Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Quote:
Originally Posted by
Zekyx
Yes, I also think one part definitely has to bend. I work with carbon fiber everyday, and carbon/epoxy mix definitely bend, otherwise it would be nearly impossible to take the part out of the aluminium molds. I''ve also built EG machine a year ago and have some experience with that too. Using only PVA, without wax, was more than enough. But I wasn't trying to replicate surface plate as I had metal inserts.
Properly cured cf parts don't have nearly enough flex to allow them to release from rigid molds without molds release. Room temp cure cf parts will flex if they are made with too few layers and not left to cure and post cure properly. Properly cured cf parts are rigid and require proper mold release to be removed from the mold cleanly.
Nobody who knows what they are talking about recommends using pva on it's own. Literally nobody. Some use just wax but never just pva.
btw, being able to flex a part or the mold does not cause it to release without mold release. It only works that way with silicone molds because silicone doesn't bond to anything except glass and silicone. Try making a flexible mold using urethane rubber without using mold release. I guarantee that thing is never coming off.
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Quote:
Originally Posted by
Goemon
If the surface plate is poor quality and has tons of voids, I wouldn't bother using it at all. You can create a flat and level surface with good bartop resin and, if needed, an isopropyl alcohol mist on top. It might even give you a flatter surface that casting against a surface plate.
Hi Goemon, can you elaborate on the bartop resin and propanol idea? What are you pouring it into? And how do you overcome the edges that want to create a fillet to the top? How would you get a flat surface with the edges that don't fillet, rather they leak over if you will?
Thanks
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Quote:
Originally Posted by
made2hack
Hi Goemon, can you elaborate on the bartop resin and propanol idea? What are you pouring it into? And how do you overcome the edges that want to create a fillet to the top? How would you get a flat surface with the edges that don't fillet, rather they leak over if you will?
Thanks
Sure.
You would build a simple mold to pour the epoxy into. It doesn't have to be anything difficult and time consuming. For example, you can buy a cheap piece of 3/4" 2 by 4 from Home Depot and 4 of their cheap pre-made shelves (two 48" x 8" and two 24" x 8"). Use them to make a box. You can drill and screw the shelves into the edges of the 2/4. Then you would want to fill in any voids or gaps in the edges (so resin does not leak out). A quick and easy way of doing this if you don't have filleting wax is to use Play Doh or Plasticine. They both work and are cheap.
Once your box / mold is made, you apply a mold release wax (or you might be better off applying mold release before making the box). I use mirror glaze mold release wax and apply it with an automotive cloth. I then buff if with an automotive buffing cloth ($4 at Walmart). When all the layers of wax are dry and buffed, apply purple pva mold release (don't buy the green one for this). You can wipe on the PVA with some folded kitchen towel.
When all of the layers of PVA are dry, you are ready to pour your resin.
One of the common problems people have with epoxy bar top resins is the phenomenon known as fish-eyes where instead of evenly coating the surface, the resin pools in some places and leaves voids in others. This is usually due to grease or other impurities in the surface. In this case, the mold release might cause problems. This won't be an issue if you don't plan to remove the resin from the 2 by 4 (because there would be no mold release) but if you do plan to release it, you need to coat the mold surface with something else first for guaranteed success.
I suggest applying a layer of epoxy black surface coat (from ACP composites) with a roller to the mold surface and let it cure until it feels dry before pouring the bar top. The black epoxy surface coat never fish eyes as it's so thick and heavy. I have only ever seen perfect results from using a quality bartop resin (like FX Poxy) over the black surface coat.
The key point of all this is that you end up with a perfectly flat surface and one that is flatter than most of us could achieve by hand scraping metal or granite.
Note, the flat surface is the one facing up that isn't touching anything, not the rear side resting on the 2x4. A good bartop resin settles flat like still water. In fact, companies use water as a mold surface to cast perfectly flat sheets of glass for this reason. It's free of man-made imperfections.
This method will only produce one flat surface. To make the other side flat, you would turn it over, use the shelves to build mold walls around the edges and then pour another layer of bartop. If all this is done on a level surface then you will end up with two flat parallel sides.
I would suggest not doing this with just resin. On it's own, large slabs of room temp cure resin might warp or bow. If you cast the resin over a bunch of steel, aluminum or carbon fiber tubes, it should be sufficiently reinforced to remain flat. Plus, using something like steel or aluminum for a majority of the bulk would probably be a lot cheaper than using enough pure resin to make it stable.
You can also cast some T-slot tracks into the surface so you have a way of attaching clamps and other work-holding devices. Resin won't hold screw threads well on it's own.
For reference, my digital dial guage is accurate to 0.01mm and it can't detect any variation in surface flatness on the surfaces I cast with bar-top resin. I haven't been able to achieve results like that with my hand scraper yet (I'm still learning).
Three final tips for using bar-top resin:
leave it next to your boiler or some other warm place overnight before using it. Cold resin become too thick and traps more air bubbles. Slightly warm is good. You can even use a hair dryer (sparingly) if you notice trapped air in the surface that doesn't seem to be coming out on it's own.
Make sure to measure exact ratios of resin to hardener. Don't get tempted to add extra hardener thinking it will make it cure faster etc. When the ratio's are wrong you start to have more trouble with trapped air bubbles.
A thin mist of isopropyl alcohol can reduce surface tension if your resin does not seem to be leveling on it's own or if it starts to fish-eye. Buy some of the 99% stuff and something to spray it with.
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Thanks Goemon for all the information. I will shortly cast some moulds and test out your ideas. Although, unfortunately I was planing to pour in the garage, but it is not heated, so I might just build the moulds there and pour inside.
I don't have any bar top resin, I'm using a 2:1 epoxy resin used for industrial flooring, the idea being to mix it with a suitable aggregate for a resistant floor. However, it is workable to 40 minutes and the viscosity is similar to that of other well flowing resins.
I still have to pick up some mould release wax and pva.
Do you know what kind of inserts people use for screws? I'm thinking of trying a few options such as hex nuts, concrete inserts, etc.
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Quote:
Originally Posted by
made2hack
Thanks Goemon for all the information. I will shortly cast some moulds and test out your ideas. Although, unfortunately I was planing to pour in the garage, but it is not heated, so I might just build the moulds there and pour inside.
I don't have any bar top resin, I'm using a 2:1 epoxy resin used for industrial flooring, the idea being to mix it with a suitable aggregate for a resistant floor. However, it is workable to 40 minutes and the viscosity is similar to that of other well flowing resins.
I still have to pick up some mould release wax and pva.
Do you know what kind of inserts people use for screws? I'm thinking of trying a few options such as hex nuts, concrete inserts, etc.
I don't bother with any inserts for screws for this type of thing. If I was making something for multiple uses then I would put more effort into the mold structure but for single use, the screws go straight into the wood using a power drill. The only thought that goes into the screws is to make sure they are shorter than the wood is thick. If they go all the way through the wood then you'd never get them out again.
I'd be careful about substituting a good bartop for any old epoxy. They might seem similar but they aren't. They won't all produce a flat bubble free surface. It is worth investing in a decent bartop for this (there are plenty of poor quality bar tips that should also be avoided).
Also, don't assume that super low viscosity equals greater flatness or less trapped air bubbles. It doesn't usually work like that. Some of the best resins I have (and I have a lot on my shelf) start out relatively thick. What matters is how they cure and release gases.
1 Attachment(s)
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
[QUOTE=Goemon;2134222]I don't bother with any inserts for screws for this type of thing. If I was making something for multiple uses then I would put more effort into the mold structure but for single use, the screws go straight into the wood using a power drill. The only thought that goes into the screws is to make sure they are shorter than the wood is thick. If they go all the way through the wood then you'd never get them out again.[QUOTE]
Hi Goemon,
I'm not sure how this would work. I've attached an image as to what I want to achieve. A linear rail that is screwed down to the epoxy / granite base. Does the epoxy granite mix not need screw inserts ? Otherwise, how do I screw the rail to the base?
Thanks,
http://www.cnczone.com/forums/attach...d=383336&stc=1
1 Attachment(s)
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Anchor a steel bar in the epoxy that the rail mounts to.
Attachment 383564
Lot more detail photos in this thread:
http://www.cnczone.com/forums/epoxy-...52784-cnc.html
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Quote:
Originally Posted by
rocketflier
Ok, are those bolts there to anchor the steel bars themselves? And, on the flip side, are the steel bars drilled and threaded? My concern is that an M6 or M8 scren (as per DIN 912) has anywhere from 24mm - 28mm thread length. So, my rail will not be threaded, it will simply have a through hole, the steel bars underneath (or whatever insert I have) has to allow at least 24 - 28mm thread, or if anything, allow the screws to go through.
Or do I have to make my rail also threaded, so that some of the thread is "above ground" and some beneath, so to speak?
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
[QUOTE=made2hack;2135894][QUOTE=Goemon;2134222]I don't bother with any inserts for screws for this type of thing. If I was making something for multiple uses then I would put more effort into the mold structure but for single use, the screws go straight into the wood using a power drill. The only thought that goes into the screws is to make sure they are shorter than the wood is thick. If they go all the way through the wood then you'd never get them out again.
Quote:
Hi Goemon,
I'm not sure how this would work. I've attached an image as to what I want to achieve. A linear rail that is screwed down to the epoxy / granite base. Does the epoxy granite mix not need screw inserts ? Otherwise, how do I screw the rail to the base?
Thanks,
http://www.cnczone.com/forums/attach...d=383336&stc=1
I was talking about the mold, not the gantry itself. For attaching linear rails (or anything else) to the gantry it's as Rocketflier suggested. You need to add steel or aluminum plates (buried under the epoxy granite mix) to allow you to drill and tap mounting holes.
In my case, I have two tool steel bars under the mounting surface which can be drilled and tapped. I chose tool steel because of it's high hardness but I regret that decision. Regular steel would have been fine and far easier to drill. Tool steel adds no stiffness or anything like that.
I am not mounting my rails directly to the gantry surface. I have a precision flat tooling plate that is being mounted to the gantry surface and this plate will hold the rails. This will allow be more opportunity for fine adjustments / squaring if needed. It's also a lot easier to drill a tool plate on my drill press than a gantry I can't lift.
I have a second tooling plate which will be mounted to back of the gantry. I have long bolts that will go through the surface plate, into the embedded tool steel bars buried in the epoxy granite, through the gantry and into the tooling plate at the back to clamp everything in place. I believe this design offers maximum strength and prevents the embedded steel bars from being ripped out of the epoxy granite under a heavy load.
It might sound like over-kill but.... if anything is going to let down an epoxy granite structure, it is the low surface hardness in comparison to iron and steel. I want to prevent the mounting screws from moving after repeated stress.
Forget trying to mount screws directly in epoxy. There are no inserts that solve the low hardness issue.
The only other thing I would say on this topic is that my embedded tool steel bars are 3/8" thick which I now think is too thin. If I was buying them again, I would have looked for at least 1/2" of threading depth.
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Yes, the rail only has a hole for the bolt to go through. Something must anchor the bolt (and thus the rail) to the casting. You can think of the bar here as the insert in your drawering; only much more surface area to resist being twisted or pulled out. The carriage bolts attached to the bar are to additionally anchor the bar. The carriage bolt head and additional welded washer help resist pullout.
If you go to the link I provided, you'll see more images including the flip side
Attachment 383634
Where they drill and tap the holes for the rail bolts. The bar has the advantage it can be machined flat and planar to the other surfaces after the pour.
When I looked at woodworking inserts, none were very impressive. Most are some sort of thin, stamped metal. The insert must bond to the epoxy to resist both twisting and pulling out of the casting. It would suck if the insert broke free as you were tightening the bolt.
Re: Epoxy Granite In Practice (Mineral Casting, Polymer Concrete)
Hi All - Old thread but I'd like to explain some things about wax and PVA. A release film separates resin from the mould surface. Resin is designed to stick really well so if you don't get your release system correct it will stick!! Styrenated resins like polyester and vinyl ester have the extra issue that styrene can permeate the film, dissolve the film and then you have no barrier and hence a stick up occurs,. But I shall stay with epoxy for now. Epoxies used for epoxy granite have no solvents in them to upset the release film unless its a solvented epoxy. Do not use epoxies with solvents as these will shrink and make your part change shape as the solvent egresses the part over time. This is what happens with styrenated resins and their shrinkage.
Now waxes provide a physical barrier to the wet resin and the wax layer has to be quite thick. Wax will not provide a gas barrier however. We think of wax as a slippery barrier but in fact the resin binds to the wax, this is why the part has to be dewaxed and the wax barrier has to be renewed. The way the release happens is that the wax layer shears, some stays on the mould some stays on the part. The initial grip of the part is due to the vacuum and mechanical grip the resin has created with the part via the wax surface. An easy way to shear the wax is to flex it using wedges or local force, once the wax is sheared and some air gets in the wax shears easily like a zipper opening. If the parts are rigid then the wax has to be softened or a water type release is very useful so the moulds and parts are not damaged. Never hit moulds or parts with rubber mallets!!
Onto PVA - PVA is impermeable to gases which makes it a very good thing to use with styrenated resins. It also dissolves in water so if you can get some water in the gap it will start to wash out the PVA and this will aid the release. Also if you use wax then PVA the PVA does not grip the wax as well as a resin would so the PVA/wax interface slips and releases easier then if just wax was used. Using PVA however does not give you a polished finish so using PVA has that downside.
Liquid release systems that use silicon or oils make a very slippery barrier. There is no mechanical grip between the part and the mould as its a liquid. These are preferred in pouring applications. But in cases where a gelcoat is used liquid systems don't work as the gelcoat will fall off before you can get the part started. So it is possible to release rigid parts from rigid moulds if the part has the correct draft and the wax system is thick enough. Many issues occur simply because the wax is not thick enough so it can;t shear easily... Cheers Peter
