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.
16 comments:
I think those caps serve specific purposes for specific products. The blue and white cap are used for drinking water. The red cap is for soda.
There might be certain air pressure injected to keep the bottle steady and to keep them maintaining their shape during winter and summer (pressure differential due to temperature difference), however the softer polymer which acts as a sealer in the red cap is design to hold higher air pressure inside the bottle. It is crucial to maintain higher level of pressurized carbonated drinks for them to maintain the taste. The air pressure in the soda bottle has two purposes, to maintain its shape and taste if the product.
Therefore, I think the blue and white cap although might be water tight and air tight, however, the red cap with softer polymer would be able to withstand higher air pressure.
Anonymous,
Thanks for your comment. I don't think there is any intentional air pressure injected in mineral water bottles, but your comment is valid for the soda bottle air pressures.
This might very well be the reason that the manufacturer of soda bottles choose the soft coating design.
Along the same argument, I think the tongue-lip seal design can be adjusted to withstand higher air pressure. The seal capacity is proportional to the amount of preload between the tongue and the lip of the bottle. By increasing this preload, the tongue-lip seal can withstand higher pressure.
For the same amount of tightening torque I would argue that the tongue-lip seal design will sustain higher pressure compared to the coated cap design.
OMG this blog is so geeky
Thanks for the compliment :)
um, i cant comment on the techy/geeky stuff - sorry - but anyways, the blue cap is not from aquafina... aquafina's cap is white. (well at least the last time i saw it around april this year, it was white).
i remember seeing something like that though. what was it - dasani?
ugh - now it's going to bother me...
I agree with the previous comment, it was from a DASANI water bottle ...
I also think a better picture will help illustrate your point, the images are too "wide". A "macro"-ish picture will better serve your post. Need to save and get a macro lens, perhaps?
For the red cap, why put the seal all over the area? Why not just the circular area around where the cap actually touches the lip of the bottle (so leave the middle area unseal)? This will cut the cost up to half of the material for sealing, I think.
I found that the water bottle's lid is harder to open. Could that be the reason why coke is using an extra material to design their lid/ cap?
Do you mind analyzing the sports drink gatorade's cap? the cap is unbelievably hard to open. what's your suggestion on their design? =)
yesich,
thanks for your comments
if you are referring to opening the bottle initially, the main load is due to the torque needed to break the safety seal (to assure that the bottle is unopened by anyone), which is composed of about 7-8 hair-size bridges between the bottom ring and the cap.
The culprit of the gatorade bottle cap, I think is not the mating surfaces between the cap and the bottle, but the sizing design of the safety seal 'bridges'.
Is this a blog only for "plastic bottle cap design" or will there be more posts?
i think the most gorgeous cap is from vitamin water bottle cap ! Its so awesome ! its much bigger and nicer to look at. Pretty cap!!!!
this blog is boring ...
No, it will continue, when the time comes...
The gaps in the threads are a safety feature. They allow pressure in the bottle to vent while the cap is still retained by the thread. If they weren't there then potentially the thread could form an airtight seal until you unscrew it all the way, then the pressure would be released suddenly and the cap could fly off.
Chris,
Interesting point on the partial thread design. I didn't think of that one. Thanks for the comment.
I have observed that though Coke and Dr. Pepper caps (and probably Pepsi) are grossly interchangeable, they do not create a seal when interchanged. What is this about? What are the critical differences?
@anonymous They might simply have different dimensions, although you can force them in, and that's why it does not create a good seal.
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