Common Rubber Moulding Defects: Why They Happen and What to Check

A rubber gasket comes out of the mould with excess material around its edges. Another batch has small air pockets, and a few parts haven't cured properly. These look like separate problems, but each can be traced back to the moulding process.

When a defect appears, the hydraulic press is often the first thing people examine. But the press isn't always responsible. The rubber compound, mould condition, material quantity and curing cycle all affect the finished part.

Through my work with rubber processing machinery and customer requirements at BBD Engineers, I've come to see why it's important to understand the application before focusing on the equipment.

Real-life case study: High Miller Rubber, Kerala

At High Miller Rubber in Kerala, we supplied a hydraulic press that was working perfectly. However, the customer experienced excessive flash around the moulded parts and incomplete curing in certain areas.

The issue highlighted why checking the mould and curing conditions is just as important as checking the machine itself.

Here are five common rubber compression moulding defects, what causes them and what to check. At the end, we cover how to tell when the press really is the problem.

1. Excess Flash Around the Moulded Part

Flash is the thin layer of excess rubber that appears along a moulded part's edges or parting lines.

Some flash is expected in compression moulding, depending on the mould design. Excessive flash, though, means extra trimming, more material waste and possibly rejected parts.

One common cause is the amount of rubber placed in the mould. If the charge is too large, the excess escapes as the mould closes.

The mould itself can also be responsible. Worn parting surfaces, poor alignment, or residue that prevents the mould from closing fully can all allow rubber toescape.

What to check

• Compare the rubber charge weight with the approved production specification.

• Inspect the mould's closing surfaces for wear or residue.

• Confirm the press is reaching the required closing force.

Note where the flash appears. Flash all around the part usually points to charge weight or overall closing force. Flash mainly on one side points to mould alignment or uneven pressure.

2. Air Pockets and Blisters in Rubber Parts

Air trapped inside a mould can leave small voids, bubbles or raised areas on the finished part.

This matters most when producing rubber seals, bushes or other parts where internal defects affect performance.

During compression moulding, air needs a way out as the rubber flows into the cavity. If the mould vents are blocked or poorly placed, some air stays trapped.

The shape and placement of the rubber preform also affect how the cavity fills. Moisture or volatile materials in the compound can add to blistering.

What to check

• Check that mould vents are clear and correctly placed.

• Review how the preform is shaped and positioned in the cavity.

• Check the compound for moisture and confirm it was stored and prepared correctly.

Some processes use a controlled bumping (mould-breathing) cycle to release trapped air before curing progresses. Whether this suits your process depends on the mould, compound and approved operating procedure. Check these points before changing any press settings.

3. Short Fills (Incomplete Mould Filling)

Sometimes a part comes out with missing corners, incomplete edges or poorly formed sections. This is a short fill, also called incomplete mould filling or non-fill.

The problem may start before the press closes. If the preform has too little material or sits in the wrong position, the compound may not reach every part of the cavity.

Rubber flow also depends on temperature, compound viscosity and the time available before curing begins. If the rubber starts curing before it reaches the thinner sections of a mould, those areas stay incomplete.

This is why raising the pressure isn't automatically the fix.

What to check

• Weigh the rubber charge and check where the preform sits.

• Check mould venting, since trapped air can block flow into corners.

• Review compound flow and mould temperature.

• Then compare press closing conditions with the established process.

4. Incomplete Curing and Uneven Rubber Properties

A moulded part may look fine when it leaves the press but still fail hardness, compression set or other quality tests. One likely reason is undercure.

Vulcanisation needs the right combination of temperature and time for the rubber formulation. If either falls short, the compound won't develop its intended properties.

Part thickness matters too. A thick rubber bush and a thin gasket may need different curing cycles, even when made from the same compound.

Check the actual mould temperature, not just the temperature on the press controller. The controller may show that the heating system has reached its set point while parts of the mould are still cooler.

What to check

• Measure mould temperature at several points with a suitable method, such as a surface pyrometer.

• Compare curing time and temperature with the compound's validated specification.

• Confirm the cure cycle suits the part's thickness.

Changing curing time without understanding the temperature and material requirements can create new problems.

5. Rubber Sticking to the Mould After Curing

A finished part should come out of the mould without damage to its edges or surface. Some compounds stick, making demoulding difficult.

The mould surface is one likely factor. Residue from earlier cycles, surface wear or unsuitable release practices can all make separation harder.

Curing conditions also play a part. An undercured part may deform or tear during demoulding.

What to check

• If sticking starts after several good cycles, inspect the mould for built-up residue.

• Compare current curing conditions with records from earlier good batches.

• Follow the approved cleaning, release-agent and maintenance procedures for the compound and tooling.

How to Tell When the Press Is the Problem

If the compound, charge weight and mould all check out, look at the press itself. These signs point to the equipment:

• Pressure drops during the cure cycle. This often means a hydraulic seal or valve leak.

• Flash or thickness differs from one side to the other. Check platen parallelism and how evenly pressure is applied.

• Cure varies with cavity position. Measure surface temperature across the platen; cooler edges or corners mean uneven heating.

• Closing time is slow or changes from cycle to cycle.

A pressure gauge log and a surface pyrometer will confirm most of these. If the press is the cause, the fix may be a repair, a heating upgrade, or a press matched to the part's size and tonnage.

How to Investigate Repeated Moulding Defects

When defects continue across several batches, changing several press settings at once makes it hard to see what caused the problem. Start by recording the defect and comparing current conditions with a batch that was accepted earlier.

Defect	Check first	Press-side check
Excessive flash	Charge weight, mould alignment, closing surfaces	Closing force, platen parallelism
Air pockets or blisters	Venting, preform placement, moisture	Closing speed, bumping cycle (if used)
Short fill	Material quantity, compound flow, mould temperature	Closing speed and pressure
Incomplete curing	Actual mould temperature, curing time, compound specification	Platen temperature uniformity
Rubber sticking to mould	Mould cleanliness, surface condition, curing	Platen temperature consistency

These checks help narrow down the cause. They are not a substitute for proper testing or an approved troubleshooting procedure.

Frequently Asked Questions

What causes flash in rubber moulding?

Flash usually comes from too much rubber in the mould, worn or misaligned parting surfaces, or a press that isn't reaching the required closing force. Where the flash appears helps narrow down the cause.

How do you prevent air bubbles in moulded rubber parts?

Keep mould vents clear, shape and place the preform so air can escape, and control moisture in the compound. Some processes also use a bumping cycle to release trapped air before curing.

How can you tell if a rubber part is undercured?

An undercured part may look normal but fail hardness or compression set tests, or tear during demoulding. Check the actual mould temperature and cure time against the compound's specification.

Is a moulding defect always a press problem?

No. Most defects start with the compound, the rubber charge or the mould. Pressure loss during cure, side-to-side variation or uneven platen heating are the signs that the press needs attention.

Final Thoughts

Rubber moulding quality depends on the compound, mould design, curing conditions and press working together.

A defect in the finished part doesn't always mean the hydraulic press needs replacing. Sometimes the fix is material preparation or mould maintenance. In other cases, press condition, heating consistency or capacity needs a closer look.

If you're reviewing your production equipment, BBD Engineers supplies hydraulic presses for rubber moulding and hydraulic presses with thermic fluid, steam or electric heating.

Facing a recurring defect and not sure if it's the press? Send us the part, compound and press details on WhatsApp or at bbdengineers2120@gmail.com, and our team will get back to you.

About the author

Basant Kaur writes for the BBD Engineers team, bringing her curiosity and hands-on involvement in client discussions, business development and technical aspects of rubber-processing machinery to her articles. Her writing focuses on manufacturing challenges, machinery applications and making technical knowledge more accessible to industry professionals.