So after a few fails, I have success
So for those that are wondering what I am doing making male and female parts, the process is -
1. The letters are drawn in CAD and then reversed. The letters are then cut into a piece of Perspex on a CNC milling machine. This gives me a female master part. The reason I don't just use this to make the letters is the difficulty of getting the letters out. And as I only have one master set of letters, I can only make one set of letters at one time. In this picture, I have poured clear elastomer on the female too to make a male master part.
http://www.therpf.com/attachment.php?attachmentid=575809&d=1452425276
2. Once I have a male master part, I then mix and pour a silicone rubber over that to make a flexible female mold that is easy to get the letters out of. I repeated the step above so I have 2 sets of letters to work with.
http://www.therpf.com/attachment.php?attachmentid=575808&d=1452425263
3. Once I had two sets of male molds made, I cut them up to better fit them in the new mold box.
This image is of the new mold that has been made from the 2 previous molds. Because I am casting the letters in their colour, this helps keep the colours from mixing and contaminating as well.
http://www.therpf.com/attachment.php?attachmentid=575806&d=1452425223
4. The cast parts are removed from the mold. You will notice these are perfect casts. To achieve this, the molds were placed back into the vacuum chamber after the pour and this ensured all the air was removed. If you look closely at the letters on my own shoes, there are tiny pits where air did get trapped during the pour. Ultimately, with an even larger chamber and slight modifications to the molds, the small pits that occur in the clear soles could also be eliminated.
http://www.therpf.com/attachment.php?attachmentid=575807&d=1452425223
If I was able to have the letter laser cut, I could have had them done as a male part and glued them to a sheet of plastic to make a male master and just poured silicone over that. This would have been easier, but would have cost more to do. If I had used a laser cutter, I could have multiple sets done and made a mold with many letters so multiple sets could be done at one time. Any this is done now, so I can make a 2nd silicone mold as I still have both sets of male masters.