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  • casting plastic nuts

    I'm about to cast some epoxy around an acme rolled thread rod for a mechanism I'm building. The plan is to apply mold release to the rod, then suspend the rod in a channel and seal the ends. Epoxy is then poured into the channel, and the end result is a plastic nut. The usual problem is then to get the rod out of the casting without damaging either. Previous projects using epoxy nuts have seemed to work out, so I have no question with the use of epoxy for this purpose. Considerable force was required to break the acme rod loose from the nuts, which were about 2 inches long at their longest. Where an easy turning of the rod was required, I ran a tap through the nut to loosen things up. Play remained very low and all was well.

    Now though the nut I'm making will be about five inches long, as it will be the only thing keeping the rod constrained to its axis. When I fill the mold, I plan to fill it only enough that the epoxy doesn't bridge across the upper part of the rod as it's laying horizontally in the mold. The result will be that the rod is roughly 3/4 surrounded by epoxy. I'm thinking that there will be enough flex then to allow the rod to break free and turn in the threads without too much friction. If the epoxy completely surrounded the acme rod, then it can't open up slightly to allow the rod to break free. This is the part of the process I'm putting up for discussion.

    An option that I see is to make the nut this way, but then if it's seen that the extra strength of a fully surrounding nut is needed, the pour can be completed after breaking the rod free and running it in and out several times to 'run it in'. It will have to be wrenched free again once the second pour is cured, but that should be easy enough as there would be less surface area to break the bond with.

    I see also that I could make this second pour adhere to one side of the nut, but lay some mold release along the other side. When cured, I would have a fully surrounding nut, but with a gap which I could pry apart slightly to aid in removal of the acme rod, and to help set up the degree of friction and play between the parts.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    How about coating the rod with PTFE spray it will then come free much easier.

    Peter
    I have tools I don't know how to use!!

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    • #3
      I have done that. I find that regular mold release agent works pretty well. Maybe I'll do a test with ptfe spray, then mold release agent. I'll often spray the acme rod with ptfe anyway, so why not try it. I do want to avoid having a release problem in a part where there's already much time invested-

      I'm trying to brainstorm some alternative ways to make custom nuts, and one thing I do prefer is a cold process- one in which there wouldn't be any expansion effects in the materials to deal with. From time to time I do think of the low melting temperature plastic pellets I got somewhere- the plastic can be heated and poured at less than 100C, and is quite slick. I think if it was poured into a mold which had numerous pockets which would act to captivate it, it would work well as a nut. Just don't overheat it-

      With my project, the epoxy is going to help bond the structure together and make it more rigid, so I'll stay with it.

      I did try mixing graphite into some epoxy a few days back to see if there would be some positive self-lubricating effects. I found that it weakened the bond to the substrate, but didn't add anything significant as far as lubricity.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Rather than doing a partial pour, would it work to put a piece of something soft - a closed-cell foam such as expanded polystyrene perhaps - along the top of the thread and do a full pour? Once the soft layer is removed (possibly by heating it) the slot would give purchase for encouraging the nut to let go of the thread.

        The low temperature plastic I know as Polymorph softens at 62*C and is excellent stuff for moulding, though I don't know how it would last as a running fit. If you heat it to actual melting temperature it is an excellent adhesive! There are some very poor quality versions on the market, so be aware of what you are buying.

        There is an even lower temperature version called Coolmorph which softens at about 40*C but I haven't worked with that.

        George

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        • #5
          i have not really "researched" this, but i once was pouring a bearing for a spindle and i put some ptfe spray into the mixture. the bearing was tight fitting, but after some time it became loose. my explanation: the solvent in the spray evaporated and made the bearing larger. maybe if you put some acetone into your epoxy you will have the same effect.

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          • #6
            I've never done this so take this for what it's worth....

            I'd bet if you look at that rod under magnification it wouldn't look very smooth. Theoretically the reason it's so hard to turn initially is because the epoxy makes a perfect mold of the rod including all the micro features on the threaded surface. If that holds true then there are a couple of things you can do to reduce break out torque.

            Use high precision acme rod - high $$$ too.
            Carefully debur and polish the rod all the way down into the root of the thread.

            Finally: How about breaking the rod loose after the epoxy sets, but before it reaches full strength? Say in the time frame of 1-2 hours after the pour. All those micro irregularities would still exist but they would deform with less force.

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            • #7
              I use clean clay (modeling clay) for a flexible parting line when molding Aluminum Devcon bushings. Plane old Johnson Wax is the best release agent I have found.
              Byron Boucher
              Burnet, TX

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              • #8
                You could make a test sample ring and meaure its deflection before breaking to see how far you can go during the release process.
                During release it might help to apply some fluid that capiliaries into the crack and helps it stay open.
                If one gap is ok how about two - ie pour only half the nut at a time, twice, then fettle and join.

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                • #9
                  Why are you using epoxy? There's a thread here on making zero-backlash threads from Delrin/Acetal. Seems pretty easy and several people have gotten great results.
                  Last edited by Tony Ennis; 07-29-2014, 08:37 AM.

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                  • #10
                    Originally posted by Abaker View Post
                    I've never done this so take this for what it's worth....

                    Finally: How about breaking the rod loose after the epoxy sets, but before it reaches full strength? Say in the time frame of 1-2 hours after the pour. All those micro irregularities would still exist but they would deform with less force.
                    To my mind, once the epoxy has begun to set, yet is till soft (if possible) give the rod a turn to break it free and maybe create the clearance that is needed to have a smooth running nuts

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                    • #11
                      What is epoxy? Not a rhetorical question. Just like there's about 20 different battery chemistries that use lithium so there is no "the" lithium battery, there must be 500 kinds of epoxy.

                      Some turn to mush at 140F and some are rated by the mfgr for continuous use at 450F.

                      Maybe a temp cycle or two would break the metal-epoxy bond. I wouldn't try it with stuff that turns to mush at 140F but if you buy stuff the mfgr rates for continuous use at 300F and alternate boiling and freezing that should break it free after a cycle (or a couple cycles)

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                      • #12
                        Perhaps coating the screw with soot from an acetylene torch or candle would do the job.

                        bob

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                        • #13
                          I've found, when bedding rifle actions in epoxy, that good old PAM cooking spray works as well as anything as a release agent. Another thing that helps in breaking them loose is putting the whole thing in the freezer for awhile.
                          Jim

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                          • #14
                            Most epoxy that you're going to get without buying specialty stuff or convincing someone like Huntsman or Momentive to send you a sample is going to be medium molecular weight DGEBA with some type of amine hardener (and possibly a reactive dilutant) and a glass transition between 80C and maybe 120C tops. When I was working on Epoxy Granite over at cnczone, we used the slow hardening kind from U.S> Composites which contained a reactive dilutant to thin it but still was of the same general properties above.

                            Mold release is definitely the way to go. Everything else is half baked at best. Why not use a product that is $10 a can and designed to do exactly what you want?

                            If I was going to make nuts which I've never done, I think I'd try a filled epoxy filled with a fair amount of hexagonal boron nitride. This is the white kind that is soft and lubricating, not the cubic kind thats used for cutting tools. The original moglice patents are reasonably findable on the internet. I'd also try to add a fair amount of oil free bronze dust.

                            I also think that Abaker is spot on. Epoxy is such a good replicant that the surface profile will match the mold to within tenths or perhaps less in this configuration and give 100 percent contact in a way you'd never get with metal. The shrinkage of epoxy is very low but it will shrink fractions of a percent which will make your nut even tighter. One might be able to fight this by using a piece of over-spec acme rod if they were feeling adventurous. A fair amount of the lubricity fillers will likely be needed and it may require levels of mixing that aren't popsicle stick in a cup achievable.

                            Anyway. Good Luck.

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                            • #15
                              I made two attempts at making an acme nut a couple months back. I used J&B Weld thinned with acetone as per manufacture recommendation. I used wax and silicone powder spray as a release agent on the first one. It came out of the mold okay (a piece of plastic rod) but it was so tight on the acme shaft it would not budge. I eventually broke it trying to break it loose. (adding acetone to the mix cause the epoxy to shrink as it cures) On my second attempt, I put the acme shaft in the lathe, heated it while spinning it with the lathe and rubbing wax on the shaft to get a thicker layer of wax. I used the silicone power spray again. The resulting nut was tight but when I heated the acme shaft, I was able to screw the nut off the shaft. After a little clean up I had good fit between the nut and the acme screw.
                              Note: adding acetone does improve J&B Weld "pourablity" but it takes much longer to cure, at least 48 hours in a warm room. I let it set out in the sun after the first 24 hours.

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