Cup — The Kitchen Series
Designing reasonably cheap utensils for the kitchen
I grew up in a very middle class family, my parents are academics but for a long period in my childhood, they had to support the extended family. As a result, we had very functional utensils. One that I remember very well was this stainless steel cup made by the company 555.
If you grew up in Nigeria in the 90s, you know this cup.
You gave it to people who came to your house asking for ‘cold water’, they could tell how cold the water was by how much sweat was around the cup. It literally could not break, you could wash and rewash a thousand times, and it was cheap. You could buy a pack of 5 at a nearby market and use it for years. Glass cups were reserved for fancier occasions.
The thing about this cup though, is that you couldn’t use it to drink hot or warm water — you would not be able to hold it. The same would apply with very cold water. Also, it lost the desired temperature pretty quickly; by the third sip a cup of cold water could be already lukewarm.
So What?
I decided to redesign this cup.
This redesign is part of a three-product series of things I would like to design for the kitchen, the basic requirement for the products is that they are relatively cheap to produce — the products should get progressively complicated also. For each product, I will write about the concept on this blog, how it can be made and how the new design is compared to the original or what’s currently available.
Concept
Thermos flasks have been available for a long time, the basic idea is that there is insulation between the two layers which trap the temperature. Thermos cups are increasingly more common, particularly in Canada where I live, because it helps you carry your coffee on the go. David’s Tea sells a really functional one.
The thermos concept serves to keep liquids at the same temperature. However, when consuming a drink (such as cold water), you want the temperature to change — just very slowly. Glass cups do that perfectly well!
Surprisingly, I had more questions than ideas. And I started to ideate:
I had essentially decided on a thermos cup without a lid, so that heat could only escape (by convection) through the top of the cup. Heat can obviously radiate through vacuum, but significantly less.
This way, it could be held even when hot. I also think this could keep the temperature at a slow decreasing rate — but this will have to be verified later, probably from an FEA simulation.
Research
I wanted to determine the best cup size for holding, and decided to measure out a couple of commercial cups — from Starbucks, Subway and Tim Hortons.
These cups are cylinders with bases smaller than the top so that they can be placed within each other. However, this reduces the volume of liquid that the cup can hold — this explains why (to hold more) the Subway cup is longest.
I decided to go with the outer circumference of the Tims cup, but with a constant outer diameter. How do I know what size thickness is best for the vacuum space — this will have to be verified when I do the thermal analysis.
My goal is to keep the outer surface of the cup at room temperature (25 degrees) when the water inside it is at boiling temperature.
On future research, however, I learnt that the thickness of the insulation does not affect vacuum’s resistance, so the thickness can be almost anything!
To determine the cup’s volume, I wanted to know the recommended daily water intake. I learnt of the 8 x 8 rule, while it’s completely arbitrary it’s a good reminder to drink eight ounces of water, eight times a day. 8 ounces of water is about 0.24 Litres, which I fixed as the desired cup volume.
To give the cup a distinctive look, I thought to add a lip (probably the wrong word) to it. Lips could direct the water when being poured out of the cup and provide a logical place for a person to drink from, an affordance in design terms.
I did a little test of a mini size and saw that with straight edges, when the water is being poured out of the cup it forms ripples at the straight edges.
While the flow is likely okay, these edges could form places where tiny particles gather and over time is likely to darken. So I switched to smoother edges.
Design
Of course, I used Onshape. I’m a huge fan.
I actually had an issue making the lip, Onshape has different operations that work differently for surfaces and parts so it threw me off a little but I got help on the Forum; very helpful.
Materials and Production
The big question was Stainless Steel or Aluminium.
Since thermos flasks are typically made with some welding it’s important to note that Stainless steel is easier to weld and costs less than Aluminium, it is also a poorer heat conductor. But sadly, it weighs more than Aluminium.
Weight may not be a bad thing here. There’s a study that shows that people assign weight to quality, which is why products like Beats By Dre had a metal arc in the head band solely to give the perception of quality.
Decided to go with 24 gauge 304 Stainless Steel, which gives it the following properties.
To give an indication of how heavy the part is, see below.
There’s a great YouTube video that shows the manufacturing process for thermos cups, for a slim design like mine it will be cut and formed from extruded stock material. The lip can be added by simple bending and the welding only at the top, probably manually.
Thermal Analysis
Finally, I wanted to check to see how well I’ve done on my goal.
I used SimScale — which runs completely in the browser — for the thermal analysis and declared boundary conditions for boiling water along the inner surface of the cup. Even though Stainless Steel is not one of the materials currently in the app, you can change the material properties to suit yours.
The results show that it does pretty well actually, the cup can be held anywhere at the base. This is surprising better than the simulations done here on a typical thermos flask — removing the need to join the different cups (thermos flasks are actually two cups) at the base eliminates conduction of heat that could move to the outer surface through the base.
Final Rendering
After watching a ton of tutorials on rendering in Fusion 360, I gave it a go.
Thanks for reading! The next item in my Kitchen Series is available here, please leave me feedback or lessons for improvement. I’m also looking for a design engineering opportunity, so hire me :)
