Compression moulding is a versatile manufacturing process. It’s used for producing durable and reliable plastic and composite components. And it’s especially useful when you need your part to perform well under stresses like heat, pressure, electrical loads, chemical exposure or demanding mechanical conditions.
Compression moulding works by placing a measured amount of material into a heated mould cavity. The mould then closes. Inside, heat and pressure are applied until the material flows and takes the shape of the tooling. The material then cures or sets inside the mould before the mould opens and the finished component is removed.
On this page, our team at Talisman Group explains the main types of materials used in compression moulding, why you might choose them, and when compression moulding may offer more benefits than injection moulding.
What materials are used in compression moulding?
The process of compression moulding, as described above, is compatible with several different material groups. However, the most common groups are as follows:
- Thermoset plastics
- SMC and BMC composite moulding compounds
- Silicone, rubber and elastomers
Each material type has its own advantages and drawbacks. Making the right choice depends on your component’s shape, strength requirements, production volume, operating environment and end use.
Thermoset plastics
Thermoset plastics are resin-based materials. They’re among the most common materials used in compression moulding. These are very different from standard thermoplastics such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). A thermoplastic can be melted, cooled and remelted over and over.
A thermoset, however, cures permanently when you expose it to heat and pressure. In other words, it can’t be melted down and reshaped. This makes thermosets useful for components that need to remain stable in demanding environments.
However, this permanent curing process also makes thermosets less forgiving than thermoplastics. Once the material has cured, you can’t remelt your product. This means you need to pay close attention to your material selection, tooling design and process control right from the start. It also makes thermosets harder to recycle.
Common thermoset plastics used in compression moulding include:
- Phenolic resin – Offers heat resistance, electrical insulation, flame resistance, dimensional stability and mechanical strength. Commonly used for electrical components, handles, knobs, switchgear parts and heat-resistant industrial components.
- Melamine formaldehyde – Offers hardness, scratch resistance, heat resistance and surface finish. Often used where the finished part needs a tough, durable, decorative or hygienic surface.
- Urea formaldehyde – Offers hardness, rigidity, electrical insulation and surface finish, although it’s less moisture-resistant than other thermosets. Commonly used for electrical fittings, housings and moulded components.
- Epoxy resin – Offers toughness, chemical resistance, low shrinkage, strong adhesion, electrical insulation and dimensional stability. Often used for more technical or higher-performance components.
- Unsaturated polyester – Offers a good balance of strength, stiffness, chemical resistance, heat resistance and cost-effectiveness. Good for electrical enclosures, equipment housings, automotive panels, covers, trays, appliance components and industrial parts.
- Vinyl ester – Similar to polyester in some ways, but usually chosen when you need improved chemical resistance, corrosion resistance, toughness or durability. Suitable for chemical-resistant tanks, ducts, pipework components, marine parts, electrical enclosures, corrosion-resistant covers and other components exposed to moisture, chemicals or harsh operating conditions.
Both unsaturated polyester and vinyl ester are commonly used with glass fibre reinforcement. Polyester is often chosen for cost-effective strength and stiffness, while vinyl ester is usually selected for tougher, more chemically- or corrosion-resistant components.
SMC and BMC composite moulding compounds
Compression moulding is also widely used with composite moulding compounds. These pre-mixed compounds generally combine thermoset resins, as discussed above, with reinforcement and filler materials. As such, they’re known as reinforced composites. The most common examples are SMC and BMC.
These compounds are made by combining a resin system with fibre reinforcement, fillers, pigments, catalysts, release agents and other additives. As such, they can be produced with specific performance characteristics. That’s a major advantage in the manufacturing process.
However, note that composite moulding compounds can be more demanding than basic thermoplastics. They need to be stored, handled, cut, charged and moulded very carefully. That’s because the fibre distribution, material flow and curing conditions vary from material to material. Using the wrong tooling design or process settings can weaken the finished component, so careful material selection and process control are essential. Common composite moulding compounds used in compression moulding include:
- SMC – Sheet Moulding Compound. Offers strength, stiffness and dimensional stability. Often used for larger, flatter or more structural components like covers, panels, housings, vehicle components or appliance parts.
- BMC – Bulk Moulding Compound. Offers strength, heat resistance, electrical insulation and dimensional stability. Often used for smaller, more detailed technical components like switchgear parts, insulators, housings, connectors, parts with ribs or components with moulded-in inserts.
Silicone, rubber, and elastomers
Although compression moulding is most commonly used with thermosets or reinforced composites, as discussed previously, it can also be compatible with flexible polymer materials such as silicone, rubber, and other elastomers.
When used with these materials, compression moulding is useful for producing flexible parts for sealing, compressing, stretching, cushioning, protecting, or resisting demanding environments. Common applications include seals, gaskets, washers, pads, caps, boots, diaphragms and protective components.
However, using elastomers in compression moulding requires careful control. The specific material, charge size, curing temperature, pressure and mould design can all affect the finished component. Poor process control can lead to inconsistent dimensions, trapped air, flash or incomplete curing.
Common silicone, rubber, and elastomer materials used in compression moulding include:
- Silicone rubber – Offers flexibility, heat resistance, sealing performance and stability across a wide temperature range. Often used for seals, gaskets, pads, medical components, food-grade parts, appliance components and protective covers.
- EPDM – Offers weather, water and ozone resistance, and outdoor durability. Often used for outdoor seals, gaskets, washers, roofing components, automotive parts and components exposed to moisture or changing weather conditions.
- Nitrile rubber – Offers resistance to oils, fuels and greases. Often used for seals, gaskets, washers, fuel-system components and oil-resistant parts.
- Neoprene – Offers a useful balance of flexibility, weather resistance, oil resistance and general durability. Often used for seals, gaskets, pads, protective parts and vibration-control components.
- Fluoroelastomers – Offers high chemical resistance, heat resistance and durability in aggressive environments. Often used for seals, gaskets, O-rings and specialist components exposed to chemicals, fuels, oils or high temperatures.
- Conductive or ESD-loaded rubbers – Offers flexible performance with added electrical conductivity or static-control properties. Often used for seals, pads, connectors, keypads and parts where electrostatic discharge control is important.
Can thermoplastics be used in compression moulding?
You can technically use thermoplastics, such as polypropylene, ABS, nylon, polyethylene and polycarbonate, in compression moulding. However, for standard high-volume thermoplastic components, it doesn’t usually make commercial or practical sense.
Thermoplastics are usually chosen when you need a high-volume, cost-effective component. In those cases, injection moulding is almost always the more efficient production choice because, by nature, it’s better suited to producing large quantities of components quickly and consistently.
Compression moulding, on the other hand, usually requires a more specific setup and careful material charging. It’s also generally a slower production process.
There may be limited scenarios where you’d consider compression moulding a thermoplastic component. For instance, it could make sense for a large, thick, relatively simple part where the material, tooling and production volume suit the process. However, in most cases, injection moulding is the better route for thermoplastic components.
What are the benefits of compression moulding over injection moulding?
Compression moulding and injection moulding are both valuable manufacturing processes. It isn’t a question of one being better than the other. Rather, it’s a question of the benefits compression moulding might bring for your specific product needs.
The best option depends on the material, part design, budget, production volume and performance requirements. Here are the potential benefits of compression moulding over injection moulding (in the right application).
Better suited to many thermosets and composites
One of the biggest benefits of compression moulding is its suitability for thermosetting plastics and composite materials. Thermosets are designed to cure under heat and pressure, which makes them a much better fit for compression moulding than injection moulding. As such, compression moulding is often the better choice if your component requires heat resistance, electrical insulation, chemical resistance or mechanical strength.
Lower tooling complexity
With injection moulding, the tool needs to manage molten plastic as it is forced into the mould at high pressure. That usually means more detailed gating, runner, sprue, cooling and ejection requirements. On the other hand, moulding works differently because the material charge is placed directly into the mould cavity before the tool closes. As such, the overall tooling is simpler by comparison.
This can make compression moulding a more cost-effective option if you need lower or medium production volumes, especially if the part doesn’t justify the upfront cost of a more complex injection moulding tool.
Useful for larger or thicker parts
In injection moulding, the material has to flow through the tool from an injection point. The larger or thicker the part, the harder this is to control. You may run into issues with flow, cooling time, shrinkage, sink marks and tooling cost.
Because compression moulding places the material directly into the mould before pressing it into shape, it can be better suited to forming strong, stable parts without relying on high-pressure injection flow.
Good material efficiency
Compression moulding reduces unnecessary waste because you’re starting with a controlled amount of material for each part. Although injection moulding is highly efficient at scale, some setups create extra waste through runners, sprues or excess material around the moulding system. That depends on the tool and production setup, but it’s still an important consideration.
When is injection moulding the better choice?
In some cases, injection moulding may absolutely be the better option. Materials such as ABS, polypropylene, polyethylene, nylon, polycarbonate, acetal and polystyrene are better suited to injection moulding. Once you know your component needs one of these materials, injection moulding is usually more cost- and time-effective, especially at higher volumes.
Injection moulding is also often better for fine detail in high-volume thermoplastic parts, fast cycle times, tight repeatability and large-scale production. In these cases, its speed and consistency usually make more sense than the slower curing and pressing process used in compression moulding.
Choosing the right material for your component
So, the best material depends on what your component needs to do. Although we’ve listed a few of the most common on this page, we’d be happy to give you more information.
Start with a clear assessment of the most important characteristics of your component. This ensures you design, develop and produce something that furthers your goal, whether it’s an electrical system cover or a large automotive component.
Working with Talisman Group removes the uncertainty around material choice and manufacturing process selection. We help you make the right decision before production begins. Our team supports your component design process, recommends suitable materials, and advises you on whether compression moulding or injection moulding is the best fit. We also bring the reliable process control and more than 100 years of plastic moulding experience needed to produce consistent components without avoidable manufacturing mistakes.
If you’re developing a new part or reviewing an existing component, reach out to us today with your material requirements, performance needs, and production goals. We’d be delighted to help you design and produce the ideal product for your needs.