HDPE

What HDPE is

High-density polyethylene with excellent chemical resistance and low moisture absorption.

Why engineers specify it

HDPE gets specified when the environment is wet, chemically aggressive or food-contact, and the loads are moderate. It shrugs off moisture and most common chemicals at a fraction of the cost of the engineering plastics. What it will not do is hold a tight tolerance or carry load without moving: it is soft, it creeps under sustained pressure and it expands and contracts noticeably with temperature. When the part needs to hold a fit, acetal is usually the move; when it needs to take abrasion, UHMW is.

Machining HDPE

HDPE cuts easily and is forgiving of speed, but it is soft and it moves, which is where the tolerances go. It expands and contracts far more than the engineering plastics, so parts should be measured at a known temperature and left to settle after machining rather than gauged straight off the machine. Sharp tooling and generous chip clearance prevent the material dragging and leaving a fuzzy edge, and long or thin parts want support so cutting pressure does not deflect them.