Two part mold, this is long

[mattb]

This will be long, it is the full description of how I do a 2 part mold with no vacuum or pressure. The piece I am making is the interior plate for the the 1/24 MPC 1966 Dodge Charger slot car. With this interior, you can convert any MPC Charger promo car to a perfect clone of the slot car issue as the bodies are the same colors, mold and cycolac plastic.
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I start with covering the bottom with clay. I use the stuff in colors from Ben Franklin, I have used this clay over and over for years. Warm it and it works easier, I use a hair dryer. You want it to cover the plastic at least 3/8 inch for the lowest area.
I use a knife and trim it to the outline of the part as shown. A little taper as it goes down is preferred.
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[mattb]

Bottom pic of the interior with the clay filling the bottom.
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Back right side up on cardboard which will be the mold base. Notice pieces of balsa shoved into the clay around the base.
This makes a nice ledge for the inner mold to rest on and remain in the exact location.
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Starting to build the mold box around the pattern. I used some balsa for this one, but cardboard works, plastic sign material, legos whatever you have hand. I use the glue gun to attach and seal all joints. All joints must have 100% integrity or the rubber will seep out of your box and you will have to re-do. You may have a bit of rubber run under your clay when you fill the mold box. No big deal, you can trim out any excess.
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[mattb]

Mold box about done. All joints sealed with the glue gun.
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Rubber has been poured in the mold box and cured for the required time, This is quickset and takes 3-6 hours. I have given it two brush coats of silicone mold release, not confused with release just for the plastic casting, this is 2 part silicone mold release. Petroleum jelly also works, but I have not tried it. A local guy told me he mixes pertroleum jelly with mineral spirts and it is the same as the commercial product. . P1010016.JPG
Now the clay is removed and the interior put back in the outer mold. I have added plastic tube, rod or sprue to allow air to bleed out from the highest parts of the part where air may be caught. I also usually insert the inner mold at angle so the liquid plastic starts at the bottom and flows to the end of the mold I have higher,
letting air flow naturally out.
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[mattb]

Rubber is poured in the inner mold, this is high strength 3, my favorite soft rubber.
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Skipping a little but the inner mold (plug) has cured and been pulled out. The original interior has been removed from the plug. The vent holes were partially filled with hardened plastic, but the interior pulled out with no problems.
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30 grams of mixed plastic has been poured into the outer mold and the inner plug inserted into the outer mold. I let it cure 15-20 minutes then pulled it apart. I try to always use more than enough. That is cheaper than throwing away a couple castings, even if you waste a bit of plastic. A thin casting takes awhile to cure. This cures in 10-15 minutes, but not rock hard when it is this thin. I demolded anyway, twisted off the vent hole plastic and then put it back in the outer mold to retain correct shape until it is good and cured.
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[HomeRacingWorld]

Excellent sir, just excellent. Thank you for this.

[mattb]

This pic is a little out of order, but it is the first casting with the vents still attached. They just twist off.P1010026.JPG
The new casting back in the mold to retain shape until it is fully cured. In front of the mold is a one sided mold that makes the bushings that mount the interior to the posts in the body and the driver head for the plate.
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This pic is out of order again, these are the extra pieces silicone glued to the bottom of a small box that is filled with silicone and makes the one sided mold for theses pieces.
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Any questions? You have to experiment with how much material you use when casting. I try to write down how much plastic it takes for a casting, then I don't guess every time. Rubbers all vary some, I like the Alumilite high strength from Hobby Lobby with the 40% discount coupon. I like Alumilite quick set plastic and buy the mold release there also. Working this way you still may get air bubbles, but usually you find a way to fill the mold or tilt it around so air is expelled as you push the plug into the mold.

I learned most of this from a great caster, Chris Etzel. Chris was happy to share his knowledge which was extensive and his Etzel Speed Classics open wheel kits were the standard by which other kits were judged. Chris passed away a few weeks ago and I just want to give him credit for helping me and being the good guy he was.

[mattb]

Harry, this Alumilite cured on time. It was purchased in Dec. The batch that took so long to cure was from Aug and Sept last year. I wouldn't think that is too old, but this cured dry, not sticky in 8-10 hours.

The best tip I can give about this stuff is buy some rubber and plastic and see what you can do. Like Jack always said, "it ain't rocket science".
Remember you kind of do everything backward.

[BARC 1]

Very nice. Especially like the idea of having another project ready beside the main subject. Seems like you always end up with more silicone then you need. Having a little side project will ensure you can make use of any extra.

On that note though, I learned here or somewhere else to use rice to fill the mold box after the pattern is in. You then pour the rice in a measuring jug and you know exactly how much silicone to mix up.