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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Potential new concepts in the process of epoxy granite castings
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  1. #1

    Potential new concepts in the process of epoxy granite castings

    Hello all,

    I have only made it through the first halfish of the semi-official EG thread, but I would like to offer up a handful of ideas I have had in the matter for constructive criticism (Or to find out if they have already been tried and if so, the results)

    In the past I have done work with composites in the realm of carbon fiber and fiberglass. For bigger layup jobs where it isn't feasible to hand wet everything, we use a process of vacuum infusion. In its most basic form, fiberglass or carbon fiber is laid up dry in its intended mold and then sealed with a very stretchy plastic film, allowing a vacuum to be pulled. Resin is then introduced from an external resivor and with the assist of the vacuum is infused throughout the part.
    Things quickly grow more complex beyond this simplistic explanation, with carefully thought out flow paths, resin injection points, the use of what amounts to perforated tubing to introduce resin to specific areas of the mold, etc. Here is a brief video of a the process successfully accomplished with a large part.
    Blah blah, boring, thats all great but how can it be beneficial to EG machine bases? In the composites world, this process allows compaction of the fabric pilings against a molding surface, 'automatic' impregnation of the correct ratio of resin into the part.

    Imagine for a moment: Begin by filling your mold to the brim with the largest particle size (1/5 the size of your overall thickness). Vibrate/agitate until absolutely no more can can be added without overflowing the mold. Move down to the next size of particles and repeat the the process. Continue the cycle, down to your smallest particle size; As long as you are adding material into the mold without increasing its overall volume, you are increasing density and approaching the ideal formula of size ratios (with smaller particles filling the voids left behind by the larger, etc, down to a fine flour particulate size).
    Seal the mold with a plastic film and pull a vacuum (compacting the filling in the process at a maximum of 14.69 psi). Introduce a thin, sow setting epoxy into strategic points of the mold, allowing the vacuum to distribute it evenly throughout the mold into whatever voids remain. Perhaps some of the metal reinforcement I have seem many people using in their castings could be hollow, used to help distribute the epoxy throughout the mold like veins in a biological body.

    Some issues I foresee: ideal ratio of particle sizes may be too finely packed to allow for resin to freely impregnate/flow throughout the mold. Perhaps mixing of the aggregates would need to be pre-rationed and mixed outside the mold and pre-wetted with epoxy.

    Thoughts, constructive criticism?

  2. #2

    Re: Potential new concepts in the process of epoxy granite castings

    A second idea, much briefer than the first:

    In the application of epoxy floor coatings to a concrete surface, one of the most crucial steps is the cleaning and preparation of the surface via acid etching/shot blasting/grinding. Muriatic acid is commonly used in a diluted form. This has probably already been thought of, but shouldn't the same concept be applied to EG castings? Pre-wash all aggregates above a certain size (Muriatic can be quite aggressive. Too small of a size would just be destroyed.).

    I can see very little, if any downsize to this idea. It can only improve/ensure mechanical bonding of epoxy to aggregate, via etching of the surface and the removal of dirt, oils, contaminants. Also, the acid needs to be neutralized afterwards, (baking soda/water wash) and probably followed up with an aggressive rinsing with clean water.

    Any thoughts, constructive criticism?

  3. #3
    Join Date
    Jun 2013

    Re: Potential new concepts in the process of epoxy granite castings

    I think you'd need more than 14.69psi to pull resin through a 200mm+ aggregate block.

    Sent from my MI 8 using Tapatalk

  4. #4

    Re: Potential new concepts in the process of epoxy granite castings

    I dont think you can get more than 14.69 psi, the pressure of 1 atmosphere. It could be done by applying pressurizing the epoxy reservoir though, completely possible. Thats actually a good idea.

  5. #5
    Join Date
    Aug 2009

    Re: Potential new concepts in the process of epoxy granite castings

    You would have to make sure the mold for the casting could handle the pressure, though. Otherwise, as the epoxy permeates the aggregate matrix, it will start to push out on the walls of the mold; and it may deform.

  6. #6
    Join Date
    Jul 2018

    Re: Potential new concepts in the process of epoxy granite castings

    Hi - Infusion of the aggregate would be quite simple using standard infusion techniques. Cleaning with acid would mean it has to be neutralised and I think this level of cleaning the aggregate is not warranted. If the mould top is level using vacuum is not needed if an infusion epoxy is used. It would seep down and self level and fill in all the gaps easily. People using std laminating resin have trouble because its too thick for this application. Pressure won't help much as the mould would have to be made very strong to take the pressure. RTM moulds are massive structures to prevent them changing shape or leaking. Peter

  7. #7

    Re: Potential new concepts in the process of epoxy granite castings

    Actually, the infusion worked pretty well. I did a 1.5"x3.5" sample, 8" long. (basicially a mold of a 2x4 wood :P ). With the injection point at one end, and the pickup at the far end, I only got about 4 inches of penetration. but those 4 inches are rock solid before it starts to peter out. A cut in half shows no air pockets/voids/dry spots, and surface finish is minimal pocketing.

    I did a few things wrong,
    -mixing generic fiberglass resin a bit hot so that I hoefully wouldnt have to wait so long to demold. Mistake.
    -No preheating to help thin out the epoxy. Potential mistake
    -only 1 injection point. Definate mistake
    -No filter on the pickup point, loss of fine sand as a result. Mistake

    I also wonder if I packed in TOO much aggregate, sand, etc as a result. Like I outline above, I first filled with the largest gravel of about .25", vibrating/slamming the heck out of it to settle it in. With that then emptied out into a plastic bucket i added and mixed/vibrated .125" pea gravel until the overall volume started to increase. Then added random sand with mixing/vibration until the overall volume began to increase.
    My question is this: With heavy vibration I started to see some stratification of material by particle size. How do you guys avoid this, especially when its in a mold and sticky with setting epoxy? I was thinking that perhaps I needed to limit vibrating motions to sided-to-side, instead of side-to-side and up-down. Might help?

  8. #8
    Join Date
    Apr 2006
    Throwing a cpl of cents in the pot.
    One could also raise the temperature of the sand to help resinflow, One might need ro eperiment on what temp tho to not impact the curing cycle in a negative way.

  9. #9
    Join Date
    Jul 2018

    Re: Potential new concepts in the process of epoxy granite castings

    Hi MFG - Stratification of the mix is a known problem. My suggestion is to use a single small grit size and not use a mix. If you use a proper epoxy infusion resin it will fill very easily. Since carbon fibre has a fibre diameter of 9um if you use a sand say at 2mm the porosity is huge compared to carbon. I have infused timber, sand, ceramics, plastic crumble, all infuse easily. CF is the hardest to infuse as the fibres are so small. For instance glass is 20um. If you are to make machine parts I suggest you use quadaxial glass cloth it will be stiffer then any granite or sand mix you can make, Use a fibreglass skin say 10mm thick and then fill with sand as sand is cheap. Peter

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