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Thread: Sprue plate lead build up

  1. #1
    Boolit Master
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    Sprue plate lead build up

    This is a consistent problem for me, that on my 8 cav aluminum MP molds I get semi lunar accumulations of lead around the sprue holes, all on the underside trailing edge. This happens after a few hundred cycles and causes finning on the boolit bases.

    The pot is PID controlled at 720° and I take care to let the sprue harden up prior to cutting. The mold top and the plate underside are pretreated and during casting retreated with a rubdown using a 2 stroke oiled rag. The sprue bolt tension is maintained by a wave washer (there's also a wave washer on the stop bolt) and the bolt is turned in enough so the the plate and its handle don't quite open by themselves when the blocks are rotated handle side down. There is also a set screw for the bolt so that it doesn't turn, for which I filed a flat into the bolt threads to set the position.

    So, any suggestions on how to keep the build up from happening and how to get rid of it now that it has? Unlike lead specks and other smears on the hot blocks or sprue plate, I can't remove these build ups with a rag or small alloy ingot. Do I need to sand the underside of the plate?

    Thanks in advance.

    Kevin C

  2. #2
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    Winger Ed.'s Avatar
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    Mine do that too if I'm running the temperature a little too hot.
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  3. #3
    Boolit Grand Master Outpost75's Avatar
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    I keep a sardine can of water and wet sponge on the bench top and watch the sprue as it hardens. When the sprue turns from shiny to frosty I lay the sprue pour of the gang mold onto the sponge, and let it steam to a count of three before opening the mold. If smearing persists, alternate between two set of blocks to give the sprue more time to harden. Remove any smears on block and sprue cutoff surfaces with a piece of washed burlap while the mold is hot.
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  4. #4
    Boolit Master

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    I use a old wash cloth folded and bullet lube to wipe some melted lube on the bottom side of the sprue plate and wipe off excess also a quick wipe on top of mould blocks sometimes

  5. #5
    Boolit Master
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    Winger: 720° is too hot?

    Outpost: I'd tried the sponge technique and ended up with rusted hardware. I do use two molds alternating and have scrubbed the mold and sprue with oiled cloth and it does get off just about everything but what is right around the sprue holes. The cloth is knit cotton, maybe the burlap is rougher and has better scrubbing action?

  6. #6
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    Quote Originally Posted by kevin c View Post
    Winger: 720° is too hot?
    I'm not all Scientific with this stuff.
    I don't even have a thermometer.
    I just do what works for me.

    When I start getting that lead smear, I turn the knob on the Pro-Melt down just a bit,
    slow down my rate to let the sprue cool a second or two more, and go on.
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  7. #7
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    Cutting the sprue too soon.
    I cast from 2 molds at a time, don't have that problem.

    Those Really Big molds take a lot longer to cool.
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  8. #8
    Boolit Master
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    Hmm, it's true I like a little matte frosting on the boolits: they seem to have the best and most flawless fill out and finish that way. And I recall now that that occasionally I get a "crumbly" sprue at the same time.

    Maybe keep the same mold/alloy temp and cadence, but also use a cooling fan or the damp sponge technique to harden the sprue more? I already try using two molds.

    And for the life of me, I can't get the buildups off the bottom of the plate, hot or not, prelubed or not. Any options besides sanding? Does deleading cloth work?
    Last edited by kevin c; 07-02-2020 at 05:39 PM.

  9. #9
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    Quote Originally Posted by kevin c View Post
    And for the life of me, I can't get the buildups off the bottom of the plate, hot or not, prelubed or not.
    I don't bother. If it hurts anything, I'm not smart enough to notice.
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  10. #10
    Boolit Grand Master Outpost75's Avatar
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    Quote Originally Posted by kevin c View Post
    Winger: 720° is too hot?

    Outpost: I'd tried the sponge technique and ended up with rusted hardware. I do use two molds alternating and have scrubbed the mold and sprue with oiled cloth and it does get off just about everything but what is right around the sprue holes. The cloth is knit cotton, maybe the burlap is rougher and has better scrubbing action?
    After each casting session I brush the iron hardware with plain mineral oil USP, which prevents rust but sublimes away when mold is pre-heated. Burlap is coarse and better scrubber, but should be washed to remove dirt in the weave.
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  11. #11
    Boolit Master
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    Thanks for the suggestions! I'll consider lowering the temp or working on cooling the sprue more, and maybe find some burlap.

    The irony is that I have very consistent bad technique: for four molds now I've cast the same way, gotten great boolits for about the same number of pours using the same alloy and cadence, and all the molds now have the same lead build ups and base fins on the casts (along with other mold issues). I'm hoping to fix up the first four, and not damage any future molds the same way.

    Any suggestions for getting the lead off the bottom of the sprue plates? I'm hoping that getting the plate bottom flat and smooth will eliminate the fins.

  12. #12
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    Quote Originally Posted by kevin c View Post
    maybe find some burlap.
    Old blue jeans make a good substitute until ya can find real burlap.

    I save the cut off legs from making short pants.
    It's pretty handy for things like you're doing.
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  13. #13
    Boolit Master
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    That's a good suggestion, Winger. I love finding use for stuff that others would throw out without a thought. Makes me a pack rat, maybe, but I can put up with being called that.

  14. #14
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    I occasionally get carried away when I get everything going nicely casting , then I cut the sprue a little to quickly , or over heat the mold - and smear the bottom of the plate or top of the mold . I have a shop rag on the bench that I use to wipe off excess oil when I apply sprue plate lube , that will usually clean the smear before it builds up while the mold is still hot , but other useful objects such as a wooden pencil when used to clean a speck of lead also coats the area with a little graphite .
    In my limited experience I have found lead likes to stick to lead so a speck of the top of the blocks or the bottom of the sprue plate will quickly grow if not taken care of early on . Continuing to cast with a lead smear will score the mold leading to more of a leading problem .
    A mold that is well lubed is pleasure to cast with .

  15. #15
    Boolit Grand Master Bazoo's Avatar
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    I don't have any experience with such a large mould. But what I use is a carpenters pencil. Since you already have a buildup, you need to remove it. Heat the mould in the lead until it's so hot that the lead melts and then wipe it clean. I will let my moulds come to pot temperature by propping them in the lead with my dipper keeping them from going too far. Maybe ten minutes. I rubber band the handles to keep the blocks closed. Then color the bottom of the sprue plate and top of the mould with the pencil. You can do this hot, and if you do it as soon as you see lead building up again the pencil will scrape a lot of it off.

  16. #16
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    When I get smear on the sprue plate [not often] I heat it evenly with a propane torch ad wipe the molten smear off with a COTTEN cloth, not synthetic.
    Some are concerned about warping the plate by heating it with a torch but I haven`t had that problem by carefully heating it evenly.
    Bullplate lube or some other synthetic 2 cycle oil applied to the plate when it is hot will help prevent smearing.
    Be sure to wipe off any excess oil before continuing casting.
    I cool my molds/sprue plates between pours with a small fan.
    Using these methods I have nearly eliminated sprue plate smears.

  17. #17
    Boolit Master


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    If I encountered the problem you describe I would get the sprue plate clean, then rub the surface with a # 2 pencil to encourage the lead not to stick. Cast on!
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  18. #18
    Boolit Grand Master Outpost75's Avatar
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    This is caused by cutting the sprue while the center of the base of the bullet is still in the “Pasty Phase”. This “Pasty Phase” can be as wide as 150 degrees. What is happening is that the antimony is crystalizing and floating in a tin/lead liquid. As the cutter swings across the opening, a little of the antimony is deposited on the cutter. This is why it takes as you say “several hundred cycles” to finally force up the wave washers. Graphite and two-cycle oil will slow this process, but not stop it. Once it starts building on the cut-off plate from one cavity it will rapidly expand to the others.

    I would only recommend sanding on a granite block or plate glass backer if the cut off plate is steel. For aluminum, try gently heating with a propane torch and scraping with a lead pencil or sliver of hard wood.

    The only cure for this situation is to wait longer before cutting the sprue. The last cavity filled must cool below the “Pasty Phase” or it will start building again. There is a reason automatic casting machines use two cavity iron moulds, thermal uniformity and controllability.
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  19. #19
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    Make sure the sprue plate is sitting flat on the mold (DO NOT OVERTIGHTEN THE SPRUE SCREW, THIS CAN MESS UP THE MOLD)

    Look for the sprue to change colors before cutting and keep an eye on the boolit basses.

    You see a divot/dimple then your not waiting long enough.

    With steel sprue plates I have carefully used a razor scraper maintaining the angle of the razor against the plate, it will easily remove the lead build-up.

    As previously mentioned, heat and a piece of wood are the safest (for the sprue plate )scraper

  20. #20
    Boolit Master
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    I found my aluminum 8 cav MP molds cast well in a fairly narrow temp range that, starting out, I tended to lose on the low side rather than overshooting. I think, as a result, I now crowd the high end of the range by cycling quickly so I can cast a lot of keepers before pouring a mess of culls. I may have made things worse by casting with two molds: being conscious of excess block cooling, I actually was doing more cycles per mold per minute than with just the one.

    So, what I'm getting here is that I'm just not waiting long enough for the sprue to harden sufficiently. My options are to wait a little longer on the sprue, cool it faster, lower the operating temp by lowering alloy temp, extending the pour to pour cycle time, cooling the mold (open empty blocks for longer before closing up, etc.), or some combination of the above.

    Miha's sprue plates are nice, hefty chunks of steel (or at least glom onto a magnet like they are), so the razor option (judiciously applied, of course) is viable. And it sounds like a couple carpenter's pencils might be good to add to the casting kit.

    Thank you all your sharing your hard earned experience and tricks of the trade. My only regret is not having asked after the first mold (been a bit frustrating )
    Last edited by kevin c; 07-03-2020 at 06:24 PM.

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