I have had quite a few castings poured over the years. The largest was the legs for a lathe. In most cases, I have prepared an origional part and use that as a pattern. My problem has been how to handle hollow parts. In simple cases, the foundry has been able to use some stock cylinders. I have been stuck dead on how to make large or complex core boxes. I have made a pattern for the outside profil of a large asymetrical cylindrical cast muffler (one piece). The closest thing would be to picture an elongated socker ball. I need to make a core box, that is a hollow box, split down the centerline, with a cavity slightly smaller than the outside shape of the muffler. I dont want to try and chissel or machine a solid wood box. So once Again, I am looking for a trick or shortcut. Might it be possable to use my outside pattern and some spray foam or similar porduct to create a cavity. Then split the halves and buildup the needed 1/4 inch thichness? Can the foundry work with a foam core box? Or some other modern miracle matterial thats easy to hand carve?
as always, thankyou all
[ 09-20-2006, 05:28 PM: Message edited by: peter ]
I'm not very familiar with casting molds but if you want to build a solid surface onto a complex curve, I do know of one trick using clay. Take a clay sheet of uniform thickness that you desire. Cut the sheet into ribbons. Lay the clay ribbons, next to each other as best as possible, over your complex shape. You'll leave voids here and there and won't always use the entire length do to the curves. Once you have covered the majority of the surface, fill in the various voids with clay and smooth everything over for a clean surface. You can experiment with various widths of clay for best fit in the different sections of curve.
For castings, you might have to use different materials. Let us know what you did in the end. Good luck.
The short cut I can think of, is to make a core shaped object (same size and shape as the core should be); then cast it in reprothane or similar. If you do one half at a time, and use good mould release on the parting line, the resulting box (reprothatne mould of your core model) should work fine for pasted up sand cores.
I've done similar merely to prove the tooling, in one case. IOW, I milled aluminum core shell core boxes, but needed to make sure the core keys mated in the mold without crushing the sand, so proved them with reprothane. But the resulting box could be used for making cores if approached from the other direction.
Be aware that with a core, you will need core prints in your pattern, and corresponding keys added onto the core itself. This is to support the core, and if the core is assymetrical, the keys may need to be designed to keep it from twisting. The core will try to float in the molten metal. The core prints and mating keys keep eveything in place.
First Put core prints on your pattern. Then make a split plaster mold off of that. Next roll out some modeling clay to the thickness you want your part to be. Line the mold with the clay but not the core prints. The clay will take up the space that your finished part will be. Coat the mold with a release agent and assemble it back together. Fill that with plaster and now you have an exact shape of the core. Make a split mold out of plaster or castable plastic off of that and you have a core box. This is a very simplefied description on one way to do it. There may be others. Check with your foundry to make sure it will work with their process.
I had to do some fairly complex cores for my Gardner gun castings. Both of the above are good suggestions to get you where you need to go. You can check out the videos at the Freeman Supply website for some great approaches to your problem. I used the repro stuff for one of my more complex core boxes and it made the job almost rediculously easy. Here is the link to their videos: http://www.freemansupply.com/video.htm
All good suggestions above. If it's a "one off", or something less than twenty cores, plaster of paris is the cheapest way, no "hard" mating surfaces necessary. You can reinforce it with #3 (3/8") rebar and/or leave it in the casting boxes, made with 2" lumber for strength.
Since your core is asymetrical, I would recommend "square" core prints to counter rotation. Of course the vertical sides of the prints would have to have a few degrees of draft each way from the parting line.
I am a big proponent of the CO2/sodium silicate core, a very strong and simple core to produce. In your case, since the core will be large, you could use a capped, hollow tube filler or even styrofoam inside to reduce the sand to a managable weight.
The reason I suggest the CO2 cores is because if you need several of them, you can produce them at home and deliver them to the foundry.
The other two common types of cores are 1, sand/binder and baked, takes something bigger than the home oven and they are heavy, (and time consuming/expensive from the foundry) 2, shell cores which have the advantage of being strong, light and generally the best finish. However, shell cores must have machined metal, (usually aluminum) core boxes, as the process utilizes high heat (big fire around the core box) and not every foundry will have a large enough shell core molder to handle a "muffler".
If the core has the proportions of a muffler, the CO2 core should be more than strong enough to support itself, if not, the foundry can provide chaplets, like tacks with heads on both ends, same material as part and as tall as the casting is thick.
Hope this helps, Bob
the first thing i would do is call the foundry and talk to the supervisor in the pattern maintenance shop.
find out the dimesions of the flask that approximates you casting size then make your matchboard in that size that way your safe as to the size of the mold...jim
If you want some real advice from someone who actually makes patterns and core boxes every day feel free to private message me.