High pressure mixing head

In a high pressure mixing head for producing a chemically reactive mixture of at least two plastics components and other mixtures, comprising

• a mixing chamber which has inlet ports for the individual plastics components and an outlet port for the plastics component mixture,
• an expulsion piston arranged in the mixing chamber and having the same cross-section as it,
• a drive cylinder,
• and a drive piston arranged in the drive cylinder and adapted to be driven by pressure medium, the drive piston being connected with the expulsion piston for moving the latter between a rear end position leaving the inlet ports uncovered and a position in the vicinity of the front inlet port, the inlet ports being simultaneously shut off from the mixing chamber
• the improvement which resides in that
• the drive piston comprises firstly an inner piston part rigidly connected with the expulsion piston and secondly an annular outer piston part arranged on the inner piston part and adapted to move between two abutment surfaces on the inner piston part, the outer annular piston part being sealed off internally with respect to the inner piston part and externally with respect to an inner surface of the drive cylinder, an end of the inner piston part in the form of a plunger piston-like projection adapted to fit into a recess in an end of the drive cylinder having the same shape and cross-section as the projection shortly before and at the end position of the drive piston,
• two connections are provided for connecting the pressure medium with the drive cylinder and one of these connections is connected with the recess,
• and the longitudinal extent, measured in the direction of the piston movement, of the recess is in a range lying below and just including the relative axial movement between the inner and outer piston parts and is in a range larger than and just including the axial distance between the expulsion piston end face in its rear position and the remote end of the inlet ports.