Large Epoxy Granite Vmc Project

[gt40]

I got the ground stock being patient on Ebay. The two 1/2"x3" x 36" pieces of Starrett Ground 0-1 were $60.00 total and the smaller 1 1/2" x 18" pieces of a2 were 30.00 total or about a $100 in ground steel for the y axis and the mount for the column

 

[RJSakowski]

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.

I worked for six years for a company which made epoxy composite slabs for calibration of medical x-ray systems. These slabs had to have extremely uniform composition, determined by scanning each with a CT scanner. An elaborate process for mixing, pouring, and curing was developed. Mixing was done in a vacuum chamber to eliminate air bubbles, pouring into molds from the bottom up, and curing purposely slowed by cooling to dissipate the heat from the exothermic reaction. After a prescribed initial cure period, the curing was "finished" in a warm oven. Finished is in quotes because I discovered that the curing process actually takes years to near completion. What we call set epoxy is actually only 10 -20% cured. I could verify this by measuring density over time. The composite would continue to shrink as it aged. Our molds typically held about 5 kg but on one occasion we had a 10kg product. Because we were concerned about temperature rise, I was using a thermocouple to monitor temperature When the temperature started to rise towards 35C, I had to take the pour outside and place it in a snowbank to slow the process.

Once epoxy is mixed and poured, there is nothing that you can practically do to control the internal temperature. The best bet is to start as cold as possible and try to move as much heat away from the exterior as possible. Cure slowly and gradually increase the ambient temperature to complete your cure. A final cure at temperatures above 120C is desirable. Large volume pours tend to cure from the inside out. We observed separations of the composite due to shrinkage during curing.

When epoxy composites are heated above the glass point, the composite becomes plastic. It can deform and if the deformation is held as the temperature is dropped it will set at its new configuration. Quartz counter top material is 93% quartz and other aggregate and 7% epoxy or polyester resin and they have to use special considerations to prevent warping. The glass point is in the vicinity of 120C and varies with formulation. My understanding of the quartz countertop manufacturing process is that a combination of vacuum, vibration, a pressure are used to make the product.

A bunch of rambling. I hope it helps you in your endeavor.

 

[H8deadW8]

I'm not sure where you can buy air release agent commercially. I can get it from BYK, but I don't think they would sell to someone without a chemical company. Most resin suppliers actually send free samples, so I bet 90% of my chemicals are free samples.

Acetone also works well to clean up uncured epoxy. Just remember that acetone goes through nitrile gloves pretty quickly and if there is resin on your gloves, the acetone takes it right though with it. Remember to use safety glasses too!

The typical epoxy system develops to glass transition temperature (Tg for short) of maybe 20-30C higher than the maximum cure temperature. The maximum attainable Tg is controlled by the chemical make up of the resin-- stiff benzene rings allow for high Tg for example. So if you buy a resin that claims a Tg of 150C, you only get that 150C if you cure to somewhere near that temperature. If you only cure at room temperature, the Tg will likely be 20-30C higher than room temperature. The resin manufacturer usually specifies a cure schedule, though I don't see a cure schedule listed on the US Composites site. If I were doing this project, I would let the casting cure for at least a week before attempting a post bake. I'd then set up a temporary oven around the casting-- maybe cardboard lined with aluminum foil-- shiny side in. Then I'd use an office space heater to blow hot air into a hole in the side of the 'oven'. Try to increase the temperature of the casting slowly, allowing time for the heat to rise uniformly within the casting. Be careful not to start a fire of course-- I once thermocoupled my space heater and was surprised to find the air coming out of it at 140C! Another option might be to leave the casting in the sun on a warm day.

A low tech method of testing the degree of cure of an epoxy is to rub it with a rag that has some acetone on it. If some sticky resin comes off on the rag then it is not very well cured. This works better if there is a filler or pigment in the resin, as it will leave a colored splotch on the rag.

I would be tempted to try mixing a test batch of epoxy granite ahead of the actual casting just to see its handling properties.

Nic

 

[RJSakowski]

Thanks for all the info. I am nearly done with the mold and have been busy radiusing all the inside corners and completed alignment and test fit of the rails - better to do this before you pour epoxy.

I have a couple of questions:

1. What would you suggest for air release agents for the US composites epoxy I already have?

2. What would you suggest as a cost effective means of mixing all of this? I leaning towards the polymer tub gas powered cement mixer from home depot- It is a fall type basically a big plastic barrel with fins inside.

3. If I went the cement mixer route, how do you clean it?

Thanks, in advance for any ideas

Re: 1, we waxed our molds. Our molds were Blanchard ground steel. I have used machinable wax on wood molds. I would think that ski wax would work. I have also used a product called Slide from McMaster Carr. It is a synthetic wax in a spray can.
Re: 3, We cleaned our mixing vessels with isopropyl alcohol. Acetone will work better but because of safety concerns, we used the alcohol. They don't really dissolve the epoxy but permit you to wipe the surfaces down. You need to clean up as quickly as possible. Once it sets up, there is no good solvent that I know of. You should be able to remove hardened epoxy from the polypropylene surfaces.

 

[gt40]

Update:

The base is curing. Big thanks to Darkzero for coming down and for all his help.

On to it...

Spent the early morning mounting 70 feet of rebar in the casting:





Next we mixed the epoxy in a five gallon bucket with a drill rotary mixer. Added the epoxy to 130lb batches of aggregate. The epoxy and hardener weighted 13lbs for 10% epoxy by weight. I went a little higher because of the mineral flour and chopped carbon I added to the mix and it seemed about perfect. It wetted out and yet left no epoxy pooled on top. Mixing:



Here is a close up of the mix:



We tamped the mix with a dirt tamping tool- basically a 8" square of steel on the end of a pole. I didn't get a pic of the tool but here is the result:



We vibrated the mold by bolting a bench grinder with an eccentric weight- this made a big difference and parts of the mix dropped as much as 3/4":



I would say definitely vibrate your molds...

After vibrating and adding more mixed aggegate, we screeded the top to provide a flat top surface:



End result was flat and I was very happy with my mix and the epoxy ratio as well as the US composite 635/slow hardener epoxy system. We had time to mix 4 batches, pack the mold and vibrate and weren't rushed. Temp was 70F/21C btw.

This what it will look like for the next 4 days during cure:



I have heard post cure by putting a box with a small heater can add to strength I will look for a box and maybe do that tomorrow. Now I just have to wait...

Thanks for looking and again, thanks to darkzero for all his help- mixing 650lbs of aggregate with epoxy is some work.

 

[darkzero]

No, thank you for having me over, even though you put me to work!

But seriously, I enjoyed helping out, it was a lot of fun helping & seeing the process. I think you're crazy but I like your motivation & what you are doing. I don't think I would have the patience and well I don't have the workspace either. Just sorry I didn't last as long as you guys mixing up the batches.

Can't wait to see the first piece completed, you as well I'm sure. I'm game for the next round as long as it's not blazing hot out & I'm free. I overheat easily.

 

[gt40]

No, thank you for having me over, even though you put me to work!

But seriously, I enjoyed helping out, it was a lot of fun helping & seeing the process. I think you're crazy but I like your motivation & what you are doing. I don't think I would have the patience and well I don't have the workspace either. Just sorry I didn't last as long as you guys mixing up the batches.

Can't wait to see the first piece completed, you as well I'm sure. I'm game for the next round as long as it's not blazing hot out & I'm free. I overheat easily.

Well I am getting old so we both got tuckered... Thanks again for the help

 

[gt40]

Update:

I came home and the slab was baked by a full day in the sun and solid, so I took a peek by removing a corner. Good news it is really tough- inadvertently smacked it a few times with sledge hammer removing the mold material and it didn't even dent, let alone crack. The 4lb sledge fairly bounced off of it. The part I removed looks good - feels and acts like a solid rock. I took a grinder to it and it grinds like a rock too- you can see that part on the upper right part of the pic.

The bad news is I didn't use enough wax or a proper mold release. Fortunately I taped the entire saddle section and all corners which are radiused and those areas are coming off easy- the flat side- not so much. I spent an hour with a chisel and crow bar with the sledge to remove the end and a little part of the long side. Any secrets to making this easier?


I forgot to mention earlier, here is the breakdown of materials by weight for the mill base:
epoxy weight 60lbs
aggregate weight 612lb
steel and reinforcements 65lbs
Base total weight 737 lbs/335kg

 

[sgisler]

Fortunately I taped the entire saddle section and all corners which are radiused and those areas are coming off easy- the flat side- not so much. I spent an hour with a chisel and crow bar with the sledge to remove the end and a little part of the long side. Any secrets to making this easier?

Maybe soak it with water? The chipboard would turn to mush, you'd still have to deal with getting the melamine surface off of the casting though.


Sent from my iPhone using Tapatalk

 

[dracozny]

I am very curious to what this thing will look like in the end. how tall is the column going to be?