With the advice given and core process understood, my direction going forward for making the core is using the sand/expoxy by weight recepie suggested by ironsides. As the tube is a compound curve; spray interior of tube with release agent, fill with silicon and let cure, remove silicon and use to make mold for core. Must revisit registering the core in the cavity. The videos and advice given is very helpful and much appreciated! Regarding shrinkage..........is there a better option? https://www.amazon.com/RUST-OLEUM-3...ocphy=9028761&hvtargid=pla-587270511996&psc=1 Thank you Forum.
If you have that much experience, please advise us on your methods including use for iron. What percentage resin to sand do you use? What resin brand(s) do you (have you) used? What sand? Do you add coal for iron cores? Do you have to do anything special to provide for the core being easy to remove post casting---similar to adding sugar to silicate cores to allow removal? Any other pointers? Denis
Some results of trying polyester resin and epoxy resin for binders in sand cores. First off, the epoxy worked great. I used 3% West System epoxy (see above) in my Olivine 100 mesh sand. The result was a VERY strong core that was permeable enough that I could blow air through it with by testing it in my mouth. The core was much stronger than that obtained in my standard sodium silicate core making process. I have not yet tried it in an iron casting. I have some questions in the above post to Ironsides regarding his process for epoxy prior to trying it on a for-real casting. Second, polyester resin curing was mysteriously inhibited by both Green Diamond sand and olivine sand. I was quite surprised by this. I am guessing it may have to do with pH factors of the dust and grains. But, I have no way to know this. I first used some resin that seemed good but was from a can that had been open six months or more. So, I bought a fresh can this AM and same results---failure to harden. However the mixed residual resin in my mixing bowls cured just as expected. I am hoping someone can shed light on the whys and wherefores of this failure or if they have contrary experience. Either epoxy resin or polyester (if it can be made to work) would be very cheap binding agents. For a six pound core only 3 ounces of resin would be needed. That would be about 3 dollars for epoxy the way I buy it. And cheaper still for polyester. The core strength of the epoxy was remarkably good. It may well be that a lower percentage of resin could be used and still get very good core strength. Denis
My guess is, the polyester resin was probably air inhibited. Polyester resin doesn't cure completely in the presence of air, ordinary hardware store resin contains a small amount of wax (~3%) which migrates to the surface of a lay up, sealing the resin off from the surrounding air thus allowing it to cure completely. Maybe the layer of resin coating the grains of sand is too thin to allow the wax to do its job.
that makes sense. I assume it is the oxygen causing trouble. So, maybe a bit of CO2 Cover might fix it. Denis
So here is a core made with polyester in a zip-lock flushed with Argon. Who knows whether it will help. I blew argon through the core in the bag hoping to displace the air in the interstices between the grains of sand. Again, how thorough that was, how well the remaining oxygen and nitrogen will diffuse out of the core, and whether the polyester will cure are all up in the air, so to speak. To be able to use polyester would be nice as it’s speed of cure can be easily increased or slowed depending on how much catalyst is added. Denis
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.
I didn't realize that one is able to control the cure time of polyester epoxy by amount of catalyst. And I will use Argon
So, this morning I opened up the polyester-bound sand core box that I let cure overnight in a zip-lock of Argon. The core had indeed cured well. Had it cured fully? I don’t know for sure but it was pretty strong. My non-quantitative observation was that this core was less than 50% the strength of the epoxy-bound core. But it was a little stronger than a comparable silicate core. So, strong enough for use, for sure. My conclusion is that rocco’s suggestion that the previous open-air cure failure of the polyester resin was due to inhibition (until he mentioned it, I had not considered that well-known effect) by air was correct. The argon environment allowed far better curing of the polyester. Another guess is that my attempts to eliminate air by using a bag and Argon was only partially successful as completely flushing air out of the packed core would be difficult. I tried by blowing argon through the core and most likely that was partially effective. But, what was done was enough to get a good enough cure so that a workable core could be made this way. Having to flush out and exclude air is cumbersome though. Given the ease of making a core with epoxy and the recent post by Ironsides above, I think I’ll be using epoxy if I use a resin binder. The best indication for using epoxy (2%) to bind cores for my use will be when high mechanical strength is needed. You could see in his videos the strength of the core she made that were may twenty times as long as the were thick and my single trial of epoxy also pointed to high core easily strength obtained using epoxy. Denis
So I guess one could summarize by saying that while polyester resin can be made to work, epoxy resin is more convenient and better, especially for those of us that don't have bottles of inert gas lying about.
Hello again As the OP, my epiphany moment is that I am duplicating an existing hollow part. The silicon mold of the core is setting up. Trying to put in words..... in making the first half of the mold I'm thinking it necessary to make a closely fitting wood outline of the part at the parting line, dam up the space between the wood and part with clay and ram sand around it. Flip over, remove wood outline then ram sand for the opposite side. Does someone have a link to this process? An other question is what is the rule of thumb in how much larger in dimension does the sand box have to be than the part being made? I've studied every response in detail and watched all of the videos in my quest to duplicate the subject matter of this thread. Thank you!
You are talking about making a “follow board.” That is a well-established practice in casting. For the parts you are dealing with, it would not be terribly difficult. Before you make your board, you will need to make cylindrical plugs in each end of the part. This plugs should protrude an inch or so and will make impressions in the sand, core prints, that will support the cores which will produce the hollows in the parts. Follow boards should fit fairly closely but can have gaps of maybe a 16th of an inch or and still work OK. In your case making a heavy paper template at the existing part lines of the existing part should get you close. Glue in bits here and there as needed. Once it’s a pretty good approximation transfer the outline to a piece of smooth plywood or Masonite and cut it out. Back up the Masonite or plywood with 1/2” plywood to reinforce the Masonite. Put the part on a table, prop the follow board with 2x2 or so legs cut at lengths so the follow board sits at the part line of the part.wax the part and use thickened epoxy or epoxy paste to fill in gaps. After the epoxy sets, pop the part out and sand/file/abrade the board outline so the part fits pretty well but pops out easily. I suspect you get the idea. The support legs or rails or whatever you use to hold the follow board at the part line will have to be solid enough to tolerate sand ramming. And the board itself will have to be pretty stiff as well. Make a flask set whose outer dimensions match the follow boards outer dimensions and sits on the follow board and you on your way. I suspect it is obvious to you that you pack your drag using the follow board and pattern, flip the rammed drag together with the follow board+pattern and then remove the follow board leaving the pattern in place. Talc, pack cope. Split cope and drag and remove pattern. Cut gates, runner, sprue, and pouring basin. Drop in core. Ready to pour. That’s my take any way. There are other methods that would work. For instance, for a one-off, or just a few parts, coping down to the part line (trace it with a sharpie to make it visible) could work too. I won’t be surprised if this description above is clear as mud. And other folks will have good ideas on how to do it more easily or better. I use follow boards often and find them to work well. As far as dimension loss is concerned, 1-2% should be close. There are ways to build up the pattern to compensate, but probably not needed. Denis Added: Here is a follow board video. Not just what you are doing, but shows how a board gets around a not-flat part line on a pattern piece.
Thanks much Denis! Yes, this is a different but more simple 2Dpart. No core involved on this one. Roughed out a"follow board" Will trace and CNC cut a tighter board. I will need to compensate for the loss in volume on this one. If I'm successful I'll make fine adjustments on my lathe. Question is; 1/2 the part is in the box "supported" by the follow board and the other half is hanging in air (picture3) How to support the part when I ram sand? Still working on getting the process down and terminology down. Thanks again!
The flippant answer is "any way you want." Sorry! But that is the truth. The part you are making begs to be a split pattern where the pattern is made by turning a wood spindle and splitting it in half either by bandsawing it or by having glued in a piece of paper in the glued up blank before turning it to a spindle. This is a common woodworking technique. But, I gather you are interested in using this as an example for your other more complex part. So, to use a follow board, I would just glue the one you have made to a similar-sized piece of 3/4 plywood that is cut out a bit bigger than the cutout of the thin board you have made. Then attach u-shaped pieces of solid wood or ply to the underside of the backing plywood so that the pattern piece only falls half way through the follow board. The u-shaped pieces might be tall enough to seerve as legs to suppot the follow board as well as the pattern. I tried to make some sort of drawing. Denis
The flippant answer is great. I'm seeing that much about this art/hobby is about improvising and "thinking out of the box" The "duh" moment was; why not reorientate the part so that a more simple follow board would be used? In other words, Stand it verically rather than horizontally. Yes, this ties into the more complex part mentioned at the beginning of this tread. Molds of my cores have been made. Next step is making those sand cores using the techniques suggested in this thread. I appreciate your time. Thank you Denis!
Most of my cores are cylindrical in shape and release easily so I do not use a release agent. Having said that if you watched my video making a impellor core it was so difficult to get out the pattern so it does need a release agent. I was thinking of using graphite like davidf did in his video making water necks.
And are those cylindrical core boxes usually plastic to which epoxy adheres poorly? The impeller was metal if I recall. Maybe the box material is a significant determinant of separation ease? I made an epoxy core in split PVC the other day. I was able to separate the core from the pipe without much trouble, but it was easier with wax. Had the box been painted wood, for example, or aluminum I am pretty sure the core would have adhered firmly. For those wanting to use a separating agent, a coating of wax, shortening, spray cooking oil, or vaseline are recommended by West Systems to prevent unwanted adhesion. One of mny nightmare scenarios has been forgetting to use a parting agent in a complex core box and then having the core stick hard so that I might have to damage the core box to remove it. Lots of work and time down the drain. Denis
