Tubular Lost Foam

Discussion in 'Lost foam casting' started by Al2O3, Aug 22, 2017.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    This is a condensed version of a four-month odyssey which is chronicled in its entirety here.

    http://www.alloyavenue.com/vb/showthread.php?12972-Gating-Tubular-Lost-Foam-Part

    Much of the above was learned from the contributions of others in that thread all of whom I hope to see here in the near future.


    I started this project on a lark that turned out to be a great learning experience, very useful tool in casting practice, and surprisingly (shockingly) effective. This was the first in a series of experiments in lost foam casting that I tried only because I had built a furnace but had no molding sand, prep equipment, nor flasks and wanted to cast something. Lost foam was the fastest path to a casting.


    The concept test piece was a water neck for an automotive intake manifold, 1 1/2" OD, 1/4" wall. I made a few jigs for my over arm router which made reproducing the foam patterns practical.

    1 Jigs.JPG
    1 Jigs.JPG


    I decided to detail the parts with wax fillet. While I was at it, I had a silicone mold of an embossed logo from a previous project so decided what the heck and used it to make a wax embossment for the castings to see if it would work.

    2 Installed Logo.JPG

    I had solicited opinions on how to gate the part on the AA forum. This orientation was the winner.

    3 Preferred Gating.jpg

    The blue foam was a Dow product that was leftover packing from items shipped to me. I researched the various foams and found two basic types; Expanded Polystyrene (EPS) and Extruded Polystyrene (EXPS or XPS). EPS is typical lower density beads that is expanded with steam in molded shapes. It’s typically white, low(er) density which is good, hot wires well, and though it cuts easily, it does not machine nor sand to good surface finishes particularly well. XPS on the other hand does machine and finish well with abrasives. I found six densities commercially available from Owens Corning which is the pink trade named as “Foamular”. The big box stores typically carry the Foamular 150 and 250. I found the Foamular 150 to be the best trade of low density, strength, machinability, and ability to achieve fine surface finishes with common abrasive paper…..amazingly so in fact. Foamular 150 is 1.3 lbs/ft3. EPS is usually 1lb/ft3 or less and many use it as sprue and gating material to take advantage of its very low density.


    For a mold, I just started out with a plastic 5-gallon bucket and to pack the loose sand after it was poured in place I had a vibration platform on springs left over from casting refractory for my furnace.


    Pattern Coatings

    I also received quite a bit of advice and experimented with a number of coatings for improved surface finish. About everything worked great but thinned drywall mud achieved as good of result as any, was the least expensive, easiest to apply, fastest drying, and just fell off the castings during quench. The above may only hold for aluminum as bronze, iron and higher temp alloys may require coatings with better refractory properties.

    Glues and Adhesives

    For gluing pieces together, you need to make sure the glue doesn't have solvent that attacks polystyrene. Many do. Hot melt is da-bomb for smaller pieces but tends to set up too fast if you need longer working time, but it does burn out very cleanly. I initially used shellac for glue by applying several coats and drying with a heat gun. Shellac is reduced with alcohol which does not attack polystyrene. When you add the second or third coat, it reactivates the first coats and behaves like contact cement. It makes a good thin joint but doesn’t burn out as clean as hot melt. The happy medium for longer working time was wood glue, sometimes slightly thinned. It also burned out very clean but doesn’t stick on contact like coated shellac so small pieces of packing tape work well to hold parts in place. You can wipe away the excess with a damp cloth or cotton swab. A very thin joint is important if you need to sand the seam. All adhesives will make sanding the glued joints evenly very difficult and load up power abrasive sanding. Hot melt is terrible in this regard. Shellac , best, thinned wood glue a good compromise.


    I had some leftover pieces so I made a couple of similar test parts from the scraps. Here they are sprued up and coated along with a couple of test coupons of the wax logo.

    4 Gated Samples.JPG

    5 3rd Coat Dry.JPG

    6 Coated Logo Coupons.JPG

    Here they are after being vibrated in loose sand. Keep in mind this is just loose sand, no binder of any kind. I also added a pouring cup to make it easier to hit and feed the sprue during pouring. Some folks call these Kush cups, I believe from the fella that popularized it.

    7 Vibe and In Bucket.jpg

    …I fluxed, did not de-gas, and poured at 1375F and here was the result. Wow, I didn’t expect it to turn out that well.

    8 As cast.JPG

    I was dumbstruck by how well the loose sand tubular core remained stable and the quality of the surface finish…….both way exceeded expectations. Now the hook was now firmly set.


    Though the results were very good, upon closer inspection, I found some casting flaws. Though opinions varied, “fold” flaws are common to the process. So I embarked on a series of refinements and process control improvements.

    9 Flaws.jpg

    Best,
    Kelly
     

    Attached Files:

    Last edited: Aug 22, 2017
    Jason and DavidF like this.
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Process Refinements




    I decided to improve my mold rig. I lifted a bunch of ideas from this video. Some amazing info in there.



    https://www.youtube.com/watch?v=6fEU3fuQd9s




    I used a metal 5-gallon can for a flask. I found if you are only doing small castings, say a couple pounds, a plastic bucket or even a wooden box is fine for a flask. But if you are going to do larger parts or multiple successive pours the sand can become hot enough to soften the bucket or ignite the wood depending upon the amount of sand between the casting and flask wall.



    Vibration and Good Mold Packing.



    You can often achieve satisfactory results by just rapping the bucket with a plastic mallet and/or jolting it against the ground. However, the right frequency and amplitude/direction of vibrations can achieve 20% higher packing density than just poured sand and when you do so, the loose sand takes on near bonded characteristics. This can become critical for stability of internal coring, undercuts, and overhangs. So I incorporated an electric vibrator on the wheeled base which I eventually converted to a more aggressive turbine vibrator. Just place the pattern, pour and fill with sand, and vibrate….done, ready to pour. No ramming, no binder conditioning, no mulling.





    I added a vacuum plenum, used a 3-stage central vac motor that could pull 7” Hg, and used polyethylene film as a seal at the top of the mold. As I write this post, I’ve only performed a few vacuum assisted pours. I intend to develop this process further but was achieving better results without vacuum with just gravity fed pours. I found the vacuum caused massive metal penetration of the sand and could destabilize the mold and even ingest sand near the sprue. It also produced lower quality surface finish on coated parts. It did significantly increase the metal propagation speed which could prove very useful on thin walled or very long travel path lost foam castings (more on this in another post). So the rest of this post focuses on gravity fed casting.



    2.1 Lost Foam Rig.jpg






    https://www.youtube.com/watch?v=gwK2kS0GEB0




    Pour Temperature Control



    This proved to be very important for success and repeatability. I have an electric resistance furnace that I can dial temperature but a metal contact pyrometer is far better so I built one. I won’t cover the details in this thread but this is a must have for good foundry work. For lost foam, pour temperatures are significantly higher than with conventional sand cavity molds. This is because the melt must surrender energy to evaporate the foam. In the Chinese video, they suggest 30-50C higher pour temps for Iron. For aluminum due to its lower specific heat, I found aluminum needs to be adjusted even higher. For this part I settled on 1540F. Several other very useful tips I received, suggested sifting and examining the used mold sand. If it appears wet or clumpy, you’re probably not hot enough.



    2.3 Lost Foam Sand Aftermath.jpg



    When I sift after good pours I usually don’t find anything other than flakes of drywall mud and a little burnt sand.



    2.4 Riddled Sand.JPG



    I’ve reused my sand for probably 40 pours and it’s nothing special….just coarse silica……but it does need to be dry. I does have a very unpleasant odor so I have a lid to store it in my bucket.



    Expendable Foil Sprue



    Another caster suggested I use foil around the sprue as it helped keep the sand stable during the initial phase of the pour. I tried this with success and eventually adapted it to use foil HVAC tape to make what I call the Expendable Foil Sprue (EFS). This was a big advancement and when implemented together with higher pour temperatures, my success rate has been essentially 100%. I had noticed that the flaws seemed to occur at about the same depth in the mold. I also noticed there was a lot of initial turbulence in the cup the instant the molten metal hit a foam sprue. Subsequently metal was drawn at very different speeds from the cup.



    2.5 EFS Mounted.JPG



    I believe there are three big advantages of the EFS:

    1. The melt does not need to give up any energy to evaporate the sprue leaving all the heat to go into the pattern.
    2. The initial turbulence in the sprue and cup is essentially eliminated and the metal quickly settles onto the smaller contact patch of the pattern and a much more uniform and tranquil propagation for top pouring. For bottom pouring, the bottom of the sprue can also be used to choke for slower metal propagation speeds.
    3. The loose sand around the sprue remains very stable. I even see the ink on the tape printed through on the casting and sometimes the foil itself survives the pour.

    I believe the above greatly reduced fold defects and now use the EFS on everything.



    So with all of the above in hand, I began producing flaw free castings typical of those shown below:



    2.6.0.JPG



    2.6.1.JPG

    Ultimately, I made these parts from the learnings. These are typical ¼” wall, 1 ½” diameter tubes. These are working parts for a vintage racing induction system and carry engine coolant.



    2.7 CB H2O and Intake.JPG



    Though when sectioned, polished, and magnified, the casting density appear very good, they did tend to sweat fluid under pressure. So I vacuum impregnated them with methacrylate. I did flux but did not degas the melts. Making the parts hermetic is can be accomplished through impregnation to but I may also try some grain modifiers and refiners in the future. If it was a dry mechanical part this would not be needed but the impregnation process can also improve machinability.



    I have some other follow-on threads that I will be posting in this subforum in the near future.


    Best,
    Kelly
     

    Attached Files:

    Last edited: Aug 22, 2017
    Michael, Jason and DavidF like this.
  3. OCD

    OCD Silver

    Will Lost Foam practices work with closed cell foam?

    Kelly got me to thinking, ut oh, we're all in trouble now. :eek:

    I got to thinking of how to go about making a POP hollow sprue mold and started thinking along the lines of foam tubing.
    The only small foam tubing I'm coming across is closed cell.

    Me thinking that I could piece together a plug out of foam, encapsulate it with POP, and then chemically dissolver/remove it.

    Not really sure if closed cell foam melts when in contact with solvents such as acetone.

    I know that floatation closed cell foam Does Not chemically melt, period.
     
    Last edited: Oct 18, 2017
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Depends upon the base material of the closed cell foam. The pink foam insulation board it extruded polystyrene closed cell foam and acetone will aggressively reduce it to a sticky residue about 2% of the foam volume and I’ve made fiberglass ducts and enclosures with that method of lost foam. If your talking flexible foam of unknown material you’ll need to experiment.

    Best,
    Kelly
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    This thread and associated parts are where lost foam casting started for me. I thought I’d update and tie a few related threads together. I’ve made many process refinements since this thread and enjoy essentially 100% success rate with these parts today. The companion thread with those process refinements is linked below.

    http://forums.thehomefoundry.org/index.php?threads/automotive-water-neck.90/

    Best,
    Kelly
     
  6. Wader

    Wader Copper

    Kelly,

    If you were using drywall mud to coat these pieces, how did you coat the inside of the tubes?
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    When I first started doing them I was brush coating, so I didn"t coat the internal surfaces at all. In more recent years since I've been dip coating, I've done them with and without coating the interior. I've found no difference in the casting (except I suppose the interir surface finish is a little rougher) however, drying the inside takes an excessive amount of time or the added complication of moving air through the tube, and then of course how to insure the refractory has been removed from the interior of the casting. For a while I used some foam plugs to exclude the interior but now I just ladle on the coating while holding the pattern in a position that excludes the interior.

    Best,
    Kelly
     

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