The Xometry app works best with JavaScript enabled!
Xometry Logo
Sign In
Search
CapabilitiesIndustriesResourcesSuppliesBecome a Supplier
Search
Resources3D Printing Design
thermoplastic parts on a table

Thermoplastics vs. Thermosets - What's the Difference?

Plastics have become so widespread that every single industry makes use of them in some manner. In broad terms, these plastics classify into two subcategories: thermoplastic and thermosets. Both of these categories of plastics have their uses. This article will break down the key differences between thermoplastics vs. thermosets and the best times to use each.

Xometry's X Logo
By Team Xometry
 5 min read
RECENT STORIES
Update: Estimated Lead Times for the 2021 Holiday Season
December 8, 2021
 1 min read
What is Low-Volume Production?
December 4, 2021
 5 min read
Manufacturing with Garolite Machining
December 3, 2021
 4 min read

What is a Thermoplastic?

Thermoplastics are the most widely used polymers due to their ease of manufacture after polymerization. The benefit of thermoplastics is they become liquid at high temperatures--in other words, they melt. Melted thermoplastics are formed into other shapes for use as raw materials as well as final products. Thermoplastics can even be reformed and often recycled. Polymerized thermoplastic pellets are raw materials for many manufacturing processes. Thermoplastic bar or rod stock can also be CNC machined. Thermoplastics come in a wide range of hardnesses spanning from Shore A rubber-like thermoplastic elastomers to Shore D flexible-to-rigid plastics.

Thermoplastics polymers vary from commodity materials like polypropylene or polystyrene to engineering plastics like PEEK. Some of the more common thermoplastics are listed below.

  • Polyamide (nylon, PA): Nylon is a common thermoplastic with many formulations. Nylon uses range from rugged mechanical parts to textiles.
  • Polyvinyl chloride (PVC): PVC is typically used for conduit, plumbing, and with the addition of plasticizers, it can be made soft enough to be turned into raincoats and cable insulation. PVC is naturally flame retardant.
  • Polyethylene: Polyethylene is one of the most common plastics for anything from the piping to low friction wear plates to drinking bottles. It comes in many forms, including HDPE (High-density polyethylene), PET (Polyethylene Terephthalate), and UHMW-PE (Ultra High Molecular Weight Polyethylene).
  • Polypropylene (PP): Polypropylene has superior heat resistance when compared to polyethylene and is standard for dishwasher-safe food packaging. Polypropylene also works well in living hinges. 
  • Polycarbonate (PC): Polycarbonate is a rigid, heat-resistant thermoplastic that can also be made optically clear. It is regularly employed in the viewing windows on machinery.

How are Thermoplastic Products Made?

Thermoplastic polymers accept a wide range of manufacturing processes. Some of the more common ones are listed below.

  • Injection molding: A considerable fraction of our everyday products are created via injection molding. Simply put, injection molding works by melting the thermoplastic or elastomer in an auger, injecting it into a mold cavity at high pressure, and cooling the part until it is ejected. This process is rapid, so it is excellent for high-volume production. However, the molds are complex, making the up-front costs expensive.
  • Extrusion: Extrusion is the process of forcing melted plastic through a die to take on a specific cross-sectional shape. This process is typically used to manufacture long, uniform components such as electrical conduits. Extrusion is also how bar and rod stock is created for milling.
  • Blow Molding: Blow molding is the method used to manufacture bottles and containers. It works by making use of preforms manufactured in an injection molding process. These preforms are then moved to a blow mold where they are heated, and air is pumped into them to take on the mold's shape but leave the inner volume empty. 
  • Machining: Many thermoplastic elastomers can be easily machined, so this is done on low-volume parts that require tight tolerances like gears or pulleys. It must be noted that the softer the material, the more challenging it is to machine; more rigid plastics are paradoxically much easier to mill. 
  • 3D printing: The majority of 3D printed plastic components use thermoplastics. The plastic filament, powder, or pellets is fused to the part's final shape layer by layer until the final part is complete. This is typically used for low-volume production or prototyping. 

The Chemistry of Thermoplastics

Thermoplastics are polymerized through a complex process that starts with distilling crude oil into naphtha. It is then further processed to create ethylene and propylene that can be polymerized and mixed to produce whichever thermoplastic formulation a user needs.

Advantages & Disadvantages of Thermoplastics
AdvantagesDisadvantages
Advantages

Highly recyclable

Disadvantages

Poor temperature resistance (For common thermoplastics)

Advantages

High volume, low-cost manufacturing

Disadvantages

More expensive than thermosets for limited runs 

Advantages

Chemically resistant

Disadvantages

Softer thermoplastics are susceptible to creep 

Advantages

Excellent surface finishes

Disadvantages

Harder thermoplastics are susceptible to cracking 

Advantages

Good impact resistance

Disadvantages

Limited abrasion resistance

What is a Thermoset?

Thermosets are a class of plastics that are typically created by reacting a pre-polymer with a curative compound. This differs from thermoplastics which melt under heat, where thermoset plastics are compounded and harden through chemical reactions. Heat and careful mixing techniques can assist in forming cross-links between the polymer chains. Full properties may be achieved in as little as an hour or may take as long as a few days. However, once that plastic has been cured it cannot be reformed; the decomposition temperature lies below the melting point. Excessive heat will break the cross-links between the molecular chains permanently. There are many forms of thermosets but the most common ones are listed below:

  • Polyurethane: Polyurethane (PU) comes in many chemical variations. Typically, polyurethane is manufactured as a casting material, as part of an adhesive, or as an expanding foam. Polyurethane is typically more abrasion-resistant than thermoplastic polymers.
  • Vinyl Ester Resin: Vinyl ester resin is used for specialized composites that require additional structural strength or which need to resist moisture absorption. 
  • Epoxy Resin: Epoxy resin is typically used for general-purpose composites like fiberglass, as well as for coatings and adhesives.

How are Thermoset Parts Made?

Thermoset polymers, both rigid and thermoset elastomers, can be manufactured using many different techniques such as:

  • Reaction Injection Molding (RIM): Reaction Injection Molding is similar to ordinary injection molding. The only difference is that thermoset compounds, typically an A+B mixture, are mixed and quickly injected into a mold. Thermosets cure in an exothermic reaction so mold temperature management is critical. Due to the lower viscosity of thermosets, this process can turn out parts with thinner walls than are achievable using thermoplastics. 
  • Resin Transfer Molding (RTM): This is the process used to create composite structures. A fiber layup is inserted into a two-part mold. The mold then closes and resin is injected into it, fully wetting the composite fabric. This results in a high-strength composite structure. Depending on the required properties, either polyester or vinyl ester resins can be used.
  • Casting: Thermoset plastics are commonly cast into an open or closed mold where they slowly harden and can be removed once they set. The process is often called urethane casting, or RTV molding and is considered soft tooling. This method is typically used to manufacture prototypes or low-volume production parts. Casting costs less than RIM for low volumes. 
Xometry's X Logo
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.