Shrinkage holes

[kelvin2164]

As a background, I'm bulding a 5" live steam loco and doing all my own castings.
Small ones are ok, but I'm trying to cast the body for the boiler hand pump in brass. It's about 28mm dia, 70mm long with a few bosses and mounting lugs here and there. (I would supply pics but cant find USB cable). I'm getting holes right through to the middle of the casting in the thicker areas. I've tried adding a large reservoir at the bottom of the riser right next to the heavy area, but this goes hard before the main casting and a hole gets sucked in right next to it.
The next item is the cylinders, 50mm dia in bronze. If I can't sort the pump. I will never manage the cylinders.

 

[ddmckee54]

Kelvin:

Are you casting this part solid or cored? If it's solid that may explain why the riser is freezing before the part does. You'd use more brass in the pour, but maybe you could try multiple feeders to the casting and multiple risers with the gates as large as possible? The good thing about casting your own, the rejects just go back in the pot for the next pour.

Don

 

[Jasonb]

How big is the runner between the riser and the part, if thats the thinnest part it will cool before the others and the metal from the resovoir won't flow into the part

 

[abby]

Pictures say much more than words but you could try using a "chill" this is a piece of metal ,probably steel in your case , embedded in the sand so that it touches the pattern at the shrink area.
The idea is to cool the molten metal more quickly so that it draws from the feeder rather than the other way round.
Moulders often used steel nails for this purpose and you will have to experiment to get the best results.
You might also check on your pouring temperature , for collected brass scrap the temperature is about right when the zinc oxide is falling like snow !!!

 

[kelvin2164]

Thanks for a he responses there guys.
I'm not using a core in this application as it would need to be about 15mm dia, to bore out to 20mm and I figured this wouldn't really help much. I will use one of course for the cylinders.
The reservoir on the riser was actually touching the fat part of the casting. Reservoir about 20mm sphere. It makes a mess of the appearance of the casting though. Can I have a large reservoir connected via a smaller, short runner and hope it stays molten. I'll try embedding some chill material for the next trial run. I also modified the pattern to get rid of excess material in the overly large mounting feet which is where the problem was.
Regarding reusing brass (which I seem to do a lot). I know I'm loosing zinc. How do I know when I need to add some more.
Pouring temp. tell me more about the "snow" part. I haven't seen any.
I was a bit discouraged before, but I can see a bit more light now.

 

[abby]

The "snow" was a bit of a joke , but in brass foundries , and often in my home foundry , when the brass is at pouring temperature the zinc is boiling off , this is why you don't need to de-gas brass , the zinc forms oxide with air and falls like snow.
Breathing the foundry atmosphere is not very healthy so I wear a good respirator and have an xpelair fan too.
As the zinc is depleted following melting the quality of the brass improves to some extent.
From a 60/40 brass you can go to 70/30 which is cartridge brass and if you were to lose an awful lot you might get to 80/20 brass which is similar to brass prior to the 18 century.
Before zinc was known in its metallic form brass was made by smelting copper with calamine (zinc ore) the maximum zinc content obtainable from this process is around 20%.

 

[kelvin2164]

SUCCESS (Well sort of)
I followed the advise given and got a casting free of any sign of shrinkage.
However, next problem. At the top of one end is some very serious porosity. It looks like lots of liitle bits of brass stuck together. I used no bits in my pot, all solid brass fittings. This goes right down to the bore for the ram. I hope the pic I attached shows this. It's at the riser end. The sprue end and most of the rest is ok.
I'm getting close, but still no prize.

 

[Lakc]

It looks like there was a bit of loose sand at the gate that got washed into the mould. Its hard to pack the sand well when your gate takes an immediate 90 bend and starts feeding the part. I like to cut a little splash area into the cope so the metal overflows into the mould cavity, pouring slowly it helps keep the gate getting washed away like that. Looks like it could be a serviceable part for you, congrats!

 

[nel2lar]

Kevin
One of the ways you might try is to give yourself extension on bothe end of your part, that was you have a good solid ramming. Measure just where you need to put you pour basin and after all rammed up cut the appropriate runners. Just don't give up, it will just one day fall right in there.

Happy New Year
Let's make some chips
Nelson Collar