Lost Foam Process
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Top Ten Lost Foam Casting Questions
Many of the initial contacts that we receive regarding the Lost Foam process concern several general questions. As an introductory feature article, we have compiled these questions and the specific information that we have developed from our experience with the process.
1. What is “Lost Foam”?
The Lost Foam casting process originated in when H.F. Shroyer was granted a patent for a cavity-less casting method, using a polystyrene foam pattern embedded in traditional green sand. The polystyrene foam pattern left in the sand is decomposed by the poured molten metal. The metal replaces the foam pattern, exactly duplicating all of the features of the original pattern. Like other investment casting methods, this requires that a pattern be produced for every casting poured because it is evaporated (“lost”) in the process.
The basic steps to the process include:
- A foam pattern and gating system are made using a foam molding press
- The foam pattern and the gating system are glued together to form a cluster of patterns
- The cluster is coated with a permeable refractory coating and dried under controlled conditions
- The dried, coated cluster is invested in a foundry flask with loose, unbonded sand that is vibrated to provide tight compaction
- The molten metal is poured on to the top of the gating system which directs the metal throughout the cluster and replaces the foam gating and patterns
- The remaining operations such as, shakeout, cut-off, grinding, heat treat, etc. are straightforward and similar to other casting processes.
2. What metals can be poured in the Lost Foam process?
Generally, all ferrous and non-ferrous materials can be successfully cast using the Lost Foam process. Because the foam pattern and gating system must be decomposed to produce a casting, metal pouring temperatures above °F are usually required. Lower temperature metals can be poured, but part size is limited. In addition, very low carbon ferrous castings will require special processing.
3. What size range of parts can be produced by the Lost Foam process?
Lost Foam castings can be produced in most all metals from a fraction of a pound up to thousands of pounds. Slightly more advanced techniques are used for very large castings.
4. What “as-cast” tolerances can be expected?
Typically, a linear tolerance of +/-.005 inches/inch is standard for the Lost Foam process. This tolerance will vary depending on part size, complexity and geometry. Subsequent straightening or coining procedures will often enable even tighter tolerances to be held on critical dimensions. A targeted effort between the foam pattern producer, the casting producer and the casting user will often result in a Lost Foam casting that substantially reduces or completely eliminates previous machining requirements.
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5. What type of surface finish can be achieved?
Because a permeable refractory coating is applied around the smooth foam pattern, the resultant finish is excellent. Each casting facility is different, but generally Lost Foam castings have a surface finish within the 60-250 RMS range. If surface finish, due to cosmetic requirements, is a critical issue then surfaces can be targeted to maintain an exceptionally smooth finish.
6. What is the cost of using the Lost Foam process?
Lost Foam castings are generally more expensive than forged parts, or parts made by other casting processes. The value inherent in the Lost Foam process versus other processes is seen in tighter tolerances, weight reduction and as-cast features which all results in less machining and cleanup time. Many castings that require milling, turning, drilling and grinding can be made in the Lost Foam process with only .020” - .030” of machine stock. It is imperative that the features to be cast are discussed by all parties to determine the net finished product cost.
7. What quantities need to be made to make the Lost Foam process practical?
The answer, simply, is not as many as you would think. Tooling amortization is a key factor in this determination. Potential overall savings for your application will aid in your decision. Generally, 500- pieces per year is the minimum production run to be economical. Prototyping runs, however, may be as few as 3-5 pieces for Fabricated Foam patterns or 20-100 pieces for Quick-Cut CNC machined aluminum tooling.
8. What about casting porosity and shrinkage problems?
Lost Foam castings are used for many critical applications, including engine heads, marine motors, high-pressure pumps and valves. X-ray and soundness testing on Lost Foam castings shows characteristics comparable to other casting processes.
9. What type of tooling is required and at what cost?
Typically, tooling is composed of a split-cavity machined aluminum die that is the negative mold from which the foam pattern is produced. The tooling is highly specialized and must be constructed by experienced tooling manufacturers familiar with the requirements of the foam molders and foundries. Most tooling for Lost Foam patterns will compare favorably with permanent and die cast tooling. Prototype and simple tools may be in the $- range while high-end tooling for complex or very large parts can be in the several hundred thousand dollar range. As a result of the materials used and the process stresses, Lost Foam tools can be expected to have 3 to 4 times the cycle life of permanent mold or die casting tools.
10. What lead times can be expected when ordering a Lost Foam casting?
As with all processes, lead times vary greatly depending on part complexity. Generally, 8 to 16 weeks is typical for completed tooling and first castings produced. After casting approval, 6 to 12 weeks is typical for production run startup. Rapid prototyping methods can produce castings in as little as 2 to 3 weeks.
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I would think all the foams have some form of toxic gas after all you are burning plastic. Some may be worse than others.
Real questions would be does it burn out easily, does it leave too much ash, does it need a separate burn before pouring or will it melt out as the metal goes in without problems?
If it did burn OK it would be an interesting material if you had a batch to cast. One printed mould to expand as many patterns in as you wanted and then use lost foam casting. Would need a good release agent as it is sticky stuff!
Urethane gives off all sorts of cyanate based toxins when burnt out.
That being said I have used it with a little water to roll cast a plaster mold with great success.
I've also, in the long past, experimented with putting a dribble in a suitable mold and placing it under vacuum, which was maintained until the PU hardened. Under vacuum it foamed up very well. Excellent details with a very very low density, far lower then the density of ambient cast stuff.
I was using casting PU not an off the shelf product.
If you played a bit with a vacuum chamber you might get something low enough in density that burns it out cleanly with minimal toxic fumes.
I never experimented with PU and sand casting, Styrofoam always did the trick for me.
An old msds sheet will give you an idea of the fumes you are dealing with.
Mixing in a little water helped the reaction speed and volume.
Good luck.
I would be very cautious of the fumes generated by burning foam.
You probably can't undo the damage that those fumes can cause, even in small amounts of exposure.
I read the MSDS (material safety data sheet) for everything I use, including sand, and I take the necessary precautions with commerical respirators and even powered respirators with a remote air source.
The industrial lost foam process uses expanded polystyrene beads (expanded with steam/heat), with a permeable outer coating that allows the gasses generated by the melting foam to pass out of the mold cavity.
I have seen a lot of recent success with the pink lost foam method used in hobby applications, and it can't be discounted, even though it is not a true expanded bead application such as used in industry.
I can't answer the question asked, ie: "Is the two part foam suitable for lost foam casting applications?".
The expanding foam thing is much like the expanding plystyrene beads application, as far as foam expanding to fill a mold.
It seems like some backyard guys can get just about anything to work in any situation, but again, I always say "Show me sections of the casting, and put it under stress, to see if it has cold joints that will fail".
Just because someone can make something work, does not necessarily mean it is the most efficient, safe, or cost effective method for others.
For non-structural, non-airtight castings, pretty much anything goes with lost foam.
Below is a commercial application of the lost foam process, to make tractor castings/parts.
For rapid prototyping, the printed resin-sand mold method is very cost effective, since there is no mold required to expand foam into, and changes can be in the 3D program, and then castings made rapidly after that with a printed sand mold.
Once the prototype design concept is proven, then permanent molds can be made for expanding the beads into.
Troll, when you say you burnt out the PU was that a separate operation or did you use the poured metal to burn it as it was poured?
I wonder if you could make the biodegradable foams that you see used in some packaging, that should expand into a printed mould and unlikely to give off toxic fumes. It could save on 3D printer time and filament as you could print a fairly thin wall negative mould with little fill and then expand the foam into that rather than have to use a lot of filament to print a more durable pattern that could be sand cast. The foam would also suit the slurry method.
Would have saved some filament for this cylinder pattern printed over the weekend that I did the CAD for, it is 13" long and as well as the other half there are 8 prints to form the required cores.
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I have sent a request to East Coast Fibreglass Supplies asking for advice. In the meantime I am trying to get my head around designing the patterns/moulds required to produce a foam part. Whem making a flywheel pattern for example, I print the two halves of the pattern, then sand and fill for a good surface. But making a pattern for foam would mean making the inverse(?) of the pattern to take a foam pattern from. If using an FDM printer, which I prefer to use for pattern making, then it would be harder to get a goos surface finish. The layer lines would be internal, not external where they are accessible. Badly explained, but I hope you get my drift!
Reply from East Coast just now, Hi Unfortunately we mainly sell this as a general void filler for example buoyancy chambers on boats. We do not have any knowledge of this being used for moulding. Kind rgds Martin
I suppose it depends on the part, take that yellow one above, that pattern has both concave and convex surfaces so some are easy to access others less so. Would be the same with a negative some easy to sand other s not so easy. If you are going to be using it as a way to produce multiple foam patterns then a bit of work put into one mould would be better than having to prep multiple prints of the same item. Also as you only need the one print you could set a smaller layer height as the extra time spent printing that one would be worth it for the overall time saving
The designing is easy just draw the patten, draw a "box" and use a boolene subtract to cut the pattern out of the box leaving a negative cavity. This is how I do Core boxes, as an example the cylinder I showe dabove has cores for the bore and also the water jacket.
Draw the water jacket
Draw your box and subtract the core. In practice you draw the box as two items one either side of your split line