Key Properties to Look for in Mold-Making Liquid Silicone Rubber
Choosing the right mold-making liquid silicone rubber (LSR) can determine whether a project succeeds or fails. I've seen many mold issues-tearing, poor detail, short service life-that trace back to one thing: the wrong silicone properties. If you understand what really matters in mold-making silicone, you can avoid costly mistakes and produce reliable, high-quality molds.
Below are the key properties I always evaluate when selecting liquid silicone rubber for mold making.
1. Tear Strength: The Foundation of Mold Durability
Tear strength determines how well a silicone mold resists ripping during demolding. This property matters even more for molds with undercuts, deep textures, or thin walls.
Low tear strength leads to edge tearing and short mold life
High tear strength allows repeated demolding without damage
For complex molds, I always recommend a silicone with enhanced tear resistance, even if it costs slightly more. The longer mold life quickly offsets the price difference.
2. Hardness (Shore A): Balance Between Flexibility and Support
Shore A hardness affects how easily a mold flexes and how well it holds its shape.
Low hardness (Shore A 10–20)
Excellent flexibility
Ideal for complex or fragile parts
Medium hardness (Shore A 20–30)
Most common for general mold making
Good balance of strength and flexibility
High hardness (Shore A 30+)
Better dimensional stability
Suitable for simple shapes and larger molds
I always choose hardness based on part geometry, not personal preference.
3. Viscosity and Flowability: Capturing Fine Details
Viscosity determines how easily the silicone flows into fine features before curing.
Low-viscosity silicone
Flows smoothly
Captures fine textures and sharp details
Often requires less vacuum degassing
High-viscosity silicone
Better for brush-on or layered molds
Less likely to leak
For detailed prototype molds, I prefer low-viscosity LSR with good self-degassing behavior.
4. Curing System: Addition Cure vs. Condensation Cure
The curing mechanism directly affects mold accuracy and longevity.
Addition Cure Silicone
No shrinkage
Excellent dimensional accuracy
Odor-free
Long mold life
Higher cost
Condensation Cure Silicone
Slight shrinkage
Shorter mold life
More tolerant of contaminants
Lower cost
When precision matters, I always choose addition cure silicone. For simple, low-budget molds, condensation cure can still work well.
5. Shrinkage Rate: Accuracy Matters
Shrinkage affects final part dimensions. Even small shrinkage can cause serious problems in precision molds.
Addition cure LSR: near zero shrinkage
Condensation cure silicone: 0.3–0.8% shrinkage
If dimensional accuracy matters-even slightly-I never compromise on low-shrink silicone.
6. Working Time and Cure Time: Control Your Process
Pot life and cure speed affect workflow efficiency.
Long pot life allows careful pouring and degassing
Fast cure time increases production efficiency
I recommend matching cure speed to your process rather than rushing for fast curing. Controlled curing always produces better molds.
7. Heat Resistance and Chemical Stability
Good mold-making silicone should withstand:
Heat from resin casting
Chemical exposure from polyurethane, epoxy, or polyester systems
High-quality LSR maintains elasticity and surface quality even after repeated exposure.
8. Mold Release and Surface Finish
A good silicone naturally releases most materials without additional release agents.
Smooth surface finish improves casting quality
Good release properties reduce mold damage
I always test compatibility with the actual casting material before committing to full production.
Final Thoughts
Mold-making liquid silicone rubber isn't a one-size-fits-all material. Tear strength, hardness, viscosity, curing system, and shrinkage all play critical roles. When I evaluate silicone for mold making, I focus on performance first, cost second-because failed molds always cost more in the long run.
If you match silicone properties to your application instead of guessing, you'll get consistent results, longer mold life, and better cast parts every time.