Material selection
The two sheets are the same polymer made two ways. On the shelf they are indistinguishable; machined, bonded, polished and left in service, they behave differently enough to decide whether a part lasts.
Cast acrylic is made by pouring monomer between glass plates and polymerising it slowly. Extruded acrylic is pushed through a die as a continuous ribbon. The chemistry is the same; the process leaves two very different internal structures, and almost every practical difference downstream comes from that.
Cast sheet polymerises slowly at low pressure, so the finished sheet is close to stress-free. Extruded sheet is formed under pressure and cooled quickly, which locks stress into the material. That stored stress is invisible until something releases it:
Cast sheet is not immune — any machining adds stress — but it starts from a much lower baseline, which is why every part that will be machined, bonded or polished for optical clarity is specified in cast.
| Property | Cast acrylic | Extruded acrylic | Consequence |
|---|---|---|---|
| Residual stress | Low | High | Extruded parts move after machining and craze at bonds |
| Thickness tolerance | ±10%, can select to ±5% | ±5% typical | Extruded is better for a stack that must close |
| Thickness range | 1.5–100 mm and beyond | 0.8–12 mm typical | Thick sections are cast only |
| Optical quality | Water-white, minimal distortion | Good, slight flow lines possible | Optical and vision parts are cast |
| Surface | Glossy, both faces from glass | Glossy, one face may be matte | A matte face is a design decision, not a defect |
| Machining | Cuts clean, polishes predictably | Tends to gum, edge chips more | Cast needs less finishing effort |
| Bonding | Solvent bond nearly invisible | Joint tends to craze | Display work is cast |
| Thermoforming | Wider forming window | Narrower, needs closer control | Extruded is fine for thin simple bends |
| Colour consistency | Batch to batch variation, needs a swatch | More consistent within a run | Long rollouts are easier in extruded |
| Price | Higher | Lower, often 25–40% less | The reason extruded exists |
Extruded acrylic is not a downgrade in every case. It is the correct choice when all of these are true:
Signage faces, simple light diffusers, cheap protective screens, vacuum-formed trays and packaging all sit comfortably in extruded sheet. A retail display with a polished edge and a bonded base does not.
There are three quick checks, and a fabricator should be willing to answer the question directly:
This is the most common complaint we investigate and it almost always has the same cause. A panel is cut from extruded sheet, a large opening is routed out of it, and a week later the panel has developed a curve. Routing removed stressed material from one face; the remaining stress pulled the panel into a new shape.
The same part in cast sheet usually stays flat, because the starting stress is low enough that removing material is not enough to bend it. If the part must be made from extruded sheet, the fixes are to route symmetrically from both faces where the design allows, to relieve stress by annealing before machining, or to accept the bow in the design.
Annealing is not a cure-all. Heating extruded acrylic to relieve stress also relaxes the sheet's flatness, so a panel that was flat can come out of the oven slightly wavy. It is more reliable to start with cast sheet than to anneal extruded sheet and hope.
Cast sheet varies from batch to batch because each batch is a separate polymerisation. That is not a defect; it is the nature of the process. For a retail rollout or any programme where parts will be compared side by side years apart, the working method is a signed physical swatch plus a retained sample per batch.
Extruded sheet is more consistent within a run, which makes it attractive for a long single-run order — but a re-run six months later is still a different batch. Nobody should promise visual colour match across batches without a retained swatch, in either material.
CAST PMMA explicitly.EXTRUDED PMMA — acceptable for non-bonded flat parts.You can, but the joint will usually craze on the extruded side because the solvent releases the stress locked into that sheet. If a bonded assembly is required, both parts should be cast, and the finished assembly should be annealed.
It is cheaper because it is faster to make, not because it is inferior material. It is the right product for flat signage and simple formed parts. It is the wrong product for a machined, bonded or optical part, where the process rather than the polymer becomes the limit.
Because it is polymerised between glass plates with the shrinkage controlled by process rather than by a die. Selected sheet can hold ±5%, but it is selected after production and costs more.
Yes, but it gums more and the edge is less predictable. A diamond-polished cast edge is water-clear; the same operation on extruded sheet can leave a slightly hazy or wavy result because the material softens and smears rather than cutting cleanly.
Either, depending on the effect. Extruded sheet gives a very consistent diffusion within a run and is cheaper. Cast sheet with a textured face or an added diffuser gives a more even glow and is the usual choice where the diffuser is looked at directly.
Send the part drawing or describe the part, and we will state cast or extruded, the grade, and the thickness tolerance we can hold.