Dip Coating Polystyrene Foam

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

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I’ve always brush coated my lost foam patterns with mildly thinned lightweight drywall joint compound, aka mud. To get to the next level of consistency for technical lost foam castings, I really need to work on coatings and namely high permeability coatings. Among other things, permeability will be a strong function of coating thickness. Brushing is quick and dries fast but coating thickness varies considerably. Controlling the viscosity and surface tension should yield consistency in coating thickness and permeability to the extent mud is permeable.

    Recently, on a more complex part, I slush molded an interior core with thinned mud. I use wax fillet to detail patterns and wax is very hydrophobic (repels water). Dippable solutions need to be thinned more and with mud being water soluble, that means more water. Even though the interior surface was covered multiple times in slushing, the wax did not wet as shown in the yellow outlined area. The exterior of the part was brushed with a thicker mud slurry.

    1 Wax Repel.JPG

    I had the presence of mind to try a sample first from the same batch of thinned mud. Here it is bare foam and wet after dipping.

    2 Dip Sample.JPG

    …..and after drying. – I thought it looked great.

    3 Sample Dried.JPG

    In preparing the slurry for the sample above, I started thick and continued to thin the mixture. Each time I wiped the mud off the sample part and recoated. But the foam sample contained no wax and the multiple coats/wipes of mud may have helped with surface tension and wetting. Then it Dawned on me, I had forgotten to add surfactant to the slurry or the pattern.

    I went into the wife’s kitchen cabinet looking for anti-spot dishwasher additive and found that but also a bottle of Dawn dish washing soap. I looked at the Dawn label and it said “contains biodegradable surfactants” …….and I thought that sounds like the ticket.

    So, I added a half dozen drops to the same 8oz slurry sample used above and prepared another foam sample pattern, this time with wax fillet. Here’s the foam pattern and still wet after first dip. There were noticeable air bubbles.

    4 Wax Sample.jpg

    After the first dip dried, the bubbles had disappeared, but the addition of the Dawn soap not only successfully wetted the wax, it also made the slurry coat noticeably thinner. So, I decided to dip again. Here is dip #1 & Dip #2 after drying.

    5 Dip 1 and 2.jpg

    I think dip #2 looks great and it may be possible to adjust the viscosity to accomplish same in one dip.

    I dried the slush molded part by placing it over a forced air furnace vent in my home with a vented cardboard box. It dried in two hours. I’m going to make a simple drying box to accomplish same for my future experiments.

    I also have “Top Coat” drywall compound. The top coats contain vinyl and spread more smoothly but are supposed to be applied thinner. I had originally shied away from them thinking vinyl wasn’t something I wanted in my casting coatings. But given I’m burning out a polystyrene pattern…what’s a little vinyl in the coating? Now I’m kind of liking the idea because I’m thinking the vinyl may burn out and increase permeability of the coating.

    I'm also trying to get my hands on some professional lost foam dip coatings but want to work both avenues. Other than trial and error with castings I’m scratching my head a bit on how to measure coating permeability. I can come up with a few ways to do so at room temp, but at casting temps it's a different ball game.

    I intend to work on this over time. All comments and suggestions welcome.

    Best,
    Kelly
     
    oldironfarmer likes this.
  2. Gippeto

    Gippeto Silver

    Looks pretty darn good to me, and timely too as I've some parts CAMed up ready to cut.

    Only the obvious thought...hit the mixture with a vacuum pump prior to dipping. Would test pour something first, maybe the vacuum is not necessary?

    Need to see what I have for a drying box, the dry time you're getting would seem to allow going from cutting foam to pouring all in the same day, which would be amazing.

    Al
     
  3. Petee716

    Petee716 Gold Banner Member

    Yes Dawn belongs on the list of wonder tools like duct tape and drywall screws. There is a lot written on the web about permeability testing but outside of a lab it seems like you may be limited to measuring fill times as a function of permeability. Maybe you have other assets up your sleeve.
    Here's some food for thought
    https://waset.org/publications/6159/behavior-of-generated-gas-in-lost-foam-casting

    Pete
     
  4. I tried putting a small piece of foam in my toaster oven set to 100F. I figured it's good for 140F attics, but the radiant heat shrank it right up. I've been using a fan coupled with a radiant heater and getting good dry times, a couple of hours.
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'll be looking forward to that.

    Thought the same. I don't think vacuum could hurt. Pretty common step for investment molds.

    Same day coating & casting is very possible. In the summer, I set the coated pattern in the sun and with a light breeze, and in 1-2 hours it's dry if there aren't any long tunnels or blind passages. If so, you need to move air through them because without circulation the vapor pressure of water remains too high and poor diffusion vastly slows localized drying.

    It doesn't take much, especially with thin coatings. A hair drier would be much more than needed but I'm sure would work. I suspect you'd need to take care not to overheat the pattern.

    I'm thinking 200w light bulb with a dimmer switch for the heat source mounted in a ceramic light socket suspended in a 3” steel tube, a little flexible duct, a muffin fan with an intake choke pushing air across the light bulb, and a cardboard box. I have a couple spare PiD controllers and TCs and could easily make it control to set point.

    Best,
    Kelly
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks for that Petee. Read it. Will post thoughts later.

    Best,
    Kelly
     
  7. Gippeto

    Gippeto Silver

    Thinking along the same lines for the drying box, but less wattage, maybe 40w to start. Won't be putting any temperature controls on it and may just duct it into a storage tote to see how it works out.

    Merry Christmas,
    Al
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The gist of the paper is higher density foams and temperatures make for larger amounts of gas and gas gaps between the molten metal and decomposing foam (which further insulates and slows the advancement of the metal front), and require more permeable coatings. Don’t think I could duplicate their permeability measurement method but there were a few useful little nuggets.

    The paper cites a coating slurry of 50 Baume and a .2mm to .35mm coating thickness (.007”-.014”). That’s thin but the high end of that is probably about where I’m at on the samples above. I don’t think any non-fired, air-dried, coatings would add appreciable shell strength at those thicknesses. The paper says the coating shell thickness prevents sand/mold collapse? Not sure I buy into that. Then how do we manage lost foam castings no coatings?

    The paper discusses foam existing in solid, liquid, and gas phase during the pour, the paper suggests 4-5mm gas gap for Al and 25-60mm for Fe but also mentions no gas gap for Al and 5mm Liquid Expended Polystyrene (LEPS) zone. When I did metal propagation tests behind glass I’d say my gas gap was very small and I could see the foam boiling in a very small zone (~1/8”) in front of the advancing metal. Those tests are discussed here.

    http://www.alloyavenue.com/vb/showthread.php?12972-Gating-Tubular-Lost-Foam-Part&p=202622&viewfull=1#post202622
    http://www.alloyavenue.com/vb/showthread.php?12972-Gating-Tubular-Lost-Foam-Part&p=202639&viewfull=1#post202639

    The paper test samples were A356 aluminum in their tests at 700-720C. 1292F-1328F. This is quite a bit cooler than I pour which is typically 1550F-1600F. I can’t get complete castings a t those temps unless they a heavy sectioned. This may in part be due to foam density and excessive gas creation. They do talk about a percentage of the foam being expelled as liquid (LEPS). I can attest because at lower pour temps, after demolding, the mold media will appear wet and clump the sand.

    The paper considers high density foam to be 20kg/m3 and low density to be 10kg/m3. The pink Owens Corning Foamular 150 and 250 extruded polystyrene (XPS) products are 20kg/m3 & 25kg/m3 respectively classing them as high-density pattern foam. This I knew but there is a trade between density and machinability and unfortunately higher density machines, finishes, and just generally fabricates better. Makes for stronger patterns too. But it does generate more gas when decomposed. The only foam I’ve worked with at 10kg/m3 is expanded polystyrene. It did not machine or finish well and was pretty flimsy.

    For me, I’m stuck with the 20kg/m3 XPS foam because is machines and works well, thus my interest in higher permeability coatings. I think my best bet for home brew coating is controlling (thin) coating thickness, good pattern wetting, and maybe some small amounts of additives that decompose upon molten metal contact, and also the use of flask vacuum.

    Best,
    Kelly
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You'll want to shoot for a temp of about ~110F. Higher wattage and heat input just means you can/will need to move more air which should yield faster drying times. There's a combination of both that will produce this equilibrium temperature as long as you aren't loosing too much heat along the way. Higher temps dry faster but risk pattern distortion. Pretty tough to beat my househould furnace vent capped with a vented cardboard box......but it wont always be Winter here in the Midwest and I won't always be patient enough.

    Best,
    Kelly
     
  10. Petee716

    Petee716 Gold Banner Member

    Thanks for your expanded thoughts on the link Kelly. Your previous comments about permeability got me thinking about how one could go about measuring. Among other things the paper mentioned the methods they used albeit briefly, so I thought it would be of interest.
    Thanks again.

    Pete
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Appreciated it Pete. Other than shell strength most of the other stuff cited was consistent if not confirming. Most of the LF coating thicknesses I see referenced are .2-.5mm and this one was as well. I think that reducing/controlling coating thickness is my first opportunity for improvement as it will likely produce greater and permeability and more consistent results.

    I poked around the site at that link and there were some other interesting topics as well.

    Best,
    Kelly
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    After experimentation, I don’t think there is any doubt that dip coating with drywall joint compound (aka mud) is viable and will make for quick and consistent pattern coating. The one surprising thing is the profound improvement to the coating caused by the addition of Dawn dishwashing soap to the slurry. It's also necessary at the thinner mixes because the slurry wont properly wet foam and especially wax, without it or comparable surfactant.

    The improved wetting effect of the surfactant was expected but the improved coating flow, self leveling, and ability to achieve thinner more uniform coatings was a real bonus. I think this could be easily controlled by mixing up a 5-gallon bucket of slurry, using a Zaun cup or trial and error to maintain a target viscosity, and stirring with a drywall paddle before each use. If the dip coat was exhibiting surface bubbles that persisted when dried, you could probably use the same bucket to de-gass the slurry with vacuum if needed.

    Being able to quickly and consistently apply coatings to foam patterns is of value. Drywall joint compound is very practical and seems to get reasonably good results with aluminum. Dipping may be a good time saver or in instances where there are very hard to reach or internal surfaces but with my most recent castings I can’t say at this point if I really see an impact on the casting quality versus brushed coating.

    I still think there is something to be gained through the use of lost foam coatings with different compositions and properties. This deserves more work and a thread of its own. -More later.

    Best,
    Kelly
     
  13. Come on, Kelly. You told me months ago to add surfactant to get the brushed coating to cover wax. Dawn was all I had and it has worked perfectly for me.

    I'm about ready to mix up a bucket to dip with now.

    Thanks for your guidance.
     
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yah-but months ago I was using glycols for surfactants.....they worked but not like Dawn. Maybe there's some inflection point as far as concentration but for me it was a dramatic difference in respect to the characteristics I noted with Dawn. It's like Petee said...it's a wonder drug.

    Best,
    Kelly
     
  15. WhoLady

    WhoLady Lead

    To be sure I understand this thread, you added Dawn detergent to thinned drywall mud, then dipped your foam pattern in and thoroughly dried it before casting?
    May I ask:
    What kind of proportions, did you decide 6 drops/8 ounces was a good recipe?
    Did you decide whether it is definitely best to coat again after drying or would one coat work well too?
    Will a coating help with keeping details on the finished casting or is it more for keeping smooth areas smooth? The last week or so we have been casting skulls. Pretty happy with the results but wondered if we could get a smoother result with a coating. We don't want to have to grind or buff too much and potentially buff off some of the details like the teeth etc. but there is definitely roughness from the sand.
    (edit: I should have said, using mostly brass and brass mixed with copper for these skulls.)
    BTW Very glad I found this forum, I am learning so much here!
    Thanks.
     
  16. dtsh

    dtsh Silver

    I've not used it for LF, but I use(d) dishwashing liquid as a surfacant for plant propagation to sterilize my tissue samples before culturing. I had started with Tween, but found Dawn as effective, easier to find, and way cheaper. I found I needed just enough to break the surface tension of the water, usually a single drop in my application, but likely not much more even here; a little bit goes a long way to make water wetter.
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes. I use ready mixed non-setting drywall joint compound. It will store indefinitely with or without thinning. The Dawn was added as a surfactant. Just stir the slurry thoroughly before each use.

    I'm casting aluminum and cannot attest as to whether drywall mud will be of sufficient refractory for Brass. Might be fine. In extremes, I've poured at nearly 1800F and the mud seemed to serve it's purpose about the same as the more usual pour temps of 1400-1575F.

    If you are presently lost foam casting in dry mesh media (not block investment slurry) with no coating, the improvement in surface finish will depend upon the mesh size of your current mold media, but if this is so, I would suspect the improvement will be dramatic. A drywall mud coating with aluminum will essentially replicate the degree of finish on the pattern......in great detail...you'll be surprised. Please report back.

    I used what amounted to a quarter sized puddle laying on top a 12oz cup of thinned mud slurry. At this level, it may have been slightly overdosed, but there seems to be little or no sensitivity to overdosing but the improvement in wetting leveling and flow is significant. In fact, I'd say enabling at the level of additional water needed to reach a dipping consistency.

    This may be somewhat dependent on the degree of thinning. I don't have a Zaun cup or means to measure viscosity, but if you dip a 1/4-1/2" rod in the slurry, it was thin enough to run off immediately and continuously for 3-5 seconds before breaking stream. One coat was sufficient for me. While wet, I could almost see through the coating in places, but dry it was opaque white. I'd estimate the coating thickness at .020"-.030". My experience has been if the coating is white and you cannot see foam color through the coating, it is sufficient.

    You may want to experiment on a small sample piece.

    You do need to make sure it is thoroughly dried before pouring. This will very a bit with coating thickness but as long as there aren't long internal voids to dry, and it's all external surface, drywall compound will readily air dry overnight, or a few hours in sunshine and light breeze.......even in Florida! You will know because it will be chalk white everywhere whereas it tints slightly grey while wet.

    Best,
    Kelly
     
  18. WhoLady

    WhoLady Lead

    @Kelly

    Great response! I am eager to try this now. I did do one skull with aluminum, and can certainly try that again with the coating.
    Looking at our skulls with a critical eye, they definitely need improvement.
    We are using normal play-sand, sifting it to take out chunks, and vibrating it really well.

    I will report back :)
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    After you demold (aluminum), while still hot, if you quench the casting in a bucket of water it will blow the cooked/burnt mud off. There may be some black carbon residue from the decomposed foam on the surface of the casting. It will readily scrub off.

    Also, you can leave the slurry a little thicker (just pourable) and just brush it on.......you don't have to dip.

    Best,
    Kelly
     
  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I dipped these three parts tonight. I’ve made a lot of these thermostat housings and they are relatively easy to make so good parts to experiment with. I had typically just brush coated the outside but being dipped these are coated inside and out. We’ll see how they cast. I have a couple variations to try.

    6 3 Dipped Tstats.JPG 7 Dipped Close Up.JPG

    Best,
    Kelly
     

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