mold and cast - inexpensively

[Kozma Minin]

This is my addition to Cole's, are you cheap as me.... series.
I want to scratch build a few box cars with scrap plastic I dig out of the garbage at work. I find the details on the ends and roof of the rail cars to be the most difficult to replicate by actually building and modeling them. I want to share a method I've used to mold and cast details for projects in the past, inexpensively.

The mold material is RTV Silicone Instant Gasket and is sold in a 40g (1.4 oz) tube. There are three formulas; Red Hi-Temp, Blue, and Black. One tube can make molds of both ends of two model box cars. Each tube is priced at $1.09 at the local household supplies store.

I sprayed the end and around the edge of the model (mold master) with an industrial mold release that I had. RTV Silicone generally doesn't stick to anything except itself, but I didn't want to take chances of it not pulling apart later.

My first attempt was applied incorrectly in one layer that was too thick.
This material requires no catalyst added to the formula, so it has to air cure.



This didn't allow the center to cure properly and made a mess. I had to clean this off of the model, so meanwhile I started a new mold on the other end of the car.


I applied three layers no thicker than 1/8 inch with 24 hours cure time before adding each layer.


The final step was to add seven blobs spaced apart to glue on the sheet metal base that was cut from a can lid. This will create air space for the hot plastic casts.

Finished mold of one end.



As you can see, it replicates all details. After cure, the consistancy of the mold is slightly softer than a Pink Pearl eraser.

[Cole Only]

WOE! thats dope. im a little unsure about how you plan to complete it though. will you melt the scrp plastic and fill in the mold some how?this looks pretty interesting. im curious to see the remainder of the steps..

[Kozma Minin]

The Red Hi Temp stuff above will be filled with melted scrap styrene sprue from the sprues I have from plastic model kits. I'll probably get to that this weekend or so.

I also made a mold using the normal temp Blue stuff that I poured and smeared in with A+B epoxy glue. This epoxy is the 30 minute setting type so I had time to work out most of the bubbles and work into the details. This epoxy is also about a dollar and is enough to cast one of both ends of an HO scale car.




This is the cast part after a light coat of spray primer.

The scrap plastic from the trash can at work will be used for the sides of the car and bed carriage.

[Kozma Minin]

I've been away from the bench for a while working on other sht. Finally had some time today.
The melted sprue cast did not go as well as I thought. It didn't work.



The scrap sprue stock on hand and then I chopped it up, ready for melt down.




The first set up on the left is outside for ventilation of fumes that this process may produce. Notice the little canister candle under the crucible. This burner did not produce enough heat fast enough to produce a pourable cast material. It was also too breezy outside with this setup.
Safety note: I had a bucket of water standing by and I was wearing safety glasses throughout this experiment.
The second setup on the right was in the garage to avoid wind. A larger candle was used as well as an aluminum can because I thought it was thinner material for a crucible and would heat up faster than the tin can method. Again, this did not produce enough consistent heat fast enough to produce a pourable plastic.

I wanted to try this method because of all the scrap sprue I already have on hand. And the red silicon mold material is for high temp applications. I guess I'll cast with A+B epoxy glue like the blue silicon posted above.