In Materials

Common Thermoset Materials and Their Industrial Applications


Thermoset plastic has many industrial applications. It’s often the go-to option due to its heat resistance and strength. Unlike thermoplastics, thermosets can’t be remelted, which means your components can stand up to use near high current and in high-heat environments.

At Dielectric, we started in 1955 with a focus on insulating (dielectric) materials. We’re here to help you understand the common thermoset materials and the best applications for each. Here are the details you should know about the different types of thermosets to make a smart selection.

Key Takeaways

  • A thermoset cures into a permanent shape and won’t melt when reheated, so it holds up on hot industrial equipment.
  • Electrical insulation is a standard benefit, so the real choice is heat, strength, moisture, and fire rating.
  • FR-4 and GPO-3 are the two thermosets we run most for electrical distribution work.
  • UL recognition varies by material and grade, so check it before specifying.

G-10 and FR-4 Thermosets

G-10 and FR-4 are glass-epoxy laminates. These are made by layering woven glass cloth and epoxy resin. They are then cured under heat and pressure to create rigid sheets, rods, and tubes. The glass gives the thermoset strength, while the epoxy binds it together, making it nearly impervious to moisture.

FR-4 is the fire-retardant grade of G-10, often used in circuit boards. While FR-4 can replace G-10 in most applications, the reverse doesn’t apply.

Components created from these thermoset materials have both mechanical and electrical strength. The combination makes them a good fit for electrical distribution, defense, and industrial machinery. These materials are strong enough for use on industrial equipment, where temperatures often run high. The added fire resistance of FR-4 makes it the go-to choice when heat safety ratings are required.

GPO-3 Thermoset

GPO-3 is another common thermoset. GPO-3 features a fiberglass-reinforced polyester sheet, typically used for electrical insulation. Like G-10 and FR-4, it’s a glass-reinforced thermoset laminate, but its binder is polyester resin instead of epoxy.

GPO-3 is one of the materials we work with most frequently; in fact, GPO-3 and FR-4 account for about 75% of our thermoset work. These thermosets offer the insulation and structural integrity that electrical distribution requires for applications in defense, industrial manufacturing, and industrial machinery.

GPO-3 is flame-retardant, which is especially important in situations where even a slight fault could ignite a component. It also features low moisture absorption for damp environments. FR-4 also has UL recognition. Some thermoset grades are UL recognized, and some aren’t; confirm the grade and thickness your application requires before specifying.

When choosing between the two thermoset options, FR-4 is the preferred choice when you need mechanical strength. GPO-3 is the choice for arc and track resistance. Of course, if you’re unsure, reach out. Our team will recommend the best grade for your specific application and machine it in low-volume, high-mix runs—in other words, many different part types made in small quantities.

Phenolic Thermoset

Phenolic is a hard, heat-resistant thermoset often used for electrical and structural parts. It’s made with phenolic resin and cured under heat and pressure, which determines how the thermoset performs.

At Dielectric, we work with several types of phenolic thermosets: Grade X paper, canvas, linen, and glass cloth. Glass cloth phenolic, also known as G-3, pairs woven glass fabric with a high-temperature phenolic resin. It offers flexural, compressive, and impact strength. It also has excellent resistance to heat and creep (creep is the tendency of a material to deform under constant load slowly). Resistance to creep matters for structural supports and machine components in applications where there is consistently high stress.

The main caution with this thermoset is moisture. You should only use G-3 in low-moisture applications, which differs from other epoxy-based laminates like G-10 and FR-4, which are nearly impervious to moisture. If your part will see humidity or frequent washdown conditions, Phenolic may not be the right selection.

Phenolic is a strong fit where heat, strength, and electrical insulation all matter and the environment stays dry. For help matching the right phenolic grade to your application, contact our team.

Epoxy Thermosets, G-11

G-11 is a glass-epoxy laminate, made by combining woven glass cloth with epoxy resin and curing under heat and pressure. The thermoset material is a close relative of G-10 and FR-4, but it’s the grade to consider when temperature is a primary concern.

Temperature is a key consideration in material selection. A material that softens or loses strength under the constant heat of industrial machines won’t work. G-11 is often the go-to choice because of its ability to hold strength at higher temperatures than standard glass epoxy.

Like other epoxy laminates, G-11 is extremely resistant to moisture, so it offers stable insulating performance even in humid or damp environments. It also has outstanding mechanical and electrical strength and good dielectric properties, so it’s a go-to for fabricated components in electrical and mechanical applications.

If your application runs hot and demands moisture resistance, G-11 is worth a look alongside G-10 and FR-4. We can help you compare grades and machine parts to your specifications.

Melamine, G-5 and G-9

Glass-melamine laminate is sold as G-5 and G-9. This thermoset combines woven glass cloth with a melamine resin binder. Melamine is one of the hardest and most rigid resins used in industrial laminates, so it’s often a choice for insulating components.

G-5 and G-9 are known for their arc resistance, meaning they resist damage from electrical arcing across the surface. They’re also valued for dimensional stability, and they resist abrasion and caustic chemicals. One caution: prolonged high heat can wear down Melamine’s mechanical and electrical strength.

G-5 and G-9 are especially suited for electrical parts, like insulating components in switchgear and other electrical distribution equipment.

Polyester (the GPO Family)

Glass polyester laminates pair a glass mat with a polyester resin instead of epoxy, and they come in three grades: GPO-1, GPO-2, and GPO-3. Each grade features a slightly different balance of properties. GPO-3 is the grade to choose when you need arc, flame, and carbon-track resistance, which is why it’s used extensively in electrical components.

The family offers a nice middle ground: strong, rigid, and electrically insulating, with polyester’s own resistance profile in place of epoxy’s. GPO-1 is the cost-effective, general-purpose grade, and GPO-2 is chosen when arc or flame resistance is needed.

How to Choose the Right Thermoset Material for Your Application

Picking the right thermoset material comes down to answering a handful of questions about your application:

  • What temperature will the part run at? Thermoset parts often sit on industrial machines where heat is constant, so start here.
  • How much mechanical and impact strength does it need? Structural parts and parts under load call for different materials than simple insulators.
  • Will it be exposed to moisture? Some thermosets absorb moisture, and others don’t; phenolic in particular is best kept dry.
  • Does it need fire resistance? If so, FR-4 is the flame-retardant choice in the glass-epoxy family.
  • Does it need UL recognition? Some grades are UL recognized, and some are not, so confirm the specific grade before you specify.

Not sure? Talk to our team. Our fabrication and machining experts can help you match a material to your requirements, and we have detailed data on each grade.

We source and machine thermoset materials for low-volume, high-mix runs. Whether you need a prototype or a mix of parts across several materials, we can supply the material and machine it to your specs. Request a quote to get started today.