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Composite Shell Hemispheres


Many of you are familiar with the two major types of shell hemispheres: kraft and plastic. Each type has its own advantages and disadvantages. However, there is one type that I have been developing that is most likely little-known: the "composite" hemi. This type of hemi is a cross between kraft- and plastic-based hemispheres, which will hopefully offer the best of both worlds.


The materials needed to make composite shell hemispheres are:

Polystyrene glue (see below)
100% Cotton fabric (not too fuzzy, not too thick, and w/ fine weave)
Hemisphere former (I use a dead lightbulb)
Razor blade


Kraft paper, whilst seeming to be an acceptable alternative for cotton fabric, is, in the author's experience, a material that does NOT work for this application. Not only can you not pull it to free it of detrimental wrinkles, but it does not adhere to the glue in the way that is wanted in this application (the fibres of the paper, unlike the cotton cloth, do not latch onto the PS glue, and therefore do not adhere with the ferocity of the cotton).

Polystyrene Glue

The polystyrene glue used in this procedure can be storebought, but this is not recommended, because the storebought "model" glues are very expensive. A MUCH cheaper alternative to paying out the ass for model glue is to [surprise surprise] make your own. To do this, you'll need:
250ml of toluene (available @ hardware stores)
2 or so packages of styrofoam coffee cups
A chopstick or similar (to stir with)
A glass jar with metal or HDPE screwtop lid.
I HIGHLY suggest you perform this procedure outside. See below.
First, pour 250ml of toluene into the glass jar. Then, take 5 or 6 styrofoam cups and tear them into postage-stamp-sized pieces. Then, dump these "chips" into the toluene, and stir vigourously for 5 minutes. The polystyrene should effervesce slightly, dissolve, and thicken the mixture somewhat.
Continue tearing up bits of styrofoam, tossing them into the toluene, and beating the crap out of them with the stir rod for 5 minutes, until this mixture takes on the viscosity of commercial polystyrene glue. At this point, cap the jar and set it aside. After an hour, come back and again ruthlessly beat the polymer solution with the stir rod. You should notice that the solution has become much more clear and homogenous in the 1 hour that it was left for.
Well, that's it. You now have about 310ml of PS (polystyrene) glue, which cost you $3 to make. If you had bought the stuff ready-made, the price would be about $40 (ouch..). This PS glue can be used whereever you would ordinarily use model glue, as well.


NOTE: The following should be performed outside or under adequate ventilation, as toluene vapours are poisonous and flammable and can form explosive mixtures with air, and toluene is suspected to be a carcinogen [!!].
1. Cut a square piece of the cotton and pull it taut over the top of the lightbulb, so as to greatly lessen wrinkles.
2. Using a paintbrush, brush PS glue liberally over the fabric, and before it dries, place another taut layer of fabric over the glue. Allow to dry.
3. Again, paint some PS glue onto the hemi's exterior, and secure a layer of fabric over it. Allow to dry, and, if nessesary, repeat.
4. Once the shell hemi has reached a suitable strength and/or thickness, take the razor blade and neatly slice around the hemisphere to remove any extra material, and to ensure that the hemi is an exact half-circle.
5. Now, if you've done it right, the shell hemisphere can be removed from the former, as if it was done correctly, the hemi should not be stuck to the former. Give the internal parts of the hemi a coat of PS glue, and set it aside to dry (outdoors, or indoors IN A FUME HOOD ONLY.).

If all was done correctly, you've ended up with a nice 'hybrid' hemisphere. Hemispheres of this type can be glued together using a coat of PS glue, then by reinforcing it with PS glue-soaked fabric strips.
These PS glue-soaked fabric strips are not only useful for holding together composite hemis. The strength of plastic hemis can be greatly improved by "pasting" their exterior with strips of this type. Not only does it make for stronger RAP* shells, but I've found it encourages better breaks and star patterns (due to the shell actually shattering, not just splitting at the seams).

Note: The ID of a composite shell hemi made in this fashion, with a standard lightbulb, is around 2.554".

*- RAP means Rapid Assembly, Plastic.