Polypropylene is moulded in greater volume than any other resin, because it is cheap, chemically inert, fatigue resistant and easy to process. Its one demanding characteristic is shrinkage: it is semi crystalline and shrinks considerably more than amorphous resins like ABS, which reaches into tool design, part design and tolerance expectations alike.
Where it is used and why
Closures, containers, automotive interior and under bonnet parts, appliance components, medical disposables, furniture and living hinges. It resists most chemicals, it does not absorb water, it fatigues extremely well in flexure, and it costs less than almost anything else that can do those things at once.
Shrinkage and its consequences
Being semi crystalline, polypropylene shrinks substantially more than amorphous resins, and the shrinkage depends on cooling rate and section thickness. The tool is cut oversize to compensate, and a part with uneven walls will shrink unevenly and warp. Filled and nucleated grades change the numbers again, so a resin substitution after the tool is cut is rarely as simple as it sounds.
The living hinge
Nothing else does this as well. A thin section of polypropylene, moulded so the polymer chains orient across the hinge as it fills, will flex for a very large number of cycles without failing. The rules are specific: the hinge must be thin, it must be gated so material flows across it rather than along it, and it should be flexed once immediately after moulding to set the orientation.
Limits
Low stiffness compared with engineering resins, poor performance at low temperature in unmodified grades where it becomes brittle, poor ultraviolet stability without stabilisers, and a surface that resists paint and adhesives without treatment. Copolymer grades fix the low temperature behaviour; the others need a coating, a filler or a different resin.
Questions people ask about polypropylene injection molding
Why is polypropylene so widely used?
It is cheap, chemically resistant, does not absorb water, fatigues very well in flexure and moulds easily, which covers an enormous range of parts.
Why does polypropylene warp?
Because it shrinks a great deal as a semi crystalline resin, and uneven wall thickness or uneven cooling makes that shrinkage uneven.
What makes a good living hinge?
A thin hinge section, gating that makes material flow across the hinge, and flexing it once immediately after moulding to orient the polymer chains.
Can polypropylene be painted or glued?
Not easily. The surface is low energy, so it needs flame, plasma or chemical treatment before most paints and adhesives will hold.