1. Selection of pre molding action（Injection Molding Technical Information

The injection molding machine generally has three options based on whether the injection seat retracts before and after the pre molding feed, i.e. whether the nozzle leaves the mold.
(1) Fixed feeding: The nozzle is always attached to the mold before and after pre molding, and the injection seat does not move.
(2) Front feeding: The nozzle is placed against the mold for pre molding feeding. After the pre molding is completed, the injection seat retreats and the nozzle leaves the mold. The purpose of selecting this method is to use the mold injection hole to support the nozzle during pre molding to prevent molten material from flowing out of the nozzle when the back pressure is high. After pre molding, it can avoid heat transfer caused by prolonged contact between the nozzle and the mold, which affects the relative stability of their respective temperatures.
(3) Post feeding: After the injection is completed, the injection seat recedes, the nozzle leaves the mold and is pre molded. After pre molding, the injection seat advances. This action is suitable for processing plastic with a particularly narrow molding temperature. Due to the short contact time between the nozzle and the mold, the loss of heat is avoided, and the solidification of molten material in the nozzle hole is also avoided.
After the injection is completed and the cooling timer is timed, the pre molding operation begins. The rotation of the screw melts the plastic and pushes it to the front of the screw head. Due to the one-way valve function of the stop ring at the front end of the screw, molten plastic accumulates in the front end of the barrel, forcing the screw backward.
When the screw retreats to a predetermined position (this position is determined by a travel switch to control the retreating distance of the screw and achieve quantitative feeding), the pre molding stops and the screw stops rotating. Next, there is a reverse contraction action, which means that the screw makes a slight axial retreat. This action can relieve the pressure of the molten material gathered at the nozzle and overcome the "salivation" phenomenon caused by the imbalance of internal and external pressure in the barrel.
If it is not necessary to retract, adjust the retract stop switch to the appropriate position, so that at the same time that the pre molding stop switch is pressed, the retract stop switch is also pressed. When the screw moves backward to press the stop switch, the backward retraction stops. Then the seat begins to recede. 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.
Generally, fixed feeding method is used in production to save the operation time of injection seat advance and retreat, and speed up the production cycle.