Unsupported Closed Loose Sand Coring In Lost Foam

This thread is a condensed version of an AA Forum post at the link below where the unabridged version and contributions of others can be found.


http://www.alloyavenue.com/vb/showthread.php?13263-More-Fun-with-Lost-Foam


So after the experience with the tubular part, which still sort of freaks me out, I thought....how about a completely unsupported enclosed core?…….would that mold remain stable too? So I made a mock four cylinder engine block and a hollow foam pill to experiment…..yes, it’s a pill……..it’s made from the tooling and fixtures I make my water necks with and is ¼” wall and 1 ½” OD.






I made this mock 4-cylinder engine foamie in a about 30 minutes. It’s four one-inch bores, and overall about 7”x2.5”x2.5” with ~3/16” wall. You can see from the picture it has water jackets around each cylinder. Several cylinders were Siamese, several were not to see how they did. The water jackets are about ¼” wide gap around the cylinders.








I drilled a ¼” hole in the patterns, made a little foil funnel with a soda straw, and filled the enclosed internal cores of the foam patterns with sand by tapping and rapping but didn’t vibrate them. I plugged the ¼” fill hole with foam plug.





I gave them both the usual treatment with hollow foil sprue and small bobs on the end of flow. I positioned both in the sand in my lost foam rig and vibrated them in place, then poured them, each with a separate cup.





Here they are after the pour. Externally they both appeared fully formed so the dissection began. I drilled a hole in each end and blew the sand out with compressed air.








A portion of the pill wall was foil thin and failed. I was going to cut it lengthwise but couldn’t grip it well enough to do so. Though it is hollow, the end that was directly under the sprue was solid for about an inch. The other end was formed. The cylindrical walls were not very consistent in thickness but were formed other than the thin section.






I sectioned the mock engine bock axially and radially through the cylinder walls. The water jacket in the lower end of the part (1 & 2) is very well formed. The core failed on one side of the second cylinder and the external walls were very thin in the upper portion of the part under the sprue.






It’s certainly not full success but also not total failure. I think I’m going to have another go at it. I was thinking about possible causes other than the obvious….that being why the heck would you expect a completely unsupported loose sand core to remain stable in the first place?




On the next go, I’m not going to completely seal the core. It will all be loose sand but will have the equivalent of core prints penetrating the walls and allow sand to continue to fill and pack the cavities when vibrated in the lost foam rig. I may have destabilized the core when I applied the high energy vibe to the external sand. Also, I think the internal features need a vent path. The internal cores can only vent through the metal when completely enclosed as in today's test.


Once Foam Patterns become cheap, so does experimentation.


Here’s the router jig I threw together to make the mock 4-cylinder engine block patterns. It’s used with an overarm pin router to cut the foamies.







Best,
Kelly

 

Here’s the second round. There were some changes from the previous pour. The pattern is slightly different and it can be seen in the last picture of the two casting attempts. The water jacket section is only ¾” tall where the previous attempt was a little over 1”. This was so I could use two 1” thick foam board sections on the router jig to make the patterns. There is ¼” gap between all cylinders and this modestly increased the overall length. I made a slight change in the in-gate to the highest point on the part and most significantly, I drilled eight 3/8” diameter holes midway up the water jacket; three on each side and one on each end. This was so the sand could simultaneously be vibrated into the water jacket cores and exterior….and these are tiny little cores with about ¼” thick cross sections.






After adding the foil sprue I placed it in the sand rig, vibrated for a couple minutes, added the cup and poured.





All the exterior features are well formed.





Here is the water jacket sectioned.











Here are the first and second attempts together.





Wow! Almost got it. Everything below the first cylinder water jacket is perfectly formed. This was a big improvement. I’m amazed……..I must try again. I’m still losing the first core area the molten metal front sees but not the lower portions of the mold. I’m thinking it’s either wash from high molten metal velocity eroding and destabilizing the core, or maybe lack of initial head pressure and or depth of sand to keep the sand in place because it’s so close to the cup and because there’s only a few inches below the surface. It would be better if it was 6-8” below the surface. …..I can put these hypothesis to the test. Are you with me?



Best,

Kelly

 

So not being the type that discourages easily, I ran a couple more of these. One part is identical to the previous run except I added two more small (1/4”) holes in the top corners of the water jacket. I had already drilled the other 8 holes in the two patterns I had or I might have just tried 3 in the top only. The second part is identical except it is buried 8” deeper in the sand and has a longer sprue for more head. Neither had a bob. I poured them in the same bucket. To improvise more sand height, I used a section of sonna tube. It didn’t fit inside my metal and rig but fit nice and tight in a 5 gal bucket.




All of the exterior features are well formed. You can see the two additional holes in the picture below.





And here they are sectioned. Everything is very well formed except the small section above Cylinder #1. Though the part with the deeper sprue is modestly better in the area where the core leaked, it is only marginally so, and I think one can conclude this is all about packing and sand depth and head aren’t the operative factors. The sand depth probably just marginally helped the packing.





If I moved the center hole from the midpoint in the water jacket to the top of the cavity it may yield some improvement. That area that I’m losing is only ¼ by ¾”. Finer media may help too because the grain size is probably only 1/10-1/20 smaller than that small 1/4" gap. Rule of thumb says media should always be at least 10x less than diameter you are filling but here you need all the friction and hold you can get. If it was a full-size part with ½” or more minimum core dimension it may run without issue.


Besides the forming around the small loose sand cores, it’s also pretty amazing how well the part fills simply gating into the edge of the upper wall. I may try one or two more in a different attitude and/or core feeding holes, but I’m feeling pretty good about the potential for lost foam and loose sand coring.

Best,
Kelly

 

Reserved for post #4

 

Thanks Joe. I hadn't seen the first video. That 5 gal bucket is my lost foam sand rig. It has a turbine vibrator mounted underneath and I am routinely achieving 15-18% compaction with 5-10 minutes of vibe. 90% of that result comes in the first 30 seconds. The thing hung on the side is a port for a 3-stage central vacuum motor. It achieves 7-8"Hg vacuum, and yes, for a seal I just use a piece of thin polyethylene film over the top with a rubber band around it. An inch or so of sand on top the film helps too. I'm actually getting much better results without vacuum and just gravity feed. I did a series of experiments with and without vacuum and also on metal speed propagation. In the near future I'll add a post here to include those. The whole saga is chronicled here.

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

Here's a short video on my lost foam rig:



Here's a vacuum assisted pour:



Best,
Kelly

 

Thanks Joe but I’m still a bit of a newb to actual casting part of the hobby. I’ve designed a lot of castings and did development work in conjunction with professional casting houses of various processes. I also have several longtime friends that own pattern shops and foundries and spent a lot of time learning and mooching from them. Between the two I’ve picked up quite a bit along the way. I’ve been a pattern maker of sorts and have soft tooled quite a few parts for my own consumption but didn’t really start pouring my own castings until the this year so mold construction and melt management is fairly new to me. I picked up quite a bit along the way in this regard too but watching it and talking about it isn’t the same as properly controlling and doing it yourself, so still much to learn for me along with some of the practical knowledge required in the home foundry setting, which brought me here and to AA.


I’m fairly handy with machinery and metal fabrication but as I mentioned, I really only picked up on lost foam because I’m yet to build flasks, muller, and molding equipment and I wanted to cast something. Once you have molten metal handling ability, lost foam really has minimal equipment needs. I’ve since become so pleasantly surprised by the results, the more I learn about it, I’ve become a bit obsessed with it and am going to continue to make parts and experiment to explore its limits before building sand and mold management equipment. Lost foam has real potential for my casting needs.


Best,

Kelly

 

There is hips filament that is a polystyrene, but its quite dense and produces alot of black smoke when burnt.
I lit a piece on fire and it reminded me of a "magic snake" fire works. There is also wax filaments for fdm printers but you cant get it to work on some model files because it doesnt bridge well. Up next is moldlay filament. Decent stuff but still not a fix all.
Im considering trying an abs printed part and autoclaving out the mold with acetone vapor before the pour. Nothing like boiling acetone to clear the sinuses or rearranging the house..

 

Agreed. 1zs and maybe a handful of parts but also complex parts that would otherwise require complex parting lines and coring. The absence of draft also can have benefit for chucking and locating for post machining.

I think that may be a pretty tall order for 3-D printing but CNC router produced parts are very practical. You just need to divide the part into easily machined pieces and glue them together. After I get a little more experience casting actual parts a CNC router build may be in my future.

Best,
Kelly