The purpose of this blog is a show how to make a 2 part mold. There are few ways to do this and because the part I am making a mold of is a complex 3D part, I had to change a few things to make it work.
The part(s) I am molding and casting are soles for the 2015 MAG replicas. These are a 2 part sole with a flat oval part at the front and the 3D part at the rear. I started by making a new master part where I created an "outrigger" style sole and added "Pyramatic" tread to both parts. The 3D rear part is complex because it has 'wings' that wrap around the side of the shoe. Traditional soles are flat and a single part, so an open back mold would be the easiest way to do those. In fact the front part of this project is done that way.
The first step is to ensure your master is as close to perfect as you can get it. It is best to spend time at this stage rather than correcting each cast part later.
There are a few materials that can be added to the master for a quick mod. I used a school grade Plasticine because it does not contain sulfur and therefore does not affect the curing of the silicone being used to make the mold.
There are also two main types of silicone - condensation curing and additive curing. I am using the additive type where part A is the silicone and part B is a curing agent. The product I have used for this project is Vario-40 and it is mixed at 10:1 in weight. It is translucent and has a shore hardness of A-40.
DO NOT EVER MIX THE TWO TYPES!
Unlike some quick set silicones, Vario-40 can be placed in a Vacuum Chamber to degas so you can have bubble free molds and capture greater detail.
So to start the two part mold, you have to work out where you want the seam lines, fill holes, breath holes and then fill in the parts you don't want silicone to get in during the pour.
In the image above, I filled in the entire inside of the sole with Plasticine and built it up so that only the external surfaces were exposed. I also shaped the block to have slightly tapered walls so the part would be like a bowl. I have also added "registration points". You can make registration points from pretty much anything that won't float away in the silicone. I have used some glass marbles pushed half way into the Plasticine. These assist with aligning the 2nd part of the mold so that it "locks into position" later.
When working with additive curing products, ALWAYS use a gram scale and measure the amount to be as close as you can. I found that I could mix a maximum of 250 grams into a 2 litre ice-cream container and that amount would degas without over flowing into the Vacuum Chamber during the rise and fall. Fill the mold until you have about 12.5mm or 1/2" covering the part.
Once this is cured, you remove all of the fill medium. Be careful NOT to disturb the or break the seal between the first pour and the master part as you DO NOT want silicone leaking in between these parts during the 2nd pour. I made my first pour like a bowl so that I would not get any leaks. The walls extend well past the highest point on the master. I have added fill and breath holes at the highest points of the 'wings'.
Once all the fill medium is removed, you must use a release agent as silicone sticks to itself and you will not be able to get your master part out if this happens. I have used Vaseline [AKA Petroleum Jelly] as it prevents the silicone from sticking to itself and can be cleaned off with warm soapy water.
Once prepped, you may then begin to fill the mold. I let a base line cure first and then placed two small jars inside the hollow which I filled around. The reason I did this is that the cavities left by the jars act as finger pull holes and this also saves on silicone.
Once this has cured, you can clean up the two parts and you are ready to begin casting.
The front oval parts are flat with texture on one side only, so I was able to mold these parts using a simple open back mold.