My first lost foam casting. Close oh so close.

The melt was too cold. Pour hotter.

I typically pour at 1375F (~750C) but, the time from when the crucible leaves the furnace to when I pour is typically <10 seconds, and, my pouring cups are insulating material and take very little heat so I can use lower melt temps and my pour temps are very nearly what I measure.

During your process, if you take the temp, skim, remove the crucible from the furnace with tongs, transfer it to a shank, then pour, there is heat loss. When you pour into a cold metalic cup, it further reduces heat and yours looks nice but perhaps a little massive. So even though you may have measured 700c, by the time you pour and it heads down the sprue, it might be considerably cooler.

Successful pours are motivating. Minimize the opportunity for heat loss and pour at 760-780c. Smaller masses of metal lose heat faster. Melt more metal than you need and fill the pouring cup rapidly full and keep it that way. If you get complete castings you can further refine your melt and pouring process.

Best,
Kelly

 

The positive thing about a soup can besides being free and easy is they are typically pretty light gauge so not very massive and rob less heat. Square is better than round for the reasons mentioned, but that said, I used straight sections of muffler pipe for the first 2-3 years I was lost foam casting with success, which by comparison to a soup can are pretty massive, but would still get occasional defects even when I seemingly did everything identical to defect-free pours.

Though I had refined other processes too, when I started using the insulated offset cups, defects became quite rare and first pass success nearly 100%. I was pleasantly surprised by that.

It's been 6 years now since I made my first insulated reusable cup, but if casting is something you will routinely do, you may eventually want to invest in making something like:

Reuasbale Offset Pouring Basin | The Home Foundry

The easiest way to fabricate one is gluing fiber board together.

In the meantime, you should be able to be successful using a metalic cup and higher pour temp.

Best,
Kelly

 

So this defect is the classic lost foam fold defect.



It appears at the bottom of your casting because it is the very first metal into your cup that starts melting foam. The initial contact of molten metal with foam creates some turbulence and trapped oxides and that dirty metal travels down through the pattern and presents itself in the bottom of the casting since it was the first metal in.

In my early days this is exactly what I battled from time to time with round cups and pouring directly onto the sprue. So my remedy was to give that initial dirty metal a place to go somewhere other than my casting by adding a feature like this to the feed system, doing something like this:



That worked pretty well, but not nearly as well as pouring beside the sprue in an offset cup.

By the way, what's this feature?



If you poured a little hotter you might still get there. One thing about lost foam; pouring hot means you get a complete casting although it may have some minor (micro) porosity. Pouring too cold means you get to try again....

Best,
Kelly

 

Yes. When you pour on the sprue, the metal tends to penetrates it in an uneven chaotic manner. The sides melt and fall into the stream and either buoyantly float, ignite, expand, etc or travel into the pattern with the molten metal. When you pour beside the sprue in an offset cup with a weir, the sprue tends to become more completely and uniformly covered by a pool of molten metal which is more likely to consume the sprue in more uniform plug flow.

In lost foam there will be a liquid and gas boundary layer between the solid foam and metal front. The gas and liquid to a lesser degree is what is expelled to the mold media as the pattern is consumed.

For a small part like that 3/4 should be fine but you might benefit from a little more sprue length (maybe 6" total length from cup to pattern). More sprue height means more metal pressure which helps expel the evaporated foam through the coating to the mold media.

This is a very good thing. Many people allow the sprue to protrude above the sand and that just becomes fuel to burn in the cup. You know you have good pouring technique and a good feed system when you have no flame smoke. When I started using the offset pouring cups, this became much more routine.

You might even go 800C. It's sort of hard to say how much heat you are losing and what your actual/net effective pouring temp is without seeing your routine, and as I mentioned, smaller melts lose heat fast (what size crucible are you using?), so melt a larger amount of metal and pour the excess into an ingot tray. If you video your pour from the time you take the metal temp to the time you complete the pour, there is usually a lot of useful information and pointers that can observed and offered.

Best,
Kelly

 

There's a lot of old not so great info out there about how to do lost foam casting hobbyist style that comes from ancient Instructables articles and such, written by people who were figuring things out on their own with much fewer online resources available at the time than we have now. Parts got made, but they weren't always pretty. Some of the stuff about venting and using thicker coatings comes from those old articles, I have no doubt. The much thicker coats of drywall mud many hobbyists were using back then likely made venting a much bigger problem to solve than with the thin coatings we now know about.

Jeff

 

The cost of these materials is 2x-3x what they were two years ago. Sign of the times I guess. I cant be without them.

You can buy ~24 x 12 x 1/2" ceramic board for about $40 delivered on Amazon. Probably similar on eBay. You want 2300F rated board. I think you could make 3 cups from that. Although I've patched and repaired them a number of times, I've been using the ones I made from moldable ceramic fiber for 5 years now. Just like other foundry equipment it's an investment.

But, you also need glue. Some have used sodium silicate or mortar, but the ceramic fiber is much better because like the fiber board, it is unwetted by aluminum and you can wipe fillets into corners of your cup for easy release of the residual aluminum and also to have some on hand for touch ups and repairs. Here's a source. The smallest quanity in tube form is $25.

Ceramic Fiberboard Moldable Mix | 2300℉ | In Stock - CeraMaterials

I think you could use some very light gauge metal (like roof flashing from big box stores), bend it into a four-sided cup open on top and bottom, and tape the corner to keep the sand from leaking in. The tape will of course fail when you pour but the surrounding sand will hold it in place. Could also just take a round can and press the four flat sides. Put the sprue at one end and pour in the other.

Best,
Kelly

 

You're using your CNC router right with a ball end mill? No coolant/cutting fluid I presume? Are you seeing any loading of the bit? What alloy is your casting stock?

Sometimes organic shapes can be difficult to mount/clamp for machining and adding a couple of features (which can potentially be removed) to the pattern/casting can be a great help.

Pouring technique and feed system can go a long ways to eliminating those fine defects but casting stock melt management is also important. A very lean burner tune, and not overpowering the melt with more burner flow than needed (at least as/after the metal begins to melt, and minimizing the exposure time after melt has occurred, all can help reduce H2 porosity.

Degassing can become necessary depending. It can do more harm than good if not done properly.

Lower density foams are best for LF casting but the density of the foam can affect how well the pattern machines and finishes. The colored insulation board at the big box stores is typically 1.3-1.5lbs/ft3 which is a good compromise of these two needs. It is extruded polystyrene (XPS) that has fine cell structure as opposed to the white expanded polystyrene (EPS) which has bonding lines between the expanded beads.

When sanded, XPS surface finish can be improved up to 320 grit but I usually start and end with 220grit. You can even polish is a bit with paste wax but make sure the solvent in your paste wax doesn't attack the foam.

Best,
Kelly