Polycarbonate FAQ: Cracking, UV Yellowing, Bullet-Resistance & More

Polycarbonate generates consistent questions from engineers and buyers: about cracking around fasteners, about UV yellowing in outdoor glazing, about whether it is genuinely bullet-resistant, and about how to form or polish it without damaging the surface. These questions come up in every project. The answers below are direct and specific — no hedged generalities.

At a glance:

• Stress cracking around holes is preventable with correct fastener design and oversize holes
• Outdoor yellowing is grade-dependent — general-purpose PC yellows; UV-stabilized grades do not
• Polycarbonate is genuinely bullet-resistant when thick enough or assembled as a laminate
• Forming temperature is 340–375°F; pre-dry 4–6 hours before thermoforming
• Flame polishing does not work on PC — use progressive wet-sanding and buffing compound
• Standard solvent cleaners and acetone will crack the surface — use mild soap and water

Will polycarbonate crack?

Polycarbonate is one of the toughest plastics available — it does not crack from impact under normal service conditions. A 0.25" panel will deform and bend before it fractures.

However, polycarbonate is susceptible to stress cracking when mechanical stress and chemical exposure occur simultaneously. Common scenarios:

1. Overtightened fasteners: drilling a hole sized exactly to the bolt diameter and tightening the nut compresses the PC aggressively. If any solvent (cutting fluid, cleaning spray) contacts the stressed area, surface cracks propagate from the fastener hole outward within hours or days.
2. Solvents on stressed parts: methylene chloride, acetone, aromatic solvents, and some mold-release sprays cause immediate crazing on stressed or residually stressed PC.
3. Bending below minimum radius: bending PC below the minimum radius for its thickness causes stress whitening and micro-cracking on the outer surface.

Prevention: drill mounting holes 1/8"–3/16" oversize with large washers; never apply solvent cleaners to installed PC. Use correct bend radii — see the machining guide.

Does polycarbonate yellow outdoors?

Yes — uncoated general-purpose polycarbonate (Lexan 9034, standard Makrolon, Tuffak GP) yellows within 6–12 months of direct UV exposure. The photooxidation of the carbonate linkage produces a yellow phenol compound visible as a progressive yellow tint that also reduces optical transmission and eventually causes surface embrittlement.

UV-stabilized grades do not yellow at the same rate. Makrolon UV, Lexan SL, and Tuffak A contain co-extruded UV-absorber layers rated for 5–10 years of outdoor service with < ΔYI 3 (barely perceptible change in yellowness index). These grades must be installed with the UV-cap face toward the weather (marked on the masking film at the mill).

Rule of thumb: for any outdoor installation expected to last more than one year, specify a UV-stabilized grade. General-purpose PC is an indoor material when aesthetics or retained impact performance matter.

Is polycarbonate truly bullet-resistant?

Yes, but resistance depends on thickness, laminate construction, and the specific threat round. A single sheet of polycarbonate stops handgun rounds at sufficient thickness; rifle rounds require laminated composites.

Monolithic thick PC "stops" the bullet by deforming elastically and plastically, dissipating kinetic energy in a cone of deformed material ("ballistic mushroom"). The limitation is spall — material ejected from the back face. Laminated constructions with polyurethane interlayers prevent spall from exiting the panel face.

UL 752 certification requires testing the complete assembly — not just sheet thickness. Use a tested and listed unit in regulated applications.

What temperature is needed to form polycarbonate?

For thermoforming (full-sheet oven forming): heat to 340–375°F (171–191°C) in a convection oven. The wide forming window makes PC forgiving — it stays in a formable state for 30–60 seconds after removal from the oven.

For line bending with a strip heater: heater surface ~340°F. Dwell time: ~60 sec/side for 0.125”; 2–3 min/side for 0.250”; 4–5 min/side for 0.375”.

Critical pre-forming step: pre-dry at 250°F for 4–6 hours (8 hours for sheet over 0.375” thick) before thermoforming. Undried sheet produces silver streaks or bubbles from moisture flashing to steam.

Can you polish polycarbonate edges?

Yes — but not with flame. Do not flame-polish polycarbonate. Localized torch heat causes surface crazing and residual stress that degrades impact resistance — a common error when coming from acrylic work.

Correct sequence:

1. Start at 120 or 220 grit to remove saw or router marks
2. Wet-sand progressively: 220 → 400 → 600 → 1000 → 2000 grit
3. Apply Novus #3 with a soft cotton buff at 1,200–1,500 RPM
4. Final pass with Novus #2 then #1 for optical clarity

A routed or saw-cut edge can be brought to near-optical clarity with this process. Diamond-polished cuts from CNC routers with finishing passes can skip several grit levels.

How do you clean polycarbonate without damaging it?

Use mild dish soap and warm water with a soft lint-free cloth or microfiber. Rinse with clean water and blot dry — do not drag a dry cloth across the surface without wetting it first, as dry wiping scratches the surface.

What to avoid:

• Acetone, ketones, aromatic solvents (toluene, xylene) — dissolve or craze PC
• Ammonia-based window cleaners — can attack PC with repeated use
• Paper towels — too abrasive for uncoated PC; use microfiber only

A dilute isopropanol solution (70% IPA / 30% water) is generally safe for incidental cleaning of installed guards. Do not soak or apply repeatedly — prolonged alcohol exposure can cause stress crazing on loaded parts. AR-coated grades are more abrasion-resistant but are still softer than glass; use the same mild soap method.

How does polycarbonate compare to acrylic for machine guards?

For machine guards where impact from tool ejecta or flying debris is possible, polycarbonate is the only correct choice. The notched Izod of 18 ft-lb/in vs. 0.4 ft-lb/in for acrylic is a 45× difference — acrylic shatters under impacts that PC barely deforms. OSHA 1910.212 compliance at appropriate thickness is met by PC, not acrylic in most high-energy applications.

Acrylic offers advantages where impact risk is low: 92% optical clarity, no UV yellowing, better scratch resistance, lower cost. Full analysis: acrylic vs. polycarbonate.

What is the service temperature limit for polycarbonate?

Continuous service limit: 240°F (116°C) under sustained structural load. Short-term peaks to 270°F are tolerated, but sustained loading above 240°F causes creep. The glass transition temperature (Tg) is 302°F (150°C) — above this, PC has no structural integrity.

For higher continuous temperatures: Polysulfone or Ultem to 300–340°F; PEEK above 340°F.

Is polycarbonate food-safe?

Standard GP polycarbonate is listed under FDA 21 CFR 177.1580 for repeated-use food contact in industrial settings — machine guards, inspection windows, and sight glasses.

BPA is the PC backbone monomer — not removable. For BPA-free applications (children's products, hot-beverage contact, EU food packaging), PETG is the standard transparent alternative. For potable water contact, request NSF/ANSI 61 documentation — not all stock PC is certified. Full details: polycarbonate FDA and food-grade guide.

Can polycarbonate be painted or printed on?

Polycarbonate accepts inkjet, screen printing, and pad printing directly — reverse-printed clear PC sheet is a common approach for backlit graphics and instrument overlays. Painting requires a PC-compatible adhesion promoter; standard automotive paints do not bond reliably without it. Two-component urethane topcoats bond well to primed PC. Test solvent-based paints for compatibility before production — many contain aromatics that attack PC.

Order Polycarbonate Sheet, Rod, or Tube

If your question isn't answered above, the polycarbonate material hub links to detailed guides on properties, grades, machining, specifications, and FDA compliance. For comparison against competing transparent materials, the polycarbonate comparisons index covers PC vs. acrylic, ABS, and PETG with side-by-side data.

More related guides

Cross-cluster suggestions to help shoppers and engineers explore adjacent topics:

Applications

• Machine Guards
• Marine Outdoor

Industries

• Marine

Compare to other materials

• Ultem Vs. Polycarbonate
• Polycarbonate Vs. Acrylic
• Polycarbonate Vs. Abs

Frequently asked questions — Polycarbonate FAQ