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SANKO GOSEI

Mold technology: screw shape molding technology

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.

ペットボトルの本体とキャップ

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.

ネジ構造の場合の金型構造

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.

中子形式でネジ山を成形する場合

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

モーターでコアを回転させる場合

This is a method whereby the core is rotated in the direction of the screw rotation to extract the product.


(3) Yose-pulling

寄せ抜きによる金型構造

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.

金型無理抜き構造の場合

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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