What is crazing in acrylic?

Key facts about crazing

• What it is: a network of fine cracks on or slightly under the surface. Sheet makers call the mechanism environmental stress cracking.
• Two ingredients: stress (from cutting, drilling, bending, clamping, or load) plus a chemical agent. Either alone usually does nothing visible.
• Laser-cut edges carry high stress. Acrylite's data sheet says a CO2 laser leaves a polished edge "with high stress levels" and recommends annealing after laser cutting.
• Load alone can craze it over time. Acrylite rates cast sheet at 10,000 psi tensile strength but advises keeping continuous design loads under 1,500 psi, because crazing can appear below the breaking stress.
• Annealing schedules vary by maker: Acrylite anneals at 176 to 180 °F (80 to 82 °C); Perspex cast at 194 °F (90 °C), 2 hours for sheet up to 1/4 in. (6 mm).
• It's permanent. Annealing prevents new crazing but doesn't close cracks that have already formed.

What does crazing look like?

Crazing shows up as many tiny, shiny cracks, often parallel or in a branching web, that catch the light when you tilt the part. Up close they're separate fine fissures rather than one big crack. Common patterns:

• Halos around holes and screws, radiating outward.
• A frosty band along a cut edge, especially laser-cut edges after gluing or wiping with sanitizer.
• Lines along a bend, on the outside of the fold.
• A cloudy patch where a cleaner, paint, or sealant touched the sheet, sometimes appearing hours or days later.

What causes crazing?

Both Perspex and Plexiglas describe the same recipe: stressed acrylic plus a solvent, solvent vapor, cement, ink, or other incompatible material. Remove either half and crazing is much less likely.

Sources of stress

• Laser cutting. The beam vaporizes a narrow kerf and the edge cools fast, locking stress into a thin zone. That's why laser-cut parts craze along the edges when they meet alcohol or solvent cement. See why laser-cut acrylic cracks later.
• Machining and drilling heat. Perspex warns that heat from machining, drilling, and cutting builds stress, and Acrylite recommends cooling the tool. How to drill acrylic without cracking.
• Overtightened fasteners and screws without clearance, which squeeze the plastic and stop it from expanding. A 48 in. panel grows and shrinks about 3/16 to 0.2 in. over a 100 °F swing.
• Bending or thermoforming too cold, or cold-forcing a flat sheet into a curve.
• Constant load, like a shelf carrying weight for years or a panel bowed into a frame.

Chemical agents

Sheet makers' chemical-resistance tables are the best guide. These come from Acrylite's cast sheet table and the Perspex Design Guide's one-year immersion tests at 68 °F (20 °C):

The ammonia result surprises people. Ammonia itself isn't what the makers warn about. Acrylite and Perspex both say not to use window-cleaning sprays, which are a blend of ingredients, and they list alcohol and solvents among the things to avoid. Treat Windex-type glass cleaners as off-limits and clean with mild soap and water. Our acrylic cleaning guide covers safe products.

How do you prevent crazing?

1. Clean with mild soap and lukewarm water or a cleaner made for acrylic, such as Novus 1 or Brillianize. No glass cleaner sprays, no strong alcohol, no solvents.
2. Drill oversize holes so the sheet can move, use washers, and tighten to snug. Acrylite says large sheets should sit in frames rather than be bolted through.
3. Use sealants, tapes, and gaskets labeled compatible with acrylic: neutral-cure silicone, and butyl or compatible EPDM gaskets, per Perspex. Test anything new on an offcut.
4. Choose the right edge. If a part will be solvent-bonded or wiped with harsh chemicals, a machined edge holds less stress than a laser edge.
5. Anneal stressed parts before gluing, painting, or screen printing (below).
6. Bend at the right temperature, heating the bend line evenly rather than forcing it. How to bend acrylic.

What is annealing, and how do you anneal acrylic?

Annealing means heating acrylic below its forming temperature, holding it long enough for the core to reach temperature, then cooling it slowly so internal stress relaxes. Use a forced-air oven, keep parts clean and dry with masking removed, and support them flat so they aren't stressed while hot. Makers' schedules differ:

Two practical notes from the makers: let solvent-cemented parts sit at least six hours before annealing so trapped solvent doesn't form bubbles (Acrylite), and if a part distorts, drop to a lower temperature and hold longer. Plexiglas adds that annealing raises the strength of some cemented joints and should come after all fabrication steps, including polishing.

Annealing is standard practice for parts that will be solvent-bonded, like aquariums and display cases. It won't undo crazing that already happened.

Can crazing be fixed?

Not really. Crazes are physical cracks inside the plastic, so polishing the surface doesn't remove them. A few cases are partly salvageable:

• Very shallow surface crazing from a cleaner can sometimes be sanded and polished out, because it only goes a few thousandths of an inch deep.
• Crazing along an edge can sometimes be removed by trimming or routing a little material off, if the part can lose a fraction of an inch.

Otherwise, replace the part, especially if it's structural or holds water. We can cut a replacement from your old part or drawing; get a price in the instant quote or upload a drawing.

Questions

Sources

• Röhm (Acrylite): ACRYLITE cell cast sheet (GP) physical properties and chemical resistance
• Acrylite: What is annealing and what are the procedures for annealing acrylic sheet?
• Perspex: Design Guide (stress, annealing, sealing, chemical resistance table)
• Acrylite: Clean, disinfect and maintain ACRYLITE acrylic sheet