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ELI5: Why glass bottles are mostly round shape and have narrow, slim, neck, when plastic bootles don't?

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15 points·1 month ago·edited 1 month ago

Both plastic and glass bottles are often made by blow-molding in mass manufacturing. But because glass and plastic have different strength and flexibility, plastic is easy to cram with lots of details like swooshes, or even threads (like for milk jug caps). It's much harder to do this with glass because it's stiff, so they usually don't bother. This means you can make plastic bottles in all sorts of strange shapes, but doing the same to glass is much more expensive.

Blow molding makes a tube-like stream of molten stuff, with an air tube in the middle of the stream. The stream gets pinched by a mold in the shape of the bottle, and then the air tube inflates the material like a balloon so it conforms to the inside shape of the mold. After that, plastic and glass differ: plastic needs to cool down immediately, glass needs to be reheated and tempered or else it cracks from thermal shock.

The molds for glass have fewer details usually because the glass, even while molten, can't fill all the details easily. It doesn't flow like plastic does. People can make really detailed bottles like the ones in the liquor store, but those might have more broken bottles during manufacture, or run at higher pressure, making the molding machine more complex.

Plastic bottles like jugs are made in a similar, but different process than bottles like coke bottles called extrusion blow molding. Coke bottles are made in a 2-stage process. The first is where the preform is made, the second is where it is re-heated, then blown into the shape of the coke bottle. In this case it is not because of the shapes you can achieve, but to achieve uniform wall thickness in order to obtain uniform cooling within the part as plastic shrink when they cool. When the part has areas cooling at different rates those chain molecules pull at each other causing sinks/voids in the part.

Relatively easy to manufacture, makes it easier to seal, and has a built in pouring method.

Just means you need a small cork as opposed to a big one, really, and to make a wider container good for pouring it has to have a lip on an edge, which makes sealing it even more difficult.

Besides which, if it doesn't have a slim neck with a narrow opening, what you have there is generally regarded as a jar, rather than a bottle.

Both plastic and glass bottles have a small opening.

He asked about the neck of the bottle, not the tip.

That’s not why.

Plastic is not as strong as glass, and it's economical to make very thin walled plastic bottles which are very weak and mostly get their structure from the pressure of the liquid inside of them.

In order to make a long tapered plastic bottle, a thicker plastic wall is required which increases costs. If you look at some of the premium juices like Simply Apple, etc they have long tapered necks like a glass bottle, but the plastic used is very thick, which increases cost. Compared to a 2 liter cola bottle which has a much thinner wall and can easily bend. Cola bottles must also contain the pressure of the carbonated liquid inside of them. The strongest shape for containing pressure is a sphere, and only slightly weaker is the cylinder, which is why 2 liter bottles are shaped the way they are. A long tapered neck in a plastic bottle, under pressure, would bulge out over time and rupture. Plastic wine containers are possible, but not plastic champagne bottles.

There are a few different ways of making a glass bottle, and only one major way to make a cost effective plastic one.

Glass used to be turned by hand, so someone making a glass, would have to make it in such a way that it could be turned and blown, which results in the classic round shape with a tapered neck. They can also turn the glass over a wet and wooden mold to change the exterior shape.

Industrially, glass can be blown onto a mold and expanded into it much the same way that plastic bottles can, but the starting product, molten glass, has very different properties than the starting product in a blow molded plastic machine, so the shape is different.

Plastic bottles are almost always made with a heated blow molding machine. A little tube of thick plastic is heated and inserted into a mold while connected to an air hose. The starting slug looks a little like a thin test tube. The hot plastic is then blasted with air to blow it up like a balloon inside of the mold and it takes on it's shape. Because it's not turned it can have a much narrower opening and a higher interior volume.

It’s actually because of the manufacturing process. Plastic bottles (like coke bottles) are formed in a two-stage process. they are first made into a pre-form that looks like a test tube with a coke bottle lip. These pre-forms are then placed into a blow molding process. The preforms are encased inside of a mold the shape of a 2 liter bottle. The preform is then heated, stretched downward by a rod, this aligns the molecule chains so they become semi-crystalline. The rod also serves as an air hose. Once the tube is stretched to the desired length, air is shot into the tube To expand the walls outward pressing them against the walls of the mold. The wall thickness of the plastic bottle is actually much slimmer than a glass bottles because plastic parts require uniform cooling in order to prevent warpage from occurring. You obtain uniform cooling with uniform wall thickness. The reason for that is if there are varying wall thicknesses within the part then different areas of the part will cool at different rates. When plastic cools it shrinks. As the slimmer areas start to cool more rapidly than the other areas they start to shrink at different rates and the molecules begin to pull at each other, causing sinks/voids. So the reason why plastic bottle are the same diameter, and thickness throughout the whole bottle is to ensure the part has uniform cooling to prevent warpage, and form a “good part”.

I think it has to do with the pressure that water puts on the surfaces. While plastic is malleable and somewhat elastic, glass is way more rigid and could shatter if the pressure was too much, so plastic bottles simply can resist the pressure while glass needs special shapes.

No, that’s not why. Although what you said of the material is correct... Read my comment above.

Thank you for the explanation. I can't know everything in the world, we are all trying to find things out I guess.

No worries! I’m a plastics engineer. It’s not common knowledge. you weren’t wrong in what you said. Just off for this specific question.

It is literally what I study for hahah. Not engineering actually, but chemistry. At least I can say that lol

Lol you say “at least” as if chemistry is an easy subject. I’ve taken a lot of chemistry classes that were focused on polymers, and I struggled a lot with them. Give yourself more credit. YOU’RE A CHEMIST!! Like I said - you were completely right about the material. When manufacturing plastic products you have to consider processing, product design, and the material itself. It seems you’re mastering your field of study.

I said "at least" because confronted to you I'm totally ignorant. Also, technically I'm not a chemist yet, I'll be one in a year! Yay! :D

Maybe. If everything falls into place.

You’re not ignorant on the subject at all. I don’t know anything about glass at the molecular level except that it can be used as a reinforcement in plastic products to improve on its physical properties. We know what our studies focus on. Congratulations on almost achieving your extremely difficult degree ;). Just out of curiosity - what country are you In? I only ask because I find it interesting when having good/civil communication with people around the world. I’m in the U.S.

I'm Italian, and still live in Italy. In the U.S. I would be a senior, hahah. One more year to go and I finally will have my degree :D I also hope to go to university and get a PhD though, I'd love to be a researcher.

If you want to talk some more we can continue in private, I'm loving this conversation!

Sure! I’m new to this so I don’t know how to initiate private discussions, so you’ll have to send me a message first lol.

Little fun fact about polymers: we produced nylon 6-6 Saturday, it was super duper fun.

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