what’s screw head
screw head is the front of the screw of the injection molding machine, which is the important part of the screw.
Function of screw head
When machine do charging, the plasticized melt can be discharged into the storage chamber, and when high-pressure injection, the melt material at the front of the screw head can be effectively closed in order to prevent backflow.
Classfication of screw head
it can be classfy to two kinds. one is with check ring, the other is without check ring. when the screw is with check ring, during the charging,the melt material of homogenization section push the check ring away,and flow into the storage chamber through the gap of the screw head.When injection machine do injection, the melt material of head position form a thrust and push the check ring back 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, it is not necessary to use the check ring, but by this way, it will generate reflux and prolong the pressure holding time.
Requirement to screw head
- it should be flexible and smooth
- there should be a suitable match clearance between the check ring and barrel. it should prevent the melt material from flowing back, but also need to be flexible.
- There should be sufficient flow cross section, and it also need to ensure the end surface of check ring has a return force, so that it can be quickly closed while injection
- the structure should be easy to disassemble and assemble, also easy to clean
- the thread of screw head is opposite to the thread direction of screw in order to prevent loosening while charging.