Material selection is the decision most often made badly in plastics design, usually by starting from a datasheet and looking for the best numbers. The reliable method runs the other way: write down what the part must survive, use that to eliminate, and only compare cost and processing behaviour between the handful of grades that are still standing.
The five eliminating questions
What is the maximum and minimum service temperature, including transport and any cleaning cycle. What load does it carry, and is it sustained or intermittent. What chemicals, solvents, oils or cleaning agents touch it. Does it see sunlight or ultraviolet light. And is there a regulatory requirement, such as food contact, medical or flammability. Between them these five usually reduce the list to a family or two.
Then narrow within the family
Inside a family the decision is between filled and unfilled, between impact modified and standard, and between flow grades. Glass filling raises stiffness and heat resistance and lowers impact strength and increases warp. Impact modifiers do the reverse. High flow grades fill thin sections at the cost of some properties. This is where the moulder's opinion is worth more than a datasheet.
What datasheets do not tell you
Datasheet values are measured on moulded test bars under controlled conditions and in one direction. Your part has weld lines, varying wall thickness, fibre orientation determined by the gate, and moulded in stress. Long term creep, fatigue and environmental stress cracking are the properties that decide real failures, and they are rarely on the first page.
Confirm before the steel is cut
Once the list is down to two or three grades, mould samples if a prototype tool exists, or machine test parts from stock if it does not, and test them against the actual requirement including any cleaning agent the customer will use. A resin change after the tool is cut means shrinkage changes, which means dimensions change, which is not a small correction.
Questions people ask about injection molding material selection guide
How do I choose a plastic for a moulded part?
Eliminate first, using temperature, load, chemical exposure, ultraviolet and any regulatory requirement. Then compare cost and processing between the few grades still standing.
Can I choose from the datasheet alone?
No. Datasheet values come from moulded test bars in one direction, and real parts have weld lines, fibre orientation and moulded in stress that change the behaviour.
Does glass filling always improve a material?
No. It raises stiffness and heat resistance and lowers impact strength, increases warp and abrades tooling. It is a trade.
Can the resin be changed after the tool is made?
Not freely. Different resins shrink differently, so the tool dimensions and the part dimensions both change. Confirm the material before the steel is cut.