The Mystery of the Popping Caps

By Gene Hawkins

I engineered for a child-resistant cap company in the 1980s and was sent to the Midwest on a complaint from a customer that our caps were cracking. This wasn’t just any customer. They packaged herbicides and insecticides for home and orchard use, and they used huge quantities of our caps to seal their bottles of assorted nasties. The caps were of a single piece of double-walled squeeze-and-turn variety with tabs hanging below the cap that engaged bottle neck lugs. All we knew was that some caps were cracking like crazy, and others were fine.

It was a sunny spring afternoon when I arrived and met with a young but no-nonsense plant foreman. I could tell he was annoyed he had to call us to troubleshoot his problem as they didn’t like strangers in their plant. The floors were white and clean, very unusual at the time. They showed great pride in their facility, but it smelled like a garden center’s chemical aisle. The automatic capping equipment was older but top-of-the-line and it was working like a Swiss watch. The boxes of caps were manually emptied into a hopper that oriented the caps into the automatic capping machines to spin onto filled moving bottles. Partial pallets of caps were in a corner of the big room near an overhead door by what used to be a loading dock. Boxes of “bad” caps were near the line, waiting for my arrival. I was worried when the boxes of “bad” caps were put into the hopper, but they ran smoothly, without any cracking. Everything was running TOO smoothly to diagnose.

I asked the foreman to tell me every detail that was different between now and when they had the cracking incident. The problems occurred when they started up on first shift, and the breaking of caps sounded like popcorn, with fractured yellow plastic shrapnel flying everywhere. This time of year they were bottling two shifts. A third shift was only maintenance. They cleaned the line, changed chemical products and got the equipment ready for first shift’s arrival. Upon further questioning, it turned out third shift often opened the overhead door when the weather was good to bring much-needed fresh air into the smelly plant.

AHA!  Knowing that cold can make polypropylene caps brittle, I had the foreman take a bag of “good” caps and put them in the break-room freezer. An hour later, he dumped the frozen caps into the hopper, and as soon as they touched the first bottle they were popping tabs everywhere! The opening of the overhead doors were great for ventilation, but the pallets of caps sitting next to the cold night air chilled the caps enough to make them brittle when quickly forced to expand over the bottle lugs. The foreman was delighted the problem was solved, and my company was happy that a good customer didn’t have to ship defective products back to us. I was happy to get back home.

After five years at the cap company, Gene Hawkins spent 2 decades engineering for medical companies.