Hi Peter,
I'd say compacting chamber gate or compacting chamber safety/pressure relief gate.

The packing ram compresses the waste into the compaction chamber against a vertical steel gate. The gate remains closed while the ram is cycling forming a dense log of refuse. When the desired log length has been reached, the vertical gate raises and the ram pushes the log into the transfer trailer.
http://www.marathonequipment.com/products/transfer-systems/b...