Understanding viscosity helps you choose the right casting resins and mold rubbers for your project.
Quick Answer
Viscosity is the measure of a fluid's resistance to flow. Think of it as how "thick" a liquid is. Water has low viscosity (1 centipoise) and pours easily. Honey has high viscosity (10,000 centipoise) and flows slowly. The higher the number, the thicker the fluid.
Understanding Viscosity: The Basics
Viscosity describes how easily a fluid flows. Technically, it's the internal friction within a liquid as it moves—the force that resists molecules sliding past each other.
Viscosity in Everyday Life
You encounter viscosity differences constantly:
Low Viscosity (Thin, Flows Easily)
- Water: 1 centipoise
- Milk: 3 centipoise
- Light cream: 20 centipoise
High Viscosity (Thick, Flows Slowly)
- Honey: 2,000-10,000 centipoise
- Ketchup: 50,000 centipoise
- Peanut butter: 250,000 centipoise
How is Viscosity Measured?
Viscosity is measured in centipoise (cPs), named after French physicist Jean Poiseuille. One centipoise equals the viscosity of water at 20°C (68°F).
Quick Reference Scale
- 1-1,000 cPs: Low viscosity (thin, flows easily)
- 1,000-10,000 cPs: Medium viscosity (moderate flow)
- 10,000-100,000+ cPs: High viscosity (thick, slow flow)
Why Temperature Matters
Viscosity changes with temperature:
For liquids like resins and rubbers:
- Warmer = LOWER viscosity (flows easier)
- Cooler = HIGHER viscosity (flows slower)
This is why all Smooth-On specifications list viscosity at 73°F (23°C). Working in a cold shop? Your material will be thicker than specified.
Pro Tip: Gently warming materials in a water bath (not exceeding manufacturer recommendations) can lower viscosity and improve flow.
Why Viscosity Matters for Your Project
For Mold Making
Low Viscosity (1,000-5,000 cPs)
Best for: Detailed sculptures, jewelry molds, complex shapes
- Flows easily into fine details
- Captures intricate textures
- De-airs quickly
Smooth-On Low Viscosity Silicones →
Medium Viscosity (5,000-15,000 cPs)
Best for: Life casting, general molding, vertical surfaces
- Balance between flow and sag resistance
- Good for most applications
- Requires vacuum degassing
Smooth-On Medium Viscosity Silicones →
High Viscosity (15,000+ cPs)
Best for: Brush-on molds, large pieces, vertical surfaces
- Stays in place without sagging
- Can be brushed or troweled
- Ideal for body casting
Smooth-On Brush-On Silicones →
For Casting Resins
Ultra-Low Viscosity (80-300 cPs)
Best for: High detail castings, production work
- Flows like water
- Captures finest details
- Naturally de-airs
Smooth-Cast Series →
Medium Viscosity (300-3,000 cPs)
Best for: General casting, medium parts
- Good flow characteristics
- May need vacuum or pressure
- Versatile for most uses
Task Series →
When to Vacuum Degas or Pressure Cast
Critical Guidelines
For Mold Rubbers: Vacuum degas if viscosity is 15,000 cPs or higher
For Casting Resins: Vacuum degas if viscosity is 7,500 cPs or higher
High viscosity materials trap air bubbles during mixing. These bubbles won't naturally rise to the surface before the material cures, resulting in defects in your mold or casting.
Vacuum Degassing Requirements
- Vacuum pump must achieve 29+ inches of mercury
- Allow material to rise, expand, and collapse 2-3 times
- Pour while under vacuum when possible
Complete Vacuum Degassing Tutorial →
Smooth-On Viscosity Scale
Understanding viscosity becomes easier when you compare materials to familiar products:
| Viscosity (cPs) |
Everyday Example |
Smooth-On Products |
| 1 |
Water |
— |
| 80-300 |
Light Cream |
Smooth-Cast 300 Series (ultra-low viscosity resins) |
| 300-1,000 |
Heavy Cream |
Smooth-Cast 320-326 Series, Task Series |
| 1,000-3,000 |
Ketchup |
PMC Urethane Rubbers |
| 3,000-10,000 |
Yogurt |
Mold Max Series, OOMOO 30 |
| 10,000-15,000 |
Sour Cream |
Tin Cure Silicones, Mold Max T-Series |
| 15,000-30,000 |
Tomato Paste |
Dragon Skin Series, Body Double |
| 30,000-50,000 |
Mayonnaise |
Thickened Platinum Silicones |
| 50,000+ |
Peanut Butter |
Rebound Series, Brush-On Silicones |
Note: All measurements at 73°F (23°C). Temperature significantly affects viscosity. Check individual Technical Bulletins for exact specifications.
Download Printable Viscosity Chart (PDF) →
Common Viscosity Questions
What does CPS stand for?
CPS stands for centipoise, the most common unit for measuring viscosity. One centipoise equals the viscosity of water at 20°C (68°F).
What is the viscosity of water?
Water has a viscosity of 1.0 centipoise (cPs) at 20°C (68°F). This makes it the reference point for all viscosity measurements.
Can I thin high-viscosity materials?
Yes, for platinum-cure silicones you can use Silicone Thinner (up to 10% by weight). You can also gently warm materials in a water bath. However, thinning reduces physical properties like tear strength.
Can I thicken low-viscosity materials?
Yes, use THI-VEX thickener for platinum silicones to create a brushable, non-sag consistency. Add 1-3% by weight for vertical surface application.
What's better: high or low viscosity?
Neither is "better"—it depends on your application. Low viscosity captures fine detail and de-airs easily. High viscosity stays in place on vertical surfaces. Choose based on what you're making.
Related Guides & Resources
Shore Hardness Scale
Understanding durometer and rubber hardness Learn More →
Vacuum Degassing Tutorial
Step-by-step guide to removing air bubbles Watch Tutorial →
Release Agents Guide
Preventing materials from sticking Read Guide →
Technical Bulletins
Detailed specs for all products Browse TB's →
