Title says it all but an example would be domestic beer bottles such as Bud Light vs imported beer bottles Dos Equis. Caps and bottles are identical so what allows the twist off to actually twist off. Dumb question but I’m curious.
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The caps are largely identical, although twist caps may use a slightly different composition of rubber sealant underneath with different properties.
The biggest difference lies in the lip of the bottle. Twist off bottles have screw threads. Pop caps simply have a lip or “bead” that is captured by the folded edges of the cap, instead of threads.
Twist caps also generally require different equipment to install vs twist.
Pop tops can be installed with a simple hand-operated lever press that crimps the edges persons, and work reliably with such simple equipment. Although almost any commercial bottling operations use automated equipment to save labor and prevent contamination.
Twist caps generally require more sophisticated, more precise tools and equipment to crimp them. Otherwise they are prone to leaking ocassionally.
Thirdly, screw top bottles themselves are significantly more difficult to form from from hot glass.
Pop top bottles can be formed in a single blowing+molding operating. The bead for the cap is an integral feature of the main mold the hot glass is inflated into.
Use of a single molding operation results in minor inconsistencies in the size and shape of the bead/lip. This isn’t a problem for pop tops and it won’t affect their ability to seal.
This was probably why the design of a crimped top was widely adopted in the late 1800’s because it was not very sensitive to the quality of the underlying glass bottles. At the time most beverage companies bought their bottles from third parties and did not have direct control of the quality of the bottle. This style mirrored the mass production of bottles with highly mechanized equipment. Before that time corks were most common because the size of the neck could vary quite a bit due to bottles being largely handmade and different manufacturers using different molds. So a bottler usually kept corks of a range of sizes in stock. But provided bottles were almost the same size, crimping metal caps was much faster.
In contrast, screw threads on bottles requires highly accurate, Highly consistent shape to avoid the problem of the occassional leaky bottle.
The lip cannot be slightly oval shaped, for example. It cannot be too big or too small due to the nature of the crimping tooling. Otherwise the top won’t be engaged into the threads deeply enough.
This makes it necessary to add a second, often a third molding operation on the hot glass to form and/or refine the shape of the threads. This is usually done by pinching the rim with a set of steel rollers to roll the threads into the hot glass, after the rough molding step.
This is not so simple to carry out because once the glass is formed into the thin walls of the bottle it rapidly cools. Suffice it to say there are many different engineering considerations in designing machines to do this. It was considered quite an engineering frat when the first screw top bottles were introduced. During the cooling process glass is even more delicate than at room temperature.
As I mentioned it wasn’t just the accuracy of the bottle, but also the precision of the cap installation equipment that was important to avoiding leakage problems with screw tops. A single leaking bottle could contaminate an entire stack of crates, requiring the offender to be found and the rest washed off.
One final side note. It needs to be mentioned that screw lidded glass mason jars for food were being produced in the 1860’s. However canning foods causes a partial vacuum to form in the jar, which sucks the lid against the lip of the jar, keeping the seal tight. Using a vacuum is a pretty reliable sealing mechanism.
Beer and soft drinks on the other hand are pressurized, and this makes keeping the lid reliably sealed against the positive pressure more difficult. This is why screw top beverages weren’t widely sold until the late 1940’s. As I mentioned this is due to a grab bag full of prickly quality control problems.
As someone else explained, it’s not the caps that are different, it’s the bottles. Bottles that have caps that screw on have threading on the lip of the bottle, non twist bottle simply have an edge that the bottle cap is pressed on.
Follow up question, what determines which is used? Is there any difference in the bottling equipment needed?
It’s not the caps that are different. It’s the bottle itself. Twist-off have grooves to actually turn the bottle cap off. Whereas the pry-off have a lip on the bottle that prevents it from being twisted.
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