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Some of my rubber molds and master patterns.

Description of techniques


Having the ability to quickly reproduce a masonry model in Hydrocal was a major step in my modeling. It took something precious, that I had spent many hours on, that I would not otherwise dare to even try to paint (my god, I'd only mess it up!), and turned it into something that it didn't matter what I did to it, I could always cast another one. This freed me up to try all sorts of things, from coloring and painting, to cutting and chopping.

I must give credit to Jack Work, the pioneer model builder, who inspired us all with his many construction articles. One such article, his Masonry and Stonework Techniques appeared in the charter issue of Mainline Modeler. (It was also reprinted in the February 1984 issue.) In it, Jack explained how to create ten Fractured Stone key pieces that went together to make larger interlocking panels. I followed his instructions and recreated my own, learning how to make rubber molds in the process.

Rubber molds.


Let's get this straight. You start off by creating your own master patterns. It is not right to steal them from someone else. That's called pirating. Without going into it specifically I have left all sorts of clues in this web site on how I do it. I hand-scribed my own original patterns, usually in fresh Hydrocal. Or I built them up from scratch, using the stock castings I've already done. This is much easier than starting blank. I have written several articles describing these techniques. Most were published in Mainline Modeler. Check out the article link. Also see the previous How 2 clinic as well as those posted for my Durango Roundhouse construction.

Silver Plume Public School, beginning the pattern, openings are outlined. Silver Plume Public School, progress on the pattern, openings are being cut out.

Pattern work in progress for the Silver Plume Public School
The patterns are being scribed and carved in freshly poured Hydrocal

For now lets just say that you've built your pattern. It can be made just about out of anything you are comfortable working with. I like to use raw Hydrocal, wood and materials that are easy to work with. You can mix materials as in the end everything will be cast in Hydrocal. You can cast in other materials too, like urethane, resins, etc. but I'm not describing that. This is just simple old plaster.

There are a couple of important rules. You want to avoid undercuts. If it is too sever it will lock the rubber in place. It needs to be able to pop out easily. Everything must be sealed. The rubber is so good it will attach itself microscopically to anything porous, like wood and Hydrocal. So it must be sealed. I use Scalecoat Sanding Sealer. I thin it some and just brush it on. Larger openings are filled with water putty or wax. The pattern is lightly wiped down with petroleum jelly just before the mold is made to act as a release agent. Just a thin coating is required.


The simplest models are flat open faced. You can have split multi-part molds and more complex ones but they go beyond the scope of this basic explanation. You'll figure those out as you gain experience. A simple flat mold, say for one of my typical 4" x 8" x 1/4" thick stock castings is no less than 3/32" thick. I place the master down on a flat 3/4" piece of plywood. It has been coated with Vaseline too. I then build a dam around the master with 1/2" x 3/4" pine lathe. Sometimes I will permanently attach the master to the base but most of the time I just stick it in place with small dabs of Vaseline. You want to avoid voids underneath as the rubber will migrate into them and lock your master in place. Leave about 1/2" one all four sides. I simply nail the wood lathe with a couple of small brads. Whatever is required to contain the rubber at a certain level all around the pattern.

a wooden mold box

Okay, we're now ready to mix our rubber. I recommend and use:

Dow Corning's 3110 RTV Silicone Rubber.

RTV stands for room temperature vulcanizing. It is a natural rubber base that is mixed with a catalyst which causes it to harden at room temperature.
Check out Dow Corning's web site:  DOW CORNING WEB SITE   for more specific information.
You can call Dow Corning at 1-989-496-4400 to find a local supplier or find the link on their web site.
My current local supplier is actually an automotive supplies (bearing) distributor. They carry Dow Corning lubricants and can order the 3110 for me. I have been using "3110 RTV Silicone Rubber Encapsulant" from this suppliers. I asked a Dow rep if there was any difference than the 3110 RTV Silicone Rubber I had traditionally bought at another supplier (at a much higher price) but there is no difference. Someone once told me that the 3110 was never meant as a mold making material which is nonsense according to Dow. It works just fine for our simple plaster molds. But no, it's not the greatest either.

The stuff is not cheap. It costs towards $200.00 for a 10-pound kit. That's nine pounds of base and one pound of No.1 catalyst. I think it's about $28-30 for a one pound kit. While it is expense, actually it is pretty cheap considering you will get about four 4" x 8" x 1/4" molds that will last a lifetime. Less than ten bucks for an average mold. That's pretty cheap. I still have some of my original molds, which have produced hundreds and hundreds of castings with little signs of wear. Others have aged and died, but you should expect 5-10 years of service.

I should explain that there are a number of other silicone rubbers available from Dow Corning and other manufacturers that might work just as well or better, or worse for that matter. The 3110 is well suited for us hobbyist as it is very fluid and does not require vacuum de-airing (though it is advantageous to do so) to get good results. Other rubbers are more flexible and tear resistant but they are more difficult to use. You can find out more on these on the Dow web site. I also like to use the HS II and HS III.

  ALUMILITE   is also a supplier of Dow Corning RTV. If you can't figure it out for yourself they are a good place to go. I think they even have instructional videos for sale.


The mix ratio of the 3110 is flexible. 10 parts base to 1 part No. 1 catalyst is the recommended mid-point. At this ratio you have about two hours working time and a full cure in 24 hours. You can speed it up by adding more catalyst or there are other faster catalysts available. I used to use the No. 4 (very fast) but suffered more bubbles because of it. Vacuum de-airing is not required however it really helps eliminate trapped air bubbles. Just like in casting our plaster, trapped bubbles are our enemy. They will result is little bubbles of plaster appearing on the surface of our castings. For years I did without a vacuum but now I have one and the results are much improved.

Nonetheless, you can still make very good molds without de-airing. Just take your time and try not to whip them up. Tap the bottom of your mold box after pouring and watch them rise to the surface.

Oh, but wait, I haven't explained that it is very important to thoroughly mix the catalyst and rubber base together. Murphy's Law says that an ever so small spot that has not been mixed will be first to fall on the surface of your pattern and not cure properly, thereby marring your castings. I used to mix my rubber on a sheet of safety glass however I now mix it in throwaway plastic cups. Whichever, I am careful to mix it thoroughly.

Once mixed (and de-aired) the rubber is gently poured over the pattern. You want just enough to slightly overfill the mold box. Of course you don't want to waste it at this price. Experience helps me estimate how much to mix and it makes little difference if you mix it in two or more batches. Just use the same ratios.

You could simply leave it level out on its own however I like to make sure the backs, what will become the bottom of my molds, are absolutely flat, so I cover it with a sheet of styrene with another sheet of plywood placed on top. As I mentioned earlier, this comes after I have tapped the air bubbles out.

Mold making process.

Okay, now we wait. You can keep the mixing cup handy with the leftovers to watch the progress of the cure. You can speed things up a bit by keeping the mold warm, like beside the furnace. But resist the temptation to de-mold it too soon. I was forced by a deadline once to pull one too soon and tore it in several places. I was able to make the few castings I needed immediately however the mold ultimately required replacement. So be patient.

I should mention the rubber has a limited shelf life. Dow recommends you use the rubber within six months of the manufacture date stamped on the pail. Of course I have pushed that from time to time but keep that in mind. If it's a year old you're probably better off buying some new material.


In the morning, or whenever it has cured, we can pull the master out of our new mold. Remember, it is the rubber that bends and not the pattern. So gently remove it, progressively pulling it away from the master. Trim the excess rubber flash as you go. Once the master is removed we should be ready to go!

I'm always very excited about a new rubber mold. I can't wait to see the new castings. In the next clinic I'll describe the casting process.

If the mold is a good one the master is no longer so important but I like to keep mine carefully stored away in case I loose the mold. My molds have given me years and years of service. Some of my older molds are now 20 years old (I can barely believe it's been that long!). The castings are still crisp and clean. Unfortunately I recently lost a bunch of molds that oddly distorted, sort of softened and picked up unwanted dents and impressions where they lay for a long period against an odd surface (another mold). I had inadvertently exposed them repeatedly to an alcohol solution. Over time this damaged the molds so be careful to avoid solvents.

Here are some tips from Dow Corning   MOLD MAKING TIPS


Occasionally I am asked if I will do molds for others. Sorry, the answer is NO. I have attempted to make molds for others previously on occasion with mixed results. The problem is I have no control over the patterns and since I am the "expert" I'm expected to overcome all sorts of amateur mistakes on the patterns. I'm not a magician and well, you can see where this is going. I have plenty of my own work to do. A local fellow still owes me $100 on a (large) mold he had me make ten years ago! I'm sure he has long forgotten about but I have not. A hundred bucks is a hundred bucks! So no, I do not make molds for others. I'm busy enough with my own work.

I've also been asked if I will do molds of other people's work for third parties. The answer to that is ABSOLUTELY NOT and I refer you to my statements on  COPYRIGHTS  and  PIRATES!  . It just drives me crazy that someone buys a bucket of rubber and they think the have the right to make a mold of anything that doesn't move (better wake up kitty). If it is a sculpted object of "intelectual property" it's hands off!

Look, it's not that hard to do yourself. I've showed you how. Start off with something very simple and work your way up. I've given you all the hints you need and probably more. Some of these solutions took me several years to figure out and I am handing them to you. The least you can do is MAKE YOUR OWN PATTERNS and respect the work of other model builders. I'm amazed that some people don't understand that this is wrong. Even main stream manufacturers and publishers! You'd think they'd know better. Sorry, I'll get down off my soapbox now- If you use it reproduce your own work it is extremely rewarding and an awfully lot of fun to brag about!

C. C. CROW 's