Extrusion is a continuous production method of manufacturing acrylic sheet. In the process, pellets of resins are fed into an extruder which heats them until they are a molten mass. This mass is then forced through a die as a molten sheet. The molten sheet is fed to calender rolls, the spacing of which determine the thickness of the sheet and in some cases the surface finish. The continuous band of sheet may then be cut or trimmed into its final size.
In the manufacture of acrylic resin there is not a 100% conversion of monomer to polymer. The monomer is the unpolymerized form of a compound, in this case methyl methacrylate, which is the major component of the polymer. The polymer is a chemical molecule formed by the successive addition/joining of monomer units to form a molecule of high molecular weight. In the case of acrylics this polymer molecule is a chain of perhaps 100-1000 units (monomer) in length. A few percent of monomer may remain in the resin if the manufacturer has not removed the major amount of it in the pellet formation. Further, resin absorbs water if it is exposed to humid air. However, the feed stock, or pellets going to the extruder, will usually contain a small amount of water, if it has not been dried thoroughly before being processed. In an effort to obtain high quality sheet with a high yield, some production lines extract, in early stages of the melting process, the monomer and water from the feed stock. This may give some observers the false impression that monomer is being added to the pellets rather than being removed.
As an aside, monomer which is left in the molten resin can cause bubbles or streaks in the extrudate. Monomer left in sheet can reduce the hardness, promote crazing when the sheet is fabricated, and in extreme cases lower physical properties such as tensile strength and modulus of elasticity (rigidity or stiffness). The severity of these phenomena will depend on the level of residual monomer.
The final product of extrusion exhibits much closer thickness tolerances than cast sheet. Because of the volume at which extruded sheet is produced, it is the most economical form you can buy. It is available in a fair selection of colors, finishes and sizes. Extruded acrylic sheet is prone to shrinking along the extruded line and expansion across it. This is of particular note if you plan to use it for thermoforming. Extruded material also has a tendency to gum during fabrication when the cutting is too fast (linear feed rate) because it has a lower molecular weight. It also may absorb fast drying solvent cements faster than cast or continuous cast material. This can result in joint failures and incomplete gluing. To remedy this, use slower drying cements and tooling designed for cutting extruded acrylic.