Q&A Bios Articles

Special Clinic:
Resin Injection Casting at Home

Marty Vaughn, MMR
Clinician for Nternet

	Photo 1: Castings: Everything you see in this photo was cast two-part resin in one or two piece mold.

     A while back I was asked to build a display for the Museum of Technology in Newark, Ohio. The display represents the Baltimore and Ohio railroad yards in Newark circa 1936. Included in this display is a model of the 25-stall roundhouse that existed at that time. The entire display was to be four by six feet and built in N scale. While the entire project had a number of challenges building the roundhouse and the other major structures was one of the most imposing.

Photo 2: Roundhouse: The entire roundhouse was made from 3 molds, one for the back wall of each stall, one for the doors and one for the smoke jacks. In addition, the majority of the tenders visible in the stalls are also castings.

     Because of the short time available to complete the project, I realized I would not be able to construct all 25 stalls from scratch and would have to use a master and make castings. In the end I also ended up casting the doors and the smoke jacks. The entire roundhouse and all the major buildings were made from just 8 masters and molds. No structure took more than 3 masters and molds.

     In the past I have primarily used Hydrocal and Cerrobend (a low temperature melting metal) for casting parts but neither of these materials would work for the roundhouse. Hydrocal would be to brittle in the size and cross sections necessary for the building and Cerrobend would be too heavy and too expensive. The alternative was to use one of the two part casting resins now available.

     For lack of a better description I like to think of casting as alchemy, there is a little bit of science and a little bit of art involved in the procedure, all topped off by a considerable amount of fiddling to make it all come out.

     For any type of casting, no matter what type of material you are casting with and no matter what type of mold material you are using, there are two critical items. These are the master that you are using and the mold itself. These two will make or break any casting job you do. If either the mold or the master are not made well, there is nothing you can do to improve your cast parts. While I normally subscribe to the "good enough" modeling philosophy, when making a master for casting that doesn't apply. Any mistakes in the master will be cast into every one of the castings. In the case of the master for the rear wall of the roundhouse stalls the master itself was composed of 37 separate parts and there was the added complication that the master had to be built in such a manner that it would mate to the next section. The left side of the master had to mate with the right side, but there was no way to test it until the master was built and a mold and casting made.

Photo 3: Boiler Repair Building: The second largest building is made from 3 molds. All the side and end walls came from a two-story 4-window master or a two-story 2 windows and lower level door master. The peaks at each end, not visible in this picture are the same as used in two other structures. Many of the boxes, barrels, wheel sets and junk are either commercial or homemade castings. The junk pile in the lower right made use of a number of poor castings.

     In the display, the masters were constructed from sheet and strip styrene and brick sheet material. The actual plans were ones I drew from an aerial photograph, not the easiest way, but it was the only photo available. The basic shape of each master was constructed and over laid with the brick material. In those cases where fancy brickwork was used on the prototype, individual layers of brick material were over laid to build up the brick. I used to use Holgate and Reynolds brick sheet material for this type of work, but I am no longer able to find a source for it. If you want to build a brick structure, Plaststruct makes a good looking brick sheet material in HO. I don't care for their N scale version, every other row of bricks is distorted, but they also make a TT scale brick sheet that will work for N scale.

     For the lintels over the doors and windows, and for the sill at the bottom of the windows, I just glue a strip of the appropriate size strip styrene directly on top of the brick material. This will work fine for concrete (as with the roundhouse) or wood. For wood you will need to scribe some wood grain into the styrene strip. Just take care that there are no crevices or gaps in the master. The mold material will flow into these, no mater how small and get trapped. Inspect your master closely and if you have any gaps or hair thin cracks, fill these with a gap filling CA glue, or liquid styrene cement for plastics or white glue if the master is made from wood.

     Once the master is finished, the next step is to make the mold. There are two different materials that I use for making molds, latex rubber (like you use for rock molds) and Room Temperature Vulcanizing (RTV) rubber. As long as you have one flat side on the master that will not be visible on the finished model, you can use latex rubber to make a one part mold. This works well for things like boxes, barrels, crates, stacks or ties, small solid buildings (sheds, outhouses) etc. that have one side that will always be hidden. Latex rubber is much cheaper than RTV. As long as your molds are properly prepped and don't include a lot of under cuts, you can easily get 20 or more castings from a latex mold.

Photo 4: Car Repair Building: This building and the one immediately behind it are nearly identical. Each was made from two masters, one with two windows and one with one door for the sides and the peak used on all four ends. Again, many of the detail castings are either commercial or homemade resin castings. The rolling stock on the display also includes a number of commercial and homemade castings.

     To make a latex rubber mold, glue the master to a flat glass or wood surface with white glue or rubber cement. Use just enough glue to stick the master to the surface, as you just want to hold it, not permanently bond the master. With a clean disposable brush, apply a thin coat of latex. The first coat is the most critical and you need to make sure you work the rubber into all the nooks and crannies of the master. After the first coat is dry, repeat this step until you build up approximately ten layers of the latex, letting it dry between each coat. Each coat should be as thin as possible while still covering the master. A thick layer of latex will actually weaken the final mold, making it easier to tear when removing the casting. Also, unlike a latex rubber rock casting, do not add gauze or cheese cloth to the final coat. Adding gauze will make the mold stronger, but it will also decrease its flexibility making it impossible to get some of the castings out of the mold. Even if you have a mold tear, you can always make another from your master.

     For two part molds or when casting very large masters, RTV rubber is necessary. Make a frame that will fit around the master leaving at least a quarter inch of space on all sides. The quarter inch is the minimum, more space is helpful, but will require more RTV rubber for the mold. The frame must be at least 50% higher than the master you are casting. You can use wood or styrene for the frame, either will work, but I use styrene because it is easier to remove from the RTV rubber when the mold is finished and easier to reuse for other mold dams.

     You will need to buy some modeling clay or the casting clay that is made by the Alumilite Company. Permoplast is another brand name. Just don't buy the kind of clay that hardens in contact with air. That won't work for this purpose. Lay out a smooth surface of the clay the same size as your frame and at least a quarter inch thick. You want this clay to be as smooth and level as possible. A rolling pin works well for this and if you cover the clay with wax paper no clay will stick to the rolling pinÉ and if you're lucky your wife will never even know that you used it! Force the frame down into the clay cutting off any clay that is outside the frame.

     Now you want to lay out your master inside the frame. Make sure that it is evenly spaced from all sides of the frame then press it down into the clay. Sometimes you will have a part of the master with an odd angle that will not lay flat, such as the outstretched arm of a figure or the bent leg of an animal. Using a toothpick, you may need to pull up some of the clay around the out stretched part to make sure that the master is completely covered on one side with clay. This will leave a depression in the clay on sides of the upraised part of the master but that won't hurt anything. Once you have the master positioned, you need to use a toothpick to outline the master in the clay. You also need to outline the edge of the frame with the toothpick. The purpose here is to press the clay tightly against the edge of the master and the frame so that no RTV can seep under the master. RTV rubber is much like casting resin in this regard, the smallest hole will allow it to leak out of your mold or under your master. If you are making a mold with lots of small parts rather than one big wall section, I find it easier to cast the pouring channels (sprues) directly into the mold. You can use wood, metal wire or even sprues from other models for this. Just embed the sprue in the clay like you did the master and make sure that the master just barely touches the sprue.

     Once the master is set, you need to make depressions in the clay around the master. These are the mold keys that will help you lock the mold together when you are casting. Two keys are the very minimum you can get by with, three is better and four or more are best. These don't have to be very deep, but should be big enough and smooth to avoid tearing the mold when casting.

     The last thing that you need to do before adding the RTV is to use a mold release. Micro Mark makes a special product for this, but you can also use Vaseline or Pam No Stick Cooking Spray. You do not want to get any of the mold release on the master (so if you use Pam, don't spray it into the mold frame). Using a small brush, lightly coat the clay surface and the sides of the mold frame. You do not want to add a thick coat, just enough to prevent the RTV from sticking to the clay.

     There are two main brands of RTV rubber on the market. The first, HSIII, is sold by Dow Corning and is available in many hobby shops. The HSIII comes in two parts, the rubber material and the catalyst. It requires you to mix 10 parts rubber to one part catalyst by weight. You can measure this with a good food scale, but it is critical that you measure the amounts correctly, otherwise the HSIII will not set up. HSIII is very flexible and strong and is very good for use with a master that has many under cuts.

     The other brand of RTV I have used is called One-to-One RTV Silicon Mold Rubber and is sold by Micro Mark. This RTV has the advantage that you mix equal parts of the material by volume. While this makes it much easier to mix than the HSIII, the disadvantage to this product is that it is extremely fragile. Small pieces will easily tear off the mold when removing the casting. I used this for one of the simple wall sections on the display. From the first mold I got one wall section and it tore. From the second mold I got one wall section and it tore. The third mold I made from HSIII and got 26 wall sections and still have a good usable mold. If anyone out there has used the Micro Mark product and had success with it, please write up your comments. I would like to know if I am doing something wrong!

     Alumilite has now come out with their own RTV. I have some but have not tried making a mold with it yet. I'll post comments when I have tried it.

     Whichever product you use, pour the rubber and catalyst together in a disposable mixing container and stir. I use a paper cut and a tongue depressor or Popsicle sticks for this. Take your time and try to keep from stirring air bubbles into the mix, but make sure that you have the two parts mixed well. Both of these RTVs change color as they are mixed, helping you ensure they are well stirred. In some cases where the master has lots of nooks and crannies, I use a disposable brush to work the RTV into the spaces, painting it on the master. This will help to ensure that the RTV gets into all the spaces and doesn't trap any air bubbles next to the surface of the master. Pour the remaining RTV in the mold. It helps to reduce air bubbles if you slowly pour in one corner of the frame and let the RTV flow over the master. Fill your mold frame to the top and tap the frame a few times to help release any air bubbles from the surface of the master. Set it aside to cure. While some products say they will cure in less than 24 hours, I have found that 24 hours is about the minimum time required.

     When the first side is cured, push the master, mold and clay out of the frame. Be careful not to damage the frame, you still need it to make the other half of the mold. Slowly peel the clay off the master and the RTV, you want to try and not pull the master out of the RTV. If there is any clay stuck to the RTV or master, just use a toothpick to scrap it off. Place the master in the RTV back in the mold frame, add the mold release, this time coating the RTV and frame (but not the master) and repeat the steps of mixing and pouring the RTV for the second half of the mold.

     Twenty four hours later you can separate you mold. Remove the master and clean up any visible mold release. It is a good idea to wash the mold with soap and water to remove any traces of the mold release agent that you used. You now have to decide whether to pour the mold from the edge where the two sides of the mold join, or whether to pour from one of the sides. It really doesn't matter which way you go, just pick the location where the part will be least visible or the easiest to clean up the sprue attachment. You will need to make two holes in the mold. The first should be big enough to just fit the nozzle of the syringe. The second should be as far as possible from the first, but still on the same side or edge of the mold. The purpose of the first hole is to inject the resin. The second hole is to allow air to escape from the mold when injecting the resin.

     Whether you are using a one part latex mold or a two-part RTV mold, you need to apply mold release before casting. For resin casting, the mold release is spray lacquer. The color doesn't matter, although if you can find a color that is appropriate for the basic color of the finished casting this will save you a step when painting. Spray the interior of the mold so that all surfaces where the resin will touch the mold are covered with a coat of lacquer. With one part latex molds it is sometimes easier to turn the mold inside out before spraying. The lacquer will coat the rubber and then chemically bond to the resin. This prevents the resin from bonding to the rubber mold and will increase the live of the mold a thousand percent. I have some latex rubber molds that I have used for over 50 castings and they are still very usable. Without the lacquer the resin will stick to the rubber mold and make it difficult to remove the casting without tearing the mold. When the lacquer is dry you are ready to try casting.

     You will need some tools for casting. The first item is an oral syringe. You can find these at most major pharmacies. You do not want a syringe with a needle. Some oral syringes have a small circular-casing cast into the syringe that covers part of the nozzle of the syringe. These needs to be cut off with a hobby knife so that you can insert the syringe into the mold. You can also find syringes at some hobby shops used for applying glues. These will work, but you need to remove the narrow pointed end to keep they from clogging. The other tools you will need are a disposable graduated mixing cup. These can also be found in some pharmacies and also in hobby shops and are used to mix equal parts of epoxy glue. Hospitals also use these small cups for dispensing medications, they are about the size of a shot glass. If you use Alumilite resin you get 2-3 of these cups with the resin. You will also need rubber bands, .80 styrene sheets, a plastic knife, fork or spoon, safety goggles, a shop apron and disposable rubber gloves. While casting resin is handy stuff, it will also stick to most anything that it touches. It will not wash off skin, it is there until it wears off or the layer of skin cells sluff off and it will not wash out of clothes. I don't even want to think about getting it in your eyes. The apron, goggles and rubber gloves may seem excessive, but they are easier to replace than clothes or eyes.

     There are three brands of two part casting resin that I have tried, Alumilite, Model Cast, and Micro Mark. All are about the same price and all work equally well.

     You need to cut pieces of the .80 styrene that are just larger than the flat sides of you two part molds. Hold the molds and styrene backing together with rubber bands. If you are pouring your molds through the sides and not the edge you will need to mark your injection hole and the air hole on the styrene and cut holes through the styrene.

Both Alumilite and Model Cast resins have a three-minute mix time. That means that at normal room temperature you have three minutes from the time that you add part A to part B before the mix will harden. The Micro Mark resin gives you about five minutes. All of these resins are temperature sensitive. If you are using the resin on a hot day in a non air-conditioned location you may have less than a minute of working time. While you need to have your protective gear on and your mold rubber banded before you begin to mix the resin, you can lengthen the mix time of any of these resins by chilling the resin. I do all of my casting in my non air-conditioned garage where we also have a deep freeze. I keep the resin in the freezer until I am ready to start casting. Freezing the resin doesn't hurt it, but it does require more vigorous shaking of the part B material before use. Freezing the resin will at least double the working time in the spring, fall and winter. In the summer I cannot keep the resin in the freezer because the humidity is so high where I live. All of these two part resins are also moisture sensitive. Any moisture in the mold or the mixing cup will cause the resin to bubble and spoil the casting. In the summer if I try to keep the resin in the freezer the condensation inside the bottle as soon as I open it puts enough moisture in the resin to spoil the casting. That is also why I use plastic utensils to mix the resin rather than a wood stick like the one provided by Alumilite. Sometimes the wood stick can retain enough moisture from humid air to cause bubbles in the casting. When casting I find that about the largest quantity that I can work with before it begins to cure is one of these small graduated mixing cups full of the 50% part A and 50% part B resin. While I don't know the chemistry, it also appears that the more resin you attempt to mix at one time the quicker it will cure. Start with small quantities to prevent waste.

     Once you have poured the two parts of the resin together, mix rapidly until it changes color. Alumilite and Model Cast will turn to light tan, the Micro Mark will be clear. Insert the syringe and suck up the casting resin and inject into the mold. Just be careful not to suck air into the syringe causing bubbles in the mold. Your mold may be large enough to require more than one injection. That's fine, just keep injecting the resin until the resin begins to seep out the injection and/or air release hole. You will want to inject a little bit more resin to force out the air. Since the resin is heavier than air, tilt the mold so that the air hole is higher than the injection hole and inject more resin until there are no more air bubbles escaping. On difficult molds, while injecting the last bit of resin, you can also squeeze the mold with your hand and slowly release it as you inject the last of the resin. Getting the air out of the mold is the "fiddling" part of casting that I mentioned earlier.

     When casting I always have an open one-piece mold that is ready to cast. That way, if I end up with a lot of over flow from the two piece mold, when I am trying to get the air bubbles out I can just tilt the two piece mold over the one piece mold and let the excess drain into it. This almost eliminates wasted resin. If you casting is larger than the volume of resin that can be mixed in one measuring cup, you can inject the first cupful and then add the second (or more) later. The only difference is that you need to make sure that you do not clog either the injection or air holes in the mold or the .80 styrene support.

     When finished injecting the resin, wipe off the outside of the syringe and set it aside. You won't be able to get the last of the liquid resin out of the nozzle at the end of the syringe while liquid. Once the resin hardens you can pull the plunger back slightly. The plugged nozzle will create a vacuum so take care not to pull the rubber gasket off the end of the plunger. While pulling back on the plunger, with a map tack you can force the hardened resin pop out of the nozzle and into the barrel of the syringe. If you don't press the plunger into the barrel firmly when injecting the resin, you will leave a little film of resin on the end of the rubber plunger. This actually is a benefit. You can peel the resin off the end of the plunger and you have a perfect HO or larger scale lampshade. If you leave a little bit of resin in the barrel intentionally, you can also create nice looking smokestack caps. These are what I used on the roundhouse. Don't bother wiping the mixing cup, once the resin has hardened you can simply peel the cast resin out of the cup. All of the syringes, styrene, mixing cups and mixing sticks are reusable.

     As the resin hardens in the molds it will change color again. The Micro Mark resin turns white and the other two will turn a lighter tan color. Wait about 10-15 minutes, depending upon the air temperature, before removing the castings from the mold. The resin will harden sooner than that, but will remain flexible for a while. If you remove the casting too soon there is the possibility of warping the casting.

     You will probably find that some of the resin is stuck on the rubber bands and on the surface of the styrene used to hold the moldings together. The rubber bands can be thrown away if too heavily coated. I use styrene to hold the molds together because it is one material that the resin does not bond to. It will stick to the styrene, but not bond and you can peel or pry the resin off with a hobby knife.

     Check your castings and remove any sprues that you had built in for mold. These sprues of resin can be broken up into small pieces and dropped into one of your open face one-piece molds. All of these casting resins will bond to themselves. They will also bond to each other, so if you switch brands in the middle of a casting that is no problem. The only noticeable difference will be the color of the two different resins in the final casting and this will be hidden by the paint.

     You can also save the dust left over from sanding resin parts, resin pieces used for the sprues, flash or any bad castings. I grind up the bigger pieces in an old blender and mixing this with the liquid resin I use it to help fill up the one-piece molds. This cuts the expense of wasted resin down considerably.

     If you find that you have a consistent spot in your casting where there is an air bubble there is one more trick to try. After the lacquer spray has dried, but before you assemble the mold, try brushing just a little bit of talc or baby powder over the area where the air bubble occurs on both sides of the mold. The resin will bond with the talc and the talc acts like a wick to help pull the resin into the mold and move the air along. Less is more with the talc, if you can barely see it in the mold that is enough.

     Sometimes you will have a casting with a visible air bubble inside, but the resin has formed all around the bubble. If the bubble doesn't weaken the part structurally it is still usable and the bubble will not be visible when the part is painted. If the bubble weakens the part, there is one more thing you can try. With a sharp knife, open the bubble area on the side of the part that is least visible on the finished casting. You want to just lift the resin up without removing it. Using glue such as Weldbond, Aleene's, or Elmer's Weather Tite Wood Glue Gel Formula, you can squeeze the glue into the void of the air bubble and lay the resin back in place. With a little care, you can strengthen the part and not be able to see the repair. You want to use a thicker glue for this, not Elmer's white glue, nor any yellow carpenter's glue, they are too thin. Do not use any of the super glues as they will crust over on the outside near the edge of the resin but will not harden on the inside. This can make for a mess later if you have to drill through that spot or accidentally break the crust of the super glue. Believe me on this, I know!

     Besides mixing the casting resin with itself to extend the resin, you can also mix it with other things. Alumilite sells little glass balloons that you can mix in the resin, but you don't have to be that fancy. A friend of mine gave me some fine casting sand, used in the glass industry, which had leaked out of a hopper car on the railroad that he works for. As long as the sand or other similar material you use is dry it will mix satisfactorily. I have used mixtures of 50% sand and 50% casting resin and that works fine. I actually use a larger mixing cup and pour the resin parts A and B over the sand. That way I mix the resin and the sand in the same step. Remember you have to work quickly. Adding sand has the advantage that it will actually strengthen the resin, making it harder. The disadvantage, as you can probably guess, is that it will be so hard that it is difficult to sand and nearly impossible to drill. It is literally as hard as a rock.

     That's injection resin casting in a nutshell. Most of the tools you probably already have. Start up costs for the RTV and resin are about $50, but you can cut this to about $30 if you start by making latex molds first until you gain some experience.

Tools:
Goggles, shop apron, rubber gloves,.80 styrene, Pam non-stick cooking spray, rubber bands, hobby knife (to cut styrene), graduated measuring cup, cheap disposable paint brushes, clay (Permoplast or Alumilite), disposable plastic utensils for stirring resin, talc or baby powder, toothpicks, and paper cups for mixing RTV.

Materials:
Molds: Dow Corning HSIII RTV, Woodland Scenics latex rubber
Casting resin: Alumilite, Model Cast, or Micro Mark two part resin