Threads on plastic products include external thread and internal thread. External thread is normally knockout by slide block, while internal thread adopts twisting tooth method for knockout.
<1> External thread
This structure is simple, parting line marks will be left on plastic product after forming. Obvious trace of parting line on the product will impair product appearance and affect thread fitting. Principle: Slide open with the help of oblique guide post, then push the product out by thimble. <2> Internal thread mould
It is can be classified into：
1. Forced thread knockout mechanism ( non-rotation type)
The mould structure is simple, which is used for plastic products with low precision requirements.
A). Carry out forced knockout using product elasticity. It applies to elastic products formed by PE, PP etc. materials.
B). Carry out force knockout using silicone rubber thread core.
Make thread core using elastic silicone rubber. During mould opening, firstly take out the core pin in the silicone rubber thread core to give shrinkage space for the core, then push product out. This kind of mould has simple structure, but the silicone rubber thread core has short service life, which applies to production of small lot products.
2. Non-forced thread knockout (rotation type)
It requires automatic mould knockout of plastic products no matter whichever plastic product or mould both rotate and move in axial direction, or only rotate or only move in axial direction. Note that in mould opening process, it shall be ensured that rotation-prevention part of plastic product shall function as usual.
Other driving method shall also include: To and from movement by oil cylinder or pneumatic cylinder, rotation of thread core by gears. When hydraulic, pneumatic, or electric motor (varying-speed motor drives the gear for mould knockout ) are adopted as power source, control devices are normally required. Normally electric motor is adopted for large thread-numbers. Use radium spoke wire to drive gear for knockout is another method sometimes adopted.
<3> Comparison between external thread mould and internal thread mould in production
Fabrication of external thread mould is convenient, but requires high work intensity with low production efficiency, it applies to production of small lot products; While the structure of internal thread mould is complicated, requires long period fabrication. It applies to production of large-lot products, and automatic production can be easily realized for this kind of mould.