Need to include scrap plastic as an output if disposal/recycling not included within scope of LCA?

Question

I'm trying to model an injection molding process in which about 3.5% of the input materials are actually turned into scrap plastic. Therefore, 1.0363 kg of polycarbonate granulate is needed to be inputted in order to get 1 kg of injection-molded polycarbonate pieces (.0363 kg /1.0363 kg = .035 kg = 3.5%). I am not including the disposal/recycling of this waste scarp within the scope of my LCA. I only want to make sure I'm correctly accounting for the extra amount of input required to get 1 kg of output. 

My question is: when defining the inputs/outputs of the polycarbonate in the openLCA process, do I need to define it in this way? 

• inputs: 1.0363 kg polycarbonate granulate 
• outputs: 1 kg of injection molded polycarbonate pieces (reference flow) AND .0363 kg polycarbonate granulate  

Or do I not need to include the .0363 kg polycarbonate as an output? So that the polycarbonate inputs/outputs of the process would be: 

• inputs: 1.0363 kg polycarbonate granulate 
• outputs: 1 kg of injection molded polycarbonate pieces (reference flow)

Answer 1

You need to model the following:

Input: 1.063 kg polycarbonate

output: 1kg injection moulded polycarbonate pieces

This way, for every 1.063 kg polycarbonate input you get 1 kg moulded plastic output. If you added the 0.063 polycarbonate to the output, you would basically subtract it from the input polycarbonate and end up with only 1kg polycarbonate consumption in total, which is contrary to what you are trying to achieve.

Comments

Got it, that makes sense. Thank you, Tim!