There are hundreds of plastics to choose from and as many applications. Where does one start you may wonder? After all, it's one thing reading about characteristics and specifications, yet another knowing what to use and how to work with and apply them.
Workshop-Publishing authors a book called - DIY VACUUM FORMING FOR THE HOBBYIST. This book offers the most complete information about the basics as a novice, with a whole chapter dedicated to plastics. It's the best collective information that I know of to date specifically for vacuum forming (pictured below).
Plastics are one of the biggest variables in vacuum forming since it heats and forms quite differently from one type to the other and one thickness to another. Equipment and ambient conditions add more variables that can effect outcomes. In uncontrolled conditions, vacuum forming becomes more of an art form than a well controlled mechanical process. Vacuum forming as a hobby requires patience and persistence, but when mastered, it's quite effective.
Most people don't ramble into vacuum forming looking for a problem to solve, rather the opposite . Though specific information is not easy to find, it's best to research plastics, thicknesses or even what brands are most popular for what you're specifically intending to do. There are endless uses for vacuum forming, so one plastic does not fit all. That said, there are a few common types of broadly used plastics that can be explored until you become familiar with what works specifically for your situation.
ABS- Is a popular lesser expensive plastic. Most commonly black, it has hair cell texture on one side and smooth on the other. ABS is a tougher more durable plastic. For that reason and the fact it can quickly and easily be bonded with Acetone makes it a go to plastic for multiple part builds. Also easy to paint.
PETG - Is a clear material that is very pliable when heated. It's very easy to form but also very easy to overheat which often results in webbing. A very useful and reasonably priced plastic for indoor uses. Clear plastics look great when they are back painted and very popular for those reasons.
POLYCARBONATE - Is also clear but is known for it's durability and UV resistance. it's very popular with Remote Control enthusiasts for car bodies. It's often more expensive than PETG and often required to be dried before use or it creates tiny bubbles in the plastic giving it a cloudy look.
HIGH IMPACT POLYSTYRENE (HIPS) - Is an easy plastic to form and is usually opaque white. Don't let the "high impact" fool you, it has more of a tendency to crack and tear but is good for many things depending on what you're using it for. It's great for duplicating molds and lighter stress projects.
KYDEX - Some plastics are known by their brand name. Kydex is one of those brands and appears similar to ABS. Kydex In the DIY world is popular for making custom holsters for hand guns. It has a blend of plastics that make it durable for its application and stays hot longer making it easier to work with for a longer period.
These are but a few of many many types of plastics. For more information on plastics and vacuum forming, the book shown below can be purchased from Workshop Publishing at build-stuff.com. I consider it the bible of vacuum forming for the hobbyist.