I have a soft spot for reusable water bottles. I had my Nalgene for 13 years before it died on a marble floor. I could not resuscitate it with heat or adhesives, and had to replace it. So when I was given a water bottle that clearly had a lot of sentimental value but a critical leak, I did my best to try to fix it.
The bottle in question wasn’t broken; it was missing a piece. Specifically, a silicone piece that stops water from escaping when the lid is closed. Replacing this part would be easier than repairing broken plastic. My first thought was to design a part, 3D print it, use that part to make a silicone mold, then cast a silicone part in the mold.
The first iteration was promising. It made a seal, but it was too weakly attached to the cap. The flexibility of the part was great for the seal but too much for affixing. Of course, epoxy wouldn’t hold it in place, either. I tried again, this time with a thicker top for holding on to the cap. This failed as well.
Next I decided to split it into two parts so that it would attach to the cap like an earring. I 3D printed the parts and gave it a shot. Naturally the bottom part wouldn’t make a seal with the hole, so I tried some all purpose Goop contact adhesive. This stuff stuck to the PETG well, and had some flexibility when cured. I spread some of it over the surface of the part. I tried multiple times, but could never get it to make a good seal, and since I was eyeballing it, I had no way of being certain until after it cured.
Finally, I decided to use the materials that were suited to each purpose. I modified the parts again, this time making the top piece PETG, longer, and with some holes through the side to allow the silicone to penetrate and mechanically secure to the peg. Then I designed and printed a PETG mold in the shape I wanted to make the silicone part.
I then mixed up the silicone and put it in the mold, put the peg through the cap, and then dipped the cap+peg into the mold, so that the silicone would cure onto the peg in the right shape. This worked! (ish). It took a few tries. The first try had a giant air bubble that prevented it from making a seal. On the second try, I was clever and used a syringe to suck up 1.5ml of the silicone before pouring, then used the plunger to create a vacuum by closing off the end. This made the bubbles expand and collapse into each other, acting as a really cheap vacuum degasser.
The second attempt worked really well, except it didn’t quite make a seal. For the third attempt I took a dremel with a sanding bit to the inside of the mold to remove a couple thousandths from the inside, giving me a slightly larger molded part. This time it worked perfectly. After removing the mold you could see it flex slightly as the cap closed, making a watertight seal. Success!
