Smooth-On Smooth-Cast™ 300Q

Article number: 751635496415
Availability: In stock (5)

Smooth-Cast™ 300Q liquid plastic is a quick setting, ultra-low viscosity casting resin that yield castings that are bright white and virtually bubble free. Vacuum degassing is not necessary. It offer the convenience of a 1A:1B by volume or 100A:90B by weight mix ratio. 

This resin readily accept fillers (such as URE-FIL™ 3 from Smooth-On) and can be colored with SO-Strong™ or Ignite™ color tints (Smooth-Cast™ 325 series accepts pigments better than the Smooth Cast™ 300 series). Fully cured castings are tough, durable, machinable and paintable. It resists moisture and mild solvents. Applications for Smooth-Cast™ 300Q Liquid Plastics include reproducing small to medium size sculptures, casting into alginate molds, making prototype models, special effect props and decorative jewelry. 

Those interested in making roto cast pieces should refer to the Smooth-Cast™ 65D technical bulletin.


  • 30 Second Pot Life - can be poured in alginate molds
  • Easy to measure and mix - 1:1 mix ratio by volume - no scale required
  • Low viscosity - captures excellent detail
  • Resin cures to a bright white finish


Safety - Materials should be stored and used in a warm environment (73° F / 23° C). These products have a limited shelf life and should be used as soon as possible. All liquid urethanes are moisture sensitive and will absorb atmospheric moisture. Mixing tools and containers should be clean and made of metal, glass or plastic. Mixing should be done in a well-ventilated area. Wear safety glasses, long sleeves and rubber gloves to minimize contamination risk. Because no two applications are quite the same, a small test application to determine suitability for your project is recommended if performance of this material is in question.

Applying A Release Agent - A release agent is necessary to facilitate demolding when casting into or over most surfaces. Use a release agent made specifically for mold making (Universal™ Mold Release or Mann’s Ease Release™ 200 available from Smooth-On or your Smooth-On distributor). A liberal coat of release agent should be applied onto all surfaces that will contact the plastic.

IMPORTANT: To ensure thorough coverage, apply release and brush with a soft brush over all surfaces. Follow with a light mist coating and let the release agent dry for 30 minutes. Smooth-On silicone rubber molds usually do not require a release agent unless casting silicone into the mold. Applying a release agent will prolong the life of the mold.

MEASURING & MIXING... Liquid urethanes are moisture sensitive and will absorb atmospheric moisture. Mixing tools and containers should be clean and made of metal, glass or plastic. Materials should be stored and used in a warm environment (73°F/23°C).

Stir or shake both Part A & Part B thoroughly before dispensing. After dispensing equal amounts of Parts A and B into mixing container (100A:90B by weight) and mix thoroughly. Stir deliberately making sure that you scrape the sides and bottom of the mixing container several times. Be careful not to splash low viscosity material out of the container.


Pouring - Pour your mixture in a single spot at the lowest point of the containment field and let the mixture seek its level. This will help minimize air entrapment.

For Best Results - Best results are obtained using a pressure casting technique. After pouring the mixed compound, the entire casting assembly (mold, dam structure, etc.) is placed in a pressure chamber and subjected to 60 PSI (4.2 kg/cm2) air pressure for the full cure time of the material.

Curing - Warning: Fumes, which may be visible as this product starts to “gel” and cure, will dissipate with adequate ventilation. Only use this product with room size ventilation and do not inhale/breathe fumes. Castings will be extremely hot immediately following cure and may burn the skin. Let cool to room temperature before handling. Smooth-Cast™ 300 will cure in 7 - 10 minutes (Smooth-Cast™ 305 in 30 – 40 minutes and Smooth-Cast™ 310 in 2 - 4 hours) depending on mass and mold configuration.

Post Cure - Castings will reach “full cure” faster and achieve maximum physical properties if post cured. Allow material to cure for recommended Cure Time at room temperature followed by 4 - 6 hours at 150°F/65°C. Allow casting to come to room temperature before handling.

Performance - Cured castings are rigid and durable. They resist moisture, moderate heat, solvents, dilute acids and can be machined, primed/ painted or bonded to other surfaces (any release agent must be removed). If machining cured material, wear dust mask or other apparatus to prevent inhalation of residual particles. Castings can be displayed outdoors after priming and painting. Unpainted castings will yellow over time - more quickly when exposed to ultra-violet light.



Pot Life @ 73° F / 23°C (ASTM D-2471): 30 Seconds
Cure Time ** @ 73° F / 23°C: 4-5 Minutes **Mass Dependent
Tensile Strength (ASTM D-638): 3,000 psi
Tensile Modulus (ASTM D-638): 139,500 psi
Elongation at Break % (ASTM D-638): 5%
Flexural Strength (ASTM D-790): 4,510 psi
Flexural Modulus (ASTM D-790): 128,000 psi
Compressive Strength (ASTM D-695): 4,000 psi
Compressive Modulus (ASTM D-695): 45,800 psi
Shrinkage in./in. (ASTM D-2566): 0.01
Mix Ratio: 1A:1B by volume or 100A:90B by weight
Mixed Viscosity (ASTM D-2393): cps; 80
Specific Gravity (ASTM D-1475): g/cc; 1.05
Specific Volume (ASTM D-1475): cu. in./lb.; 26.4
Color: White
Shore D Hardness (ASTM D-2240): 70
Heat Deflection Temp (ASTM D-648): 120°F/50°C
All values measured after 7 days at 73°F/23°C:
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