Re: direct displacement sand casting

Posted by bruce paul fink on December 04, 1998 at 10:43:13:

In Reply to: direct displacement sand casting posted by bill wolff on November 23, 1998 at 18:44:35:

An example of styrofoam form castings which were welded together for final assemblage....

This method works well with a foundry sand 'green sand' which is composed of a sand and small amount of powdered clay and a touch of water to hold it together. Undercuts of course are limited but do exist well if the sand can hold it's shape against the flowing metal without joining the metal flow or breaking into it. That is also the reason I would prefer the oil bonded types mentioned by others or the sodium silicate / CO2 gased type, or any of the investments that are more firm but still need no burnout to pour. (How many options are there? Maybe others can add to this list os sands or mold media.)

Heating a welding rod or such to a red hot and plunging it down the center of the sprue (a reason to keep the sprue form straight) will also start the flow into the mold faster and help feed the bulk in a quicker way. A larger than normal cup also helps maintain the feed flow (good idea with the clay pot form, just be sure it is bone dry or better).

Aluminum will transfer the heat through it's whole mass more readily than the bronzes so the lesser BTU capable transfer is not necessarily a disadvantage at all and in some cases it seems to fill better in extremes than the bronzes due to this fact as it will not chill as readily.

The biggest thing I see lacking in this short comment notation group is the number one start of how to shape the styrofoam to your most desired form and still have the surface detailing to the max. Yes wax may be added to the surfaces but do remember that it has to also become a gas in the burning and will also have to be expelled out through the walls of the sand mold so additional venting can be an assist (sometimes via soda straws or even longer welding rod pushed holes around the form but not always necessarily having to touch it).

Other forms of working the styrofoam initial form is a really new blade in a sharp razor knife giving you detailed shaving capabilities;

of using a heat gun or larger heat source to thrust the shape into and then quickly hand twist of form it;

or using water based and wet media to draw upon it and then subject it to a heat source to shrink the styro wherever the water does NOT cover;

etc. etc. etc.

You may want to look up the Artmetal Chat that covered a lot of this and more including styrofoam forming at:

http://www.artmetal.com/village/chat/main/transcrp/980423MN.htm

For an example of a finished work after assembled welding of many smaller castings in styrofoam check out http://www.fink.com/bpfink/gate1a.jpg

or detailed others at http://www.fink.com/bpfink

Good luck and just analize any failures for the reasons of cause and you should be able to quickly get great castings with the minimum of molding costs and time.