Cast vs Extruded Acrylic: Which Should You Use?

How They Are Made

Cast acrylic (cell-cast or GS-grade) is produced by pouring liquid monomer between two polished glass plates sealed at the edges and curing slowly under controlled heat — typically 50–80°C over several hours. The slow polymerisation preserves long molecular chains, producing a denser, harder, more dimensionally stable sheet.

Extruded acrylic (XT-grade) is produced by feeding PMMA pellets through a heated die at continuous speed. The process is faster and more economical, but produces shorter molecular chains and a softer, slightly less stable sheet. That trade-off is the root of every difference between the two types.

Optical Clarity

Both achieve approximately 92% light transmission — comparable to optical glass, and visually indistinguishable in most applications. Cast acrylic holds an edge in optical homogeneity: slow, uniform curing minimises internal stress, producing lower birefringence (colour distortion when light passes at an angle). This matters for display cases, optical instruments, and backlit signage where visual uniformity is critical.

Extruded acrylic can exhibit faint optical stress lines when viewed at oblique angles, more noticeable in thinner gauges under 4mm. In most practical applications this is not perceptible.

Surface Hardness

Cast acrylic registers approximately 90 on the Rockwell M scale; extruded approximately 85. That 5-point difference is significant in use: cast acrylic resists surface scratching more effectively, polishes to a glass-like edge finish, and forms stronger solvent cement bonds due to its higher molecular weight. For applications involving polished edges, display-facing surfaces, or elements under regular contact, cast is the correct specification.

Thickness Tolerance

This is where the two types diverge most significantly. Extruded acrylic, produced through a fixed die, achieves a tight dimensional tolerance of approximately ±3–5% of nominal thickness. Cast acrylic, manufactured to ISO 7823-1, has a wider inherent tolerance: ±(0.4 + 0.1 × nominal thickness in mm).

For a 3mm cast sheet, this means actual thickness may fall between 2.3mm and 3.7mm — a nominal variation of up to ±23%. This is not a quality defect; it is inherent to the cell-cast process, caused by the thermal expansion of the glass moulds during curing. For most applications, the variation is inconsequential. Where tight dimensional consistency is critical — precision assemblies, grooved frames, stepped joints — extruded or post-machined cast acrylic is the appropriate solution.

Thermoforming Performance

Extruded acrylic thermoforms more easily. Shorter molecular chains soften more uniformly at lower temperatures and permit tighter forming radii without stress cracking — making XT-grade the standard for vacuum-formed signage, lighting diffusers, and high-volume bent forms. Minimum bend radius for extruded acrylic is typically 20–30× sheet thickness.

Cast acrylic can be thermoformed but requires a wider minimum bend radius — approximately 50–100× sheet thickness — and more careful temperature control (150–165°C through-thickness). Its advantage post-forming is better dimensional stability over time, owing to its higher molecular weight.

Price

Extruded acrylic is typically 10–20% less expensive than cast at equivalent thickness. The faster production cycle, lower energy input, and higher process consistency all contribute. For volume commodity shapes — standard rectangular panels, light diffusers, simple bent forms — the cost saving is meaningful.

Which to Choose

For furniture, architectural glazing, display cases, museum-quality work, and anything requiring polished edges or long-term UV stability: specify cast. For backlit signage, thermoformed housings, diffusers, and cost-sensitive volume production: specify extruded.