Injection moulding vs compression moulding: What’s the difference?

There’s no ‘best’ process when it comes to injection moulding vs compression moulding. Rather, it’s about finding which best fits the job you need doing.

Injection moulding and compression moulding both exist because they solve different problems. The differences between them come down to avoiding over-engineering, controlling cost and getting appropriate parts into production with minimal compromises.

Here at Talisman Group, we offer both injection moulding and compression moulding. We’ll help you decide which is best for your needs or decide if a mix of both is the right choice. In the meantime, this page gives you a brief overview of the main differences between injection moulding and compression moulding.

 

Injection moulding vs compression moulding: key differences

• The material choice dictates the process. Injection uses thermoplastics, compression uses thermosets and composites.
• Injection moulding is good for fast cycles and high output. Compression moulding tends to be slower, with longer cure times.
• Injection tooling costs more upfront. Compression tooling is more adaptable and lowers your initial investment.
• Injection excels at thin walls and fine detail. Compression handles thicker sections and large, structurally important parts.

 

How injection moulding works

The process of injection moulding forms parts by injecting molten thermoplastic into a closed mould at high pressure. The material flows through runners and gates and fills the cavity. Then, it cools and solidifies. Finally, the mould opens and ejects the part, where it can be sent to the next stages of production.

Injection moulding is best-suited to fast, repeatable cycles. Once the tool is built and the process is set, injection moulding can produce thousands, even millions of parts, per year, even in moderate factories.

 

How compression moulding works

Compression moulding forms parts using a measured charge of material placed directly into a heated mould. The mould closes, pressure is applied, and the material flows to fill the cavity while curing.

Most compression moulding uses thermoset resins or fibre-reinforced composites. Over their time in the mould, these materials cure permanently under heat and pressure. Once set, they don’t melt again, and you can’t reuse them, so it’s vital to get it right first time. However, the result is a strong and durable component, uniform to very small tolerances across multiple runs.

 

Typical materials used

Injection moulding primarily uses thermoplastics such as polypropylene, polyethylene, ABS, polycarbonate, nylon, POM and TPEs. These materials can melt and solidify repeatedly, so they’re great for recycling, regrinding and reusing, within reason.

Compression moulding typically uses thermosets and composites. Common materials include phenolics, epoxies, polyesters, vinyl esters, silicone rubber, and SMC and BMC. The main benefits of these materials are heat resistance, dimensional stability and structural performance.

The material choice depends on what you need your component to do. So, your choice of material usually dictates the process. If you’re likely to need thermosets or composite materials for high-stress environments, compression moulding is likely best. If you need thermoplastics for significant production runs, you probably need injection moulding.

 

Cycle times and production volumes

Injection moulding is a fast production process. The cycle times are very short, and multi-cavity tools can significantly increase output. As such, injection moulding is often the better choice for medium- to high-volume production runs, especially when you have to keep the individual unit cost to a minimum.

Compression moulding processes are slower. They cure with time, heat and pressure, rather than cooling, and this takes longer. Output volumes are better suited for low-to-medium production runs, although you could still develop large-format parts and composite components to justify compression moulding on a larger scale.

Both injection moulding and compression moulding are efficient processes, but they achieve different things.

 

Tooling requirements

The tooling requirements for injection moulding are complex. Injection moulding requires hot runners, gates and cooling circuits. All these add costs and lead times, and if you need to make changes, there are more steps involved. However, this initial investment pays you back via the sheer volume of low-cost output units.

With no runners or gates to manage, compression mould tooling is simpler. Aside from the press and its systems, all you need is the mould and the dedicated control settings. Overall, this means a lower initial investment, and a reduced risk for when you need to make changes to the product’s structure. As such, compression moulding is better for low-to-medium volumes and evolving R&D or prototype work.

 

Part complexity and design freedom

One of the main advantages of injection moulding is how well it forms fine details such as thin walls, snap-fits, or complex internal geometry. You get a strong surface finish and cosmetic consistency across all the output units.

Compression moulding is excellent at producing thick sections, large surface areas and otherwise strong and durable structural features. With composite or thermoset materials, fibre reinforcement and material distribution remain controlled without high-shear flow. This makes compression moulding well suited to parts that need ribs, bosses and other integrated features.

 

Cost considerations and trade-offs

Injection moulding almost always delivers the lowest unit cost at scale. However, it does require a higher upfront tooling investment, and if you need to make changes, the reduced flexibility also increases costs here.

With compression moulding, you get a reduced tooling cost and less material waste, but the cycle times take longer. If it comes down to cost, we suggest weighing up the following:

 

• Volume
• Material requirements
• Performance demands
• Tooling budget
• Likelihood of design changes

 

Choosing the right process

And finally, you don’t have to lock in to just one of these two processes. Injection moulding and compression moulding can be complementary. Many assemblies use both, with different parts produced using the process that best suits its function.

And that’s the key takeaway when you’re deciding between injection moulding vs compression moulding. Function. What does the output unit need to do? Work backwards from there, and you’ll find the right materials, quantities, volumes and other requirements. These help you narrow in on one process or the other, or both.

To discuss your requirements further, and how each process applies to your needs, speak to the Talisman Group team today. With facilities around the UK specialising in both injection moulding and compression moulding, we’ll help you find the optimal manufacturing route for your product.