Thermoset Injection Moulding Guide

By Ollie Cockman | 01/03/2022


What is thermoset injection moulding?

Thermoset injection moulding takes plastic and moulds it into various shapes and forms, making them irreversibly formed and are usually stronger than thermoplastic moulding.

Thermoset materials consist of resins supplemented by a ratio of either organic or non-organic catalysts and fillers. Non-organic fillers include products such as carbon fibre or glass beads, whilst organic fillers, such as cellulose or cotton, all are employed to improve the physical properties of the compound. These combinations of materials are mixed to form a granular compound placed into the material hopper before being fed through a hot screw and barrel. It can be stored at temperatures of around 93 degrees celsius. This compound is then injected into a mould which is heated to temperatures of up to 204 degrees celsius via a sprue and runner system, where it fills the voids and creates the final form.

Black rubber dust cover - an example of a thermoset injection moulded product (Above)

Thermoset polymers are permanently cured once they have been cooled and cannot be remelted due to the catalysts' molecular chemical reactions. The polymers become cross-linked and form an unbreakable bond, increasing the strength of the material. However, this means in practice that thermoset plastics can not easily be recycled – instead, they can only be ground down and used as filler for other purposes.

Bulk Moulding Compounds (BMCs) are glass-fibre reinforced thermoset polymer materials that come in a paste form and can be injection moulded. The process is slightly different – forcing the paste to the screw and barrel using a hydraulic stuffer ram before injecting, moulding, and curing in the same way.

To see the specific melting temperature ranges of common plastic materials used within this process, see the table below.

MaterialMelt temperature range (Celsius)Mould temperature range (Celsius)
Polycarbonate 275-325 80-125
Nylon 235-295 40-95
Acrylic 225-255 45-85
HDPE 215-275 20-65
Polypropylene 205-285 30-85
ABS 195-275 40-85
Acetal 185-215 50-125
Polystyrene 175-285 30-65
Thermoset material melting temperature range and mould temperature range (degrees celsius).

What is the difference between thermoset injection moulding and thermoplastic injection moulding?

Thermoplastics like polycarbonate, ABS, polythene and polypropylene are solid at room temperature; more information on these materials can be found here. They soften as they are heated and moulded in the tool until cool. Materials used for thermoplastic injection moulding are excellent for recycling because they can be melted and reused multiple times. However, this ability to melt also demonstrates a lack of heat resistance compared to alternative thermoset materials. On the other hand, Thermoset plastics are usually a gel at room temperature. As they are heated, shaped or moulded, they solidify permanently into their final shape. Although they are poor in terms of recyclability, thermoset plastics tend to be resistant to outside influences, which makes them useful in many applications, including electrical or subsea.

Benefits of thermoset injection moulding

Thermoset injection moulding has several specific applications which depend on the original polymer or compound. In general, thermoset plastics provide thermal and electrical insulation – which is useful for electronic applications. Find out all the benefits below:

✅ Heat resistant

Thermoset plastics can withstand elevated temperatures because they will not remelt – some as high as 370 degrees celsius, and some as low as way below 0 degrees celsius.

✅ Excellent load-bearing capacity

Lightweight thermoset polymers often have a surprising load-bearing capacity, both in compression and shear, especially when flexible.

✅ Shock absorbant

Thermoset plastics tend to have significant impact resistance and can be shock-absorbing. The final properties of the compound can range in hardness and flexibility.

✅ Resistant to harsh chemicals

They are also often resistant to chemicals and solvents, proving reliable for use in applications that include liquids, like the medical industry, because they don’t swell or deteriorate.

✅ Good properties for medical applications

Some thermoset compounds, specifically silicone, are approved for medical applications because they have low toxicity and do not react with the skin or internal organs or fluids. Thermoset injection moulded silicone can even create implantable items for surgical use.

Alongside all these benefits, products that utilise thermoset injection moulding can easily maintain tighter tolerances during the moulding process, which means less room for errors and better quality control than when using similar thermoplastic materials.

Common thermoset injection moulding materials

There are several different thermoset injection moulding materials, and each has slightly different applications because of the bonds they make.


Epoxy resin is one of the most commonly used thermosets, and it is frequently found as both an adhesive and a moulding compound. It is a low-cost resin that can be moulded in many ways, and either forms cross-bonds with itself as it heats up or with other materials when used as an almost unbreakable adhesive.

With high strength and low shrinkage, it is often used to create a high gloss finish on surfaces and even create art. It has good electrical insulation properties, which makes it useful in electric applications and has both chemical and solvent resistance.

However, epoxy tends to be brittle and has a higher moisture sensitivity level than other thermoset injection moulding materials.


Often found as a flexible foam at room temperature, polyurethane is a polymer that can be injection moulded to have various levels of hardness in its final form. This means that it is perfect for applications that need flexibility, and this is the compound of choice for load-bearing and shock-absorbing, even in a harder form.

Lightweight and abrasion-resistant, polyurethane has so many applications depending on the original formulation, staying flexible at low temperatures and being able to resist heat up to 248F (120C) constantly in some cases.

You’ll find thermoset polyurethane in situations where water immersion is necessary, like water-based bearings, because they won’t seize due to swelling from taking in water.


Made through the reaction of phenol and formaldehyde to create a resin, thermoset phenolic is often used as a bonding material on brakes or abrasive wheels. Originally known as Bakelite, this polymer was one of the first plastics to be commercially available and was used most famously in the traditional black telephone.

Lightweight with impressive tensile and impact strength, the phenol-formaldehyde resin can flex and compress, so it can be used in laminating paper and cardboard.

The additional materials added to the phenolic compound make different finished products – including high-speed bearings and balls for tabletop games such as billiards.

Liquid Silicone Rubber

There are many applications that silicone is perfect for, including the medical field. Silicone has a remarkably high heat tolerance, able to deal with constant temperatures up to 316 degrees celsius and intermittent temperatures of up to 371 degrees celsius.

As the compounds are available in different durometers (hardness), some can even maintain flexibility at extremely low temperatures, even double digits below zero degrees celsius.

Of course, the applications for medical use, thanks to low toxicity and no reactions with chemical or solvents, make liquid silicone rubber a sought-after material – but you are just as likely to find it in a vehicle repair shop as you are in a hospital.

Explore our guide: Common items made using plastic injection moulding

You can explore our entire range of injection moulding services here, or head to our technical toolbox for more industry insight.

Ollie Cockman

Head of Content. Football fan ⚽️

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