Lost Foam Metal Casting (LFMC) Notes
Types of Foam Used in LFMC:
There are many types of rigid, closed-cell foams available to the hobbyist. However, only a few are suitable for the LFMC process. The two most popular foams used in LFMC are (white) polystyrene “bead” foam like that used in packing electronics and appliances and the other is extruded polystyrene (EPS) “insulation” foam as found at your local builder’s supply houses like Lowe’s and Home Depot. Without going into a huge philosophical discussion, I recommend the use of EPS foam (Owens Corning “Pink” or Dow “Blue”) rather than bead foam whenever possible. While the EPS is slightly too dense to be “perfect” for LFMC, it is substantially more rigid, cuts cleaner, is generally straighter and provides much nicer casting finishes than bead foam. Whatever you do, DO NOT USE POLYURETHANE (URETHANE) FOAMS UNDER ANY CIRCUMSTANCES!! The byproducts that result from burning urethane foam produce rather large amounts of potentially deadly toxins!!!! Urethane foams have absolutely no advantages over EPS and the dangers in its use for LFMC cannot be ignored. YOU HAVE BEEN WARNED!!!
LFMC Patterns and Pattern Making:
LFMC pattern making is (for me anyway) much more fun than wood pattern making (I hate woodworking) and it presents opportunities to explore other areas of interest to increase your pattern-making prowess.
The first thing that should be covered is section thickness. Because foam patterns do not have the strength that wood patterns do, you must be cautious that you are not trying to create patterns that have sections that are too thin. I have found that maintaining dimensional accuracy with LFMC patterns demands fairly thick pattern sections or the weight of the sand itself will deform your pattern, often ruining your casting. The rule of thumb that I apply is that all sections in a LFMC pattern should be a minimum of .375" (3/8" or 9.5mm) thick. If this can't be achieved, then consideration should be given to investing in a pattern constructed of a more permanent and robust material (e.g., wood).
Perhaps the most interesting and important “area of interest” involved in LFMC pattern making for me is hot-wire foam cutting. This is a fun process that allows virtually unlimited shape and contour options to be achieved and the machinery to get started is downright cheap. Like most technologies, the options to expand are many and the cost to move toward something like a CNC foam cutter can be considerable.
Manual Hot-Wire Cutters
There are many options available for either buying or preferably building a simple, yet effective manual hot-wire foam cutter for use in LFMC pattern making. The one thing to bear in mind is that patterns cut with a manual cutter will never be as smooth, consistent and accurate as those produced by even a basic CNC hot-wire foam cutting machine. However, if you only intend to produce a few LFMC patterns and are willing to spend a bit more time finishing your castings, there is no reason a simple manual hot-wire foam cutter won’t work very well. One thing to consider is that the success and finish of your foam patterns may benefit from using a thin pattern template affixed to your foam stock to guide the cutting wire. Cutting templates can be made of many materials including poster board, thin plywood, masonite, MDF, etc. Just remember that the template should be able to tolerate the wire heat without burning or melting and it must be non-conductive!
CNC Hot-Wire Cutters
If LFMC pattern making will become a larger part of your foundry activity, then it may be advantageous to consider the construction of a computer numeric controlled (CNC) foam cutting machine. These rather simple, yet no less ingenious machines have been quite an eye opener for me on many more levels than just foam pattern making.
Like most, I began making my LFMC patterns using templates and a manual hot-wire cutter as described above. While this technique was more than functionally adequate, it (as well as I) lacked the subtle ability to produce attractive patterns that would reflect their form in a nicely finished casting.
After much research and rumination, I decided to build a simple and inexpensive CNC foam cutter that I could employ to produce LFMC patterns of much higher quality and complexity than I could have successfully produced using my manual cutter. The plans I settled on can be found here.
One thing to note is that there are perhaps hundreds of CNC foam cutter designs that incorporate varying levels of technology and expense. However, when you boil the subject down to its salt, one truth comes clear: Simple and inexpensive does not equate to incapable and of poor quality. The design I settled on to construct my first CNC foam cutter incorporates components and materials to which many would turn up their noses. However, this is definitely one of those “don’t knock it ‘til you’ve tried it” opportunities. As a first foray into home built CNC machines, a CNC foam cutter is a great project that does not need to be exorbitant in its expense. I built my machine for about $300 and it makes patterns of exceptional quality and accuracy. The one area that I would strongly recommend you consider (unless you’re an electronics “geek”) is the purchase of a packaged CNC controller/motor kit. I looked long and hard at what was involved in making the controller myself and decided that it just wasn’t my cup of tea. Then I set out to find out what was available in kit form. My research lead me to Dave Rigotti’s HobbyCNC site where hobby CNC projects are his only game. Dave is a great guy and his CNC foam packages are excellent. It took me about a week to assemble his controller board and about two more weeks to assemble the gantries (yep, I work S-L-O-W). But once it was finished and after a short learning curve with the FoamWorks software package, I was making beautiful foam patterns that I simply couldn’t even begin to match by any manual means. In fact, my simple machine made with industrial drawer slides and MDF will produce extremely accurate LFMC patterns all day long and repeatable to less than .010” in most cases. In fact the biggest variable to repeatability is the control of the wire heat rather than any mechanical limitations of the machine.
The links section of my site has numerous references to CNC hot-wire foam cutting resources for your reference and research. WARNING: Home built CNC machines have a funny way of getting you addicted. So much so, that your “dependency” is likely to drive you to want to build more!
Getting “Wired” for Hot-Wire Foam Cutting
There is a certain “secret” to getting excellent hot-wire foam cutting results that is rarely discussed. This secret is quite simply the type of wire to use for foam cutting. Just about everywhere you look, the overwhelming recommendation for cutting wire is nichrome (nickel-chromium) wire. Of course, there are exceptions that have been used with varying degrees of success (e.g., guitar strings, stainless steel, etc.), but nichrome wire is by far the most recommended for the purpose. While nichrome will certainly do the job, it tends to require a lot of current to heat and that tends force the use of larger, more expensive power supplies, it makes for very large kerfs and makes the job of producing nice clean cuts extremely difficult.
The clever folks in the R/C aircraft hobby have been expanding the frontiers of hot-wire foam cutting for many years. As part of their developments, much work went into finding a more suitable wire for use as the cutting element in the hot-wire foam cutters used to produce foam wing cores. These folks discovered that a very thin strand of inconel wire vastly outperformed nichrome on all fronts. The advantages of inconel in hot-wire foam cutting are many. Inconel wire can be much thinner allowing much finer detail to be reproduced and requiring substantially less current to produce nice clean cuts. All of the foam cutting I do is accomplished with .012” Inconel X750 wire. After struggling with the fragile nature, expense and the high average heat required to use nichrome wire, moving to inconel X750 cutting wire was like a beacon of sunlight in a dark room. The difference really is that dramatic and you would be doing yourself an injustice to not make this your first choice in wire material. Check here for a reliable, online inconel wire supplier.
CNC Router Options
As nice as CNC foam cutters are, they have their limitations. For example, if you like making plaques or sundials, the CNC foam cutter will do a great job of cutting out the outline of the part, but it simply can't produce the details and features you need in the middle of the part, like numbers, pictures, words, etc. A nifty solution to this problem is the CNC router. While a bit more expensive and demanding of higher quality componentry than a CNC foam cutter, a CNC router is another wonderful CNC project. In fact, electronically, a CNC router is generally a bit cheaper and simpler to construct as routers typically are only 3 axis machines where foam cutters are 4 axis machines. However, a CNC router will demand more precision from its movement to produce accurate and repeatable results. This pretty much eliminates drawer slides from consideration in most cases.
Since there are literally hundreds of options when constructing a CNC Router, I won’t try to recommend any one of them here. However, as mentioned before, the links section of my site will identify numerous resources for demonstrations, plans, software options and ideas to build your own CNC router that will suit your needs. The good news is that, once again, Dave Rigotti at HobbyCNC has got you covered with a very nice packages electronics system that will get you going very quickly.
Adhesives
In the face of convention, I will start by strongly recommending against the use of hot-melt glue for LFMC pattern assembly. Even the low-temp products tend to leave quite a bit of residue in the colder parts of the melt that often show up as metal “BBs” on the surface of the casting. Additionally, some glue formulations seem to create additional outgas products when they vaporize that other adhesives do not.
By far, my two favorite LFMC pattern adhesives are Elmer’s No-Wrinkle Rubber Cement and Elmer’s Glue-All Multi-Purpose Glue. Both of these products work very well on LFMC patterns and feature low shrinkage, good workability and firm bond strength. Both adhesives are readily available at most of the larger chain stores (Wal Mart, Target, K-Mart, etc.) and can often be found with the school supplies or in the home improvement areas. Overall, I like the rubber cement best as it dries quickly and grips quite firmly. By using a few hat pins as anchors, a typical foam pattern will be ready for finishing overnight.
If you need to smooth a pattern or add some fillets in corners, etc., a wax material like that described on my Foundry page is an excellent choice for an easy to apply and shape pattern filler material.
Surface Finish Enhancement (Loose-Sand Molds)
For LFMC patterns intended for loose sand molds, it is often desirable to completely coat the pattern with a “shell” to enhance finish quality and improve the mold quality around the pattern. This shell mixture can be made by thinning pre-mixed drywall mud with water in a 1:1 ratio (about the consistency of a thick soup). This coating can be brushed onto the pattern, sprue, gates and runners in several thin coats, or the entire pattern can be dipped in the mixture and hung from a drying rack so the excess material can drip off of the pattern while it dries. In is critical that each coat be allowed to dry completely before the next is applied and that the pattern is bone dry before casting. Normally, a shell thickness of 1/16” to 1/8” is sufficient. Coatings that are too thick tend to prevent the mold from “breathing” during the pour and can often ruin a casting that would have been successful if the shell had been a bit thinner. Remember that the coating is primarily to improve surface finish in loose sand molds and not to provide strength to the mold surfaces.
LFMC Molding & Pouring Considerations:
Loose-Sand Molds
There are some important considerations than must be applied to any LFMC pattern intended for loose-sand molding.
 | The nature of this type of mold demands that the molten metal be poured “fast and hard.” To achieve this, the pattern must include sufficient sprue, gate and riser area to allow fast metal flow and sufficient escape pathways for the gaseous byproducts of the vaporized foam. Generally, the size of the required sprues, gates and rises used in a LFMC pattern is about twice that used in a conventional drawn-pattern sand mold. | ||||
| The sprue configuration is often most successful when it is capable of delivering metal to as many areas of the mold at the same time. This can be achieved quite successfully when the main sprue is split into “runners” than feed several of the heaviest areas of the pattern (much like a single-plane intake manifold on a small-block V-8). | ||||
| In conventional (e.g., loose sand) molds, ALWAYS mold
and pour the pattern vertically as opposed to the standard bonded sand mold
which is rammed, drawn and poured horizontally. The reason for this are:
| ||||
| The head pressure of the molten metal entering the mold is critical to getting larger LFMC molds to fill properly. A remarkably simple tool for increasing the pouring head pressure is demonstrated here. Dave Kush, is a noted LFMC practitioner and his site is a worthwhile educational resource for anyone interested in LFMC. |
Bonded-sand Molds
While likely to stir quite a bit of controversy among the hobby casting community, I believe there is no place for water-bonded sand molds in LFMC. Quite simply, water of any kind has no place whatsoever where any sort of LFMC pattern is used. Having said that, all hope is not lost. LFMC patterns can be used with tremendous success in petrobond (oil-sand) molds of virtually any configuration. In fact, petrobond LFMC molds are often much more successful and produce more faithful and accurate results than the loose sand molds are capable of. I try to do as much of my lost foam castings using petrobond molds as possible and the results are generally excellent.
As with loose-sand molds, there are unique considerations that must be applied to LFMC patterns intended for petrobond sand molding:
| Due to the increased volume of gaseous byproducts produced by the vaporized foam, venting a petrobond mold requires particular attention. A good rule of thumb is to vent a petrobond mold twice as much with a vent wire twice as large. |
| Sprue and gate sizes and placement can generally be configured in the same way as “standard” drawn-pattern sand molds. However, I recommend twice the number of risers. Risers act as large vents early in the pour and as a “filling indicator” at the end of the pour. Quite simply, when the risers are full, chances are, so is the mold. |
| Because gravity doesn’t provide the extent of assistance in filling the mold as it does in a vertical loose-sand mold, larger LFMC patterns may require two (or possibly more) sprues and a “double-pour” technique to properly and completely fill the mold. |
| Ramming a petrobond LFMC sand mold requires a slightly different approach as well. LFMC patterns prefer to be rammed fairly lightly in comparison to wood patterns. I prefer to riddle most of the sand into the mold with my fine ¼” riddle. I peen the sand around the periphery of the mold as normal, but as I move over the pattern, I compact the sand using firm finger pressure to set the pattern and fill the pattern details. Once the pattern is covered with about 2” of finger-pressed sand, I riddle and ram the remainder of the sand with a “closed-fist” ramming technique rather than using a standard ramming tool |
Pouring Temps
Because the molten metal is doing double-duty by both vaporizing the foam pattern and filling the mold cavity in LFMC, the pouring temperature has to be a bit higher than with a typical open-cavity bonded-sand mold. For aluminum, I have had good results with pouring temperature between 1350° and 1400°F. Basically, you want to pour as cold as possible while still filling the mold completely yet still flashing off all pattern materials. This will help keep gassing to a minimum and provide good surface finish.
Perhaps one of the most useful tools the home metal caster can invest in is a genuine foundry pyrometer. Commercial units are available from MIFCO for a few hundred bucks, but perfectly adequate and capable pyrometers can be built right in your shop using the thermocouple from a commercial unit and an inexpensive digital VOM with a Type K thermocouple input. There is nothing particularly fancy required and the structure of my pyrometer was constructed from simple ½” plumbing parts and a bit of scrap wood. It really is worth the investment no matter what type of casting you do.
