Automotive work uses acrylic where you need to see through a fixture or a guard and still hold a dimension. The material is fine; the failures come from heat near a motor or a lamp, and from coolant that was never mentioned at quotation stage.
Four things come up on almost every enquiry in this sector. None of them is solved by choosing a thicker sheet.
Acrylic is service-rated at 70–80 °C. A panel that sits near an exhaust or a curing lamp will bow. We ask for the temperature at the panel and specify polycarbonate where acrylic cannot hold.
Mineral oils are mild on acrylic; some synthetic coolants and brake fluids are not. A coupon test with your actual fluid costs a day and prevents a crazed guard six months later.
An inspection gauge machined from cast sheet moves if it is not annealed. Gauges are rough-machined, annealed, then finished, and the flatness is recorded.
Optical prototypes are diamond-polished on the working face and vapour-polished where a moulded look is needed. We will say when a machined part cannot reach a moulded surface.
All of the below run on the same machines, in the same plant, with no operation subcontracted. Full detail sits on the Industrial & machinery line page.
Automotive and new-energy work asks three questions of a plastic part that most sectors do not: how hot does it get, what is it in contact with, and will part 5,000 match part one.
Cast acrylic is service-rated to about 70 degrees C continuous and 80 degrees C short-term. Above that it does not melt abruptly — it creeps. A panel near an exhaust manifold or under a curing lamp bows toward the heat over weeks, and holes go oval. The engineering answer is to measure the air temperature at the panel rather than at the source, and then either add an air gap, add a heat shield, or change to polycarbonate, which handles 100 degrees C and beyond.
Mineral oil and most hydraulic fluids are mild on acrylic; brake fluid and glycol coolants are not, and they attack a machined edge within hours. Synthetic coolants vary by manufacturer and cannot be judged by name, so the only reliable answer is a 24-hour coupon soak with the actual fluid. We ask for a safety data sheet or 20 ml of the fluid on any under-bonnet enquiry.
| Application | Material | Governing limit |
|---|---|---|
| Bench test-rig cover | Cast acrylic, 5–8 mm | Panel air temperature below 70 degrees C |
| Under-bonnet cover | Polycarbonate | Radiant heat and brake-fluid exposure |
| Inspection gauge | Cast acrylic, annealed | Dimensional stability over the shop year |
| Display and showroom part | Cast acrylic, UV-stabilised | Yellowing under showroom lighting |
| Battery-pack viewing panel | Cast acrylic or PC | Electrolyte splash resistance and impact |
An automotive buyer's real question is whether part 5,000 matches the first article. That is answered by process control rather than by inspection: the same sheet batch held for the run, the same jig, the same parameter card recording spindle speed and clamping, and a first-article record that a rerun pulls rather than reinvents. Without those, "consistent" is an opinion.
Automotive programmes are priced on consistency and documentation as much as on the part. On a typical gauge, cover or showroom piece: sheet 20–30 percent, machining 25–35 percent, annealing and fixturing 10–15 percent, first-article inspection and records 8–12 percent, packing 8–12 percent.
The largest saving available is consolidating part numbers. A programme with six similar covers in three thicknesses pays for six setups and carries six sheet stocks; the same programme reduced to two thicknesses usually costs 15–20 percent less with no functional change. That conversation is worth having at design stage, not after the first PO.
The second lever is the acceptance standard. A gauge held to ±0.1 mm on every feature costs more than one held to ±0.1 mm on the two features that actually locate a part and ±0.3 mm elsewhere. Writing the tolerance map onto the drawing rather than applying a blanket tolerance is free and usually reduces both price and argument.
What we do not compress is the first-article record. On a programme that runs for years, the record is what makes the fifth rerun identical to the first, and rebuilding that knowledge costs more than keeping it.
| Item | What we need | Why it changes the price |
|---|---|---|
| Panel air temperature | Measured at the panel, continuous and peak | Below 70 degrees C acrylic; above it, polycarbonate |
| Fluid contact | Fluid name or an SDS | Brake fluid will craze a machined edge within hours |
| Volume and repeat | Units per year and whether the part repeats | A repeating part justifies a fixed parameter card and a stored sheet batch |
| Acceptance standard | Which features are gauged, and to what | A tolerance map is cheaper than a blanket callout |
| First-article requirement | Whether an FAI report is needed, and in what format | Adds 2–3 days; keeps reruns identical |
For a programme that will run for years, the most useful thing you can send is the first-article record from the current supplier. It tells us what "identical" means to your quality team before we quote rather than after the first rejection.
The acceptance points below are recorded per batch, and the record travels with the packing list.
Measure the air temperature at the panel, not at the engine. Acrylic is service-rated to about 70 °C continuous and 80 °C short-term; above that it does not melt, it creeps — the panel bows toward the heat source and holes go oval over weeks. In practice that means a clear cover on a bench test rig is fine, a cover within 150 mm of an exhaust manifold is not, and a cover under a curing lamp needs either a 30 mm air gap or a polycarbonate substitution. We ask for the measured temperature and the distance to the source; with those two numbers the material choice is decided in one reply.
Depends on the fluid, and the answer is not the same for all three groups. Mineral oil and most hydraulic fluids are mild: a splash wiped off within the shift leaves no trace. Brake fluid (DOT 3/4) and glycol-based coolants attack acrylic and will craze a machined edge within hours, so a cover that sits above a brake reservoir needs a polycarbonate panel or a deflecting lip. Synthetic coolants vary by manufacturer and cannot be judged by name — send the safety data sheet or 20 ml of the fluid and we run a 24-hour coupon soak test, which is the only way to answer this without guessing.
Yes, in cast sheet up to about 20 mm, machined and annealed so the dimensions stay put. We hold ±0.1 mm on critical features and record flatness on a granite plate. A gauge is a measuring device, so we ask which dimensions are functional and leave cosmetic ones loose — that is what keeps the price sane.
Yes. Engraved datum marks are permanent and do not wear off the way a printed label does, which matters on a gauge that is handled every shift. We engrave after annealing so the mark is not distorted by stress relief, and we record the datum position on the inspection sheet so a replacement gauge references the same point.
Where the programme justifies it, yes. Colour and thickness consistency across a long run depends on using sheet from one batch, so on a repeating programme we buy the sheet for the expected volume up front and store it, and record the mill certificate against the batch card. That is the only reliable way to keep year-two parts matching year-one parts.
