What is breathing in compression moulding?

Manufacturers rely on  compression moulding to create consistent, high‑performance parts.

But the process brings its own challenges, especially during curing.

Compression moulding involves placing a precisely measured charge into a mould, which then applies heat and pressure. Under these conditions, the charge forms into the intended shape, and also cures.

But this high pressure can cause problems. As gases develop due to chemical reactions and as moisture vaporises, it can create voids and other structural weaknesses in the component.

Breathing is a way to stop those gases from damaging the finished product. Here’s how it works.

 

What is breathing in compression moulding?

Breathing is a deliberate, controlled opening and re-closing of the mould during the early stages of the compression cycle.

In compression moulding, the mould first closes. It then applies pressure, allowing the material to flow and fill the cavity. Next, the mould briefly opens by a small, preset amount. Then, it closes again to continue the curing process.

This is known as breathing. It’s controlled by the press settings, set by an operator before production begins. It may happen once or several times, depending on the material and part geometry.

The breathing process allows trapped air to escape, releases volatiles during heating to vent and expels moisture from the cavity.

Breathing only happens before the plastic fully cures. Once the component reaches this stage, it’s only giving off negligible amounts of gas. In fact, breathing too late could lead to defects.

Breathing is most common with thermoset resins, fibre-reinforced materials such as SMC and BMC, and parts with large surface areas or varying section thicknesses.

 

Why breathing is used and how it improves part quality

When any material heats up, chemical reactions start to take place. Compression moulding is no exception. Inside the mould, especially when using thermoset materials, those reactions release gases. At the same time, residual moisture in the material expands and converts into steam.

If any of those gases stay trapped, they cause problems with the finished product. Without breathing in compression moulding, manufacturers are likely to see:

 

• Surface blistering
• Internal voids
• Porosity
• Inconsistent density
• Reduced mechanical strength

 

Breathing releases those trapped gases early to prevent pressure pockets from forming inside the part. This improves the internal structure and surface finish. Fibre wet-out (for composites) becomes more uniform. Resin distribution stays more consistent throughout the cavity.

Breathing also makes the entire process more stable and repeatable. Without breathing, the gases affect the pressure levels and behaviour inside the mould. However, letting them escape in a controlled way allows you to stay on top of how the mould behaves. This reduces variation between cycles and wear on the mould itself.

 

When breathing is most beneficial in production

Compression moulding doesn’t always need breathing. It’s most beneficial when parts have large surface areas, section thickness varies across the component, the fibre content is high, the resin releases significant volatiles, or moisture is a concern.

 

For example, SMC and BMC components often benefit from breathing due to their fibre structure and resin content. Electrical components and industrial housings also see gains, particularly where voids would otherwise compromise their performance.

However, even though you don’t necessarily need breathing in every instance, it does widen the allowable process window. In other words, breathing makes the compression moulding process more forgiving and more tolerant of minor variations.

Breathing also supports more complex geometries. As designers come up with larger or more complex parts, the effects of gas build-up are harder to manage. Breathing helps combat this.

 

How Talisman Group provides top-quality compression moulding

If you’re developing a plastic component that needs strength, stability and consistency across multiple runs, compression moulding could be the best production method for you. With the right setup, material choice and tooling, you’ll develop a part that performs reliably in service.

To discuss your requirements and whether compression moulding is the right fit for your application, speak to the Talisman Group team today. We’ll help you decide if compression moulding is the right way to go (or if injection moulding might be better), what materials to opt for, and the best plan to scale and improve your components.