Key takeaways
- Polycarbonate grades are selected primarily by melt flow rate (MFR, ISO 1133, 300 °C / 1.2 kg), which ranges from roughly 3 g/10 min for high-impact extrusion/sheet grades up to 60–80 g/10 min for thin-wall, high-cavitation moulding — higher flow trades impact strength and molecular weight for fill.
- Bisphenol-A polycarbonate offers near-glass clarity (light transmission around 88–90%) but is intrinsically UV-sensitive; outdoor and lighting applications require UV-stabilised optical grades, and unmodified PC will yellow and surface-craze without that additive package.
- Flame-retardant PC reaches UL94 V-0 (commonly at 1.5 mm, with thinner-wall V-0 grades available) and glass-fibre reinforcement at 10–30% raises tensile modulus and dimensional stability at the cost of clarity and a higher anisotropic shrinkage/warpage risk.
- PC is strongly hygroscopic — it must be dried to below ~0.02% moisture, typically around 4 hours at 120 °C in a desiccant dryer, or hydrolytic chain scission during melt processing permanently degrades impact strength and causes splay; this is a non-negotiable processing step, not an optional one.
Polycarbonate is not one material — it is a family of bisphenol-A (BPA) based resins sold against a small set of variables that determine almost everything about how a grade behaves: melt flow rate, the additive package (UV, release, flame retardant), and reinforcement (glass or mineral). A buyer who specifies 'PC, natural' has specified almost nothing. The same base chemistry covers everything from 3 mm safety glazing to a sub-millimetre LED light guide, and the grade code is where the real spec lives.
PC earns its premium on a combination engineering plastics rarely deliver together: near-glass transparency, high heat deflection (HDT around 130–140 °C on unfilled grades), and exceptional impact strength — it is the resin behind riot shields, machine guards, and headlamp lenses. The trade-offs are not trivial: it is intrinsically UV-sensitive, notch-sensitive, prone to environmental stress cracking against certain chemicals, and aggressively hygroscopic. Getting the grade right means understanding which lever each application pulls.
MFR (ISO 1133, measured at 300 °C / 1.2 kg for PC) is the single most useful number on a PC datasheet and the proxy for molecular weight. Lower MFR means higher molecular weight, higher melt viscosity, and tougher parts; higher MFR means easier fill and lower impact. It is the same physics covered in melt flow index explained, but the PC test condition is hotter and the practical bands are specific to this resin.
| MFR band (g/10 min) | Relative MW / impact | Typical use |
|---|---|---|
| ~3–6 | Highest MW, highest impact | Sheet, extrusion, glazing, blow moulding |
| ~6–12 | High impact | Thick-wall mouldings, helmets, machine guards |
| ~12–22 | Balanced | General injection moulding, housings |
| ~22–40 | Easy flow | Thin-wall, multi-cavity, electronics |
| ~40–80 | Highest flow, lowest impact | Thin-wall optics, light guides, high cavitation |
The working rule: specify the lowest MFR that will reliably fill the tool. Reaching for a high-flow grade to cure a short shot trades away impact strength and environmental stress-crack resistance you may need in service. Flow length, wall thickness, and gate design should drive the choice — not a habit of ordering one familiar grade.
General-purpose PC is transparent — light transmission of roughly 88–90% on a clear, unfilled grade — but 'clear' and 'optical' are not interchangeable on a purchase order. Optical grades are higher-purity, tightly melt-filtered resins with controlled gel count, low haze, and a consistent refractive index (≈1.585), formulated for lenses, light pipes, and LED optics where a single inclusion is a reject. Cosmetic and contamination specs are far tighter than on a general-purpose clear grade.
Separately — and this catches buyers out — unmodified PC is UV-sensitive. Without a UV stabiliser package it yellows and surface-crazes outdoors or under lighting. Any glazing, automotive lens, signage, or luminaire grade must be UV-stabilised; many carry a co-extruded or in-mould UV layer for multi-year outdoor warranties. Always confirm whether UV protection is intrinsic to the grade or applied as a cap layer, and ask for the weathering data (e.g. ISO 4892 xenon-arc) behind any outdoor claim.
Specify the lowest MFR the tool will fill, confirm the UV package, and dry the resin properly — get those three right and most PC problems never reach the moulding floor.
Flame-retardant PC is specified by UL94 rating at a stated thickness. The common targets are V-2, V-0, and 5VA/5VB, with V-0 the workhorse for enclosures and electrical parts. Critically, a UL94 rating is meaningless without its thickness: V-0 at 3.0 mm is an easier achievement than V-0 at 1.5 mm. Always demand the UL Yellow Card and the rated minimum thickness, and check whether the FR system is halogen-free if your end market (EU electronics, for instance) requires it — see REACH/RoHS polymer compliance.
Glass-fibre reinforcement (typically 10%, 20%, or 30% by weight) raises tensile modulus and strength, sharply improves dimensional stability and creep resistance, and lifts HDT. The costs are loss of transparency (filled PC is opaque), reduced impact toughness, a more abrasive melt (tool wear), and anisotropic shrinkage — fibres orient with flow, so shrinkage differs along and across the flow direction, creating warpage risk that gate placement must manage. The same reinforcement trade-offs run across the family; see glass-filled grades explained.
| Grade type | Clarity | Impact | Stiffness / HDT | Watch-out |
|---|---|---|---|---|
| Unfilled GP / optical | Transparent | Very high | Baseline | UV yellowing; needs UV pkg outdoors |
| UV-stabilised | Transparent | Very high | Baseline | Confirm intrinsic vs cap-layer UV |
| FR (UL94 V-0) | Tinted / opaque | High | Baseline–slightly lower | Rating valid only at stated thickness |
| 10–30% glass-filled | Opaque | Lower | Much higher | Anisotropic shrinkage / warpage; tool wear |
| PC/ABS blend | Opaque | High (esp. low-temp) | Lower HDT | Less heat resistance than neat PC |
PC/ABS blends combine PC's toughness and heat resistance with ABS's flow, processability, and surface finish at a lower cost point. They are opaque and give up some heat deflection versus neat PC, but deliver excellent low-temperature impact and paintable, platable surfaces — which is why they dominate automotive interiors, IT and telecom housings, and appliance parts. If your part does not need transparency or maximum HDT, a PC/ABS blend is often the smarter buy; weigh it against straight ABS grades and the broader engineering plastics field.
Form matters too. Extrusion and sheet grades are low-MFR (high-MW) for melt strength and impact — solid and multiwall sheet, profiles, glazing. Injection moulding grades span the full MFR range to suit cavity count and wall section. Sheet is also where co-extruded UV-protected and abrasion/hard-coated products live. Do not cross-spec: a high-flow moulding grade will not extrude into quality sheet, and a stiff extrusion grade will struggle to fill a thin-wall tool. Confirm the intended process with the supplier before you fix the grade.
PC is strongly hygroscopic, and moisture is its most common processing failure. Absorbed water causes hydrolytic chain scission in the melt — the polymer chains are permanently cut, molecular weight drops, and impact strength is lost for good. Visible symptoms are splay (silver streaks), bubbles, and brittleness, but the molecular damage is irreversible even where the part looks acceptable. This is chemistry, not a cosmetic issue.
- Dry to below ~0.02% (200 ppm) moisture before processing.
- Typical schedule: around 4 hours at 120 °C (some grades 110–125 °C); glass-filled grades may need longer.
- Use a desiccant / dehumidifying dryer at a dewpoint around −40 °C — a hot-air oven cannot reach the required dryness.
- Do not over-dry at excessive temperature; protect dried resin from re-absorbing moisture before the throat.
- Verify incoming moisture and MFR against the Certificate of Analysis on every lot.
The practical buyer's checklist for PC is short but unforgiving: pin the MFR to the wall section, confirm the UV and FR packages with their test thickness and Yellow Card, decide filled vs unfilled against the clarity/stiffness trade, choose neat PC vs PC/ABS on heat and transparency needs, and treat drying as a hard processing requirement rather than a suggestion. Get those right at the order stage and most PC defects are designed out before resin ever reaches the press. For tailored grade selection and qualified supply, talk to the OmniaStrata engineering plastics desk or contact us.
Frequently asked
Questions on the desk
How do I choose the right polycarbonate MFR for my part?
Match MFR to wall thickness and flow length, not to a single 'best' number. Low-flow grades (MFR ~3–10 g/10 min at 300 °C/1.2 kg) carry the highest molecular weight and impact strength — use them for sheet, extrusion, glazing, and thick or impact-critical mouldings. High-flow grades (MFR ~20–80) fill thin walls and high-cavitation tools but give up impact and ESCR. As a rule, pick the lowest MFR your tool will fill cleanly.
Does polycarbonate need to be dried before moulding?
Yes, always. PC is hygroscopic and absorbs enough atmospheric moisture to cause hydrolytic chain scission in the melt, which permanently lowers molecular weight and impact strength and shows up as splay and bubbles. Dry to below roughly 0.02% (200 ppm) moisture — typically around 4 hours at 120 °C in a desiccant (dehumidifying) dryer, never a hot-air oven. See [reading a polymer CoA](/blog/reading-polymer-coa) for how moisture and MFR appear on documentation.
What is the difference between optical and standard polycarbonate grades?
Optical grades are higher-purity, tightly filtered (gel- and particle-controlled) resins formulated for low haze and consistent refractive index, used in lenses, light guides, and LED optics. General-purpose 'clear' grades are transparent but carry looser cosmetic specifications. Both need UV stabilisation for any outdoor or lighting service, since unmodified PC yellows under UV.
Is PC/ABS better than straight polycarbonate?
It depends on the requirement. PC/ABS blends sacrifice some heat resistance and clarity (they are opaque) but flow more easily, cost less, and offer excellent low-temperature impact and finish — which is why they dominate automotive interiors, IT housings, and appliances. Straight PC wins where you need transparency, maximum heat deflection, or the highest impact in thick sections. Compare against the wider family in [engineering plastics compared](/blog/engineering-plastics-compared).
Can flame-retardant polycarbonate still be transparent?
Some FR PC grades retain transparency or translucency while meeting UL94 V-2, but the most common V-0 grades (especially glass-filled or high-FR loadings) are tinted or opaque. If you need both V-0 and clarity, specify it explicitly and request the UL Yellow Card listing the rated thickness — V-0 at 3.0 mm is not the same as V-0 at 1.5 mm.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.