Epoxy granite but graphite? - Page 2


Page 2 of 2 FirstFirst 12
Results 13 to 17 of 17

Thread: Epoxy granite but graphite?

  1. #13
    Member
    Join Date
    Nov 2017
    Location
    United States
    Posts
    458
    Downloads
    0
    Uploads
    0

    Default Re: Epoxy granite but graphite?

    One thing to add, if you cast the test blocks, don't just add weight until they fail and record the force needed for failure. In this case I don't think that data is very relevant. What you want is force vs deflection. Slowly add weight and record deflection, preferably with a dial indicator. Graph out the points and compare the graphs of the different castings. I don't think failure point is all that important for this application.

    I have also considered composites in a machine, but not powder or chopped. actual carbon fiber layup. For a base structure that doesnt need to move, I don't think there's any reason to use anything other than epoxy granite, but for special machine layouts where you need extreme strength in moving structures, I think a carbon fiber layup could be beneficial to get the best strength possible and keep weight down. Another added benefit is thermal expansion which is basically zero with carbon fiber.



  2. #14
    Member peteeng's Avatar
    Join Date
    Jul 2018
    Location
    dum dum
    Posts
    744
    Downloads
    0
    Uploads
    0

    Default Re: Epoxy granite but graphite?

    Hi Yobster - Granite is used because its available and its often used in monolithic form for precision machines. It's bulk modulus is about 50GPa some say 10-70GPa. Now epoxy is 3.5GPa so you need to use a high volume ratio of granite or anything else for that matter to create something stiff. Something to look into is steel metal fibres. They are increasingly being used in concrete reinforcement. Since they are 200GPa stiffness you need less to get to a reasonable bulk stiffness. These should be available through your local concrete supplier or they should know where to get them. They also should be cheap as they are mass produced for concrete. I did a quick search and there's lots of info on it so should be straightfwd to track down. eg

    https://www.bekaert.com/en/products/...-reinforcement

    There are plastic fibres as well but these are same stiffness as epoxy so not good for what you want. Plastic fibres are used for crack control, steel fibres are used to make structural slabs without steel mesh.

    If you pack the fibres into your mould very well and get say a 40% fibre volume ratio (you can figure this out with a small test mould) you will get 0.4x200=80Ppa which is stiffer then aluminium at 70Gpa. If you pack really well I'd expect to get to 50%Vf. Steel is the same stiffness as carbon fibre yet its much less cost. Moulds can be made from formply and use PVA as a release PVA is polyvinyl alcohol not PVA glue which is polyvinyl acetate and should be noted as PVAc. In fact if steel wool is cheap that's just as good!! So an epoxy steel composite will be quite damp and very stiff. Good Luck. Peter

    Oh yes - epoxy. Do not get a laminating epoxy they have thixotropics added. Which means they are thickened so the resin does not sag when you laminate on a vertical surface. You need infusion or pouring resins for benchtops that are clear. These will not need vibration as they will pour nearly like water. If you warm the mould surface and the steel wool through with a heat gun prior to pouring it will release bubbles easily. If you have access to vacuum pumps then you can put a vac bag over your job and it will fill perfectly as you can remove all the air and moisture from the mould prior to the pour. Use the vacuum to suck the resin in. Look up infusion it's a great process.

    If there is a machine shop nearby you could use the steel swarf. But it will be sharp and have oil on it so you would have to wash it first. Don't use the family washing machine for this!! Peter



  3. #15
    Member peteeng's Avatar
    Join Date
    Jul 2018
    Location
    dum dum
    Posts
    744
    Downloads
    0
    Uploads
    0

    Default Re: Epoxy granite but graphite?

    Hi Yobster - Granite has a modulus of around 50GPa. (Some say 10-70GPa) Epoxy is 3.5GPa so you need to have a high volume ratio of granite or any other stiff material to get a high bulk modulus material. Granite is used because you can get it in size grades and there is a ratio of grades to create a close pack arrangement. But I suggest you look at steel fibres used in concrete. Steel has a modulus of 200GPa so if you achieved a 40% Vf you would have 0.4*200=80GPa material stiffer then aluminium at 70GPa. You could also look at using steel wool. Easy to use friendly fibres and if you can get it at a good price then your away.

    https://www.bekaert.com/en/products/...-reinforcement

    use formply to make moulds and use PVA from the concrete guys as a release agent. Do not use PVA glue!! PVA glue is polyvinyl acetate and should be written PVAc. PVA is polyvinyl alcohol and is water soluble so it's easy to release things and is easy to wash off. Again the concreters use tonnes of this stuff so should be easy to get.

    If you have a machine shop nearby you could use the swarf or chips if its free. But it will be sharp and oily. So will need to be washed. If its from a CNC machine it will be in small strings not long strings ideal. But do not wash it in the family washing machine, guarantee that's trouble. Good luck. Peter



    sorry about the double up, first one didn't appear for some time so I thought it was lost.

    Oh yes Epoxy - Do not use a laminating epoxy as these are thickened so it does not sag on non horizontal surfaces. Use an infusion or pouring grade epoxy that has ZERO thickeners (thixotropics) . This will pour into and around your fibres and will only need a few bumps vs lots of vibration. If you have a vacuum pump you could infuse the mould. Look up infusion its a great process. if you go this way use a heat gun to warm your fibre stack and mould surfaces a little bit before the pour, this will help it flow and release bubbles.... again good luck

    On the subject of graphite it is often added to epoxy to make bearings, it is soft and slippery so I think its out for your project.

    Sorry for double up, was slow to come up and I thought it was lost....

    But I think that reconstituted steel from swarf is very worthwhile to pursue. Need a catchy name for it "newsteel" or Eswarf or Eric or rustystuff ...

    Last edited by peteeng; 04-18-2019 at 11:18 PM.


  4. #16
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    797
    Downloads
    0
    Uploads
    0

    Default Re: Epoxy granite but graphite?

    I just finished a CNC build using carbon fiber and epoxy granite. As far as composites go, it is an excellent combo for machine bases. Both have excellent properties (for milling) individually and they work even better together. The end result (if designed right), is incredibly strong, stiff and with better vibration damping properties than iron, steel or aluminum.

    While some people use the terms "graphite" and "carbon fiber" interchangeably, they are very different. For a good idea of graphite properties, think pencil lead. I.e. It's brittle and weak with none of the desirable properties for CNC that carbon fiber, has.


    Btw With carbon fiber, the type of lay-up has no relevance to the fiber to resin ratio. A hand lay-up just means you are laying carbon fiber in a mold, by hand. A hand lay-up is just a common when using prepreg.


    For applications where the ratio of resin to solids is important, relative precision is easily achieved by weighing out the resin and solids beforehand. I regularly do this when making my own pre-preg.



  5. #17
    Member
    Join Date
    Feb 2005
    Location
    Australia
    Posts
    2
    Downloads
    1
    Uploads
    0

    Default Re: Epoxy granite but graphite?

    Goemon, can you provide any information on the ratios of epoxy/carbon fiber/granite that you used for your CNC build ? I'm interested in a similar project.



Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Epoxy granite but graphite?

Epoxy granite but graphite?

Epoxy granite but graphite?