High density polyethylene is chosen where chemical resistance and low temperature toughness matter more than stiffness or heat resistance. It is the resin of chemical containers, crates, fuel tanks and cutting boards, and its two awkward characteristics are large shrinkage and a surface that almost nothing sticks to.
What it does well
Excellent chemical resistance including to acids, alkalis and most solvents. Toughness that holds up at low temperature, where unmodified polypropylene becomes brittle. Very low moisture absorption. Good impact strength and a naturally low friction surface. It is also easy to mould and inexpensive, which is why it is produced in such volume.
Shrinkage and tolerances
It is highly crystalline and shrinks a great deal, more than polypropylene and far more than ABS. That means the tool is cut significantly oversize, and it means moulding tolerances on HDPE parts are looser than on amorphous resins. Uneven walls warp badly, so uniform section is not just good practice here but close to mandatory.
The bonding problem
The surface is very low energy, so paints, inks and adhesives do not adhere without surface treatment such as flame, corona or plasma. That is why HDPE containers are printed after treatment and why HDPE parts are usually joined by welding or mechanical means rather than glued. Design the joint around this rather than discovering it in assembly.
HDPE against polypropylene
Polypropylene is stiffer, has better heat resistance and makes living hinges. HDPE is tougher at low temperature, more chemically resistant and has a lower friction surface. Where the part must survive cold or aggressive chemicals, HDPE; where it must be rigid, warm or hinged, polypropylene. Both shrink a lot and both resist bonding.
Questions people ask about hdpe injection molding
What is HDPE used for?
Chemical containers, crates, fuel tanks, pipe fittings, cutting boards and any part where chemical resistance and low temperature toughness matter more than stiffness.
Why is HDPE hard to glue?
Its surface energy is very low, so adhesives do not wet it. Flame, corona or plasma treatment is needed, or the joint is made by welding or mechanically.
How much does HDPE shrink?
A great deal, more than polypropylene and far more than amorphous resins. Tools are cut significantly oversize and moulding tolerances are correspondingly looser.
HDPE or polypropylene?
HDPE for cold toughness and chemical resistance. Polypropylene for stiffness, heat and living hinges. Both are cheap, both shrink and neither bonds easily.