How to Reduce Flash and Defects in Liquid Silicone Rubber Injection Molding

Liquid Silicone Rubber (LSR) injection molding is one of the most precise ways to produce silicone components for medical devices, automotive seals, baby-care products and consumer electronics. The material offers outstanding flexibility, heat resistance and long-term stability, and modern LSR machines can hold tolerances that conventional compression molding cannot match.

In real production, however, quality problems still appear. Flash at the parting line, incomplete filling, air bubbles, dimensional drift and sticky or under-cured parts are the most frequent complaints in daily LSR manufacturing. In most cases the silicone material itself is not the cause. The defects usually trace back to the interaction between clamping force, mold condition, dosing accuracy, injection parameters, temperature control and material handling.

This article explains where these defects come from and which practical measures manufacturers can take to reduce them. The focus is on actions that can be applied to an existing LSR production line without replacing the entire system.

1. Why LSR Is Less Forgiving Than Conventional Rubber

Before discussing individual defects, it helps to understand why LSR production demands tighter control than solid silicone rubber molding.

Low viscosity. LSR is supplied as a flowable two-component liquid. It easily enters gaps that solid silicone would never reach, which is excellent for filling complex cavities, but it also means that any imperfection at the parting line becomes flash.

Two-component chemistry. Component A and Component B are metered and mixed in a defined ratio, usually 1:1. Even a small deviation in ratio changes curing speed, hardness and mechanical strength. Consistent products therefore depend on consistent dosing.

Heat-triggered, fast curing. LSR cures quickly at mold temperature. The material has to reach the cavity and fill it before scorch begins. Injection speed, mold temperature and venting all interact directly with the curing reaction.

High material cost and costly deflashing. Silicone is an expensive raw material, and removing flash from a finished silicone part is time-consuming and can damage the surface. Preventing flash is always cheaper than removing it.

These four characteristics explain why process discipline matters more in LSR than in almost any other molding process.

2. The Most Common LSR Molding Defects

Most production issues fall into a small number of categories:

• Flash / burrs at the parting line or around ejector pins
• Incomplete filling (short shot), especially in thin walls and long flow paths
• Air entrapment, appearing as bubbles, voids or burnt spots
• Dimensional variation and unexpected shrinkage
• Surface defects such as flow marks, white spots, contamination and oil bleeding
• Under-cure, resulting in sticky parts, low tear strength and compression set failure
• Over-cure, resulting in brittle parts and tearing during demolding

Each defect has more than one possible cause, and several of them can appear at the same time. A systematic approach is therefore more effective than adjusting single parameters at random.

3. How to Reduce Flash

Flash is the most common and most costly LSR defect. Because the material is so fluid, flash appears whenever the cavity pressure exceeds the sealing capability of the mold.

Check the clamping force margin. LSR injection requires high clamping force relative to injection pressure. If the machine is running close to its clamping limit, cavities will open slightly during injection. Selecting a machine with adequate clamping force for the projected area of the mold is the first step.

Inspect and maintain the parting surface. A worn, damaged or poorly fitted parting line cannot seal low-viscosity silicone. Mold maintenance intervals should be based on shot count, and the parting surface should be inspected and re-fitted before flash starts to appear.

Control shot size and dosing accuracy. Oversized shots push excess material into the parting line. A closed-loop dosing system that monitors the A/B ratio and injects an accurate shot volume is far more reliable than manual adjustment.

Balance injection pressure and speed. Excessive injection pressure or an aggressive speed profile raises cavity pressure beyond what the clamp can hold. Profiled injection, with a controlled fill phase, reduces peak pressure without lengthening the cycle.

Keep mold temperature within the validated window. Higher mold temperature reduces viscosity further and accelerates curing in the runner, raising the risk of both flash and scorch.

Verify vent depth. Venting that is too deep allows silicone to escape; venting that is too shallow traps air. Vent depth should be specified for LSR, not copied from a solid rubber mold.

4. Preventing Incomplete Filling and Air Entrapment

Short shots and bubbles usually share the same root cause: air or cured material reaching the cavity before the silicone does.

Use vacuum-assisted molding. Evacuating the cavity before injection removes the air that would otherwise be compressed into voids. Vacuum performance should be checked regularly, including seals, hoses and pump condition.

Design venting for the flow path. Air must be able to leave exactly where the flow front arrives last. Vent location should follow flow analysis rather than habit, and vents should be cleaned on a defined schedule.

Profile the injection speed. A controlled initial fill, followed by a faster main fill, reduces turbulence and air entrapment while still beating scorch. The correct profile depends on wall thickness, flow length and material hardness.

Balance multi-cavity molds. In unbalanced molds, some cavities fill later at higher pressure, producing a mix of short shots and flash in the same cycle. Runner and gate balance should be verified with short-shot studies.

Maintain the dosing and mixing system. A worn static mixer, a leaking check valve or an unstable A/B ratio produces inconsistent viscosity and cure behavior. Mixer condition and ratio calibration belong in the preventive maintenance plan.

Control material handling. Partly used drums, contaminated containers and incorrect storage temperature all affect viscosity and pot life. Material should be conditioned before use and consumed within its working window.

5. Dimensional Stability and Curing Control

Dimensional problems in LSR are rarely random. They usually follow a pattern linked to temperature or cure.

Temperature uniformity across the mold. Each heating zone should be verified with a surface probe, not only with the machine display. A difference of a few degrees between cavities is enough to change shrinkage and hardness.

Correct cure time. Under-cured parts are soft, sticky and dimensionally unstable; over-cured parts become brittle. Cure time should be established by a cure curve test on the actual mold, not estimated.

Post-cure where the application requires it. Some medical and automotive specifications require post-curing to remove volatiles and stabilize properties. This should be defined in the process sheet.

Catalyst poisoning. Sulfur, tin, amine-based release agents, certain gloves and some cleaning solvents inhibit platinum cure. Handling discipline in the mold area directly affects cure consistency.

Demolding and part handling. Parts are still soft when they leave the cavity. Ejection speed, ejector pin contact area, air-assisted demolding and robotic removal all influence tearing and deformation.

6. Equipment and Process Control Factors

Defect-free production is largely a matter of repeatability, and repeatability comes from equipment capability and process documentation.

A modern LSR system should provide closed-loop dosing with ratio monitoring, injection pressure and position feedback, multi-zone temperature control, vacuum control and recipe management with data logging. The logged data is what allows a manufacturer to trace a defect batch back to a specific parameter drift instead of guessing.

Preventive maintenance should cover dosing cylinders, check valves, static mixers, seals, vacuum lines, heating elements and thermocouples. Most "sudden" defect outbreaks are the visible end of a component that has been degrading for weeks.

7. Quick Troubleshooting Checklist

• Flash at parting line - verify clamping force margin, inspect parting surface, check shot size and injection pressure
• Short shot - check venting and vacuum, review injection speed profile, verify material supply and ratio
• Bubbles or voids - check vacuum level, vent depth and vent cleanliness, review injection profile
• Sticky or under-cured parts - verify A/B ratio, mold temperature and cure time, check for catalyst contamination
• Brittle or torn parts - check for over-cure, review demolding method and ejector contact
• Dimensional drift - verify mold temperature uniformity, cure time and post-cure conditions
• Inconsistent hardness between batches - audit dosing calibration and material storage

8. Building a Stable, Low-Defect Process

Prevention is a routine, not a reaction. Four practices make the biggest difference:

1. First article inspection after every mold change, with dimensions, hardness, appearance and part weight recorded.
2. Part weight monitoring as the primary indicator of dosing stability. A drift in part weight usually precedes a visible defect.
3. Scheduled mold maintenance based on shot count, with parting line and vent condition checked each time.
4. Operator training on material handling, mold cleaning and defect recognition, so that problems are reported at the first occurrence rather than after a full batch.

Conclusion

Flash, short shots, bubbles and curing problems in LSR injection molding are not inevitable. In most cases they can be traced to specific, correctable factors: insufficient clamping force, a worn parting line, unstable dosing, inadequate venting, uneven mold temperature or contaminated material.

Manufacturers who treat LSR as a precision process - matching machine capability to mold design, maintaining dosing and vacuum systems, and documenting parameters - consistently produce parts with less flash, fewer rejects and shorter cycle times.

If you are planning a new LSR project or trying to solve a persistent defect problem on an existing line, the right starting point is a technical review of your product, mold, material and equipment configuration.

Struggling with flash or defects in your LSR production?

Contact our technical team with your product drawing, material grade and current process parameters. We can help evaluate the application and recommend a suitable machine and dosing configuration for your project.