In Part 6 of my series on the making of the Wall-Saver safety cylinder, I discussed my difficulties with forming the center hold that fits over the crane and making ridged inserts to hold the brass in place. I also introduced the problem of plastic shrinkage, which basically obviated my entire attempt to use the real cylinder as a mold master.
What I needed to overcome the shrinkage problem was a new mold master, with outer dimensions like the original cylinder, but 1% or so larger to allow for a freshly –poured plastic cylinder to shrink to size. As long as I’m dreaming, I’d like it to incorporate a solid front face and uniform chambers properly sized for springs.
No such thing exists…so I made one. A CAD model was surprisingly easy to make (took about a full workday) and then, thanks to the magic of CAD, to scale up by 1%. Then I just needed tool paths and chunk of plastic and…I’d have my master. I made probably a dozen iterations of toolpaths, and crashed tools, and so on, but in the end got something that looked a heck of a lot like I wanted. I cut the first versions into some scrap HDPE to see if it looked right, and (after the many revisions and crashes) it did! I had a lathe by this point but it still wasn’t set up, so I chucked a piece of Delrin into the mill and turned it down to the outer dimensions for the cylinder and tried it out. Discovered my ¼” endmill, nearly brand-new, is too dull to cut plastic unless REALLY rigidly chucked, ruined the first try. But I ran the operations even so to make sure it works like I expected—and also found a better endmill that would work.
I was really pleased with that first master, and got a mold made from it into operation. The size was just right after shrinkage. But it never made it into production, because I ran into three major problems. First, it was very, very hard to get the cylinders to come out without bubbles; even the relatively thin liquid plastic had trouble getting into the mold and chasing all the air out. The second was that I had sized the hole through which the extractor rod passes improperly, and the extractor could rotate a fair amount. And finally, the center rod, used to form the hole for the crane, created problems. If it was too short, it wouldn’t seal against the top and bottom, resulting in excess plastic that had to be cut away (and, if at the top, trapped the rod in there and made it impossible to extract). Too long, and it would deform the thin structure at the bottom and result in an even worse fit for the extractor (as well as plastic so thin it broke very easily). The other issue was the general difficulty of extracting the center rod—I was using an threaded eyelet to screw in and pull, but that resulted in a limited lifespan for center rods, and also could occasionally fail completely.
Next time we’ll talk about how I solved all of these issues.