Resin Casting Aircraft Parts

Model Airplane News, Jul 2004 by Mosher, Vance

Guns, exhaust stacks and other small parts that are commonly used on a typical scale model are easy to cast out of urethane resin, using silicone mold-making materials. I used this method to make guns, exhaust stacks and wingtips for a large Fairey Firefly and machine guns for a Heinkel 111. Parts for the Firefly featured In the March 2003 issue of Model Airplane News were made by the same casting process. You can make parts of almost any size.

Here's an outline of the process:

* Make patterns of the parts you want to mold.

* Attach sprues to the patterns, if necessary.

* Attach the patterns, or the sprues, that hold the patterns to a smooth surface.

* Build dams around the patterns to contain the molding material.

* Pour the molding material around the patterns and let it cure.

* Extract the patterns from the molding material.

* Make parts by pouring urethane casting plastic into the molds.

* Extract the cast parts from the molds.

For frequently needed parts such as exhaust stacks, you can assemble several cast parts into a more complex pattern, add sprues, and then pour molding material around the larger pattern.

This process may seem complicated, but it's really only repetitious; at most, you have to carve only one left and one right part. The rest is pouring and waiting. It sure beats building eight machine guns or carving 12 exhaust stacks! It's even better than carving two air scoops.

CREATE THE PATTERNS

Your patterns don't need to be strong; dense foam covered with lightweight spackle will work, and so will balsa, basswood, white pine, etc. You can even make patterns out of modeling clay and plasticine; they're really stiff when cold. Good pattern material is easy to carve and smooth (any surface grain or texture on the pattern will be faithfully recorded and will make the pattern and parts more difficult to remove from the mold!). It also helps if you can glue small details to the material.

The exhaust pipe patterns shown have small beads of Elmer's glue down their sides and around the ends to simulate the welding line. I applied them to the patterns with a toothpick. Keep details such as these very subtle.

PREP WORK

If your part needs to be molded on all sides, you must add sprues to the patterns. You can make sprues out of modeling clay, dowels, sticks, pencils, etc. They are used to leave a hole in the mold into which you can pour the casting plastic and let the air escape. Stick the pattern sprue to the pattern in a position that allows you to easily cut the molded sprue off the finished part. Glue sprues on fully cast parts (such as the machine guns shown) to a surface near the end of the part, so the casting material can flow into all the dead ends of the part. It is helpful to have sprues that will let air bubbles escape from dead ends, even when you don't need them for pouring the part. A sprue should have a wide outer end, too, so it can act as a funnel for pouring the part. If you have to pull the sprue through the mold, don't leave the sprue full of molding material when you cast the part; the wide end will hold it in the mold!

Secure one side of the pattern directly to the work base with 3M 77 spray contact cement. It must only stick well enough to prevent the mold material from leaking under the pattern during the mold pour.

Build a dam around the pattern to contain the mold-making material. The dam can be any stiff material, such as balsa sheet or cardboard, and it must be about ¼ inch taller than the pattern, so your mold will have a flat bottom to rest on. Build the dam so that the mold walls are relatively thin; about ¼ inch is ideal. If they're thinner than that, they may tear easily when you remove the parts from them. Thin, flexible walls make it easier to remove the part from the mold; you can't always do this, but keep it in mind. When you build mold dams, remember that simple rectangular or circular outsides are easy to work with and that thin walls can bend and leak. Make sure that the dam's bottom doesn't leak; the molding material is designed to get into small spaces so it will find any cracks under it. be sure to glue the joints of the dam so they don't leak.

MAKE THE MOLD

The mold-making material is room-temperature vulcanizing (RTV) silicone molding rubber. You can buy it from Micro-Mark (item no. 82083) or from your local patternmaker's supply shop. Mix the molding material as directed.

A one-piece mold is made upside-down. Pour the material over the pattern and into the mold dam. This requires patience. Air bubbles can be trapped when you mix the two parts of the molding material together, and you don't want any of them to come in contact with your pattern because they will cause the molded part to cure with little bubble-shaped lumps on it. The best method is to pour the silicone over the pattern in a thin coat and let the bubbles work their way out. Then pour the material down the sides of the dam, so the bubbles can escape. Pour more material as slowly as possible wherever the material touches the pattern. You have a time limit, though: the silicone material will start to gel in about 10 minutes.