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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Epoxy granite - has anyone tested for Young's modulus?
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  1. #21
    Join Date
    Oct 2017

    Re: Epoxy granite - has anyone tested for Young's modulus?

    Ok, news. Yes, as suspected, their quoted modulus is compressive only (though they do not state this). I independently corroborated the test values by updating an FE model. EG is just naff in flexure. Here ends the investigation.

  2. #22
    Join Date
    Mar 2017

    Re: Epoxy granite - has anyone tested for Young's modulus?

    Quote Originally Posted by peteeng View Post
    Hi Goeman - To discuss your comment a little bit more:

    Obviously machine designers need to understand if their machine is going to be stiff enough for the intended purpose. Yes very much so

    I'd just have zero confidence in using any general epoxy granite data or any specific epoxy granite suppliers results. Zero is a bit harsh but it is good to be sceptical

    One of the (many) things that makes resin different from metals is that stiffness is massively dependent on how it's cured and the conditions it will be used in. Actually stiffness is partially dependent on cure but strength is very dependent on cure If you select a resin that can achieve a "crisp" cure at ambient then that's all you really need. The aggregate being the dominant stiff additive does most of the work

    In general, the optimum stated stiffness and hardness is only achieved with high temp curing that follows a specific schedule. You then add a lot more variables when you use resin as part of a composite. Optimum stiffness of the epoxy can be achieved at ambient if you pick one that does not need special post cure. By optimum I mean it is suitable for CNC machine parts. Having said that yes epoxy does benefit from post cure if you can do it

    Re -epoxy curing. Heres the data sheet for an epoxy being used by one the forum members. It states it fully cures at 15deg C.

    Trying to figure out how epoxy granite compares to steel or aluminum in a Young's modulus table would not be massively useful for a machine designer IMO. Epoxy granite is an entirely different material requiring a different design. Could you propose how to design then if it requires a different approach?

    For one thing, epoxy granite will not bend much before if breaks (unless you mess up the curing). If I was reproducing my Epoxy granite machine base to sell, the only data I'd have any confidence in is from stress testing my specific design. All the companies making EG and UHPC machines regularly test their materials for strength and stiffness and use this information for machine design. Testing is done with representative samples that allow accurate stiffness results.

    Testing how much force could be exerted on the center of the gantry before if snaps would be far more useful. Breaking the gantry would not give anyone any useful information about the machine except perhaps how many elephants it may hold up .

    Testing a small piece would not be that useful as it's performance is not linear. As it's mixed with stones you couldn't simply extrapolate that a 10" thick slab would be five times a stiff as 2" thick piece. Testing the right size piece is critical to get accurate modulus results small is a relative statement and we have to presume the people doing these tests know what size is required for the purpose. For instance I do calculations on very large composite yachts, say 20-50m long or more and we design on coupons 25mm wide by 120mm long which are not very big compared to the yacht...

    In the event that a design isn't strong and stiff enough, there are more options than simply increasing thickness with epoxy granite. It can be easily reinforced with steel tubes, for example, like they do with concrete. Using steel in concrete is to control cracks and to improve the concretes tensile strength. Both are not needed in a CNC machine. Plus you introduce stiff loadpaths to the machine that may be counter productive

    To conclude - The stiffness of any material used is needed to design a machine. The stiffness can be tested for and if the recipe is consistent and the casting is consistent then the resultant modulus is consistent. The rules or domain of static linear analysis apply and the strains in most stiff cnc parts is very small and nowhere near the failure or yield point of the material. If there is another approach to machine design, I and others would like to know about that so please share Peter

    attached is a dat6a sheet from an epoxy being used by one of the members. It states it fully cures at 15degC but it does recommend a post cure schedule of 40C for 15h. If you need a high Tg or HDT, which machines don't unless they are operating on ovens then it needs to be high temperature cured....

    Designing parts made using epoxy where stiffness is the key factor is what I do for a living. Your understanding of the properties of resin and what effects them is off.

    Particularly on the importance of using resin as part of a composite and the effect of temperature on cured and curing resin.

    Just because a resin is designed to do it's initial cure at room temp does not mean it achieves maximum stiffness from a room temp cure.

    There is a reason why we post cure parts at high temperatures even if the initial cure was at room temp. Resin cured at room temp is far less temperature stable. That is universally true.

    In other words, even the best room temp resin will start to soften in a warm room. Fully cured resin can become flexible if you leave it in a car on a sunny day (or around heat generating machinery).

    Curing or post curing at higher temperatures increases the stiffness of the part and also raises the temperature that the resin would start to lose stability (I.e. soften and become flexible).

    Someone expecting to build pro grade epoxy granite machines for a living should be capable of building a simple curing oven.

    Aside from following an optimal curing schedule, the best method of compensating for undesirable properties of resin (I.e. that it's soft, brittle and expensive) is using it as part of a composite.

    Adding stones makes it cheaper and harder. I can say from personal experience that epoxy granite marries extremely well to a steel tube skeleton too.

    Steel square tubes are strong, stiff and hold screw threads well but they have horrible acoustics. Epoxy granite has exceptional acoustics but is potentially brittle and poor at holding screws. Together you have the best of both.

    If you look at the various online epoxy granite how to vids and tutorials, you'll see that most cast epoxy granite machine bases around a steel or aluminum skeleton. It's just easier for attaching rails and ball-screws (without cracking the epoxy granite when you tighten screws).

  3. #23
    Join Date
    Aug 2006

    Re: Epoxy granite - has anyone tested for Young's modulus?

    Quote Originally Posted by Mud_ View Post
    Ok, news. Yes, as suspected, their quoted modulus is compressive only (though they do not state this). I independently corroborated the test values by updating an FE model. EG is just naff in flexure. Here ends the investigation.
    But isn't this the case with all materials of this sort(UHPC, EG etc)...?

  4. #24
    Join Date
    Jul 2018

    Re: Epoxy granite - has anyone tested for Young's modulus?

    Hi Geomon - I have no issue with the fact that post cured epoxy is better then ambient cured. This is like saying a stress relived steel frame is better then one that is not SR. These are relative conditions.

    Also epoxy is a great adhesive and it sticks to metals really well so using metal inserts makes sense for machining purposes. But DIY Makers need a simpler path then what the "pros" use. My current view is that there is not enough consistent "proper" engineering data out there on EG that makes sense (which agrees with one of your statements). Another OP has posted this article and its good as the researchers have tested in tensile, flexure and compression. Unfortunately they did not publish the compression stiffness. But the key point is that the tensile and flexural modulus are very different. If they published compression stiffness this would be a good indicator of the non-isotropic behavior if this is the case (or poor testing for the purpose). This could be a testing bias in that the tests measure the strain over different intervals. This is why I am testing my own material to determine if its worthwhile using EG or UHPC or another composite. My current stance is that although fibreglass is low modulus (30GPa) at least I have decades of correlated data that shows the laminates I design have the same flexural, tensile and compressive stiffness so I can assume they are isotropic making design a reliable process. That's all that I want - a reliable design process or reliable data so good decisions can be made. Peter

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