Optical and precision acrylic parts, machined and stress-relieved
Light guides, lenses, flow-cell windows and precision curved parts — where the specification is usually about what you can see through the part rather than the part itself.
Light guides, lenses, flow-cell windows and precision curved parts — where the specification is usually about what you can see through the part rather than the part itself.
Optical grade cast acrylic is a specification about internal clarity, not about surface finish. It means the sheet has low haze, minimal internal stress and few inclusions. Two sheets can be dimensionally identical and visually very different.
For a light guide or a window that will be looked through, the sheet grade decides the baseline. Machining then either preserves that baseline or destroys it: a poorly machined edge scatters light, and a polished edge on a light guide is not cosmetic but functional, because light extraction depends on edge quality.
We specify optical grade sheet where the part transmits an image or carries light, and standard cast sheet where it only needs to be clear. The price difference is real, and paying for optical grade on a dust cover is wasted money — we will say so.
Machining removes material unevenly and leaves stress behind. In a thin, flat optical part, that stress gradually relaxes and the part bows — sometimes by more than the flatness tolerance it was machined to. The bow appears days or weeks after delivery, which is the worst possible time.
Annealing fixes this by heating the part to a temperature below its softening point, holding it long enough for stress to relax, then cooling slowly enough that the cooling itself does not reintroduce stress. The cooling ramp is the part that cannot be hurried; a fast cool produces a part that is stress-relieved in the middle and stressed at the surface.
For optical and precision parts we anneal after machining as standard. It costs a day and it is the difference between a part that measures correctly on delivery and one that measures correctly six months later.
Acrylic surface specifications are usually written in terms of scratches and digs, borrowed from glass standards. Applying a glass scratch-dig standard to a polymer is imperfect, because acrylic is softer and a "scratch" in acrylic behaves differently from one in glass.
What we can do reliably: polish to a visually clear finish, inspect under grazing light against a defined illumination, and reject parts with visible scratches, bubbles or polishing marks in the clear aperture. What we cannot do is certify a numeric scratch-dig grade, because we do not have the interferometric metrology that a true optics shop would use.
If your application genuinely needs a certified optical specification, that is a specialist optics supplier rather than a plastics fabricator, and we will tell you so. For the great majority of light guides, windows and sensor covers, a defined visual inspection under agreed lighting is the practical and honest standard.
Acrylic transmits about 92% of visible light, similar to glass, at roughly half the weight and with far greater impact resistance. It can be machined and polished into shapes that would be expensive in glass, and it does not shatter.
Glass still wins on scratch resistance, on temperature resistance, and on index stability over time. Acrylic also has a higher refractive index and more chromatic dispersion, so it is a poor choice for imaging optics where colour correction matters.
For a light guide, a protective window or a sensor cover, acrylic is usually the right answer. For a lens that has to form a sharp image, it usually is not — and we will say so before you spend money on a quote.
Questions on this line
We can guarantee dimensional tolerance, flatness measured on a granite plate, and visual quality inspected under defined lighting. We cannot certify a numeric scratch-dig or wavefront specification, because that requires interferometric metrology. If your application needs it, an optics house is the correct supplier.
Machining leaves internal stress. In a thin flat part that stress gradually relaxes and the part bows, often weeks after delivery. Annealing releases the stress before it can move the part, which is why we do it as standard on optical and precision work.
0.05–0.1 mm across an optical face up to about 200 mm is achievable with annealing and a final finishing pass. Tighter than that, or larger faces, moves into specialist optics territory rather than plastics fabrication.
Whether the part transmits an image, carries light, or only needs to look clear decides the sheet grade and the finishing route. Tell us which, and the quote will be accurate the first time.