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Clinic - Casting with Resin & RTV Molds

The process for creating a mold it very easy.  RTV rubber is available at most hobby shops and can be ordered direct from the manufacturer if need be.  I was using Alumilite's casting resin and RTV.  I have also used Micro-Marks resin and rubber.  I have recently switched RTV suppliers to a product called OOMOO 30 from Smooth-On.  The link to there site is on my links page.  This rubber is much more flexible and makes taking the casting out of the mold much easier.

I)       Creating the mold.

1)  Create a box with whatever materials you wish.  I have used styrene and foam core board. I use Duct Tape to seal the edges of the box.  Measure the part you are going to use and allow at least 3/8 inch of space around each part. 

2)   Attach the original part to the bottom of the box with rubber cement (Use the directions for a temporary bond!).

3)   Mix the RTV Rubber according to the directions.  Do not slop it around when mixing.  This causes air bubbles to be introduced into the rubber and they might cause problems later.  Once the rubber and activator have been mixed pour it into the box making sure to surround the part completely.

4)   Vibrate the box to shake any air bubbles away from the part.   I use a foot massager.  Create a ring of Duct Tape with the sticky side out.  Attach it to the bottom of the box and then to the surface of whatever you use.  A sander works……anything that would shake the box a little.  The rubber is thick enough that unless it is a violent shaking it should not come out of the box.

5)   Leave the mold to cure for 24 hours.  Some rubber states that it is ready in 4 to 6 hours.  I have tried and I was not comfortable using the mold yet.  I always leave the mold to cure overnight. 


II)     Casting.

1)   Mix the Alumilite 50/50 according to directions.  As you stir the mixture it will turn a milky brown and then start turning a transparent brown.  If you are using hydrocal then mix to a tomato soup thickness.

2)   Pour into the mold.  With Alumilite you don't have much time to work so mix in very small quantities.  If you don't have enough to fill a mold don't worry.  The next batch can be added on top of the other (Once the first has cured) without creating a seam.  You may see a color difference if the mixture is off.   It will still paint up just fine.  On any part that is very thin, pour the Alumilite into the mold and then place a piece of glass or a flat surface on top of the mold.  This will press any extra resin out and it can be cut off easily.  If you are casting with hydrocal let the mold sit until the plaster has setup.  On most molds it takes 15 to 20 minutes depending on the size of the casting.

3)  When using resin the casting will need to be washed with a regular dish washing soap to cut any oils off the part.  Alumilite leaves an oily film on the casting due to the curing of the resin.  Make sure that it has been washed away before attempting to paint.  For hydrocal you are ready to paint once the casting has had it's flash cleaned off.


That's all there is to it.  You can make any scene come alive with ton of extra detail with very little effort.  

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