Wouldn’t it make more sense to have the cup be a consistent width throughout? Or, even make the cup wider at the bottom to make it more stable when sitting on a surface?
In: Engineering
For enjoyable drinking games obviously. You ever try to play beer pong with the skinny rim cups? Impossible to have fun with those.
If the cup get’s condensation, a thicker top assists in ensuring the cup does not slip through our hands.
You don’t have to tip the glass as much to drink out of it, leaving more room for your nose.
Beside the stackable factor, holding a glass that gets wider towards the top secures your grip on it, should it slip.
Manufacturing has a lot to do with it. Most mass manufactured products are molded, that is they are made in a mold.
A mold has 2 halves, a core and a cavity. [Here is a graphic example](https://media.springernature.com/original/springer-static/image/chp%3A10.1007%2F978-94-017-7324-9_3/MediaObjects/331685_1_En_3_Fig5_HTML.gif). When you press 2 halves of a mold together there is a void left, an empty space. Liquid hot material (like plastic) is pumped in between the core and cavity at high pressure, this fills the void. Rapid cooling is applied so the plastic keeps it shape. This also makes the product shrink a little (about 5% depending on the plastic). The molds open and the new product pops out.
The direction that the mold opens is called the direction of draw.
[Direction of draw with draft angle](https://s3-eu-west-1.amazonaws.com/3dhubs-knowledgebase/design-injection-molding/IM+5+-+Draft+angle.png)
If the product is designed so that its walls are straight up and down (at a 90 degree angle to the direction of draw) then the product can get stuck on the core or in the cavity as it cools because of its shrinking. If there is a 90 degree angle then this is called having no draft angle.
[incorrect core at right angle](https://www.natechplastics.com/wp-content/uploads/2018/12/Zero-Draft-Angle.jpg)
[incorrect cavity with no draft angle](https://www.natechplastics.com/wp-content/uploads/2018/12/Draft-Angle-Stuck.jpg)
To stop the parts from getting stuck in/on the mold a draft angle between 2 and 5 degrees is introduced. When the part shrinks it forces itself off of the core.
[Correct draft angle](https://www.natechplastics.com/wp-content/uploads/2018/12/Draft-Angle-Extreme.jpg)
Look at any plastic product and you’ll see a draft angle, it might be really slight, but it will be there. There is also the added benefit that the product will stack with others like it, but it is a pleasant side effect rather than the reason.
If you are interested in mold design and want to learn more ( I teach 2 semesters of plastics design and an entire semester of how to create molds in a CAD program, so there is a LOT to it) here’s a pretty good resource:
[https://www.3dhubs.com/knowledge-base/how-design-parts-injection-molding](https://www.3dhubs.com/knowledge-base/how-design-parts-injection-molding)
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