These parts are small, precise and surface-critical, which makes them easy to treat as a trivial job. But one coated lens marked by a fixture costs more than the entire batch of acrylic. On this work the process concentrates on contact surfaces, not on outline.
The risk on small parts is not whether they can be made, but whether they damage the lens they hold.
Acrylic is softer than glass and will not mark a coating in normal contact. The real risk is burrs, fly edges and residual polishing compound. Polishing every contact face to a mirror and rounding edges to R0.5 does more than moving to a more expensive material.
In a dry room, plain acrylic tribocharges to several kilovolts — enough to damage sensitive components in the module. Lens trays and transfer racks are made in ESD sheet, with surface resistivity held between 10⁶ and 10⁹ Ω.
Standard acrylic runs 1–2% haze, and three shelves stacked read visibly grey, worst against a dark frame. Optical grade holds haze below 0.5%; both shelf and lightbox face are cut from it.
Below 20 mm, tool radius and clamping distortion dominate, and a conventional setup will run out. Small-diameter tooling, depth-of-cut stepping and vacuum holding instead of mechanical clamping hold ±0.05 mm.
Small parts are machined in the temperature-controlled area with contact faces hand-polished. Capability limits sit on the Optical & precision page.
Smart eyewear and AI glasses put an acrylic part in front of a lens, a sensor or a camera. In that position the material stops being structural and becomes part of the optical path, and the specification has to be written in optical terms rather than mechanical ones.
Standard cast acrylic runs 1–2 percent haze with 2–3 percent transmission variation between batches. Optical grade holds under 0.5 percent haze and within 1 percent transmission. On a display stand that difference is invisible; on a test window between a camera and a target it is the difference between a calibration that repeats and one that drifts with every sheet change. Where the part is in the optical path, we specify optical grade and state the batch on the inspection sheet so a later replacement matches.
A routed acrylic edge carries a burr of 20–40 microns. Against an anti-reflective coating that burr is an abrasive, and the mark it leaves is permanent. Every edge on a part that touches a lens gets flame-polished or diamond-drawn to about 0.2 microns, faces stay masked until install, and contact faces carry a soft pad rather than bare acrylic.
| Part | Requirement | Specification |
|---|---|---|
| Lens display stand | Will not mark a coating | Soft-pad contact faces, polished edges, 3 mm minimum pad thickness |
| Optical test window | Repeatable transmission | Optical grade, under 0.5 % haze, batch recorded per piece |
| ESD tray | Protect a bare board | 10^6–10^9 ohms surface, or volume-conductive where cleaned daily |
| Inspection fixture pad | No witness mark on a housing | Felt or EVA overlay, edge broken 0.5 mm |
| Display shelving | Uniform lighting behind product | 3–5 mm opal diffusing, thickness set by LED pitch |
Any part near a bare board, a sensor or a flex cable is a static risk. Standard acrylic holds several kilovolts after a dry wipe, so a tray or a cover in an assembly area needs a dissipative grade. Surface grades decay over six to twelve months of wiping; volume-conductive sheet lasts but costs about double. On a line that wipes every shift, we specify volume-conductive.
These parts are small, precise and often inspected under magnification, so price is dominated by finishing and inspection rather than material. On a typical display stand or test window: sheet 15–25 percent, machining 25–35 percent, edge and optical finishing 25–35 percent, inspection 10–15 percent, packing 8–12 percent.
The optical grade is the main material premium — roughly 20–30 percent over standard cast — and it is only worth paying where the part is genuinely in the light path. A display stand does not need it; a test window does. That single distinction is the most useful cost lever in this sector and it is frequently missed, with buyers specifying optical sheet for a part that only has to look good.
Second lever: the number of polished faces. Polishing all six faces of a small block is significantly more labour than polishing the two that are visible, and on an internal fixture part the other four are never seen. Third: batch the inspection. Checking every piece under a magnifier is appropriate for an optical window and wasteful for a display stand, and specifying which is which in the PO saves real money.
| Item | What we need | Why it changes the price |
|---|---|---|
| Optical role | Is the part in the light path, or only in front of the customer? | Optical grade costs 20–30 % more and is wasted on a display stand |
| Contact surfaces | Whether the part touches a coated lens | Requires polished edges and a soft pad rather than bare acrylic |
| Static risk | Whether bare boards or sensors are nearby | A dissipative grade may be mandatory; standard PMMA holds several kV |
| Cleaning | Wipe frequency and chemistry | Daily IPA is fine; solvent degreasers craze the surface |
| Cosmetic standard | Which faces are inspected, and at what distance | Polishing six faces instead of two is a large labour difference |
Send a lens sample if the part touches one. Coating hardness varies and a burr that is harmless on one lens will mark another, so we test against the real thing.
Every contact face inspected; ESD parts measured and recorded.
Not through contact alone — the damage comes from the edge, not the face. A routed edge left unpolished carries a burr of 20–40 µm that will score an AR coating the first time a tray slides past it, and dried polishing compound behaves like a fine abrasive. So edges on any part that touches a lens get flame-polished or diamond-drawn to a 0.2 µm finish, faces stay masked in PE film until install, and we wipe with an IPA-and-water mix rather than a dry cloth. If the display holds loose lenses in a tray, specify a flock or EVA lining: it costs about 8 % more and removes the contact question entirely.
Surface-type ESD layers decay with wiping, typically over six to twelve months; volume-conductive sheet lasts longer but costs more than double. Measure surface resistivity quarterly rather than trusting the label — a sheet marked 10⁶ Ω can read 10¹² after three months of cleaning.
Haze and batch consistency. Standard runs 1–2% haze with 2–3% transmission variation between batches; optical grade holds under 0.5% haze and within 1% across a batch. It is obvious on lightboxes and stacked shelving, invisible on structural parts and back panels — so there is no reason to pay for it there.
Yes. The tray is machined from static-dissipative sheet at 10^6 to 10^9 ohms, with 24 cradles on a machined comb so the pitch is even, and a soft-pad seat on each so a coated lens never touches bare acrylic. Above about 200 trays a year we recommend the volume-conductive grade, because surface grades decay with wiping over six to twelve months.
2 mm is the practical floor for a polished edge. Below that the sheet flexes during polishing and the edge rounds unevenly, so a 1.5 mm part is usually supplied with a sawn or flame-polished edge rather than a diamond finish. Where a thin part needs an optical edge, the answer is normally to step up to 3 mm and adjust the surrounding geometry.
