The “Lab Rat” Water Bottle Tough Enough for Harvard University
A client came to us needing a laboratory animal water bottle. OK, that’s easy enough. You can get one at the pet store for a couple of bucks. What’s the big deal?
Turns out there were two big deals.
These bottles were needed in big university labs, including Harvard and Baylor Universities’ cancer research programs. The two requirements that had never been achieved in animal water bottles were:
- The bottle had to withstand repeated high-heat autoclaving (sterilization) to keep the lab germ-free. (Isn’t it amazing that cancer is so hard to keep alive in the lab?)
- The bottle had to be shaped to fit into the top of the caging system from the outside, so it could be accessed, filled, and replaced without disturbing the lab environment.
OK, so now we understood why our client came to us, instead of PetSmart. I’d like to tell you we nailed the solution right away, but in fact we had to go through several iterations of the process before we got it right.
We began by solving the problem of autoclaving, since the choice of material determines everything else. Our first thought was “Polycarbonate!” – you know, like those red and blue unbreakable water bottles – but it turned out that polycarbonate couldn’t withstand repeated autoclaving. The bottles would crack after just a couple of runs in the sterilizer.
We switched to polysulphone and polyphenylsulphone, both high-temp engineered plastics (as opposed to the commodity plastics that make up most of the stuff around us).
We next designed the bottle so it could easily be accessed and filled without disturbing the animal’s environment. Turned out not to be so straightforward. As you can see from the photo of the mold below, the parting line (the “seam” in the plastic) has a double dog-leg to allow the bottle to slide in and out of the cage. The outer edge is mostly flush, except for an indent near the bottom. And the threads have to be engineered precisely so the screw cap works without a problem. (If you’ve ever tried to screw a nut only a bolt with badly cut threads, you know how frustrating this is.)
Now we had to figure out how to blow-mold the polysulphone into that very tricky shape. The big challenge was the asymmetry of the shape. Without giving you a PhD in blow molding, let’s just say that it’s nearly impossible to maintain uniform wall thickness when one side of a product is bigger. The molten plastic has to get stretched thinner on one side than the other.
In this case, we were lucky: if one side was a bit too thick, it didn’t matter. The key was making sure that the other side wasn’t too thin.
Still – the shape of the bottle required some blow-molding gymnastics that would probably give you a headache to grok. Here’s how I describe it to myself: because of the double dog-leg, the parison hits the vertical mold before the blow action starts.
(I realize that makes no sense to 99.999% of the world, but I’m happy to explain it to you if you have a strong desire to be part of the 0.001%. Otherwise, just know that it’s a little bit like laying the track while driving the train.)
One additional requirement that came up (a lot of the time, the specs are a moving target because we don’t know what we don’t know until we create the product and test it in the real world) was smoothness. No part of the bottle could be sharp enough to puncture a latex glove.
We discovered that in order to achieve the shape, drainage (total liquid drainage was crucial), and chamfer size that was required, we had to trade off a nice rounded edge to the bottle neck. The bottles came out of production sharp enough to slice off bits of skin. So we added a secondary chamfer operation on the neck (think high-tech emory boards) to get the necessary smoothness.
We did it! Here’s a photo of the bottle.
But wait… there’s more. In blow molding, there’s no such thing as a happy ending. Because there is no ending…
Ongoing Quality Control:
Here’s the wonderfully frustrating thing about blow molding: you never get to rest on your laurels. Even after we nail the design and process, we have to recalibrate the equipment and could potentially deal with several dozen failures every single time we set up a production run.
So QC is also an ongoing process. Here’s what it looks like for the water bottle:
Parts are chemically tested for their ability to resist cracking.
- Every drop of water must drain from the upside-down bottle. In blow-molding, there’s a dreaded phenomenon known as “pushdown,” where excess plastic accumulates along the lower inside surface of the bottle neck (that’s why it’s called a “bottleneck”) and traps some liquid along its lip. We test bottles from each run for pushdown by filling them and draining them completely. (Not exactly high-tech, but it’s crucial and has to be done on a consistent basis.)
- Wall thickness is tested with a very cool tool called a magnetic micrometer. We drop a tiny steel ball bearing into the bottle and then use a magnetic wand to measure the pull from the outside. If any part of the wall is too thin, we get a force reading that’s too high. We chuck all of the bottles in question and tell the production manager to stop the machine until we figure out what’s going on.
- Weight is checked with a very precise scale.
- Size of the chamfer of the neck is measured.
- Check for contamination.
- Check for damaged threads, so the tops can screw on and off without a hitch.
- Check for dents and scuff marks.
I hope you have a sense of some of the challenges of a blow-molding project, and what sort of capabilities are required to overcome them to bring a perfect product to market. Aside from the machinery, the plastic, and the knowledge, did you notice the importance of meticulous process? And the necessity of creative problem-solving?
I often say that blow molding is as much craft as science. Without being immodest, I can say that there are very few, if any other, blow molding operations in the world who could have turned out this product, in this material, in this shape.
If you have a project that requires this level of blow molding expertise, give me a shout at 401-723-3000.
As you can probably tell, I love talking about blow molding, and will be happy to talk about your project and see if we can help.
Wishing you success,
Tom Boyd (Captain Meticulous)
Blow Molded Specialties
535 Prospect Street
Pawtucket, RI 02860