Sprues for LF

Discussion in 'Lost foam casting' started by Gippeto, Dec 31, 2018.

  1. Gippeto

    Gippeto Silver

    Still waiting for the cutters for the little cnc but itching to get rolling with some LF. :)

    Knocked together a quick hotwire and printed the scaled cad drawings on card stock to make templates. Have to admit it worked better than I expected it too. Have some filling (wax) to do on the trigger frame yet, but giving a little thought to how it should be sprued up...posting some pics to see what the hive mind here thinks.. ;) Sprues are not attached yet.

    Also wondering about a Kush head and how much head (head pressure) I should be shooting for.

    For the curious, the rifle will be similar to the Air Force Condor...bore size yet to be determined.

    TIA,

    Al
     

    Attached Files:

  2. DavidF

    DavidF Administrator Staff Member Banner Member

    4" of sprue height will give you about half a psi of head. You should be able to get away with half that.
    I'm not liking how the sprue is on the upper half. Kelly is the undisputed lost foam expert here, I'm sure he will chime in shortly and give you some guidance.
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    They're both relatively small and simple parts and I suspect they would pour fine the way you have shown.

    Different orientations offer different benefits. In most cases, there are multiple approaches that will work fine.

    Besides choosing an orientation that is beneficial to packing and flow/filling, I like to minimize the casting clean up casting near net shape parts, so if possible I also gate into a (to be) machined surface.

    Depending on the degree of (vibratory) mold packing, there also might be a tendency for the sand to become a little unstable in the cavities under your gate in the three contact part. I'd probably be inclined to Sprue and orient it this way. Small risk of short pour if you pour too cold.

    Gippetto 2.jpg
    Similar thinking on the other part....all assuming I've identified machined surfaces accordingly and that aspect is of any value to you.
    Gippetto 4.jpg
    Best,
    Kelly
     
  4. Gippeto

    Gippeto Silver

    Thanks guys.

    Those are indeed machined surfaces. I didn't really like how I had the scope mount rail either, but was thinking it would help keep the part straight. Admittedly, I gave little consideration (as in...didn't even think about it lol) to the gate collapsing. The trigger housing is actually partially hollow to save some machining in the recess for the trigger bits...it's a 1/4"wide x 5/8"deep slot, not sure if that would change your recommendation. The rear where you show a sprue is solid.

    Only a bit of foam and not much to lose...will get them sprued up as suggested, mudded and have a go. Worst that can happen is I make funny shaped ingots. ;)

    Al
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If there is a slot under where I sprued, that would change things. For narrow slots, I'd shift priority to having the canted and open toward the top of the mold to insure that packed as well as possible.

    Best,
    Kelly
     
  6. Gippeto

    Gippeto Silver

    Thank you Kelly. Company has departed, and will slip out in a bit to play with this more. Will attach the sprue to the same place, but flip the trigger housing over so the slot will be "up" in the sand, and pack the slot before placing it.

    Al
     
  7. Gippeto

    Gippeto Silver

    Took some play time this afternoon. :) Not the prettiest, and not 100% success, but I think I learned a little and as the song goes..."Two out of three ain't bad."
    Poured the scope mount rail and trigger housing shown above, and a body for an air vibrator.

    Bet you can guess which one failed by looking at the pail...one of the things I learned was that I need a bigger pail...or more pails. There were gases coming up the sprues and bubbling molten metal off the side of the pail...not desirable. Will also be checking the height of sprued parts compared to the pail before coating next time, having the pouring cups largely buried in the sand with more room around them would likely have kept the metal in the pail where it belongs.

    The failed part was due to the usual failure mode folks report, thought it was full and moved on...it wasn't.

    All things considered, these were simple parts, but what really gets me is the outstanding potential for rapid prototyping. These parts were all in essence drawn in 2d cad, printed on card stock and hot wired using the card stock templates, coated and poured. Talk about minimal equipment. :)

    If a fellow gets a drying setup together along the lines of what was mentioned in Kelly's dip coating thread, a fellow could literally be cutting off the sprue before a fellow doing wood patterns/sand casting had dry paint on a pattern. The air vibrator literally took 3-4 minutes to do up in cad and print, and was glued together, sprued and with a first coat of mud in less than a half hour.

    Lit the furnace with a full crucible at 3:58, a couple minutes on low power and then ramp-ed up to 15psi. Added muffin ingots as there was room until crucible was largely full, reached 1508F at 4:17 and shut the furnace down. Skimmed a little dross while the crucible was still in the furnace then pulled the crucible and right to pouring.

    Still have the top to pour for the air vibrator, and because it's such an easy part, playing with some different sprue setups. Found a couple references to feeding LF from the bottom to avoid trapping gases...makes sense, so worth at least trying.

    Thank you for your help Kelly and the rest of the community here, much appreciate you taking the time.

    Al
     

    Attached Files:

    Mark's castings likes this.
  8. Isn't gratifying how easy the lost foam is? Simple shapes like that I just lay out on the foam with a sharp pencil and then cut to the pencil line. I have a little trouble making a smooth cut next to a pattern and do better freehanding to a line. And I'm gluing while you're printing.:D

    I use plastic buckets. So after burning one I bury the pouring cone deep and make a nice trench around to hold the metal and keep spills from running to the plastic.

    If you get aggressive you can pull the sprue off while it's hot short and make trimming easier.

    Enjoy seeing your progress!
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Pretty Darn good first experience I would say. Out of curiosity, did you pour the scope mount last? Might have been cooler if so. I mentioned that one was probably the higher risk of pouring short. Another +75F of temp may have gotten you there.

    When you use the Styrofoam cups, do they fill with metal the shape of the cup or does the sand collapse around them? The steep angle is beyond the angle of repose and the sand will move if you don't keep the cup full of molten metal and with that classic stutter in the fill rate of lost foam, that can be tricky.

    Most folks use a soup can. I use a short piece of 2 1/2" muffler pipe. When the aluminum cools it shrinks enough, with the smooth walls, I can pull them off the slug and reuse them. They build a little buffer volume of molten metal so when the hesitation in the pour presents, you have a little more reaction time to add metal before the cup runs dry and it prevents collapse of the pouring cup. Just install it around your sprue at the surface and add some more sand to mostly bury the cup and that will prevent run-outs and keep your feed continuous.

    Looks like the hook has been set....Lol

    Best,
    Kelly
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I've done both and cant say I've noticed much if any difference in results. There's pros/cons to each. The biggest issue may be the additional travel length for the metal through the foam. With the denser foams and low permeability coatings we use, I've found I start to have a lot more short pours and defects at >12" of travel length, but that varies depending on the surface area/volume ratio of the part and the rate the melt loses heat in the mold. Pouring hotter, using lower desnity foam, more permeable (or no)coatings, or adding vacuum to the flask, of combinations of all can extend this.

    Happy to help. Now you're hooked.

    Take care,
    Kelly
     
  11. Gippeto

    Gippeto Silver

    Old iron....I enjoy seeing my progress too! lol

    Kelly... Scope rail was poured first, so at this time I'm just marking it up to part of the learning curve. If it fails next time, I will be looking for other reasons.

    The cups filled as far as the height of the sand and then burned off. They were also coated with drywall mud.

    Think I'm having soup for lunch tomorrow...daddy needs a new can. ;)

    Hook is very much set lol

    Al
     
  12. I think I would have sprued the scope rail like you did but without the added runner. I bet it will fill fine.

    You might try 1,550F too. You lost a few degrees skimming and getting to pour position.

    For most small stuff I just make a longer sprue and bury deeper in the sand.

    I built a simple plywood box for stuff which is too big for a bucket.

    IMG_4748.JPG

    That's not me, we were pouring a transmission crossmember for his 57 Chevy project.
     
  13. Gippeto

    Gippeto Silver

    The sprue position was Kellys suggestion and worked well. I put that runner there primarily to keep the scope rail straight...was worried about the sand pushing on it.

    Will try a little hotter as that seems to be key. I also need to get some better alloy and see if that helps my porosity issues. Had blue flame licking out the furnace opening, don't think I was running rich this time. If melting pistons doesn't solve the porosity, the degassing lance gets bumped up on the priority list I guess.

    The casting did actually machine fairly nicely despite the porosity...undecided whether to toss it back in the crucible and try again yet. Might be interesting to attempt casting the trigger housing with the trigger opening in place.

    And I need to spend more time making the foam "purdy". ;)


    Al
     

    Attached Files:

  14. Jason

    Jason Gold

    3ft tall, flat head and PISTOL GRIP EARS..... Oh wrong forum... :D
     
  15. Do you know my girlfriend?:confused:

    Show some respect...
     
    Jason likes this.
  16. Keeping it straight is why I recommended trying it without the runner, just to avoid removal time and potential defects at the intersections. I've found the foam maintains it's shape and also does not seem to shrink like sand cast does. The ears on this piece did not move during casting.

    IMG_3444.JPG

    Also, when you want to slice a thin layer of foam to make a thin section, it's best to slice off the factory surface first or the sheet will warp slightly. By taking off the factory surface you can get a nice flat sheet of very thin material.

    It is interesting how the aluminum looks exactly like the foam, fingernail marks and all. That's a function of the sheetrock mud.

    I don't get a lot of big porosity, but it is there. Some recommend only melting virgin ingots, suggesting that each time a piece is melted it gains porosity. I wonder if it's not more like ice melting, where the air will escape at the melt front only to be captured again. I know other materials are like that, the porosity escapes as the melt front progresses. Minimum time melted seems a critical factor and you talked about pretty short cycle time. Is the level of porosity you have detrimental to the functionality of your parts?
     
  17. FishbonzWV

    FishbonzWV Silver Banner Member

    Al,
    With Aluminum, if you have flames coming out of the vent, then you are pumping in more hydro-carbons than the furnace can burn. I don't care what color they are, there is only so much room in the furnace for complete combustion and if you exceed that you get porosity.
    One more time, turn your gas down. Listen to the roar, if it's throaty sounding, it's too rich. Dial it back until it gets to the highest pitch.
    This is not theory talking, it's real life.
     
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Changing alloy won't help hydrogen porosity. It may help fluidity. You didn't mention what your feed stock was other than ingot from a previous melts. If it was wrought or not a casting alloy, that will make it quite a bit more difficult to cast smaller cross section castings for a given pour temp. If you are going to process/recycle castings or want highest aluminum metal quality, a degassing lance is a good investment. Whether or not it's needed depends on what you're casting and your personal standards.

    If I'm the source of that, the primary reason I use virgin ingot is certainty in metallurgy for mechanical properties and post treatment. Grain modifiers and refiners can also be used with alloys containing silicon for further benefit but it's no better or worse for porosity for a given set of conditions and casting cycles. If you don't know the alloy your using, you're probably not making good use of your time to subject it to specific a heat treat schedule which can have a major affect on properties and machinability. Most of the 3xx series alloys used in automotive casting will respond to post treatment and something that resembles a T5 schedule is fairly easy to do and will noticeably improve strength and machinability over most as cast conditions. Natural aging can do so too....they will harden up with time.

    Best,
    Kelly
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hey OIF, I think you have been holding out on us. What's going on here? Another IFB furnace??.......new thread please....!

    OIFs rig.JPG

    Best,
    Kelly
     
  20. Gippeto

    Gippeto Silver

    OIF... I will try one without the runner and see how flat it stays. As things go, these are very lightly stressed parts and more than a little thicker than needed.

    The level of porosity currently showing would not hurt the functionality at all, purely a cosmetic thing and a coat of primer and paint would also make them disappear. I can't get too carried away, but want to steadily improve the results, and there's a bunch of fine tuning to do, variables to find and eliminate. As far as melt times go, 19minutes is the longest I've ever taken...does seem like I may be about to slow that down more though.

    FB...It's a work in progress, I am paying attention and believe I'm making progress. I did tweak the burner to lean it out some already and will keep tweaking it. Thanks for sticking with me.

    Kelly... Changing the alloy is just a thought, trying to eliminate some variables as far as my "junk" alloy that's been melted and re-melted so many times now. The stuff I'm using is a mix of various castings and bar stock bits, trading up to an alloy that was at least meant for casting should be a move forward...maybe.

    I will be moving forward with a simple degassing lance. Without exception, every video or paper I come across where it's "industry" melting aluminum, they have some sort of degassing going on, and industry is notably frugal when deciding whether it's necessary to spend a dollar. How much of an effect it will have for the home gamer IS in question, but a fellow never really knows until he tries.

    Right now all I know for certain is that there's room for improvement, and there are a handful of things to look into over the next while; burner tuning, degassing, graphite crucibles, alloys and of course procedures. It's going to be a journey, and it may not always be a smooth one but I'm buckled in for the ride. ;)

    Regards,
    Al
     

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