1. Air Bubbles and Voids

Air bubbles are probably one of the first defects operators notice in an LSR molded part.

They can appear as small visible bubbles, internal voids, cloudy areas, or larger air pockets trapped inside the product.

Common causes

Several things can create air entrapment:

Air remains in the mold cavity.

The mold venting is insufficient.

Injection speed is too high for the mold design.

The material contains trapped air.

The injection system is not properly degassed or maintained.

The injection point is poorly positioned.

The material flow creates an area where air cannot escape.

A common mistake is to immediately reduce injection speed.

Sometimes that works. Sometimes it makes the problem worse.

The better approach is to understand where the air is coming from and where it is supposed to go.

Possible solutions

First check the mold venting. If the cavity cannot release displaced air, adjusting the machine will only have a limited effect.

Then check the injection process and material feeding system.

It is also important to inspect the material itself. LSR normally has very good flow characteristics, but poor handling, contamination, improper storage, or problems in the metering and mixing system can introduce process instability.

For complex parts, mold venting and injection gate design should be considered together rather than separately.

A bubble problem is often a mold-flow problem, not simply a silicone problem.

2. Flashing

Flash is another very common LSR molding defect.

It appears as a thin unwanted layer of silicone around the parting line, inserts, or other areas of the mold.

Because LSR has relatively low viscosity before curing, it can enter very small gaps in the mold.

Why does LSR produce flash?

Typical causes include:

Excessive injection pressure.

Mold clamping force is insufficient.

The mold parting line is damaged or worn.

The mold has excessive clearance.

Injection volume is too high.

Material viscosity is not suitable for the application.

Mold temperature or curing behavior is not properly controlled.

But there is an important point here:

Increasing clamping pressure is not always the correct solution.

If the mold itself has excessive clearance or a damaged parting line, simply increasing clamping force may hide the problem temporarily while increasing mold wear.

How to solve it

Start by inspecting the mold.

Check:

Parting-line condition.

Mold alignment.

Insert fit.

Clamping force.

Injection pressure and speed.

Shot size.

If flash suddenly appears after the mold has been running normally for a long period, mold wear should be one of the first things to investigate.

3. Short Shots

A short shot occurs when the silicone does not completely fill the mold cavity.

The result may be an incomplete part, missing edges, or insufficient filling around thin sections.

Common reasons

Injection pressure is too low.

Injection speed is too slow.

The material starts curing too early.

Mold temperature is too high in certain areas.

The gate or runner is too small.

The material cannot flow properly through a thin section.

Air is trapped inside the cavity.

The injection volume is insufficient.

This is where LSR molding becomes more interesting.

It is tempting to simply increase injection pressure.

But if the real problem is premature curing, increasing pressure may not solve the fundamental issue.

The material may still fail to reach the end of the cavity before its flow becomes restricted by curing.

What should be checked?

Look at the entire filling process:

Material → Metering → Mixing → Runner → Gate → Cavity → Venting → Cure

A short shot can originate from any point in this chain.

For a new mold, mold-flow analysis can be very useful. For an existing mold, comparing the current process with historical production data can help identify whether the problem is related to the mold, equipment, or material.

4. Uneven Curing

Uneven curing can be particularly frustrating because the molded parts may look acceptable at first glance.

After demolding, however, some areas may remain soft while other areas are fully cured.

In more serious cases, the finished part may have inconsistent mechanical properties.

Possible causes

Uneven mold temperature.

Poor temperature control.

Different cavity temperatures.

Incorrect curing parameters.

Inconsistent material mixing.

Contamination.

Problems with the platinum catalyst system.

Excessive variation in injection cycle time.

LSR curing is highly dependent on temperature.

That means temperature control is not simply about setting the machine to a certain number.

What matters is the actual temperature inside the mold and whether that temperature remains stable throughout production.

A machine display showing 170°C does not automatically mean every part of the cavity is actually at 170°C.

For multi-cavity molds, temperature balance becomes even more important.

5. Silicone Sticking to the Mold

Ideally, the cured silicone part should release cleanly from the mold.

When it sticks, production becomes slower and the operator may need to manually remove parts. Repeated sticking can also damage the product or mold.

Why does sticking happen?

Possible reasons include:

Incorrect mold surface treatment.

Poor mold surface condition.

Incorrect curing conditions.

Contamination on the mold.

Incompatible mold release conditions.

Part geometry that creates mechanical locking.

Excessive adhesion between the silicone and mold surface.

Before adding a mold release agent, I would recommend checking the process first.

Mold release can be useful, but it should not become a substitute for solving a fundamental molding problem.

For products that require a clean surface, especially medical, food-contact, or high-performance applications, the use of release agents must also be evaluated carefully.

6. Poor Demolding and Part Deformation

Sometimes the silicone does not stick to the mold, but the part still does not come out correctly.

The product may stretch, bend, tear, or lose its designed dimensions during demolding.

This is especially common with thin-wall or soft LSR parts.

Possible causes

The part is demolded before it has sufficient strength.

Mold temperature is not uniform.

The product design has insufficient draft.

Undercuts are too aggressive.

Demolding force is too high.

The silicone hardness is not suitable for the product.

The mold opening sequence is not appropriate.

This is a good example of why material selection and mold design cannot be separated.

A very soft silicone may be excellent for flexibility, but that does not mean it will automatically be easy to demold from every mold design.

When developing a new LSR product, the silicone hardness, tear strength, elongation, shrinkage, and mold geometry should be considered together.

7. Uneven Surface or Poor Surface Finish

LSR products can have very smooth and attractive surfaces, but surface defects are still common.

Typical problems include:

Dull areas.

Gloss variation.

Flow marks.

Weld lines.

Surface bubbles.

Uneven texture.

Contamination marks.

What causes these problems?

The mold surface is often the first thing to check.

If the mold has different surface finishes in different areas, the silicone will reproduce those differences.

But surface quality is not always a mold issue.

Injection speed, material temperature, mold temperature, contamination, and curing behavior can all affect the final appearance.

For high-end consumer products, medical components, or optical-related applications, surface appearance can become a major quality criterion.

In these cases, it is better to define a measurable surface standard instead of simply saying the product should have a "good appearance."

8. Weld Lines

Weld lines occur when two or more silicone flow fronts meet during filling.

They are particularly noticeable around holes, inserts, ribs, and complex geometries.

Depending on the product design, a weld line may only be a cosmetic issue. In other applications, it can become a mechanical weak point.

How can weld lines be reduced?

Possible approaches include:

Changing the gate position.

Adjusting injection speed.

Improving mold temperature uniformity.

Modifying the runner system.

Changing the product geometry.

Improving venting.

The key point is that weld lines are often created before the material enters the mold.

If the gate location forces the silicone to split into two flow fronts and then meet in a critical area, changing the material may not eliminate the problem.

Sometimes the best solution is simply to redesign the gate.

9. Dimensional Variation

LSR molded products are often used in applications where dimensional consistency matters.

Examples include:

Seals.

Gaskets.

Medical components.

Automotive components.

Connectors.

Membranes.

Precision molded silicone parts.

Dimensional variation can result from:

Mold temperature variation.

Material shrinkage.

Different curing conditions.

Inconsistent injection volume.

Mold wear.

Machine repeatability issues.

Incorrect mold design compensation.

One important point is that silicone does not behave exactly like a rigid thermoplastic.

Its flexibility, thermal expansion, curing shrinkage, and deformation during demolding all need to be considered.

Therefore, measuring the part immediately after demolding may not always tell the whole story.

For precision applications, manufacturers should define when and how the dimensions are measured.

10. Curing Problems Caused by Contamination

This is one defect that deserves more attention.

LSR systems, especially platinum-cured systems, can be sensitive to certain contaminants.

In production, contamination may come from:

Gloves.

Lubricants.

Mold release agents.

Cleaning chemicals.

Other rubber materials.

Certain plastics.

Adhesives.

Poorly cleaned equipment.

The result can be incomplete curing, sticky surfaces, soft spots, or inconsistent mechanical properties.

This is one reason why an LSR production line should not be treated like a general-purpose rubber or plastic processing line.

Material compatibility matters.

When a curing problem appears suddenly, do not immediately assume that the silicone batch is defective. Check what changed in the production environment first.

A Practical Way to Troubleshoot LSR Molding Defects

When a defect appears, changing three or four parameters at the same time is usually a bad idea.

You may get a better part, but you will not know why.

A more reliable troubleshooting method is to work through the process systematically.

Step 1: Define the defect

Do not simply record "bad molding."

Record exactly what happened.

For example:

Flash on cavity No. 4.

Bubbles near the gate.

Short shot at the end of the flow path.

Soft area around the insert.

Part sticking after 20 cycles.

The more specific the description, the easier it becomes to find the cause.

Step 2: Check whether the problem is new

Ask:

Was this mold producing good parts before?

If yes, look for recent changes.

For example:

New silicone batch.

New operator.

New mold maintenance.

New temperature setting.

Machine maintenance.

Change in ambient conditions.

New release agent.

Change in cycle time.

This simple question can save a lot of time.

Step 3: Change one variable at a time

If you increase pressure, lower the mold temperature, change injection speed, and modify the curing time simultaneously, you lose the ability to identify the real cause.

Controlled adjustments are slower at first but much faster in the long run.

Step 4: Look at the complete molding system

An LSR molding process is not just a material.

It is a system:

LSR Material → Metering → Mixing → Injection → Mold → Temperature → Venting → Curing → Demolding

A defect can originate at any point.

Choosing the Right LSR Is Only Part of the Solution

There is a tendency in the silicone industry to ask:

"Which LSR should I use?"

That is an important question, but it is only the beginning.

The right material depends on the actual application.

A suitable LSR may need to provide:

Appropriate hardness.

Good flowability.

High tear strength.

High elongation.

Low compression set.

Suitable curing speed.

Good release behavior.

High transparency or suitable colorability.

Resistance to heat, chemicals, or aging.

Compliance with application-specific requirements.

For medical applications, the requirements are even more demanding. Material selection, curing conditions, processing equipment, cleanliness, and regulatory requirements all need to be considered together.

For automotive or industrial applications, mechanical performance, temperature resistance, compression set, bonding, and long-term durability may become more important.

There is no single "best LSR."

There is only LSR that is appropriate-or inappropriate-for a particular application and process.

Final Thoughts

Liquid Silicone Rubber molding is a mature technology, but that does not mean every molding problem has a simple answer.

A bubble does not always mean bad silicone.

Flash does not always mean excessive pressure.

A short shot does not always mean insufficient injection pressure.

And a curing problem does not always mean the material is defective.

The most effective troubleshooting approach is to look at the entire molding system rather than blaming one parameter.

At ANYSIL SILICONE, we believe that material suppliers should do more than simply provide an LSR product and a technical data sheet.

A good silicone material needs to work with the customer's machine, mold, process, and final application.

When a customer encounters a molding problem, the right question is not simply:

"Which silicone should we change to?"

The better question is:

"What is actually happening inside the molding process?"

Once that question is answered, the right solution is usually much easier to find.

Need Help With Your LSR Molding Process?

ANYSIL SILICONE supplies Liquid Silicone Rubber for a wide range of industrial applications, including precision molding, automotive components, medical applications, silicone parts, and other demanding applications.

If you are experiencing problems such as flash, bubbles, short shots, poor demolding, incomplete curing, dimensional variation, or inconsistent surface quality, share your material grade, mold information, processing conditions, and the defect you are seeing.

The more information we have, the easier it is to determine whether the real problem comes from the LSR material, mold design, equipment, or processing conditions.

ANYSIL SILICONE - Liquid Silicone Rubber Solutions for Industrial Applications.