Coroplast Safety Cutters

These little beasties I made over 9 years ago and posted to Thingiverse, a practical design I designed to help build my entry to the 2015 Hackaday Challenge. With my printer at the time - IIRC, a Microcenter Makerbot (a FlashForge Creator something) clone. I wanted to give it a shot with my Bambu Lab P1P with 0.4mm nozzle. 

….and I must say, WOW. Perfection on the first P1P print. Razor blades just slid right in with perfect clearance - enough friction to hold the blade in place, but an easy assembly. The cross-cutter took a little tapping to make sure it was fully seated (perhaps the notch in the bottom cutter cover was not deep enough). Note when you have used the cutters enough to dull them, the blade can be turned around for a second set of cutting edges.

What is coroplast? If you are not familiar with it, it is plastic corrugated cardboard. Its most common application (in the USA) is political signs… which means after every election, if you are clever, you can source tons of the material for free. A friend of mine has been using Coroplast to make light duty shelving for his hobby storage bins. My use for coroplast was for an “entropy wheel” heat exchanger. I still have not completed that project in 8-9 years, but, you know, one day. Link: 

https://hackaday.io/project/2517-entropy-wheel-heat-recovery-ventilator

in case you are curious… In 2015 I moved from Maryland to Arizona then in 2018 I moved to California so the climate changed >>rather dramatically<< from what I had been used to in Maryland, so the efficacy of a heat exchanger readjusted my priorities. Odd things you learn: most of the USA has issues with radon in houses with basements… something I learned when I moved and sold the house which then had to be tested for radon… and failed… and needed $$$ spent on a ventilation system to mitigate the risk. Arizona, OTOH, has a relatively low radon content in its very-common granite (the whole state is like several miles deep of solid and decomposed granite), and also basements are rare there. Most parts of the USA, though, a heat recovery ventilation system 1) saves money, 2) might reduce lung cancer risk. My theory is that the small channels of the coroplast will make a fairly decent heat exchanger. Entropy wheel exchangers are very common in large buildings that have high requirements for fresh air exchanges (domed stadiums, office towers). Commercial units (up to about 14ft diameter) claim efficiencies of like 80-90% when using hygroscopic coatings. Hard to believe, so I had to try it myself. FYI, Panasonic makes bathroom fans with heat exchangers, and other companies make whole-house fresh air exchangers that only get 50-60% efficiencies due to their limited size, IIRC. Counter-flow exchangers need to be HUGE (like 100ft long) to reach 90-ish% efficiency, but commercial entropy wheels are typically only about 14 inches thick… the efficiency claims seem impossible. Key to the efficiency and the nominal difference between an “entropy wheel” and an “enthalpy wheel” is special coatings that also preserve internal to external humidity ratios. Relative humidity has an enormous role in the overall efficiency, and accurate humidity measurement is very difficult. Typical cheap sensors are around 5-10% accuracy while high efficiency requires more like 0.5% accuracy. New 2023-era sensors are MUCH closer to meeting that specification.

Useful information about Coroplast: 3M High Strength #90 is one of the few adhesives that will work on it. Coroplast is a low-surface-energy polypropylene, which means glue generally does not like it. Typically it is treated with high voltage plasma to “activate” it so adhesives will work.