Types of Plastics Used in Injection Molding

With over 80,000 material options to use in plastic injection molding, it can be overwhelming to weigh the differences and find the right plastic for your injection molding needs. 

To understand the difference in materials, it’s important to first understand the categories into which plastics are grouped. 

Plastics can be broken down into 3 main categories:

  • Thermoplastic 
  • Thermoset
  • Elastomer

When using thermoplastic in injection molding, the material starts in pellet form and is melted and injected into a cool mold. This is entirely different than the injection molding process for thermoset plastics, in which cold material is injected into a hot mold and the part cures to form its permanent shape. 


Thermoplastic becomes liquid when heated. Then, as it cools down, it solidifies and becomes inflexible. Thermoplastic parts can be re-melted and reused. Thermoplastic is seen as an all-purpose material and is very common in plastic injection molding. 

plastic injection molded utensils

There are two main types of thermoplastic: amorphous thermoplastic and semi-crystalline thermoplastic.

Amorphous thermoplastic tends to be transparent and glassy in appearance. It softens slowly and gradually as it is heated.  Examples include, Plexiglass, Polystyrene, and Polycarbonate.

Semi-crystalline thermoplastic, on the other hand, tends to be more opaque. As opposed to liquifying gradually, semi-crystalline thermoplastic stays solid until it reaches a certain temperature. Examples of semi-crystalline thermoplastic include Polypropylene, Polyethylene, and Nylon. 


Thermoset material can withstand much higher temperatures than thermoplastic material. After curing, it forms a permanent shape and cannot be melted and reused again. Thermoset material has electrical and thermal insulation properties that make it suitable for electrical and electronic applications. A commonly used form of thermoset is Phenolic Formaldehyde.


Elastomer, as implied by the name, is a polymer material with elastic properties. This category can include both natural and synthetic manufactured polymer. Elastomer can be considered a subset of the other two plastic categories. Some elastomers are thermoplastics, while others are thermosets. 

Much like thermoset, as Elastomer cures it forms a permanent shape and cannot be reused. It can withstand extremely high temperatures and is very flexible. Examples include Rubber and Liquid Silicone Rubber. 

What Materials are Most Suitable for Injection Molding?

There are many factors you need to consider when selecting which plastic material to use in injection molding your part(s). 

The best material for you depends on the desired functionality of your part. For example, some parts need to be thin and flexible, while others need to be thick and rigid. The design and complexity of your mold also plays a large role in determining which materials are best suited for your particular injection molding process.

Some molds require plastic with a  specific material component makeup. In these cases, factors such as melting point, cooling point and viscosity play an important role. 

There are also hazards and risks that need to be taken into consideration when selecting materials, as certain plastics can be toxic (especially when used in food or medical functions). 

Once you have a good idea of what features and functionality your part needs to include, you can begin looking for a material that meets your needs at the lowest cost possible. There’s no sense in spending extra money on a material that features premium qualities your part doesn’t require. 

Ready to Take the Next Step? 

At Tailor Made Products, we specialize in TPE/TPR (thermoplastic elastomer and thermoplastic rubber) plastic injection molding. 

Request a Quote today or reach out to learn what we can do for you.