Often customers experience what is termed “stripping” or “jumping” when their closure jumps the bottle thread when tightened onto their bottle.

Common causes of this will be:

• Lack of Thread engagement
• Bottle neck thickness
• Bottle material and its manufacturing process
• Thread form of the closure
• “wet” necks and “slippery” liners
• Over torquing

Thread engagement refers to the degrees of rotation that the closure thread and bottle thread are in contact. We recommend a minimum of 270 degrees of engagement. For simple closures such as a basic flat refill cap that should be sufficient. For a complex closure such as a child resistant closure or a closure with an induction foil seal, this needs to be greater than 360 degrees. One item to be careful of is when you change your liner material. Even if it is the same type of liner (e.g., Expanded PE foam) and the identical thickness (1.00 mm) the new foam may compress very differently or have a different surface finish so that under on-torque the resultant amount of thread engagement will change.

The thickness of the wall of the bottle neck is important, particularly for extruded-blown PE bottles. The hoop strength of the bottle neck as the closure is tightened may not be sufficient to remain circular, so the neck may momentarily “collapse” allowing the cap to jump the thread and therefore not be tight on the filled pack.

Bottle materials and the process they are manufactured will give widely varying neck strength, size tolerances and thread forms. Ranking them from the highest performing to lowest performing combinations:

1. PET 2 stage process (very rigid and accurate sizing)
2. PET single stage process
3. HDPE injection-blown process
4. HDPE extrusion-blown process
5. LDPE extrusion-blow process (relatively soft and inaccurate sized thread forms)

For the PET 2 stage process, you can apply a very high on torque that may be beyond the on-torque capability of most capping heads.

For the other extreme of LDPE extrusion-blown bottles, you will have to carefully set your maximum on torque of your capping heads as over torquing the combination of a strong closure and these soft bottles, the closure will jump the bottle thread quite easily by partially collapsing the neck.

For the best results, the bottle should have a “dry” clean neck. For some filling applications, particularly hot fill in the beverage and food markets and in the pharmaceutical markets such as cough syrups, there is often residue products from the filling process left on the neck. This will radically change the friction level, so you may need to set the on torque based on wet necks (then the dry necks will simply have a higher on torque). A liner with a slippery facing will have the same affect. You may need to set a lower on torque as the momentum of your spinning capping head with a slippery liner face may lead to the closures being over torqued and then stress cracking over time.

Over torquing will occur if you have a strong bottle held securely in the capping line and a rigid cap also held securely in the capping head, the on torque of the closure can be too high. This can lead to a white “ring” over time on the top face of the cap that can lead to a crack of the closure at this white line over time.

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Refer to our article on Torque Guidelines Here