Hi
We have done something similar for a company, NAV production orders allow you to post negative consumption in the consumption journal
Create a new item, call it metal scrap, set it to be standard cost and the cost of 0, this means that should anyone actually post a cost with some kind of adjustment it will just go through as a variance and not affect the cost of the scrap, it also means that when you sell the scrap you will record it as a profit, lastly you can use inventory posting groups to determine where the postings on this scrap item end up in the GL.
On your consumption journal specify the scrap item and use a negative qty, this will actually increase the stock of scrap, at the cost you have specified.
The benefit to doing it this way, is that the output cost wont get allocated to the scrap, and you will be able to record multiple scrap entries as the process is followed not just at the end of the process.
Hope this helps
Nev