Hello gang. My first time post here. I was doing alum castings for quite a long time, mostly lost foam type. Nothing fancy, just sqare'ish or round'ish parts like pulleys or mounting blocks. Recently I was trying to cast thin walls item - step motor bracket - and it was a failure. Was it cold aluminum ? something with the sprue ? Appreciate you input. P.S. part should be something like that, not exactly but close
You've shown the shape but not the size of the part so there's really no way to gage without some reference dimensions. When you say thin, how thick are the various sections? Regardless of that, the pictures show pretty classic signs of cold shut, so I'd say yes, way too cold. What are you using to measure the molten metal temp? Are you coating the foam pattern? If so with what? Are you using a pouring cup? If so what? Best, Kelly
Thanks Kelly here are some photos with rule for comparison All the sections are the same thickness - 4mm, slightly more than 1/8 fo an inch . I am not controlling temperature - just by the feel of it. I know this is not scientific - sorry.
After gaining a lot of experience (also known as experiencing a lot of failures) and familiarity with their equipment, some folks are able to guestimate pour temperature. I'm not one of them. You should buy or build yourself a contact pyrometer. They can be built inexpensively and there are threads here on the forum if you search. Cheap Pyrometer parts list | The Home Foundry 1/8" is getting pretty thin for any kind of sand casting if the metal needs to flow far but should be doable for a small part like that. The parts have so little mass they lose heat to the cold mold rapidly and freeze. However, I've done 1/16" thickness but had to resort to some extreme methods. Saucer Cup & Spoon – The Lost Foam Edition | The Home Foundry Lost foam method can be somewhat of an aggravation to casting thin parts. Although it does take a little energy to evaporate the foam, the real issue is doing so slows the flow and this gives it more time to lose heat to the mold while filling and prematurely freeze. You didn't answer my other questions which could all be contributing factors to success, but one more question would be the alloy? Was the aluminum casting or wrought stock? Wrought alloys can have poor fluidity and behave more viscously at a given temperature. Depending upon these other factors, most likely, your part would successfully cast at a higher pour temperature. I typically cast 1350-1375F. Since you don't measure temp, that wont do much for you. -If you don't know where you're going, it doesn't matter what route you take to get there. Best, Kelly
The problem is that aluminum is so light. With bronze / copper / cast iron I get decent results with 3mm (1/8") or even 2mm thin walls. These alloys are almost three times more dense than aluminum.
Thanks Kelly, fair enough. Indeed, I need improvement in my casting process. Thanks again for pointing to the DIY pyrometer stuff - will be my next (sub)project . Regarding remaining questions - I am not using any kind of compound to cover styrofoam - just haven't had a need of it. No pouring cup - just an appropriate size of soup can. Relatively rough surface was not a problem - I am machining casting to fit another part. Stock that I am using is mostly scrap that I am recovering from broken engines, alloy wheels, other castings. Just making sure it is not extrusions or rolled profiles. Everything that was previously casted or die-casted.
You'll be glad you did. It's not the best kind of cup IMO but still a cup in my book and better than none. I usually recommend >25% of pour volume to avoid being bitten by the lost foam pause and gapping your pour. Besides surface finish there are several other reasons for using a coating. It will slow the rate at which the decomposed foam can escape to the mold providing a more controlled pour and less chaotic filling of the mold. It also prevents sand from becoming imbedded in the metal surface which can wreak havoc on you cutting tools when machining. You might take a look at this thread. I need to update it now that it's 9 years on but still some useful tidbits in it. Confessions of a Lost Foam Caster, 5 Years on. | The Home Foundry Best, Kelly
Thanks Kelly. It was very good reading. I was trying another cast y'day night having all your recommendations. (No temperature control yet, though) It was much better but still fail I increased wall sickness and also temperature of the molten aluminum by pushing more gas trough the burner on last several minutes. Seems it made the trick. There is still a problem though - part came out way out of square. Looks it was distorted while I was vibrating the sand . On following picture it is really warped is there anything can be done to keep it square more or less ?
Yes, vibrate in stages of depth (or continuously) as you fill, and keep the depth of the sand at a uniform height around your pattern as you fill. By this I mean, if the initial depth of sand in your flask before you place the pattern is 4", make sure you vibrate it first to settle it. Then place your pattern and add an inch or two of sand while/or then vibrate and repeat until full. There are multiple reasons for this method. Vibrated sand behaves like a liquid and exerts pressure on your pattern surface as a function of sand depth. Vibrating in stages or as you fill allows you to more effectively vibrate the lower mass of the partially filled mold. As the mold fills, it becomes more massive and more difficult to pack without higher energy vibration, but the top of the mold is usually just simple geometry of sprue and cup and thus easier to pack and less critical than the pattern shape. If you have decent vibration method, you can expect the sand to settle 10-15% of your flask height. If you fill the flask full then vibe, vs vibrating as you fill the sand will move downward 1'-1.5" for every 10" of mold height and your pattern must be able to withstand this movement and be uniformly supported while doing so or it can/will distort. Whether or not this happens can also depend on the position and geometry of your pattern. If you vibrate as you fill the dependence is much less for the reasons that follow. The positioning of the pattern in the mold is very important with respect to how well it will pack during vibration and consequently how stable the mold will be during the pour. Below are three possible flask positions for your part from least to most preferred positioning, feeding/sprued from the arrows. Position 1: Very bad because it's difficult to vibrate the sand up into the hollow. The mold media does not fully conform/pack to the pattern shape and the casting will typically just fill above the red line unless you employ a very sophisticated vibrating technique. Position 2: Better but horizontal overhangs are undesirable because they are also more difficult to vibratory pack, especially as a hobbyist. This is probably why your pattern is distorting, especially if you filled the mold full and vibrated as opposed to vibrating in stages and as you fill because your pattern is unsupported under the overhang and the sand beneath the overhang settles faster than the sand above it. Position 3: Best. The sand/mold media slides down the sloped pattern and exerts very little to no pressure on the pattern (or at least equal pressure on opposing sides) as the mold fills, thus the pattern does not deform. Hope that helps. Best, Kelly
@3gger , the parts are looking better for sure! I am wondering what the outcome would be with a wax-filleted and coated pattern. I may be looking at a similar style part to adapt a planetary gear to my current z-axis movement.
I am preparing for the next casting attempt with coating. Already attached sprue as per Kelly's recommendations. Will also fill up flask with sand and vibrate it in stages. I don't hade any suitable wax to do a filleting - will candle paraffin work ?
I have mostly used half beeswax and half paraffin, melted in a small double boiler and poured into something to initiate a long skinny shape. The wax rings from setting toilets are also an option, some are soft enough to use without heat. I went down the rabbit hole so to speak with getting an alcohol lamp and making filleting tools from inexpensive metal painting/cake decorating ball tools but I have not ordered 'real' filleting wax from a foundry supply.
Fresh from the furnace This time it was OK and surely workable part. I put sprue , compacted sand in stages and covered model with drywall compound as Kelly suggested. Geometrically it is very good - no warpage or twisting I am still seeing some molten metal flow artifacts, but this is acceptable for this part. BTW - what are this flow marks ? In any case - Kelly, Thanks a lot . I am surely making progress.
The short answer is your feed system and pouring technique need refinement.......but the devil is in the details. A long reply, but if you want nice LF castings, these things matter, and they really don't take any more effort. I know I previously asked and you said you are using a pouring cup, but some of your pictures don't show a cup at the top of the sprue and there are potential clues missing in its absence. Are you removing it when it is hot/short? If so, there is no reason to wait. If you want to separate the can from the cup metal, just pull it immediately after the pour. By that time the casting is what it is going to be and the metal pressure remains the same whether the cup is slipping out or not so there is no harm in doing so. If I had to guess, I'd say it the cup metal has fold defects, black carbon, and lots of dross, and there was black smoke and fire in the cup while you were pouring. Most of the defects in your pictures are what are known as fold defects, which is a common LF defect. They are usually produced by turbulence in the feed system or pattern while filling. They tend to be less at hotter pour temps and worse at lower pour temps.......but we don't know your pour temp. Turbulence creates pockets of gas, and worse, air which allows for oxides to form on the surface of the pockets, and when the metal flows back together the oxide layers prevent metal fusion and form very pronounce knit lines. The most likely contributor in your latest pour is turbulence which starts in your cup, especially if you had the sprue protruding above the sand in the cup. That excess sprue is fuel. It immediately is melted, floats, ignites, and creates a lot of black smoke and turbulence that you must pour through. Round cups and (especially cones) tend to aspirate air due to Coriolis affect and also, you tend to pour right onto the sprue. With the latter, you get deep metal penetration into the sprue before you have the cup full and stable. This is why I use square(ish) offset pouring cups with a weir. Initially I was amazed at how well they seemed to eliminate flaws and improve the quality of my pours and castings. The offset cup allows you to pour away from the sprue so the metal doesn't rapidly penetrate the sprue. Filling the cup rapidly to the top allows you to get a few inches of metal pressure above the sprue. It then begins consuming the sprue in a more organized manner like plugged flow. Be at the ready to add metal......it will seem to pause until the metal reaches the pattern and then take metal faster. -Don't be caught sleeping! You know you have a good feed system when you have no flame or black smoke during a pour. Black smoke means the polystyrene is reacting/burning in air. White smoke is decomposed polystyrene that has been denied oxygen, which is what you usually see coming off the sand when you demold. My pouring cups are reusable and made from moldable ceramic fiber or ceramic fiber board. http://forums.thehomefoundry.org/in...le-offset-pouring-basin.688/page-4#post-52874 But they don't have to be. They can be expendable and made from green sand, resin bonded sand, or a square metal can if you can scrounge one. If so just cut a hole in the bottom of one end, fill the mold to the top of your sprue, place the can with the hole over the sprue, fill the mold to the top of the can. You can remove and empty the can/cup after the pour while the metal is still molten and reuse it. If you very slowly remove it the metal just seems to magically stay put in the shape of the cup. It's not as good as one of my reusable insulated cups (but better than the round cans) because the metal robs heat from the initial pour, and there is no weir (dam) in the flour. The weirs tend to separate the flow in the cup and float contaminates away from the metal feeding the sprue. It can also just simply be sheet metal, folded into the square shape without a floor. Pour on the sand. You can use a piece of tape to seal the corner. The tape will of course immediately be destroyed but the sand will hold the cup in place. Remove it while the metal is molten and retape/reuse it. Your mold should look like this: If you distribute the sprue across a wider thinner contact area on the pattern, it will help distribute the metal across the pattern and promote a more organized less chaotic fill while consuming the pattern: After you get comfortable with the all of the above, it becomes all about making nice patterns. If you are using white foam it is likely EPS (Expanded PolyStyrene), which is ok, but the density can vary quite a bit depending upon the source. The colored foam insulation board (Extruded PolyStyrene, XPS) is slightly denser and machines and sands better than EPS, though EPS can be sanded too. Best, Kelly
Thanks a lot Kelly. I have yet another similar part to make and I will surely follow your suggestions.
One more attempt - incorporating suggestions, tips and tricks . Part came out with defect but usable. I will work on it and , I think, it will be OK. There is a noticeable defect right below the whole. I believe it is because molten metal was flowing around hole, formed two streams that were too cold to fuse completely with each other. A bit disappointing, but I can live with it for now. Another angle - result is OK'ish
With those corner edges weakly formed and unfused knit line at the hole, it still looks like too cold of pour temp to me. I might become suspicious of the alloy having poor fluidity too, but without knowing the pour temp, I wouldn't even go there. It's not a challenging part IMO, and although I'd tend to say it deserves a little more sprue height, even so, as is it should succeed every time at an appropriate pour temp. Motivated enough to build your contact pyrometer yet? My 2 cents Best, Kelly
