- Defense — bullet-resistant glazing (often laminated PC + glass + PC sandwich), military helmet visors, vehicle armor windows.
- Medical — IV connectors, blood-contact devices, surgical instrument housings, dialysis components. Gamma-sterilizable medical grades.
- Consumer electronics — phone bodies, laptop housings (often PC-ABS blend), power tool bodies, appliance trim.
- Optical — CD/Blu-ray substrate (huge legacy volume), camera lens bodies, optical waveguides, light pipes.