Haff's, Theo is correct. HDPE stands for high-density polyethylene; a linear non-branched polymer. Low-density polyethylene, LDPE, is a branched, cheaper to make version of PE. You can imagine how a linear collection of polymer molecules can pack easier than a bunch of branched molecules; hence the term "high density". Imagine filling the bed of your pickup truck with all the branches that you just trimmed off of the back yard oak tree, low-density oak (LDO), as compared with loading a cord of oak firewood, high-density oak (HDO) that is well trimmed and cut straight. The firewood will have a greater density and hence weigh more. However, if you really want "heavy duty" go for UHMWPE. This is ultra-high molecular weight linear polyethylene that is a 3 to 6 million molecular weight polymer. These fibers are so strong that they have replaced Kevlar in bullet-proof vests...
