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
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)
You don’t have to tip the glass as much to drink out of it, leaving more room for your nose.
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.
Less material is needed, generally the bottom is thick and the brim is thin.
Fluid dynamics also has a role, narrowing the opening will accelerate the fluid. Wider opening offers a lower flow which you can control better with your lips for make an opening you feel comfortable with.
You don’t have to lift the cup high to get the last drops.
And last but not least: stacking and molding both like to have narrow bottom and wider opening… which is the biggest reason: all of the above has exceptions, straight glass works just as well and many glass types have wide at the middle and narrow at the top.. Bottom being thick has more to do with balance than structural integrity but the bottom will always be thicker just because that is the surface that hits the table all the time. Multiple things are in play but wider opening glasses are usually always stackable and they are WAY easier (and cheaper) to manufacture without losing too much material. Long narrow glasses tend to have the problem that the liquid comes out too fast and unpredictable where as wider rim does everything “slower”. Wide in the middle is sort of compromise between the two, the distance from bottom to top is short but the opening is narrower (with cognac/brandy glasses, you need to tilt it more than usual, which puts your nose inside the glass… so there are many, many reasons for the shape)
Making the cups narrower at the bottom and wider at the top gives them a slight “wedge” shape, which makes it easier to put them into cupholders.
Without the wedge shape, if you try to fit a cup into a cupholder that’s too small, you can’t. With the wedge shape, even if the cupholder can’t accommodate the whole cup, it can probably fit enough of it to keep it in place.
There’s also the fact that it’s more ergonomic. Your shortest finger, your pinky, is at the bottom of your hand. When you curl your hand, the area inside is roughly conical, hence the roughly conical cups.
When molding plastic it helps to have a “draft” or an angle, so when you pull the 2 halves of the molding tool apart, they separate easily without rubbing on the part being made, like the Cup. It simplifies the tooling and reduces the amount of time per mold cycle by speeding up the release. It also helps with stackability of the items.
While some of these reasons are true, the main reason is that stacking the cups takes up less space during transport and storage, saving money for the manufacturer, distributor, and retailer.
The main reason to make larger at top is so that you can stack glasses to store them more efficiently in the cupboard.
Latest Answers