Material selection
Five transparent materials, seven dimensions, and one decision that decides most of the rest: what is going to hit the part, and what is going to clean it. One naming note first — acrylic, plexiglass, Perspex, Lucite and PMMA are five names for the same polymer.
Cast acrylic for almost everything we make. Polycarbonate when something will hit the part. PETG when aggressive solvents will clean it. Polystyrene almost never. Glass when scratch resistance or temperature outweighs weight and breakage.
That is the whole decision for most projects. The rest of this page is why, and where the simple rule breaks down.
Impact resistance is the single biggest differentiator between these materials and the one buyers most often get wrong in both directions.
Acrylic is brittle. A sharp impact cracks it, and a crack in acrylic propagates. Polycarbonate is roughly two hundred times more impact resistant and deforms rather than cracking — a polycarbonate panel that takes a heavy hit will dent and stay in one piece.
The confusion usually runs the other way, though: buyers specify polycarbonate for a display case because “it is stronger”, and end up with a part that scratches in a week, yellows in two years, and cannot be polished to a clear edge. If nothing is going to hit it, polycarbonate’s advantage is irrelevant and its drawbacks are not.
A useful test question: is the part protecting something from impact, or is it being looked at? If it is being looked at and nothing strikes it, acrylic. If it is a barrier against flying debris, polycarbonate.
The second differentiator, and the one that causes the slowest and most confusing failures. A part that is chemically attacked does not break — it clouds, crazes, or develops fine surface cracks over months, and by the time anyone notices, the cause is long past.
Acrylic survives alcohol-based sanitisers reasonably well. It does not survive acetone, MEK, chlorinated solvents, ammonia glass cleaner, or citrus degreasers. Polycarbonate is attacked by ketones and some chlorinated solvents. PETG resists a wider range than either. Clear rigid PVC resists more still, but discolours with heat.
Two practical points. First, edge finish matters as much as grade: a flame-polished or roughly machined edge crazes faster under chemical attack than a diamond-polished edge, because surface stress gives the solvent a starting point. Second, most damage in commercial settings comes from staff substituting whatever cleaning product is on the shelf. A written approved-and-prohibited list prevents more failures than any material upgrade.
Both are acrylic and they are not interchangeable for fabricated parts.
Cast sheet is poured between glass plates and cured. Higher molecular weight, better optical quality, far better resistance to stress cracking, and it machines and polishes cleanly. Extruded sheet is pushed through a die continuously. Cheaper, tighter thickness tolerance, available in long lengths — and it stress-cracks around holes, machines less cleanly, and looks dull next to cast.
Where extruded is genuinely fine: flat, unpolished, indoor parts where nobody inspects the edge and cost is the driver. Signs, simple dividers, non-critical backing panels.
Where extruded fails: anything bent, anything solvent-bonded, anything with holes near an edge, anything polished. If a quotation is unexpectedly low, cast-versus-extruded is one of the first questions to ask.
There is also a third category worth knowing: virgin versus regrind. Regrind sheet contains reprocessed material. It looks acceptable when new and yellows faster, and its mechanical properties vary between lots. We do not use regrind in production parts, and for medical or optical work we specify virgin material with a mill certificate.
Numbers below are typical values for standard grades, not specifications.
| Property | Cast acrylic | Polycarbonate | PETG | Glass |
|---|---|---|---|---|
| Light transmission | 92% | 88–90% | 85–88% | 90–91% |
| Impact | Low, brittle | Very high | Medium-high | Very low |
| Scratch resistance | Good | Poor without coat | Fair | Excellent |
| Machining | Excellent | Good, melts easily | Fair, gummy | Cut and grind only |
| Solvent bonding | Excellent | Poor | Fair | UV adhesive only |
| Polished edge | Excellent | Fair | Poor | Factory edge |
| Max service temp. | 70–80 °C | 115–125 °C | 65–70 °C | 150 °C+ |
Concrete cases, because general rules are easier to argue with than specifics.
| Application | Material | Why |
|---|---|---|
| Retail display stand | Cast acrylic | Polished edge, colour match, nothing hits it |
| Machine guard, fast spindle | Polycarbonate | Swarf and tool breakage are real hazards |
| Lab enclosure, wiped daily | Cast acrylic | Alcohol-safe, clear, bondable; not autoclaved |
| Chemical splash shield | PETG or clear PVC | Broader solvent resistance than acrylic |
| Outdoor sign, 5+ years | UV-stabilised acrylic | Better UV life than PC, lighter than glass |
| Museum vitrine | Optical cast acrylic | Low haze, no off-gassing concerns, lighter than glass |
| Autoclave window | Glass | 121 °C steam rules out all the plastics |
| Vandal-prone panel | Polycarbonate | Will not shatter; replace on scratching, not breakage |
1. Specifying “clear plastic”. That could be four materials with a 3× price spread and completely different failure modes. Name the material or describe the environment.
2. Using polycarbonate for its strength on a part that is looked at. You get scratches, yellowing and a dull edge, and no benefit from the impact resistance you paid for.
3. Using acrylic where solvents are involved. The failure takes months and looks like poor quality rather than a material mismatch.
4. Buying on price without asking cast or extruded. A 20% saving that produces stress cracks at the holes is not a saving.
5. Substituting a material on an approved design without telling the fabricator. The reason for the change is usually where the failure is. If you are switching because the original was unavailable or too expensive, say so — the substitute may need a design change to work.
You can, but you probably should not. Polycarbonate scratches more easily, yellows faster and cannot be polished to a clear edge, so a display made from it looks worse within a year. Its advantage — impact resistance — does nothing for a part that sits on a counter.
For flat, unpolished, indoor parts, no. For anything bent, bonded, polished or drilled near an edge, yes — extruded stress-cracks and machines less cleanly. The price difference is 10–20%, which is small relative to a failed batch.
UV-stabilised cast acrylic is the usual answer: better UV life than polycarbonate and far lighter than glass. Be realistic about the timeline — stabilised acrylic will still show measurable yellowing in four to six years outdoors. Nothing transparent stays optically clear outdoors indefinitely except glass.
Acrylic is used in food service, but we do not hold food-contact certification. If your market requires it, testing has to be arranged against the specific standard — FDA 21 CFR, EU 10/2011, or GB 4806.7 in China — and we quote that as a separate line rather than implying a certification we do not have.
What hits the part and what cleans it decide the material faster than any dimension. Tell us both and the quote will be right the first time.
