Casting failure ... :(

Hi all,

I'm conscious this is a very noob question and I'm almost embarrassed to ask it but I'm hoping you guys will appreciate I'm still learning this craft and won't take the piss too much!

I had a little play over the weekend and poured some very simple moulds in brass with varying degrees of success. The patterns were various small trinket type thingies including a couple of key fobs. I had a central sprue which I gated off in a radial fashion to each of the items. I used Mansbond oil sand. Some of the items formed reasonably well but others refused to fill completely.



I attempted this one a second time and while it did a little better it was still incomplete. Second time around I gated onto the back of the mould rather than onto the bottom edge so that the molten brass could run into it unimpeded. The pattern itself is approx. 60mm x 20mm x 3mm.

I didn't use a riser because I didn't think it would be necessary for something this small - am I correct in thinking risers become more critical the larger and more complex the piece is? I also didn't use vents but would that alone give rise the failure seen here?

As an aside, I have learnt that molten brass gives off Zinc Oxide fumes which aren't great for the health. My workshop/garage is well ventilated - garage doors wide open plus a side door, so plenty of through draught - but do you reckon I need to take more precautions?

Many thanks in advance.

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I'll let somebody else tell you it was not hot enough.

I want to emphasize a respirator is very important when pouring brass outside. Inside it is imperative. When you get the zinc flu it will pass but it is miserable. Drink lots of milk. Lots. It gets the zinc out of your blood stream.

I suggest a 3M 6200 (medium) or 6300 (large) half mask respirator with an activated charcoal filter like a 3M 6001 organic vapor cartridge. You will not get sick wearing them. The whole system is cheap.

 

Ah cool. If that's all it is I guess I can just learn to recognise when the metal is at the correct temperature by colour. I have also been reading around thermocouples so may well go down that route at some point too.

Many thanks for the clarification around the zinc oxide poisoning. I will definitely be purchasing a respirator as I can see myself working with brass reasonably frequently as I may have found a good source of cheap raw material.

I appreciate you guys typically work on quite large and/or complex projects and I can fully understand the need for getting the runners/gating/venting/etc. all optimal. But what I would still like to know is how critical the venting and risers etc. are for such small casting jobs?! I'm assuming much less so and guess experimentation will teach me much here.

 

Okay, so I haven't learnt yet it seems.

I tried another brass cast today - source material was some .22 bullet casings (which created a lot of dross btw) - and I had exactly the same problem despite me thinking the brass was hot enough. Looking into the crucible prior to pouring, I could see a fine layer of oxidation on the surface of the melt which was breaking up. Through the cracks I could see the mirror-like surface of the molten brass gentle rippling with the heat. It genuinely looked hot enough to pour to me ... clearly I was wrong.

Are there any other signs I should be looking for to indicate it's at pouring temperature (or not as the case may be)?!

I would be grateful for any pointers folks.

Cheers,
Gavin

 

Assuming you're using a small crucible there is a problem of heat loss. The brass would almost need to be literally boiling/yellow hot to compensate for the loss by the time you start pouring. Putting the crucible back in the furnace to heat back up after skimming the oxides and dross could help a bit as well as pouring as fast as possible.

 

The crucible is A5 so not massive. Plus I was using only a small quantity of material, ~350g of scrap so I guess what you say makes sense. I did put it back in the furnace after removing the dross as you say. Maybe if I used a greater quantity of brass, the larger body of molten metal would retain its heat better when removed from the furnace.

LOL ... there was plenty of smoke coming off these things most of the time! I don't know whether bullet casings are lacquered but I assumed it was possibly this and other impurities burning off (as well as the zinc fumes of course).

 

I've never poured brass before (disclaimer), but looking at your picture in the first post you're asking that metal to flow through 3 tiny points of contact. There's just no way that's going to fill. Sometimes you have to make provisions in your mold to facilitate filling. which will require some extra cleanup of the casting. That's just the nature of the beast. Attached are 4 pictures. (I have no idea how they'll sort themselves in the post).

One is a circular bronze plaque casting 1/4" thick. It filled the first time with just two gates with blind risers cut into the cope. Note the shrinkage in the risers. I don't know if they fed the casting or the gating.

Next is a picture of a cope and drag just before closing. You can see where I had cut multiple channels in the cope to maximize flow across the mold. It was a bastard to clean up (metal is hard!) but it was my third attempt and just cutting a couple of gates at the edge wasn't enough.

The last 2 pics are of my latest tree casting. Also 1/4" thick. Instead of cutting into the cope I decided to go totally flat-back and gate into multiple points through the edges only. It filled but there is a flaw. In the upper right portion of the tree in the shined up picture there is an area where 2 metal fronts met. One of them had already mostly solidified when the second one met it and caused a defect that can be seen on both sides of the casting. It's going to have to be ok.

Venting can't hurt

So, pick your poison.

Pete

 

If the brass is hot enough there's white smoke coming off that will leave powdery deposits of zinc oxide (including your sinus and lungs) not to mention a pale blue flame as the zinc burns off the surface of the metal. Also the boiling point of zinc is 907 deg C versus the melting point of brass at 930 deg C so it's actually possible to boil the zinc and have it bubble through the molten brass if you get it hot enough. The zinc oxide powder is yellow when hot and white when cooled (which fooled me into thinking I had yellow lead oxide once).

 

Thanks to all for your replies so far.

@Petee716 Cheers for the explanation and the photos. Very helpful.

So what I am trying to cast here are some small plaques which measure 50mm x 30mm x 2mm (at their thinnest point). They are a couple millimetres thicker around the outside edge due to the border.



I've arranged them in a circular fashion and gated them radially from a centralised sprue. Hopefully the attached photos will make it clear.





And this has consistently been the final result ...



Sounds like I need to gate differently, add runners, and use risers. I did vent each piece through the cope but that clearly didn't help.

As @DavidF has already suggested, maybe I need to pour directly and vertically into the mould. This would mean I could only pour one at a time. I was trying to be efficient here and pour all six at once.

Cheers,
Gavin

 

Yes, I'm seeing that yellow/white powder accumulate on the top of my furnace. And I am using breathing apparatus while I do this so hopefully my lungs and sinus will remain unharmed.

 

@DavidF Understood. I will give this a go - I assume to be poured vertically as you have drawn? Are the pieces at an angle for a reason or could they be at right-angles to the central runner?

 

Cool thanks. And what reason would that be?! I'm trying to visualise what will happen when the molten metal is poured in and I think I can see how the angled moulds will fill better than if they were at right-angles (allows air to be expelled better??).

One other question please - what purpose does the 'tail' on that vertical runner serve? That one is eluding me right now.

 

@DavidF Awesome ... thank you for the advice.

 

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I think I may need to practice my controlled dumping.