Large Epoxy Granite Vmc Project

[rwm]

This is awesome! What do you use for mold release? Is the melonite enough?
I agree about reinforcing those sides some more. They may bulge and become convex. How about a perimeter of angle iron?
R

 

[gt40]

This is awesome! What do you use for mold release? Is the melonite enough?
I agree about reinforcing those sides some more. They may bulge and become convex. How about a perimeter of angle iron?
R

I will be reinforcing all castings with re-bar. As far as mold release, carnauba wax works with the melamine coating pretty well.

 

[jumps4]

this is a kewl project
I was interested in a build like this years ago but never did it
thanks for posting your build
Steve

 

[H8deadW8]

Neat project! I formulate epoxy resins professionally, and I enjoy seeing new uses for my favorite polymer!

What epoxy are you using? I would suggest you have a plan in case the resin exotherms. I have personally had a 1L batch of resin with a quoted potlife of 4 hours exotherm in a half hour. The worst case scenario for exotherm is mixing a big batch of resin in an insulating container. Using a metal mixing container should help, as will adding the filler. If it does run away, there won't be much you could do to stop it-- a garden hose might be the best option.

We generally use 5mil nitrile gloves and safety glasses as the minimum personal protective equipment in our polymer lab. The amine curatives (B-side) can burn skin and eyes. The actual epoxy (A-side) tends to be more benign, however you can become sensitized to it with repeated exposure. If you get any resin on your skin by accident, wash with soap and water-- never acetone, as this allows the chemicals to penetrate your skin.

The A-side and B-side really should be weighed out on a scale prior to mixing to make sure you get the proper mix ratio. This ensures that you get the best cured properties. Also, thorough mixing is critical.

Sometimes we have filled samples that settle during cure. Essentially, the particles fall to the bottom of the sample, producing a resin rich layer on top and a filler rich layer on the bottom. Since epoxy shrinks on cure (though less than almost all other polymers), the resin rich layer contracts more than the filler rich layer, and the sample bows. You are shooting for a very high filler loading, which should prevent that, but if the resin does pool on the surface of the casting, it may be worth removing before it fully cures.

Hopefully the resin manufacturer provides a recommendation for cure time. Most ambient cure systems I have come across continue to gain strength over the first week or 2. Heated post cure improves the properties of most epoxy formulations, but you have to make sure the heat is applied slowly and evenly to prevent the possibility of warping.

I wish I could come help out with this project, but I'm on the other coast. Please let me know if I can be of help remotely.

Nic

 

[gt40]

Thanks for your interest. I have made a few things as a hobby and some boat projects with composites but nothing on this scale. I have had the exotherm experience and am concerned about it. I will have to mix about 7 gallons worth of epoxy and then put that into a premixed aggregate mix of around 600 or so lbs. I am shooting for 12% epoxy to aggregate ratio by weight. From what I have been researching, the aggregate will slow down the curing and limit/prevent exotherm.

I bought us composites 635 with the slow hardener, based upon successful application in a few builds on cnczone and the price wasn't as expensive as some. http://www.uscomposites.com/epoxy.html

I have spoken to the manufacturer and to someone who used it in this application and this setup offers low viscosity and long curing time. from US composites website:
"Our slow hardener should only be used at temperatures above 80 degrees unless an extremely long curing time is desired. Temperatures below 70F during the cure can result in a 2-3 day drying time."

My plan is to pre mix the aggregate with a 6 cf polymer barrel concrete mixer, mix the epoxy and add to the rotating mixer as fast as I can mix each gallon of epoxy. Alternatively, I could put the epoxy in a large container mix it at once and then add to the aggregate. I will use a large rotary mixer and properly mix the a&b portions. after it is the right consistency, I will pour into the mold and pack it + attach a bench grinder motor with eccentric weights to vibrate it bolted to the base of the mold.

If you have any suggestions, or concerns about how best to do this, I would welcome any input.

 

[dracozny]

normally I would suggest a vacuum Chamber instead of a vibrating table on such a thing but due to it's size that may prove difficult. It would remove any air trapped in the epoxy in extremely short order.

 

[brav65]

Would a vacuum bag work? You can buy bag material on a roll then just seal the ends and apply the vacuum with a pump.

 

[gt40]

I have a pretty serious vacuum pump and have done some infusion composite stuff but consensus on the cnc zone monster thread is that the thickness of the piece and viscosity prevent large epoxy granite stuff from vacuum. I am going to pack the mold and vibrate. The mix should only be 12-14% epoxy

 

[dracozny]

I have a pretty serious vacuum pump and have done some infusion composite stuff but consensus on the cnc zone monster thread is that the thickness of the piece and viscosity prevent large epoxy granite stuff from vacuum. I am going to pack the mold and vibrate. The mix should only be 12-14% epoxy

I haven't seen such a thread but they make large rigid boat hulls under vacuum. over at this vid they are using a vacuum to degas before the pour of a granite sink.

the main thing I have seen from some people making test videos of such a thing is failure to apply a proper release agent to the mold itself. A combination of vacuum chamber and vibrating table is not a bad idea either. The vacuum is going to remove all of the trapped air. The vibrating table will help settle any clumped aggregate.

 

[gt40]

I haven't seen such a thread but they make large rigid boat hulls under vacuum. over at this vid they are using a vacuum to degas before the pour of a granite sink.

the main thing I have seen from some people making test videos of such a thing is failure to apply a proper release agent to the mold itself. A combination of vacuum chamber and vibrating table is not a bad idea either. The vacuum is going to remove all of the trapped air. The vibrating table will help settle any clumped aggregate.

I saw that video- the viscosity for the mix I am using is a lot thicker and has different aggregates.

Here is a good commercial example of the process. They show the mixed aggregate being packed into molds: