According to whether the injection seat retreats before and after pre-plastic feeding, that is, whether the nozzle leaves the mold, injection molding machines generally have three options.
(1) Fixed feeding: The nozzle is always stuck into the mold before and after pre-plasticizing, and the injection seat does not move.
(2) Pre-feeding: The nozzle is pressed against the mold for pre-plasticizing and feeding. After the pre-plasticizing is completed, the injection seat will move back and the nozzle will leave the mold. The purpose of choosing this method is to use the injection hole of the mold to resist the nozzle during pre-molding to prevent the melt from flowing out of the nozzle when the back pressure is high. After pre-molding, it can avoid long-term contact between the nozzle and the mold to generate heat transfer and affect them. The relative stability of their respective temperatures.
(3) Post-feeding: After the injection is completed, the injection seat moves backward, the nozzle leaves the mold and then preforms, and the injection seat advances after preforming. This action is suitable for processing plastics with a particularly narrow molding temperature. Due to the short contact time between the nozzle and the mold, the loss of heat and the solidification of the melt in the nozzle hole are avoided.
After the injection is finished and the cooling timer is finished, the pre-plastic action starts. The screw rotates to squeeze the molten plastic material to the front of the screw head. Due to the action of the one-way valve at the front end of the screw, molten plastic accumulates at the front end of the barrel, forcing the screw back. When the screw retreats to a predetermined position (this position is determined by the travel switch, which controls the distance of the screw retreat to achieve quantitative feeding), the pre-plasticizing stops and the screw stops rotating. This is followed by a retraction action. Retraction means that the screw makes a slight axial retreat. This action can relieve the pressure of the melt gathered at the nozzle and overcome the "salivation" phenomenon caused by the imbalance of pressure inside and outside the barrel. . If there is no need to retract, the retract should be stopped and the switch should be adjusted to the appropriate position to stop the pre-plasticization. The same moment the switch is pressed, the retract stop switch is also pressed. When the screw retreats to the stop switch when it retracts, the retraction stops. Then the betting began to retreat. When the injection seat retreats until the stop switch is pressed, the injection seat stops retreating. If a fixed feeding method is used, attention should be paid to adjusting the position of the travel switch.More about:Injection Machine