Complete Step-by-Step Guide to Liquid Silicone Rubber (LSR) Molding

Phase 1: Pre-Production Preparation

Step 1: Material Selection & Handling

Select the right LSR grade based on:

Required Shore hardness (e.g., 30A, 50A, 70A)

Required certifications (medical, food-grade, UL)

Color (pigmented or natural)

Special properties (high transparency, high/low temp resistance)

Proper material storage:

Store Component A (base) and Component B (catalyst) at ≤ 25°C (77°F)

Keep containers sealed to prevent moisture contamination

Rotate stock (FIFO - first in, first out)

Avoid contamination from foreign materials

Step 2: Machine Setup

Select appropriate equipment:

Use LSR-dedicated injection molding machine

Ensure proper material ratio (typically 1:1, but verify)

Check static mixer condition (clean or replace if needed)

Prepare material feed system:

Use air-operated piston pumps or gear pumps

Ensure material lines are clean and dedicated to LSR

Set Material A and Material B temperatures (typically 20-25°C/68-77°F)

Phase 2: Mold Preparation

Step 3: Mold Installation & Preparation

Clean the mold thoroughly:

Use isopropyl alcohol or specialized mold cleaners

Ensure vents are clear of debris

Check for damage to parting lines

Install mold and connect systems:

Mount mold to platen

Connect cooling lines to temperature control unit

Connect vacuum lines if mold has integrated vacuum system

Ensure proper alignment

Set mold temperature:

Heat mold to 150-200°C (300-390°F) depending on material

Allow sufficient time for thermal stabilization (typically 30-60 minutes)

Verify uniform temperature across mold faces

Phase 3: Process Setup & Startup

Step 4: Machine Parameter Setup

Injection parameters:

1st Stage (Fill): Set to 10-30% of max injection speed

2nd Stage (Pack/Hold): Set to 30-70 bar (435-1015 psi)

Hold time: 20-50% of total cycle time

Curing parameters:

Set cure time based on wall thickness (rule: ~30-60 seconds per mm)

Start with material supplier's recommendations

Clamping force:

Calculate required force: Projected area (cm²) × 10-15 kN/cm²

Set clamp force to calculated value + 10% safety margin

Step 5: Startup Procedure

Purging the system:

Run material through lines to remove air bubbles

Purge until consistent, bubble-free material flows

Check mix ratio visually (should be homogeneous)

Initial shot setup:

Set shot size to 80% of estimated cavity volume

Use "shot by shot" mode for initial setup

Adjust shot size until slight flash appears

Phase 4: Process Optimization

Step 6: First Articles & Adjustment

Run initial shots:

Produce 5-10 shots for evaluation

Allow process to stabilize

Evaluate and adjust:

Short shots? → Increase injection speed, temperature, or shot size

Flash? → Reduce injection pressure, check clamp force, verify mold closure

Air traps? → Activate vacuum, reduce injection speed, check venting

Sticking? → Increase cure time, check mold surface, adjust draft

Finalize parameters:

Document optimized settings

Ensure process is stable and repeatable

Phase 5: Production Run

Step 7: Running Production

Start production:

Switch to automatic cycle mode

Monitor first 50 parts closely

Check critical dimensions every 15-30 minutes initially

Quality checks:

Visual inspection for defects

Dimensional checks per control plan

Durometer checks (if specified)

Weight checks to detect fill variations

Step 8: In-Process Monitoring

Monitor key parameters:

Mold temperature (both halves)

Material temperature

Cycle time consistency

Injection pressure profile

Documentation:

Record SPC data if applicable

Document any process adjustments

Maintain production log

Phase 6: Shutdown & Maintenance

Step 9: End of Run Procedure

Material system cleanup:

Purge with appropriate purging compound

Drain material tanks if extended shutdown

Cap material lines to prevent contamination

Mold removal/storage:

Cool mold gradually (avoid thermal shock)

Apply mold protective coating if storing

Document any mold damage or wear

Step 10: Preventive Maintenance

Regular maintenance tasks:

Daily: Clean vents, check static mixer

Weekly: Inspect material lines and pumps

Monthly: Calibrate temperature controls

Quarterly: Full system maintenance

Critical Success Factors & Troubleshooting

Common Issues & Immediate Actions:

Process Window Guidelines:

Mold Temp: ±5°C tolerance from optimal

Cure Time: ±5% tolerance

Injection Speed: Adjust in 5% increments

Hold Pressure: Adjust in 10% increments

Safety Considerations:

Material Safety: LSR components can cause skin/eye irritation

Thermal Safety: Molds operate at 150-200°C

Machine Safety: Follow lockout/tagout procedures

Ventilation: Ensure adequate ventilation for any fumes

Quick-Reference Process Checklist

Before Production:

Material at correct temperature

Static mixer clean/replaced

Mold cleaned and vents clear

Mold at correct temperature

Vacuum system operational (if equipped)

Material lines purged

During Production:

First article approval completed

Process parameters documented

Quality checks scheduled

Material levels sufficient

After Production:

System purged

Mold cleaned and protected

Production data recorded

Maintenance performed as needed

Pro Tips:

Consistency is key - LSR processes best with stable, repeatable conditions

Don't rush cure time - Under-curing causes most quality issues

Invest in vacuum - It solves 80% of air-related defects

Keep it clean - Contamination is the hardest problem to fix mid-run

Document everything - LSR processes are reproducible when well-documented

This systematic approach will yield consistent, high-quality LSR parts with minimal defects and downtime. Always refer to your material supplier's specific processing guidelines, as formulations can vary significantly.