Plastic bottle cap design

Here is my first real post in this blog. Let's pick a typical plastic bottle cap that most of us interact with everyday from soda or mineral water. What we have here is three different bottle caps. The left one I found on the parking lot, probably from aquafina, the middle one is from ice mountain bottled water I drink everyday in the office, and the right one is from the coke I bought for the sake of writing this blog :)

The first thing I would like to comment on is the sealing design from the left and middle ones. To seal the bottle, they used a short 'tongue', probably about 1/8" inch high that mates into the bottle mouthpiece from the inside. The coke bottle cap accomplish this by coating the mating surface of the cap with a softer polymer. It has the feel of the rubberized coating popular in cell phone housing these days. I think using the 'tongue' to seal the bottle is a clever idea. It eliminates the need for coating the cap with a softer material (which eliminates a whole set of machines in your assembly line). As a consequence, the material used in the left and middle caps in the picture is also softer. This is acceptable since they are probably cheaper as well.

In addition, the tongue seal is more robust with respect to the variation in the cap preload. The surface seal style in the coke bottle cap relies heavily on preloading the softer material against the mouthpiece in the vertical direction. The tongue seal delivers equal performance as well as the tongue is engaged to the mouthpiece in the radial direction regardless of the preload from the cap tightening torque. The sealing direction are indicated in the next picture. This might not be an issue from manufacturing perspective because the assembly station will ensure a certain tightening load. But as a consumer, this means that you need to make sure you tighthen the coke bottle all the way while for the mineral water bbttle a snug twist is adequate.

Is it possible that the sealing capabilities of the coated cap (from coke) has better sealing performance, and hence used in carbonated beverages? Possibly. But I would argue that the tongue seal is capable of high pressures too. This is because the hydrostatic pressure due to the soda will exert force on the tongue against the bottle mouthpiece from the inside, and thereby increasing the seal as opposed to weakening it.

Plastic bottles is typically made with Polyethylene Terephthalate (PET) while its caps are made from high density polyethylene (HDPE). You can notice this by the resin identification code.

The second thing I always wonder is the broken threads. I have few guesses on why they do not have continuous threads around the cap, but I'm not sure. I think it has to do with the molding process. I will leave this up to the readers to comment on.