Clinic Notes

Making Cast Model Parts and Molds


Making Models and Parts by Casting

By Bill Porter

It frequently comes our way that we would like to have multiple copies of some model or part of a model. If money is no object and there is a supply of commercially available items, we can just buy them. For must of us, money is an object a huge impediment to the progress we would like to enjoy with our hobby.

In the past, I have repaired rolling stock that had been damaged. If stock material with the same surface detail is available, I would use that. In some cases, there was no appropriate stock material available. The solution was to create a mold and to cast replacement parts or suitable stock material.

Cutting, gluing and filing is always an alternative, but, it does take a lot of time. It also takes skill, tools and space to make a bunch of items come out looking just alike. That may be why so many of us do not choose to build models from scratch.

One way to make many items alike is to place a fluid material in a carefully-shaped container and then to induce a state change to solid. That process is called casting. The container is called the mold. The fluid may be liquid or a finely-divided powder. We may induce the state change by subtracting heat, adding heat temporarily or by means of a chemical reaction.

This article discusses some of the common materials and techniques used to make molds and then to make the castings in our homes and workshops.

Legal note: The physical design of commercial products is often protected as intellectual property by way of design patents or copyrights. You must be careful in your use of mold-making and casting to avoid legal entanglements. Each case is different and I do not pretend to offer legal advice. My personal guidelines are that I will not make a mold or castings for anyone who does not own the original. If I buy something, I am willing to duplicate it for my own use, but not for sale. If I make my own master, I feel free to do as I wish.

I       Molds

            A       Plaster good for higher-temperature casting such as lead and bismuth mixtures. Be very careful if you use lead or lead-based alloys. The low-temp allows made for hobby use are much preferable. Plaster molds are rigid. That means, all cavities must be made with no undercutting. Otherwise, the part will not release or the mold will be damaged.

Soap is a good release compound for making multi-part, plaster molds. When the first part is cured and has dried, cover the mating surface with liquid soap. Then, apply the wet plaster that will become the second part. Once it cures, the new mold piece will release from the old mold piece.

Plaster shrinks a little as it cures. I use plaster of Paris and it works well. When I get it, I also use Hydrocal. Neither of these plasters shrinks, much. They do shrink a little bit as they cure and finally dry out. When I was a kid, there was a plaster product with the brand name: Expand O Tite. It swelled, slightly as it cured. I have not found that stuff in many years.

            B       Latex is good for low temperature casting material such as plaster, resin or epoxy. It is very flexible, so, it will permit some undercutting of the cavity. With latex, we can make some things with a one-part mold that would require a two-part mold in a rigid material.

The liquid latex material must be fresh. If it is lumpy, it can no longer be used. The latex sets by way of a chemical reaction that is not easily reversed. So, once it begins to ‘dry’, it has become unusable for making molds. Buy only what you can use in a short time. Check the containers at the store before you buy one. It does not keep once it starts to cure.

The consistency is a lot like that of thick paint. That is really what it is. The liquid form looks like an off-white paint slightly tan.

We also need some mold-release compound for most applications. If we want to make a mold of some object, we want to be sure that the mold material will not adhere to the master once the latex has cured.

Apply a coat of the mold release compound. It is mixed with water to become a very thin solution. It seems a bit like soap or detergent. Once the release is applied and dry, you can start applying the latex right away. If you wait a day or longer, you may accidentally wipe off some of the nearly invisible stuff and that can cause big problems when the mold is nearly complete.

Brush on a thin layer of latex and allow it to dry. Let each coat dry completely before the next coat. After the first layer, the others can be a little thicker. Apply three coats and then begin the cheese-cloth or gauze layers. Apply a coat of latex and press the cloth into it. Immediately apply latex on top to encapsulate the cloth. You need a couple of these layers of cloth. The fabric holds the mold together and lengthens the usable life of the mold.

Smaller pieces with a maximum dimension of about 3 inches do very well with 8 layers of latex. Larger molds should have 12 layers. For the big ones, I like to use 3 layers of latex, a layer of cloth with latex, four more layers of latex, a second layer of cloth and the final three layers of latex. The resulting mold is sturdy enough to handle and to use.

Latex dries and cures at the same time. Since it loses water in the process, the material shrinks. It is vital that the layers of mold material be thin in order to minimize the distortion caused by the shrinkage. We do need to allow for the shrinkage in thickness of the mold material. That is one reason for using thin coats. Masters should be slightly oversize to allow for the shrinkage of the mold material and the probable shrinkage of the casting material, later in the process.

            C       Room Temperature Vulcanizing (RTV) silicone has middle strength, middle temperature resistance. I have heard that it can be used to make metal castings using the low-temperature, modeling alloys. I have not tried it.

I did use the silicone that comes in tubes, a few years ago. Sadly, I did not apply a mold-release compound to the master before I squeezed the silicone goo onto the surface. Once it cured, I could not get the silicone mold away from the master and had to throw the whole thing out.

RTV material is more expensive and a little harder to use than latex. You mix the proper proportions of two parts of the mold compound. It sets by way of a chemical reaction. It is handy for making molds rapidly because you can make thick layers of the mold material.

RTV rubbers have good dimensional stability. That means the molds will not shrink as the material cures. Recently there have been a couple of articles in national magazines on the use of this material. I have an example of such a mold and of the parts cast in it. After only two parts, I began to find cracks in the RTV mold. That may mean molds made from it are not very durable or that they must be handled very carefully.

          D       Modeling clay

For some very simple castings, we can press the master part into a lump of modeling clay. Carefully remove the master piece so as to not disturb the impression in the clay. Then, pour in the casting material. Normally, such a mold can only be used one time. But, since the material is readily recycled, and since the process of making the cavity is so fast, that is OK.

The mold should not shrink if it is used right away. Be sure to test the material and the clay you are using. I have tried some plastic materials with Kleen Klay and had poor results. The clay stuck to the plastic even when mold release compound had been applied.

          E       One-part and two-part molds

                   1        Keys

When I make multi-part molds from plaster, I use a countersink tool to machine a concave ‘key’ or two in the mating surface of the first part. Then the second part will be created with the matching, convex key when the time comes to use the mold. That improves the reproducibility of the mold registration.

                   2        Sprues and runners

When pouring melted metal, it is sometimes a good idea to have one mold with multiple cavities. The cavities are connected by way of ‘sewer lines’ cut in the surfaces of the mold. Metal flows through the sewer lines and fills the cavities. That way, one pour and freeze cycle can be used to produce more than one part. The metal will also freeze in the sewer lines. We commonly call those resulting sticks of metal: sprues. My professional casting colleagues tell me that is not precise use of the term. However, it is pretty common usage for model railroaders. In fact, there are tools identified as ‘sprue- nippers’.

                   3        Vents

If the mold has a closed cavity, the liquid material may not be able to get to all of the places in it. For such molds, it is a good idea to make a way for the material to work its way out the other side. That might be a second hole leading up to the top of the mold or it might be another cavity, beyond the official mold cavity. The liquid material can fill the cavity that is the shape of the part we want and overflow a little into the vent cavity. That can help prevent bubbles due to trapped air.

          F       Support

A mold made from flexible material is handy for our amateur work. We can make cavities with some undercutting or with other features that make it hard to remove the finished piece. A flexible mold can be stretched a bit to release the parts. That is the good news. The bad news is that flexible molds may bend a bit before the part hardens. I saw an article a few years ago about a man who made warped flatcars every time he tried to cast them. The solution is to provide removable bracing for the mold. It will help the mold stay in shape as the part hardens. Then, the braces can be removed to allow the mold to flex. Finally, the part can be removed.

II      Casting Materials

          A       Plaster

Most of us have made castings using plaster as the material. We can buy commercial, rubber molds into which we pour a soupy mixture of water and plaster powder. Plaster of Paris has been around since the earliest days of model railroading. It is a good material to use as it is inexpensive, readily available, sets quickly, is fairly light when cured and dry, takes paint and stain well and is easily attached to the layout using white glue, yellow glue, uncured plaster, drywall compound or almost any of the adhesives available to us.

Another type of plaster that works well is sold under the brand name Hydrocal. It is a product of US Gypsum. It is slightly less easy to find and a little more expensive than Plaster of Paris. It is reputed to be harder and stronger than Plaster of Paris and has been promoted in numerous model railroading publications. I find Hydrocal to be a bit denser than Plaster of Paris and it is a little more resistant to staining. Since it has been popularized, we often find commercial castings alleged to be made from Hydrocal.

Other kinds of plaster can be used, but, are not good choices due to the extended cure times. We do not want to pour the material and then wait two or three days before removing the part.

          B       Epoxy

Two-part epoxy resin is a convenient and economical, casting material. It can be sticky, so, be sure to apply mold release compound to the mold if epoxy sticks to the mold material or if the mold is rigid. Flexible molds, such as latex offer more opportunities for recovery.

Mix the resin according to the instructions and pour into the mold. The material should be mixed until it is clear. The two parts have differing indexes of refraction. So, when they are first brought into contact, the mixture appears translucent. Items within or beneath the mixture are blurred. Stir the mixture until the blurriness is gone. That assures us that the chemical reaction will proceed all the way for all of the material. If some parts are not mixed – if there are ‘lumps’ of one part or the other – those parts may not take part in the reaction and will not set, properly.

I like to use disposable cups and plastic spoons for my mixing. Before I start, I pour a measured amount of water into each of the two cups. Mark the top of the water on the inside of foam cups or the outside of clear ones. I like to mark the top level at four places around the surface. That makes the job of pouring the epoxy a touch easier and more precise. Throw out the water and dry the cups.

Be sure that you only open one container of epoxy material at a time. Use what you need and then put the top back on. That prevents the possibility of switching the caps and ruining the unused material.

Pour one type of epoxy resin into one of the cups and the other type into the other cup. Do not use the same cup for both parts. Now, you can either pour the contents of both cups into a third vessel or combine all of the material in one of the measured cups, if there is room. Mix with the plastic spoon or wooden stick and be sure to dig all of the material out of all the corners and from all of the surfaces.

As the material begins to react, it will give off heat. More heat speeds the reaction, so, you have a couple of things to consider. First, you can add heat to the reaction by pre-heating the two parts. That makes them less viscous and they will pour more readily. You can also add heat using heat lamps or light bulbs. Second, the amount of surface area determines the rate at which the heat escapes. So, if you are pouring a lake, river or harbor, the thin layer will take longer to cure. If you have the mixture in a mixing cup for any length of time, the reduced surface area will retain the heat and help the stuff to set faster. You really want to pour as soon as you can.

Epoxy mixing and pouring always results in bubbles. The bubbles will rise to the surface. There are several, good techniques to remove them. First, you can breathe on the bubbles. Our High School band director always wanted the players to use ‘warm air’ as they played their instruments. That means to exhale from deep in your lungs. The warm, moist air will cause the bubbles to break. Another good technique is to use a hair dryer. It blows warm, dry air and that works nearly as well. It also allows you to keep your clothes, skin and beard out of the liquid epoxy. Finally, I have had success with stubborn bubbles using a wooden match. Light it and bring it close, above the bubbles. Be very careful since the epoxy will burn.

When the epoxy has nearly cured, it will still be somewhat flexible. If you remove the casting from the mold at that point, it may come out more easily. It may also bend in the process. Perhaps the mold was not completely flat when it received the liquid material. In either case, place the part on a flat surface protected with wax paper. Place weights on the casting to help it resume the appropriate planar surface. Then, allow the casting to cure for a couple of days. The result will be rigid and true.

          C       Low-temp alloys

These alloys are meant to melt at temperatures that are commonly found in a home environment. Some melt at temperatures below the boiling point of water. One readily available alloy is electrical solder. Most solders do contain significant amounts of lead. So, be very careful in handling the liquid melts and the resulting parts. Solder has an advantage over metallic lead in that it melts at a lower temperature.

Cerro Corporation makes a variety of low-temperature alloys. The one I am most familiar with is named “Cerro low”. There are other alloys with varying melt temperatures, hardness and other qualities. The Walthers part number is 949-525 for a small ingot.

Some alloys are very sensitive to the wrong types and amounts of impurities. Many of the die cast locomotives from the 1920’s and 1930’s will crumble to pieces now. The mixture of metals in the alloy was wrong and that resulted in unexpected chemical reactions within the locomotive shell. I have seen this sort of deterioration on some metal frames produced for locomotives in the 1970’s as well.

D       Casting Resin

I recently used some two-part plastic resin for castings made in an RTV mold. The material I used takes 16 hours to cure sufficiently for removal from the mold. Full strength is projected after a total of 48 hours. This material comes in a variety of formulations with varying properties. You will need to experiment. Mixing must be done with some precision – sloppiness can lead to brittle parts or castings that do not set properly.

Stir the mixture with wooden utensils. I use pine sticks I make from scraps. Popsicle sticks would also work and are available at most craft stores. I also prefer to use clear plastic drinking cups as the mixing vessels. That way, I can see the sides from outside and be sure I have mixed everything.

III      Making the parts

          A       Mold Release compounds

There are some commercially-produced mold release compounds. These are normally superior in performance to the various home-made releases. They normally cost not much more than kitchen sprays or other ad hoc compounds. I recommend them. Depending on temperature and the materials in question, you can use talc, carbon black, cooking spray or liquid soap if you do not have commercial release agents available.

          B       Timing

Each material requires its own recipe and time factors. The basic idea is to cause the fluid material to become solid before we remove it from the mold. Some materials change state by cooling down. For these materials, we must be sure that the part is cooled sufficiently to survive removal from the mold and the handling to place it in storage. For those materials that require a chemical reaction, be sure to follow the instructions. Epoxy resin is typical of this class of molding compound. The quick-setting epoxy may become hard to the touch in 5 minutes, but, it is still flexible. If you remove it from the mold too soon, it may bend as you do so or it may slump while sitting in storage. Since we often mix very small batches of this material, we can easily cause unexpected mixtures which will each have their own setting time. When I cast using most epoxy, I prefer to let quick-setting material sit for an hour. Slow setting epoxy should be allowed to harden at least overnight and perhaps longer.

Plaster has different constraints. As the chemical reaction is proceeding, the material will be warm. After the reaction is complete, the warmth will dissipate, but, the plaster will still be wet. At this stage, it is normally strong enough to remove from the mold and to place in storage to allow time for the material to dry out. Wet plaster is not as strong as dry plaster. I have never seen it ‘slump’ as epoxy will do.

I have even made a few castings out of hot glue. These castings are not strong since the material is not very strong. Once they are cool to the touch, they can be removed from the mold.

          C       Safety

The lawyers and your spouses will get after me if I neglect to mention safety. Please be careful and follow any instructions that come with the materials you use. Melted metal is hot and it can burn. It is also dense and very fluid. So, if is spills, it will burn most things it touches and it will touch a bunch of things as it seeks low ground. Be sure that your tools and work area have back ups and fail safe features to contain any hot metal.

Hot glue is also hot and sticky. If you get it in the wrong place it will burn you and keep on burning you until it cools. Be careful. Keep your work area clear and keep some cold water nearby.

Plaster, epoxy and other plastic resins are all potentially hazardous. Some people are allergic to the compounds, some give off irritating fumes as they set and all are chemically reactive since that is how they work. Be aware of the dangers, take care to avoid troublesome situations and keep remedies and cleanup materials handy. Rubber gloves can be especially handy. Most latex releases ammonia as it cures. Some rubber releases acetic acid.

          D       Clean up and what that means for preparation

Many of the materials we use for making molds and for casting parts change to solids by way of chemical reactions. We want to be sure that we do not place the residues from the projects in places where they might cause trouble. That is especially true of plaster.

Caution should be exercised in the cleanup of any plaster project. The material sets by completing a chemical reaction while immersed in water. That means it will also set in your plumbing if you wash the bowls, spatulas, brushes and other utensils in your sink. I know this fact from experience and from payments to a plumber. Now, I prefer to clean my utensils using a garden hose out on my lawn. The grass likes the stuff much better than my house does.

Epoxy can be diluted using some organic solvents such as lacquer thinner or mineral spirits. These chemicals are not good for us. So, be sure that the area is well-ventilated and that you have all you need with which to clean up. I like to have thick rubber gloves on and plenty of paper towels handy when I work with some of these solvents.

Since I know I will be cleaning up, I try to always have what I need to clean up all ready to go and nearby before I start work. That saves time and trouble later. Normally, I tear off 3 paper towels and put them part-way into a pocket before I start. Then, I have cleaning materials conveniently accessible.


IV     Sources of Materials

          A       Glue Products West Palm Beach

                        4015 Georgia Avenue

                        West Palm Beach, FL 33405

This business has epoxy, Fiberglas, RTV and a variety of other fascinating chemicals. They have tools, measuring and mixing vessels and they are very helpful. If you live in other parts of the State, check your Yellow Pages for similar businesses. I also found them using Google on the web.

            B       Michael’s craft stores on the web at www.michaels.com

This chain has stores all over the place. I found that they carry a fresh supply of latex mold making compound.

            C       Rag Shop craft stores on the web at www.ragshop.com

            D       Micro Mark mail order business on the web at www.micromark.com

This business carries a line of mold making rubbers and release agents. The prices are sometimes a little high, but, they provide small quantities for those who want to experiment and get started.

            E       Smooth-On on the web at www.smooth-on.com

This business carries a variety of rubbers and release agents. They sell in quantities suitable for hobbyists.

            F       Your favorite hobby shop is a good place for small quantities. The Walther’s number for the low-temperature metal is 949-525.

            G       Wal-Mart, K-Mart and many other large retailers will often have a craft section containing some of the mold-making and using supplies you can use.

V      Some Handy Arithmetic How much stuff do you need?

Many of the materials we need for making molds and castings come in packages sold by volume. How much do we need for our projects? Pardon the old math teacher for the following explanation. Feel free to skip it if you are familiar with this subject matter.

A pint’s a pound the world around. That is a mnemonic device for remembering that a pint is equivalent to 16 fluid ounces of volume. From that you can deduce that a cup is 8 fluid ounces; a quart is 32 fluid ounces and a gallon is 128 fluid ounces. Measuring cups are frequently calibrated in that sort of unit and the materials we buy are often sold that way as well.

So how big is a fluid ounce? I remember it this way. A 15 gallon fish tank is about two cubic feet. The measures for these commonly available aquaria are 12 inches wide by 12 inches deep by 24 inches long. So, 12 * 12 * 24 is a volume of 3456 cubic inches. Divide by the 15 gallon volume and you will get about 230 cubic inches per gallon. The official conversion is 231. For our work, either number is close enough. Calculators and spreadsheets don’t care which we use. That leads us to the following conversion table.

 

 

Fluid Ounces

Cubic Inches

Gallon

128

231

Quart

32

57.8

Pint

16

28.9

Cup

8

14.4

 

To figure out how much stuff we need to fill a cavity,

[1] Measure the length, width and height of the cavity in inches.

[2] Then, multiply the three numbers together to obtain the volume in cubic inches.

[3] Divide by 14.4 to determine the number of cups you will need to pour in to the cavity.

For bigger volumes, divide by the bigger, common measures from the table. When you buy a kit of material, buy the amount that equals or exceeds your calculated amount.

For instance, suppose a harbor is about 24 inches wide and 18 inches long. Further, we expect to fill it ¼ inch deep with epoxy. Multiply 14 times 18 times ¼ and you will get 108 cubic inches. That means I would need to buy at least that much epoxy. Probably, I would buy two quarts of epoxy.

That is about it for the calculations and conversions. There are a few odd cases to consider and I include them for completeness.

What if the cavity is not a rectangular solid? We only need to be close, so, the preceding process might work. If the shape is very different, for instance it might be a round shape, and then we need to dig back into our high school arithmetic. To calculate volume, we always multiple the horizontal area by the height. So, if the cavity is round, you must calculate the area of the circle (pi times the radius squared) and then by the height.

If the shape is even less regular, you can look up the formulas for pyramids or spheres and use one of them as an approximation. If the horizontal area is more like a triangle than a rectangle or circle, use the formula for the area of a triangle and then use the height.

Another method is a little like cheating. If you already have the mold, you can fill it with water. Then, pour the water into a measuring cup and read the volume right from the scale.

What if the material is sold using metric volume measures such as liters? In the days of my youth, I remember watching the super stock cars race each other on the drag strips. In those days, the motors with the biggest displacements, produced by the Detroit manufacturers, were about 427 cubic inches. Sometimes these big engines were referred to as being 7 liters. That helps me remember that 427 cubic inches is 7 liters. That means one liter is 61 cubic inches. So, if I have a cavity that is 7 inches by 4 inches and it must be filled 2 inches deep, I can see that I need about 1 liter of stuff to pour into it. (That is, 7 times 4 times 2 equal 56 cubic inches which is a little less than one liter.)



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