PLA filament + heat = trouble

My card reader had been working pretty well. Then I put it in the back seat of my car to take it to show someone, but before the demo it sat  outside, on a hot day, for a couple of hours. As pet owners know, the interior of a car can easily get to 120ºF or more, and it did. The result was serious deformation of some of the printed parts made from PLA -- Polylactic acid -- the most common and easiest to print plastic filament that I've been using for almost everything. 

Right angles and straight edges under the least bit of tension became terribly distorted, and of course the card reader stopped working.

This permanent bending of PLA-printed parts when exposed to heat was well known to the 3D printing aficionados I sobbed to, who recommended switching from PLA to PETG, Polyethylene Terephthalate Glycol, which is good up to at least 160ºF. Even better would be ASA, Acrylonitrile Styrene Acrylate, but that is more difficult to print. I took the baby step and reprinted the deformed parts using PETG. At the same time I made several other changes to improve reliability:

• The original design had the supports for the control rod assembly parts bolted to the bottom support plate. That is not sufficiently rigid, particularly for tall parts, and also provides the opportunity for slight misalignment. The new design uses a rigid printed frame for the parts in the control rod assembly.
• The alignment pointers on the prism were not entirely inserted into the alignment channel before the rods contact the cards, which means that the alignment could be slightly off and a rod might hit the circumference of a hole. The new design added 0.2" of travel to make sure the alignment was complete before the card was touched.
• The control rod lock shape wasn't sufficiently wide at the mouth to force alignment for rods that aren't completely pushed in, perhaps because of waviness or bumps in the card. The new locks are wider.

• The cards were sometimes hanging up on the prism alignment pins. So I smoothed the pins -- I otherwise do almost no post-print finishing -- and coated both the pins and the alignment holes of the cards with a teflon lubricant. That helps a lot.
• I made a better alignment jig for sewing the cards, by 3D-printing sections of 6 that can be lapped and glued together. That makes it easier to sew long strings of cards.

• I changed the sewing regimen to double-knot the upper and lower strings in between the cards. With the strings just interlaced in sewing-machine style, it was too easy for the cards to slide along the strings, which changes the inter-card distance too much.

Here's a video of it running, again with the cards coded for testing with two 6-bit characters on each. When used in the real Engine they will of course be coded with instructions and memory addresses instead.