Using Epoxy (and maybe polyester) to Make Cores

On another current thread (http://forums.thehomefoundry.org/index.php?threads/replicating-a-motorcycle-intake.1916/) as a sidebar we have been discussing epoxy and polyester as binding resins for cores. I am unaware of much previous discussion of these binders in this forum. So, since tow very experienced folks, Ironsides and Tom Cobett, have provided important information on the subject I decided to make a thread compiling that information here. Otherwise it can be hard to find the information as it would be buried in a thread not primarily dealing with core making.

First, the information I received today in answer to a question I emailed to Tom Cobett concerning epoxy as a core binder. He has considerable personal experience with this in his work as a professor and foundry industry consultant:

"Epoxy Resins are probably the strongest (and the most expensive) foundry sand binders that you can use. Their normal usage rate in foundry sand is between 1.0% and 2.0%. Because they are organic chemicals, the provide for complete burnout out. They shakeout like dry sand. Because they are organic chemicals, they also provide for a Lustrous Carbon formation which gives terrific surface finish on the castings. And, because they are organic chemicals, the mold will smell like a burning TV set after it has been poured. They really stink. But ...



They are the highest performance sand binders that you can get.



One of the largest manufacturers of Epoxy Resins and Curing Agents is Hexion, Inc. The standard Epoxy Resin is EPON 828. Unfortunately, it is way too thick to useful as a foundry sand binder. You can thin the EPON 828 with Acetone, Xylene, or Propylene Carbonate. A better starting point is to use EPON 813 as the Binder. This version is already diluted to a reasonably thin liquid.



As the Curing Agent, I would use EPIKURE 3234 (also from Hexion). It will give you a mix that can be stripped in less than 30 minutes. DO NOT MIX THE EPON AND EPIKURE TOGETHER WITHOUT PUTTING THEM INTO THE SAND FIRST. There is a rapid exotherm that can get up to 400 degrees F in less than 3 minutes. When mixed with sand, the reaction rate is much slower because the sand absorbs much of the heat that is generated.



I suggest starting with a mix of 2% EPON 813 Resin (by weight of the sand) and 17% EPIKURE 3234 Curing Agent (by weight of the RESIN). Add the Curing agent to the sand and mix for a minute or two with a hand held kitchen mixer. Then add the Epoxy Resin and mix at high speed for just one minute. You need to have your core box all set and ready to go. This mix is going to harden weather you want it to or not.



Wear disposable rubber gloves. When Epoxy cures it is really difficult to get it off of your skin and clothes.



Since this is a chemical reaction, I would not recommend using anything except silica sand. The high pH of olivine sand may prevent the complete hardening of the sand mix.



Here is a recipe for a 1000 g. Epoxy Resin mix.



1000 g. dry silica sand (70 to 100 AFS Grain Fineness Number)

20 g. EPON 813 Epoxy Resin

3.4 g. EPIKURE 3234 Curing Agent



1 Add the sand to a mixing bowl.

2. Add the Curing Agent to the sand and mix at high speed for 1 to 2 minutes. Make sure the mix is not lumpy. No reaction will occur until you also add the Epoxy Resin.

3. Add the Epoxy Resin to the sand and mix at high speed for just 1 minute. The mix may feel dry, but, it will set up to be extremely rigid. If the sand actually feels wet, you may have added too much binder. That extra binder will cause the core to stick to the core box!

4. Fill the core box by tucking the sand tightly. Pack any extra sand into a paper cup. When the sand in the cup is hard, the core will also be hard enough to strip.

5. The final core will not pick up any moisture and will last for years! I have used this mix to make a lot of decorative "rock" pieces.



If you want to go faster or slower, you can use other Curing Agents. Each Curing Agent is used at different addition rates. Always ask for a Technical Data Sheet.



Where do you buy this magic Sand Binder System?



www.miller-stephenson.com - BEST SOURCE for small quantities of Epoxy Resin and Curing Agents.



www.skygeek.com - They sell epoxy resins and curing agents to people who want to make or repair their own aircraft!



www.pilotshq.com - More aircraft guys.



www.theepoxyresinstore.com - Sell several Resins and Curing Agents for home hobbyists.



I hope that this will help you get started on the right foot.



Tom Cobett

T. Cobett and Associates"

I'll be adding in Ironsides comments, and other relevant posts from other threads later---running out of time right now.

Denis

 

Myford boy has considerable experience in this area. He posted this in a related thread:

http://forums.thehomefoundry.org/index.php?threads/replicating-a-motorcycle-intake.1916/#post-41640
I dont think it says so in that video, but he is using epoxy binder for his core.

I did something silmilar a while ago.


http://myfordboy.blogspot.com

 

And then later:


Rocco asked "does anyone here know if normal polyester resin could be used in the same way? I've never seen it used and I'm wondering why, it's cheap, universally available, you'd think, if it works, it would be near the top of the list of preferred binders for the hobbyist."

I tried polyester resin once, it hardens too quickly for using as a binder for sand cores. I have been using epoxy resin for cores for about 25 years and would not use anything else.

 

And a little later:
"When I tried the polyester experiment I used some resin from my brother, so if the experiment failed for any reason I would not try it again. Another reason why I was not very happy with polyester, the label says it contains styrene which smells really bad when it burns. Epoxy resin has a low odour when used as a core binder. When I did this experiment I had been using epoxy resin for 7 years without failure so it was a proven method and I could not see any point trying another resin when epoxy worked so well.

The very first core I made with epoxy resin it was 4% and was super hard and strong so it was used with iron. Post casting the core was still intact but was so much easier to remove than S.S. With the second core the percentage was halved to 2%. Post casting the core binder was completely burn out so the casting was tipped on its side, all the sand flowed out like water. I tried lower percentages but the cores became too fragile to handle. Fast forward about 3 years I went to a foundry closing down sale and got some core sand. A magnifying glass was used to see what proper core sand looks like and it was so much better than the sand I buy from a local sand pit. My sand has very sharp edges compared to the core sand which was very rounded like river rocks. So I tried 1.5% with the core sand and it was a lot harder and stronger than the 2% mix using local sand. The reason why that happens is sand grains with sharp edges takes more binder than than sand grains that are rounded. Post casting was excellent as the binder was completely burnt out so the core sand flowed out like water from the cored hole. A long time ago I toured through the Caterpiller iron foundry in Peoria Ill. It was explained to me that water jacket cores in engine blocks have to become like a powder after casting because if it does not how do you get the core sand out?

The brand of resin I use comes from the local hardware shop which always seem to change brands if one does not sell well. So what is available in Australia will never be seen in the U.S.A.

This the most asked question I get in my videos, what sand do I use? My green sand for iron is swimming pool filter sand. Do I know what classification it is, no idea. The same goes for the core sand I got.

No need to add coal to epoxy resin cores.

Same as above,adding too much epoxy resin will prevent the core sand flowing out of the cored hole.

GJL have a look at these two videos of mine showing how I use epoxy cores for iron castings.




Ironsides, Thursday at 8:29 PM

 

I am following these discussions closely. I am still unsure as to how I can make a mould for my model A7 engine block. It has so many ins and outs for features and cores that I was at a standstill until I found out about resin bonded sand. I am looking at the possibility of not only making the 10 cores for the mould from resin bonded sand but the mould itself. I am now trying to figure out how many sections to make the mould in to get it to release from the pattern. At the moment it stands at 6, but I am wondering if I can make the pattern in sections, that might mean I can have a 3 piece mould. A one piece outer shell and a separate top and bottom. More thinking and reading to come, but I have bought a litre of epoxy resin (£28), no idea if it is suitable for this application. I have also started to prepare an old 3D printed cylinder head to act as the trial casting. I will take an epoxy/sand mould from it and pour some aluminium. Should I use a release agent on the pattern? I have sodium carbonate or I could buy some wax.
Lee

 

Lee let us know if your epoxy is a success (or failure) and what the name of it is. Not sure what an A7 is but here is an Austin Healey block.

https://healeyfactory.com.au/services/dmd-australia/dmd-aliminium-engine-block-pattern-making/
https://healeyfactory.com.au/services/dmd-australia/dmd-casting-process/

 

You really do need a release agent. One of Epoxy's primary purposes is adhesion. For me, the most reliable release agent for silicate and epoxy has been paste wax. Johnson's Paste wax is the brand I use but have tried no others so there may be better ones out there. I follow advice given by West Systems and wax two or three times to be sure you I do not miss an area. Also buffing lightly after applying wax seems to result in a easier and cleaner release. I know some folks also use talc as an adjunct to relaease agents. I have tried on multiple occasions using silicone spray and it works pretty well but wax seems better for epoxy and and is defintiely much better for silicate. Silicate is highly reactive and will bind aggressively to waxed shellac and somewhat to waxed lacquer.

Added: I looked at West Systems site for advice on release agents. They say: "Vaseline, paste wax, shortening, and cooking sprays can be applied to objects to contaminate the surface to keep the epoxy from sticking" I had not thought about cooking sprays, shortening, or vaseline. Nice to haver some additional options over paste wax as I worry about getting past wax into every nook and cranny.

 

Sorry Pal! I am so used to using abbreviations I forget sometimes. A7 = Austin Seven. I have visited that website and would like to be able to do something similar.
Lee

 

I bought some West System 105 epoxy with 205 fast hardener. I don't like resins that cure too fast. I will get some wax soon. I used to have a large tub of wax for when I was making fibreglass car bodies. I have moved house since then and I think I threw it away. Give me a day or three and I will try and get the head moulded, which will mean a custom made flask, then I can pour it. My first pour NOT using green sand!
Thanks for the advice Denis.
Lee

 

There is a commercially available epoxy accelerator called DMP-30 which is used a lot for pattern making and can speed cure to minutes in warmer climates with a release of lots of heat. It's commonly available, fairly toxic, irritates eyes, skin and lungs. It can only be shipped by road here as "Dangerous Goods". Only a few percent/drops are needed to work, it's mainly used for pattern making but I don't see why it can't be used for cores. The silica sand we use for making non-ferrous moulds is what you'd call sharp sand and as a result we have to use roughly twice as much phenolic urethane resin recommended by the manufacturer to get decent strength.

The release agent is based on fine aluminium powder paint, it's a commercially sold product and several similar products are sold by foundry supply shops. We use xylene to thin it as no other solvent seems to mix with it and spray on a coat and allow to dry like paint. It lasts for quite a while and lets you make 20+ sand moulds off the patterns, after a while it will come off the pattern in sheets like old paint and you have to gently blast it with media like walnut shells to strip it for recoating. Aluminium will rub off in small amounts on your fingertips if you handle the coated patterns. It's obviously incompatible with alkaline sodium silicate binders so graphite would be a better option for that.

 

I did a little searching on DMP-30 and it looks like there may be more than one product with that designation produced by different manufacturers. If you get a chance could you report which manufacturer's product you are using?

Good information, Mark. Likely of interest to many.

Denis

 

That's going to be hard to do with a small brown bottle with DMP written in felt pen on it . It does seem to be commonly sold by scientific supply companies in small bottles for use embedding microscopy samples in epoxy.


Looks like it works with polyurethanes too:
https://kautschuk.com/products/epoxy/isoqure-dmp-30
http://www.chemcoms.com/Tris(dimethylaminomethyl)phenol.html

Sellers:
https://proscitech.com.au/products/dmp-30-dg
https://www.2spi.com/item/02823-da/

Edit: This page states BDMA: https://proscitech.com.au/collections/accelerators-catalysts/products/bdma-dg is now the preferred accelerator over DMP-30 as it has a longer shelf life.

https://www.agarscientific.com/accelerators-for-epoxy-resins.html

 

Yes they are very strong and at the time I thought about putting in re enforcing wire but decided not to and it shows how strong they are. If epoxy is used for cores in aluminium castings it will not burn out the epoxy so the cores will remain in one piece. The pouring temperature for aluminium is only half of cast iron so the epoxy is largely uneffected. If you are going to use large cores for aluminium castings, I use styro foam packing to hollow out the cores so it makes it easier to remove the core and does not waste epoxy making a full core. Another possibility is cutting back on the amount of epoxy used to the barest minimum so the core can be removed.

In the original thread the question was asked"how do you make a core box"? I made a video a long time ago answering that question using plaster for a core box. If you want a quick cure for epoxy put the plaster core box in a microwave oven and the heat will make the epoxy harden a lot more quickly. As long as the plaster has plenty of moisture it will heat up the epoxy. Have a look at the video

 

Very nicely done! And I see you used 1.25% epoxy and I noticed the core sand simply drained out as you indicated in a prior post. Awesome.

Denis

 

Here is an interesting video for you. He said there was very little information on youtube and forums about using epoxy resin for cores so he saw some of my videos about epoxy resin and had a go at making some. He tried to make sodium silicate for cores and as you will see it was not very successful. The video shows how easy it was for him to make a epoxy core the first time around and says at the end of the video that he would never use sodium silicate again. You will also see that the core does not breakdown to a powder because he is pouring aluminium. He also uses a release agent.

 

Since this thread was started to try to get as much epoxy core making info as possible in one spot that might also be easily found on a search, I will mention one more release agent for use on cores made with epoxy. I came across liquid PTFE release agents that are used in a variety of molding operations. They can be incredibly expensive and probably not necessary for most folks' needs. That said there are Teflon dry powder sprays that might be useful and some aerosol PTFE sprays that are reasonable in cost.

Here is one maker: https://miller-stephenson.com/chemicals/mold-release-agents/ptfe-release-agents/?gclid=CjwKCAjw7fuJBhBdEiwA2lLMYeDXhf4uCy-umnf6Jz2lMcXRc9U4r2DxVOx7B78MveXyL3v6FkZ-oBoCRzkQAvD_BwE

Denis

Added: If this fellow in the above post video were interested in making silicate molds we could have helped him have success. That core would have been very easy had he known proper technique. There are several threads and willing people here that would have given him the tools he would have needed to make very fine cores with silicate. Too bad he did not find the information. But good that he figured out a system that worked for him and that allowed him to make the part he needed.

 

No it's not an epoxy core. I use sodium silicate and C02 for my core making.

 

My mistake. Sorry. You do good work by whatever methods you use. (I also recall you were generous in providing information to me when I was building my first furnace.) I too have used silicate exclusively. But I am going to try epoxy soon to see how useful it may be in light of some of the information above in this thread.

Denis

Denis

 

Unfortunately Miller-Stephenson is no longer selling the Epon 813 epoxy unless you place a rather large order.

 

That is unfortunate. It would be nice to follow Tom’s suggestion to the letter. But, it does look like 828 is available. The main advantage of 813 over 828 according to Cobett is that the 813 is diluted to make its viscosity lower. Cobett does suggest that you can use 828 and dilute it with various agents to cut it and 828 is available.

I am inclined to follow Ironsides suggestion, based on his many years of experience, and use whatever epoxy is convenient. For me, that is West System at least as a starting point. If it doesn’t work out, I’ll start looking around. I’d be very interested to hear what does or doesn’t work well for you and anyone else trying epoxy as a binder. Looks like everyone is in pretty good agreement that between 1 and 2% resin is a good place to be.

Denis