Work in progress: sports bottle top

This one has taken me a while. We have a couple of these nice glass bottles in rubber sleeves. The problem is that the caps screw off and are not tethered. Not convenient for one-handed use such as when driving. I know, "first world problem", right? My wife suggested a flip-up top as a project. Imagine the flip-up top on your shampoo or syrup bottle, on steroids. It seemed like a good project to learn some techniques. It sounds simple, but it's actually fairly complicated. I didn't find anything like it on Thingiverse, so it's not trivial (or not in much demand). It needs to be relatively attractive.

There are several "moving parts" that need to fit and work just right, and it needs to be 3D-printable:

1. The screw-on part needs to be just the right depth to seat correctly so it will not leak fluid around the neck.
2. The threads need to match the glass bottle neck.
3. The flip-up top needs to snap on and off easily yet securely.
4. The top needs to have a plug that fits into the drinking hole so it will not leak fluid into the cap when closed.
5. The flip-up part (top) needs to be hinged to the screw-on part (bottom).

(I recently bought an electronic caliper. This would have been much easier if I had had it all along!)

I started thinking about it in late January. I've been designing and printing trial parts since mid-February, or about 6 weeks off and on around other projects and real life. I'm working in Fusion 360 and trying to design it completely parametrically. I now have a working design which meets 4 of the 5 requirements. I'll discuss each in turn and then update it as I finish the design and testing.

1. The screw-on part needs to be just the right depth to seat correctly so it will not leak fluid around the neck.

Right now it's about 2mm too deep, so the bottle top doesn't quite seat. But since I'm working with PLA, even if it seated I don't think it would seal tightly, because PLA is hard and doesn't "give". So I'm thinking a nylon gasket or washer up in the top would do the trick. Depending on the thickness I find, I might need to adjust the depth again. I've thought about spreading some sort of food-safe silicone around the inner top to form a gasket. I've also thought about switching to Nylon for the whole project. The texture would be better and it should seal properly, but it's more expensive and issues with moisture in storage.

So this part is not done yet.

2. The threads need to match the glass bottle neck.

This turned out to be tricky. None of the threads that come in Fusion 360 seemed to be quite right. It turns out that glass bottle neck thread standards are established by the Glass Packaging Institute. All the info is freely available on their web site, if a bit hard to interpret. Eventually I figured out the neck size and thread specs in their terms. Then I had to learn to create threads "by hand" in Fusion 360, following tutorials and examples. It requires creating a spiral of the right length and number of turns, creating a profile for the thread, and then "sweep"ing the thread profile along the spiral. It worked.

I printed some little slices of the bottom in order to more quickly test the threads without printing the whole model.

Later I'll talk about the printing orientation. It turns out the design needs support material inside the bottom adjacent to the threads. I was afraid the support would be difficult to remove and jam up the threads, but it actually worked out OK.

3. The flip-up top needs to snap on and off easily yet securely.

I designed a "step" around the top of the bottom, and an extension (not sure what to call it) around the bottom of the top, so the top will snap down onto the bottom. The diameters and heights need to be just right. I used a bit of chamfer to make them seat into each other. With a tolerance in the parameters, I can adjust the tightness of the fit. The latest sample is fitting just a little too tight, but I think it'll break itself in.

4. The top needs to have a plug that fits into the drinking hole so it will not leak fluid into the cap when closed.

Kind of like the rim, these need to be the right height and diameter so the plug will fit the hole. The centers of the hole and plug need to be the same distance from the center (or rim) so they will fit together when the top rotates down on its hinge. This fit contributes to the total friction holding it closed. There's a chamfer on the plug so it will guide itself in. So far so good. Sometimes it needs some filing to smooth it out after printing. 

5. The flip-up part (top) needs to be hinged to the screw-on part (bottom).

This has been challenging and strongly influences the overall design and printing orientation. I'm now on my third hinge idea. I wanted to figure it out without minimal influence from others's design.

1. Many such objects use a "living hinge" which is basically a thin section of the part which will flex without breaking. I printed a couple trials, then I found some (often-repeated) guidance on living hinge dimensions. I tried a couple variations of this fastened to the top of the bottom and the bottom of the top. Again I used slicing to print just the relevant section without the full depth of the bottom and top. It worked... kind of, with a couple of problems. PLA is just too stiff for this to work well. It fractures instead of flexing. My samples would break after about 300 bends (150 open-close cycles).
   
2. I then designed a separate living hinge with tabs that would fit into slots on the top and bottom. This tried to solve two problems: the hinge would be replaceable after it inevitable broke. And it would allow the top and bottom to be printed separately, which would solve a couple of other problems. This kind of worked but required some post-print filing to get the tabs to insert easily.
   
   
   
3. About that time I saw someone's design for an object with what I'll call a "ball and socket hinge". I think of it as a bump hinge. The bottom has hemispherical bumps on the ends of a long piece. The top has hemispherical holes in arms. The bumps and holes differ in size by a tiny margin. When printed, the bumps are in the holes but (hopefully) separated from it by a tiny tolerance. (3D printing has problems with spherical shapes due to the overhang issue, but when they are this tiny that's not a factor.) If it all works, it will come off the printer as a working hinge. In practice there has been a tiny amount of fusion that easily breaks loose. Nice!

Except... I just opened it forcefully and one of the holes broke out. PLA is brittle when small. I'll need to adjust it to give it a little more material around the holes. Nylon is looking better and better...








Now some thoughts about the printing orientation:

• If the bottom is printed right-side-up, it needs support material because the big "deck" that seals the bottle and supports the drinking hole is a big bridge.
• If the bottom is printed upside-down, the drinking spout would cause the deck to be slightly off the bed. Ugly support problems.
• If the top is printed right-side up, the deck supporting the plug would need thin support. Yuck.
• If the top is printed upside-down, it works out best. The top-of-the-top would be on the bed and so be nice and smooth. The deck would need support, but that turns out to be completely internal to the cylinder.

So the chosen solution is to print the bottom right-side up, with the top upside-down (open position), hinged together. The only downside of this is that to print it, the top needs to be be as tall as the bottom, which it otherwise would not need to be. That makes the whole assembly a little taller than aesthetically ideal. Oh well, I'll live with it.

I'm thinking to make the top not so tall-and-ugly by embossing my initial onto the top of it, in a fat font which will not create a lot of overhang. That will kind of visually break it up. And I'm thinking of printing the initial in a different color to add interest. The latest version of Mattercontrol enables you to pause at a specific layer to change filaments, so this might work out well. The bottom of the bottom would also be printed in that other color, which would give the whole thing a three-stripe appearance which might be cool.

My next steps include:

• Figure out the gasket issue, and/or adjust the bottom depth, and/or switch to all Nylon.
• Maybe adjust the hinge size for strength. I might reverse the parts so the bumps are on the arms and the holes in the "shaft", which would shift the strain to the heftier part. But in Nylon it might flex and be more resilient, which might solve the problem.
• I'd like to cut a little depression in the front where the top and bottom come together, to give one's thumb a little purchase when flipping the top open. It can't be too deep... I might need to beef up the inside a little to allow it to intrude on the cylinder wall. Still thinking about this part.
• Then figure out the initial on the top of the top.