How to Glue Plastic Shells Using Thickened Methylene Chloride
By Ned Gorski
Properly joining plastic aerial shell parts is both critical to the fireworks shell performing as desired in the air, and to the shell not self-destructing on the ground. Failure to properly seal a shell can result in the shell exploding in the mortar ("flower pot") or just as it exits the mortar ("muzzle break"). Both of these are extremely dangerous to the person lighting the shells and potentially to the audience. Worse, one shell exploding at or near the ground can cause the rest of a fireworks display to be prematurely ignited, multiplying the risks and dangers to the display operators, audience, and property.
Since such risks can include injury, damage to property, fires, and/or loss of life, learning to join shells properly is something every shell maker must master.
When I started making aerial shells using plastic casings some 20 years ago, I followed Bill Ofca's recommendations in his Volume 2 of Bill Ofca's Technique in Fire series of fireworks-making books, "Design and Quick Assembly of 3, 4, and 5-Inch Plastic Ball Shells."
For the glue to join the shell parts together, Bill recommends, "I made my glue by placing several broken pieces of plastic ball shells into a coffee can, then pouring Xylene into the coffee can until the plastic pieces were submerged. After 24 to 48 hours, the plastic pieces will be very soft and will dissolve with stirring. This process is repeated until the solvent thickens like molasses.... Xylene by itself is not very good at gluing the ball shells together, but this process of dissolving the ball shell plastic into Xylene makes a superior, strong glue."
One advantage to using a thickened glue like Bill recommends is that it will fill and seal all cracks and voids between the shell parts, preventing the lift gasses from entering the shell when it is fired from the mortar. Thin solvent, even if it softens the shell casing plastic, will often not completely seal all those gaps at first.
I used homemade glue made this way for quite a while. Then, later on, I started simply using PVC plumbing cement, which seemed to work just fine, and was readily available, pre-mixed.
Another method of welding plastic shell parts together is to simply use methylene chloride, applied between those parts. The methylene chloride softens the plastic parts and allows them to harden together, making a solid joint. One real advantage to methylene chloride solvent is that it is one of the fastest-evaporating ones, and will quickly dry, leaving the shell ready to fire almost immediately.
Skylighter #CH8193 Methylene Chloride
Joints glued with unthickened methylene chloride are likely to have some unsealed voids in them, since the solvent is so thin. So care must be taken to go back and apply more methylene chloride, visually inspecting for locations that may need more of the solvent. [See Harry Gilliam's method for solving this problem at the end of this article.]
There is a way to thicken that methylene chloride, though, so that is serves as a super "plastic welder," and also seals all the cracks and voids at the same time. And, in the process we get the benefit of that rapid drying of methylene-chloride-based glue.
Plastic shell parts (usually made of ABS plastic) may be dissolved in the methylene chloride in the same way as Bill recommends with Xylene, but, as he states, that process can take days.
Here's a fast way to thicken methylene chloride in minutes, if not seconds. Dissolve some Styrofoam (polystyrene) packing peanuts in it. The peanuts have to be the polystyrene kind, not the bio-degradable corn-starch kind which will dissolve in water, or in your mouth. (Don't ask me how I know that. Ha! Great trick to play on the kids.)
Styrofoam Packing Peanuts
The Styrofoam peanuts will immediately dissolve in the methylene chloride, but will not be fazed by water. So that's one way to check to make sure you have the right kind.
Methylene chloride, thickened this way, will perform in the same way as Bill's thickened homemade glue, or the PVC plumbing cement, welding the shell parts together, and sealing all cracks and voids as long as enough of it is used.
Methylene chloride, like any volatile solvent, quickly puts toxic and very flammable fumes into the air. Please perform the following mixing and gluing operations in an outdoor, well-ventilated area, wearing a respirator, and in a location where those fumes cannot accumulate and be ignited or cause an explosion. Seriously.
Getting the peanuts
My local UPS store sells bags of the Styrofoam peanuts shown in the shot above. Staples only sells the water-soluble bio-degradable kind.
Other materials you'll need
You'll also need:
- A small paper cup
- Some Q-tips
- The shell casings which are to be glued
- Nitrile gloves
- Paper towel
Materials for Making and Using Thickened Methylene-Chloride Glue
Mixing the glue
Pour about a tablespoonful of the methylene chloride into the paper cup.
Pouring Methylene-Chloride into Paper Cup
Start adding peanuts one at a time, swirling the solvent in the cup as you do so. The peanuts will immediately dissolve in the methylene chloride.
Adding Styrofoam Peanuts to the Methylene-Chloride
Keep adding peanuts until the desired consistency is reached. To prevent the glue from evaporating too quickly, it's best to strive for a fairly thick glue, about the consistency of pancake syrup or a little thicker like molasses.
If you have to, the final peanuts that are added can be mixed into the glue using the end of one of the Q-tips.
I like to mix up just enough of the thickened glue for the job at hand. But you could get a small wide mouth Mason jar, an empty, new, clean one-quart metal paint can from Home Depot or Lowes, and mix up a larger amount of the thickened glue, and save it for future use.
Using the glue
Apply the thick glue liberally to the plastic-casing joint surfaces that are to be glued together using a Q-tip.
Applying the Thickened Glue to the Joint Surfaces of a Shell Casing
This assumes you already have your shell filled with stars and burst. The empty shell halves in the photos are for illustration only. Do this quickly so that the glue does not dry before the parts are mated.
Press the glued parts together completely, twisting them slightly as you do so. A thin bead of the melted plastic glue should extrude from the glued joint. Hold the parts together for a half minute or so.
Closing the Shell, Pressing and Twisting the Glued Surfaces Together
Use a paper towel to wipe the excess glue off of the plastic casing.
Visually inspect the glued joint for any open voids or cracks in the glue. Apply more glue to the outside surface as necessary to ensure complete sealing of the casing.
Harry Gilliam's plastic shell gluing method
I use a slightly different method than Ned does. It is not my invention. It was taught to me by a commercial shell maker (back when we had such people in the US!).
I fill the two shell halves with stars and burst. I do not use plastic, tissue, or cardboard to "trap" stars in one half so they don't fall out. With both hemispheres loaded with stars and burst, I quickly snap the two halves together.
Surprisingly, by the second try I could do this without spilling a single grain of burst or stars.
Holding the two hemispheres tightly together, I push them more tightly together. I then put them into my shell clamp (which is an excellent tool and project in and of itself). The clamp holds both parts of the shell for me, with constant pressure being put on the two halves.
I gently tap the two halves with the plastic handle of a screwdriver. This serves to settle the contents and gradually the tapping and the pressure of the shell clamp causes the two halves to close up.
Once the lip of the two halves have come together to the point where they are only about a millimeter apart, using one of Skylighter's needle applicators, I squirt a little (non-thickened) methylene chloride all around and the equator and into the shell.
Doesn't take much to do the job. What's the job?
The methylene chloride will soften the ABS shell plastic. No, it will not hurt the contents of your shell.
Next, with the shell still in my clamp, I keep tapping all around the shell until the two halves are tightly joined, with no gap in between.
At that point, I leave the shell in the clamp, and set it aside to dry.
After 30 minutes, I remove the shell from the clamp. Then I eyeball the equator and look for little air gaps between the two halves. They are easy to see. Using a Q-Tip, I then dab a little of the thickened methylene chloride into the little gaps. These will effectively seal the shell so that no flames will enter the shell during launch.
I set the shell aside and let it dry (no clamp needed at this point) for at least an hour, often more.
The shell joined this way looks neater and more professional. There is less sloppy stuff squishing out between the shell halves, and that sticky stuff is not messing up whatever it touches. I personally think the process is quite fast. It was, after all, developed by somebody who manufactured plastic ball shells commercially, and who needed to be highly productive.
The acid test of effective shell joins is what your shell looks like in the air, and what the shell debris looks like on the ground. What you want is non-symmetrical shell fragments. What you do not want is intact shell hemispheres.
If you are getting intact hemispheres, your shell is not glued tightly enough. Then use more glue, or try a different glue. But if you use either Ned's technique, mine, or some combination, you should get good shell patterns, and the right kind of fragmentation.
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