Common problems with LSR and how i solve them?
Excellent question. Liquid Silicone Rubber (LSR) is a fantastic material but requires precise control. Here are the most common problems in LSR injection molding and how to solve them, broken down by category.
1. Air Traps and Voids
Problem: Air gets trapped in the mold cavity, causing shiny surface blemishes, bubbles, or incomplete fills, especially in thin sections or complex geometries.
Causes: Improper venting, too-fast injection speed, high material viscosity.
Solutions:
Optimize Venting: Ensure vents are in the last-to-fill areas, are deep enough (typically 5-15 µm), and are regularly cleaned.
Vacuum the Mold: The single most effective solution. Use a mold-integrated vacuum system to evacuate air from the cavity before injection.
Adjust Process: Use a slower first-stage injection speed to allow air to escape through vents before final pack. Increase mold temperature to lower viscosity.
2. Part Sticking (in Mold or to Itself)
Problem: Parts tear or are difficult to demold, or they fuse together post-molding ("blocking").
Causes: Undercuts, insufficient draft, mold surface finish, incomplete curing, or post-molding migration of unreacted components.
Solutions:
Mold Design: Ensure adequate draft (at least 1° per side). Use a polished (often EDM or polished) mold surface – LSR sticks to rough surfaces. Avoid undercuts.
Process: Increase cure time/temperature to ensure full cross-linking. Under-cured LSR is sticky.
Post-Processing: Wash parts to remove silicone bloom (migrated unreacted oligomers). Store in a cool, dry place. Use interleaving paper if necessary.
3. Flash (Excess Thin Material on Part Edges)
Problem: Very thin, hair-like silicone on parting lines or around ejector pins.
Causes: Worn or damaged mold, insufficient clamp force, injection pressure too high, or mold overheating.
Solutions:
Mold Maintenance: Inspect and maintain parting lines. Ensure mold is clean and free of debris. Check for wear.
Process: Verify clamp force is sufficient (rule of thumb: 2-3 tons per square inch of projected area). Slightly reduce pack/hold pressure if possible. Ensure mold temperature is stable and not excessive.
4. Incomplete Fill or Short Shots
Problem: The mold cavity doesn't fill completely.
Causes: Insufficient material, too low injection speed/pressure, cold mold, blocked or frozen gates, air traps.
Solutions:
Process: Increase injection speed (but balance with air evacuation). Raise mold temperature (typical range 150-200°C / 300-390°F). Increase shot size.
Check Hardware: Ensure the material cylinder is not starved. Verify gates are not blocked (especially with cured material from previous shots).
5. Contamination and Inclusions
Problem: Black specs, foreign particles, or cured silicone chunks in the part.
Causes: Poor material handling, contaminated drums/hoses, degraded material in dead spots of the machine (especially in the mixing static mixer or nozzle).
Solutions:
Cleanliness Protocol: Keep material containers sealed. Use dedicated, clean hoses. Purge the system thoroughly after any stoppage.
Machine Setup: Use LSR-dedicated machines if possible. Ensure the static mixer is cleaned/replaced regularly. Design the runner system to be self-cleaning with each shot.
6. Dimensional Instability
Problem: Parts are out of spec, showing shrinkage or warpage.
Causes: Inconsistent cure, varying mold temps, incorrect pressure profiles, or post-molding shrinkage.
Solutions:
Process Consistency: Ensure stable and even mold temperatures. Optimize and hold cure time constant. LSR has very low shrinkage (~2-4%), but it must be consistent.
Post-Curing: For critical tolerances, implement a controlled post-cure cycle (e.g., 1-4 hours at 150-200°C) to stabilize properties and dimensions.
7. Poor Mechanical Properties
Problem: Parts are weak, tear easily, or feel "rubbery" and under-cured.
Causes: Insufficient cure (most common), wrong material grade, or contamination.
Solutions:
Verify Cure: Increase mold temperature and/or cure time. Use a Durometer tester and Tear Strength tester to quantify. LSR requires both heat and time to fully cross-link.
Check Material: Ensure you're using the correct grade (e.g., 30 ShA vs. 70 ShA have vastly different strengths).
General Prevention & Best Practices
Material Handling: LSR is sensitive to platinum inhibitors. Avoid contact with PVC, sulfur-based compounds, tin-cured rubbers, and some adhesives. Even trace amounts can cause "poisoning," leading to uncured spots.
Cold Runner vs. Hot Runner: Use a temperature-controlled cold runner system. True hot runners will cure the material in the channels. Keep runners cool (20-40°C) while the cavity is hot.
Mold Design: Prioritize simple, direct fills. Use diaphragm or tunnel gates. Ensure excellent venting from the start.
Process Documentation: Record all parameters (temps, times, pressures, speeds) for every job. When a problem arises, you can return to a known good baseline.
Start with a Complete Process: Don't try to fix a short shot by just adding more material. Diagnose the root cause (air, temperature, speed).
Troubleshooting Flowchart for Common Issues:
Is the part sticking or tearing? → Check mold surface, draft, and INCREASE CURE TIME/TEMP.
Are there bubbles or shiny spots? → CHECK VENTS AND APPLY VACUUM.
Is there flash? → Check mold condition and clamp force.
Is the part weak or under-cured? → INCREASE MOLD TEMPERATURE AND CURE TIME.
Are there black specs? → Clean the machine, hoses, and replace the static mixer.
By systematically addressing these areas-focusing especially on complete curing, effective air evacuation, and meticulous cleanliness-you can solve the vast majority of LSR molding problems.