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Tech Tips

What Dentistry Can Do for the Modeller

Part 2: Casting

n by Frank Spahr


Taking Impressions and Casting Resin

Taking impressions is a very basic dental technique which I found useful even in modelling. A modeller won't need dentist's impression material. More useful is the material that laboratory technicians use for duplicating parts - normally the substructures for the bases of partial dentures. These materials are 2-component-silicones, they are available from various manufacturers, and you won't need the more expensive brands. The requirements are obvious:

  • fine representation of surface details 
  • little formation of bubbles (but that's rather depending on your skills) 
  • structural stability

The basic outline of the moulding process used in dentistry should sound familiar to many scale modellers.

  1. Provide a pattern (master) part for duplication
  2. Embed your pattern part in the silicone mixture to produce a mould.
  3. Fill the mould with resin to produce a duplicate
  4. Clean up the duplicated part

Resin casting is an effective way  of replicating parts, such as this radial engine.
 

A sensible way of going through the process is as follows.

Preparing a Pattern

Pick your pattern part. Usually your master would be a kit part - like a propeller blade, engine exhausts - or something that you have scratch-bulit yourself.

Mount your pattern on a shaft. The shaft will provide an opening in the mould for filling it with resin, and will result in a resin "sprue". It is important that the pattern part is securely mounted on a shaft, yet is easily and clearly detachable from the part. A propeller blade for instance, should be mounted vertically on a cylindrical shaft with an easily recognizable demarcation line for sawing it off. Dental wax can be used for this purpose.  Laboratory technicians use special gluing waxes, use these too. 

More complex patterns may require multiple shafts and some thinking in advance. You can cope with only a limited amount of undercuts when removing the resin part from the form, so it is up to the individual case how you mount it and whether it might be better to build it from separate components.

Making a Mould

Now mount the pattern part to the base plate. I recommend fashioning the base plate form Lego bricks (we've all had those, haven't we?). For a propeller blade, attach it's shaft to the plate, the blade itself standing in upright position. Again, you may attach the pattern to the base plate with wax. 

With the pattern steadily attached to the base plate, complete the form by building up walls around it to the appropriate height. Lego bricks are excellent for this purpose because they fit tight yet allow for building a form of any shape you need. 

Here is the original part securely attached to the base plate, 
the form being built around it with Lego bricks.

Now you are ready for the silicone application. After proper mixing, let the goo flow steadily until it fills the form. You might use a disposable brush (available for dental adhesives) to apply silicone to all details and recesses of the pattern part to avoid bubbles. In my eyes it is the best alternative to desktop vibrators that technicians would use :-)

The form filled with sillicone...

After the material's setting time, it's time to remove the pattern from the mould. Start by turning your entire assembly upside down and removing the base plate from the silicone cube. Now you can pull the pattern part out of the silicone, a quite hair-raising moment. Again, patterns more complex in shape  will make it necessary to cut the mould open (very carefully) to remove them. If cut into pieces, the mould can be later reassembled in your Lego cradle. If everything worked out fine, you will arrive at the undamaged silicone mould or at least one that can be properly reassembled, with a clean opening where the base plate was.

The mould removed from the Lego craddle and ready for use.

Casting

Now comes the real tense part. You will need a dental resin used for casting models, such as G-C's Pattern Resin, Kulzer's Palavit G or - better - a cheap copycat material. Mix it in a silicone cup rather thinly, so that it flows with consistency somewhere between condensed milk and honey and let it flow gently but steadily into the opening of your mould. 

The two components of Kulzer Palavit G, and a mixing cup made of silicone.

Now this is a moment where a laboratory vibrator would really come handy... Ask your dentist, or perhaps you know a laboratory technician, maybe they help you getting one. The trick is to avoid air bubbles being trapped inside the mould, and as always: the damn thing takes practice - and a dose of luck!

Refer to the instructions on the package to determine the setting time of the resin. Rather wait a bit longer than you might deem necessary before trying to pull the part from the form. Dental resin described above hardens in less than 2 minutes, but usually you will have it mixed in less-than-perfect ratio and it will harden in the mould with only minimal exposure to air, so 5-10 minutes of waiting won't hurt you nor the part.

Now remove the resin part from the mould just as you did before with your pattern.

Finishing

Having removed the part from the form, you have to finish it by sanding. Working with resin is different than working with styrene. Resin is much harder, so you can't cut or sand it as easily as styrene, but it won't smear as soon as styrene, and its stability is far superior. This also means that it's rather brittle, so it won't bend but rather break on stress.

Any air bubbles left may be filled with wax as previously described, but only after you've finished with the sanding process - or the wax will melt.

Resin parts ready after clean-up, and painted.

Casting in Hard Metals

Don't you really dislike the styrene's poor stability? Haven't you ever cursed the stuff when you needed a firm part but hadn't it? Soft, bending, breaking? I won't even speak of all the (non-) "working" features of the kits in our youth, such as retractable landing gears, closing wheel well doors, movable control surfaces, most of which had to be glued tight sooner or later for structural fatigue. 

One of the most unnerving things to me has always been rigging plastic model ships, especially since I've also built wooden ones and can compare. In plastic kits, the upper spars are so soft that you have to be very careful during rigging in order not to bend the masts. And if you succeeded in doing so, this does not prevent from material fatigue over the years, until one clumsy touch can break your USS Constitution's bowsprit and leave you with your hands more than full of work - as happened to me recently.

If you have got good contact with a laboratory technician, you might somehow talk her or him into casting some of your most critical styrene stuff into metal, a thing with obvious benefits. With a little luck, you might even arrive at a price that satisfies both parties... 

Dental technicians cast not only gold alloys, but also non precious metals, such as a Cobalt-Chromium-Molybdene alloy that is very stable yet lightweight. Even very thin parts may be cast into a very rigid copies. 

The other possibility is casting in brass, which is used as "phantom" metal for training purposes instead of gold alloy. Brass is much softer than Co-Cr-Mo, so that you might finish the parts yourself - the latter would rather require industry-standard equipment to obtain the smooth surface finish. Still the brass parts should be sufficiently rigid  in most cases. 

There is no problem in using styrene parts as masters for such casting. You should only remember to polish the styrene part to good surface finish as possible before casting, this implies less finishing work!

Since the casting itself needs to be done by a professional, we don't need to get into details of it. The technician will also normally remove the rests of the molding material with acids and sandblasting before he hands you the cast. Using fine-grit sandblasting already leaves surfaces that are not all too bad.

Finishing process on your side will involve separating the part from the sprue, removing any seams or bubbles and smoothing the part. Since the metal is much harder that plastic, you will need a Dremel tool or the like for the job. The sprues can be separated with a carbide disk, excesses then trimmed with grinding stones. 

The surface can then be finished using rotating brass brushes and finally rubber polishers of varying coarseness, depending on the degree of shine desired.

Summary

Hard-metal casting is by no means everyday technique, nor is it cheap.  Still it might pay off with models that require more rigidity for antennae, landing gears, tail hooks, masts and so on. Just remember all the items on your models that have broken over time, and ask yourself if your next work of art should be able to withstand some handling during years after it's finished... And you might team with several fellow modellers in order to reduce costs.

n


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