In plastic fabrication, thermoplastics and thermosets are two primary types of plastic materials, commonly used in injection molding, CNC machining, 3D printing, and extrusion. Both are made of polymers, which consist of long, repeating chains of molecules. Under a microscope, thermoplastics look like tangled, free-flowing ropes, while thermosets resemble a tightly woven network. The polymer’s structure has a direct influence on the properties and performance. The main difference between thermoplastics and thermosets is how they respond to heat. Beyond this, they have many other differences. Reading on, you’ll find more details of comparison about them in this article.
What Is a Thermoplastic?
A thermoplastic is a resin that is solid at room temperature. When heated, the pellets soften and eventually become fluid, either due to crystal melting or crossing the glass transition temperature. Upon cooling, they harden into a desired shape without chemical bonding involved. This makes it possible for thermoplastics to be easily reheated, reshaped, and recycled multiple times without undergoing any permanent chemical change.
They generally tend to resist shrinking and offer good strength and elasticity.They are broadly classified into two types based on their molecular structure:
Amorphous thermoplastics, with a less ordered structure, offer lower heat resistance but better toughness at low temperatures. Some are transparent, suitable for applications requiring optical clarity.
Semi-crystalline thermoplastics have a more ordered structure, which provides them with greater heat and chemical resistance. They also have enhanced strength and stability, making them well-suited for high-performance applications.
Thermoplastics are often used in injection molding, extrusion, thermoforming, blow molding, rotational molding, and many other fabrication techniques. Some commonly used thermoplastics include:
Can be reshaped and recycled with little impact on material properties.
Easy to process.
High-quality aesthetic finish.
High impact resistance.
Highly resistant to chemicals.
Good electrical insulation.
Generally lower production and processing costs.
High flexibility.
Cons
Not suitable for high-temperature applications because it may soften upon heating.
May deform under prolonged high temperature or continuous load.
In injection molding, thermoplastics are more prone to flash due to their lower viscosity when melted and the high pressures used in processing.
What Is a Thermoset?
A thermoset (or thermosetting plastic) is a polymer that generally exists as a liquid resin (or occasionally as a soft solid) at room temperature. When heated or mixed with a catalyst, it undergoes a curing process that forms an irreversible, rigid, cross-linked structure. This permanent structure gives thermosets superior resistance to heat, enhanced corrosion resistance, and greater resistance to creep—the tendency of a material to permanently deform under mechanical stress—compared to thermoplastics. This makes thermosetting polymers an excellent choice for parts that require outstanding dimensional stability at elevated temperatures, like air intake manifolds in automobiles or brake calipers in heavy machinery.
However, thermosets have some limitations. Once cured, they cannot be remelted or reshaped like thermoplastics, and they tend to be less impact-resistant and more prone to shattering. Disposing of thermosets is more challenging since they cannot be easily reprocessed and recycled, though some can be ground into filler for composite materials.
Moreover, their high hardness and brittleness make surface finishing (such as polishing and grinding) and mechanical machining more difficult. During injection molding, it is crucial to prevent a thermoset from reaching its cross-linking temperature until it has fully filled the mold, as premature curing can cause defects and hinder proper shaping.
Thermosetting plastics are more often found in urethane casting, compression molding, resin transfer molding (RTM), reaction injection molding (RIM), and filament winding. Some commonly used thermosets include:
Amino resin
Epoxy resin
Vulcanized rubber
Phenolic resin
Melamine formaldehyde
Polyurethane polymer
Silicone resin
Pros and Cons of Thermosets
Pros
Excellent heat resistance.
High dimensional stability.
Good mechanical strength.
Superior chemical resistance.
Outstanding electrical insulation and dielectric strength.
Cons
Cannot be remelted or reshaped once cured, making recycling difficult.
Damaged parts are challenging to repair or modify.
Complex processing requirements and the need for specialized equipment can increase costs.
May be more brittle and prone to fracture under impact.
What’s the Difference Between Thermoplastic and Thermoset?
There are hundreds of different thermoplastics and thermosets. Depending on the specific materials compared, their properties can be nearly identical or vastly different. In the following discussion, we will focus on the typical differences—not the universal ones—between the two classes of polymers.
Microstructure
Thermoplastics consist of long, linear or branched polymer chains that are not chemically cross-linked to each other, forming a structure that can be either amorphous or semi-crystalline. This results in a relatively loose, entangled arrangement that allows for flexibility and remolding. In comparison, thermosets have a densely cross-linked network where the polymer chains are permanently bonded. This rigid, interlocked framework strengthens the heat resistance and structural stability of thermosets.
Heat Deflection Temperature
Heat deflection temperature (HDT) measures a plastic’s ability to maintain its shape under load at elevated temperatures. This parameter is closely linked to the material’s molecular structure, distinguishing thermoplastics from thermosets. Thermoplastics—composed of linear or weakly branched chains—typically exhibit lower HDT values that remain well below their melting points. For instance, PA66 nylon begins to deform under load between 70°C and 90°C, even though its melting point is around 260°C; this progressive softening results from the mobility of its polymer chains, which can slide past one another when heated. In contrast, thermosets such as epoxy resins maintain dimensional stability up to 200–300°C or higher, thanks to their permanently cross-linked structure that acts as a molecular “lock,” resisting deformation until thermal decomposition occurs.
Corrosion Resistance
Thermosets typically offer superior resistance to chemicals, acids, and bases. Their highly cross-linked three-dimensional network minimizes the penetration of chemical substances, enhancing material stability in harsh environments. While many thermoplastics (such as PVDF and PTFE) also have good chemical resistance, their relatively loose molecular structure makes them more susceptible to swelling or degradation when exposed to highly corrosive media or prolonged chemical contact.
Durability
Thermosetting plastics have highly stable structures that provide excellent durability even under prolonged exposure to high temperatures and corrosive environments. However, they have lower toughness and may crack or fracture under low temperatures or high-impact loads. Moreover, once cured, they cannot be reprocessed by heating; if damaged, they generally must be replaced rather than repaired or recycled, which can be seen as a limitation in some long-term applications.
Thermoplastics, by contrast, offer superior toughness and impact resistance due to their physically entangled polymer chains. This structure allows for significant deformation capacity and enables repeated melting and reshaping for repair or recycling. Nonetheless, their performance may gradually degrade under sustained high temperatures or repeated mechanical stress.
Aesthetics
Thermoplastics generally offer greater flexibility and versatility in terms of color, transparency, and detailing.They are traditionally popular in consumer products for achieving a high-quality finish. In contrast, thermosetting plastics focus on structural strength and durability. Although their appearance is generally more fixed, the RIM and RTM processes offer a unique opportunity to ensure superior aesthetics. Many RIM/RTM techniques allow for in-mold coating and painting before the thermoset resin is injected, creating strong adhesion between the coating and the plastic surface. This robust bond helps prevent issues such as chipping, flaking, cracking, and other injection molding defects.
Moreover, certain thermosetting materials—such as epoxy resin—typically have lower viscosity, allowing them to capture even the tiniest aesthetic details, such as intricate patterns or the addition of logos. This often results in finely detailed textures, high gloss, and excellent visual appeal.
Applications
Both thermoplastic and thermoset materials can be used effectively across various applications. For some uses, the unique attributes of one type make it the optimal choice, while for others, both can serve similar functions.
Thermoplastic-Exclusive Applications:
For applications that require recyclability, high-quality finishes and optical clarity, these products are typically manufactured from thermoplastics. Below are the common uses:
Consumer goods (toys, toothbrushes, textiles, and storage containers)
Packaging (food containers, bottles, and flexible films)
Medical and science laboratory equipment
Windows and lighting fixtures
Thermoset-Exclusive Applications:
Some applications require the exceptional heat resistance, dimensional stability, and robust chemical resistance offered primarily by thermosets. These include:
Construction materials (adhesives, sealants, and insulation materials)
Heat shields
Panels, housings, and doors
High voltage insulators
Light watercraft components (boats, kayaks, and jet skis)
Applications Where Both Are Used:
Lightweight components in automobiles, aircraft, and heavy machinery
Cookware and kitchenware (handles, utensils, and surfaces)
Electronic components (circuit boards, housings, and electrical insulation parts)
Piping and plumbing
Cost
Thermoplastics are generally more cost-effective in both raw materials and processing. Their formulations are relatively simple, typically requiring fewer crosslinking agents, curing agents, or additional fillers. They can be remelted and reprocessed, eliminating the need for the long curing times required by thermosets. This leads to shorter production cycles, greater flexibility, and lower tooling and energy costs in high-volume manufacturing. Additionally, scrap material can be recycled, further reducing production costs.
In contrast, thermosetting plastics typically have higher material costs, particularly for specialty resins like epoxies. Their processing involves a curing step with chemical crosslinking, which requires specialized equipment and molds, as well as tighter process control. Furthermore, once cured, thermosets cannot be reprocessed or recycled, leading to higher material waste and disposal costs.
Thermoplastic vs. Thermoset: Which One Is Your Best Option?
Until now, we believe that you have an overall understanding of the differences between thermoplastics and thermosetting plastics. Here’s a concise breakdown to help you determine which material might be the best option for your needs:
Choose Thermoplastics If:
Recyclability is important. Thermoplastics can be melted and reshaped multiple times, making them environmentally friendly and cost-effective for reusable products.
Flexibility and impact resistance are needed. Their inherent durability, elasticity, and resistance to impact make them perfect for applications requiring rugged performance.
Aesthetic appeal is a priority. They offer high-quality surface finishes, and can be molded into sleek, visually appealing forms relatively easily.
Ease of processing is key. Thermoplastics are well-suited for high-speed manufacturing processes such as injection molding, extrusion, and blow molding.
Cost-effectiveness is critical. Their recyclability and efficient processing generally make them more affordable for mass production.
Choose Thermosets If:
High-temperature resistance is required. They perform quite well in applications where heat resistance and stability are needed, as they do not melt or deform under high temperatures.
Superior strength and durability are demanded. Their cross-linked structure provides exceptional mechanical strength, rigidity, and resistance to mechanical creep.
Chemical and corrosion resistance are essential. Thermosets offer outstanding resistance to harsh environments—such as chemical processing or electrical insulation.
Long-term performance is a priority. They are suited for applications that demand long-lasting stability under stress or extreme conditions.
Conclusion
Thermoplastics vs. thermoset plastics provides a quick overview of the differences between polymer-based materials. There are hundreds of individual thermoplastics and thermosets, however, and each material has specific properties. Because there are so many options, it helps to have expert advice before choosing a plastic.
Chiggo is always ready to bail you out. In addition to helping you choose the right plastic, we offer robust design of manufacturing (DFM) advice and feedback, and can get you T1 samples in as fast as 2 weeks! Ready to get started? Get your instant quote today.
FAQ
Is polyester a thermoplastic or thermoset?
Polyester includes different types of polymers. For instance, polyethylene terephthalate (PET) is a thermoplastic widely used in beverage bottles and textiles, while unsaturated polyester resins (UPR) or vinyl ester resin, often used in fiberglass-reinforced plastics, are thermosetting. The answer depends on the specific type of polyester in question.
What will happen to a thermoset plastic if it is reheated?
Thermoset plastics cannot melt or soften when reheated due to their permanent cross-linked structure formed during curing. Instead, they maintain their solid structure even at elevated temperatures, and if exposed to excessive heat, they will decompose or char.
