I'm looking for some input as to what went wrong with my lost foam pour. These are a transition piece from the flue outlet of a PRR caboose stove to a harp for a lamp. I turned the patterns on my wood lathe and hollowed out the inside with a die grinder. Coated with drywall mud. I'm thinking too cold or too hot. My pyrometer was out of commission at the time so I couldn't check metal temp. Any help appreciated.
Assuming those are hollow, and you sprued/poured into the top, the one with the worst defects and to a lesser degree the others are caused by the sand in the underside being inadequately packed and consequently moving during the pour, which in turn allowed the sand on top to move and create those defects. An interupted pour could have potentially contributed to the worst defects. An "overhanging hollow" is very hard to pack with dry sand and vibration. Does the area where you sprued in get drilled out? Even a small hole (1/4-1/2"D) in that location would allow that cavity to fill and pack with adequate vibration and you could feed the part elsewhere. Alternately, you could sprue into the same location and reposition the pattern in the mold so it is turned on its side (but still at and angle) so as to expose the underside feature to enable the sand to flow and pack better into that feature. What are you using to vibrate and pack the sand? Might be nice to get a look at your feed system. What is the height of the sprue? Are you using a pouring cup? A third option would be to pack the underside with bound sand but as a lost foam caster, I resist that because it's rarely necessary! Best, Kelly
Hello Kelly, Thanks for your input. I did have the patterns on about a 45 degree angle, and vibrated with the 5 gal. bucket on a piece of plywood with my air chisel and a blunt end tool. The sprue was about 7 ish inches long and I poured into a soup can. I considered packing the underside with P-bond but obviously didn't. I also considered molding them upside down, but that meant difficulty removing the sprue. I molded 4 of them hoping to get 1 good one, and there is one good enough to use, with a little putty work, but I wanted to know what went wrong if there is a next time for something like this. I've added a picture of the underside that shows proof they didn't pack adequately. I'm happy you responded, you are the lost foam master, and I'm in awe of what you are able to do.
Dave, I was envisioning a conical underside. That looks more like a disc/cylindrical hollow. I think that needs to be canted past vertical exposing part of the face of that hollow to the top surface of the mold. What are you using for vibration and sand? I'd say you are underpowered and/or the sand isnt flowing easily (wet for example). With decent vibe and good dry sand, that should be able to be done standing on it's side. If you vibe as you fill in stages before the flask gets heavy, it will pack the portion of th emold where the pattern resides better. The sprue is easy to pack and less critical. Also, dont underestimate how much a small hole where you sprued would help packing. In general, ifsand can flow though a cavity with multiple contact points to the rest of th emold, it becomes more like a core held in core prints. I've had some similar failures. It's pretty crazy because in some cases, both sides of those surfaces would just shift downward in the mold. Check this one out. Look familiar? It was going on 5 years ago and I've moved on quite a bit from this time. Use a different coating and pouring cup, but it was along the time I was figuring it all out. http://forums.thehomefoundry.org/in...r-upright-wheel-carrier.543/page-2#post-11032 Best, Kelly
Thanks Kelly, my sand is definitely dry, but the vibration was definitely not sufficient. I can see now that orienting it on its side would definitely improve my chances for a good pour. Thanks again for the help, much appreciated. Dave
