There are perhaps a dozen resins behind the overwhelming majority of molded parts, and choosing between them is a question about service conditions rather than about price. Temperature, load, impact, chemicals, sunlight and any regulatory requirement narrow the list to two or three, and only then does cost sensibly decide.
The commodity resins
Polypropylene is cheap, chemically resistant, flexes without breaking and makes excellent living hinges. Polyethylene is cheaper still, tougher at low temperature and harder to bond or print on. Polystyrene is rigid, clear in its general purpose form, cheap and brittle. These three cover most containers, closures, housings and consumer parts and are the default until something rules them out.
Injection molding plastic types: the engineering resins
ABS is rigid, tough, easy to mold and easy to finish, which is why it dominates housings. Polycarbonate is far tougher and optically clear, and it is notch sensitive and attacked by some solvents. Nylon is strong, wear resistant and absorbs moisture, which changes its dimensions and its properties. Acetal is stiff, dimensionally stable and self lubricating, which is why gears and bearings use it.
Fillers change the material more than the grade does
Glass fibre raises stiffness and heat resistance and lowers impact strength, increases warp because shrinkage becomes directional, and abrades tooling. Mineral fill improves dimensional stability more cheaply. Impact modifiers do the reverse of glass. A filled grade of one resin can behave less like its unfilled sibling than like a different polymer entirely.
Choosing plastic injection molding materials without a laboratory
Write down the maximum and minimum service temperature, the load and whether it is sustained, what the part touches, whether it sees ultraviolet, whether it must be transparent or colored, and any regulatory requirement. Send that to the moulder or the resin supplier. Two or three grades will come back, and molding a sample in each is far cheaper than choosing wrongly.
Questions people ask about common injection molding plastics
What is the most common injection moulding plastic?
Polypropylene by volume, because it is cheap, chemically resistant and molds well. ABS dominates rigid consumer housings and polyethylene dominates packaging.
Which plastic is strongest?
It depends on what strength means. Polycarbonate leads on impact, nylon and acetal on stiffness and wear, and glass filled grades of several resins on rigidity and heat.
Does glass filling always help?
No. It raises stiffness and heat resistance and lowers impact strength, increases warp and wears tooling. It is a trade, not an upgrade.
How do I choose an injection molding material?
From service conditions first: temperature, load, chemicals, ultraviolet, transparency and regulation. Narrow to two or three grades, then let cost and molding behavior decide.