The Society of the Plastics Industry introduced the Resin Identification Code in 1988, separating plastic resins into seven groups. The number stamped inside the triangle on the bottom of a moulded part is that code.
It is widely mistaken for a recycling symbol. It is not one, and that confusion was serious enough that the standard was changed. ASTM D7611 replaced the chasing arrows with a plain solid triangle in 2013, precisely because the arrows implied that anything carrying one was recyclable. Plenty of it is not. If you are marking a new part, use the solid triangle.
Here are all seven, what they are, and what each is like to work with in a moulding shop.
1. PET or PETE, polyethylene terephthalate
Invented by J. Rex Whinfield and James T. Dickson in 1941 while they were working for the Calico Printers Association. Called Terylene, it was first manufactured by ICI. Engineer Nathaniel Wyeth patented the PET bottle in 1973, the version that could hold the pressure of a carbonated drink.
Transparent, stiff and light. Most of what you see carrying a 1 is blow moulded bottles and thermoformed trays rather than injection moulded parts, though the bottle preform itself is injection moulded before it is blown.
Moulding it: needs thorough drying. PET hydrolyses in the melt if there is moisture in the granule, and the damage is permanent, so the part comes out brittle and you cannot process your way out of it.
Recycling: bottle-grade PET genuinely is recycled at scale. Trays and textiles largely are not, so "PET is recyclable" is true of one form and not the others.
2. HDPE, high-density polyethylene
Milk bottles, chemical drums, pipe, crates, closures.
Moulding it: easy to fill and very forgiving, but it shrinks a lot, typically a couple of percent and not the same in every direction. On a large flat part that shows up as warp. It also needs no drying, which makes it one of the cheapest materials to run.
Recycling: one of the two codes kerbside collection actually wants.
3. PVC, polyvinyl chloride
Window profiles, pipe, cable insulation, flooring.
Moulding it: rarely injection moulded at a general trade moulder, and there is a good reason. PVC degrades if it is held hot, and what it gives off attacks steel. A shop that runs it needs dedicated screws and barrels. We do not run it.
Recycling: difficult, and the additive packages vary enormously between grades.
4. LDPE, low-density polyethylene
Film, bags, squeeze bottles, flexible lids.
Moulding it: soft and very flexible, which makes it good for living hinges and snap-fit lids. Same shrinkage behaviour as HDPE. Mostly extruded as film rather than moulded.
Recycling: collected in some places, not in others.
5. PP, polypropylene
The workhorse. Automotive interior parts, closures, housings, crates, containers, anything with a living hinge.
Moulding it: the material a moulder reaches for by default, and probably the most moulded polymer in the world. Cheap, tough, chemically resistant and no drying needed. It shrinks more than the engineering polymers and it is semi-crystalline, so cooling rate changes the dimensions, which is where process control earns its money. Glass and mineral filled grades take the shrinkage down substantially.
Recycling: increasingly collected, and recycled PP grades are now good enough for a lot of production parts.
6. PS, polystyrene
General purpose polystyrene is the rigid clear one. High impact polystyrene, HIPS, is the opaque tougher version. Disposable cutlery, CD cases, yoghurt pots, appliance housings.
Moulding it: fills beautifully and holds detail well, which is why it is used for model kits. Brittle in its general purpose form. Expanded polystyrene carries the same code and is a different thing entirely.
Recycling: poor. Bulky, low value, and rarely collected kerbside.
7. Other
Everything else. Polycarbonate, ABS, nylon, acetal, PMMA, PEEK, PLA, and every blend and filled compound.
This is the code most engineering parts carry, and it is where the system stops being useful. A 7 tells a recycler almost nothing. It could be a PEEK bearing or a compostable coffee cup lid.
Moulding it: this is most of what we run. PP through to PEEK, with glass, talc and mineral filled grades. Each material in here has its own drying, melt temperature and shrinkage behaviour, and the differences between them are far larger than the differences between codes 1 to 6.
Recycling: essentially never, from kerbside. Reprocessing engineering polymers happens, but through industrial routes rather than domestic collection.
What this means for a part you are having moulded
If the part is a consumer product that will end up in a household bin, the code matters and codes 2 and 5 give it the best chance of actually being recycled.
If it is an engineering part, it will carry a 7, and the code tells nobody anything useful. What matters instead is the grade, the filler and the specification, and that belongs on the drawing rather than on the part.
Either way, the marking has to be designed into the tool. Adding it later means cutting steel, and where it sits affects gate position and ejection. It is a five minute conversation at the design stage and an expensive one afterwards.