Machine guards, hinged covers, sneeze guards, dust covers and observation windows — where ±0.1 mm on a mounting hole decides whether the guard fits, and annealing decides whether it survives a year of vibration.
This is the first fork in the road and it is often decided by habit rather than by requirement. Acrylic is clearer and scratches less visibly, but it is brittle: a sharp impact cracks it. Polycarbonate is roughly 200 times more impact resistant, yellows faster under UV, and scratches more easily.
The practical rule we use: if the guard is there to keep hands out and nothing is thrown at it, acrylic gives better visibility and a longer clear life. If the guard faces flying swarf, a tool breakage or a projectile, polycarbonate is the correct choice and we will say so even though we machine far more acrylic.
There is a third option worth knowing about: impact-modified acrylic. It sits between the two — better impact resistance than standard acrylic, better clarity and UV life than polycarbonate. For guards on faster machinery it is often the right compromise.
Bending acrylic puts stress into the material. On a 3 mm panel the residual stress is modest. On a 20 mm panel bent to a tight radius, it can be enough that a small scratch or a drilling operation triggers a crack weeks later — long after the part passed inspection.
Annealing releases that stress. The part is heated to a controlled temperature below the forming range, held long enough for the polymer chains to relax, then cooled slowly enough that no new stress is introduced. For thick bent guards we anneal as standard and we say so in the quotation, because it is a real cost that a cheaper supplier will have omitted.
The other crack source is holes. A drilled hole in stressed acrylic has a stress concentration at its edge. We rout or drill with the correct geometry and, where the design allows, keep hole edges at least 1.5× the hole diameter from the panel edge. Threads are never cut directly into acrylic for anything load-bearing — a threaded insert or a through-bolt with a washer is used instead.
Acrylic moves roughly seven times more than steel per degree Celsius. A 500 mm guard machined at 20 °C and measured at 30 °C will read about 0.5 mm different — which is five times the tolerance we just claimed. Precision on acrylic is meaningless without an agreed reference temperature.
So we ask two questions before quoting tight tolerances: what is the ambient temperature on the machine, and what is the reference dimension that actually has to fit? Usually only two or three dimensions matter — the mounting hole pattern and one locating edge. Everything else can be ±0.3 mm and the part still assembles.
Where a tight tolerance genuinely matters, we machine it in a temperature-stable part of the shop, check it on a granite plate at the agreed reference, and record the reading on the inspection sheet that ships with the parts.
Electronics assembly and painting lines need guards that do not hold a charge. Anti-static acrylic is available and we keep it in stock for repeat work; note that the additive that provides conductivity also slightly reduces clarity, so it is specified only where needed.
UV-stabilised sheet slows yellowing on guards exposed to daylight. It does not stop it. A realistic expectation is that a standard guard facing a south window will show measurable yellowing in two to three years, and a stabilised one in four to six.
For cleanroom applications we cut and pack in a controlled area, seal edges, and supply the parts double-bagged. We do not claim a cleanroom certification for the plant; we can meet a specified cleaning and packing protocol if you give us the standard.
Questions on this line
We can cut a thread, but we do not recommend it for anything that will be tightened more than a few times or carries load. Acrylic threads strip and craze. The standard solutions are a threaded insert, a through-bolt with a washer and a compliant pad, or a metal bracket bonded or bolted to the panel. We will mark this on the DFM note.
On a typical 10–20 mm bent guard, annealing adds roughly 8–15% to the part price and about one working day to the schedule, because the cooling cycle cannot be rushed. On thin flat parts it is usually unnecessary and we will not add it.
Yes. A dimensional inspection sheet ships with each batch, recording the critical dimensions and the reference temperature at which they were measured. Mill certificates for the sheet and the raw-material RoHS report are available on request.
Ambient temperature, what the guard is protecting against, and which dimensions must fit are more useful than a fully toleranced drawing. Send what you have and we will come back with a DFM note.
