Mortars are the tubes used to launch fireworks up into the air, they can be made for a single use (cardboard) or made to use over and over again. Simple mortars are made for one-time shells; many store-bought fireworks will come with their own disposable mortar, while multi-shot firework racks are usually incorporated for firework shows and grand finales.
The only type of plastic that should be used for tubes is HDPE, also known as High-density polyethylene.  HDPE is a sturdy, dense material for mortars that features the safety benefit of flexible polymer construction that actually gives and bends during the firework expulsion (despite seeming rock solid).  Accidents can happen using any other type of plastic for mortars. For example, ABS and PVC plastic seem like they would make good mortars just looking at the tubes, but they are actually very brittle, not flexible like HDPE. If a shot became stuck in the middle of a non-HDPE tube, the shell may experience a disappointing low break, or worse, the force of the burst charge would cause the shell to burst open the pipe and send sharp shards flying in every direction.
As a general rule of thumb, mortars should be the same diameter as the shell (well, actually a hair less) and three times as long. The shell should fit in the tube snuggly, so that gases can build up underneath and not escape until the appropriate time. Experts recommend a 1 7/8” HDPE tube as that is the standard diameter of most consumer firework shells. Additionally, all HDPE mortars require a plug in the bottom to prevent lift charge gases from escaping from underneath.  The plug is usually made of 3/4 – 1″ thick wood cut into a circular shape. Typically, plugs will come with consumer-grade mortars, but if you are making your own, you’ll need to purchase or make plugs with a tight fit.
Planning lots of aerial fireworks?  You’ll need more than just one mortar and a convenient way to organize them all. A mortar rack consists of a number of individual tubes fused together so that the shells fire off in the desired sequence with the electronic lighting of only one fuse. Complex displays may even be fused directly to a computer that is manned from a safe distance.