How to calculate the carbon footprint of an injection-moulded part

Whoever injection-moulds plastic parts receives more and more footprint requests from automotive, packaging, electronics or consumer goods customers. Usually there is no per-part figure.

1. ResinPolymer, virgin or recycled.
2. MouldingEnergy per cycle and per machine.
3. ScrapSprue, flash and rejected parts.
4. TransportFrom resin to your plant and from part to customer.
5. PartValue per unit, with boundary and standard.

What goes into the footprint of an injection-moulded part?

• The resin: producing it has an emissions cost that depends on the polymer and whether it is virgin or recycled.
• Process energy: each injection cycle uses energy, varying with the method and the machine.
• Scrap: sprue, flash and rejected parts, which are recovered or lost.
• Transport: of the resin to your plant and of the finished part to the customer.

What data do you need?

1. The part drawing or data sheet.
2. The weight of the part and of the sprue.
3. The polymer and, if known, the recycled content.
4. Machine consumption per cycle, if you have it.

The drawing and weights are enough to start: physical factors are applied and, when in doubt, the most conservative one.

How do you improve the result?

With primary data: the actual machine consumption per cycle, the origin of the resin and the recycled percentage. Each replaces a conservative assumption with a measurement.

And the plant’s purchases?

Resin, compounds, masterbatch, inserts and packaging are your plant’s Scope 3. See how it is measured in Scope 3 for purchased goods and the detail on the plastics page.

What mistakes are common in moulding?

• Counting only the weight of the part. The sprue and scrap are also produced and must be allocated.
• Using a generic “plastic” figure. Each polymer has its own footprint, and recycled content changes the result.
• Forgetting cycle energy. A more efficient machine or a better mould shows up in the footprint, but only if the calculation captures it.
• Calculating by spend. If you move to a more efficient process and the price rises, spend punishes it.

What changes if you use recycled material?

The origin of the raw material changes and, with it, part of the resin footprint. The effect depends on the type of polymer, the recycling process and the energy used, so there is no universal rule: it has to be calculated with the data of the material you actually buy. If your supplier gives you its recycled percentage and origin, they replace the generic values.

Data checklist per part

• Drawing or data sheet of the part.
• Weight of the part and of the sprue.
• Polymer and recycled percentage, if known.
• Machine consumption per cycle or per hour, if measured.
• Origin of the resin and inbound transport mode.

What can you do this week?

Pick the part you are asked for most or that bills the most and locate its drawing, its weight and that of the sprue. If you have the machine consumption per cycle, note it down; if not, that is fine: the first calculation uses physical factors and the most conservative one where data is missing. What matters is starting with a real part and not a category.

Then talk to your resin supplier and ask for the exact polymer, the recycled percentage and the origin. These are three data points any serious distributor should know, and they are the ones that move the result most. With them and the drawing you can already show your customer a value per part, with its boundary and standard, and decide whether it is worth measuring machine consumption to refine it.

How does Xain do it?

Xain interprets the technical documentation of the part and builds a process tree: each step (resin, moulding, scrap, transport) carries its physical factor, and where data is missing the most conservative one applies. Plastics, with injection, extrusion and machining, is one of the sectors included in the LRQA verification. See the plastics page or talk to us.