Bases: Making an RTV mold

Time to create the mold for my set of 10 25mm bases.  The type of mold I work with is generally refered to as one part, open face or dump mold.  The mold is a single RTV piece, with open cavities for casting.  Makes for easy mold building, but does require a bit more attention when pouring the resin and may require a bit of sanding on the completed cast.

The RTV used for hobby mold making is a two part kit, mixed by weight or volume, and cured at room temperature in 24 hours.  My rubber of choice is Mold Max 20 from Smooth On, a 10-1 mix by weight with good strength and a lot of stretchability.   For a while I was using a one to one Micro Mark product, easy to use but lacking the durability of the Mold Max.

Mold box 

Step 1:  Glue the bases to a firm base, like quarter inch MDF or a thick sheet of plastic card.  Surround the masters with a barrier.  In this case, I used Legos; handy, reusable and quick to assemble.  Other options include build a box out of foam core board, recycled plastic containers or any other frame that can be made water tight.  Tacky glue (a thick white glue) was used to attach all elements.  The gap filling glue is handy for a project like this and its easy to tear apart at the end of the project.

Once the glue was dry, a coat of Future Floor polish was painted over everything.  The wax will seal any porous surfaces and assist in removing the RTV from  the masters, mold box and base board.

RTV supplies pour

Step 2:  A postage scale, a few cups & a mixing stick is all that’s needed.  The RTV comes in two parts.  The ’10’ is a thick white goop, the ‘1’ is a red fluid.  Don’t let the goop scare you, despite the thickness of the mixture the RTV does a fine job of penetrating small spaces.   For this project I mixed 190 grams of white with 19 grams of red. 

How much to mix up? – good question.  I’m prone to just guessing & then mixing up additional batches as needed.  The RTV takes hours to set up, leaving plenty of time add more to the mix.  If you want a better answer, try filling the master mold with sand, then pour out the sand into your measuring cup to determine the volume of RTV needed to fill the space. When choosing the thickness of the mold I like about an eight of an inch of rubber, less if only small points of the cavity reach any depth, a bit more if the cavity has a large open space of uniform depth.


Step 3:

Mix the two parts of the RTV well.  Try not to fold in bubbles as you stir the rubber.  Pour the RTV in a thin stream into the mold box.  The thinner the stream, the more bubbles you’ll break as the RTV is poured.  

mold in box

Tomorrow night, I’ll be able to take down the mold box & try a first cast.  Always a bit exciting to see if the mold comes out right.

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