Acrylic (PMMA) CNC Machining

Clarity is the specification

Acrylic is chosen for what it lets through. At around 92% light transmission in the visible range it is clearer than glass, and at a density of about 1.19 g/cm³ it is roughly half the weight. For a guard, a window, a light guide or a display, those two numbers are the reason the material is on the drawing, and everything else is a constraint to be managed.

The properties that support the clarity are real:

Property	Roughly	What it means
Light transmission	~92%	Clearer than glass in the visible range
Density	~1.19 g/cm³	About half the weight of glass
Tensile strength	~70 MPa	Reasonably strong for a plastic
Modulus	~3,000–3,300 MPa	Stiff — noticeably stiffer than ABS or polypropylene
Service temperature	~80 °C	Adequate for indoor and display service

And the properties that constrain it are equally real:

It is brittle. Acrylic does not yield and deform the way most plastics do; it cracks. That makes it notch sensitive — a sharp internal corner, a machining mark or a rough drilled edge concentrates stress, and the material has little capacity to redistribute it.

It crazes. Under stress, or in contact with a solvent, fine cracks appear at the surface and spread. This is the characteristic acrylic failure, and it is what most of the practical guidance about the material is designed to prevent.

Stress is the enemy, and annealing is the answer

A machined acrylic part carries internal stress. Cutting forces, clamping pressure, and the thermal gradient from friction all leave stress locked in the material, and the part may look perfect and measure correctly while holding it.

The problem shows up later. Expose the stressed part to a solvent, to a warm environment, or to a load, and the stress finds a release: crazing — a network of fine surface cracks — which spreads into a fracture.

Annealing is the standard preventive step. It is a controlled heat cycle — bringing the part up to around 80 °C, holding it long enough for the chains to relax, then cooling it very slowly. The slow cooling matters as much as the heating: a fast cool re-introduces a thermal gradient and puts stress back in.

On a visible, stressed or solvent-exposed acrylic part, annealing is not a refinement. It is part of making the part correctly, and a drawing that specifies acrylic without considering it is a drawing that will produce a part that fails in service for no visible reason.

The other half of stress control happens during machining:

• Radius sharp internal corners wherever the design allows. A sharp corner in acrylic is a crack initiation site.
• Keep clamping gentle. Acrylic marks and stresses under clamping pressure, and clamp stress is itself a crazing risk.
• Support the work. Acrylic chatters and chips where it is unsupported, and a chipped edge is both a defect and a stress raiser.
• Control the heat. The material melts and smears if the cut runs hot, and a smeared surface is an optically unacceptable finish.

Cast versus extruded

This distinction matters more in acrylic than in most plastics, and it decides whether a job goes well.

	Cast acrylic	Extruded acrylic
Molecular weight	Higher	Lower
Machining	Cleaner cuts, better optical finish	Gummier, more smearing
Solvent and crazing resistance	Better	Lower
Thickness range	Wider, including thick sections	Generally thinner
Cost	Higher	Lower

For a machined part with an optical requirement — a light guide, a lens, a window with a visible edge — specify cast. The extra material cost is small against the cost of a part that will not polish or that crazes in service.

Machining acrylic

The core discipline is the same as for any plastic — sharp high-rake tooling, high spindle speeds, moderate feeds, clear chip evacuation, air blast rather than heavy coolant — with three acrylic-specific cautions.

Heat control is critical. Acrylic has a low softening point and poor thermal conductivity, so friction heat stays at the edge, and a hot cut produces a smeared, cloudy surface that cannot be polished out. Sharp tooling and a fast, light cut are the answer.

Tool geometry drives the finish. On an optical part the tool geometry and the finishing pass determine whether the part is transparent at the cut edge or cloudy. This is one of the few plastic operations where a specific finishing pass is specified for optical rather than dimensional reasons.

Drilling needs care. Acrylic cracks from drilled holes more readily than most materials. The standard practice is a sharp drill with the correct point geometry, a modest feed, support behind the exit face so the material does not blow out, and a chamfer or radius on the hole edge afterwards to remove the stress raiser.

One further point on handling: protective film stays on until the last possible moment. Acrylic scratches in handling, and a scratch on an optical part is a permanent defect — the film is not packaging, it is part of the process.

Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold.

Where acrylic is the right choice

• Machine guards and windows, where clarity and light weight are wanted
• Light pipes and light guides, where the material’s transmission and edge glow are the function
• Displays, signage and point-of-sale components
• Lenses, covers and optical windows
• Fluidic manifolds, lab-on-chip and microfluidic components, where transparency lets the flow be seen
• Cosmetic panels and bezels where a glossy, clear or coloured finish matters
• Models and prototypes that need to look like the production part
• Any part that has to transmit light, which is a requirement few other engineering plastics meet

Where acrylic is the wrong choice

The part has to absorb impact. Acrylic is brittle and shatters. Where a guard has to stop something rather than merely enclose it, the material is polycarbonate — far tougher, at a cost.

The part meets alcohol-based cleaners or sanitisers. This catches people out constantly, because alcohol wipes are the default in so many environments. Alcohol attacks acrylic and causes crazing, especially on a stressed part. Name the cleaning regime at the enquiry.

The part meets ketones, esters or chlorinated solvents. All attack it. Name the chemical.

The service temperature is above roughly 80 °C. Acrylic softens and distorts.

A sharp internal corner is unavoidable and the part is stressed. Acrylic has little tolerance for stress concentration. Where the geometry cannot be radiused and the part carries load, a different material is the safer answer.

The part needs to be both clear and tough. That combination is polycarbonate’s territory, not acrylic’s.

Finishing acrylic

Acrylic finishes well, and on optical parts the finishing route is part of the specification.

• Annealing — the stress-relief cycle described above. On a machined optical part this is a process step, not an option
• Flame polishing — a flame or hot-air pass that melts the surface layer so it flows smooth and clear. Effective on edges and on simple forms
• Vapour polishing — a solvent vapour that softens the surface. Done through our vetted partner network, with proper solvent handling
• Mechanical polishing — for flat faces and simple geometry, working through grades to an optical finish
• Solvent cementing — for assembly; the joint can be optically clear with good face preparation
• Sand blasting — a uniform matte or frosted finish, used where diffusion rather than transmission is wanted
• Painting and screen printing — acrylic takes both readily

One caution on cleaning and packaging, because it is where finished optical parts get spoiled: a mild soap solution and a soft cloth is the safe cleaning method. Solvent cleaners, alcohol wipes and abrasive cloths all damage the surface, and the damage is not recoverable. Finished parts are shipped with film in place for the same reason.

All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.

What to put on the drawing

Instead of	Write
Acrylic	Acrylic (PMMA), and specify cast for a machined optical part
Acrylic, machined	Say whether the cut edge must be optically clear
Acrylic, stressed or visible part	Specify annealing — stress left in the part becomes crazing later
Acrylic, sharp internal corners	Radius them — a sharp corner is a crack initiation site
Acrylic, cleaning regime	Name it — alcohol-based cleaners cause crazing
Acrylic, meets ketones or solvents	Wrong material — those chemicals attack it
Acrylic, must absorb impact	Wrong material — polycarbonate is the impact plastic
Acrylic above ~80 °C	Wrong material — it softens and distorts

The three lines that decide most acrylic specifications are whether the cut edge must be optically clear, whether the part is stressed or solvent-exposed in service, and whether it has to absorb impact. The first sets the material grade and the finishing route. The second decides whether annealing is required. The third decides whether the part belongs in acrylic at all.

Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade and stock form to its mill lot plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the cast or extruded choice, the optical requirement and the annealing step where any of them need settling.