Machining Acrylic Without Crazing: What Actually Decides the Result
News
Acrylic — polymethyl methacrylate, or PMMA — is one of the most practical plastics to machine and one of the easiest to ruin. It cuts cleanly at the right feed and spindle speed, and crazes, chips or melts at the wrong ones. We machine acrylic components to drawing, then deburr, polish and — where the drawing calls for it — anneal them, so the finished part looks the way the drawing intended rather than like a part that survived the process.
What we make in acrylic
- Clear display parts, point-of-sale fixtures, lens covers and light guides
- Machine guards, safety shields and inspection windows
- Fluidic manifolds, jigs and lab fixtures for prototype builds
- Signage blanks, bezels and mounting plates
- Bonded acrylic assemblies with joints that are meant to disappear
Why acrylic is not machined like metal
Three properties change how the job has to be run:
- Heat is the limiting factor, not tool wear. Acrylic softens and then melts onto the cutter. Feed rates, spindle speed and coolant have to be set for chip clearance rather than for removing material as fast as possible.
- Tool marks are permanent. A scratch or a machining line on a clear part cannot be sanded out without changing the surface geometry. Cutting strategy is decided before the first pass, not corrected afterwards.
- Internal stress shows up later. Machining releases stress locked into the sheet. A part that measures correctly on the bench can bow or craze days later. Where the geometry is sensitive we relieve stress by annealing.
Materials and finishes
We machine cast and extruded PMMA sheet and rod, and routinely combine acrylic with the other plastics in our range — POM and PEEK — where a part needs a bearing surface or higher temperature resistance.
Available finishing steps include machined-edge polishing to restore optical clarity, sandblasting for a matte or diffusing surface, and bonding with solvent cements matched to the material grade.
Standard capability
| Item | What we work to |
|---|---|
| Tolerance | ±0.05 mm on machined features |
| Materials | Cast and extruded PMMA, plus POM and PEEK for mixed assemblies |
| Surface finish | Polished, sandblasted, or left as machined |
| Order size | 1 pc prototype through to volume production |
| Quality system | ISO 9001 / ISO-TS 16949, in-house dimensional inspection |
How an order runs
- You send a drawing (STEP, DXF or a dimensioned PDF) with the material grade and the finish you need.
- We review it for machinability and quote within one business day.
- On approval we machine the sample or first batch and inspect it in-house before it ships.
- Mass production follows the same process and inspection steps as the approved sample.
Questions buyers ask
Can you match the clarity of a moulded part?
On flat faces and machined edges, yes — polishing brings the surface back to optical clarity. On complex internal geometry, machined acrylic will not match an injection-moulded part, because the tool cannot reach everywhere. Where clarity is critical, send the drawing and we will tell you which surfaces can be polished and which cannot.
Will the part craze after machining?
Crazing comes from residual stress plus solvent contact. Annealing after machining and using the correct cement grade for the material both reduce the risk substantially. If the part will be exposed to cleaning solvents in service, say so when you send the drawing — it changes the material recommendation.
Do you supply the sheet or do we?
Either. We can source cast or extruded PMMA to the grade on your drawing, or machine material you supply. If you supply, please confirm the sheet has been stored flat and dry — warped stock cannot be machined to a flatness callout.
What do you need to quote?
A drawing, the quantity, the material grade and the finish. A photo of a similar part helps us flag any feature that will be difficult before the quote rather than after.