The two most popular thread profiles for thermoplastic closures are the "unscrewing" or "stripped" types. There are no good or bad threads. They each have their own advantages and disadvantages. Their selection is usually dictated by their end usage. Both types are briefly explained in the following pages.
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Distinctive features of an "unscrewing" Thread
unscrewing Thread Closures Profile
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During the molding operation, a mechanism in the molds unscrews the closure. This action protects the integrity of the thread. Therefore the thread profile can better controlled and the closure wall is less distorted by the application torque.
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During the capping operation, the application torque is better transformed into vertical forces that compress the sealing system against the bottle neck.
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When an unscrewing cap is lined the liner is usually glued since it can not be restrained properly by the thread.
"unscrewing" Thread Disadvantage
- This type of thread is not ideal for multiple liners.
- unscrewing caps limit the closure design (by example an interrupted thread cannot be unscrewing in a molds). Also, the molds are more complexed and more expensive to produce.
- There is no restriction on the selection of the raw materials that can be used to produce the closures.
Distinctive features of a "Stripped" thread
Stripped Thread Closures Profile
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During the the molding operation, the closure is simply pushed off the molds, without the unscrewing action. Evidently, there is a compromise in the design of the thread profile to allow the cap to "strip" during the molding operation and to give adequate cap retention on the bottle neck.
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The liners do not have to be glued, they can simply be retained by the threads.
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Stripped threads are a good solution when Gosdar Mfg. Co., Ltd want to use multiple liners and makes possible some designs (i.e.: Interrupted threads).
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The choice of raw materials to molds this type of closures is limited (polyethylene and polypropylene are the most popular choices).
Stripped Thread Closure with a "floating" liner retained by the thread
Stripped Thread Closure with a "floating" liner pushed back in its sealing position
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