What Are Amorphous Thermoplastics?

With Contributing Expertise From: simtec

Thermoplastics are a broad class of materials which are solid and brittle at room temperature but, when heated, become soft and pliable. Thermoplastics are generally separated into one of two categories:

  • Amorphous thermoplastics
  • Semi-crystalline thermoplastics

The key distinction between the two is that while semi-crystalline plastics become hard (crystallize) at a specific temperature, amorphous plastics transition between soft and hard states more gradually. Semi-crystalline thermoplastics include commonly used materials such as nylon and polypropylene. Acrylic, polystyrene and polycarbonate are some of the better-known amorphous thermoplastics.


Amorphous thermoplastics are easy to thermoform and as such are an excellent choice for most injection molding applications. Whereas semi-crystalline plastics tend to be opaque in their solid state, amorphous thermoplastics are usually translucent. For this reason, they are often used in optical products such a ski and swim goggles.

Amorphous thermoplastics also bond well with the use of adhesives or solvents. They are less prone to shrinkage than other plastics, which makes them good for applications in which high dimensional tolerances are required. However, amorphous thermoplastics are more prone to stress cracking and other types of material fatigue, while semi-crystalline plastics can be more durable.


Amorphous thermoplastics include a range of products and grades. High-performance varieties, which are more expensive, will be more resistant to extreme temperatures, chemical exposure and other forms of wear. As such, amorphous thermoplastics can have a range of potential applications. As noted above, their translucency makes them an excellent choice for anything see-through, including not just goggles but also medical devices, kitchen implements and more.

Other applications that often use amorphous thermoplastics include automotive component manufacturing, consumer goods and electronics, irrigation and filtration equipment, and more.

Amorphous Thermoplastics and Liquid Silicone Rubber

The distinct properties of amorphous thermoplastics distinguish them from thermoset elastomers such as Liquid Silicone Rubber (LSR), which are generally softer and more flexible. To get the benefits of both materials, it’s common for Liquid Silicone Rubber (LSR) to be overmolded onto an amorphous thermoplastic base, or vice-versa. In doing so, you avoid the expense of assembling a product consisting of two parts.

Overmolding LSR onto an amorphous thermoplastic base is something we have extensive experience with at SIMTEC. To learn more about the benefits of molding LSR onto an amorphous thermoplastic, or about the 2-Shot molding process in general, contact a representative directly.



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