Making resin castings of small parts is probably much easier than you might think, especially if the part has a flat side.  Parts with a flat side, like many wheel inserts, can be duplicated with a one-piece mold.  The RTV mold compound I most commonly use is Polytek Platsil 71-20 Silicone RTV Rubber.

I have used other RTV compounds with good results but the Polytek is my favorite.  For small mold construction, nothing beats Legos: molds of any shape can be easily assembled and disassembled.  I start with a flat plate and attach thin smooth caps over what will become the bottom of the mold.

I then build the walls of the mold around the base.

Next, I pour two equal amounts of the A and B RTV compounds into mixing cups.

After mixing thoroughly (but quickly), scraping the sides of the mixing cups, I pour the RTV into the mold.

Although the Polytek RTV is usually bubble-free, it is a good idea to tap the sides of the mold to allow any bubbles to rise to the surface of the mold.

After letting the RTV cure for 4 hours or so, the mold can be disassembled.  The great thing about most RTV mold rubbers is that they do not stick to anything except more RTV.

I use curved fingernail scissors to trim flash from the rubber mold.

After trimming, the part can be removed from the mold.

After removing the original part I trim the open area of the mold.

Now, I reassemble the mold box around the inverted RTV mold.

I use Polytek Easyflo 60 Polyurethane Resin.  Again I have had excellent results with this product.

I have shown the A and B parts of the resin in seperate cups, but I usually mix in the same cup, using the measuring marks on the side of the cup.  (I mix the RTV in seperate cups because it is very thick and takes a while to level out).  I mix quickly, scraping the sides of the container and trying not to introduce bubbles into the mix.  There is not a lot of working time with this resin.  I pour it into the mold until the mold is level full.

Resin cures in a thermosetting chemical reaction; heat will generate during the curing process.  The greater the volume of the mixed resin, the more heat is generated, so it may heat (and cure) faster in the mixing cup than in the mold.  When the clear resin starts to turn milky white, it is “kicking” and pouring time is ending.  The part can be demolded in 15 to 30 minutes.  

The original part is on the left, the copy is on the right.  Parts with a flat side can be molded in this manner. Other parts, like a driver’s head, will require a two-part mold.  See “Resin Casting Small Parts, The Sequel”, coming soon.

Russell Mill