What Causes Voids Inside Rubber Parts?

I know the sinking feeling when you cut a rubber part for inspection and see an ugly void staring back at you from the middle.

Voids inside rubber parts usually come from trapped air, moisture, poor flow, wrong cure profile, or volatile contamination. The root cause is a mix of part design, compound design, and molding process settings.

When I run projects with new molds in my own factory, I never treat voids as simple “bubbles”. I look at the pattern, the location, and the process history. Then the real cause becomes much easier to see and fix.

What kinds of voids do I normally see inside rubber parts?

Many people call every defect a “bubble”, but different void patterns tell very different stories about the root cause.

You usually see three main types of voids: trapped air pockets, shrinkage voids from poor packing, and random bubbles from moisture or volatiles. Each type has its own typical location and shape.

I like to classify voids before I touch any machine settings. This simple step already filters out a lot of noise and stops people from changing ten things at once.

Common void types and what they mean

When I cut cross-sections¹ of suspect parts in our lab, I often see repeating patterns. These patterns are my first clues.

1. Trapped air pockets²

These look like smooth, round or slightly stretched cavities. They often sit near the last area to fill in the cavity, or near thick sections where air can get trapped.
✅ They usually point to poor venting³, low clamp tonnage, or bad pre-form design in compression molding⁴.

2. Shrinkage voids⁵

These voids often sit near the thickest part of the section and may not be round. They can look like a sunken core or a star-shaped gap near the center.
✅ They usually indicate that the rubber flowed in, started to cure, and then shrank without enough extra material to “feed” that area.

3. Random gas bubbles⁶

These voids look scattered. They might be small, irregular, and not tied to one flow pattern.
✅ They often come from moisture in the compound, dirty molds, or decomposing additives that release gas during cure.

I sometimes use a simple table with my team to speed up diagnosis:

When you see voids in your own parts, try to match what you see to this simple map first. Then your next actions become much more focused.

How does the molding process⁷ create voids inside rubber parts?

I often see teams tweak pressure or temperature randomly when voids appear, and sometimes the parts even get worse. The process has a clear logic, and voids follow that logic.

The molding process creates voids when the cavity does not vent, the rubber does not pack and cure evenly, or the press cycle pushes air and volatiles into thick sections instead of out through vents.

When I troubleshoot at the press, I always connect voids to the actual process curve: how the press closes, how pressure builds, and how long the part stays under heat and pressure.

Where process parameters⁸ go wrong

Rubber molding looks simple from the outside, but the press recipe is a dance between flow, pressure, and cure.

Compression and transfer molding

In compression and transfer molding, trapped air is a classic problem. If the pre-form is too big, too small, or in the wrong shape, it can block vents or leave empty pockets. When the press closes too fast, air has no time to escape.
🛠️ A slower closing profile or a short “breathe” step (close, open slightly, then close again) sometimes lets air out before full pressure.

If cure starts too early because the mold is very hot or the compound is very “fast”, the rubber begins to stiffen before you reach full packing pressure. In that case, the outside skin cures, the core shrinks, and a void opens up. Here, I may reduce the mold temperature slightly or extend the cure time, so pressure and cure line up better.

Injection molding

In injection molding⁹, voids often show up at the last fill areas or deep behind ribs. Poor runner or gate design can cause jetting and trapped air. Incorrect injection speed can also drag air into corners.

I always review the fill pattern first. If the part fills from one side and has long, thin flow paths, the last area to fill is a natural risk zone for voids. Better gate location, balanced runners, and proper venting³ near the end of fill are key.

Simple process checklist for void reduction

When I make changes, I change one or two items at a time and keep clear notes. This way, I can see which adjustment really helps and avoid chasing ghosts.

If you want a deeper look at basic molding process⁷ setup, you can also link this topic to a more general guide like “how to set up rubber molding parameters for consistent quality” on your own blog, so readers can move from defects to prevention.

Can compound design and raw materials cause internal voids?

Sometimes the process looks fine, but voids still pop up. In those cases, I always turn back to the compound and raw material handling.

Compound design and material handling can cause voids when moisture, incompatible additives, or unstable blowing reactions create gas during cure. Even the wrong hardness or filler level can make shrinkage voids worse.

I have seen projects where three different molders had the same void problem with the same compound. That is when I know the issue starts in the compound, not in the press.

How the recipe and mixing affect voids

Rubber compounds are not just polymer and sulfur. They include fillers, oils, accelerators, and sometimes blowing agents or specialty additives. These all affect gas generation and shrinkage.

Moisture and contamination

If the compound absorbs moisture during storage, or if we mix moist filler into it, that water can turn into steam during cure. The result is small, scattered bubbles, often near the hottest zones.
✅ I reduce this risk by storing compounds in controlled conditions¹⁰ and by using dry fillers. For sensitive projects, I limit long open times at the press, so the compound does not sit in humid air for hours.

Volatile additives and overloading

Excess oil or low-boiling plasticizers can vaporize under heat and create gas pockets. I once saw a case where a customer pushed for a very soft compound and requested a lot of oil. The parts felt great, but they started to show internal bubbles at higher cure temperatures. We had to re-balance the formulation to keep softness while reducing volatile load.

Shrinkage behavior and hardness

Different compounds shrink in different ways. High filler loading or very high hardness levels can increase internal stress and shrinkage. If the mold does not give enough extra rubber to feed the core during cure, the material can pull away from the center and leave a void.

Here is how I summarize compound-related risks:

If I suspect a compound issue, I often ask our lab to press small test sheets at different temperatures and times. We then cut them and look for bubbles. This simple test isolates the material from the mold and press, and it often confirms if the recipe itself generates voids.

On your website, this section can link to a more detailed article such as “rubber compound design¹¹ basics for industrial seals”, so purchasing officers and engineers can understand why some materials are more sensitive than others.

How can I prevent voids in future production runs?

Most teams focus on sorting out bad parts once a void problem appears. I prefer to design the whole system so that voids become rare, not normal.

You prevent voids by combining better part and mold design, clean and stable compounding, controlled molding parameters, and a simple inspection and feedback loop that catches issues before they scale.

When I work with long-term partners, we treat void prevention as a shared task. The designer, compounder, molder, and inspector all have a role.

A practical prevention roadmap

I like to build prevention into four pillars: design, material, process, and feedback.

1. Part and mold design

During the design stage, we try to avoid very thick cores without feed paths. We add proper radii instead of sharp corners. We plan vent locations near last-fill areas. For injection molds, we balance runners so the part does not fill from one corner only. Small changes here can reduce risk a lot.

2. Material and mixing

We define compound recipes that match the temperature and pressure profile of the planned process. We avoid unnecessary volatile content and keep moisture-sensitive fillers under control. We also set clear storage rules for mixed rubber. In my factory, we track batch age and keep compounds wrapped until use.

3. Process windows and work instructions

We do not run presses on “feel” alone. We define closing speed, pressure stages, cure time, and mold temperature in simple, clear instructions. We also specify a regular cleaning schedule for vents and cavities. When operators see voids, they do not guess. They check a short troubleshooting¹² sheet that points them to likely causes in order.

4. Inspection, cutting plans, and feedback

For critical parts, we plan regular destructive tests. We cut parts on planned intervals and check cross-sections. We record void patterns and link them back to batch numbers and press records. This feedback closes the loop and allows us to adjust before customers see defects.

Here is a simple summary table you can adapt into an internal checklist or a downloadable PDF on your site:

At Julong Rubber, I follow this roadmap when I develop and produce parts for HVAC, machinery, and other industrial systems. I discuss void risk with customers early, not only when something goes wrong. If you have drawings or defect photos you want to review, you can always reach me through www.rubberandseal.com or by email at info@rubberandseal.com. I am happy to look at your case and suggest both quick fixes and long-term prevention ideas.

Conclusion

Voids inside rubber parts come from a mix of design, material, and process issues; when you classify the voids and fix these three pillars, they become rare instead of a constant headache.