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.