by Suzie Fenton
If you are using a closure with a plug/valve seal, then the relationship with bottle’s inside neck finish diameter (I.D.) and it are critical to the packages functionality. Typically, a closure manufacturer will call out the recommended I.D. of the bottle to be used with their closure on a Recommended Neck finish drawing. It is important that that information be transferred correctly to your bottle drawing so that the bottle supplier knows that this dimension needs to be controlled.
(To see a drawing showing the area of these dimensions, please look at our blog “Cap Drag – What Is It? How to Avoid It”)
By controlled, I mean the dimension needs to have a target and a tolerance. For example, (.518+/-.005”) means that the bottle I.D. can range from .523” to .513”. Using this example the closure Plug Seal O.D. could be (.534+/-.008”), meaning that the closure Plug seal can range from .542” to .526”. For example, if the bottle I.D. was large (.523”) and the closure Plug Seal O.D. was small (.526”) we would still have .003” in a worst case scenario.
In addition to having the targets and tolerances correct, the bottle I.D. needs to be smooth and free of nicks and any wrinkles. Everything can be in spec with the measurements, but if the bottle I.D. has a vertical wrinkle it could be the easiest path to leakage.
Another item that comes up in this sealing feature of a bottle is having a chamfer on the lead-in of the bottle I.D. and the start of the Plug seal. To understand the idea of a chamfer, think of a beveled edge. Picture a cross-section of a bottle finish, looking at the inside wall at the top where the land and inside wall meet in a corner and imagine a beveled edge instead of a 90 degree angle.
If you have both a chamfer on the lead in to the bottle I.D. and a taper on the start of the plug when the closure is applied, the effective seal area is reduced. Typically what I have found is that a chamfer on one is needed but if both the finish and the closure plug have them it can prove to be counterproductive in effective seal area. You want to able to get the closure Plug started into the bottle I.D. but you do not want to lose any interference fit that you have worked to create.
Lastly, you want the bottle I.D. to be controlled as we spoke of above, but you also may want to have it controlled to a depth that corresponds to the length of the plug seal in many cases as well. This way you can avoid the bottle I.D. choking in too soon and making the closure difficult to go on and in the same way have good dimensional control over the length of the plug seal.