Lost Foam Process and My Rig

Thought I’d start this thread on my lost foam rig and further discuss the V-Process post Mark made here.

http://forums.thehomefoundry.org/in...ider-halloween-casting-contest.142/#post-1893

I've seen and am very interested in the V-Process as well. The two aspects that interest me the most are the use of dry sand, high vacuum, and combined the potential benefits to casting quality that offers due to both the vacuum and absence of binder off gassing. However, V-Process still requires similar degree of tooling as conventional sand casting and lost foam much less so IMO.


There are some similarities between V-Process and the Lost Foam vacuum assist method. Here's my LF rig. I built this as a process development platform from which to scale. I plan on building a larger vacuum plenum base to accommodate larger parts that can accept various flasks sizes. The flask may be nothing more than wooden box set on top the flat vacuum plenum with a gasket. When not in use the plenum can simply hang on the wall.


Here’s a diagram of my development rig.


I have a turbine vibrator on a wheeled base. The bucket sits on the base so I can prepare multiple flasks for a pouring sessions. Most of the time I just use plastic 5-gal buckets for flasks.


The plenum has a filter plate that isolates the sand with expanded metal with a coarse and fine layer of fiberglass cloth sewn on.


The vacuum motor just sits on a gasket and lifts on and off because I use it on other machinery including forced air for a burner.


The loose dry sand is vibrated in place around the pattern. I get 15% compaction typically in just a couple minutes which is pretty good and sufficient for most all parts. With more effort I can approach 20% and do this when difficult packing conditions exist. This is measured in settling depth from an initial state of just poured in sand. Most parts only require 4-6” of sand cover for mold stability. For large cross section parts that are buried deep in the flask, the packing is so good when I grab the cup after a pour with pliers, I can lift the entire flask containing 50lbs of sand. In this short video I poured a small cup of fresh sand on top an already packed flask to show the sand fluidity that vibration provides even at low energy levels.



I’ve begun just dumping the sand on my concrete driveway to cool and then sieving through window screen to remove debris. My sieve is just window screen on a frame that fits my 5-gal bucket and I shovel sand from the driveway into the bucket through the sieve. I have snap lids for storage so I don’t have to smell the aftermath.





For coated foam patterns the sand can be very coarse and low quality as long as it is dry, of sufficient refractory, and relatively free of any fluxing debris. I have 50+ pours through my sand thus far without changing. All of this thread refers to LF aluminum. Higher refractory mold material is preferred for higher temp allows.

Here's a short video of the vacuum assist rig.




The top seal is just some thin poly film with a cut hole stretched over the pouring cup and an inch or two of sand on top, and secure around the perimeter. In the video the film is secured with a rubber band but most of the time I don't even secure the perimeter because the sand provides adequate seal and small leak path. When you pour the film does of course burn away from the cup but only slightly creating maybe an 1/8" or so annular leak around the cup. This could be prevented by making a jacketed cup. The sand on top of the film also protects it from pouring spills. -It's elegantly simple and I haven't had any urgency to create higher vacuum because it's already producing high metal speeds. Compared to the V-Process, you still get the potential metal quality benefit of dry sand contact, save the fact you're vaporizing polystyrene.


I haven’t been degassing my melts because I figured why bother with all the foam vapor present but I now believe that to be flawed thinking because hydrogen is still just as soluble in the molten metal and that’s a separate matter from managing the vaporized foam. In the initial phase of the pour you get a rapid oxidizing burn of the foam with black smoke and some byproducts from decomposition of the foam float to the top of the cup and continue to burn there. This all comes from the initial molten metal contacting the foam in free air. Once you get the melt settled on the gating cross section, I typically see no bubbling in the cup and with vacuum assist I don’t see any from start to finish.


It’s actually a very small percentage of the metal that is exposed to the foam vapor, that primarily being the initial leading edge of the molten metal front. The foam vapor diffuses into the sand and does so very effectively and efficiently with vacuum assist. When I dump a flask shortly after a pour, it liberates white smoke captured in the sand. I presume it's white as opposed to the initial black smoke at the onset of the pour because it is somewhat deprived of oxygen as it is vaporized....air can't diffuse to the vaporization site quickly enough to support combustion. When I dump a flask after a vacuum assisted pour I see no smoke because it has already been evacuated.


I’m intrigued and fascinated by the process and I’m sure it shows. I lifted much of the above from this company's process. It's a good video to take in and lot's of good technical information if you pay close attention.



Best,
Kelly

 

Yes I do, at least for the Owens Corning Products. I pasted the data sheet below. The good news is, as long as the base material is polystyrene, most any of the commonly available stuff works well as far as burn out. The most significant differences are in how workable/shapable the foam is,and strength. You want the lowest density material that still machines, sands, shapes well, and has reasonable strength for pattern making. Unfortunately, the lower density, the poorer the workability so there is a compromise to be struck.

Expanded Polystyrene (EPS) and Extruded Polystyrene (EXPS or XPS).

EPS is typically lower density beads that is expanded with steam in molded shapes. It’s usually white, low(er) density which is good, hot wires well, and though it cuts easily, compared to XPS it does not machine well nor easily sand to good surface finishes particularly well. I only use it for sprue and gating stock. The packing forms can vary greatly in density but are usually <1lb/ft3. In a commercial manufacturing scenario it is the choice for pattern molding stock.

XPS on the other hand does machine and finish well with common abrasives. 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. All of my local retailers offer the Owens Corning product. The 1" thick sheets are 150 and the 2" thick sheets are 250. I'm told sheets up to 4" thick can be special ordered. When I searched it I actually found six densities commercially available from Owens Corning. Their 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 250 finishes even better but may modestly limit the size of the part because of it being slightly higher density.

Since it is marketed as insulation, their is plenty of info on it's insulating properties and use as insulation but I had to dig pretty hard to find other mechanical properties. Such as density and strength. One other thing you may find if you buy a sheet, it is often scored so it brakes which is a pain for patterns because you need to cut around those for pattern making. Here's the Owens Foamular data sheets. I couldn't get the pdf you loaded above to open. I can send (or maybe load) pdfs if these images aren't readable.

 

That's actually quite a bit more dense compared to the Foamular 150 (20 kg/m3) and 250 (25 kg/m3) I have been using. I suspect it will make for stronger patterns and perhaps machine and sand a little better but higher density will also produce more burn out residue and take more energy from the melt to evaporate the pattern.

How significant of a difference I can't say because I haven't experimented with that high of density. It will mean for a given part it will require a (slightly?) higher pour temperature or be capable of a (slightly?) lesser travel distance through the foam for a given pour temp......or in other words, potentially limit you to a smaller part size for a given pour temp. I seem to be able to get about 12" of travel but these are fairly thin walled (1/4") parts with lots of surface area so they give up lots of heat to the mold too because the evaporation of the foam can slow the propagation of metal through the mold compared to empty cavity sand casting.

This may affect more massive or lower surface area to volume parts less but you also have more gas to expel with less surface area. There's one sure way to find out......let us know how you do.

Best,
Kelly

 

I have more to learn with the vacuum assisted pours. I don't use vacuum unless I have an intricate part or one that is pouring short. I get very good finishes without vacuum but with even 7-8" Hg, I get significant metal penetration into the sand and duller finishes where drywall mud or coatings are applied but it certainly does increase the metal propagation speed and improve the ability to fill more challenging features. In that Ruiou video attached above, it appears they may be pulling as much as 1/2 atmosphere vacuum. I think the key to high vacuum assisted pours is a porous coating that breathes well. I need to experiment more and a dippable coating like they show in the video sure would be nice. That stuff looks to wet and lay down on the foam very nicely. Must be pretty decent refractory too cuz they're pouring iron. Of course then I'd have yet another barrel of stuff hanging around. I do have some suspend slurry I've been meaning to try just for grins. I think you'll like LF......except the odor.

Best,
Kelly