Casting defect - any idea what happened here?

i would say cold shut, possibly combined with you needing a bit of a vent

I'll explain,
you have that perfect surface finish aluminum gets when it is almost too cold, i believe SWdweeb, or someone else on youtube, goes into alot of explaining about surface tension, and micro-gas formation causing a better then possible surface finish
now if your metal was that cold, and the last place to fill had to fight with a little gas, you could get that defect. so if you had been 10degrees hotter you probably would have gotten it , if you had happened to have a vent right there, you would have gotten it

Now about that gating system???? nope not going to talk about it, you playing with the Campbell stuff, i'm not going there

V/r HT1

P.S. nothing wrong with Campbells results, problem is he says we are doing it wrong, but wont(specifically) say what is right ... well he says to use a million dollar software simulator obviously thats a NoGo

 

That is a weird one.
My guess is your gate sucked a little out as it was cooling. Try thinning the gate down, width and thickness.
Edit: Or increase the thickness of the runner to feed the gate as it cools.

 

Simple: The dragon farted? ;-)

Your plaques are looking mighty good, save that smnall defect.

Denis

 

To me it looks like that spot was the last to solidify and had the contraction of the metal, your feeder is thinner than the plaque itself so it cooled and solidified before the plaque. The triangular flat castings in the photo below show a similar effect in places, like a gentle wavy surface. The three holes have rims slightly thicker than the rest of the casting but it's not critical to the finished product as it gets machined anyway. Nearly all the castings in the photos have small gating and feeders as they end up being machined to final shape so contraction isn't a problem. Also it's possible to cook your aluminium for a long period in the furnace and end up with so much hydrogen porosity that it counteracts the shrinkage during cooling. This happens when bystanders walk up and demand to know what you're doing in the middle of a session while you're trying to get them away from the danger.

 

I will.
What's with the spin trap? Is that a Campbell thing or a Dweeb? I never understood complicating a runner with that thing when a tapered runner would stop a reverb wave.
When I use a molded runner, I make them with a sharp taper on the end. There is no brick wall to bounce the metal off of.
Any other thoughts on this?

 

It looks to me like shrinkage, too. You still get a nice sand pattern in shrinkage areas.

Chvorinov's rule does work. If you have a little fat gate with a nice blind riser between it and the runner you can feed a defect like that every time. Without it, it's hit and miss. I never tire of seeing a big shrink in a blind riser next to a casting with no visible shrinkage.

 

Maybe I'm wrong, but I've been under the impression that metal should have a clear entry and egress through the casting. In this case the riser would be opposite the sprue on the other side of the plate. Is there a reason this isn't done as much I expect? Seems like a lot of people keep everything on one side but that doesn't make sense to me if you assume there are gasses in the way of the metal front.

 

I think what you're envisioning as a vent is really a riser. They can be used as a vent over on the other side of the mold cavity, or else a guy might put one there so he can tell when the mold is getting full. They can be strategically placed anywhere on the mold if done correctly, but they are often placed between runner and gate or directly in contact with the pattern as part of a directional solidification strategy. Meaning: thinnest and most distant part of the mold solidifies first and progresses toward the sprue. The riser remains molten till last and therefore can supply metal to the casting to the finish. Venting as I've learned it though is accomplished by poking a number of holes into the cope with a wire to within a 1/4 inch or so (not touching the pattern so they dont fill with metal as HT1 continually instructs). I think of it more as a function of the sands permeability.

Pete

 

Are you going to talk about anything I said or just correct my terminology

 

It could be shrink...hard to say from just pics... weird that it would be in the drag and your rim around the outside being thicker should have fed it if it was shrink. I've seen weird shrink defects like that before. If you had something in your sand it could be gas. We get those same smooth round gas bubble looking inclusions when there is a foreign substance in the sand which causes a gas bubble or if there is a clay ball on the surface of the sand where there is metal contact. Usually if it is gas there will be discoloration around it but that is in cast iron.

 

I think this is not a vent/riser but a spin trap at the end of the runner. He vents through the sand with holes poked close to the casting but not touching so metal does not fill them. A vent at the opposite side of the ingates would be the best place to put one if you need one as you suggest. Is that what you were asking....if that was the best location for a vent?

 

I do the same and angle them upwards...vertical molds.

 

You have been listening to 100 year old Brits talking about legerdemain

1) in this case, there is ZERO need for a riser! Period, it's a plaque, a quite thin one, probably under 3/8 inch at it's thickest.
2) Jeff already admitted to not having a pyrometer . It's a cold shut its at the farthest point away from the sprue, so the coldest metal
3) everyone needs to go read up on permeability and gas, particularly in Petrobond , petrobond is very permeable, and produces very little gas, Gas in green sand molds is mostly water vapor petrobond not having water produces less gas, combined with it's excellent permeability petrobond has few gas issues
4) I would also contend, that the sprue, and runner are too small, but since Jeff has had success with the pattern before, I assume his gating system at least has the potential to succeed, I would use a larger sprue, not much a 1/2 inch sprue a 1/4 by 3/4 runner,( Literally I would probably put draft on a paint stir stick, and ram it up in the mold ) with the ingate about the same as the runner, the bigger ingate is fine, other then wasting metal , what you dont want is the ingates to choke the metal flow, pressurizing the actual casting before it is full
5) Andy is completely right about Chvorinov's rule, risers need to have the HOTTEST metal in them, to work the best poor cold risers(risers on the side of the casting away from the sprue) can actually cause casting defects by drawing metal out of the casting rather then feeding it. especially a side riser !

People over compensate for gas, by using open risers, when a simple vent will work , here's a crazy idea, decrease the moisture in your sand ( not an issue here) , but yes 100 years ago, molders working on experience rather then science, might ram up some" wet sand" get gas defects, so come up with some crazy risering/venting plan implement it, if they get a good casting, that was the new SOP , well if someone wrote a book, now you have the industry standard hell this is what Campbell/Puhhaka say is going on to this very day , I definitely saw it when i was in school in 91 we where taught AFS then shown X-ray video of pours, that showed us the metal was not acting the way we where told it was supposed to act in the mold, but no one changed.

My issue with Campbell/Puhhaka is their solution is a million dollar software program, which is great for aerospace work , not going to fly in the Plaque shop


V/r HT1

 

To answer your question directly, yes there is a reason.
Because a riser is ordinarily not used as a means of egress, assuming the egress you are referring to pertains to gasses preceding the metal front. It is used as a reservoir of molten metal to control shrinkage and is placed where it can be of maximum advantage to serve that purpose which is quite often nearer the sprue.
So yes, I indeed tried to address your question and topic by correctly defining the use of a riser and why it is often placed where you evidently didn't think it should go, as well as explaining how the actual process of venting the gasses is done.
Did you read my post?