Acrylic is the part everyone can see. A machined bracket in acetal can carry a tool mark nobody will ever notice; an acrylic display case, light guide or sight window is judged on its edges, its holes and its clarity from across the room. And acrylic fails in ways that show: a chipped edge, a cracked hole, or a web of fine crazing lines that turns up a week after the part shipped.
None of that is bad luck. Here is what acrylic does at the cutter and after it, and how to specify a part that comes out clear and stays that way.
What acrylic is
Acrylic is polymethyl methacrylate (PMMA), sold under trade names such as Plexiglas® and often spelled "plexiglass". It is a hard, glassy, amorphous plastic: the clearest material we stock, it holds a polished edge, it resists scratching better than polycarbonate, and it does not yellow outdoors.
It is also brittle. That single fact drives most machining decisions. The values below are the cast-acrylic rows on our acrylic material page:
| Property | Cast acrylic sheet | What it means at the machine |
|---|---|---|
| Notched Izod impact (ASTM D256) | 0.3 ft-lb/in | Notch-sensitive; a chip or a sharp corner is where a crack starts |
| Hardness (ASTM D785) | Rockwell M100 | Hard enough to polish to a gloss, hard enough to chip |
| Thermal expansion (ASTM E831) | 3.9 × 10⁻⁵ in/in/°F | About six times steel |
| Heat deflection @ 264 psi (ASTM D648) | 205 °F | Softens and gums if the cutter rubs instead of cutting |
Treat these as typical published values for cell-cast sheet, not a specification. Extruded sheet and other grades publish different numbers.
Cast or extruded: decide before you quote
Acrylic sheet is made two ways, and they do not machine the same.
- Cast acrylic is polymerised in a mould. It is the better sheet for machining: it cuts to a cleaner edge, it is less inclined to soften and gum at the cutter, and it takes a polish well. It is the usual choice for thick sections and for parts with a lot of machined detail.
- Extruded acrylic is pushed through a die. It is generally cheaper and more uniform in thickness, but it softens more readily under the cutter and carries more built-in stress from the extrusion process, which makes it quicker to craze when solvents reach a machined edge.
If the part is mostly a cut-out with a few holes, extruded may be fine. If it has pockets, threads, tight internal corners or polished edges, say "cast" on the print.
At the cutter: chips one way, melts the other
Acrylic has two ways to ruin an edge, and they pull in opposite directions.
- Push too hard or use a dull edge and it chips. Brittle material fractures ahead of the tool, leaving a ragged, frosted edge with small conchoidal chips — each one a crack starter.
- Rub instead of cut and it melts. Acrylic softens at a modest temperature. A tool that is dull, turning too fast for the feed, or recutting its own chips builds heat, and the softened material welds back onto the edge or the tool.
What keeps it in the window:
- Sharp tooling, kept sharp. A worn edge is the most common cause of both failures.
- A steady, real chip load. Enough feed that the tool takes a chip and carries the heat away with it, without forcing the cut.
- Clear the chips. Air blast keeps chips out of the cut. If coolant is needed, use plain water, detergent in water, or a soluble oil the sheet maker lists as compatible with acrylic. Other cutting fluids can contain chemicals that attack acrylic, especially at a stressed edge.
- Support the work. Thin sheet vibrates and chatter chips the edge. Hold it flat and fully supported.
- Leave the masking on. The protective film on the sheet is cheap insurance against scratches from the fixture and the chips; peel it at final inspection.
Drilling: where acrylic parts are lost
A standard twist drill is ground for metal. Its sharp, positive cutting lips grab acrylic as the point breaks through the far side, pull the drill into the sheet, and crack it — often a crack you will not see until the part is in service.
- Modify the point. Drills for acrylic have the cutting lips ground to a zero rake angle, so the drill scrapes rather than grabs.
- Back up the work with scrap material so the exit side is supported.
- Ease off at breakthrough, and clear chips on deep holes so they do not pack and heat the hole wall.
- Keep holes away from edges. A hole close to an edge leaves a thin web that cracks under the first fastener.
- Do not overtighten hardware. Clamp loads concentrated at a hole are a common source of later cracks; washers and shoulder hardware spread the load.
Edge finishes, and what each one costs
Specify the edge you actually need. In rising order of effort:
- Machined edge. Clean and square, with a satin, tool-marked appearance. Right for edges that are hidden, bonded or framed.
- Sanded or scraped edge. Removes tool marks to a uniform matte finish.
- Buffed edge. Polished on a wheel after sanding; glossy, near-clear.
- Flame-polished edge. A quick pass with a flame glazes the edge to a clear, glossy finish. It is fast and it looks good, but it puts stress into the edge, and a flame-polished edge is a poor candidate for solvent bonding or exposure to cleaners. Do not flame-polish an edge that will be glued.
Crazing: the failure that arrives later
Crazing is a network of fine surface cracks that appears when a solvent reaches a stressed area of the part. Machining, drilling, flame polishing and forming all leave stress behind, and the edge you just made is where it concentrates. The solvent can be anything from a bonding cement to an aggressive cleaner, wiped on in the customer's own building.
- Anneal stressed parts. Heating the finished part slowly in an oven to below its softening point, holding it, and cooling it slowly relieves machining stress. Sheet makers publish annealing cycles for their grades; it is the standard step before solvent bonding and for heavily machined parts.
- Clean with mild soap and water. Keep solvents such as acetone, lacquer thinner and gasoline, and household window cleaners, away from acrylic. Use only cleaners the sheet maker approves for acrylic.
- Mind the downstream chemistry. Thread-lockers, some adhesives and some gasket materials will craze a stressed acrylic part. Put them on the drawing review, not the field-failure report.
Tolerances an acrylic part can hold
Acrylic is stiff and expands at roughly half the rate of PTFE or polyethylene, so it holds size better than the soft plastics. Two things still matter on the print:
- Temperature. At about six times steel, a 24" acrylic panel that warms 10 °F grows about 0.009" (24 in × 39 µin/in·°F × 10 °F). Panels mounted in metal frames need room to move.
- Sheet thickness. Cast sheet varies in thickness across the sheet more than extruded does. If thickness is a critical dimension, call it out so it can be machined to size rather than taken as supplied.
As with every plastic, say which dimensions are critical and put a sensible general tolerance on the rest.
When acrylic is the right call
- Display cases, retail fixtures and point-of-sale.
- Light diffusers, lenses, light guides and signage faces.
- Glazing, sight windows and viewing panels where impact is not the risk.
- Covers, guards and enclosures on non-impact applications, where clarity and appearance matter.
When it is not
- Anything that gets hit. Machine guards, safety glazing, anything a person could walk into: that is polycarbonate.
- Parts that carry a load at a hole or a snap fit. It is notch-sensitive and cracks rather than bends.
- Heat. Acrylic softens at a lower temperature than polycarbonate and the high-performance plastics; keep continuous service well below its 205 °F heat-deflection temperature.
- Solvent exposure in service. See crazing, above.
How we quote acrylic parts
Send the drawing and tell us what the part does, how the edges must look, whether it will be bonded, and how it will be cleaned. We fabricate acrylic sheet up to 96" × 48" and up to 4" thick — cutting, routing, drilling, bending and forming, and solvent or adhesive bonding — in Linden, NJ, from a single piece to production quantities. Properties and stock forms are on our acrylic material page, and our fabrication department covers bending and bonding.