Overall these techniques apply to most scales of model resin casting: 1/87 to 1/35, etc. Taking into consideration drying times the whole process make may take a week or so from start to pulling a beautiful casting from your mold. I will not address Squash Casting or white metal casting, which are covered at some of the websites I have listed at the end of this article. I do not cast commercially and am not an expert, so will approach it from that end.

It is easier to make a number of small molds and casts in one sitting. Since your chemicals are humidity sensitive after opening you will want to use it up before it goes bad (2 to 6 months or less after opening!). From personal experience, do not use anything that has started to polymerize in the bottle. The moldmaking compound will not mix or cure thoroughly and the casting resin compound may cure brittle and fracture in the mold when you try to pull it out. Read any number or articles and directions over several times, and start amassing items to cast, tools, and small containers for mixing and forming molds. Study commercial castings to see how they do it.

Molding & Casting products: There are a number of companies out there producing mold making and casting products. All of them likely produce good products otherwise they would not be around for any length of time. You should stick to one product in the beginning so as to become intimately familiar with its instructions and properties. I use SMOOTH-ON products at this time, as that is what I found on the shelves of the hobby shops in my area, but I will stick with them as they do the job I want them to do well [this is not meant as an endorsement]. They come in two pint-containers per kit and mix in equal measures/volumes (1 part A with 1 part B). Unless you have a scientific gram scale, stay away from the products that need exact measurements based on weight (such as 5 grams Part A with 50 grams Part B!). Some of these latter molding and casting materials do have superior properties though, such as being less prone to air bubbles or making tougher, longer lasting molds.

These products work on the basis of a catalyst. That is, a material added to a second material where they interact to form, in this case, a solid. Often this reaction gives off a little heat, which is referred to as "exothermic". Common injection plastic models are produced by the cooling of hot liquid styrene plastic injected into a mold to make polystyrene.

SMOOTH-ON sells a nice booklet titled How To Make Molds & Castings (And Live To Tell About It!) that comes free when you buy a sampler kit. This 40-page booklet introduces the choice of products, methods and covers the most common questions. Besides SMOOTH-ON, other molding and casting products can be acquired though Alumilite Corp., Synair Corporation and Micro-Mark catalog.

Basically, you need

1. Two containers of mold making compound: parts A & B, which when mixed, will solidify to a firm but flexible mold around the thing you wish to reproduce. Something like a firm Jell-O-like consistency. Its elasticity will allow you to remove the original or copy from a mold opening smaller than the cast object, such as a T-34-85 turret. The flexible mold-making materials I have used include PMC-121/30 which contains a release agent but takes overnight to 24 hrs to solidify and PMC-121/30 DRY which does not contain a release agent but solidifies in only several hours (read the instructions first). Latex or Nitrile surgical gloves are needed when mixing either the moldmaking rubber or resin since it can be irritating to your skin.
2. Two containers of casting resin, parts A & B: that when mixed will solidify into a strong, hard, opaque material, with nearly the same detail and texture as your original. The casting resins I use have a "pot" life of 2-minutes and one with a pot life of seven minutes. By pot life, I mean the amount of time you have to work with the liquid resin before it gets too thick to pour. For getting into small recesses and trying to do a lot of small parts at one time, a longer pot life may be better. The resin instructions say that you may de-mold in 15 minutes or so, but the resin may still be soft and pliable and may deform. I like to wait an hour or more.

I have tried a resin with a longer (7-minute) pot life and am not really pleased with it. It pours like water and gives off more heat than the resin with the 2-minute pot life, but has tended to froth & expand, and form many tiny bubbles during the curing. Perhaps it is my technique?

3. Molding containers: To keep your mold making material from flowing all over the table when you pour it on your original, you’ll need to prepare a mold container. Ideally something big enough to hold your model (about 1 cm clearance on all sides) but not too big so that you waste a lot of mold material. What I find ideal for small parts like crates, turret hatches or small-scale turrets are the small, vacuformed plastic containers that my son’s Matchbox trucks come in. They come in a variety of small sizes and are clear so we can see how the mold is curing.

For larger pieces, such as a small-scale M4 Sherman hull or 1/35 turret, you may have to build a box around the item to be duplicated. Molders often do this by building a box out of thick Evergreen styrene sheets glued together, or using something like Lego blocks. If using Lego or Duplo-type blocks, use white glue to secure the blocks down to a smooth base and fill in any cracks between the blocks. White glue (i.e. Elmer’s) may take a day or more to dry.

5. Centrifugal Force Contrivance: to get that thick resin into all those recesses and corners without air bubbles. More prolific and professional casters use special vacuum pumps and pressure vessels that sucks any air, including the air bubbles in your casting, out before the casting resin has hardened. For what I do I rely on gravity, careful pouring; and centrifugal force to get the resin into all the corners and protuberances of the mold. Use nothing more that a small, tall-sided stout plastic box attached securely to four lengths of string about 25 cm long (see photo #3).
6. Mixing Containers: disposable and cheap. Ideal are disposable clean, small plastic yogurt cups, or three to five ounce plastic cups sold in craft stores. No paper, wood or other permeable containers! Size depends on the amount of mold-making rubber or resin you need. [Don’t mix too much at a time, this will be wasteful.]

Associated with the mixing containers will be your mixing implements. I prefer jumbo size paperclips or a piece of coathanger wire with a short L-shape at the end for scraping the bottom of the container.

7. Lubrication/Release: This is necessary to get your original and duplicates out of the mold. Some prefer lubricants such as cooking oil spray, or mixed Vaseline and paint thinner. I prefer a commercial lubricant made for molds as I am sure of its qualities and compatibility. A commercial release agent is harder to clean off than veggie-oil so try a pine-oil cleaner or automatic dishwashing gel [protect your hands and eyes when doing this].
8. Paper Towels & Newspaper: Keep these on hand to clean your tools & hands, and to protect your work area. A little industrial hand-cleaner is also good to get any spilled resin off your skin before it cures.

 In the foreground are two small clear vacuform containers I like to use as molds. Behind them are two plastic cups scrounged from my kitchen. In the rear are two 1 pound containers of liquid plastic compound (resin). Atop the left cup is my bent paperclip stirring-implement. A bent piece of heavier coathanger wire works better for large quantities.

1. Preparation: Prior to making the mold of your item you will have to prepare it and your molding vessel. This means sealing up any holes to keep the molding liquid from getting into where you do not want it. If you’re doing a turret, for example, use white glue to seal the joints around the gun mantlet, gun barrel opening, periscope openings, etc. This may take a day to dry properly. The nice thing about white glue is that it can be washed off your original by soaking in water for a day or so.

Filling large hollow spaces like turrets or tank bodies with soft modeling clay is often necessary to keep the piece from floating up in the liquid resin after poring. The clay eliminates the air, helps the model to stick to the container, as well as adding weight.

Set everything up in a quite clean place to work. Newspaper on top of your worktable will protect from the eventual spillage.

At this stage let me explain that I only make one part molds and have not found a need for two part molds like is used in injection molding. The flexibility of the rubber compound makes it easy enough to remove even complex shapes from a mold.

2. Lubrication: Once a part is glued to the bottom of your molding container it can be difficult to spray release agent onto the underneath of overhanging surfaces. Overhangs like under turrets and chassis should be lubricated before gluing down to the molding box. Then after the glue has dried, a light lube over the entire model part and box. Do not allow the lubricant to puddle anywhere on your part! Let the release agent dry for 15 minutes or so before pouring the liquid in.
3. Mixing the rubber moldmaking compound: This is also called RTV (Room Temperature Vulcanization). Vulcanization is the process of "treating rubber-like materials to create a useful product" and has nothing to do with the planet Vulcan. First read the instructions that come with your materials. In clean, non-porous (i. e. plastic) cups measure out equal amounts of A & B, and mix well and I mean real well, a minute or more. Failure to adequately mix can leave you with a semi-hardened sticky blob that sticks to the areas of your piece in all the places that is hard to get out of. Scrape the sides and bottom of your mixing cup well and often.
4. Pouring: Pour slowly at the lowest part of the mold and allow the liquid to slowly rise around the item till it gets about a centimeter above (this is too late to find out that your mold box is too low).
5. Curing: Allow the rubber moldmaking compound to cure as per the instructions. If it is mixed adequately and the compound hasn’t gone bad, all should be well. After curing, slowly pull the mold out of the box, then slowly pull your piece out of the mold and admire the crisply detailed cavity that you will soon fill with resin. If the rubber mold did not cure properly throw it out and try and figure out what went wrong!

  The original Sherman hull and turret to be duplicated are to the left and my light-cream colored duplicates on the right. Unfortunately this photo is not sharp enough to compare the overall quality of the duplicates. In the background are the amber molds for these castings.   Note the spillage of resin on the outside of the mold. This is not a problem as it easily peals off if you lubed the outside of the mold lightly.

After securing the resin filled mold to the box-on-a-string (your Centrifugal Force Contrivance), swing the mold around for 15 to 30-seconds or so to force the still liquid resin in and air bubbles out; and to entertain friends and family. Allow the mold to fall out of the box while swinging and see what a mess you have! Some liquid resin is bound to smear out of the mold so be prepared to add more or spread it back into the mold with a card or wire. When you first pour it will be like maple syrup. You want to work with it before it is like cool molasses.

After swinging allow the resin to cure as per instructions and maybe a little longer. Then carefully pry the casting out of the mold. If it is at all soft, such as being able to dig your fingernail into it, leave it alone. A butterknife and sharp #11 blade may help here to remove it from the mold after curing.

After popping the newly cast part out of the mold, the release agent needs to be cleaned off previous to gluing and painting. If a light vegetable oil has been used to lubricate the mold then simple dishwashing liquid and light scrub with a soft toothbrush will suffice to clean it. As I use a commercial release agent I have to use a stronger detergent such as a pine oil cleaner (i. e. Lestoil, Pinesol, etc.) or automatic dishwashing liquid. Wear rubber gloves when doing this as these cleaners can be harsh on your skin. Rinse very well and then let the pieces dry. If paint won’t adhere than the part needs better cleaning.

Cutting off small-unwanted parts can be done with a razor saw, Exacto type saw blade and file. Large plugs can be removed with a Dremil type tool or hacksaw. Be careful with the resin dust as it can be harmful to breathe in. Cheap paper dust masks are available in hardware stores, or the resin can be cut wet to minimize dust. [Blocks of scrap resin need not be thrown away! Broken into the right size pieces, it makes great building-rubble for a diorama.]

Pinholes in the resin can be filled with regular model putties that come in a tube. Large bubble holes are best filled with good quality epoxy putty that has greater strength and can be filed to shape. On some areas the epoxy putty may need to be glued in with a little super glue to keep it from falling out.

Solvent model glues will not adhere to polyester resin so you need to turn to cyanoacrylate glues (Super Glue, Crazy Glue, etc.) and two-part 5-minute epoxies to assemble your parts. The epoxy will form a stronger bond in my opinion though takes longer to dry. White glue also has its place when you want a light bond or are gluing on a clear windshield that will be crazed (fogged) by the cyanoacrylate glue. Experiment and use what is best for you and the circumstance.

Before painting the model its finish color, I highly recommend a primer to help the pinholes; defects and blemishes show up. I prefer a light gray auto primer though this may depend on the color of your final topcoat of paint. For example, if you are doing a late war Panther in factory primer, just use a red-oxide primer.

Prior to using the mold again, be sure to clean out your mold of any dust and debris is may have collected. My experience is that a polyurethane mold may survive in good shape from 15 to 20 castings before it wears out. The simpler the mold, without corners and overhangs, the longer it will last.

For further information and articles about the world of resin casting I recommend you check out the following web sites:

www.Alumilite.com (The Alumilite Company, molding & casting products)