How do I choose the right material for my injection molded part?

The short answer

Filter on duty before you open a datasheet

Most material mistakes are made in the first five minutes, when a familiar or cheap resin is chosen and the requirements are reverse-engineered afterwards. The order below costs an hour and can save a mould. Temperature comes first because it removes more resins than any other single factor. A datasheet heat deflection temperature is measured under a stated load, usually 0.45 MPa or 1.82 MPa, and it is not the same as the temperature at which the part stops working. Write down the continuous service temperature, the peak excursion and how long the peak lasts before you compare anything else. A garden-tool housing that reaches 60 C on a hot day and a bracket bolted to a gearbox at 120 C are not in the same selection exercise at all.

Chemicals, UV and moisture come second

Environment is where the second round of candidates falls away, and where the cheapest option is often disqualified.

• Chemical exposure. Polypropylene and polyethylene resist most aqueous chemicals, alcohols and detergents and are the default for cleaning products and medical fluid handling. Polycarbonate is strong and transparent but is vulnerable to solvent crazing and to alkaline cleaners, and it can fail by environmental stress cracking at loads far below its tensile limit. Any part that will be wiped with a solvent needs the specific chemical listed on the drawing, not the word chemical-resistant.
• UV and weather. Unprotected ABS yellows and chalks outdoors because its butadiene phase degrades in sunlight. ABS used outside needs a UV-stabilised grade, an opaque coating, or a swap to ASA, which keeps ABS's appearance and processing behaviour with a weather-resistant rubber phase.
• Moisture in service. Nylon absorbs water from the air, and a moulded PA6 part can pick up around 2 to 3 percent of its own weight over time, which moves dimensions and softens the part. For a tight-tolerance assembly in a humid climate that is often enough to disqualify nylon in favour of POM or PBT, which take up far less water.
• Contact and hygiene. Food contact, skin contact and medical use are grade-level questions, not family-level ones. General-purpose ABS and polystyrene are normally not approved because of residual monomer, while food-contact grades of PP, PE, PET and PC exist and have to be named explicitly.

Then the mechanical duty

Tensile strength on a datasheet is the least useful number in isolation. For a part that is loaded once and never again, static strength governs. For a bracket, a clip or a pressurised housing, creep under sustained load governs, and creep is a function of temperature and time rather than of short-term strength. For anything that is snapped together repeatedly, fatigue and notch sensitivity govern. The spread inside one family is large: unfilled polypropylene sits around 30 to 40 MPa tensile, ABS around 40 to 50 MPa, polycarbonate around 60 to 70 MPa, POM around 60 to 70 MPa with excellent fatigue life, and a 30 percent glass-filled PA66 reaches roughly 150 to 180 MPa with a modulus in the 8,000 to 10,000 MPa range. Glass fibre also brings rules of its own: it is abrasive, so gate inserts wear, and it reduces and directs shrinkage, which changes the tool more than it changes the part drawing.

Appearance, regulation and cost come last

Cosmetics are a material decision because they interact with every earlier stage. A painted part needs a substrate that takes paint without plasma or flame pre-treatment, which points to ABS, PC or a PC/ABS blend and away from the olefins. A plated bezel needs an ABS plating grade rather than a general-purpose one, because those grades are formulated for metal adhesion. A moulded-in-colour outdoor panel points to ASA so that painting is not needed at all. Regulation narrows the list again: a flame-retardant grade is required for an electrical enclosure, it costs materially more than the general-purpose grade, and it runs in a different processing window. Cost belongs at the end of the sequence, not the beginning, because the total cost of a part is dominated by the wall thickness, the cycle time and the scrap rate that the resin decision implies. As a rough planning rule, polypropylene runs around USD 1.2 to 1.8 per kilogram, ABS around 2.0 to 2.6, polycarbonate around 3.0 to 4.0 and glass-filled PA66 around 4.0 to 5.5, so a resin that costs twice as much per kilogram is rarely the reason a part is expensive or cheap.

The shortlist in practice

Once the four stages are written down, the shortlist is usually two or three resins, and the remaining work is on the drawing rather than in the material catalogue.

Limits: when the right resin is still the wrong part

The material cannot rescue a design, and it cannot be swapped late without consequence.

• Shrinkage is a tooling number. A part dimensioned for ABS at 0.5 percent shrinkage is undersized if the resin is changed to PP at 1.8 percent without reworking the cavity. Material changes after tool approval are one of the common causes of a full cavity replacement.
• Wall thickness rules win. Most resins are comfortable between roughly 1.0 mm and 3.5 mm. Below that, flow and strength suffer; above it, cooling time, sink marks and warpage dominate and no resin choice fixes it. Ribs at 50 to 70 percent of the nominal wall and a draft of at least 1 degree per side matter more than the brand of resin.
• Some families need their own equipment. PVC and some thermoplastic elastomers need corrosion-resistant barrels and tooling, and POM needs good venting and corrosion-resistant steel because it releases formaldehyde when overheated.
• Drying is not optional. PA, PC, PBT and PET are hygroscopic and must be dried to a stated moisture level before moulding. Skipping it produces splay, voids and a measurable loss of strength.
• Fire, food and medical ratings are grade-specific. A flame-retardant version of a resin is a different product with a different price and processing window. Never assume the general-purpose grade carries the approval.
• Optics and wear have ceilings. PC and PMMA both scratch. If a lens has to survive abrasion, the answer is usually a coating or a different material system, not a thicker part.

Turning the shortlist into a quoted part

A comparable quotation needs the resin named and the reason recorded. Send the 3D model and the 2D drawing with the continuous and peak service temperature, the chemicals and cleaners the part will meet, whether it lives indoors or outdoors, the load and whether it is sustained or intermittent, which surfaces are cosmetic and to what finish, and any flame, food, medical or VOC requirement that applies. With that we can name a grade rather than a family, flag the shrinkage and ejection implications before the tool is cut, and price the part so that the wall thickness and cycle time are visible on the page rather than discovered at the trial shot. See plastic injection molding for the process and its design rules, surface finishing for painting, texturing and plating, and metal stamping where a formed metal part competes with a moulded one.