In the injection screw, the function of the screw head is to discharge the plasticized melt into the storage chamber during pre-molding, and to effectively seal the melt at the front of the screw head during high-pressure injection to prevent Backflow.
Screw heads are divided into two categories, those with check rings and those without check rings. bimetallic screw barrel For those with check rings, the melt in the homogenization section of the screw will push the check rings away during pre-plasticizing, and pass through the screw head. The formed gap flows into the storage chamber. During injection, the melt pressure on the screw head forms a thrust to block the non-return ring backflow channel to prevent backflow.
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 non-return ring can be omitted, but such injection will produce backflow and prolong the pressure holding. time.
Requirements for screw head:
① The screw head should be flexible and smooth;
②The clearance between the check ring and the barrel should be appropriate, that is, to prevent the melt from returning, but also to be flexible;
③There is enough flow cross-section, and it is necessary to ensure that the end face of the anti-return ring has a return force, so that it can be quickly closed during injection;
④The structure should be easy to disassemble and assemble, and easy to clean;
⑤The thread of the screw head is opposite to that of the screw to prevent the screw head from loosening during pre-molding.
Function of injection molding machine accessory nozzle:
The nozzle is an important part connecting the plasticizing device and the mold runner. The nozzle has multiple functions:
①When pre-plasticizing, establish back pressure to drive out gas, prevent the melt from drooling, and improve the plasticizing ability and measurement accuracy;
②When injecting, it forms contact pressure with the main sprue of the mold, keeps the nozzle in good contact with the sprue, forms a closed flow channel, and prevents the plastic melt from overflowing under high pressure
③During injection, establish melt pressure, increase shear stress, convert pressure head into speed head, increase shear speed and temperature rise, and strengthen mixing effect and homogenization;
④ Change the nozzle structure to match the mold and plasticizing device to form a new runner type or injection molding system;
⑤The nozzle also undertakes the functions of temperature adjustment, heat preservation and material cut-off;
⑥ Reduce the viscoelastic effect and eddy current loss of the melt at the inlet and outlet to stabilize its flow;
⑦When maintaining pressure, it is convenient to add materials to the mold product, and when cooling and shaping, it increases the reflux resistance to reduce or prevent the melt in the cavity from flowing back.