I am currently experimenting with making plastic parts via rubber molds for HO scale buildings and structures. The results have been encouraging so far. I am interested in the experiences of others who have used molding of parts to build structures of their own.
Does anyone know a website or anyone here that would like to trade their molded parts for structures for mine? I have molded a couple of buildings and would be happy to trade for other molded parts and structures.
I’m highly interested in doing some molding but I haven’t gotten to that point in my layout effort yet. My goal is to get the layout decks built, the track laid, and some “make-do” buildings in-place before turning my attention to the “finer” aspects.
How have you made your molds? What techniques do you use for pouring? How do you handle bubbles and such? How much does it cost-- in both time and money-- to do one (a building, let’s say) from scratch including making the molds?
John
To get almost perfect parts -
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Make sure you have at least 1/2 inch mold material around all sides of your master parts. For large size parts like a boxcar, it should be thicker. Tiny parts can be less.
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Vacuum the RTV to eliminate bubbles in the mix. If you don’t, they will show up as small BB’s on the parts you cast.
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Cast under pressure. Pour the resin in the mold, then put it in a pressure pot to cure. ( I use an old paint pot and 60 psi.)
The equipment does cost a few bucks but will make all the difference in the world in getting quality castings.
Also, consider using Leggo’s to build your mold boxes from. Then you can reuse them.
Can you explain these two items in more detail? Especially, what/how do you mean ‘cast under pressure’ ??
I have been using Micromart’s One-To-One RTV rubber for making molds for many years with very few bubbles as this compound was designed to create fewer bubbles/air pockets. I also use a tooth pick to run around edges and crevases to completely eleminate bubbles.
To reduce the chances of bubbles went casting the resin, pour some resin in the mold, then run a tooth pick around all the crevases and corners, then fill the mold the rest of the way. It’s not perfect but will tend to eleminate 99% of the bubbles without having to resort to buy additional equipment.
To keep it simple, it goes like this:
For the Mold:
Mix the RTV with the catalyst in a disposable cup. I use plastic drink cups. Put it in a vacuum chamber and pump all the air out. Take it down to 28hg using a vacuum pump. The RTV will rise up, (trapped air expanding) the air will be expelled, and the RTV will collapse. Then let the air back in slowly, take the cup out and pour the mold.
Casting the Part:
When casting the part, I use a resin that has a working life of 10 minutes. I mix the resin with the hardener in a plastic cup and pour it into the RTV mold which is sitting on a Plexiglass plate. I then put it into a one gallon Paint Pot, close the lid, and pressurize it to 60 psi using an air compressor. I keep in there for about an hour while I go work on the RR. Then I slowly let the air out, remove the mold and de-mold the part.
What NOT to do:
Don’t use paper or wood products for mixing or storage. They trap moisture in them and that will get into the resin. When moisture gets into the resin, it will create air bubbles. I use plastic drink cups and metal spatulas for mixing RTV and resin.
Using this technique I get NO BUBBLES or FLAWS which results in a professional product. If you want more info, just ask or PM me.
At the risk of sounding like a cheapskate (who, me?!?) does a vaccum cleaner or shop vac have an application here, or is it more complicated?
and I don’t re-use paper towels! Well, not the real dirty ones anyway!
Jim
Sorry, no.
A vacuum cleaner or shop vac will not work. You have to use some type of real vacuum source. A vacuum pump works best, but you may be able to use one of those venturi things that you can attach to an air compressor, but I don’t know that for sure. If you want to test it, go buy an automotive vacuum gauge and connect it to whatever vacuum source you are trying. It needs to go down to 23hg as a minimum, and 28hg is better.