Taken from www.fiberglasswarehouse.com with permission.
We have a YouTube video that shows the process of casting with a two part polyurethane liquid plastic in a silicone mold. If you prefer to read about it, then this post is for you.
In this demonstration we used a rigid polyurethane in the silicone mold we showed in a previous post. You can read about making a silicone mold here.
The supplies you will need:
-Two part polyurethane liquid plastic
-A mold release if you want to preserve your mold for multiple casts
-Stir paddle or a paint mixer attachment for a drill (purchased at Home Depot).
-Rubber bands or tape to hold the silicone mold together.
SharkThane 70-20 is a two part polyurethane with a 1:1 mixing ratio. The shore hardness of the cured material is 70D. The finished product will be very hard. It creates a white cast with almost no air bubbles.
First, add mold release (if you want). It is not necessary but your mold will last longer.
Next, you will want to use something to hold your silicone mold together such as rubber bands or tape.
The polyurethane in this demonstration is a 1:1 mix ratio. Measure out equal amounts of part A and then part B. Figure out how much you will need to fill the mold beforehand.
A stir stick can be used to mix the two parts together. Also, a paint mixer could be used to speed up the mixing time. If you choose to use a paint mixer, be VERY careful because it can create air bubbles. The paint mixer needs to be submerged in the liquid before turning it on. If you have a vacuum pump, suction out the air bubbles before pouring in the mold. DO NOT use a paint mixer with more flexible polyurethanes as it will introduce way too many air bubbles into the mixture. But for the 70-20, it worked fine.
Mix quickly, as the polyurethane will start to gel between 90 to 120 seconds.
If you have a large part, use several smaller batches. If you don't work in smaller batches, the polyurethane may cure before it is poured into the mold.
Pour the polyurethane slowly into the mold to allow air bubbles to work themselves up to the top. Within a minute or two you will see the polyurethane turn white as it gels. The polyurethane exotherms, meaning it heats up, as the polyurethane starts to cure.
Here is the leftover polyurethane cured in the mixing cup. The handling cure time is 30-40 minutes. Work the cast out of the mold once it comes to room temperature.
Here is the finished product including the polyurethane that gelled inside the cup.
Fully cured castings made with rigid polyurethane, are tough, durable, machinable and paintable. This makes it perfect for point-of-purchase displays, rapid prototypes, special effects and sculpture reproductions.