Closet Casting Part I: Better Living Through Chemistry!

     I guess my first exposure to the concept ofresin casting came in 1996 or 1997 in an issue of what was then OutdoorRailroader.  It was called the "Squash School of Casting" and wasa pretty good primer on what has been described to me as the Black Artof Casting.  I was just getting into Large Scale Railroading at thetime, and the idea of being able to make dozens and dozens of detail partsfor just pennies was very attractive.  The problem was that the sourceslisted in the article all required bulk quantities, that were cheap perpart, but the initial startup was a bit daunting.

     Still the idea intrigued me, so I kept researchingthe topic.  The Internet is a wonderful tool for such things, andI came across many sites for what the Japanese call "garage kitting." These private sites were valuable primers for a prospective caster suchas myself, and eventually I ran across a few suppliers that could furnishmy needs at a cost that a young father of 2 with limited finances couldaccept.  As a result of rushing to get started, my first time outI ordered a resin that was totally unsuitable for my needs.  I gainedexperience with creating molds, but the actual casting process ended upa disappointment.

 Finally a friend of mine who also did a little casting tippedme to some products that he had discovered and had worked well for him. So out came the credit card and we were off to the races.  While Ihave used other products since, I still keep coming back to old reliable. The materials I recommend for the beginning caster comes from Micro Mark(A list of suppliers and links to their sites will be provided at the endof this article).  They even have a kit for beginners that includeeverything you need to get started.

     Before we proceed too much further, this isprobably a good time to discuss some terminology that we're going to beusing later on:

Resin-  Generally a 2 part chemical compound that when mixedin the proper ratio results in a chemical reaction   transformingthe liquid into a solid.


Room Temperature Vulcanizing Rubber.  This is the primary materialthat molds are made from, they come in various formulas but all resultfrom a 2 part mixture to set up to a solid.

 Mold Release

This is a compound that is used to keep either the RTV or resin fromadhering to a surface, it can be a commercial product or something madeup in shop.

 Open Mold

Or a one part mold, the simplest type of mold generally a mold thatrequires only that the resin be poured in and allowed to set.

 2 Part Mold

Self explanatory I would think, this is a more complex mold that ismade of two halves.


The actual thickness of the resin while pouring.  The key herewhen reviewing resin for detail part casting are the words "water thin"this allows the resin to flow into all parts of the mold and for trappedair bubbles to escape

 Pot Life

The time that a resin has after mixing the two parts before the chemicalreaction becomes solidifies.  This is how long you have to pour thestuff from the mixing vessel into the mold.

 Demold Time

The time you need to wait before trying to remove a part from the mold. Some resins will demold in a few minutes others will take several hours,make sure of what you are getting to avoid disappointment.


The original part to be replicated that is used to make a mold

         That all being said,lets get started!!  As I stated before, I like using the productsthat MicroMark sells, these are produced by the Smooth On Corporation andcan be purchased directly from there, but since I generally end up gettingseveral other items at the same time I stick with Micro Mark.  MMhas a kit aimed directly at the beginning caster that gives you everythingyou need, but really the only absolute necessities are the 2 part RTV,2 part resin, and some equipment to mix them with.  I use the CR-300variety because of the rapid demold time (I'm not a very patient person)but if you want to have a little extra time to pour your resin the CR-600might be for you.  The important thing is to research thoroughly beforeyou jump in.


The two part resin (left) and two part RTV rubber (right)




     Sensibly, the first thing you need is the part tobe replicated.  In casting terms this is called a master.  Masterscan be made of any material, wood, plastic, metal, whatever, and then castin resin.  The only caution I might add is that wood parts that aregoing to be used to represent metal objects be thoroughly sealed and sandedto keep the wood grain from being picked up in the mold making process. For this article I'm going to be making a mold of a locomotive couplerpocket for link and pin couplers.  My master is made of a combinationof wood for the actual pockets and a styrene back plate.  After Iassembled the piece I sanded it down and then sealed it with several coatsof Krylon Ruddy Brown Primer to eliminate the wood grain.  The resultingtexture I think is a pretty good representation of cast iron.

Wood and styrene master to make locomotive coupler pockets




         What I like to do is affixthe master to a clean flat surface, such as a piece of scrap plexiglass. I used CA and make sure that I leave enough spaces around the master fora dam to be placed.  The dam should be taller than the master so thatit can be completely immersed in RTV.  I use sheet styrene for thisbut thick posterboard or card board will work.  I like styrene forseveral reasons, first of all, it can be reused for many different molds,it can be bonded with ordinary modelers glue, and is a smooth non poroussurface allowing easy removal of the mold.




          After the dam is assembledI glue it to the plexiglass at the corners, then to keep the RTV from runningunder it I add some modeling clay around the bottom to seal it.  Tomake it easier to pull the mold off the master I give it a light coatingof mold release.  In my case I use Vasoline dissolved in mineral spiritsand brush it on with a paintbrush.  There's no real formula for this,I just use as much as the spirits will dissolve.  Make sure that youbrush release on the master, the plexiglass and the sides of the dam justto make things easier all the way around.  Now you're ready to poursome rubber!!!

Using a paintbrush to apply a small amount of mold release






          The Micro Mark stuffis really easy to use, its just a simple 1 to 1 mixture of parts A andB.  I use small plastic cups I bought at the supermarket for mixing,and just pour in the amounts til they are equal.  Depending on theamount of RTV I'm pouring, I might use a third cup to mix the parts together,but more commonly I use a plastic margarine container to do the honors. After pouring in both parts, I use a stick to stir it up with, generallyjust a piece of rough strip wood that happens to be laying around. One characteristic of the RTV is that one part is white while the otherpart is a light blue in color, when blended together (with the consistencyof pancake batter) it should turn a uniform grey or blue color.  Youhave to be careful not to let any of these chemicals come in contact withyour skin or clothing of course, and wearing of rubber gloves is HIGHLYrecommended.



          Once the RTV is mixed,its time to get it into the mold.  Get as close as possible to itand begin your pour in a corner rather that right on top of the master. A master with deep recesses such as the coupler pockets here might trapair and deform the mold.  Filling from a corner is best and just letthe  RTV flow in and around the master.  Make sure that the partis completely covered with RTV and has at least 1/8 - 1/4 inch of RTV aboveit.  You will note that bubbles appear to be popping out as the rubbersets.  The Micro Mark RTV debubbles itself with no special equipmentrequired, In the years I've been using it, never had a problem.  Nowset the mold aside and let it cure for several hours according to the instructions,overnight is a good rule that I use.



          After the mold iscured, you can remove the forms and pull the mold away from the master. The more intricate the mold, the more care need to be taken to avoid rippingit, but hopefully the mold release should make this as painless as possible. And there you have it. Your first open face mold.  Now you don't haveany excuses, makeup some masters, and get some RTV and get started. In our next installment were going to cover the basics of actually castingsome parts!


 Until next time!

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