I tried this the other day in lost foam. It failed. The pattern was made from blue house foam. Here is the coated pattern: I couldn't find a metal bucket so I made a flask out of vent pipe Casting came out like this: Before you just go ahead and say "short pour" I have to tell you it was more complicated than that. I poured metal until my pouring cup was filled. During the pour I noticed gas coming up the sprue and then the pattern stopped filling. After I finished pouring the liquid aluminum in the pouring cup suddenly dropped down into the sprue and left me this outcome. I suspect I either poured too cold or I had a vapor lock in the sprue because the pattern would not vent right. I think I used the wrong dry wall compound and then put it on too heavily. I heated this metal in my electric furnace and poured at 1260 deg F. That has worked fine in the past with sand casting. I know that this is a big chunk of aluminum and I was definitely expecting some shrink that I would need to machine off. I allowed for this. I now have proper dry wall compound. Any other thoughts? Interestingly, this machines pretty well:
Hi Robert, images are not coming through for me. Would you be able to load them directly to the forum with the 'upload a file' button?
Apparently I don't know how to do this on this site? Is there a way to make the pics appear in the correct place with the text? When I go under the pic icon, I only see "insert URL"
It is a little bit fussy, but you can shove them around a bit if needed. I uploaded the casting pic second, then moved its attach code to first position while in the preview function under more options (pics will disappear but you have bracket code markers...)
Let's say you want to post 3 pictures. Use the 'Upload a File' button at the bottom of the post 3 times. Your pics will be a thumbnail at the bottom. Type a couple sentences and hit Carriage Return to get the curser on the left below the wording. Now go to your first pic you uploaded and choose either Full Image or Thumbnail. The pic will appear and the curser will be at the lower right corner of the pic. Hit CR and the curser will be on the left for more typing. If you hit CR a second time you get a space between the pic and the next paragraph. And repeat the process. Oh, a thinner coat of mud and a sprue 5 or 6 inches long.
Cant really tell from the pictures but sure looks like an interupted pour with a sprue or feed system too small for the volume of the casting. When you have a relatively thick casting, you need a generous sprue cross section and cup volume to keep up. A little additional head/sprue height helps too. On drywall mud coating thickness, no more than needed to cover the colored foam and appear white when dried. Best, Kelly
Thanks for the tech support Tops! OK- so thinner drywall and longer sprue. I'll give it another shot for another part.
I would want to be in the low to mid 1300's F and would defer to anyone with more experience, I have only done a few lost foam pours. We had one with an uncoated part and sprue fail in a similar fashion a couple weeks ago in Tamarack. The neighboring coated parts did fine. https://maritime.org/doc/foundry/part3.php#pg141 Proper pouring temperatures for a given metal vary with the casting size, design, and desired rate of pouring. For this reason, the pouring ranges given below should be taken as a general guide only: Metal Pouring-Temperature Range Steel 2850°F. to 2950°F. Gray iron 2300°F. to 2600°F. Aluminum 1250°F. to 1400°F. Manganese bronze 1875°F. to 1975°F. Compositions G & M 2000°F. to 2200°F. In general, thin-walled castings are poured on the high side of the range and thick-walled castings on the low side.
Just to elaborate a little Robert, some of the missing information for me would be: The approximate size of the pattern LxWxH Yes, the sprue length but also the cross section of the sprue The position in the flask Whether a pouring cup was used. All I see is a shape where a sprue was attached presumably to the top, and looks like it was dangerously close to pouring short, or there was an interupted pour which caused partial mold collapse in the vicinity of feed system. That's a result usually associated with a sprue contained within a conical depression in the sand and no pouring cup. I do preach the benefits of offset pouring cups but second best is a kush cup (soup can), although they do tend to aspirate air. If it was belching gas through the molten metal in the sprue and you were pouring through flame and cloud of black smoke, that creates a lot of turbulence in the pouring stream, and consequently casting defects. A longer sprue and cup, can also help minimize this. When I have a pour that produces no flame and smoke, I know I have a good feed and mold system. Knowing you, and assuming this is a machining blank, these things matter. Most likely no. Hotter, but I agree with Top's range and generalization. I'd need to know a little more about the size of the part and feed system, but by guess, it would likely be in the lower end of the stated range as Tops suggested. Hotter will almost always yield higher first pass success in lost foam, but at the potential cost of metal quality from superheating the melt. Best, Kelly
Didn't see this in your post when I replied above. -The dreaded lost foam pause. It's accentuated with chunkier castings. The fail is most likely caused by interupted pour and consequently localized mold collapse around your sprue. Longer sprue, bigger cup, maybe bigger sprue cross section depending upon answers to Qs. Best, Kelly
Thanks Kelly- I used a pouring cup made from aluminum duct tape as you have done before. The casting size is about 7 x 3.5 x 1.25". The sprue was 1 x 1.5" in cross section and 3.5" long. The pouring cup was directly over the sprue so everything was pretty much vertical. Yes, I was pouring through flames and black soot. My crucible and pouring shank were filthy. That part is a base for a tool post grinder to attach at the cross slide. It would be machined to final dimension. It will not see much force in use.
When I was using those, they were just the lower cone. I still used steel tube as a pouring cup and packed sand to the outer top of the cup to prevent run outs and gain a little extra head. An all foil cup leaves the walls vulnerable to collapse if the cup starts to empty. That all seems reasonable but would benefit from more sprue height, maybe 6" total sprue or so without cup height. The combination of that additional head from sprue height and a cup that has about 25% of the part volume helps create enough pressure at the metal front to promote the gas to escape through the mold instead of back up the sprue. For reasons I'm not fully able to explain, my parts always benefit from being slightly tilted as opposed to vertically oriented in the flask. It's kind of like tilting a cope drag mold when filling flat plaques. I use low density EPS foam for sprues just because it evaporates faster and gets the melt settled down and some head on top the part, though probably not a big deal for a part like this. That bigger cup volume gives that extra buffer time when the mold starts taking metal more rapidly which is especially helpful if your view becomes obscurred by flame and smoke.....but you should have less of it.......and the goal is none of it. Best, Kelly
Pour temp 1300 Sprue 1" dia x 7" long Weight with sprue 1Kg. The entire system weighs in at 1 Kg. I poured at 1300 deg from my stainless crucible which had excess capacity. I did not want to be short. As you can see from the pic, I still had trouble getting metal to go down the sprue. I initially though it had failed! Why do I have all the metal protrusions like berries on the sides? Is that a function of too coarse sand or not enough dry wall compound or something else? One issue is that the sides warped. I assume distortion from the weight of the sand. I could have used a cross brace and cut it way later. Even a really skinny brace that did not fill would hold the sides in place during packing. Learning. I'm gonna put this on the mill and face the sides flat and square. Seems like a good way to make project boxes. I would just add a front panel +/- rear panel.
A more generous pouring cup rather than just a foil cone is much more forgiving, especially if it allows you to get 2-3" of molten metal head on top the sprue. That inital contact ignites the foam and creates turbulence in the feed system. A little initial head pressure helps a lot and reduces the flame and smoke. It could be a combination of coating thickness, and sand size, but more likely inadequate packing. If the sand isn't vibrated it can be unstable in areas and that allows the metal to locally penetrate areas of thin coating. You can usually easily knock them off with a light tap on a chisel or screw driver. With adequate packing, I usuallly say enough drywall mud to turn the pink/blue pattern foam white is sufficient thickness. What's as or more important than sand mesh size is that the sand be very dry. The "play sand" at the big box stores is total crap because it's moist and has huge scatter in mesh size and even pebbles. Moist sand will not flow (well) under vibe whereas dry sand will. I don't now how coarse your sand is but mine is the stuff labled fine/very fine (though that's not fine for foundry sand) at the big box stores. I classified it with sieves it and the vast majority is in the 55 mesh range with some scatter above and below. A coarser mesh with insufficient vibe will aggrevate the tendacy for these little metal penetration (through the coating) surface flaws. If you vibrate as you fill, or at stages, it really helps avoid pattern distortion. I'll put a couple inches of sand in the flask, then vibrate. Then place the pattern in the flask and ladle/shower in some more sand a liter at a time until the patttern gets covered then vibe again. After the bottom few inches of the pattern get covered/vibed, it's position is secured in the mold then you can just continually shower in the sand while vibing until full. If you completely (loosely) fill the flask then vibe, the sand around and above the pattern will settle, compress, and have greater tendancy to distort the pattern as the whole mold settles and moves. If you vibe/compact in stages or continuously, this will eliminate pattern distrotion. The commercial processes continuously shower sand into the flask while vibrating. You need a vibrtaor. It needs to be fairly energetic. A 5 gallon bucket of sand will weigh ~70 lbs. If you vibrate in stages the mold is only a fraction of that weight and the packing is better for a given vibrator energy. The lower part of the mold where your pattern resides is much more important than the top half that just contains the sprue and cup. Best, Kelly
Reading through this post reminded me of a video I made. There are 2 pours in it, both with offset pouring basins inspired by Kelly. The 2nd is way more tranquil. Looking closely it gets kind of easy to see why. I was still doing a lot wrong (pouring into the basin turbulently from like the moon, metal crucible etc), but for most of the 2nd pour the basin was full, with no room for gas to bubble up through and ignite. Hopefully, this will help with future pours. I know it changed mine for the better. Also a little discussion at the end about the differences between coat, and no coat.
I was surprised how much the offset pouring cup improved the quality of my pours. With there use, most of the time I achieve no smoke or flame at all. Prior to their use, I would occassionally get small oxide and fold flaws deep in the mold/casting and I believe this was from turbulence early in the pour. The offset pouring cup, along with good pouring technique as BattyZ mentioned, eliminated this. Here's a thread on the pouring cups with several different methods for fabricating them. http://forums.thehomefoundry.org/in...le-offset-pouring-basin.688/page-3#post-44381 Having said this, they won't eliminate metal penetration flaws. That's packing and coating related. My surface flaws tend to be related to air bubbles in the coating surface because I dip coat. That's a surface tension and coating viscosity issue. It produces tiny bee-bees on the surface. They can also usually be knocked or scraped off, but are better eliminated by using surfactents (soapy water) on the pattern. All of this is really more the nuance stuff that collectively matters when you want to make high quality castings. It's not limited to lost foam. There are analogous discussions for every casting method. Best, Kelly
