We have been in production of a product for about a year and a half and never had any issues but a couple weeks ago we started seeing a strange issue in every single unit coming off the production line. By swapping out parts we have narrowed it down to the polyurethane tires which are compression molded in China. The product is like a mini segway robot inside of a spherical polycarbonate shell.
The issue is that after 1 to 2 hours of the little robot driving around on the inside of the shell the shell becomes very sticky. Not like in the sense of a glue sticky but sticky as in high friction. On the other side of the internal mechanism is a slip bearing so if the inside of the shell becomes sticky the unit functions very poorly.
One of the first things I tried was to clean the inside of the shell to see if there was something on it. At first I used isopropyl alcohol and that didn't seem to help (maybe even made it worse?). Next I used vegetable oil, let it sit for 30 minutes, then washed it off 6-7 times with liquid hand soap. The oil actually seemed to dissolved whatever was on the shell and it was much lower friction. In both cases I was careful not to rub the shell since I didn't want to accidentally polish the shell, only clean it. Perhaps being dissolved by oil and not alcohol is a clue.
We had the compression molding supplier mold two new materials from their material supplier and both did the same thing. We also had them order material from a US supplier and that also didn't help. Finally our CM got some injection moldable PU in three types and one of them wasn't to bad but was still a little sticky.
Finally we think that moisture has always been trapped in the shells because they are air tight after being bonded and it's quite humid in southern china where they are made. However, with the new tires they seem to be affected by this moisture whereas the old tires didn't seem to be. By bonding shells at our office in Colorado where the air is much drier the new tires appear to perform much better but still sticky. Maybe that is another clue?
Out of 100+ units we have tested we built 10 using all the latest production materials except for the tires in which we found some old ones. These 10 with the old tires are perfect even after 60 hours of use and the humid air in the shell. The other 90+ units with newer tire materials are all bad and sticky.
It seems like something that is common in PU must be causing our issues and our original material must not have had this common component. After searching online I found mentions of emissions even after molding but that is the only idea I have. My background is in electrical engineering so I'm definitely not a materials expert.
The material is a 70A durometer PU with a carbon toughener (though the original material vendor has been completely unresponsive so that's all we know about it). We've also tried a material without carbon that used a natural color toughener and that didn't seem to help so I don't think it's the toughener.
Our line has been down for three weeks now, I'm in China working 16 hours a day trying to test every possible thing I can think of but I'm totally out of ideas. I will seriously figure out a way to mail you a pack of beer if you can help me figure this out. Let me know what other info I can provide to help. Thanks in advance!