Acrylic (PMMA, sold under names like Plexiglas® and Lucite®) is the most requested transparent engineering plastic for machined parts — and also the one most often ruined by careless shops. Machined right, it gives you 92% light transmission (clearer than glass), crisp polished edges and dimensional stability. Machined wrong, it cracks at the drill exit, crazes around screws, or arrives with hazy, chip-welded edges.
This guide covers what buyers should specify for CNC-machined acrylic parts, how cast and extruded sheet differ, and how our shop avoids the classic PMMA failure modes.
Acrylic (PMMA) Key Properties
| Property | Typical value (cast PMMA) | Why it matters |
|---|---|---|
| Light transmission (clear, 3 mm) | ≈ 92% | Higher than glass — ideal for displays & light guides |
| Density | ≈ 1.18 g/cm³ | Half the weight of glass |
| Continuous service temperature | ≈ 70 – 80 °C | Not for hot zones — choose PC there |
| Heat deflection temperature | ≈ 95 °C | Limit for loaded parts |
| Water absorption | ≈ 0.3% | Stable in humid environments |
| Refractive index | 1.49 | Predictable optics for light guides |
| Surface hardness | Higher than PC | Better scratch resistance for exposed panels |
Cast vs Extruded Acrylic for Machining
This is the first decision we make when quoting, and it changes both price and outcome:
| Aspect | Cast acrylic | Extruded acrylic |
|---|---|---|
| Internal stress | Lower — machines cleanly, holds flatness | Higher — prone to stress cracking if abused |
| Thickness tolerance | Looser (sheet-to-sheet variation) | Tighter and more uniform |
| Machining quality | Preferred for CNC parts | Acceptable for simple cut parts |
| Laser cutting | Good | Good |
| Cost | Higher | Lower (especially thin gauges) |
Shop recommendation: for CNC-machined parts with holes, pockets or polished edges, we default to cast. Extruded wins for high-volume laser-cut panels where uniform thickness matters more than machinability.
Where Machined Acrylic Parts Are Used
- Light guide panels (LGP) and diffusers for LED lighting and displays
- Inspection covers and sight windows on equipment
- Display stands, cosmetic front panels and signage
- Device windows where scratch resistance matters more than impact strength
- Prototyping and fixtures needing a transparent, rigid view of the work
How We Machine Acrylic Without Cracks
PMMA is brittle and heat-sensitive — the failure modes are well known and fully controllable:
- Sharp, plastic-grade tooling — polished single-flute/O-flute cutters; dull tools tear the surface and generate heat.
- Heat control — moderate feeds with air blast or coolant to prevent chip welding and melting.
- Stress relief (annealing) before precision machining and before bonding — the single best prevention against stress crazing around screws and solvent adhesives.
- Drilling discipline — peck cycles, proper point geometry, and a backing board so the bit exits cleanly instead of grabbing and chipping the last layer.
- Edge finishing — flame polishing or diamond-edge polishing for optically glossy transparent edges; specify which you want at RFQ.
Realistic tolerances for machined acrylic
| Feature | Standard | Precision option |
|---|---|---|
| Thickness (machined to size) | ±0.15 mm | ±0.05 mm |
| Hole diameter | ±0.10 mm | ±0.05 mm |
| Profile / outline | ±0.20 mm | ±0.10 mm |
| Flatness (per 100 mm) | 0.4 mm | 0.15 mm (stress-relieved) |
Acrylic vs Polycarbonate: The 30-Second Answer
Buyers constantly ask which clear sheet to use. Short version:
| Requirement | Winner |
|---|---|
| Impact strength (guards, protective windows) | Polycarbonate — by a huge margin |
| Scratch resistance, long-lasting clarity | Acrylic |
| Outdoor UV stability (no yellowing) | Acrylic |
| Higher temperature (up to ~120 °C) | Polycarbonate |
| Cosmetic transparent edge polish | Acrylic |
Full details in our Polycarbonate (PC) sheet guide — and if the part is structural and must insulate electrically, see the FR4 fiberglass sheet guide instead.
FAQ
Q: Can acrylic be CNC machined at all, or is it only laser-cut? A: Fully machinable — milling, drilling, threading, engraving and edge polishing. Laser cutting is just faster for flat panels without depth features.
Q: Why did my last supplier's acrylic parts crack around the screws weeks later? A: Almost always residual machining stress plus solvent exposure — fixable with correct tooling, annealing, and torque-friendly hole design. This is a process problem, not a material problem.
Q: Can you polish edges on every part? A: Yes, flame or diamond polishing on straight edges. Complex contoured edges are hand-finished — flag it at RFQ.
Q: Do you supply colored or frosted acrylic? A: Yes — clear, opal/white, black, tinted and matte finishes, subject to sheet availability.
Work With the Fabricator Directly
ChaoRong Electronics (潮荣电子， Dongguan, China) CNC-machines acrylic, PC, FR4, carbon fiber and electrical board into finished parts — display components, insulation plates and precision panels — from prototypes to scheduled volumes.
→ Send your drawing for an acrylic machining quote · Acrylic product page
Related reading: Polycarbonate (PC) Sheet Guide · FR4 Fiberglass Sheet Guide · Carbon Fiber Board vs FR4 vs Phenolic