It was my first job right out of school. I was working for Hercules Chemical, a company that no longer exists, although you have to blame that on someone else. I was in the Packaging Films Group, making multilayer polypropylene films for food packaging. The film had a heat-seal adhesive on one side of the polypropylene base. One of our larger clients used our films to make potato chip bags. The problem they had with our existing films was that the they seal was too weak. The client's chip-making plants were located west of the Rocky Mountains, so when trucks would drive their chips out to California, some of the seals would open up due to the pressure difference between the high altitude air and the air sealed inside the bag. And so they needed a stronger seal from us, which was then passed down to me.
Other options besides a stronger seal are technically possible but not economically feasible. Potato chip bags are made on a vertical form-fill-seal(VFFS) machine. The preprinted film is unrolled and shaped to form a tube. A seal is made along the tube forming the back of the bag, and a seal is also made at 90 degrees to this back seal, pinching the tube and forming the bottom of the bag. The chips are then added to the bag. This is actually a very cool process that is more complicated than you might imagine. The chips are fed to a number of weigh-pans located just above the bag opening. Each pan has a fraction of the total weight to be added, say one-eighth. A computer then decides which combination of eight pans are to be dumped into the bag so as to most closely match the desired value. While it would be much cheaper to have a single pan machine, having the additional pans very quickly pay for themselves. All of this is done at high speed. I would love to post a video of a VFFS machine, but I've not ever found one that really shows the process very well to someone who's not seen one.
The point here is that while technical options exist to prevent premature opening of the bag, such as reducing the initial air pressure in the bag, attempting to add this to the existing processing equipment would be a nightmare. So it was necessary to increase the seal strength.
In a heat seal, you are attempting to melt the adhesive polymer and get it to flow into the other layer. Upon cooling, the two layers are now entangled and show adhesion. The strength of a heat-seal depends on three and only three variables: time, temperature and pressure. Increasing any of this will increase the strength of the bond, but most manufacturing engineers are really only open to increasing pressure. Increasing sealing time slows the entire process, and increase the sealing temperature also slows the process since it takes longer to heat the adhesives to the higher temperature, that adds to the time as well. The best option was to develop an adhesive that sealed at a lower temperature, something that was successfully accomplished, or so I'm led to believe from all the complaints that colleagues pile on me now that they know I'm that guy.
Republished from It's The Rheo Thing with kind permission from John Spevacek.