MORTAR WORKSHOP
Selection of Pipe
HDPE: High-density polyethylene is increasingly the preferred type mortar for launching fireworks since it's high density polymer construction gives it a very high strength to weight ratio, makes it very pliable in longer lengths, and also makes it very safe to use with fireworks. If a shell explodes prematurely inside an HDPE mortar, the HDPE would absorb much of the shock "flower-potting" rather than exploding and creating shrapnel. The durability of HDPE also means it can be used for many years or many displays without much wear.
PVC pipes are extremely brittle and cannot withstand the explosive force created by the burst charge of a shell. If a shell fails to leave a PVC mortar due to upside-down loading, faulty fusing or over tight fitting, the burst charge would shatter the PVC into extremely sharp, highly dangerous shrapnel.
Cardboard tubes can also be used. Most small shell kits come with a cardboard tube, but it should not be used to fire more shells than the kit contained. Thicker cardboard tubes are also available from some dealers.
Paper mortars are lighter (about 1/3 the weight), cheaper (about 1/4 the cost) and produce less dangerous debris than steel mortars. However, the service life of paper mortars is generally quite short; probably about ten firings on average. Spiral tubes soon tend to have inner wraps peeling up which can bind a shell. All paper mortars will delaminate inner wraps if the mortars are even slightly damp when fired. Even treated paper mortars can be ruined by a single exposure to rain.
Metal Pipes are mainly used to launch very large shells, i.e., around 12" and up. They're not used very often anymore for smaller shells simply because of their weight. If you use metal pipe for your shells, make sure it's thick enough (at least 1/8 of an inch). Service life is long, but high initial cost is another problem. Finally, even though steel mortars are quite strong, there is the possibility that a shell detonation may cause the production of dangerous mortar debris.
Mortar Type Comparisons
| Weight | Cost | Strength | No: of Fragments If Over-stressed |
Danger of Fragments |
|
| Steel | High | High | High | Few | High |
| Paper | Low | Modest | Low | Few | Low |
| PVC | Low | Modest | Low | Many | Moderate |
| HDPE | Low | Modest | Moderate | Few | Low |
Diameter
The inside diameter of the Mortar needs to be slightly larger than the outside diameter of your re-loadable shells, but not too much. Too small, and the mortar might get stuck in the tube: too large, and the lift gasses will escape preventing the shell from reaching a safe height before the burst charge fires. Most consumer (1.4G) shells are ~1.75" outside diameter, though there are also re-loadable shells that are ~1.5" and smaller available. Professional shells (1.3G) come in sizes of 2", 2.5", 3", 4" and larger.
For most of the consumer fireworks at 1.75" (44.5mm) in diameter, Mortars with an inside diameter of 1 7/8" (47.6mm) are used as this is exactly the size necessary to launch these shells. Common sizes for HDPE mortars are:
-
2"IPS-DR11 with an inside diameter of ~1.9". This is fine for all ~1.75" shells and is size most commonly used in the US. Generally this is not for use with ~1.5" or smaller shells.
- 2"IPS-DR9 with an inside diameter of ~1.8". Will work for most 1.75" shells, however some 1.75" canister shells wont fit. For those shells that do fit, using DR9 mortars will provide louder lift and higher breaks than DR11. SDR9 can be used for ~1.5" shells, but is not recommended for smaller shells.
In case the only diameter of mortar available is a little too wide, a possible remedy is to slightly increase the diameter of the shell itself. Unwrap the fuse of the shell, and take it out of the string loop at the top and gently bend it back so it's out of the way. Use several 6" long strips of duct tape torn in half lengthwise to wrap around the middle of the ball of the shell, keeping track of how many you put on. Insert the shell into the tube occasionally to make sure it still fits and stop adding tape when it barely fits. Put the same number of tape strips on each shell you plan to launch. >top
Length
For single-break shells, 1-foot lengths are usually long enough, even though about an inch of that will be taken up by the plug in the bottom. The remaining 11 or so inches is long enough to send the shell to a safe height when fired. To gain a little more height make the tubes about half a foot longer. This will cost more, however, and the racks will be heavier. For multi-break shells, the mortars should be at least 15" long, and ideally 18" long.
Aerial Shells - An overall length of 12" is fine for most shells although higher breaks are possible with 15" mortars. Importantly, the fuses on many shells are not long enough to reach the top of a 15" tube, so you would need to add a short piece of safety fuse the shell in order to be able to light it safely. Longer mortars, ~15" or so, are generally recommended for multi-break shells.
Mines - Mortars for mines should be 6-10" long. Shorter mortars result in lower and "fatter" spreads of the stars and material, longer mortars result in taller and skinnier spreads of stars. Shorter than 6", and the mine may be too wide and low for safety. Many mines have shorter fuses than aerials, so they won't be able to reach the top of a mortar tube longer than 10". >top
Plugging Mortars
All HDPE mortars need to be fitted with a plug in the bottom to prevent lift charge gases from escaping. The plug is usually made of 3/4 - 1" (20-25mm) thick wood cut into a circular shape. Unless you bought pre-plugged mortars, you will need to make your own plugs. The best way to do this is to buy a hole saw (shown at right) that has an inside diameter that's the same as your mortars. Remember that the walls of the bit have some thickness too, which will make the plug slightly smaller than the size of the drill bit. Because of that, you may want to use a bit that's 1/8" (3mm) or so larger. A good test is to try to insert the bit into the tube - if it fits in at all, the plug it produces would be too small.
Once you've got the right bit, find some scrap wood that's at least 3/4" thick. Do not use particle board or MDF. While you can drill them out using a hand held drill, it's much better to use a drill press. Drilling completely through the wood will cause the plug to stick inside of the bit, which will then need to be removed before drilling the next hole. To shorten the process, set the depth stop so that the bit comes within a fraction of an inch of cutting the plug completely out. This way, the plug will stay behind, and you can go on to the next one. When you're done, the plugs can be easily pushed out of the wood.
The drilled plugs should fit into the tube with little or no free space around the edges. To mount them, run a continuous bead of Liquid Nails adhesive around the inside and as close to the edge of the tube as possible then insert the plug. The glue will help to secure the plug and will create an airtight seal. Before the glue dries, the plug needs to be fastened into the tube from the outside using either screws or staples. Either method is good but an air-powered staple gun with 1" staples is much faster. Two or three staples or screws is enough to ensure a permanent fix. >top