Dip Coating Polystyrene Foam

Discussion in 'Lost foam casting' started by Al2O3, Dec 23, 2018.

  1. Gippeto

    Gippeto Silver

    Parts look good!

    "Board with a nail in it" drying rack seems to be working fine too. ;)

    Al
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thank Gippeto!

    Then you'll really like my drying cabinet (from one of my favorites)........sitting on top the heating vent. They'll be drier than Southern Arizona by morning.

    8 Drying Box.JPG

    Best,
    Kelly
     
  3. That looks like excellent coverage.

    I think I've had defects where I have bubbles, although those are pretty small. It still would be nice to see results of that exact piece in that orientation.

    I still plan to dig out pictures of my just-cast lost foam aluminum.

    I'm aobut ready to try lost foam brass.
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The viscosity of this slurry mix was slightly more than the sample pieces earlier in the thread. I'm going to cast with varying degrees of flask vacuum so I wanted a little more coating thickness. I don't think dipping alone will have any appreciable affect on the casting compared to brushing. It's vary fast and in the case of this part, easily handles coating the interior surfaces if that's what you want to accomplish.

    I've been reading everything I can find on LF pattern coatings and they all talk about the coatings being required to provide an insulating refractory layer to slow heat loss to the mold, a shell barrier to prevent metal penetration of the sand, and achieving a specific level of permeability. Supposedly if the coating is too permeable it leads to localized mold collapse and various forms of flaws. Too low of permeability, and it creates big insulating gas gaps between the pattern and advancing metal front, limits the mold filling, and invites a different set of flaws.

    The thing I don't get about all that, if you don't care about surface finish or some imbedded sand on the casting surface, I've never had any problems mentioned when casting bare foam patterns and that is the ultimate level of coating permeability....no coating barrier at all and 30-50 mesh mold media! Now, metal penetration is a whole other issue, and with vacuum depending can occur to a vary high degree. Check this one out from a couple years ago....and this was only 7" Hg or so on a bare pattern.

    9 Vac no Coating.JPG 10 Vac no Coating.JPG 11 Vac no Coating.JPG

    ......and here, the same casting conditions only with the pattern coated inside and out with drywall compound.

    12 Coated inside and out.JPG
    13 coated Inside and out 1.JPG

    So having these parts coated on all surfaces is very significant for vacuum casting.

    If the bubbles are under the coating and on the pattern surface they will definitely cast through.....especially with vacuum assist. I didn't take the time to expose the slurry to high vacuum before dipping. It does entrain air during mixing and the slurry is too thick to naturally allow the buoyant release. It may take both vacuum and stirring to completely eliminate them.

    I think there can be things of interest in regard to the condition of the coating, and the degree and location of the carbon residue on the castings.

    If you will have much time invested in the pattern, you may want to try a simple sample part first. Brass and bronzes are commonly done in LF but not sure if drywall compound is of sufficient refractory. If you go to the US Gypsum website, drill into the drywall compound products and look at the MSDS sheets, there is a surprising degree of variation in the compositions of the various joint compound products. However, most of them are various forms of processed limestone (aka plaster), talcum, small amounts of clay. The melting points of some of the materials vary but some appear marginal for even the pour temps of aluminum. That said, I've never had a single problem with it and have poured some intricate parts at 1800F and couldn't detect any notable differences from lower temp pours as far as coating behavior. At worse, brasses/bronzes may just require some home brew coatings that have higher refractory base materials like mullite, alumina, silica flour...etc to handle the high temps.

    I've been fiddling around a bit with coating compositions and application methods. Might post a thread in the future if the results warrant it and there is enough interest. Since we're getting another 8" of snow today, I'm not really in a position to cast another automotive upright (too much sand to dump and clean up with a wet icy driveway) but I can easily cast smaller items so I thought I'd experiment a bit before casting the rest of the automotive uprights.

    Best,
    Kelly
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Dried. Look pretty good. If I like the casting results, I will vacuum degas the next slurry.

    14 Dried.JPG

    Best,
    Kelly
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    More snow this weekend and 5 degrees F. Had to cast something. So I fired up my electric furnace inside and charged my A10 with 5lbs of A356 re-melt.

    I prepped two of the thermostat housing patterns.

    15 Prepped.JPG

    Proceeded to prep my lost foam rig for vacuum assist cast. The film is vacuum barrier and get’s buried under 3” or so of sand to seal it against the flask.

    16 Vacuum Prep.JPG

    I put a speed control on the vacuum motor, dialed 5”Hg. It can pull about 8”Hg maximum. If I stage two of them in series I can pull ½ atm of vacuum. I pulled the A10 at 1575F, poured both from the same melt, and poured the vacuum second, so it was probably a little cooler.

    17 Cast.JPG

    Here they are the instant I pulled them from the sand with the mud still intact. They were both light brown. I can’t say there was any appreciable difference in color. I don’t know if mud turns brown from heat or if it is discolored by the vaporized foam. May have to cook some mud on a piece of steel.

    18 Compare Coating.JPG

    Here the two are after being quenched in a bucket of water, which blew the mud off of them. I can’t see any appreciable difference between the two as far as finish and amount of carbon residue. The only obvious indication of which one had some vacuum applied is where the foil sprue was located. Since the sprue was uncoated, you can see metal penetration of the sand mold media on the vacuum assisted part.

    19.JPG 20.JPG 21.JPG

    …..and here are the castings degated and after communing for couple minutes with a stainless wire wheel. They’re decent parts, but I can’t detect any difference between the two.

    22.JPG 23.JPG 24.JPG

    These thermostat housings were easy and convenient, but they aren’t very challenging parts. I can probably pour ~100F or more cooler with that level of vacuum and if I have a part that is pouring short, a little vacuum usually solves that. I don’t know how permeable the mud coating is. It must be to a degree because of the ability to fill more complex molds and pour slightly colder with vacuum. I don’t see any advantage of vacuum for simple parts, at least not at 5" Hg.

    Best,
    Kelly
     
    kcb_1983, Gippeto and oldironfarmer like this.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Here's a Video discussing lost foam refractory coating materials and application methods.



    Best,
    Kelly
     
    Tobho Mott likes this.
  8. Melterskelter

    Melterskelter Gold Banner Member

    Another superbly informative video. Nice work, Kelly.
    Denis
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks MS. I had a comment I answered on another video and noticed comments had been blocked on this one. Apparently my default settings had changed to "made for kids" on my last three videos and consequently YouTube turned off the comments section. I have no recollection of making that change to the default setting. I went in and corrected the settings and restored ability to comment.

    Best,
    Kelly
     
  10. Mach

    Mach Silver

  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    No experience using it as LF coating. Seems to be more dense of a coating. One thing about Mud and Poly-Cap, it comes off easy and becomes re-wetted after casting. Not sure about SS in that regard. One way to find out ;)

    Best,
    Kelly
     
  12. Mach

    Mach Silver

    Yup, once the weather cools off enough then I'll give a go.
     

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