rotomolded vs. blow molded

[Smurfwarrior]

Excellent info! Thanks for posting that

[Barskekarsten]

For my part I'm curious as to why no-one has tried Vac-Forming PE canoes

I think that is because PE is a semi-crystalline thermoplastic. That gives it a very limited temperature range where it is possible to vacuum form it. The glass transition temperature of PE is close to it's melting temperature.(T_g close to T_m)
http://en.wikipedia.org/wiki/Glass_transition. It basicaly goes from stiff to completely melted very fast. ABS has a much wider temperature where it is soft enouh to form but does not melt.

[plasticman]

PE can be thermoformed (read: vacuum-molded), but is generally only used for shapes that require a lot less stretching of the material from a flat sheet into the molded shape. The narrow temperature range between "softened" and "melted" is one of the main factors that make that material relatively difficult to thermoform. Another problem with thermoforming deep draw parts in PE is that the material even when heated perfectly evenly and not melted is that it is generally too soft for the molder to control where the stretching happens.

PE is the one of the most commonly blow-molded materials and almost certainly the most commonly used roto-molding materials because both of these processes depend on melting the material (the relative closeness of the T_g and T_m is not a factor) and because it is very cheap relative to most other types of plastic material.