Cast vs Extruded Acrylic: Which Should You Buy for Windows?

The makers' own data sheets say cast and extruded acrylic are identical in clarity, hardness, impact, and expansion, and that extruded holds the tighter thickness tolerance. Here is what actually differs, what each is used for, what the premium buys, and how to tell them apart.

Every acrylic sheet is one of two things: cast or extruded. Our reviews mention the distinction constantly, the buying guide has a paragraph on it, and every plastics retailer has a blog post explaining why cast is better. Most of those posts are wrong in the same ways. We pulled the technical data sheets from the two companies that make most of the acrylic sold in North America, Polyvantis (ACRYLITE here, PLEXIGLAS in Europe) and Plaskolite (OPTIX), and compared cast and extruded on the same maker's numbers under the same test standards. The result is narrower than the folklore: cast wins on heat, on thickness, and on a few fabrication jobs, and extruded wins on price, tolerance, and forming. For a flat window, extruded is usually the right buy.

How each is made

Cell cast acrylic is made one sheet at a time. Liquid monomer is poured between two sheets of toughened glass sealed with a gasket and heated until it polymerizes. The polymer chains that form are long, which is where every real cast advantage comes from, and every cell has to be assembled and taken apart by hand, which is where the price comes from. ACRYLITE calls its cast line GP, Röhm calls it PLEXIGLAS GS, and Spartech's Polycast is the aerospace and bullet-resistant grade.

Extruded acrylic is made continuously: pre-polymerized pellets are melted and pushed through a die into an endless ribbon that is cooled, cut, and masked. The polymer is lower in molecular weight so it will flow, and the sheet has a grain, a manufacturing direction that matters when you heat it. ACRYLITE Premium FF, PLEXIGLAS XT, and Plaskolite OPTIX are the extruded lines, and extruded is what the home centers sell.

There is a third process worth knowing about. Continuous cast (Plaskolite's OPTIX-L) polymerizes monomer between moving steel belts. Plaskolite ranks it between the other two on molecular weight and craze resistance, and aquarium builders use it alongside cell cast for thick panels. The binary is a simplification, but for buying windows it is close enough.

What the data sheets say

Here are the properties that matter for a window, from Röhm's PLEXIGLAS 211-1 technical bulletin, which publishes GS (cast) and XT (extruded) side by side under identical ISO tests:

Property	Cast (GS)	Extruded (XT)
Light transmission	~92%	~92%
Refractive index	1.491	1.491
Density	1.19 g/cm³	1.19 g/cm³
Indentation hardness	175 MPa	175 MPa
Taber abrasion (haze)	20–30%	20–30%
Charpy impact	15 kJ/m²	15 kJ/m²
Notched Izod	1.6 kJ/m²	1.6 kJ/m²
Modulus of elasticity	3,300 MPa	3,300 MPa
Thermal expansion	7 × 10⁻⁵ /K	7 × 10⁻⁵ /K
Tensile strength	80 MPa	72 MPa
Flexural strength	115 MPa	105 MPa
Vicat softening point	115°C	103°C
Heat deflection (1.8 MPa)	105–107°C	95°C
Max continuous service	80°C (176°F)	70°C (158°F)
Forming temperature	160–175°C	150–160°C
Chemical resistance	Good to dilute acids and alkalis, limited to organic solvents	Same
Weathering	"Unequalled resistance"	Same wording

ACRYLITE's US data sheets tell the same story for GP against FF: both 92 percent transmission, both 10,000 psi tensile, both 400,000 psi modulus, both the same notched Izod, both 0.00004 in/in/°F expansion, Rockwell hardness M-94 against M-93. Flexural strength is a shade higher for the extruded sheet. Heat is the one clear gap: cast is rated for 180°F continuous service against 160°F for extruded, with a deflection temperature about 15°F higher.

So the honest summary: cast and extruded acrylic are the same material optically, mechanically, and thermally, except that cast tolerates about 10°C more heat and is about 10 percent stronger in tension and bending. Everything else in the comparison comes down to fabrication behavior and availability.

Where the folklore is wrong

"Cast is clearer." Both are 92 percent. Plaskolite says its extruded OPTIX has high optical clarity and that its cast lines add craze resistance "without sacrificing on optical clarity." Clarity is where they are equal.

"Cast is harder and more scratch-resistant." Identical hardness and identical Taber abrasion on Röhm's sheet, one Rockwell point on ACRYLITE's. If you want a scratch-resistant window, you buy a hard-coated grade, not a casting process; see the scratch repair guide for why acrylic scratches at all.

"Cast is tougher." Identical Charpy and Izod figures. Impact resistance comes from grade, not process: Röhm's impact-modified PLEXIGLAS Resist, an extruded family, is three times to many times tougher than standard cast. The ACRYLITE greenhouse panels we reviewed are exactly that, impact-modified extruded sheet.

"Cast has the tighter thickness tolerance." Backwards. ACRYLITE publishes ±5 percent for extruded FF from 0.150 inch up (±10 percent for the thinnest gauges) and a much wider band for cast GP made to ISO 7823-1: a nominal 3 mm cast sheet may measure anywhere from 2.3 to 3.7 mm, about ±23 percent, narrowing to about ±12 percent at 24 mm. Polyvantis markets extruded FF against imported cast on this exact point, and Spartech sells a separate "close tolerance" cell-cast line, which tells you standard cast tolerance is loose enough to be a product category.

"Extruded can't go outdoors." ACRYLITE recommends its extruded FF for greenhouse glazing and for thermoformed skylights, warrants clear FF for 30 years on light transmission, and writes the same weathering sentence on both data sheets. The vendor-blog claim that extruded yellows in five to seven years has no test behind it.

"Cast solvent-welds better." Röhm recommends solvent adhesives specifically for extruded XT and reaction adhesives for cast GS. ACRYLITE says extruded cements and thermoforms faster. Aquarium builders insist on cast for bonded tanks, but for craze resistance under permanent load in water, not because the joint forms more easily.

Where cast genuinely wins

Thickness. Extrusion stops at about 1 inch (25 mm). Cast goes to 160 mm in Röhm's catalog and to the 27-inch panels at Reynolds Polymer. Anything over an inch is cast because nothing else exists. US Plastic's catalog shows the split in practice: extruded from 0.118 inch, cast from 0.354 inch up.

Heat. Ten degrees Celsius more service temperature and a higher softening point. It matters for a dark-tinted panel in a south-facing frame, a sheet near a heat source, or a hot-climate skylight, and not much elsewhere.

Craze resistance under sustained stress. This is the property the long polymer chains actually buy. Plaskolite ranks its three processes by molecular weight and by craze resistance in the same breath, cell cast highest. An independent failure analysis of acrylic aquariums notes that large, thick panels are cast for stress-free, high-molecular-weight material that holds up under years of water load, and that a 0.01-inch scratch halves acrylic's tensile strength. Cast's edge is that it tolerates that abuse a little longer. For a flat pane in a frame with proper expansion clearance, the stress is low enough that extruded is fine; the cutting and mounting guide explains how low.

Machining. ACRYLITE says cast machines without melting or chipping as readily. Extruded's lower melting point is why a slow feed on a table saw gums the kerf and why laser engraving on extruded comes out dull grey instead of frosted white. Cast is the fabricator's material for routed, polished, and engraved work.

Isotropic forming. Heat an unclamped cast sheet and it shrinks up to 2 percent equally in both directions. Extruded shrinks up to 3 percent along its manufacturing direction (6 percent for thin or textured sheet) and nothing across it, so a free-formed dome from extruded sheet comes out slightly oval unless the fabricator plans for it. Skylight makers use extruded anyway, because it forms at a lower temperature in a shorter cycle; they just account for the grain.

A tighter warranty. Both ACRYLITE lines carry 30-year warranties on light transmission, but cast GP is warranted to lose no more than 3 percent, while extruded FF is warranted to lose no more than 2 to 15 percent. Same duration, different guaranteed band. This is the one documented durability difference, and it is about guaranteed retention, not about the sheet turning yellow.

Colors. Over 40 standard colors in cast against about 20 in extruded, per Röhm.

Where extruded wins

Price, tolerance, and forming. Extruded is cheaper because the process is continuous, holds a tighter thickness, comes cleaner (ACRYLITE lists less dirt, lint, and particulate contamination as an extruded advantage), thermoforms at 290 to 320°F instead of 340 to 380°F, and solvent-welds faster. Röhm describes cast as "easy to thermoform over a wide range of conditions" and extruded as "very easy to thermoform under optimum, constant conditions." That is the trade: extruded forms more readily but forgives less variation. Extruded FF also has little memory after forming; it will not spring back flat if reheated.

What each is used for in windows

Storm window inserts: nominal 1/8-inch sheet, extruded gauge, from Indow, Climate Seal, and the DIY kits. None of the vendors says cast or extruded; the thickness says extruded.
Replacement panes and DIY storm panels: extruded. Acme Plastics puts it plainly: for limited-fabrication uses like windows, extruded is the cost-effective choice, and cast earns its premium in CNC, polishing, and gluing work. See the replacement panel roundup.
Skylights: thermoformed from extruded sheet at lower temperatures and shorter cycles, by the maker's own recommendation. Forming a dome also stiffens the sheet and reduces the slow sag (cold flow) that flat acrylic shows over time. Our VELUX FXG review covers the buyable one.
Greenhouses: extruded, and usually impact-modified extruded. ACRYLITE recommends Premium FF for glazing and its Resist multi-skin for hail.
Boats: the trade default is cast, and the reason given is craze resistance in salt water and sun plus the thicknesses a windshield needs. We could not find the ABYC or ISO text that vendors cite for this, so treat it as industry practice rather than a standard. ACRYLITE's marine-durability language appears on both data sheets. See the boat window roundup.
Aquariums and pools: cast, cell or continuous, for thickness and for stress-free material under permanent water load, bonded with monomer cements and annealed before assembly. Clear-For-Life and SeaClear both say cast on the label.
Bullet-resistant glazing: cell cast, always. The 1¼-inch UL 752 sheet in our Plaskolite OPTIX G BR review is monolithic cell cast because nothing else is made that thick and stress-free.
Noise walls and thick architectural panels: both. ACRYLITE Soundstop sells an extruded XT grade for ground-mounted walls and a cast GS CC grade with embedded threads for bridges.

What the premium costs

Retail spot checks on September 15, 2026: a 1/4-inch 24 × 48 inch clear sheet was $41.52 extruded at Acme Plastics (list $55.37) and $65.95 cast at ToolsToday (list $72.45), which works out to a cast premium of about 60 percent at sale prices and 30 percent at list. Vendor rules of thumb range from 20 to 60 percent. The premium narrows in thin gauges and vanishes above 1/2 inch, where extruded stops being offered; several major distributors do not stock 1/8-inch cast at all in a 24 × 48 sheet, which is itself the practical answer for thin window glazing. Ballpark per square foot: $3 to $5 extruded, $6 to $10 cast. The cost guide has the full price table, and the supplier roundup covers who sells what.

How to tell them apart

Read the masking. Paper masking almost always means cast; film masking can be either. Retailers encode it in their SKUs (ePlastics uses PM and FM suffixes, eStreet writes "Paper Mask" and "Film Mask" in the product name). Yale's fabrication shop notes that paper masking can stay on through laser cutting and improves the finish, which is the same tell from another angle. It is a strong probability, not a guarantee; Interstate Plastics sells cast in both.

Measure it. A nominal 1/8-inch (3 mm) sheet that reads within about 5 to 10 percent of nominal is consistent with extruded. One that reads 2.4 or 3.6 mm is almost certainly cast, because only cast tolerance is that wide.

Check the thickness on the label. Over 1 inch, it is cast.

Look for the grain arrow. Extruded sheet labels indicate the manufacturing direction (on ACRYLITE it runs top to bottom when you read the label). Cast has no grain and no arrow.

Engrave a scrap. If you have access to a laser, cast engraves bright frosted white and extruded engraves dull grey. This is the test fabrication shops rely on.

Two tests you will read about do not work. The acetone test has no discriminating power: both makers rate cast and extruded identically on solvent resistance. The flame test is worse: burn rates are 1.2 and 1.0 inches per minute, ignition temperatures differ by five degrees, and you are setting fire to a combustible plastic that gives off irritating fumes. Skip both.

The verdict for a window buyer

Buy extruded for any flat pane, insert, storm panel, replacement light, or greenhouse glazing up to about 1/2 inch. It is the same clarity, hardness, and toughness at a lower price with a tighter thickness, and the maker will warrant it outdoors for 30 years.

Buy cast when you need more than 1/2 inch, when the panel will live under permanent load in water, when it will see sustained heat above about 160°F, when you plan heavy routing, polishing, or engraving, or when the application (boat windshields, ballistic glazing) has settled on cast for reasons of craze resistance you should not second-guess.

And whichever you buy, the mounting rules in the cutting and mounting guide matter more than the process: a cast sheet screwed down tight will crack before an extruded sheet floating in a proper rabbet ever does.