An enclosed mixing machine for rubber or other suitable material, inside or which are two (2) heavy mixing rotors which revolve in opposite directions with a small clearance between themselves and tine enclosing walls. The mixing chamber is jacketed or otherwise arranged for water-cooling, and is provided with a feeding hopper which can be closed by means of a pneumatically operated, vertical ram. Leading examples are the Banbury, the Boiling and the Shaw mixers.