Cast vs Extruded PMMA: What is the difference between cast and extruded acrylic?

The material properties between cast and extruded PMMA differ enough at the molecular level to make a difference in highly specialized applications—but the surprising truth is that in most industrial applications, the differences between the two types of PMMA don’t really matter.

Therefore, the choice of PMMA depends much more on factors such as availability – especially when it comes to specifications such as:

• Colors
• Thicknesses
• Order volumes
• Lead time for non-stock material

Cast PMMA usually excels in available colors, thin thicknesses, and lower volumes. Lead times are also generally shorter for non-stock colors and thicknesses for cast PMMA. Therefore, casting is a flexible option where the customer can pick and choose thickness, volume, and color as needed. However, if the desired color and thickness are readily available as an extruded material, choosing the extruded solution is often more cost-efficient.

Extruded PMMA is a less flexible manufacturing process than casting PMMA sheets because extrusion is a continuous process by nature. This means that any changes in thickness or color produce a lot of scrap material and take considerable time and energy compared to manufacturing cast PMMA.

For this reason, the choice between extruded and cast PMMA isn’t as important as the abovementioned considerations. Even though extruded PMMA is generally considered the more cost-efficient manufacturing solution, the factors mentioned above might exclude the choice of extruded PMMA.

PMMA is made by polymerizing methyl methacrylate monomers into long strands through cell-casting or a reactor process.

The two polymerization methods give the finished PMMA different mechanical properties:

• Cast PMMA is made by placing the monomers in a flat mold with a catalyzing agent. The agent polymerizes the monomers into sheets with the desired dimensions. Because the polymerized PMMA strands are extremely long and 100% amorphous organized, the PMMA sheet causes no optical distortions.
• Extruded PMMA is made by polymerizing the monomers in a reactor system and adding them to a solvent, where polymerization starts. The polymer strands are filtered out of the solvent and extruded into pellets, which are used as raw materials in a continuous sheet extrusion process or injection molding. Both these processes involve melting the pellets and forcing the material through a die, creating a sheet or cavity with the desired shape. When the material solidifies with this manufacturing process, stress is embedded into the material. This stress is visible when inspected under polarized light because it causes lower light transmission and optical distortions. This effect is known as birefringence or retardation.

Acrylic manufacturing processes don’t just involve casting or extrusion. Once the PMMA has been cast or extruded, you can improve its mechanical and optical properties with coatings that add unique qualities to the PMMA. Examples include coatings that enhance wear-, cleaning- and chemical resistance or coatings that reduce glare and reflections.

This type of processing is suitable for both cast and extruded PMMA, and in many cases, adding the right coating or other treatment is more important than choosing between cast and extruded PMMA. The reason is simple: While the material properties of cast and extruded PMMA are very similar, the right coating can significantly improve the material properties of the PMMA in certain areas and add value to your application.

Before you decide on a PMMA type, you need to know the technical requirements of your application to choose the most efficient way of meeting those requirements.

Here are some considerations you should make when choosing the base material:

• If you need the flexibility to choose an exact color or thickness, quick delivery, or low MOQ because you are prototyping a new device or solution, cast acrylic is most likely a better option.
• If you actually need the specific mechanical properties provided by cast PMMA mentioned above, you should also consider cast PMMA.
• If you do not need the specific material properties provided by cast PMMA, if you are working with large production volumes and production cost is a key driver, or if you do not have special requirements regarding color or thickness, you should consider using extruded PMMA since it is generally the most cost-effective option.
• For both cast and extruded PMMA, you should consider applying one or more coatings or other special treatments that improve the material properties of the PMMA and allow your device or solution to deliver the required performance in a broader range of operating conditions—for example, by increasing its scratch resistance.
• As a side note, there are considerations to make if your device works at higher temperatures. Cast PMMA is the better option if your device operates at up to 85-90 degrees Celsius. This is where material deformation occurs in cast PMMA, whereas the limit at which extruded PMMA is not recommended for applications is 75-80 degrees Celsius.

If you have an application utilizing circularly polarized light (CPL), cast PMMA is the better option because it is 100% amorphous by nature. As mentioned above, this means there is no risk of optical distortions of the CPL passing back and forth through the material. Extruded acrylic is unsuited for this type of application because of the retardation in the material. This distorts the circularly polarized light when passing through the material.

Imagine, for example, that you are developing a high-end barcode reader utilizing embedded CPL illumination for hard-to-read information, i.e., laser-etched markings on highly polished stainless steel parts. The distortions from extruded material will significantly reduce the scanning system’s signal quality or signal-to-noise ratio, making cast acrylic the only viable option.

• Not sure what signal-to-noise ratio is? Then click right here to read our article about it.

In conclusion, the choice between cast and extruded PMMA is about availability or cost considerations in most cases, not mechanical or optical properties. There are a few cases where cast acrylic is the only option, but these cases are definitely few and far between.

At PSC, we have decades of experience developing and manufacturing PMMA to our customer´s exact specifications, using casting and extrusion processes, and applying various coatings and other special treatments. We use that experience to give you a PMMA solution engineered precisely for your application—no more and no less.

Contact us to discuss your requirements and options, no matter what kind of project you’re working on.