First of all, analyze the piece and see how you do the dividing line of the mold.
See all angles.
Plastiline clay for mold making
start making the bed of the mold, leave at least 1/2 in from the side walls.
in this case we have to close the hole when the axes of the engine goes, the parting line goes on the botton of the model.
place the model on the bed and let the plastiline as smooth as possible.
atention to the line between the model and the bed, carefull not to damage your model.
when the bed it´s done make the box around the bed.
make some keys to assure that your mold is aligned every time.
For those who are beginning this is one of the parties that generate more questions. Tip: if you don´t know how mutch rubber to use, make small batches.
According to the size of your piece, catalyze a small amount of rubber. WEIGH RUBBER AND ADD THE AMOUNT OF CATALYST RELEVANT TO WEIGHT ACCORDING TO INSTRUCTIONS FROM YOUR SUPPLIER OF RUBBER. Mix well with a rubber with a flat spatula until the catalyst is mix with the rubber, scraping sides of bowl, avoid using cylindrical objects that are not good for this.
Pour the rubber over the mold in a thin and constant to avoid the formation of bubbles that can compromise the molding.
keep pouring until you cover all the model
cover the plastiline as well.
you can use old molds to fill parts of the mold saving some silicone, now when the silicone it´s catalyzed you can pour the second layer.
Again using old molds, chop the mold into small pieces and mix with some fresh silicone, this is a tecnic that help you save alot of silicone.
Here you can see the rubber mix with the recycled silicone
Catalize the silicone and pour the mix on top on the 1st layer of silicone
When the rubber is cured, turn the mold and remove the plastiline
Make some cleaning if needed
THIS IS ESSENTIAL: aply a layer of solid vaseline on top of silicon, to prevent it from sticking to the silicone you will pour next.
Avoid vaseline over the play, it will generate an undesirable texture.
Repeat the first steps you did on the other side of the mold. In this side i add an entry to fill the mold at a point that can be sanded removing the excess, it will be hidden.
When cured remove the box.
Open the mold carefully..
Remove the model of the mold and now you are ready to make some castings.
Using Dim Clay to make your model you see another vantage here the mold came out clean at the end of the process.
Depending on the complexity of your piece and the tipe of clay you are using, chances are the your model came out undeformed of your molding process.
Well this is one of the techniques to make a good mold silicone, spending as little as possible, I used 5,3 pounds of rubber in the mold, and 1.3 lbs was recycled rubber. Not counting the 4 pieces that used to put in the corners.