Platelet Factories subtract one box of damage anytime you take two or more boxes of Physical damage. It subtracts it before it get's applied, really, so the it's almost damage reduction, except that you have to take two or more boxed before it kicks in.
Trauma Damper will switch one box of Physical to the Stun track and subtracts one box of Stun from each Stun damage received.
If you're taking 4 Physical Damage, then Trauma Damper would kick in, moving one to Stun. Then Platelets kick in and subtract one from the Physical damage, so you'd wind up with 2 Physical boxes and 1 Stun box filled in.
If you take 4 Stun Damage, then the Trauma Damper kicks in, making it 3 Stun. The Platelets wouldn't affect it.
If you take 2 Physical Damage, then Trauma Damper would move one to the Stun track and the Platelets would NOT kick in, since you have only 1 Physical left. You'd take 1 Physical and 1 Stun.