Advice appreciated: making another attempt at lost foam

Discussion in 'Lost foam casting' started by Mark's castings, Apr 14, 2018 at 9:37 PM.

  1. I'm planning another attempt at lost foam casting tomorrow. The foam pattern is for a 10 inch wheel bench grinder guard and this time I've made some added three vents around the rim for excess aluminium and gasses to exit. My first attempt at this casting was made from expanded polystyrene and PVA glue which was a bit too flexible when burying in the sand and it failed as it wasn't deep enough in the sand to prevent a such a large area from developing pressure and leaking. This new version has yellow extruded polystyrene sheet and white expanded styrofoam glued with hot melt glue.

    What I'd appreciate guidance on is the pouring sprue, the last one was made from a few layers of foil which melted and collapsed fairly quickly. I have some 0.3mm soft sheet on a roll which I thought might last a bit longer during the pour but I'm completely ignorant about what's required regarding the sprue and the venting. Do I just have a short stub of foam and wrap the sheet tightly around the stub to form a cone to the surface for the sprue and the vents?. Does the loose sand need any venting anyway: I guess it would let you know you've poured enough metal when they melt. Any help will be greatly appreciated.

    Edit: I have watched Kelley's video on the topic of using the foil tape.

    grinder guard.jpg grinder guard2.jpg aluminium sheet.jpg
     
    Last edited: Apr 15, 2018 at 6:14 AM
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Feed systems for lost foam can be a bit counter intuitive compared to traditional sand casting and top feed systems work best for most instances.

    No vents or risers will be required. IMO traditional vents are never useful because the entire mold is a vent and the vent cannot be a vent until molten metal evaporates the foam obstructing it.

    I'd recommend the orientation below with slight preference for the one on the right.

    Marks Grinder.jpg

    The cup would be about 2" in diameter and 4" tall. The gate would just be a round foam button about and inch in diameter. The reasons for this orientation are a more involved discussion but the primary reasons would be:

    1. Provide good sand packing and minimize any overhangs and blind cavities that would tend to prevent sand from naturally filling.
    2. Reduces the projected area so there is less tendency to float the sand. For less dense alloys like aluminum this is not as critical.
    3. Provide as continuous path of molten metal flow through the part as possible

    Top fed systems were a bit strange to me but they work very well in LF. The hydrostatic pressure of the molten metal supports the mold as the foam is vaporized. If the part is flimsy, you can form small pieces of sheet metal and glue them onto the pattern to support it in the mold. Use vibration to pack the sand well. Coat with thin layer of drywall mud for improved surface finish.

    Best,
    Kelly
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I forgot the most important bit of advice.....pour hot! My go-to pour temp for aluminum LF is 1540F. Evaporating the foam takes energy from the melt so it must be poured hotter than conventional sand casting

    Best,
    Kelly
     
  4. Thanks Kelly, I'd modify the pattern accordingly, pour it hot and let you know how it goes. I should have a suitable container to bury it deep enough, along with some small cast iron 2Kg weights I can put on top as it has a relatively large surface area.
     
  5. ESC

    ESC Copper Banner Member

    Gravity is your friend. I usually orient vertically for that reason and it proved out with Kelly's goosenecks and the orientation in his sketches above. That being said, while my wheel guard was slightly smaller than yours, for a 7.25" wheel, it was poured horizontally and cast nicely. I eliminated the bore so there was a reservoir of hot metal to insure complete fill. There was a small void on the underside from unpacked sand, but I intended to machine the interior anyway. The Kush head is still on the casting and I use it without a cone and just extend the foam sprue several inches above the sand.
    Like Kelly says, "pour hot".

    IMG_20171118_170054.jpg
     
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  6. Thanks to the experienced advice on this forum, here's the modified sprue and cup arrangement. The old runner and risers had to be cut off, the glue has stuck too well and would damage the foam so I'll just machine it off if/when it casts successfully. The 0.3mm sheet was a little bit difficult to shape but being dead soft it's easier than say a 0.09mm soda can. I've used cable ties to hold it in position while I use wire to tie the sprue with more heat resistance than the tie. The larger sheet opening just has the sheet notched and folded back on itself.

    It's raining today and the neighbour next door got home jetlagged so I'll wait a day or so until the skies clear up a bit.

    sprue cup 1.jpg sprue cup 2.jpg sprue cup 3.jpg
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I just tape mine in place. I use that aluminum foil tape. It has aggressive high tack adhesive but you could use packing tape too. Not sure about the cable tie.....might cause some problems when it melts. Orientation looks good. I fill with sand a quart cup at a time until I get the pattern supported well in position. Good luck with the pour. Vibrate the sand...pour hot!

    Best,
    Kelly
     
    Mark's castings likes this.

  8. I'll cut off the tie, it's a lot of plastic and the wire should hold it in place now.
     

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