A sign fabricator asked for cast vs extruded acrylic advice after quoting a project twice: once at 6 mm extruded and once at 6 mm cast, with the two numbers twelve per cent apart. The job was a set of illuminated letters with solvent-bonded returns, installed on a coast. The answer was not a preference; it was four facts about the job.
Deciding cast vs extruded acrylic on four questions
Cast acrylic is poured between glass plates or into a cell and polymerised in place. It has no directional stress, holds thickness well, and machines and bonds predictably. Extruded acrylic is pushed through a die as a continuous sheet, cooled on rollers, and cut to length. It is faster, cheaper, and carries a faint flow line along the machine direction.
What the difference does optically
Both are clear, but cast sheet is clearer at thickness. An extruded 10 mm sheet shows a slight haze and a visible flow pattern under raking light; cast at the same thickness reads almost like glass. For a light guide, a lens or a display case, that difference is the product.
What the difference does mechanically
- Machining: cast drills and routes without stress lines; extruded can split along its direction
- Bonding: cast tolerates solvent cement; extruded crazes at joints under load
- Thermoforming: extruded forms at lower temperature and holds detail marginally better
- Annealing: cast responds predictably; extruded is already partly oriented
- Thickness range: cast to 20 mm and beyond; extruded economic to about 12 mm
The bonding line is the one that ends projects. A display case built from extruded sheet with solvent joints will look perfect for six weeks and then craze around the joints, usually in a warm window.
What the difference does outdoors
Neither polymer resists UV on its own; the grade does. A cast sheet with a UV absorber holds colour and clarity for a decade or more in northern Europe and noticeably less in high-altitude sun. An extruded sheet without an absorber will yellow in two to three summers, and no coating applied afterwards will reverse it.
So the outdoor question is not cast versus extruded, it is whether the sheet you are buying carries UV protection at all. The yellowing and outdoor lifespan guide covers what to look for in the datasheet.
Where extruded genuinely wins
Short-life signage, retail displays, protective packaging, vacuum-formed trays, and anything under about 3 mm where the price difference is a large share of the total. Extruded sheet is not a compromise in those duties; it is the correct engineering choice, and pretending otherwise raises cost without raising performance.
It also cuts and forms faster, which matters in a shop that turns artwork around in three days.
How to decide in ten minutes
Ask four questions: will the part be machined or solvent bonded, will it be outdoors for more than two years, will it be thicker than 8 mm, and will someone look through it closely for a decade. Two or more yes answers mean cast. Zero or one means extruded is probably right.
Then specify the grade, not just the material: UV absorber, flame rating, colour, tolerance band. The cut-to-size note shows how to carry those grades through to finished panels without losing the distinction at the saw.
Tell us the application, thickness and quantity at oversales@rpplexiglass.com and we will recommend the grade with the reason written down.
Frequently Asked Questions
Is extruded acrylic weaker than cast?
It has lower resistance to stress cracking and carries a directional stress pattern, but its impact strength is comparable. The practical difference shows up after machining or solvent bonding, where extruded sheet crazes sooner.
Can extruded sheet be used outdoors?
Yes, with a UV absorber. Both routes can carry UV protection; the question is whether the grade you are buying includes it. General purpose extruded sheet will yellow and chalk within two to three summers.
Why is cast sheet available in thicker sections?
Cast cells are built around a thickness and can pour 20 mm and beyond in one cycle. Extrusion is economic up to roughly 12 mm, after which the cooling and the downstream equipment become impractical.