All roads lead to plaster Master                    Dan Does It 5                                            Page 7                             

                  
Work at Home Part II

Setting up for Production  



If  we are ever to produce mass quantities of castings we will need many molds.  Now that we have 1 mold   we can pour plaster into it and make as many  models as we want from it. We can take those plaster models and brush rubber on them to make more latex molds.  Here's how it's done.

  Pouring the Models

The plaster castings we use to make molds are called models. To pour our plaster models we must first spray the mold with a surface wetting agent.

Prepare a mixture of  3 gallons of water with about 1/4cup of Palmolive Dish Detergent or use a mixture of  50% Windex and 50% water.  Either of these preparations can be sprayed into the mold to help the plaster flow over the inner surface of the mold. The rubber mold has a bit of surface tension (so who doesn't) that prevents the plaster from flowing into every crevice. This little bit of surface tension can leave your casting full of tiny air holes. Don't drown the mold with spray. Use only enough spray to just  wet the surface. To much spray will cause water legs on your castings or leave pitted out sections where the water lays on the bottom of the mold.

I turn my molds over so any excess water will run out and they won't dry out in the open air while I'm preparing my plaster. A few moments before I am ready to pour I turn them right side up again . Notice the universal banding strap I use to hold the two halves of the back up shell together.

If you need information on mixing plaster go to Dan does it 2. To figure the amount of water you will need , you can estimate or fill the mold with water and empty it into a bucket. That is slightly more than what it will take to fill your mold when you mix plaster in it.  You should always mix a little more than what you will need. Mix the plaster according to the directions in the Dan Does it 2 episode.  Appropriate plasters for your model would be Moulding Plaster, Casting Plaster or #1 Pottery Plaster.

Bubbles, Bubbles,Toil and Troubles

In the process of mixing your plaster some air will get mixed into the slurry no matter how careful you are. (A Hanson Mixer is designed to introduce a minimal amount of air into the mix.  Some mold makers stir the mix with a stick for a few seconds after mixing to break up air bubbles just prior to pouring. Pour your plaster slowly into the mold.  It should be poured in a thin stream to help release air bubbles in the mix. Mold makers sometimes blow on the stream of plaster to break air bubbles as they are coming out. They also  bump the side of the table with their leg to jar or shake the plaster in the mold as they are pouring. This loosens entrapped air and helps it rise to the surface.

When the mold is full tap it with your hand to level the plaster and remove any remaining air bubbles.

Put the mold aside to let the plaster harden for an hour

Getting the casting out of the latex mold

Apply a drop or two of dish detergent to the mold.
 

Wet your fingers and spread the dish detergent around on the mold.An alternative to this is to brush on castor oil thinned with 8 parts denatured alcohol. Castor oil is the only oil to apply to latex molds. Some mold companies will not honor their warranty if anything other than castor oil is put on their molds.

Start the removal process by pulling  the latex mold open and  then in one quick continuous motion force the mold latex down over itself. Your casting will be almost entirely free of the mold.

Remove the plaster casting.

You should have a perfect casting without air holes. Now place the rubber mold  in a bucket of warm water or hold it under running water and scrub it  inside and out with a scrub brush. A thin film of residue will develop on the mold if you do not scrub it  regularly.

Pour three or four  more models or as many as you think you will need to make the necessary amount of  production molds. Wash the mold after each casting.

NEXT     

Custom Molds | Dan's Molds | Dan Does It Archives