Screws are often thought of as being made of metal, but there are many plastic products with screw shapes in our everyday lives.
For example, PET bottles. If you look closely, you can see that both the bottle itself and the lid have a screw shape.
By the way, how are these formed?
Threads = undercuts, so they cannot be molded with standard molds.
The body of a PET bottle (with an external thread) can be molded using slide mold technology. What about the lid of a PET bottle (with an internal thread)?
It needs to be slid inwards, but it cannot simply be slid because it has screw threads all round.
Therefore, the following four methods can be considered for internal threads.
(1) Release the moulded part together with the core and remove the part using a jig.
This is used for low-volume production to reduce the cost of mould making, but its disadvantages are
Longer moulding cycles. Damage to the cores can occur.
(2) Core rotation by motor
This is a method whereby the core is rotated in the direction of the screw rotation to extract the product.
(3) Yose-pulling
The method of extraction is based on the use of a sloping core. However, areas with no or small threads will be created.
(4) Forced extraction.
A more aggressive method is to forcibly pull out the moulding by force of ejection by making the threads shallower or by dropping the corners. In this case, PP (polypropylene), which is a relatively soft and tough material, is suitable.
Plastic bottle lids can be expected to be moulded in this way, partly because they are round with shallow threads and partly because they are made of PP.
How are plastic products around us made? It may be interesting to find out how plastic products around us are made.
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