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About the Injection Molding Screw Head and Nozzle

About the Screw Head

Functions of the Screw Head

In the injection screw, the screw head has the following functions:

  • During pre-plasticizing, the plasticized melt can be discharged into the storage room.
  • And during high-pressure injection, it can effectively seal the melt at the front of the screw head to prevent backflow.

Categories of the Screw Head

The screw heads are divided into two categories:

(1) Screw heads with check rings

For those with check rings, during pre-molding, the melt in the homogenizing section of the screw pushes the check ring away, and flows into the storage chamber through the gap formed with the screw head. 

During injection, the melt pressure at the screw head forms a thrust, which returns the check ring to the flow channel to block and prevent backflow.

(2) Screw heads without check rings

For some high-viscosity materials such as PMMA, PC, AC or PVC with poor thermal stability, in order to reduce the shearing effect and the residence time of the material, the check ring may not be used. But such injection will produce reflux and prolong the holding pressure time.

Requirements for the screw head

 The screw head should be flexible and smooth.

 The gap between the check ring and the barrel should be suitable. So that it can prevent the melt from flowing back, and can be flexible.

 Not only have sufficient flow cross-section, but also ensure that the end face of the check ring has a return force, so that it can be quickly closed during injection.

 The structure should be easy to disassemble and assemble, and easy to clean.

The thread direction of the screw head is opposite to that of the screw to prevent the screw head from loosening during pre-molding.

Injection Molding Screw Head and Nozzle

About the Nozzle

Functions of the Nozzle

The nozzle is an important part connecting the plasticizing device and the flow channel of the mold. The nozzle has multiple functions:

 When pre-plasticizing, establish back pressure, drive off gas, prevent melt from drooling, improve plasticizing ability and measurement accuracy.

② When injecting, form a contact pressure with the main casting sleeve of the mold, keep the nozzle in good contact with the casting sleeve, form a closed flow channel, and prevent the plastic melt from overflowing under high pressure.

③ When injecting, establish the melt pressure, increase the shear stress, and change the pressure head into a speed head, increase the shear speed and temperature rise, and strengthen the mixing effect and homogenization.

Change the nozzle structure to match the mold and plasticizing device to form a new runner type or injection molding system.

The nozzle also undertakes the functions of temperature adjustment, heat preservation and material cutting.

Reduce the viscoelastic effect and eddy current loss of the melt at the inlet and outlet to stabilize its flow.

When maintaining pressure, it is convenient to add material to the mold product, and when cooling and setting, the return resistance is increased to reduce or prevent the melt in the mold cavity from flowing back.