Acrylic is the material most clients specify with a single word: plexi. Two sheets of the same thickness and the same clear look will behave differently on the router, in the glue line and on a wall that faces the afternoon sun. Almost every acrylic failure I am asked to look at started as a specification decision taken weeks before the first cut: the wrong sheet type, a hole drilled with a metal bit, or a solvent brushed onto an edge that was still full of stress.
Cast or extruded: decide before you draw
Cast and extruded acrylic are the same polymer, polymethyl methacrylate. Casting gives a higher-molecular-weight sheet; extrusion pushes the same chemistry through a die and leaves some internal stress behind. In the trade they are usually called GS and XT, and a supplier will quote both for "3 mm clear".
| Cast | Extruded | |
|---|---|---|
| Solid sheet thickness | 2 to 160 mm | 1.5 to 25 mm |
| Thickness tolerance | allowance grows with thickness, roughly four tenths of a millimetre plus a tenth per millimetre of thickness | ±10 % up to 3 mm, ±5 % above |
| Maximum permanent service temperature | about 80 °C | about 70 °C |
| Thermoforming temperature | 160 to 175 °C | 150 to 160 °C |
| Forming behaviour | easy over a wide window | very easy, but only in a narrow constant window |
| Bonding | reaction (two-part) adhesives | also solvent cement |
| Standard colours | more than 40 | more than 20 |
Both are clear at about 92 % light transmission, both weigh 1.19 g per cubic centimetre, and both move the same amount with temperature, so the choice is not about looks. It is about what happens next in the workshop. Thermoformed letters, deep returns, polished edges, engraving and anything over 25 mm go to cast. Flat printed faces, glazing panels, thin parts that get solvent-cemented and budget jobs go to extruded, which is also the sheet with the honest thickness: a 3 mm extruded sheet is held to three tenths, a 3 mm cast sheet is allowed seven. If a part has to slide into a slot or a channel letter return, I size the slot from the sheet in front of me, never from the nominal number.
One figure governs everything else: acrylic stretches about 5 % before it breaks, and it does not yield first. Steel bends and warns you, acrylic cracks. Impact-modified grades do take a knock, but they are less rigid and move more with heat, so they are a decision, not a free upgrade.
Movement: five millimetres per metre
Acrylic expands 0.07 mm per metre for every degree. A one-metre part changes length by 1.4 mm between a cold morning at 10 °C and a warm afternoon at 30 °C, which is why two "identical" parts measured in the yard and in the workshop are not identical. Add the moisture the sheet picks up, and the working allowance is 5 mm per metre. A 2.5 m shopfront face needs 12 mm somewhere to go.
That allowance has to be designed in, not hoped for:
- clearance holes oversized and the fixing shouldered, so the screw holds the sheet without pinching it;
- fixings tight at one point and floating everywhere else, so the panel grows from a known datum;
- no continuous bead of hard sealant that locks a face into its frame;
- dark faces in direct sun run far hotter than the air around them, and an 80 °C or 70 °C ceiling is not generous once a sealed light box is involved;
- panel joints wide enough to still be joints in August.
Holes: a metal drill is the wrong tool
A twist drill ground for steel will grab, melt and chip its way out of an acrylic sheet. The geometry has to be reground: the point angle comes down from the usual 120° to between 60° and 90°, the rake angle goes to between 4° and 0°, the clearance angle stays at 3° or more. The drill should scrape rather than cut, which is what stops the hole blowing out on the exit side. From 5 mm upwards, use a coolant or an acrylic-compatible drilling emulsion, and back thin sheet with a flat solid support.
The rest is detail that costs nothing and saves parts:
- chamfer or countersink every hole, because a sharp hole edge is a notch and acrylic cracks from notches;
- predrill the corners of any cut-out or recess before routing into them;
- mark out on the masking film, never with a scriber or a punch;
- keep tools for acrylic only, and keep them sharp: a blunt cutter puts heat and stress into the edge;
- avoid cut threads altogether in extruded sheet; fasten through a hole, clamp, bond, or fit a metal threaded insert, in that order of preference.
Stress plus solvent equals crazing
This is the failure that arrives weeks late and looks like a material defect. Acrylic tolerates considerable tension on its own, and it tolerates most of the chemicals a sign shop uses on its own. Put the two together and the surface fills with fine cracks.
Tension comes from sawing, routing, turning and sanding, from line bending, from uneven heating, from adhesive shrinkage, from clamping and drilling, from a badly ground tool overheating one spot, from blocked expansion, and in extruded sheet from the extrusion itself. The media that then attack it are everyday: solvents and thinners in bonding, printing and painting, monomer vapour from laser cutting and flame polishing, plasticisers migrating out of PVC insulation and films, some sealants, and aggressive cleaners. A stress-free part shrugs all of that off.
So the rule I apply is simple: if a machined or laser-cut edge is going to meet a solvent, it gets annealed first. Heat cast sheet to 80 °C, extruded to between 70 °C and 80 °C, hold it for the thickness in millimetres divided by three in hours with a minimum of two hours, then cool it slowly, no faster than 15 °C per hour, and do not take it out of the oven above 60 °C. Sheet up to 3 mm that was scored and snapped carries almost no stress and needs none of this. A routed, laser-cut, glued and then sprayed part needs all of it.
Light: specify the grade, not "white plexi"
An illuminated face is an optical component. The diffusing grades made for backlighting and for edge lighting exist precisely because a face has to hide the LEDs without eating the output. Thickness variation shows up here too: an uneven sheet reads as a dirty, blotchy sign once it is lit. A lit face belongs on the drawing with its grade and transmission, the cavity depth and the LED pitch it was chosen for, not as "opal plexi".
Fire: it burns like hardwood
Acrylic burns roughly like hardwood, with very little smoke and with combustion gases that are neither toxic nor corrosive, and it ignites in the region of 425 °C. That is a fair description for a sign face or a decorative detail. It is not a description that belongs in a lining to a corridor, a stair or a ceiling in a public building without the fire designer saying so first, in Ireland as in Poland. Swapping to polycarbonate is not a silent substitution either: it takes an impact that would crack acrylic, but it scratches more easily, it needs its own surface protection outdoors, and the whole fixing and movement design changes with it.
What to put on the drawing
- cast or extruded, stated, not implied by a thickness;
- grade and colour reference, and for a lit face the light transmission;
- edge condition: sawn, routed, flame polished or hand polished;
- annealing where a machined edge will be bonded, printed or painted;
- hole diameters with the movement allowance, and which fixing is the fixed datum;
- 5 mm per metre of movement in the joints, with the detail that absorbs it;
- fire class, confirmed with whoever is responsible for it.
None of that takes longer than writing "3 mm plexi" on a layout, and it is the difference between a face that still looks new in its third summer and one that crazes after the first service visit with the wrong cleaner. Making those decisions at drawing stage rather than on site is part of what I do in production support.
