The cheapest resin on the quote is often the most expensive decision in the programme.
Selecting an injection-moulding polymer on piece price alone usually ends one of two ways. The part warps out of the tool. Or it looks fine in goods-in and snaps the first time an end user applies real torque, drop or heat.
A data sheet tells you tensile strength in a dry laboratory bar. It does not tell you how that grade fills your tool, how it shrinks against your steel, how moisture or UV will move the dimensions after packing, or what the A-surface will look like once glass fibre is in the mix. Those are process and field questions. They belong in the material choice, not in a later concession report.
The material-selection trap
Two substitutions show up more than any others.
ABS is specified because it moulds cleanly and takes paint or texture well. Then the product lives outdoors or near a window, and the unstabilised grade chalks and yellows. The piece price looked fine. The warranty cost did not.
Nylon is specified because the data-sheet stiffness looks like a metal replacement. Then the part is measured dry after moulding, ships, sits in a UK warehouse, takes on moisture, and the snap fits that were tight at T1 go sloppy - or the ones that were free start to bind. Moisture normalisation is not a footnote. It is the dimension.
If the only number in the comparison table is pence per kilogram, you are not comparing materials. You are comparing invoices.
Piece price starts here. Processing window, moisture and cosmetics decide the real cost.
PP - living hinges, talc, and the cheap-looking trap
Polypropylene earns its place. Chemical resistance, low density, living hinges that actually live, and a price that makes procurement comfortable. Homopolymer and copolymer behave differently in the cold and at the hinge. Talc-filled grades buy stiffness and reduce shrink, then they change impact and the way the part takes a scratch.
PP also hides problems. Sink over ribs. Warp on large lids. UV that nobody budgeted because the first samples sat under warehouse lights. If the hinge is the product, prototype and tool the production grade, not a "close enough" copolymer from the 3D-print analogue.
Living hinges only live if you tool and prove the production PP grade - not a close-enough copolymer.
PC - optics, printing and residual stress
Polycarbonate is tough and, in the right grade, optically useful. It is also hygroscopic in the hopper, notch-sensitive if the design is sharp, and unforgiving when the gate and speed produce jetting on a show surface.
Clear PC that will be printed, coated or hard-coated needs a process window and a drying discipline, not just a pretty first-off. Residual stress that looks invisible at T1 shows up later as crazing under cleaner, fastener load or thermal cycle. If cosmetics or optics are the reason you chose PC, pay for the grade and the process that protect them. A cheaper opaque blend is a different product.
POM - slip, squeak and the environments that attack it
Acetal (POM) is often the right answer for gears, sliders and low-friction interfaces. Dimensional stability is the point. So is tribology. Pair POM with the wrong counterface and you buy squeak. Pair it with the wrong chemicals - strong acids, chlorine, some cleaners - and you buy stress cracking.
POM is a poor outdoor cosmetic resin without protection. It is a poor "it was cheaper than nylon" substitute when the duty cycle is wear plus moisture plus UV. Use it where sliding and stability matter, then design the interface, not only the piece.
PA - strength that moves with water
Nylon's useful toughness often appears after conditioning, not in the dry as-moulded state. That is why a dim report taken too early can bless a part that will not assemble the same way in October as it did in a dry toolroom in June.
Glass-filled PA behaves differently again. Stiffness goes up. Anisotropic shrink goes with it. Flow direction, gate position and knit lines start to decide strength more than the headline fibre percentage. If you are replacing a metal bracket, ask where the load path sits relative to fibre orientation - not what the data-sheet modulus is.
Glass fibre - when it saves the structure and when it wrecks the A-surface
Glass fibre is not a free upgrade. It can take a housing or bracket out of metal. It also wears the tool, marks the show face, and warps along the fibre. A 30 percent GF grade that looks perfect in a tensile bar can refuse to sit flat in a large panel.
Use GF when the structure needs it and the cosmetics can tolerate it - or when you can hide the filled surface. Do not add fibre to "make it feel more premium" on a Class-A cover. You will spend the sampling loops trying to polish a problem the resin choice created.
Glass fibre can save a structure and wreck an A-surface. Cosmetics and fibre are a trade-off, not a free upgrade.
What to decide before anyone cuts steel
Write the duty, not just the polymer family. Indoor or outdoor. Dry or wet. Torque, drop, chemical wipe, living hinge cycles, food or medical contact if they apply. Then pick the grade, the filler, and the colour/UV package against that list.
Name the production resin on the drawing. "ABS" is not a specification. The melt flow, the UV package, the recycled content and the fibre level change the tool and the part.
Plan measurement in the state the customer will see. Dry nylon numbers are not field nylon numbers — see also nylon, moisture and late inspection. Conditioned samples belong in the T1 conversation.
Run mould-flow and warp on the intended grade, not a generic stand-in. Gate, cooling and fibre orientation are material decisions as much as tool decisions.
Piece price still matters. It is one column. Processing window, tool wear, scrap, cosmetics, assembly yield and field returns are the rest of the row. The cheap resin that needs a second tool or a product recall was never cheap.