How a Foundry Can Diversify Into Lost Foam Casting at Negligible ...

Lost foam casting offers tremendous benefits but has underperformed on commercialization. Part of that is due to the perception of the process requiring large upfront capital and tooling expenses that make it a process only appropriate for large-volume production.

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But you don’t have to invest in capital equipment or even tooling to diversify into lost foam casting. For the modest cost of the necessary raw materials, any foundry producing iron, steel, aluminum, or brass castings can quickly produce prototypes and short-run lost foam castings. This lost foam method could be used as an alternative to additive manufacturing or for producing tooling for prototypes, replacement parts, or other small volumes.

Lost Foam Advantages

The benefits of lost foam casting have been well documented. The near net shape process is known to permit complex shapes, and Department of Energy (DOE) research has reported that compared to traditional casting methods, lost foam provides 25%−30% energy savings, 46% savings in labor productivity, 7% less materials used, and production cost reductions of 20%−25%. Less solid waste as well as less particulate air emissions and greenhouse gases are created compared to traditional processes. An additional benefit is that with proper gating design, casting yields over 70% are common, and yields over 80% are feasible.

Based on the authors’ measurements, tolerances of +/- 0.003 in.-per-in. are typical (0.076 mm/mm) and +/-0.002 in.-per-in. (0.05 mm/mm) are feasible in some cases. For the machined foam approach, the tolerance is dependent on the machining accuracy and typically 0.002 in. (0.05 mm) plus the machining tolerance. Recent DOE funded research concerning thin-walled ductile iron showed that even at 0.040 in. (1 mm), the tolerance for lost foam was +/-0. in. (0.039 mm). This key benefit means that lost foam castings can either be used with zero or minimal metal machining. With proper component redesign this can result in enormous cost savings that offset the slightly higher cost of the process compared to traditional green sand or nobake casting.

As shown in Figures 1−3, lost foam provides net or near net shapes that can have complex geometries such as interior channels, blind holes, and true position. Components can have zero or alternating draft (Figure 4). The authors have recently even developed a process for as-cast threads as shown in Figure 5. And lost foam can eliminate the need for tooling; foam can be machined and cast (Figure 6).

Steps to Trying Lost Foam

The lost foam process is shown in Figure 7. The key to having lost foam with minimal capital investment is to eliminate the need for expensive tooling and automated lines. This is done by machining the foam from foam stock and creating a fluidized bed by manually compacting the flasks. The remaining steps are essentially the same as standard lost foam casting.

Step 1: OBTAIN FOAM

The first step is to obtain foam blocks for machining. Lost foam typically uses expanded polystyrene (EPS) foam. The ideal density is 1-1.5 lbs. per cu.ft. (0.016 g/cm3- 0.024 g/cm3).

If carbon control is important such as in iron casting, using Clearcast is desirable—it is a co-polymer made of expanded polystyrene (EPS) and polymethyl methacrylate (PMMA). This can also be obtained in block form for machining.

One essential safety factor: Ensure that any foam used does not contain flame retardant. Foams containing flame retardant tend to explode during lost foam casting because they are designed to not burn. Foundries should verify with their foam provider that the foam does not contain flame retardants, and they should also conduct a flame test themselves.

If the machined foam won’t be used immediately, foundries are advised to age it for at least three weeks prior to use. Newly blown foam is not dimensionally stable due to the moisture slowly evaporating. Depending on climate, after three weeks and possibly sooner, the foam will stop changing dimensionally.

Step 2: PATTERN MACHINING

A CNC machine is necessary for machining the foam pattern. A wide range of machine tools are feasible. It may be necessary to conduct several trials as the feeds, speeds, depth of cut, etc. are going to depend on the machine, the foam, and the cutting tools used.

The key is that the foam can be cut, and tiny chips are formed. If the feed rate is too fast, the depth of cut too deep, or the speed to slow, the foam will tend to tear. This is especially true for blown foam because the foam beads will pull out from the material rather than cut. As a general rule, higher speeds and lower in-feeds and depth of cut are necessary.

When going from the part’s CAD design to CAM and machining, it is important to account for the metal shrinkage that will occur due to the coefficient of thermal expansion for the given metal used. However, due to the lack of mold wall movement, only metal shrinkage is necessary for this adjustment factor. For alloys such as aluminum with its high shrinkage upon solidification, risers can be used or the gating design itself can serve as the riser. For iron alloys, risers are not typically used. For aluminum and steels, the risers are approximately one-third the sand riser size.

Step 3: ADHESIVE

One of the reasons lost foam can permit highly complex shapes is the foam can be joined prior to casting. This means several parts can be designed to be joined together rather than machined or blown as one finished shape. There can be zero draft, alternating drafts, complex channels, or blind holes. One can even cast interlocking components such as chain—so long as they have separate ingates.

The joining of foams can be done using supplies as basic as school glue or tape, but most lost foam foundries use Foam-Lok 70-12-11, a specialty adhesive designed for lost foam. This material is heated and then rapidly solidifies on cooling. Adhesive can be applied by a brush or by dipping.

Step 4: GATING

Typically, lost foam uses a consumable ceramic down-sprue funnel. This is glued to a gating system that feeds the part with one or more smaller in-gates. Often, these are notched to facilitate the breakoff process in finishing. There is no gating ratio and there should not be a choke point in the gating system.

A gating pattern is used to blow the foam gating system for all castings, but the gating system also can be cut out of stock foam.

Gating design is a complex subject and highly dependent on the exact casting design requirements. A thorough coverage of this is beyond the scope of this article. Suffice it to say, the gating rules for normal sand casting do not apply to lost foam.

Aluminum is typically top fed—meaning the castings are underneath the gating system. Iron, steel, and brass are typically bottom fed; the castings are above the gating system.

Minimal glue should be used, as it will produce excess gas. Excess glue also will be cast into the final part, so drips on the foam will result in drips on the casting that need to be ground off. Ensure the stability of the gating system particularly for thin parts.

Finally, the most important thing to keep in mind for gating is to plan how the sand will flow around the part. If lost foam is done properly, it will obtain a near net shape casting with high tolerance control. If it is not done properly, the result will be a mass of metal mixed with sand with little resemblance to the desired part. A key aspect of this is the compaction process.

The gating design directly impacts how the sand can and will flow. While green sand and nobake casting are nearly always designed vertically or symmetrically, you want the opposite in lost foam. Parts should always be somewhat tilted. Otherwise, the sand will not flow into holes, channels, and other features. In addition, sand will only go uphill approximately 0.25 in. (6.35 mm), so gating designs should not require the sand to flow upward.

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Step 5: COATING

A variety of coating manufacturers and coatings designed for lost foam are available. The key to coating is having a consistent process. If a foundry is not using a ready-to-use coating,  it will need to invest in a viscometer to ensure a consistent coating is achieved. The coating must be mixed thoroughly and then used quickly, as it can settle within a few minutes, which will result in an inconsistent coating and poor casting results. While it is essential to mix the coating, overmixing can cause bubbles in the coating. Those will dry and result as holes in the coating and will cause surface defects and burnt-on sand.

The foams can be dipped into the coating, or the coating can be poured over the foam.

Step 6: DRYING

Moisture in the coatings can cause burn-on sand or even steam explosions. Drying depends on the climate and conditions of the plant. Some places will use fans to circulate air. Others will have special drying rooms with heated and dehumidified air.

Step 7: COATING INSPECTION

Cracked coating will cause burnt-on sand and can cause a complete collapse of the mold. The component and gating system should be inspected for any cracks and touched-up, if necessary, with coating and allowed to redry. Zircon coating also can be used to fill in the cracks. It is not typically used for the overall coating due to expense.

It is dangerous to re-dip or repaint the entire foam assembly. If the coating is too thick, the vaporized foam gas will not be able to escape through the coating walls into the sand as designed. If the gas cannot go through the walls, then the gas may go up the down sprue. If that occurs, the gas can blow the molten metal up out the down sprue and into the operator’s face.

Step 8: COMPACTION

While very expensive, lost foam compaction lines are available and similar to an automated green sand molding investment. Foundries can also opt to make molds by hand. All that is necessary for lost foam casting is to fluidize the sand so it vibrates and flows like water. This causes the sand to pack tightly around the foam. The sand backs up the coating so the system doesn’t collapse when the foam is vaporized in advance of the molten metal front, but it allows the vaporized gas to dissipate.

In manual compaction, a container is needed to serve as a flask, such as a clean 55-gallon steel drum. Depending on the size of the gating system, the drums are often cut to remove one third to half of the height to make pouring easier.

Using ceramic beads for compaction reduces silica dust. However, standard lost foam sand (olivine) can also be used. One can also use regular silica sand, but this adversely impacts the dimensional control. The key is that the sand is unbonded, dry, cooled, and sifted to remove any contaminants. Sand or beads that are used with low melting point metals such as aluminum tend to have EPS residue build up on them over time and then glom together into chunks that need to be removed.

After the drum is filled with 2 in. (50 mm) of beads/sand, it is compacted by repeatedly hitting the drum with a rubber mallet. The sand will become hard to the touch.

The foam assembly is placed onto the starting sand, and sand is added while hitting the drum with the rubber mallet so the sand flows around the part. The sand should not be directly poured onto the part to prevent the coating from wearing off. Also, for thin sections, sand should be filled on both sides so the weight of the sand doesn’t break through the foam.

Once the part is buried, the flask continues to be filled with sand, which is also compacted.
At least 10 in. (25.4 cm) of sand on top of the part is necessary; otherwise the entire mold can float and cause erratic dimensions or form an undefined mass of metal.

Step 9: CASTING

The melting process for lost foam is the same as in other foundry processes. The key is to ensure sufficient melt temperature; a higher-than-normal superheat is required to vaporize the foam compared to pouring into an empty mold cavity. Typically, an additional 50-100F (30-55 C) is needed depending on the precise alloy. Too much superheat also can cause problems. For example, in iron alloys, the metal can boil, which is extremely dangerous; and in aluminum, oxide formation increases at higher melting temperatures.

Lost foam depends on getting the vaporized gas out of the system without the mold wall collapsing. Unlike automated lines, this method of casting does not have a vacuum system to help remove the gas. The molten metal removes the gas by pushing it out of the system using gravity. Pour as fast as possible to make sure there is sufficient weight of metal to offset any gas pressure.

Pouring too slowly can result in mold wall collapse or explosions. The down sprue should always be full during pouring.

Flames may occur around the flask during pouring. That is the styrene gas catching on fire and is normal.

Step 10: FINISHING

The casting should be allowed to solidify at least 30 minutes for aluminum and one hour for iron or steel. At that point, the sand and metal will still be hot, but the sand can be removed and the parts shaken out. Waiting until the parts are fully cooled is also an option. For some metals, this will cause the parts to self-anneal since the lost foam mold is highly insulative.

Assuming the above process steps were done properly, finishing is the same as in sand casting. Typically, the blast time is only one third of the time for a sand casting.

Unfortunately, if an issue occurs in the process, lost foam castings generally are not reworkable. The resulting casting is an undefined mass that only somewhat resembles the starting point and will be full of burnt-on sand as shown in Figure 8.

Ready, Set, Go

For foundries with a need for rapid prototypes or customers desiring small volumes, the method of machined foam and manual compaction for lost foam is an opportunity to diversify with minimal capital expense. The melting, casting, and finishing processes are essentially the same as in their current processes except for needing a higher pour temperature. The molding process for lost foam, while different, is something that can be done on a small scale to first conduct trials. This could even be done with metal that would otherwise be pigged out.

Typically, the machined foam approach works well from a cost perspective for volumes under 100 castings. For over 1,000 castings (total use) it usually makes sense to manufacture foam mold tooling at some point.

An existing sand foundry could conduct trials with the simple process first; once they have had success with prototypes and larger batch sizes are ordered, it may be worthwhile to consider the larger investments in tooling, foam blowing equipment, and an automated compaction line.    

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I thought I’d throw this one out there in response to a question that was posed about the merits of lost foam casting. I’d be interested in other member’s views on the subject. For the reasons that follow, I think it’s probably the easiest and most economical point of entry to the casting hobby for most backyard gamers.

Pro’s

• No large heavy tooling to make or store……For me the router templates which are usually just profiles are small, light, and store in very little space. In many cases you can build a library of templates applicable to multiple projects.
• Mold packing is very quick and easy compared to conventional sand casting ….no ramming or heavy lifting and a flask is any container rigid enough to hold the sand. This usually means you don’t have to handle heavy molds because you can scoop it in in manageable amounts. A vibrator is beneficial but wrapping/tapping works to a degree. Now a days, I just dump the spent mold sand on my drive way to let it cool, off-gas, and then shovel it back into my buckets through my sifter. So other than this there is no binder management or sand conditioning to be concerned about…..i.e. no muller is required and other than sifting, no sand conditioning is required.
• If you use a pattern coating such as drywall compound you can use about anything for sand as long as it’s dry and of sufficient refractory.
• Foam castings do not require draft which can be helpful for designing in tooling features from which to locate and grip for post machining of castings.
• Parts that would otherwise have complex coring or parting lines in conventional sand casting are easily handled by gluing pattern features together.
• Pattern Detail: With liquid coatings, casting detail appears to rival lost wax and shell. Foam works very easily but I’ve had to introduce wax (although the foam pattern itself cannot rival the detail of wax) for the finer details such as fillets, embossments, and Lettering/Part marking.

Cons

• Odor: I store my LF sand in 5-gal buckets and with plastic snap lids so there is no storage odor. The odor while casting outdoors is very short lived. I would not under any circumstances recommend casting LF indoors….even small parts.
• Brush coating drywall compound can be slow but it’s very inexpensive and readily air-dries. I need to work on a dippable process, but then you have that to store. Frankly, I can’t see why it just can’t be an 5-gal bucket full of thinned mud that you agitate with a drill-powered paddle before use.
• Machining patterns can be messy. The foam dust travels. I spent this Saturday making improvements to my arm router which included a dust collection system. I also made hot-wire cutter for roughing out machine patterns to reduce the amount of foam that gets machined away. Together the two should make a big difference in speed of fabricating patterns, reducing dust, and clean up. My woodworking machinery earned its space in my shop many years ago so it’s there for the using without additional space.
• Pattern strength: Depends upon the shape. Foam is strong in compression but in general is fragile. Thin shapes can distort during molding and it’s easy to dent or damage a pattern during handling.
• Pattern Detail: This is a relative thing and why I list it as both a Pro and a Con. I don’t think the level of detail that can be reproduced in wax or harder materials is possible…..at least with my techniques. That’s why I introduced wax for fillets and embossments so I have sort of a hybrid process.
• Failure means you not only get to re-make the mold but the pattern too

Areas for (my) further development:

• Dip coating: Might be as simple as lower viscosity mud in a 5-gal bucket….but may also require elevated temp or longer drying cycles.
• Vacuum Assisted Lost Foam. In 90%+ of the cases I think gravity fed pours are adequate. However, I think vacuum assisted LF has real promise to further control and accelerate metal propagation speed which might increase the envelope for LF castings as far as thinner wall thickness, (larger) parts, and (lower) pouring temps.
• Metal Quality: I honestly don’t know if this is a pro or con yet. My preliminary results suggest it’s on par with conventional sand casting.

What sayeth all of you?

Best,
Kelly

The couple of my LF castings I've sectioned and examined under magnification looked as good as any sand casting with which I did the same (years ago) thus my comment above about being "on par". All castings are porous....it's just a matter of degree. Even with close furnace atmosphere control H2 will be present. Degassing certainly helps. Some of my water neck castings "sweat" hot coolant under pressure.......but I don't de-gas and I should, but I still think the issue would persist at least statistically. FWIW, I think conventional sand castings would do the same thing for the same reason and because of the thin walls. Vacuum impregnating castings is pretty much the norm for such in industry.

On machining, I think the selection of alloy and heat treating (if heat treatable and not intended for as cast machining) is more important in this regard than sand or LF process. As soon as the metal in the cup freezes, I de-mold and quench in water to knock the coating off the casting but there's a natural degree of solution heat treatment realized in doing so because it's still very hot. I can easily do the precipitation/age bit because my furnace is electric, but having sat around for a few weeks there's also a natural degree of that as well.

T5 is in reach at home but T6 is more difficult. I think you really must have the need for that extra little bit of strength to warrant T6. T5 machines and polishes well but you're never going to get to the level of the best wrought aluminum alloys. My castings thus far have machined satisfactorily and hold threads as well as any aluminum castings I've used. Neither LF nor conventional sand casting is the best process for critically stressed parts and that's why the design guides recommend higher safety factors be applied for parts manufactured with these processes due to alloys, porosity, and flaw population.

I'm accumulating a quite few parts to be machined and I'll post up results in the near future.

Best,
Kelly

I must say I am pretty surprised at the success you are having with the foam.
Your methods sort of take it from the very crude backyard "YT" sort of thing to a very viable process.

Your statement above is also true, and points out he benefit of a pattern with green sand; ie: you can tweek a wood pattern and make a new mold pretty quickly.

I think part of it is in what you are set up to do.
I am set up for green sand type molds, and I would have to rethink the whole process in order to go to foam.
If someone was set up for foam, then it would make a lot more sense to use that process.

One solution to the heavy sand/mold/flask situation is to use bound sand with a custom-fit flask, and make the mold very thin; no more than 1" thick in any one spot.
I generally start with a square wood flask, and add spacers inside to reduce the amount of sand used. The spacers can be taped in place if just a few molds need to be made, which greatly speeds the custom flask process.

And I plane 3/4" thick wood to standard heights, such as 1", 1.5", 2", etc., so that I can easily add filler pieces into a flask.
My flasks (generally the cope) are often only 3/4" thick, and relatively lightweight, and the bound sand allows for this.

But seeing your foam work really demonstrates the potential of LF-type castings, and proves that there is a whole lot that can be done in that arena, and far more can be done than I think most people realize.

Agreed, horses for courses. That's part of what I was trying to get at with the post, applicability of the process. I would not use it for jewelry either and that's probably a close cousin to your emblems. The other bit is just that there doesn't seem to be many people doing LF, at least here and AA. A little more on YT but most of that doesn't seem to be a very serious attempt at making quality parts.

If I knew I was making 50-100 of my thermostat housings that would probably be core and sand too, but ya know, having made the router templates (which also didn't take long), it doesn't take me long at all to make those small foamies with the jigs and if it was only that many per year and I was only going to cast 10-20 at time, it might not be so clear cut. For now it is because I'm not set up for traditional sand casting.

The tubular water necks I've made are somewhat a different matter because that would be a much more complicated pattern and core box for even 50+ pcs. That is one thing about lost foam....it sort of offers the design freedoms of lost wax investment or shell and maybe even a little more so because my method of making a foam pattern seems less laborious then making a master then a rubber or plaster mold, then wax patterns, then shelling, and casting them. LF would be a lot less laborious without detailing and coating the foam patterns but also yield a lower quality result. The coating labor could be greatly reduced by dip versus brush and I intend to work on that. I try to machine in all fillets where possible but at complex intersections of assembled pieces I'm stuck with wiping in wax fillets and embossment applique. So I see LF can sort of be a tweener between LW shell and sand casting as far as economy and ease of use.

I think the potential metal quality and finish is greater with LW and shell. I think LF is less total labor than LW and way less without detailing and coating the pattern.
Coated LF seems to be on par with Petrobond as far as finish potential but LF seems limited by pattern finish whereas Petrobond more so by the mold.
In volume, traditional sand casting and hard tools is the clear economic winner, but unless the part is small, few have the home resources to process that much sand, molds, and metal.

I spent a day making the templates & a set of foam patterns for the carb plenum. Now I can reproduce multiples of those foam patterns in an hour......the pattern detailing is more involved. When I shared that piece with my old foundry friends, their first impression from ten feet was they were looking at investment casting. Then when they look closer and see it isn't quite as refined, they ask how the expendable pattern was made. And then when I tell them it's LF and I have 8-12 hours of total invested time, and the next ones will come much easier......they like it.

Best,
Kelly

I have been playing around with lost foam for a few years, though less so in the more recent past. Kelly, you have covered pretty much all the pros and cons I can think of. I'll chip in my 2¢ here, but you covered it pretty well from where I sit.

Pros...

Easiest way to get into casting. All you need other than a way to melt and pour metal is a bucket of sand and some styrofoam. You can use free EPS packaging material for sprues and pieces where the beaded texture won't matter or if you're a really gentle touch with the sandpaper, or maybe buy a sheet of 2" XPS blue/pink foam insulation board that could last for a few years of weekend fun. It's easy to machine without needing much special equipment - you can manually mill out patterns for plaques and such with just a small drill press with a cutter bit in it, or even with a Dremel tool with a cheap plastic router attachment screwed onto it, or even just carve it by hand with sharp blades and files and sandpaper, if you are careful. Glue guns can be had at the dollar store and work great for assembling patterns. A paper towel or TP tube can be used as a sprue to get the metal right into the part similar to your metallic tape cones (but less conical). Hot wire cutters are easy to build on the cheap if you don't have a bandsaw to cut out patterns with. It can be so simple, or you can put more into it if you want to, just like with any other casting method - better sand, better foam carving tools, experimenting with coatings, sand vibration tools like your bucket or I've even seen where some people have tried getting some interesting sand fluidizing air hose setups to work, etc. So there's lots of room to have fun growing into it.

Cons...

Remaking foam patterns because you're too stubborn to figure out how to copy one of several cheap and easy to build pyrometer threads and poured too cold again really SUCKS! But that is not really lost foam's fault, is it? I would have had more to say here about how reliable results and thin walled parts are impossible for the home gamer if this were several months ago, but meanwhile you've gone and proved that simply isn't so. You're definitely not wrong about how messy and smelly it can be either!

Possible areas needing more research...

About the crusty sand lumps that come out of the lost foam bucket. Were you saying or implying in one of your lost foam posts somewhere that you don't get those when you pour hot enough, or am I misremembering? That is very interesting, something to watch out for. I remember on AA, metalbynevin (a guy with a good number of successful lost foam pours under his belt) had a theory that polystyrene vapour residue builds up in the sand, and that once it reaches a certain concentration, it can cause or aggravate sand float defects which sometimes occur if you pour the soup can Kush tool too full when the pattern isn't buried under a heavy enough load of sand to counteract the hydraulic pressure. If true, I can see how it would be easy to misdiagnose as such. I don't know if there is any validity to that, it was only briefly discussed, but if so, maybe it is possible the sand may need to be replaced (or burned clean?) every so often. If that is a real thing, perhaps it is only of concern if you've been pouring cool enough to see a lot of those tell-tale sand lumps?

Jeff

My 1/2 tubes are machined on both sides. I machined a female plug they just press into with friction fit, but I use small amounts of packing tape so they don't get sucked out of the fixture. My carb plenum is also machined on both sides and its just a friction fit on an internal MDF plug. I think the next time I may just try PoP and vacuum. If you machine the one side of the foam pattern and cast it at the parting line in PoP, then either fire or just use acetone or lacquer thinner to remove the foam pattern and you should have a real nice fitting machining fixture. You can use a lost foam plug immersed in the plaster that is cooked out to create a vacuum plenum in the plaster or simply set the plaster fixture on a wooden plenum and drill small wholes through the PoP for the vacuum. In some of my fixtures the foam fits so snug that I have to drill a couple finger holes in the back side so I can remove the foam patterns without destroying them.

I used the above method to make machine fixtures for tough to grip castings but I powder coated the casting and then used urethane resin instead of PoP. The powder coat increased the surface dimensions enough to easily accommodate casting variation yet accurately position it. I used toggle clamps instead of vacuum for clamping. -Worked slick.

Best,
Kelly