How to Pour Smooth-Cast 300 Liquid Plastic Casting Resin Into a Mold

Written by alex smith
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How to Pour Smooth-Cast 300 Liquid Plastic Casting Resin Into a Mold
All sorts of things can be made with Smooth-Cast 300 (Plastic Octopus image by pognyc from

Liquid plastics are commonly used materials for creating multiple copies of sculptures, artwork, props and other small items. Smooth-Cast 300 is a series of 2 part liquid plastics manufactured by Smooth-On. They create bright white casts that are practically bubble-free. These plastics are easy to mix and pour. Working times vary from one product to another, but once the plastic hardens it can be painted, sanded, drilled and tooled, and it resists moisture and mild solvents.

Skill level:

Things you need

  • Smooth-Cast 300 liquid plastic
  • Mold of object being replicated
  • Release agent
  • Brush
  • Paper mixing cups
  • Mixing sticks
  • Rubber gloves
  • Pressure pot (optional)

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  1. 1

    Paint the mould with a generous coat of release agent. Allow it to dry completely.

  2. 2

    Measure equal volumes of part A and part B of the Smooth-Cast 300 and mix them thoroughly.

  3. 3

    Pour the plastic slowly into the lowest point in the mould, allowing it to fill completely. Pour only in one spot; don't drizzle the plastic all over, as this traps air bubbles.

  4. 4

    Tap the sides of the mould to release any trapped air bubbles.

  5. 5

    Place the mould in a pressure pot and pressurise to 60 PSI until the plastic has cured. Smooth-Cast 300 can be used without a pressure pot if you don't have one, but pressurising it will yield better results.

Tips and warnings

  • Demold times: Smooth-Cast 300Q = 4 to 5 minutes. Smooth-Cast 300 = 10 minutes. Smooth-Cast 305 = 30 minutes. Smooth-Cast 310 = 2 to 4 hours
  • Always wear rubber gloves when working with liquid urethanes. Work in a well-ventilated area.
  • Mix any liquid plastics in disposable paper cups only. Once it has been poured, there will be a film of plastic on the cup that could permanantly bond to plastic or glass cups.

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