Got the patterns updated this morning (lowering the cope pattern turned into a bit of a faff), and also took the opportunity to blank off some of my cope flask to save sand and weight... Sadly no casting today, spent all afternoon moulding up but the back part of the cope came out with the pattern when I pulled it and it was too late to redo and not end up running the furnace outside in the dark. I think I got a bit carried away ramming sand in between the buttresses on the back (one of which is just about visible on the right hand side of the second pic) and they didn't want to let go Might have another crack at it tomorrow if I'm feeling keen.
Are there any issues re-using sand that has been rammed into a mould as is? Do I need to run it all through the muller again to fluff it up?
You'll have to break up the clmups of rammed sand, so the muller might end up being faster. IMO if the sand hasn't dried out too much you're probably fine though, I got along ok without a muller for a while before I built mine. Jeff
Faaaaaail. I went a bit far on lowering the cope pattern, the bit with the hole in is 3.81mm thick and it should be a hair over 4mm. Still, 3.81mm should be enough for it to fill. The bobs didn't work at all - the bobs themselves are perfect, and the bosses they were supposed to feed have nasty shrinks: And I'm still struggling with whatever this defect is - presumed shrinkage along the edge of the flange, but only on the pouring side: Some good news though, the feeder worked and kept the thick section fed nicely: Including some cheeky hand-carved bosses on the inside that I was hoping would mean I could have blind bolt holes there: The vent wires on the fins seemed to work nicely, and this is the nicest the writing has come out. Might hang this one on the wall ) At this point I'm pretty fed up with spending all my time on this, so I'm probably just going to have the first one welded and move on with something else. However if I'm suddenly struck with motivation to have a 3rd try I'd be interested to hear any thoughts on further improvements!
The simplest advice for improving filling is pour hotter, especially in thin walled castings. Do you know the pour temp? This could aggravate shrink defects if that's what they are. The risers are too small and need to be 3-4+ times the mass, a shape that minimizes surface area/volume (typically round), and be placed very close to the feature of the part they are to feed. On a part like that, I would make a U-shaped runner along 3 sides of the part sprue placed in the middle of the runner on the short side (though other may suit if you don't have room), and feed it with at least two gates on each of the long sides making sure the mold is level when poured, or very slightly canted with vent's poked at the high end of the sump. This will have the affect of providing a more uniform and hotter average metal temp for a given pour temp. You might also find that your shrink defects disappear without the need for bobs because of this, and in general will allow the lowest pouring temp, from which only goodness shall come. I realize this is what you figured would naturally happen feeding the flange feature but it is not large enough in cross section to effectively do so. I don't know what the distribution of vent holes was on this attempt but if they were uniform across the sump, the spikes and their height are indicative of hotter metal and or when it arrived. Also, and this is a minor detail, I know everyone says runners in drag and gates in the cope, but on this part I'd put the gates in the drag too so they feed through the surface of the flange that gets machined and not contact the visible surfaces of the finished casting. Cuts down on finishing and keeps the casting features clean. Never give in. Once you optimize the right features is will run consistently. Best, Kelly
You may find the shrink defects at the bobs go away if you remove the bobs. Looks to me that the bobs are the last to freeze and are sucking metal from the thick sections. When you get this mastered you'll have lessons which will serve you for years. Don't give up.
I was paying closer attention to temp this time after my overshoot on the previous attempt. I shut the furnace down the moment I hit 790, then faffed around for a bit skimming dross and lifting the crucible into my pouring shank. I didn't measure temp again, just went ahead and poured. Those corner bosses had nasty defects when they were bob-less, so I figure they need "something" at the least, but my current solution is obviously poor. I'll have a rethink and see what else might fit - it's quite tricky ramming around them. Maybe I could just cut some bobs by hand (with a pipe) that sit right on top of the bosses? When I first started designing this I had thought of a runner going around multiple sides, however I don't have the capacity required to do that at the moment. I won back some by getting the cope pattern lowered, but still not enough for a runner on 3 sides. I figure the best I'll be able to do is have a longer runner on one long side, and pour into a sprue in the middle of it with a decent-sized gate on either side of the sprue. Failing that, it's build-a-new-furnace time and I was hoping to put that job off until I have a new shed to put it all in, which is many months down the road. Thanks for the encouragement both!
Having weighed the ingots left over from today's pour, I should have enough metal for something like this (assuming I don't spill any more than I did today): (with the 22mm pouring sprue landing on that button in the middle of the runner) I understand your point about putting the gates in the drag Kelly, but I think I would end up needing extra metal to get the same kind of gradually-decreasing-mass thing going on, and I'm short of metal!
Dumb question guy here.... Please tell me that pattern was actually made and not a pan you are copying? Hate to see you nail this and go to screw it on and find it magically SHRUNK by 5%.
I like the new gating much better. That should help it fill much better. Along with Dumb questions guys response, would love to know how you made the pattern.. CNC??
Yes I made the pattern, and it's 3D printed... in many parts, glued together, and back-filled with plaster of paris, then sanded and painted and sanded and painted and broken and repaired... not sure I'd really recommend this method, but I don't have any other machines! Slightly annoyingly, I printed it at 102% scale only to find that LM4 only shrinks by 1.3% (no idea where I got 2% from, must've read it on a forum somewhere) so the castings are actually ever so slightly too big, but it's not a big enough error to be a problem.
Yea thats been floating around for some time now and has caused many a headache. Think that is the shrink rate for pure aluminum and not an alloy??? So what ya running for a 3d printer??
I have an Anycubic i3 Mega. There's a couple of vids about it on my YT: I've got a vid about making the pattern in the works, but I've been spending all my spare time in the foundry rather than editing videos!
Thanks Funkster... Did you add your YT page to our list? http://forums.thehomefoundry.org/index.php?threads/members-with-youtube-channels.188/
You narrated that like the guy from the 80's TV show, The secret life of machines. https://www.exploratorium.edu/ronh/SLOM/
Huh. I've just realised that the recommended pouring temp for LM4 is actually 720 degrees... another case of getting a number stuck in my brain and not checking against the original sources. I think 790 came from the degassing tablets. Next pour will be colder!
Well, the good news is that after unsuccessfully trying to fix it with bobs, I seem to have sorted the issue with the bosses on the non-fed side of the part by making them hollow (this will have a slight effect on their use as dowel holes but it's not critical): And my current gating has sorted the shrink in the corner of the front flange: The bad news is that I'm really strugging to get this bit to fill, I think I'll have to make it thicker: Also the rear flange is full of shrinkage, it'll need a gate also: Regards from the ragged edge!
