Lost foam casting is much like lost wax casting.
The differences are:
- The use of an EPS (expanded poly styrene) model instead of a wax model,
- sand instead of investment plaster,
- and no burnout.
|Click here to download drawing.|
I like to cut the foam using my hotwire 'jig-saw'. Foam in different thicknesses can be purchased at building supply places.
A pattern cut from stiff paper pinned to the foam will help to guide the cut.
Foam pieces may be joined in many ways. I like to use hot melt glue.
This is the pattern for a sand casting flask. The center section is made from 2" thick foam and the blue flanges are made from 1/2" foam.
The foam pattern is softly rammed in casting sand. Cut the gateing system in the usual way. Use lots of vents!
I pour aluminum at 1375F. The hot metal vaporizes the foam as you pour it in. That's why it is good to have lots of vents to let the gas escape.
Here is the part as cast.
And here is the finished flask, cope and drag.
Lost foam can also be done using loose sand. You will have to incorporate your gateing system in your foam model.
You can use a cardboard box for a flask. I used a milk case lined with a garbage bag. Put a couple inches of sand in the bottom and set the pattern on that. Then fill up the flask evenly with sand to avoid moving the pattern or warping a thin section.
The foam pieces you see out near the edges were to take up space as I was running low on sand. There is 100 lbs. of sandblast sand in the milk case.
Here is the 8" x 10" cope that was cast in the milk case.
I have experimented with some coatings on the foam to get a smoother surface. I tried several things including paint, low temperature wax, paraffin, ladle coat, investment plaster and plaster of paris. The approach is that a burnable coating such as wax will burn out when cast and will be part of the casting. A refractory coating will not burn out and will be part of the mold.
To smooth the model, I find dipping in melted paraffin to be the easiest and least expensive. To realize the advantages of that smooth model in coarse sand, a plaster of paris skin on the wax is most successful.
In the pictures above, the sample on the left had the plaster skin over wax. The sample on the right had no plaster, only wax. The numbers on the map indicate how many wax dips each area received. Both samples were cast side-by-side in loose sand blasting sand and poured at 1400F.
My conclusion is three dips in a 160F paraffin bath followed by a coating of plaster of paris will yield a very smooth surface.
Since the above was written, I have tried the wax dip on full patterns with poor results. I think the wax generates too much gas during the pour. However, small amounts of wax, like for fillets and filling gaps, seems to be fine.
Here is a lost foam cast-in-place burner bell I am now using. We got our gas line pressure brought up from 10psi to 30psi. Here are links to more photos of this lost foam burner bell:
Photo 1, Photo 2, Photo 3, Photo 4, Photo 5, Photo 6, Photo 8
Here are some tips:
- You can use masking tape to cover holes and cracks in your foam pattern.
- Hot melt glue or bee's wax makes nice fillets on inside corners.
- With the loose sand method, a little moist sand around the sprue will help to keep it from collapsing during the pour.
- The cardboard tubes that paper towels and toilet paper come on make good hollow sprues. Glue the bottom to your gate system and seal the top with masking tape or cap with a paper condiment cup from TacoBell.
- Wrap the sprue with a couple of layers of aluminum foil.
- When you pour, pour hard and fast keeping the sprue choked. That is full of metal. The foam system needs this head pressure to keep horizontal runs from collapsing.