On 2/23/2015 9:46 AM, Kirk Wallace wrote: > I am using this site as a reference for making some telescope eyepieces: > http://home.fuse.net/astronomy/ > > Larry chose to use urethane to make the blanks for machining. My > experience with urethane is that recipes range from rubbery soft to > pretty darn hard, but not fully hard. I need something that can take a > .6 mm pitch thread and stand up to assembly cycles. PVC pipe is the > wrong color, threads okay, but the threads smear easily. Acetal works > very well, but can't be bonded or painted. Polyester resin might work. > I'm also considering injection molding blanks from PLA. > > I'm wondering, are there forms of urethane that have the same > characteristics as acetal? Are there other materials that would be better?
Try Smooth-On's Onyx. It's available in two cure speeds. Too darn fast and nearly too darn fast. ;-) It's also extremely black and can take a high gloss polish. It doesn't need post curing but baking it for a while in a 200F convection oven will make it stronger, a process that ordinarily takes a couple of weeks or longer to reach maximum hardness. Post curing of urethane resins should be done with convection heat. Circa 2001, when I first got into casting urethane resins, I initially tried an ordinary oven and direct radiant heat was always a failure. If you don't have a convection oven, a food dehydrator (which typically max out around 150F) will do if you heat soak the castings for several hours. A useful property of many urethane resins is they won't soften from heat below the temperature they are heat cured at, unless formulated to be soft at lower temperatures. That's good for the replicas of old car lenses I cast. I use a dehydrator for most of the curing and if the ambient temperature ever gets over 150F there will be more to worry about than a saggy tail light lens. That brings to mind another useful property of urethane resins, until the cured resin has aged long enough, higher heat can soften the castings. I've made several straight items from originals that were distorted. Make a mold of the warped original, make a casting and let it cure then dunk the crooked parts in 200F water until it softens just enough, then straighten and hold until it hardens, speeding up hardening with cold water if needed. Then I make another mold off the straightened item. Some times a couple of iterations are required to achieve a fully straightened master item. At 200F a pair of medium duty nitrile gloves are enough protection from the hot water for short immersions. Since two part urethanes are similar to thermoset plastics, heated and reformed items will tend to return to their as-cast shape if allowed to get too hot. One part I make for 1940 Nash cars seems to have warping built into its design. I did the molding, casting, straightening and molding process yet when the straight cast parts are post cured they warp just like the original Tenite* parts. Fortunately a bit of work with 200F water and straightening on my stainless steel stove top** sets them right. Since the part is being coerced back to its original cast shape, it stays that shape even inside a hot car - at least I assume so since nobody has complained about the part warping during a car show out in the summer sun. *Tenite is a cellulosic plastic made by Eastman. It's crystal clear unless pigments, dyes or other additives are put in and easy to injection mold due to its low melting temperature. Unfortunately that temperature is pretty close to what can be reached inside a closed car in direct sunlight on a sunny summer day. The formulation of Tenite used in the 1940's wasn't too stable. Even inside a dark garage the stuff would slowly deteriorate, turning yellow, then brown. Given enough time and the wrong conditions it will shrivel up and eventually crumble. But that's not the worst... noooo. The worst part about old Tenite is the smell as it goes bad. Imagine someone chugged the vilest booze you can think of, along with a limburger cheese sandwich, then vomited that into a cat's litter box that hasn't been cleaned for a month - than set that in the sun for a week. That might get close to the stench that plastic gives off. Tenite could be weaponized for area denial, scatter carefully aged chunks of it around places you don't want anyone going into... How such stank could come from a product made from wood? **It's a very appropriate stove for the process as it's a Nash Kelvinator freestanding stove with double ovens made by American Motors Corporation in the 1960's. Kitchen appliances just aren't made that durable anymore. --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
