Just for discussion I thought I’d post about some changes in riser design that I’ve made of late and that I like. The thing that’s most important to me about a riser is that it fail where you want it to contract. That is usually the center of the top of the riser is the area that one hopes will cave in as metal contracts. Most of the time that’s the way things work out with a fairly conventionally shaped riser but that is not always the case. I have had a situations where risers have collapsed at their base where they joined with the part and I’ve had situation where the risers have collapsed mostly on top but have drawn in a little around the bottom as well In order to address these problems I have gone to a somewhat different riser shape and I think it is working for me. Here are a couple of pictures. The depressed top is pretty obvious and the fairly steeply drafted lower edge of the riser is less obvious but I think also important. That lower edge modification makes it less likely that there will be drawing of the riser where it meets the part. I also like to use registration pins as much as possible for my risers as they can tend to skate around a little bit with packing even though one tries to tuck them in with sand before ramming them firmly. And since it’s easy and in Onshape to model a spike hole I added that as well. Denis
We used to do a lot of this kind of trickery.LOL I found creating a premature shrinkage in a riser will usually help it feed better. Sharp edges can cause the shrink which is what the chamfer around the bottom helped with. We used a big radius where it contacts the casting to do the same thing. I use to make the riser round and just cut a drafted slot into the top creating a v across the top. The sharper the v is at the bottom and the deeper it is the hotter it will be and the better it will feed.
Nice ti have some validation that I am not off on a false lead. Thanks. I cast a couple more wheels using it today and it worked just as desired failing about 1/2 tsp or so in the top center with no distortion of the casting at the intersection of the riser and the wheel. I am having fun casting the wheel but may need an "intervention" to help me quit casting more wheels. Our weather has been fabulous and my furnace is running very well (8 melts since revamping it.) It's hard not to keep at it. Denis
Looks good Denis. The last set of pics explained it much mo' better. It seems quite small for the amount of metal in the hub. I thought it was a pouring cup.
I've read that a V-shaped slot cut in the top of a blind riser pattern helps feeding in 2 ways: 1) it allows atmospheric pressure to act on the riser, rather than letting a vacuum form and work against feeding 2) the thin point of sand on the V gets really hot and keeps the riser molten longer. I guess your dent in the top might be doing something similar. I seem to recall someone describing this V-shaped feature and calling it a "cracker". Maybe HT1? Not sure... I have only used this once and did not try the riser without the cracker first, so I have no basis to compare the results. But fwiw the castings were successful despite the patterns having no way to promote proper directional solidification (thickest by far in the middle and surrounded by thinner sections all the way around) unless I ruined its appearance by putting a fat riser right on top of the part. Jeff
I agree that the riser is quite small. But a very the size of my riser depending on how I want it to work. In this case that Central hub is pretty massive and left without a riser was almost certain to contract on top in the center just where that small riser is. The riser itself did not need to control solidification but simply provided sacrificial metal for expected contraction. Prior experience with this pattern showed that the amount of actual contraction that required metal donation was small and that’s why I made this one so small. On the same pour I also poured a 18 inch prism/straight edge. For that pattern I use a riser that is probably 20 times heavier than this little riser for the wheel. The reason the straight edge riser is so large is that I need to have it not only provide a little bit of sacrificial metal but also to control certification and draw that last area of solidification into the riser and out of the part itself. Experience has shown that I get a vacuum defect where the gate enters the part otherwise. I’ll see if I can find a picture in my file of that riser set up. Otherwise when I go out and dig out that mold today I’ll photograph the riser and maybe try to make a drawing of what I am trying to say about moving that last area so if Acacian outwards towards the end into the riser. When I am reporting what I am doing I am not thinking of these ideas as somehow authoritative or necessarily novel. They are not novel (and certainly not authoritative!) as you guys have commented but the idea of indentation on top of the riser is one that I have slowly come to over time as helpful. And the bent and the reason for posting about it is simply to get it on other people’s radar screens if it isn’t already and to learn how other folks approach the problem. And 3-D printing of risers has made it easier by far to explore variations in shape like the flared skirt and indentation on top. From now on I think I will always flare the base of risers that are applied to a surface of a casting. That is because I have had quite a number of incidents of minor contraction at the juncture of a vertical riser surface with a horizontal casting surface. FWIW, Denis
Found some pictures, but not great ones. I think I was trying out blind feeders because open feeders would have been a huge amount more metal and I wanted to pour 2 molds without running out. I forgot that I did try a blind riser with no V first. That was... not a great pour, it ran out. The mold filled somehow, but the parts had shrink on them. Hard to see the shrink in the pic though. This probably proves nothing. Even without the runout the riser was a different shape and probably just wrong. Pic with the "crackers" in use. These all came out usable (even if a couple of the very tips were a little short. I blame the spin traps I was experimenting with at the time and abandoned soon after). Horrible molds and patterns, a favour for a friend type of deal. The central ridge is fully an inch thick butted right against the 1/4" thick tang (which had no draft on its sides). Castings weighed almost 800g each, probably enough metal in each one for 3 or 4 usable tips if I had to guess... Buddy's men's group needed 5 of them and to be as involved in the process as possible. Tried to explain patternmaking over email to their 3d printer guy halfway across the province and this was the result of the patterns I got in the mail... Anyway, it did feed better this way, in my case. I don't use a lot of blind risers but I too would be interested to know how many others have used a trick like (or unlike) this to improve feeding. At least I didn't have to grind, polish, sharpen, or put a handle on any of them... Jeff
Here are pics of the larger riser I mentioned above and a drawing of my intention of pulling the area of final solidification back from the gate while also preventing premature solidification of the gate. In pouring the prism early-on I had a good sized riser but I think it froze off at the gate before the part itself froze entirely. So, the riser, in that case, was ineffective at preventing vacuum defects. The massive riser concentrates a lot of heat at the gate area helping it to not freeze too early. If I were only needing to provide sacrificial metal, the riser could have been 1/10th the size used. I hand-carve the sand away from the side of the riser closest to the part to increase its mass there and to get that heat still closer to the casting. That is why the riser cavity is sloping just above the triple-hatched area. Denis
So, I decideed to model the riser/gate combo in ONshape and print it so that I don't have to cut the gate everyt time. This should save some time and ensure consistent gating each time. Denis
This morning I cast another (final I think) wheel and used the tiny "saggy" riser. It certainly did what it was supposed to do. You can see that it collapsed most in the center and drug down the top edge almost all the way arounbd as it collapsed. I know such a small riser will not always be apropriate as discussed above. But for a "massive" cylinder that just needs encouragement to colllapse right in the center, I think this is a good solution. Denis
And I just now dug out two 18” straight edges that were fitted with the truncated-boot-like riser show above. The printed riser drew nicely from the sand after about 5 mins of hand sanding and a 3 coats of lacquer The also collapsed as desired centrally. Best of all, there were no vacuum defects in castings when I sliced off the gate. The more refined riser designs made possible by printing them seems like a real plus, so far. Both risers had deep collapse (piping) centrally. Nice that it was located there and not in the gate! Denis
Very interesting to see your work. I must later write some thread about gating calculatins and feeder size.
