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Patent idea: scoring soda cans so that they readily compact when twisted.
The buckling pattern of an axially compressed cylindrical shell is a pattern of diagonal ridges and valleys. If one explicitly makes a diagonal dent in such a can and then twists the top and bottom in opposite directions, other parallel dents and ridges form, the ends of the can are pulled together, and the can collapses. This makes for neater disposal and more efficient recycling.
Although an appropriate starting dent creates a better collapse than explicit crushing or folding, there is nevertheless some disorder and jamming in the folding process. The necessary parallel ridges don't appear spontaneously in the optimal places.
One can surely improve the process by optimising the initial perturbation that guides the initial wrinkling. Then one can hope to have a can that will collapse reproducibly and compactly when one twists it.
WHen one applies a shear stress \gamma_xy, the buckling waves have a wavevector in the (1 1) compression direction. The wavelength must be about the same as for axial buckling. This is known from the work of Champneys and many before him.
How to perturb the can
The can needs to be perturbed so that it buckles with a commensurate period.
All the wrinkles should occur at once
The perturbation should not weaken the can before it is used.
The perturbation should be easily adaptable as an after-process on the currently manufactured cans.
some possible methods
grooves.
a series of parallel grooves at the places of maximum curvature in the desired wrinkle pattern would tend to encourage wrinkling there.
Downside. It weakens the can. However, this type of weakening need not weaken it much under its normal conditions of hydrostatic pressure. Still, it surely decreases the yield stress when extra stress is applied to the full can.
Downside: it would lead to sharp creases instead of smooth bends. Would that collapse as neatly?
Upside: it is easy to score the grooves after the can is made.
Pre-wrinkling.
Before the can is filled when the lid is off, one could crimp it slightly in the desired wrinkle pattern. The crimping is supposed to be so slight that it doesn't appreciably deform the top rim.
when the can is filled and pressurized, this crimping deformation must disappear. Still, the inhomogeneity has been imprinted by the ductile flow that occurred during the crimping.
Downside: It is not at all clear that the mild crimping that is possible is also sufficient to affect the response to twist.
Instructions on can
In this scheme one simply indicates by marks on the can where one should press with the fingers before one twists the can.
Downside: requires more effort from the user.
downside: it is less magical
downside: it is easy to copy. The initial advantage of adoption would soon be lost.
Upside: the feasibility of this option should be assessed, becuase it would surely be tried by competitors if not by the initial adopter.
downside: it might not be suffieient. to give a clean buckling.
Next steps
Search online for
twist compaction cylinder and
twist compaction beverage can
Try simple twisting with no predisposition
to see how many grooves it wants to make.
Get some pop cans by raiding the recycle bins at work.
Make diagonal dents in a used pop can.
Try two opposite grooves on opposite sides of the can.
Make them by pressing a pencil into the can
Verify the number of grooves it wants to make.
perhaps 5 or 6 grooves would be better.
try scoring the surface
Make diagonal score lines with eg a glass cutter or a rat-tail file.
Try pre-denting with fingers in more than one place.
When I tried before I only made one groove I think.
if it works and looks promising and doesn't seem to be recognized.
prepare a good demo movie.
Consult with Adam, to find out if he has any ideas about marketing it.
Decide if this should be done with University resources or separately. It doesn't seem to need university resources.
Target: go to upscale supermarket and see what products are in soda cans.
Nov 21 session
Alternative versions found on the internet
Looked in Yahoo under crush twist "aluminum can"
NNewan from Yanko designs
http://www.yankodesign.com/2009/10/28/twist-and-trash-the-can/
It has spiraling ridge, must be about 10 of them.
I wasn't convinced that they had really implemented this. It certainly wasn't an after-treatment of standard soda cans. I don't think they worried about the design considerations for the existing soda cans.
EHow blurb
Tells how to crush a soda can with your hands.
It includes twisting.
UTube has a movie where a brawny guy is crushing a soda can by twisting. He gets it pretty small.
My trials.
I made grooves in the can by pressing it against a pencil lying on the table.
I tried 2, 3, 4 and 5 grooves.
I tried steep grooves as close to the cylindrical axis as possible, and shallow grooves.
Examination
I untwisted and looked at the main folds.
Most trials were difficult to twist, and they didn't crush down well.
The reason was that there were big extra vertices that were not at the ends of the grooves I made.
Best case was 5 grooves.
Here my pencil-made dents didn't all stay in place. When I made one, the previous one popped out.
However, I could tell where the grooves were supposed to be. I could then re-make them with my hands. With pressure i the grooves I also pushed the vertices towards the bottom and top of the can.
When I did this, it was easiest to crush, and all the major ridges were the five I had deliberately made.
Subsidiary structure. There were a lot of extra ridges and holes.
Scoring groves.
I made 5 parallel equidistant straight spiral lines on a can.
With scissors I scored a scratch along each grove. These were fairly steep grooves with vertices near the neck at the top and the base at the bottom. I formed five grooves following the five score marks.
then I twisted the top and bottom.
This resulted in the best crush of all my trials.
easier to start
Smallest final height
least subsidiary ridges; most regular (and pretty) folding pattern.
Morphology
It folded along the grooves.
In two places the can cracked along the grooves, showing the strong forces in the twisting, or maybe the unexpected weakening of the can.
There was one hole at the top and two cracks in the middle
See pictures in this folder: red christmas coke can.
next steps
Look up patents
Look under "aluminum can" and twist (and crush)
Devise an easy and reproducible way to make the grooves.
Eg 5 glass-cutters mounted on a circular jig at the proper angle
determine optimal depth of grooves.
Try different groove depths on full cans
perform crushing tests on these under axial static loading.
First determine crushing strength of intact cans.
Make movie comparing best crushing with unprepared can against prepared can.