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PMMA (Acrylic / Plexiglass) CNC Machining Material Manual

Cnc plastic materials
Last updated: May 23, 2026

PMMA (Acrylic / Plexiglass) — CNC Machining Material Manual

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Rating legend — ★★★★★ best · ★☆☆☆☆ worst. For machinability/wear/heat resistance more stars = better; for cost, fewer stars = cheaper.

📋 Material Quick-Reference Card

┌──────────────────────────────────────────┐
│  Material Name: PMMA (Acrylic)             │
│  (Plexiglass / Perspex / Lucite)           │
│  Category: Amorphous transparent plastic   │
│            (optical-grade thermoplastic)   │
│  Density: 1.17~1.20 g/cm³                  │
│  Tensile Strength: 50~77 MPa               │
│  Flexural Strength: 100~115 MPa            │
│  Hardness: M80~M100 (Rockwell M)           │
│  Service Temp.: 65~80 ℃ continuous         │
│  Machinability: ★★★★☆ (easy but brittle)  │
│  Optical Clarity: ★★★★★ (~92% light trans.)│
│  Chemical Resistance: ★★☆☆☆               │
│  Cost: ★★★☆☆ (moderate, optical value)   │
│  Keywords: transparent, weather-resistant, │
│            polishable, brittle, light guide│
└──────────────────────────────────────────┘

1. Material Overview

1.1 Introduction

PMMA (Polymethyl Methacrylate), commonly called acrylic or Plexiglass, is a high-transparency, weather-resistant, polishable engineering plastic used where optical appearance is more important than impact toughness. It transmits about 92% of visible light, often better than ordinary glass, while being lighter and easier to CNC machine.

  • English Name: Polymethyl Methacrylate / PMMA / Acrylic
  • Common Nicknames: Acrylic glass, Plexiglass, organic glass
  • Famous Brand Names: Plexiglas, Perspex, Lucite, Acrylite

1.2 Two Main Types ⭐ Important

Type Full Name Characteristics
Cast PMMA Cell-cast acrylic sheet/block Better CNC machinability, lower internal stress, better optical polishing, preferred for precision and optical parts ⭐ Common for CNC
Extruded PMMA Extruded acrylic sheet/rod Lower cost and more uniform thickness; softer, lower melting tendency, more residual stress, more prone to cracking/crazing during machining

💡 Cast PMMA is strongly preferred for CNC machining when optical edges, drilling, tapping, or flame polishing are required. Extruded PMMA is economical but must be machined more gently.

1.3 Raw Material Forms

Common forms for CNC machining:

  • PMMA Sheet/Plate: display panels, covers, light guides, windows
  • PMMA Rod/Tube: turned optical parts, spacers, lab components
  • PMMA Block: lenses, thick transparent housings, aquarium/window parts
  • Common colors: clear, opal/white, black, tinted transparent colors

2. Composition & Physical Properties

2.1 Material Composition

PMMA is an amorphous thermoplastic polymer made from methyl methacrylate monomer. Unlike crystalline plastics such as POM or nylon, PMMA has no sharp melting point; it softens gradually with heat. Its amorphous structure gives it excellent transparency and polishability but also makes it sensitive to internal stress and solvent crazing.

Type Molecular / Processing Structure
Cast PMMA Polymerized in molds/sheets; lower residual stress, better optical quality
Extruded PMMA Melt-extruded; more economical, but higher orientation and residual stress

2.2 Physical Properties

Property Value
Density 1.17~1.20 g/cm³
Melting Point No true melting point (amorphous; softens with heat)
Heat Deflection Temp. 95~105 ℃
Long-term Service Temp. 65~80 ℃
Thermal Conductivity 0.18~0.20 W/(m·K)
Water Absorption 0.2~0.4% (low)
Coefficient of Thermal Expansion 70~90×10⁻⁶ /℃
Light Transmission ~92% (clear grade)

💡 PMMA is one of the clearest machinable plastics. However, transparency also exposes every tool mark, chip, crack, and scratch, so handling and finishing discipline are critical.

3. Mechanical & Chemical Properties

3.1 Mechanical Properties

Property Value
Tensile Strength 50~77 MPa
Flexural Strength 100~115 MPa
Elastic Modulus 2400~3300 MPa
Elongation 2~7%
Hardness M80~M100 (Rockwell M)
Impact Strength Low to moderate; brittle and notch-sensitive
Coefficient of Friction 0.4~0.6

⚠️ PMMA is stiff and strong, but not tough. For impact-loaded transparent parts, choose PC instead of PMMA.

3.2 Chemical Resistance

Medium Resistance
Water, dilute acids, dilute bases ✅ Good
UV light / outdoor weathering ✅ Excellent
Mineral oils, aliphatic hydrocarbons ⚠️ Fair to good
Alcohols ⚠️ Fair to poor; may cause crazing under stress
Ketones, esters, aromatics, chlorinated solvents ❌ Poor; swelling, cracking, or crazing likely
Acetone, many paint thinners, aggressive cleaners ❌ Poor; rapid surface damage

3.3 Notable Characteristics

  • Excellent optical clarity: about 92% light transmission, suitable for display and light-guide parts
  • Excellent UV/weather resistance: better outdoor aging performance than many transparent plastics
  • Highly polishable: machined edges can be restored to optical clarity by polishing
  • Brittle and notch-sensitive: sharp internal corners, aggressive drilling, or poor clamping can initiate cracks
  • Solvent-crazing sensitive: residual stress plus solvent exposure can create white cracks/crazing

4. CNC Machining Process ⭐⭐ Core

4.1 Machinability Rating

★★★★☆ Easy to cut, but brittle — PMMA machines cleanly when tooling is sharp and heat is controlled:

  • Produces transparent, crisp edges with proper tools and feeds
  • Cast PMMA can be machined and polished to near optical clarity
  • High RPM and sharp single-flute/O-flute tools reduce cutting load and chipping
  • Main risks are edge chipping, cracking, melting/gumming, and solvent/stress crazing
Item Recommendation
Tool Material Polished carbide preferred; very sharp HSS acceptable for light work
Cutting Edge Extremely sharp, polished edge; avoid worn or honed tools
Rake Angle Positive rake (10°~20°) to shear cleanly and reduce heat
Helix Angle O-flute or high-helix geometry for chip evacuation
Flutes Single-flute or 2-flute; large chip pocket, especially for routing/milling
Operation Spindle Speed (RPM) Feed Rate (mm/min) Depth of Cut (mm)
Rough Milling 12000~24000 800~2500 0.5~3
Finish Milling 16000~30000 400~1500 0.1~0.5
Turning 1000~3000 0.05~0.2/rev 0.2~1.5
Drilling 1000~4000 30~150 —

📌 Parameters are for reference only; adjust based on machine rigidity, tool diameter, part geometry, acrylic grade, and whether the stock is cast or extruded.

4.4 Machining Challenges & Solutions

Challenge Cause Solution
Edge chipping Brittle material, dull tool, excessive feed, unsupported exit edge Use sharp O-flute/single-flute tools, climb milling, support the exit side, reduce chip load
Cracking during drilling/tapping Wedge action, high stress concentration, no relief Use plastic-point drills, peck drilling, large clearance, pre-anneal, avoid forcing taps
Melting / gumming Heat buildup, rubbing, poor chip evacuation Increase chip load slightly, use air blast, reduce dwell, use polished flutes
Crazing after machining Residual stress plus solvent/coolant exposure Anneal before machining, avoid alcohol/solvent coolants, use dry air or compatible coolant only
Scratches on optical surfaces Poor handling, chips trapped under film or clamps Keep protective film on, use soft jaws, clean chips frequently, avoid sliding parts on the table

4.5 Annealing Recommendation ⭐

To reduce cracking and solvent crazing, annealing is recommended before machining thick, drilled, threaded, polished, or precision PMMA parts:

Reference Annealing Process:
• Temperature: approx. 80 ℃
• Time: approx. 2~4 hours for thin parts; longer for thick sections
• Cooling: slow cooling in oven or still air (avoid rapid temperature shock)

💡 For high-clarity parts, the annealing → rough machining → stress relief if needed → finish machining → polishing workflow reduces delayed cracks and improves optical quality.

4.6 Cooling Methods

  • Air blast: most common and safest; removes chips without solvent exposure
  • Dry machining: acceptable if chip evacuation is excellent and the tool is sharp
  • Water mist / compatible coolant: possible for heat control, but verify compatibility first
  • ❌ Avoid alcohol, acetone, chlorinated solvent, or aggressive cleaner contact during/after machining

4.7 Polishing & Optical Edge Restoration ⭐

PMMA is one of the best plastics for restoring machined edges to clear optical appearance:

Process Use Notes
Mechanical polishing Flat edges, optical windows, visible surfaces Sand progressively, then buff with plastic polishing compound
Flame polishing Quick clear edge on cast acrylic Fast and attractive, but can add surface stress; not ideal before solvent exposure
Vapor polishing Complex edges, high clarity Dichloromethane/methylene chloride vapor can produce clear edges but requires strict ventilation/PPE
Diamond machining Optical parts, lenses, light guides Best surface quality when equipment is available

⚠️ Flame or vapor polishing can make parts look perfect immediately but may increase or reveal stress. For critical transparent parts, anneal before and/or after polishing according to the part requirement.

5. Surface Treatment

PMMA has excellent natural appearance, so most surface treatment focuses on polishing, marking, printing, and protecting optical surfaces:

Process Feasibility Notes
Mechanical polishing ✅ Excellent Can achieve optical clarity on edges and faces
Flame polishing ✅ Excellent for cast PMMA Very fast edge clearing; operator skill required to avoid bubbles, stress, or burning
Vapor polishing ✅ Excellent but controlled Dichloromethane can restore clarity; ventilation and PPE are mandatory
Laser cutting / laser marking ✅ Excellent Produces glossy edges; marking is common on signs and panels
Screen printing / UV printing ✅ Good Common for display panels, scales, logos, and covers
Painting ⚠️ Feasible with compatible paint Test first; solvents in paint can craze acrylic
Solvent bonding ✅ Excellent Produces clear joints; requires stress control and compatible cement
Electroplating ❌ Not typical Choose ABS if plating is required

💡 PMMA is normally selected because the material itself is beautiful and transparent. Protect the surface film until the final operation whenever possible.

6. Applications & Material Selection

6.1 Typical Application Industries

Industry Application Parts
Display / POP / retail Display stands, product covers, nameplates, transparent panels
Lighting / optics Light guides, diffuser plates, lenses, LED covers
Electronics enclosures Transparent covers, windows, instrument panels
Signage / advertising Letters, signs, illuminated panels, plaques
Aquarium / architectural Aquariums, viewing windows, protective barriers
Medical / laboratory Transparent guards, sample holders, lab fixtures, inspection windows
Automotive / transportation Interior light covers, indicator lenses, decorative transparent parts

6.2 Pros & Cons Summary

✅ Advantages ❌ Disadvantages
Excellent transparency (~92% light transmission) Brittle, notch-sensitive, poor impact resistance vs PC
Excellent UV and outdoor weather resistance Prone to chipping/cracking if machined incorrectly
Easy CNC machining with sharp tools Heat buildup causes melting, gumming, and poor edges
Edges can be polished to optical clarity Poor resistance to many solvents and cleaners
Lightweight alternative to glass Solvent crazing risk under residual stress
Good surface appearance and printable surface Limited continuous service temperature (65~80℃)

6.3 Material Selection Guide

✔ Recommended for PMMA:

  • Transparent covers, display windows, signage, and POP/display components
  • Light guides, lenses, LED covers, and parts requiring high optical clarity
  • Outdoor transparent parts requiring excellent UV/weather resistance
  • Parts that need polished, glossy, glass-like edges
  • Medical/lab transparent guards or fixtures with low impact load

✘ Not recommended for:

  • High impact resistance → choose PC
  • High temperature (>90℃ continuous) → choose PEEK, PPS, or high-temperature PC grades
  • Solvent-rich environments, acetone/alcohol cleaning, or paint-thinner exposure → choose PC, PP, PTFE, or PVDF depending on the chemical
  • Flexible living hinges or snap-fits → choose PP, POM, or nylon
  • Parts requiring electroplating → choose ABS

⚠️ Safety & Handling Notes

Hazard Detail Precaution
Combustible dust Fine PMMA chips/dust can be combustible, especially in dry collection systems Use dust extraction, avoid ignition sources, clean dust accumulation regularly
Flammability PMMA burns readily and supports combustion; it burns relatively cleanly with no halogen acid gases Keep away from open flame and hot chips; do not allow smoldering in the cut
Solvent polishing Dichloromethane/methylene chloride vapor is hazardous and can be absorbed by inhalation/skin contact Use local exhaust ventilation, chemical gloves, eye protection, and approved procedures only
Static PMMA is static-prone and attracts dust/chips to optical surfaces Use ionized air or antistatic cleaning methods; avoid wiping dry with abrasive cloth
Surface scratching Clear acrylic scratches easily, and scratches are highly visible Keep protective film on, use soft jaws/clean fixtures, handle with gloves
Crazing risk Alcohols, acetone, and many solvents can cause white cracks under stress Avoid incompatible cleaners/coolants; anneal stressed parts before bonding or polishing

⚠️ Never use aggressive solvent cleaners on stressed PMMA. A part may look acceptable after machining, but alcohol, acetone, or vapor exposure can trigger delayed crazing and cracking.

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