Working With Cast Resin Parts

Introduction

When you buy a model kit these days, chances are almost one hundred percent the material used in casting the pieces will be polystyrene plastic. But when it comes to kits manufactured by smaller companies and aftermarket suppliers, you will find that the majority of them are cast in a slightly different material known as polyethylene resin.
       When it comes to assembly, resin parts require almost the same approach as your standard styrene kits, with a few little quirks. That's where this tutorial comes in. Here you will learn the basics of the resin casting process, parts preparation, defects to watch out for, and how to repair them.


Skill level: Intermediate
Time required: Variable
Precision tools required: None
Cost: Varies by kit (>$10-over $200+)

The Resin Casting Process

The first step in casting a resin part is creating a master. This is generally done using wood, plaster, or styrene plastic carved to shape, then sanded smooth. In some cases, a hobbyist that wishes to secure extra copies of an existing part may also use a kit piece as their master. (If you do this don't try to sell them, you may end up in a heap of legal trouble.) 
     The next step is to create a mold. The easiest way to do this is by securing a complete starter kit such as the one found here from Micro-Mark which includes everything you need to cast parts in urethane resin. The caster will then create a simple box around the part, usually cardboard or balsa (but Lego's will work too) as a reservoir to hold the silicone rubber material as it's poured around the master.
     The mold material itself is a room temperature vulcanizing or RTV rubber which, as the name suggests, is a two part liquid that cures at room temperature. Equal parts of the parts of the two part material are mixed together in a disposable container, then poured into the box which was formed around the part earlier. When the rubber cools, the master is removed, leaving a hollow cube which can be reused multiple times.
     The last step is to actually cast the part. The techniques here may vary, depending on the material the caster uses. Most often, the resin itself is a two part material which is mixed together, then heated before pouring; a process which needs to be done quickly before the resin has a chance to cool. Once the material is mixed, it's a simple matter of carefully filling the mold, then allowing the material to cool. After cooling, the part is removed from the mold, packaged, and ready to add to your latest creation.

Tools Required

 The tools required for working with resin are fairly basic and should be found in most everyone's collection to begin with: A razor saw for removing pour stubs, a hobby knife with a stock of #11 blades, several shapes and sizes of files, sandpaper for smoothing imperfections, and finally a good CA glue or epoxy. Plastic cement will not bond cast resin, so if you must attach a resin piece to some other material, you will need to use CA glue.

 Another handy device to have is a disposable foil pan with a sheet of sandpaper glued to the bottom, sitting in about an inch of water. Sawing and sanding resin will generate ALOT of dust, which is extremely irritating and harmful to the respiratory tract and nasal passages, not to mention highly toxic. Wet sanding will turn this harmful dust into a slurry that is easily wiped away with a paper towel, or simply folded up in the pan and thrown away. This works best for large pieces such as aircraft fuselages and ship hulls that need to be kept flat. Also, make sure you have a good dust mask handy for those times when you do need to dry sand.
Common Defects In Resin Parts

 Pour Stubs

Pour stubs will be found on pretty much every part you pick up. These are a result of the casting process and are the equivalent of the sprue on a plastic kit. Some are small and easily removed, however, others like the one seen to the left can be fairly large.
     The best way to take care of these is with a good, sharp razor saw. Keep in mind, resin is a fairly brittle material which can and will break under pressure, so take it slow and let the weight of the blade do most of the work.Once the pour stub is removed, sand or file away the excess to smooth out the remaining roughness like you would any other part.

 Air Bubbles & Pin Holes

Air bubbles and  pin holes are yet another result of the casting process and are formed as a result of air making it's way into the mixture due to mixing or pouring the resin into the mold too quickly. They usually appear as small hollow holes that look as though the material has been pricked with a pin (hence "pinholes"), though sometimes, they may show up as much larger craters with a thin film of resin covering them. The smaller holes can simply be filled in with the putty of your choice (I prefer Bondo spot & glazing) or accelerated CA glue. For the larger craters, the technique is much akin to getting a cavity filled at the dentist. Using your hobby knife or a drill, carve away the excess resin skin, then fill the hole back in with putty or gap filling superglue.

Flash

Flash is exactly what it sounds like: a thin film of resin surrounding a part as a result of a two piece mold that didn't fit together tightly enough. It's pretty much the same thing you find on a standard plastic kit and is removed exactly the same way.

Shrinking and warpage

Just like any heated material that solidifies, resin can be prone to shrinking and warpage. Sink marks are usually found on thicker pieces and can be filled in with the putty of your choice, then smoothed over with sandpaper. Warps, on the other hand, can be slightly trickier to deal with. Just as it is with a plastic kit, warps are generally found on long, thin pieces such as aircraft wings, ship hulls, and vehicle chassis. The best method I have found for dealing with this is to get a pot of boiling water, then dunk the part for 5-10 seconds and GENTLY straighten it back to shape. It may take a few tries to get there, so patience is the key. Another trick you can use is to run a heat gun over the piece briefly, then bend it to shape.

Assembly Techniques

The first step in assembling a resin kit is to remove any mold release residue that may interfere with paint adhesion. This is best done by soaking the parts in a solution like Wesley's Bleach White, then washing the parts with warm water and dish soap.
     The next step is the same standard preparation you'd follow on any other model, watching for the defects listed above. Keep in mind that when it comes to adhering resin to itself or another material, you can not use plastic cement. All assembly must be done with CA glue or epoxy. Also, bear in mind some hobby putties may not bond to resin, so you may want to get a good auto body filler such as 3M Acryl Red or Bondo Spot & Glazing putty for seam filling and repairs.
     Once the kit is assembled, wash it once more in warm, soapy water and head off to the paint booth. On a side note, I've tried most types of paints on resin with good results, however, you may still want to use a good blocking primer if you plan to use a hot solvent automotive lacquer.

Conclusion

When it comes to detail parts and kits of odd subjects, cast resin will open the door to a myriad of new options for your hobby, and as you can see, it's not all that difficult a material to work with. Go ahead and give it a try, you might just fall in love with the stuff.