Sunday, May 20, 2012

Controlling the spread

One of the challenging aspects that was solved early in the history of Old Pol in Cambridge was the problem of getting an even and consistent spread of reagent, or "goop".  The problem is further compounded by the fact that over time, the pods that contain the water based reagent have a tendency to dry out. If something "dries out" that is a sure sign that water is escaping.

Impervious materials exist, and some are more impervious than others! Such is the case with anything that contains water, as even the tiniest gap can allow a very slow release of the water over a long time. Old Pol used lead as a pod material, since lead is very well known for its impervious nature. Later, as lead became something that people didn't want in their hands, aluminized plastics were employed, and techniques to encapsulate them by folding and heat sealing were developed. Today, a large and growing pod "sachet" industry exists to put ketchup and hand cream and many things on the shelf for you to use, one at a time.

Getting back to porosity, potato chip bags are a good example of an aluminized plastic that prevents nitrogen egress. Potato chips are often packed in nitrogen gas with almost no oxygen, as that retards spoilage.  Nitrogen is a fairly small atom and can make it through many thin materials, so a coating of aluminum is used to seal the holes. Other sealants include Parylene, and various co-extruded or chemical vapor deposited materials.

Knowing the egress rate of an average pod is important: That tells you how long it will take for it to dry out and become useless. Since all pods do lose a little water, the pods are initially overfilled, so, a fresh pod has more goop in it than one that has been around for a while.  Excess goop can be expected from a fresh pod. All this affects the spread, and designers have to set the spacers, or rails, to make sure an even and complete layer of goop is spread each and every time.

The example above is a good one, edge to edge, and evenly spread. Amazing what careful hand-building of things can do. Now, the next step is to use the same quantities, spacings etc, in a more controlled environment of a factory. There is a big jump in investment to go from hand assembly to mechanization, and that has been our problem so far, but we are getting closer to an answer.


Denise Ross said...


I'm a huge fan of your site and project. Marvelous. Inspiring. Fun!

Congrats and keep up the great work!

Bob Crowley said...

Denise, thanks! Likewise, I been a fan of for quite a while. Viewers can go there by clicking on your name, above. Denise has created a great resource with a wonderful sense of fun and discovery!

Jason Edwards said...

Spread is lookin good!
I was amazed at a recent discovery of a sheet of type 57 from probably the 70s, in which I found the chemical pod still sealed and relatively full of chemistry... it seems that the old Polaroid pods were much more robust and keep sealed much longer than anything Polaroid made post-1999-ish. Perhaps it is because of the use of lead pouches in the vintage years, as you say?