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Machinable epoxy

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  • Machinable epoxy

    I want to make a non-conductible prototype that will eventually be made of injected plastic. Making and modifying the injection mold is the hard way to do it, so I want an epoxy prototype that can be altered easily if I want to change design details. It would just require mixing up some more 2 part epoxy and filling the holes, then re-machining.

    Is 2 part epoxy easily machinable? Use low speed or high speed? Is it tough on cutting tools?

  • #2
    Belive it or not I have had really good results using Bondo to make prototypes. It is really soft right out of the can but I mix in a bit more polyester resin and it machines pretty well. Still soft but easy to add to and cheap. I have used it to make sand castings and latex moulds. I dont' know if it is conductive.



    • #3
      There are so many different forms of epoxy about, that it would be difficult for someone to tell you ...
      I've had some stuff that is like boiled sugar sweats (candy).......same as, in every way ...very brittle and shatters as soon as you look at it.

      and there is other stuff that never quite sets,,,,,,,,,it would be like machining rubber.

      so epoxy is different and varied.

      all the best.mark


      • #4
        I'll second the bondo.

        Check boating sites for various fillers used for epoxy and polyester resins, they've got hundreds of different types.

        Bondo itself is pretty machinable, easily filed and sanded, but should use water for a cutting fluid as it will get sticky sometimes when warmed up. Very easy to add material or fill unwanted holes when desired.

        I think the typical filler in bondo is talc so you can essentially tailor it to your needs by adding more talc or resin.



        • #5
          Two that I've used are Devcon Epoxy( F) Aluminum and Devcon plastic steel(A) both cure over night and machine quite well dry.


          • #6

            Duct Taper:
            One thing you did not mention is size of the Item. Hercules brand epoxy plumbers putty should work for small items. For larger stuff go with the bondo.
            I worked 6 years in and R&D shop at a yacht yard . we made all of the patterns of bondo wood and polyester paint. We built with wood added fillets and filled screw holes with bondo sanding was done at 80 grit. We then painted with gray primer. The form was refined with 180 grit and small inperfections were filled with evercoat a 100% talc filler polyester putty. The form was then painted black and wet sanded with 320, 400, 600 some areas were buffed and everything was waxed with five coats of mold release wax. Some of the proto type patterns were 50 foot long .
            If you are making something of all bondo consider a fiberglass reinforced formula. A Stanley sure form plane AKA cheese grater is great for roughing the shape when the bondo is half kicked. Be careful if you mix a lot or have a heavy cross section it can get real hot. Also piles of shavings can smoke DAMHIK .
            There are lots of formulas of polyester putty. Basically it is a mixture of casting resin laminating resin and various fillers; ground walnut shell,micro-beads ,fumed silica ,fiberglass chop,talc etc
            Another trick is making your own putty of aerosil Fumed silica and gel coat to touch up small holes and low spots.

            And as the others have mentioned it is possible to customize the putty by adding fillers or thinners like resin or styrene monomer.
            Tin Falcon
            Last edited by Tin Falcon; 08-13-2007, 08:01 PM.
            Ad maiorem dei gloriam - Ad vitam paramus


            • #7
              Originally posted by kendall
              I'll second the bondo.

              Check boating sites for various fillers used for epoxy and polyester resins, they've got hundreds of different types.

              Bondo itself is pretty machinable, easily filed and sanded, but should use water for a cutting fluid as it will get sticky sometimes when warmed up. Very easy to add material or fill unwanted holes when desired.

              I think the typical filler in bondo is talc so you can essentially tailor it to your needs by adding more talc or resin.


              Ken : Talc is one of the fillers it is considered a premium filler. A premium product with 100% talc typicaly cost 4 times as much as regular bondo. It is twice the money for a can half the size . However for filling small inferfestions it is well worht the extra expense it cures fast 5 minutes as apposed to 20 and it sands easily with less loading of the sandpaper. A premium bondo such as evercoat would be excelent for filling inperfections in fabricated steel assemblies that are being painted.

              Water would be OK for a coolant on a marine grade bondo . Automotive bondo uses filler that absorbs water. that could be a problem if filling holes with more bondo or painting.
              Ad maiorem dei gloriam - Ad vitam paramus


              • #8
                Thanks for the input. My prototype will be small, 3" x 2.5" x 1" and box shaped. There will be electrical wiring in a recess on one side of the bottom that will then be filled with epoxy when complete. I will start with a block of material a little larger than the finished item and mill out all the recesses with my Bridgeport. The box is limited to those outside dimensions so I need to try different configurations of the components so that everything fits inside the box. Once I know that then I can make the injection mold.

                I mentioned epoxy because I have 15 gallons of the stuff and the Part A is coagulating in the jug so I want to use it up. But I do also have automotive polyester filler and have used that for a few things. Tonight I mixed up some epoxy and put it into a square container to let it harden. I will check it tomorrow to see if it has hardened enough to mill. If not, I guess I will try the bondo.

                Then I will make my million bucks... or just have fun making the thing!


                • #9
                  old epoxy has a crystallised marzipan substance on the surface.
                  important.... do not skim this off.

                  mix this in well.

                  put epoxy container into hot water hot as you can get it ...
                  if you don't, the stuff that makes it work will be crystallised in the mix ...and separated out .

                  if spraying you may loose the chrystals content by filtering dont want to do that either.

                  when the mixture has warmed up stir ....and change the surrounding hot water for another batch.

                  the Chrystal's will melt and be absorbed into the stuff....they are needed to make it set-up.

                  this is the way of reviving old epoxy

                  all the best.mark
                  Last edited by aboard_epsilon; 08-13-2007, 08:47 PM.


                  • #10
                    Thanks Mark! I was ready to throw the crystalized stuff away. I could see $$$ going to the dump. One of my 5 gallon jugs has about 3 gallons of crystalized epoxy left. I poured off about a gallon of it that was not crystalized and gave it to a friend. I will try the hot water bath to see if I can save it.

                    You said the crystals were on the top, but this stuff is on the bottom of the jug. If that matters. My second jug has about 3" of white crystals on the bottom so I will give that the hot water treatment too.

                    The jug of Part B seems to have no problem.


                    • #11
                      I don't know if you can get your hands on this easily, but I think it'll do the job you want...
                      The next is expensive but allows very fine detailing...


                      • #12
                        Machinable Wax

                        Have you considered machinable wax? It is available from ENCO, Page 683 of 2007 Master Catalog. The wax comes in rectangles and cylinders and can be "glued" together with cyanoacrylate instant adhensives. Advantages are that they can be very easily machined and can be used again. Just melt incorrect shape, along with the machine off scraps and recast to desired shape and machine again.

                        I haven't not used them but am considering them for a future project.


                        Being ROAD KILL on the Information Super Highway and Electronically Challenged really SUCKS!!

                        Every problem can be solved through the proper application of explosives, duct tape, teflon, WD-40, or any combo of the aforementioned items.


                        • #13
                          Machining recently cured epoxy requires some care. Epoxy hardens well before the cure is complete so active hardener will be included in the dust produced from machining. Skin contact (or breathing) this dust can induce an allergic reaction, where the reaction varies based on one's body chemistry. There is a "transfer lag" between exposure and reaction for sensitized individuals because it takes some time for the immune response. Symptoms often develop 8-16 hours after exposure so the cause of the reaction may not be recognized.

                          Common symptoms in sensitized individuals are swelling and itching of exposed skin, sometimes increasing to overall itching and swelling. Benadryl is helpful in managing mild symptoms.

                          Most people don't develop an epoxy allergy unless exposure is prolonged and some never develop an allergy but it is something to consider.

                          Location: Newtown, CT USA


                          • #14
                            A few points.
                            There is a excellent machinable epoxy made for the Foundry/Patternmakers
                            industry. I is used many times for core boxes and as a filler.
                            Don't know the name however.

                            Years ago, my company did what you are attempting now.
                            We spoke to a epoxy chemist and here are some of his comments.
                            Epoxy cast blocks or objects can vary in size, based on cure.
                            The heavier the crossection, the more heat generated since the
                            surface area cannot dissipate it fast enough.
                            This can cause shrink cracks, and distortion....even after machining.
                            To prevent the above, Aluminum or other metal fillers are added to help conduct thermo energy to the surface ! This can improve the strength of the epoxy as well.
                            Heat accelerates curing of epoxies.
                            Cooling epoxies lengthens the cure time, but provides a more stable material in the long run
                            True Epoxies are 1 to 1 or 2 to 1 mix ratios.
                            Polyesters and urethanes are mixed at higher ratios like 10 to 1.
                            Water/moisture accelerates curing of Urethanes.
                            Water/moisture affects curing of Polyesthers
                            Bondo absorbs water , and should be sealed/painted within 24 hours to stablize size.
                            If epoxies are still soft or sticky after cure, a low level of heat should be applied for 48 hours (160-180F !) to solve tyhe issue.
                            hope this helps
                            Green Bay, WI


                            • #15
                              I've been doing hoards of research on epoxy on the Epoxy Granite thread at Cnczone. While I've been designing a mixture of epoxy and additives to make machine tools, I've encountered a lot of epoxy information including a diamond filled thermal epoxy! There are a number of machinable epoxy composites for tooling over at

                              These guys seemed to have the most interesting selection of filled epoxies for tooling. If you get a slow enough setting one, the exotherm problems are smaller. Many of these benefit from several hours in an oven at 250F after they harden in order to achieve full cure and highest glass transition temperature.