Using Epoxy (and maybe polyester) to Make Cores

In the search for lower epoxy resins take a look at systems designed for vacuum infusion and room temperature cure. For those that want to experiment and don't want to spend $150+ for a gallon kit this can be found at some hardware stores and has chemistry very similar to the other epoxy systems.

SYSTEM THREE ROTFIX

 

I hope that stuff works for our purposes. I love the mixing instructions, put the epoxy and hardener (2:1) in a bottle, cap the bottle, shake for one minute, empty bottle within 15 minutes and if you want to reuse the bottle, immediately rinse it out with vinegar then water. It doesn't get much simpler than that!

 

It would be nice if the West system or System Three works out since its available in many places and sizes. I always look at the SDS and there are just so many different recipes.

 

Here is the very first epoxy resin I tried for sand cores. Very expensive but I only needed a small amount to see if it would work. I have tried many brands and they all seem to work well.
https://www.woolworths.com.au/shop/productdetails/76793/selleys-adhesive-araldite-5-minute-dispenser
Smoking Shoe, That is exactly the same amount I get from my hardware shop, similar price but different brand to yours. I suspect that most retailers in Australia buy large amounts from the same company and repackage it into smaller containers for consumers.

 

I finally made my first core with epoxy last night. I used olivine 100 mesh and West System epoxy at 1.5% by weight. The resulting mix set up just fine and separated from the corebox perfectly. (One of my nightmares is to have an epoxy core stick tight to the box and be very hard to remove) I poured the casting today. We'll see in the morning the results. I can say the material stinks after the metal is poured in and it seemed like the amount of flame that was generated by the hot metal vaporizing the epoxy was noticably more than a comparable silicate core. This core was 6 pounds in a 28 pound casting. I poured a silicate core of the same design at the same time. Good for comparison. I am expecting little difference. We'll see.... I used coal in the silicate core but none, per you guys' adivce, in the epoxy core.

Denis

 

I'll be very interested to see how the casting went. For what it's worth, even the commercial product has lots of smoke and you have to be careful to stay upwind of it, when the mould is sitting on a metal surface there's also condensed droplets of moisture and resin by-products underneath the mould. There's also a hint of caramelized burnt sugar in the smell so possibly there's some to aid shakeout afterwards

 

I was very happy with the performance of the epoxy core. It did its job well. The surface finish was just slightly matte compared to the silicate/coal core. I suspect adding 2% coal to the epoxy sand would result in a glossier finish. I’ll try that next time. The core was quite soft and crumbly and fell out completely with just a few rings from a hammer on the casting.

The epoxy core was on the left.
The two together.

Denis

 

How did your actual work time/strip time compare to what the West 105/205 calls out?

 

I used Johnson’s paste wax lightly buffed and a dusting of graphite. Buffing helps separation.

I think it set up about like it does when in a glueline. The mass of the sand probably prevents the sort of accelerated cure due to heating commonly seen in a in a pot of epoxy. I just mixed it up, checked a few hours later (soft but starting to set) and then the next morning when it was hard. It was 72 deg F when mixed and curing.

Denis

 

What is the name of the commercial product? My epoxy cores are small so they don't produce any smoke and only have a slight odour.

 

West system 105 with fast 205 hardener. I used about 1.8 oz of resin in the core.

Denis

 

I don't even know what it's called these days, we just buy it from a local foundry as it's only sold in those 1000 litre bulk containers. It's a 3 component resin, so something like a 2 component urethane with an accelerator/crosslinker additive, it doesn't last longer than 6 months in the tropics so we just get what we can use. The original product was "Pepset" but that hasn't been made in years. Also because we use fine "sharp" silica sand, you need twice as much resin as normally recommended by the manufacturer to get decent strength for handling.

The ratio of the third component drastically affects cure time: You do a few test cure times to get the third component ratio right. There is a hand made dosing system which contains two resin components (one component has the third premixed in) above the sand mixer. It's all made from tin cans, glass bottles and teflon tubing so it can either be cleaned with strong solvents or discarded and replaced easily. In operation you measure the sand in with a scoop to get the correct volume, add one component to the sand mixer and let it get stirred in thoroughly, add the second component, start the two minute timer, dump it into the bucket, dump it into the pattern and tamp it down with a trowel and screed off the excess before the timer goes off. The loose resin coated sand has to be in the pattern before the timer goes off or it will have already gone off and won't stick to the other sand grains.

I speculate that a 2 component urethane resin with Bostik Desmodur RFE crosslinker for extra strength and heat resistance would go pretty close but I haven't tested it myself.

 

Kind of sounds like a vinyl ester resin system?
Resin/promoter/initiator

 

The original product was described as phenolic urethane no-bake "PUNB" resin. I know the third component is light sensitive, moisture sensitive and heat sensitive which kind of matches cross linker additives like Desmodur RFE. If you filled the bottle with dry CO2 or Argon and stored it cold you'd extend the life a fair bit. The proper foundry resin is actually the cheapest way to buy resin compared to two component urethanes and epoxies, it's just nobody wants to import and sell something with such a short shelf life (unless you buy a couple of tonnes).

 

A while ago a friend told me he was using Fenotec resin binder made by foseco.

https://www.vesuvius.com/content/da...ers/brochures/fenotec-e.pdf.downloadasset.pdf

So I purchased two liters of resin and some hardener of him to compare it with epoxy resin and found the same thing as you, I needed a lot more resin to make a strong core. ( 4% resin ) I went to use it again a couple of months later and the resin was completely solid. so it has a short shelf life. My friend also said he had to throw out the resin in the 200 liter drum and he had only used about half of the 200 liters. He also said to get a good surface finish on his iron castings he had to use a mold paint. The surface finish was outstanding. He also used a graded foundry sand and needed less binder. So what I learnt from this experiment was if you don't make molds every day to use up the resin and use graded foundry sand it can be a very expensive way to make molds and cores. So epoxy resin is the best way for me to make cores.

 

In contrast to the binders you guys are working with, 5:1 epoxies have very long shelf lives. I found some several-year- old orange-colored-due-to-age West System resin in a cupboard and figured it was done for. But I also noted the West said even old discolored resin should still work well. (I was amazed they did not just tell you to throw it out and BUY new stuff.) I have used it up and it worked the same as fresh stuff.

I figure the resin cost for my 6-pound core was about 2 dollars. That is more expensive than silicate and lot more expensive than a green sand core. But, it is nice to have the option and know it will work well. I have one casting to do that will require a very strong core as it is 15 times as long as it is thick. Epoxy may be just the thing. Silicate cores tend to break as this core needs to be horizontal and supported only at the ends. I may need to mix it at a higher per cent for more strength. But the whole core weighs less than a pound so the resin costs will be 25 cents or so.

When I get going on it, I will post how it works out.

Denis

 

David, if you are familiar with the vinyl esther resin products, how about (1)a few more clues like brand names and (2)sources for the resin, cobalt, and MEKP that you use. (3)In what ratios do you use them? I see some lisitngs online (like Amazon) for the resin, but not cobalt. (4)I suppose the MEKP is the same as used for polyesther resins.

(5)It looks like the vinyl esther is intermediate in cost between epoxy and polyesther. (6)I suppose there is good shelf life with vinyl esthers?

Denis

 

Derakane VE resin is pretty popular, but finding small quantities of the un-promoted resin is kind of hard to do. Shelf life for the promoted resin is in the 3 to 6 month range - for structural parts. You 'might' get a little more usable life for cores.
Keeping in mind that I have never used this method for cores (and haven't cast anything in over a decade*) I'd probably get the cheapest polyester/VE/Epoxy I could find locally. I've used a lot of EZ Poxy and I know it has a usable shelf life that can be measured in years - for non-structural applications. Same with the System 3 resins I've used. For aluminum casting I kind of suspect that systems with a lower glass transition temperature would make core removal easier.
I've also been pondering using 'glue' made with acetone and Styrofoam packing material as a core binder. My one very quick experiment with very low SF/solvent ratio indicated it may be possible. A lower vapor pressure solvent blend with higher SF/solvent ratio would probably work better. Working time was short and the 'core' had strength similar to very dry, but usable, green sand.

Will give me something to do while the new kiln is curing - as if I need more to do.


*about a month away from starting the build of a new 6kW A20 sized kiln for melting primarily aluminum.