Seams like Thanksgiving, don't they?

[rjlittlefield]

The title may be easier to interpret if I tell you that this post is about Thanksgiving, and about seams. It's also about Christmas, odd as that may sound.

The background here is that last Christmas, I received as a present one of those fancy new side-cutting can openers.

Now, I thought I understood pretty much how the old top-cutting can openers work -- they shove a blade down through the lid, run it around the can, and hey presto, the top of the can falls out. Sure, it's kind of sharp around the edges, and yes, there's usually a sharp lip left inside the can itself, but that's the cost of doing business, right?

Side-cutting can opener, now that's another story. I simply did not understand how a side-cutting can opener was supposed to work. OK, it did, but how?

Answering that question sent me on a search that turned out to be mostly about how cans are sealed, anyway. I found the answer to be very interesting and thought I would share it with you. Oh, and I did figure out how side-cutting can openers work.

Here's the pictorial explanation. This is from a can of pumpkin, left over from making pie for Thanksgiving. (You knew all this would tie together, right? )

Canon 300D, Olympus 38mm bellows lens @f/8, stacked at 0.005".


Very briefly, the lid of a standard metal can is formed by a process called "double seaming", which folds the lid and the side of the can around each other to form a five-layered metal sandwich, with some plastic compound stuck between to complete the seal. You can find more detail at this web site and much more information by searching the Web for words like metal cans double seam.

I'm pretty pleased with how well these pictures illustrate how the double seam looks, and how the side-cutting opener gets it apart.

It's interesting to note that although these are fairly deep stacks, most of the time needed to get these pictures went into preparing the specimens.

The challenge is that whenever you cut and grind metal, it forms a burr on the "downstream" side of the grinding action. In a structure like this crimped sandwich, the burr often ends up completely obscuring the layers of metal. What you see here is the result of several iterations -- grind some, remove burr (using an extremely fine-tipped needle under a dissecting scope), grind a little more, remove smaller burr, grind even less, remove even smaller burr, stop when structure is sufficiently clear.

There's a bit of burr showing at the right side of the bottom "V" in the right-hand frame. (I'd like to claim that I left it for illustration, but of course the truth is, I didn't notice it in time to make it go away.)

An amusing exercise... I hope you enjoy!

--Rik

Edit 12/24/2016, replaced obsolete link http://www.dixiecanner.com/1120.htm with current equivalent in the Internet Archive.

[beetleman]

Really superb photos Rik. I never knew any of this info on cans. Now this is a classic case of scientific investigation.
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[Ken Ramos]

Though I don't think it will be coverd by CNN or Fox, this is some good investigative reporting you have done here Rik . Never crossed my mind how they seal these cans but it seems that opening anything these days from a plastic bag to a tin can requires a degree in engineering.

[rjlittlefield]

"Investigative reporting", eh? Hhmm... I wonder what Woodward and Bernstein could have done with Google?

Ken, regarding the difficulty of getting into stuff, did you ever hear the "origin story" about Mylar? If I remember correctly (having heard the story about 40 years ago), the first commercial application for Mylar film was bread wrappers. A great use, the company thought -- until they started getting reports from angry housewives who couldn't get into the bread!

Of course I felt compelled to try finding this story with Google. I couldn't find it.

I did, however, turn up a great article about the material itself, at http://en.wikipedia.org/wiki/Mylar. It seems that these metallized "mylar" balloons you see at parties are actually nylon, not Mylar. Completely different chemistry. On the other hand, Mylar film and plastic drink bottles use the same chemistry (polyethylene terephthalate -- PETE -- Type 1 recyclable), but the film is physically processed in a different way that "biaxially orients" the molecules for maximum stability.

Hhmm. I wonder if I'll ever have a use for that little snippet of information? Oh well, it's probably better than reading cereal boxes.

By the way, in the spirit of "truth in advertising" and "things aren't always as they appear", it's probably worth mentioning that the plastic sealing material used in the can seams is actually cream-colored, not black. The black appearance in these photos is due to innumerable tiny particles of metal, produced by the grinding operation, that are now coating the surface of the sealing material.

--Rik

[Ken Ramos]

Dupont Chemical !?...well mix my napalm would ya , acutaully I have mixed a batch or two many years ago, a very easy reciepe . Sticks as good as super glue and burns a lot longer. No wonder we can't get into things! An interesting bit of info on Wiki there Rik, that explains potatoe chips having been thrown across the room once you do manage to get the bag open and lets not forget anything that you may purchase from good ol' Wal Mart and have to open out in the shop with a carbon blade saw. Well we know who to blame now.

I read a story once where a guy took about 15 of those "so called" Mylar ballons and tied them to the wifes little yapping dog and took it outside and let it go. He got fined of course, probably didn't have a "launch window."

[rjlittlefield]

> "...Mylar balloons and tied them to the wifes little yapping dog..."

Well, that one's new to me. Sounds a bit extreme, though I can certainly symphathize with all the participants...

The story about the guy in the lawn chair is well documented, however: http://en.wikipedia.org/wiki/Larry_Walters.

--Rik

[Ken Ramos]

A very sad but interesting story there Rik. It is a shame that his life came to such a tragic end after all of that. Oh by the way, the dog story was featured on "Paul Harvey, good day!"

[Mike B in OKlahoma]

Very interesting bit of research, and fine photos to document them. I gotta learn about stacking....
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