Plasticising unit for a screw injection moulding machine having improved volume adjustment means

A plasticizing unit for plasticizing material, comprising:

• a barrel having a plasticizing zone portion of a substantially constant inner diameter, an inlet, an outlet axially spaced from the inlet, a generally cylindrical reservoir zone portion axially intermediate the plasticizing zone portion and the outlet, a continuous helical groove formed in the plasticizing zone portion which varies from zero area to full area in the axial direction toward the outlet, and a flow channel communicating the continuous helical groove in the plasticizing zone portion with the reservoir zone portion for flow of plasticized material from the continuous helical groove into the reservoir zone portion;
• a screw disposed in the barrel and having a substantially constant outer diameter portion in which a continuous helical groove is formed, the continuous helical groove varying in cross-section from full area to zero in the axial direction toward the outlet;
• means for selectively axially moving the screw relative to the barrel between a plasticizing axial position in which the continuous helical grooves of the barrel and the screw cooperate with one another to transfer material between one another and thereby effect plasticizing of the material and a material ejection axial position in which the screw is axially closer to the outlet than in its plasticizing axial position, the screw being axially movable from its plasticizing axial position to its material ejection axial position to thereby displace plasticized material out of the reservoir zone portion through the outlet;
• and
• means for adjustably varying the volumetric capacity of the reservoir zone portion while the screw is maintained in its plasticizing axial position, whereby the volume of plasticized material which is received in the reservoir zone portion can be selectively controlled to a desired shot volume of plasticized material which is subsequently displaced from the reservoir zone portion by the screw.