Injection Molding Pre-Molding Actions and Back Pressure Control

Selection of Pre-Molding Actions

Depending on whether the nozzle retracts from the mold before and after pre-molding, injection molding machines typically offer three choices:

Pre-Molding Actions and Back Pressure Control-1

(1) Fixed Feeding:

The nozzle remains in contact with the mold both before and after pre-molding, and the injection carriage does not move.

(2) Feeding Before

The nozzle is in contact with the mold during pre-molding feeding. After pre-molding, the injection carriage retracts, and the nozzle moves away from the mold.

The purpose of choosing this method is to use the mold injection orifice to press against the nozzle during pre-molding. This prevents molten material from flowing out of the nozzle when the back pressure is high. 

After pre-molding, it prevents prolonged contact between the nozzle and the mold, which can transfer heat and affect the relative temperature stability of both.

(3) Feeding After:

After injection, the injection carriage retracts, and the nozzle moves away from the mold. Then, pre-molding takes place, and after pre-molding, the injection carriage moves forward again.

This action is suitable for plastics with a very narrow processing temperature range.

Since the nozzle has limited contact time with the mold, it prevents heat loss and the solidification of molten material in the nozzle orifice.

Pre-Molding Actions and Back Pressure Control-2

After injection completion and once the cooling timer has run its course, the pre-molding action begins.

The screw rotates, melting the plastic and pushing it toward the front of the screw. Due to the action of a one-way valve, created by a check ring at the front of the screw, the molten plastic accumulates at the front of the barrel, forcing the screw backward. When the screw reaches a predetermined position (determined by a stroke switch, controlling the distance of screw retraction to achieve precise metering), the pre-molding stops, and the screw halts its rotation. This is followed by the backflow action.

Backflow, in this context, refers to a slight axial retraction of the screw. This action relieves the pressure of the molten plastic gathered at the nozzle, overcoming the problem of “drooling” caused by an imbalance in pressure between the inside and outside of the barrel.

If backflow is not required, the backflow stop switch should be adjusted to the appropriate position. Allow the pre-molding stop switch to be activated at the same time as the retraction stop switch.

When the screw, during the backflow action, retracts to the point of activating the stop switch, the backflow stops.

Following this, the injection carriage begins to retract. When the injection carriage retracts to the position of the stop switch, the injection carriage stops retracting.

If the fixed feed method is used, attention should be paid to adjusting the position of the stroke switch.

In general production, the fixed feeding method is often used to save time on injection carriage operations and expedite the production cycle.

Screw Backpressure and Speed Control

High backpressure can provide strong shearing force to the molten plastic, while low speed can provide longer plasticization time in the barrel. Therefore, control programs that simultaneously adjust both backpressure and speed are commonly used.

For example:

During the screw metering full stroke, start with high speed and low backpressure,

then switch to lower speed and higher backpressure.

Next, switch to high backpressure and low speed.

Finally, perform plasticization under low backpressure and low speed.

This way, most of the pressure in the front part of the screw’s molten plastic is released, reducing the rotational inertia of the screw, thus improving the accuracy of screw metering.

Excessive backpressure often leads to: 

  • Increased colorant discoloration. 
  • Greater mechanical wear on the pre-plasticization mechanism, barrel, and screw. 
  • Prolonged pre-plasticization cycles, resulting in reduced production efficiency. 
  • Increased likelihood of nozzle drooling and greater usage of recycled materials.

Even when using a self-locking nozzle, if the backpressure exceeds the designed spring locking pressure, it can still lead to fatigue damage. Therefore, backpressure must be adjusted appropriately.

Leave a Comment

Contact Us