This intructable is on how to make the shell casings for your aerial shells. The method used in this instructable is pretty much the same method used in almost all Chinese fireworks factories (just adapted for people without huge, expensive tools-like me) and so is very quick and easy to do, and you can use this method to create loads of hemispheres in a short period of time.
Step 1: You Will Need...
For this all you need is:
Some PVA glue
Half a ping-pong (table tannis) ball
Some scissors (preferably small ones)
Some thin card with the template printed on
And a marble that fits inside the ping-pong ball.
Step 2: Cutting...
For each hemisphere you will need to cut out two of the circles. Just cut the outer-most circle, don't cut any other lines.
You will end up with two, pritty much identical circles.
Step 3: More Cutting...
Now take your two circles and place them over the top of each other with the lines on each lined up (see the picture if you dont understand)
Cut two of the lines that are going in towards the center of the circle. Cut two that are next to each other.
You can now fold down this 'tab' that you have created. This will help keep the circles from moving in relation to each other.
Finally, cut the rest of the straight lines.
Step 4: Shaping...
Take one of your circles and line the ping pong ball up with the inner ring on it.
Push the card into the ping-pong ball and smooth it out with your fingers.
Use the marble to help achieve the hemisphere shape.
Remove the card and do the same with the other template.
Step 5: Glueing!
Choose one of you now off-flat templates, and put a good dollop of PVA in the middle of it (on the inside of the curve side).
Now draw lines from the centre to the edge of the template with glue. Do this on each 'tab'.
Step 6: Squishing
Take your now-gluey template and push it into you ping-pong ball gluey side up. Do this by the edges so you don't get glue all over your fingers. It doesn't matter if the template isn't in tight or snug, this will soon be rectified.
Now, take the other template and push it into the ball, so the two templates are both inside the ball, with a layer of glue between them.
Squish the templates into the sides of the ball, to get them both to stick to each other and to become the right shape.
It doesn't matter if glue spurts out, just smear it on the top edge of the hemisphere-it will make the hemisphere stronger.
Step 7: Finishing Up
Remove the soon-to-be hemisphere from the ball, and look around the edges for any loose bits. If there are any, just put a bit of glue under it, put the hemisphere back in the mould and squeeze.
Finally, after everything is finished, just remove the hemisphere from the ball and sit it down on a desk to dry. I leave mine overnight.
Then, once the shell is dry, place it back in the ping-pong ball, and cut around the top of the hemisphere, using the ping-pong ball as a guide.
Step 8: Final Thoughts
In theory you can use this method for any sized shell, as long as you find the right sized ball, and you scale up or down the template and the amount of card. A good one to try is using a tennis ball cut in half although, if you used this method for a shell that size i would suggest using three layers of card over two.
Here is the template i used for my hemispheres.