The Much Maligned Clam Shell Blister Pack


No, not that kind of clam shell. No one has a problem with that. You know what I’m talking about, the clear, impossible to open, nearly sliced your hand off while trying to open it with a butcher knife, packaging that everything from toys to batteries seems to arrive in. This kind:


I’m not going to try to dissuade you from despising this type of packaging. I just want to explain what they are made of and why they are used.

The thing that surprises people about blister packs is what they are made of. They are made of polyethylene terephthalate, also known by the acronym PET. This material is best known as good old polyester. Yes, this is the same exact polyester that is used for leisure suits, water bottles, soda bottles, and a lot of carpeting. Polyester is pretty versatile stuff.

PET is used for this packaging because it is surprisingly tough, has good clarity and is relatively inexpensive. The other part of the equation is the manufacturing process. Having a mold built to injection mold packaging would be very expensive. The blister pack is made in a process called thermoforming. The equipment and tooling for thermoforming is a lot less costly than that needed for injection molding.

In thermoforming, they start out with sheet of PET on a big roll. The sheet gets heated and then formed using vacuum into a relatively inexpensive mold (compared to an injection mold). Here is a video of some blister packs being made.

The other interesting part is how this packaging gets sealed shut. It is typically done with a process called sonic welding. Sonic welding uses sound waves to weld plastic together. The sound actually vibrates the molecules in the plastic. Here is a video of a sonic welder being used. If you turn your sound on, you can hear the high pitched noise that it makes. You know how well this works when you try to open one.

The advantage of sonic welding is the speed at which is can be performed. The strength of the bond is both a blessing and a curse.

I know this information will be of little consolation the next time you visit the ER after stabbing yourself with a butcher knife trying to open your kids birthday present. Unfortunately, PET blister packs are here to stay.

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