The Making of a New Mold

The following sequence of photos shows a few of the many steps required to make a mold from RTV Silicone.

The first 3 photos shows the hard master. This master was machined from hard plastic and has been fastened to the base. Both the master and the base have been coated to make them as slick and shiny as possible. The smoother and shinier the master the shinier the castings the mold will produce. Hours of time and effort go into this part of the process and after the mold is made, these parts will not used again.

The 4th photo shows the master after the mold box has been built around it. I made this box as small as possible because this mold will only be used for very few castings, so thick walls around the master were not necessary. As long as I support the walls to prevent deflection during the casting process, this mold will give satisfactory results. The reason for the thin walls is to use as little RTV silicone as possible. The intent of this mold is to replicate the hard master with several copies that are identical. These multiple copies will then be used to make a multi cavity production mold.

With the master and the mold box ready, its time to mix the silicone. This particular RTV is a 10 to 1 mix. 10 parts of RTV to 1 part catalyst, by weight. In order to get the proper mix I weigh the RTV and then add the correct amount of catalyst.
Photo 5 shows the RTV.
Photo 6 is after the catalyst is added.
Photo 7 is the beginning to the mixing process. You must mix the two parts until a consistent color is reached.
Photo 8 shows the mixed RTV and ready to be poured into the mold box. You can see the RTV de-gassing itself by the presence of air bubbles coming to the top. This de-gassing will continue for quite some time and will cause no problems in the mold as long as none of the bubbles gets trapped against the master as the RTV cures.

Photo 9 through 12 shows the RTV being slowly poured into the mold box until the master is completely submerged.

Now we wait. It takes about 24 hours for the RTV to cure before we can remove the master to see how our new mold turned out.
I'll post some more of this process in a later post.