Hmm normally it should automatically fill it up, until the required amount is received.
You will see that it is full, when something of the metal remains on it and has to be hauled back to an empty storage. It shouldn't half fill it, except when you have one of your metal storage set to critical.
But that doesn't explain, why it works shortly after restarting it.
It should work like this:
You start it
It scans the weapon/other stuff and defines the needed material
A storage field is created that accepts metal and silver (or is it only metal at the printer? )
It is filled, until one of it is full, then this one is deactivated and the other is filled up (if it needs 2)
It is full the storage is removed and the printing process starts
When it is done, you have a new weapon and the printer resets to start.
If it doesn't work like this, I need to look into it and try to find the problem..