Clamshell Cope and Drag Hinge (No Pins) Al parts to be cast.

So, you are suggesting pouring directly into the mold right down the middle? I suspect that is not what you mean, but that is the way I read you post.

Denis

 

I think the problem with my large central riser is that I simply made a tee joint as Andy said and I alluded. That moved the area to freeze off last right to the junction of the riser and the pattern itself. However, if I neck down the riser where it joins the pattern that will allow the pattern portion of the cavity to cool off somewhat independently of the riser. Of course, the proximity of the riser does add heat to that central area of the part. But, if the riser is blockier (has a higher Andy modulus) than the part, the riser will freeze last and be able to feed the part. The trick is timing the freeze of the contact between the riser and the part. I would guess that Chvorinov may have addressed that somewhat more complex scenario over and above simply calculating the modulus of individual pieces in isolation from each other. It would seem that those sorts of calculations could become quite complex and involve the specific heat of the metal and sand thermal mass of the sand, its conductivity, the duration and flow rate of hot metal previously through that sand and other nearby radiating masses of molten metal. For me, not having the knowledge needed nor the computing power needed, that is where intuition will help a bit, I hope. So, I am thinking of making some silicate blind risers that will be "square" cylinders with a neck 1/2 the diameter of the cylinder and length of neck that is 1/3 cylinder height. I will situate the riser in the same location as my prior "chimney" riser and then hope. ;-)

I have a feeling it will work.

I am thinking of making those risers using a lost wax method. I am thinking that will be a novel, relatively easy, and interesting approach. Then I can just place the pre-formed riser on the pattern and pack sand. Sounds easy. As long as the silicate riser doesn't crack when packing the sand. Watch out, Kelly will be jumpimng in and suggesting lost foam necked-down risers. They might just work and would be super easy to make.

Denis

 

So, you are suggesting pouring directly into the mold right down the middle? I suspect that is not what you mean, but that is the way I read you post.

Yes, but only for the flask pieces. I'll try to get shots of coreplates I have done that way when I get to the shop.



You can see where the riser caused slight erosion, but nothing around the whistler in the other pad. I cut the sprue with a cutter rather than with a sprue peg so missed slightly.

 

I see I have been unclear about which approach I would make to which pattern I was referring to above. Sorry.

For the horseshoe-shaped 50-piece project I intend to thicken the gates and use a small riser at each gate and that gating and riser system will be set up as a 4-gate unit that can be rammed up in conjunction with the main patterns.

On the flask pieces where I used the ineffective chimney-like riser and where only a few pieces total will be cast, I am going to play with necked-down risers to replace the chimney risers. I’ve been thinking various times about applying necked-down gates to castings and want to test a system of making them. Commercial fiber and silicate necked-down riser shells are available as such a shape for risers has frequent commercial application. But it is not practical to buy a case of one size and a case of another size for each pattern application I have in mind. Maybe it can be easy to make silicate ones as needed. It is for this application that I was toying with the lost foam idea.

Denis

 

We started discussing two different castings so I'd like to comment on this one first. You can apply Chvorinov's rules with simplicity. While I don't have your dimensions I can guess from the tape the central chunk is 1-1/2" long by 2-1/2" wide by 1" thick. That makes it 3.75 cu in. If it were hanging in space it would have a surface area of 15.5 sq in. Dividing volume by area you get a modulus of 0.24 in. But it is not hanging in space. It has casting on both ends, maybe 1.5 sq in so you are left with 14 sq in of cooling area. That brings the modulus to 3.75/14=.268 in.

I'm guessing your riser is 1" diameter so that's 0.785 sq in. Take that off of 14 and you are left with 13.2 sq in and a modulus of 0.28" for the chunk of casting which shrunk. A 1" by 2" high riser (with no bottom area because it contacts the casting) has a volume of 1.57 cu in and an area of 7,07 sq in, modulus is 1.57/7.07=0.22 in. It will freeze before the chunk does with a modulus of 0.268. The feeder has to have a modulus larger than the fed section.

If you made the riser 1-1/4" in diameter it's modulus goes up to 0.269 in but you take away area on the fed section so it's modulus goes up too.

A 1-1/2" riser gets you to a modulus of 0.316 and the fed part grows to a modulus of 0.313. You would have to have something to feed the big cylinder but it might work. As you can see ever increasing feeders contacting the part to be cast are counterproductive on shrinkage because with each increase of contact area you are reducing cooling are and raising the modulus of the part you are trying to protect.

If you come in the side of your casting (like your original gates) but right in the middle with a gate 1/4" high and 1" wide and 1/4" long from a large bob you only have the sides of the gate to cool it. It would be 0.0625 cu in with a surface cooling area of 0.625 sq in for a modulus of 0.1 in. The center chunk now has an area that goes down to 13.75 sq in (from 14) and the modulus is 0.27 in. Unless this gate is shielded from cooling a gate like that just does not work. If you extend a gate like that another 1/4" down into the sand you double the volume to 0.125 cu in and increase the cooling area to 1 sq in for a modulus of 0.125 in. You didn't increase the contact area so the modulus of the cast part is unchanged but you are still short. So hang a 1" cube off the side of your casting, touching the casting only by 1/4" by 1". Now you have 1 cu in of volume and 4-3/4 sq in of area (one side of the cube comes from a large riser and 1" by 1/4" is on the casting). 1" divided by 4.75=0.210 in. That still does not feed the center cast part with a modulus of 0.27 in.

If you make your feeder 2" deep, 3" long, and 1" wide contacting the casting over a 1" by 1/4" gate area you will have 6 cu in of volume. The cooling area will be 21.75 sq in (if it is connected to the sprue by a tiny gate like 0.25"x0.1") and you will have a 0.276 modulus. That will probably work. Increase the size to a 3" box 1" wide and you have 9 cu in with 29.75 sq in of cooling. 9/29.75=0.30 in modulus. I think that would work well. You have more feeder to remelt. A thicker contact area with the casting helps but the contact area gate tends to be hot, and the internal corners don't offer as much cooling so the gate will not freeze so long as metal is moving through it.

I know, too many words and too many numbers but this stuff really does work and you don't have to be precise to make it work. The flat emblem I was making in the video I posted was a difficult shape to avoid shrinkage and going by the numbers worked. I have cast 80 of those with a match plate and absolutely no shrinkage failures.

 

If that doesn't bore you to tears give me the dimensions of the chunks of sewing machine casting which shrank and I'll comment on what I would feed it with.

 

Interesting work Dennis.

Looking at the riser on this photo, I notice you have sharp corners where it meets the pattern, this can create shrinkage. A smaller diameter riser with a generous fillet should stop that happening and encourage piping on the riser instead. I had a very similar problem and solved it that way.

However, I think the best approach, is to calculate the modulus as Andy has explained. I will certainly be doing that in the future,

Cheers Charlie

 

This is a shot of a flask piece. No shrink with this size gate from a central runner that also fed a long side mold.

 

Looks good, ESC. I had good luck with a similar pattern 26” long and 3.5” wide with a single gate 3/16 thick and 1” wide at one end of the mold. The forgiving thing about your pattern and my 26” is they are of relatively uniform modulus throughout. So they freeze up more or less uniformly. I am sure mine was hottest near the gate and began freezing at the far end. The pattern is thin in cross section so the top and bottom surface could just pull in toward each other a skosh. No visible shrinks resulted. And no feeding needed during solidification.





But my short pieces are thick in the center and that area freezes last and needs feeding. A big gate won’t solve that. So I am making necked risers as we speak. They will be combined sand and silicate plus pink foam. I’ll post more on that later.

Denis

 

Here are the necked down risers that I am making. You can see that they consist of a pink foam shell that I cut out of a block a foam simply using an aluminum pipe of appropriate dimension that was sanded down on the end to make it sharp. Then I wrapped a piece of manila paper around the foam and filled in the resulting gap with sand plus sodium silicate and then gassed it for 30 seconds to solidify the sand. I expect to use these on the short sided flask piece in place of the chimney-like riser.

I I drilled a quarter inch hole down the center of the foam for two reasons one to help act as a vent, though I don’t think it really needs that, and, two, I’ll put a little dowel on the pattern to help locate the riser.

Denis

 

LOL. When foam's involved, YOU are everyone's prime suspect.

 

Here is what I’m gonna try. For my lift and flip setup the boss on the end pieces is needed. There are other ways it could be provided, but casting is what I want to do.



I’ll make some measurements and get them up so Andy can tell me if intuition is leading me astray here.

Denis

 

I'm no expert!

But I'm not sure why you think a giant gate won't work.

My intuition is that you will have a nice necked down gate with shrinkage on the casting. I hope I'm wrong.

When do we pour?

 

Denis, I have had decent luck using a sprue cutter to make a riser in similar situations. Pushing it in until it contacts the pattern, then cutting a basin around it. Once it shows metal, it can be topped off with hot metal directly from the crucible.

 

Pouring will not be for a "few" days as I have a few projects going right now that have deadlines. I should not be screwing around with silicate foam risers, but the idea is intriguing.

I may, indeed, end up wiith nice risers and shrinks in my part, but I hope not. Here is why I think it will work. The foam riser shown has the highest ratio of mass to surface area of any of the components of the mold. So it should freeze last. Freezing last does not, however, guarantee success though. I am counting on the combined heat of the central thick section of the part and the heat of the riser to keep the neck in a liquid state long enough to allow the part to freeze off more prior to the riser and yet maintaining a channel of liquid metal in the center (pipe) of riser neck. I am using a riser neck diameter to riser diameter rule of thumb offered by a mentor. that rule of the thumb is that the neck should be 1/3rd to 1/2 the riser diameter and should be "not very long." Now that is not so satifying as a geometric calculations but it does take into account the interaction of the hot bodies and their cummulative effect on sand and cooling/freezing. Those cummulative effects would have to be difficult to calculate. I can not help but think that simple volume/area calculations not also acknowledging the effect fo nearby bodies is incomplete. And I will be the first to say that, though incomplete, it probably is considerably better than pure guesswork. Nevertheless, the riser and the part boss are in close proximity in this case and must strongly influence each others freezing and collapse/shrinkage/piping.

So, in effect you would be making an open necked-down riser. I guess that would be a simple way to address this issue if I could accurately hit the boss with a spruie cutter. Hmm, something to look at. One way to make sure the sprue cuttter was accurately place for a riser would be to use a sprue pattern with a central nub on the sprue pattern end that could be placed in a corresponding recess in the primary pattern. That would keep it centered and located while packing sand. Then, as you suggest, carve around it to make the body of the riser and then pull the sprue pattern. For some reason I really like blind risers. I think because I worried that I would accidentally splash early metal into open risersor might have crud fall into open risers. In this case they may not be worth it though. I do use blind risers on almost all of my iron casting and they are effectively necked down as the gate connectring them to the cavity is about 1/3 to 1/2 tyheir diameter allowing good piping as the main part freezes off. If I forget to place one (happens!) I pay the price of a spoiled casting. Uggh.

Measurements in the morning. tired..

Denis

 

I think the necked riser has a good chance of working, so long as the riser does freeze last. How big is it?

You are very correct in that the amount of cold sand each part sees greatly affects it's cooling rate. That's why you can simplify most shapes and get good results with Chvorinov's Rule.

 

I made a runner/riser system for the mold. It worked very nicely. No shrink seen. Happy happy.

 

I also poured the flask sides with the crazy foam/silicate blind risers.

Also worked out just fine. Interesting spout of smoke from the foam. End result was good.


Gates were a max of 3/16" thick and about 1" long


Denis