Stacked Plaster mold

Discussion in 'Investment casting Block method' started by magno_grail, Jan 18, 2021.

  1. New to this. Has anyone tried stacking sections of plaster together to make the mold cavity? Looking at possibly casting an air cooled cylinder head and the easiest way to make the cavity for the fins is to machine plates of Plexiglas for each section through the head to build a solid model of the head then cast plaster between each plate and stack the plates together to create the mold.
    The original head's fins are about 0.1" thick at the root.
     
  2. Haven't had any success with casting aluminium in plaster as the heat drives off enough steam to ruin the casting. Maybe some higher temperature investment casting coating would work, but then you'd just do a lost wax casting anyway.
     
  3. rocco

    rocco Silver

    There are plasters specifically designed for non-ferrous casting. Here is the technical bulletin for one such plaster called Hydroperm. I'm not aware of anyone here that's used this type of plaster so read over this bulletin to see if it might meet your needs. BTW, this stuff is pretty reasonably priced at less than $30/50lbs, maybe it'd be worth it to you to just buy some and experiment with it. And if you do, please take lots of pictures and report back here with your results.
     

    Attached Files:

    Last edited: Jan 18, 2021
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    30 years ago I plaster cast a lot of intricate aluminum parts and my father and grandfather were plaster casting aluminum model engine and tether car parts in the 1950s & 60s. It's fairly weak mold material compared to today's investment materials. How big is the part you want to cast and how many do you want to cast?

    Best,
    Kelly
     
  5. rocco

    rocco Silver

    Kelly, do you remember what types of plaster you, our father and grandfather used? And do you know anything about these specialty metal casting plasters?
     
  6. Kelly, elaborate a little, please.
     
  7. OMM

    OMM Silver

    I've been playing with dura bond 90 and 45, lightly mixed with regular drywall mud. I have been trying plaster of Paris mix as well..... but, don't like it. Plaster of Paris is great in my opinion for open steal moulds..... With very thin coats. I have been testing as a release agent with metal and it works great. I have yet to test any of these with sand backing.
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    As far as my Father and Grandfather, I don't know. When I asked my Dad, he looked at me funny and said.....plaster, so I suspect it was whatever they had on hand. I wasn't born yet but years later I did have many of the resulting model engine castings, and for the most part, they were intricate and pretty nice.

    As far as my use, it was a USG product and I used both tooling plaster (now called the Hydrocal or xxx-Cal) and probably what they are now calling Hydroperm. Back then they just had a product/formulation number, no fancy marketing name.

    Cement & Plaster For Prototype Casting | USG

    Select Hydroperm and have a look at the installation guide. These are high strength tooling plasters and the metal casting formulation supposedly have materials added that more closely match the thermal expansion of aluminum, increase refractory, make it stronger, and more permeable.

    For what I was doing, I'm not sure I could honestly tell the difference between the two because it was gravity fed two parts molds. If I was actually investment casting I suspect I may have been able to appreciate the difference. I still have a half a bag of the tooling plaster. It's 25 years old but I haven't touched it for 8 years so it may be a rock. I had friends that owned pattern shops and they had oodles of it. It was dirt cheap and they swepped more of it off the floor than I ever used. They used the tooling plaster for making proofing plugs to proof core box tooling and fit up, but also making large low use tooling. The reinforcement was hemp fiber......also dirt cheap. They used the aluminum casting plaster for making match plates.

    This all was occurring on during the shop's transition to CNC machining all patterns and even though they still had tons of pattern wood, urethane, and woodworking machinery, it was only being used to repair old tooling, and journeymen pattern makers were being replaced by programmers and machines that ran 24/7.

    The first plaster mold I ever made wasn't dried thoroughly enough and promptly ejected the molten aluminum right back out the sprue at me......which was attention getting. After that, I made crude plugs to hollow the back side and reduce the mold wall thickness which greatly increased the rate at which they could be dried, and dried them over night in programmable ovens the pattern shop had. Those worked great.

    .....but I should add for the OP, I don't think plaster would be a good choice for a motorcycle head unless using the commercial metal casting material done as block investment. Even then, big project, thus my questions to him. There was a fella on AA that successfully used bound sand forhis cylinders....Henise.

    Best,
    Kelly
     
  9. Tobho Mott

    Tobho Mott Gold Banner Member

    FWIW here is Bill Jurgenson's recipe for plaster based investment mold material. Note that he was using one piece block molds for lost wax casting, not multi piece molds as described here. But maybe it could be a starting point if you try this:

    20210119_114356.jpg

    "Fill mixing container to about a third" means to fill it with water, I'm quite sure.

    Good luck!

    Jeff
     
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  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That looks very similar to what Rasper uses and recommends, and IIRC, that was handed down to him from one of his acquaintances and has been/is a very commonly investment recipe used by art casters. The discussion piqued my curiosity and I discovered I had a number of USG files stored. These two may be of interest.

    Best,
    Kelly

    Edit, I see rocco had posted same......that one got by me rocco.
     
    Last edited: Jan 19, 2021
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  11. Tobho Mott

    Tobho Mott Gold Banner Member

    I think Rasper has some red clay brick dust grog in his recipe, and Bill's burnout temperature may be a bit lower. Or maybe not. He told me he used to go higher but eventually found 700 works fine and with less cracking.

    But I also noticed the similarity and mentioned it to Bill. He perked up and replied that one of his students lives in Mexico these days... Now, I realize it's hard to imagine Rasper hanging out up here in the frozen north for any length of time, and of course there must be way more than just one guy doing lost wax casting in Mexico, but just then in the moment the world suddenly felt really small... but of course that other guy's name isn't Richard. :D

    Anyhow, I forgot to mention that recipe was used for casting silicon bronze. I've never used investment molds so take this with a grain of salt, but for aluminum (if that is the target here), maybe the mold parts should be allowed to cool down a bit more before pouring. Maybe they don't even need to be kept hot at all, which would make stacking the mold parts easier. However that could destroy them for all I know. Rasper and Bill don't post here super often, but maybe someone else who's used investment molds could say?

    Several of our members seem to very much prefer and recommend (at least for fairly small parts) premixed commercial investment powders that look much like plaster and come with detailed instructions and schedules for burnout, etc., over a diy plaster sand and ludo mix, so that is probably worth looking into as well. But it isn't cheap; that stuff might be probably be prohibitively expensive for making the sort of very large sculpture molds such as Bill and Rasper have cast.

    I really don't know if stacked plaster is the best way to cast your part or not, but it sure sounds like fun to watch and see if it works... Pictures please, however you end up going about it! ;)

    Jeff
     
  12. Billy Elmore

    Billy Elmore Silver

    What is typically used in production a shell core?
     
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Don't know about these days, but my friends that owned foundries used Furan as shell core binder. Dead simple. Polished cast iron core box with natural gas torch applied to backside of core box. Core box had an investment hole and was vented. Sand/binder mix blown in with compressed air and heat activates the binder. Demold on timed cycle. Repeat. This was non ferrous foundry. Not sure about iron guys. The only iron foundry in town went tits up about 15 years ago after 100 years in operation.

    My local steel foundry uses cold box binders with washes applied afterward.......but they're not shell and usually large castings.

    http://forums.thehomefoundry.org/index.php?threads/tour-of-sivyer-steel-castings.720/

    Best,
    Kelly
     
    Last edited: Jan 19, 2021
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Wow, I was much higher......1100-1200F. After that first experience I found religion.

    After that first experience I programmed the furnace to rest at 500F after an 18hr cycle. I took the molds from the oven directly to furnace and poured them. Think I've seen David recommend similar mold temp for aluminum.

    Best,
    Kelly
     
  15. Billy Elmore

    Billy Elmore Silver

    I meant do they usually use a shell core to do the cylinder heads in production? Most of the ones I have seen look like some sort of permanent mold or shell core...except for DA engines..they machine all of their parts from solid stock.
     
  16. rocco

    rocco Silver

    It's all good. The pdf I posted is different than either of the ones you posted, mine goes into pretty specific detail on the use of hydroperm.
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Sorry Billy, different wavelength again. I guess shell and core threw me. Probably just me and a terminology thing but I equate shell mold and shell core to the heat cured bond sand and shell to evaporative pattern ceramic shell casting.

    There was a really cool thread with pictures and video of WWI era aircraft cylinder head casting. The castings were magnificent. I couldn't find it here.....it might have been at AA. Anyone? Some of those were shell. Some were investment. Some bound sand but not the finest and thinnest.

    It wasn't unusual to find sand cylinder castings on <1960s motorcycles but not much after that. I can't imagine any air cooled engine cylinders produced in any numbers today being anything but die/permanent mold excepting maybe small model engines.

    I was waiting to see what the OP said about the number of castings he wanted to make. It would be nice to see a picture of something similar.

    BTW, not to get too far off track here but what's a DA engine?

    Best,
    Kelly
     
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yah mine was a more general document about everything in that general product line. I did read yours and must say the lines in my forehead got deeper when I read the process for controlling permeability.......an ad hoc process for entraining air......that happens on all plaster when you mix it doesn't it?....it expands.

    The other thing I think unusual, for stressing the importance of drying, the document is pretty vague about parameters for drying the molds. The Hydroperm doc seems to suggest only 350-500F for pour temps above 1500F.....? Wow, that is low. No temp or time schedule? What about pouring temps below 1500F or whether the mold should be heated or room temp? Just says when the mold weight reduces circa ~50% or the internal mold temp reaches 220F you're there. The latter wouldn't have worked for me because they were halves and there was no internal cavity.

    But your Hydroperm doc is much more detailed than the one I had (see attached).

    Not planning any plaster casting in the near future but there's a big difference between a max cure temp of 350 and 1100F, but Bill's comment about 700F is in the middle of that range.......and casting at a 500F mold temp.

    Best,
    Kelly
     
  19. Billy Elmore

    Billy Elmore Silver

    Desert Aircraft...model and drone engines. They are the top of the line in model aircraft and make engines for military drones. I dont think they cast anything.LOL
     
  20. rocco

    rocco Silver

    I know what you mean, that caught my attention as well and also the contrast between that process and the vacuum de-aerating that's standard procedure with normal lost-wax block investment.
    I think you misread that part, it recommends embedding a thermocouple into the thickest portion of the mold, NOT inside the mold cavity.
     

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