Found Dreaming

I'm trying to cast an aluminium cylinder, about 35mm OD x 22mm ID x 70mm long. That is, my core is 22 diameter x 90 long over the prints.

My core mix is sand + flour + a dash of water, squashed into a plastic water-pipe (which gives it a very smooth finish, by the way) and baked in the domestic oven for several hours. I start the heat low (120~150 C) and gradually increase it to flat-out (250C) for an hour.

My problem is that the core 'gasses' and the gas rips through the casting - a riser on top was spectacularly un-successful, so today I tried a riser at the opposite-end from my pour, together with several small vents along the top of the mold. Still NFG.

It seems to me that the volume of 'stuff' in the core is big compared to the amount of metal, so next I'm thinking of making the core hollow but ...

(A) how can I vent from the hollow core?
(B) what wall-thickness (or thin-ness) should I aim for?

BTW, having just found Jammer's article on making Sodium Silicate at Home, I wonder what gassing problems arise from such cores?

Geoff

put a vent hole in your core, by inserting a steel rod into it when you mold it but coat it with some type of release agent say Pam, so you can remove it after the core is hardened and before its used. also use a sprue with plenty of height and gate into one end with a air vent at the other,

You didn't say which but are you pouring horizontal or vertical, either way put that vent into the core so it can degas when the hot metal hits it.


BTW, having just found Jammer's article on making Sodium Silicate at Home, I wonder what gassing problems arise from such cores?

you would have very little trouble with gas using a SS/Sand mix for the core, one reason is there is nothing there to create gas with and the core that is made with this method is very porous not so that the hot metal will flow in between the grains of sand but so that any gas created escapes freely. it will be interesting to hear about your trials and tribulations of finding the ingredients to make Jammer's formula in Thailand.

happy casting

Finding the ingredients:
By some miracle, a Thai company (Power Dry Co Ltd.) have responded to my emails, and I have ordered 1Kg of silica gel SiO2 for the princely sum of 100 baht (3 dollars, US, 2 pounds real money) plus post (another 50 baht.)
Sodium hydroxide is easy - I have had a kg of Soda Fie (soda-that-burns) for several years. It's still good - I broke it out today for some printed-circuit board work.

Shortly the fun can commence - fingers crossed.
Geoff

Geoff, what percentage of flour to sand are you using? Should be no more than 3% of flour to sand volume, preferably use bentonite clay rather than flour, or a combination of clay and flour with no more than 3% water. Also... before you extrude that core from the mold, take a coat hanger and prod several holes in it from end to end this will aid in relieving gas build up.

And on your other note of making a hollow core, yes, that's easily done, too. And depending upon its diameter and length you may not even have to use flour and bake it and could just as easily use your normal green sand, clay and water mix. I've done this often before. Remember, too much moisture you get too much gas... too much burnables in the mix you get too much gas. Multiple perferations in the core, though, will help dramatically, too.

Oh dear! F.C., can't I persuade you to have some more flour ..... lots of it?
I've been mixing about 1 cup of flour with 2-cups of sand (30%). No wonder the ants enjoy my cores when I set them in the sun to begin 'curing'.

Since my green-sand contains bentonite, what you are suggesting is effectively to just add a little flour to some green-sand for making cores? Nothing special about it after all?

Thank you for that enlightenment.

Geoff

Personally, if you need a stiff core that will withstand some manipulation and stress you can mix some of your dried bentonite/sand mix with no more than 3% to 5% solution of linseed oil added (that's in relation to the volume of sand, by the way). Mix it real good so each sand grain has a micro thin coating. Then pack it well hard in the tube, punch a couple or three length wise holes with a coat hanger, extrude the core onto a cookie sheet and bake at 400 degrees till nice and golden brown. Once it's used in a cast (expecially if you core the middle out of the core, itself, before baking it, that sand will be easily recovered to use back into your molding sand. But there are also numerous ways to get a green sand core to span a distance without cracking or collapsing. Just gotta be creative is all.

As for coring the middle out of your solid core, what's the diameter and length of your intended core? Maybe I can help you sort this out without a lot of complications or effort.

By the way... corn flour works best in green sand mixes and aids in binding properties more than what you get with wheat flour. I suppose rice flour would work as well as corn flour, too.

Last first: strangely enough, I haven't seen Rice flour over here but corn flour is on most shops' shelves.

My core will be 22mm OD x 90mm long, of which 10mm each end sit in the core-prints. I have a piece of 3/4 (nominal) PVC water-pipe, whose bore is just about 22mm, and I have been 'extruding' my core by stuffing the sand-mix into it then shoving it out with a 1/2-inch (21mm OD) pipe.

Unless I pack it fairly hard, the core falls apart during the extrusion. I have tried leaving it in the pipe to dry/set in the sun for a while but then its a b***ger to push out, even if I lube the tube (with Mazola corn-oil spray) before filling.

Thanks, H.C., for your help and advice in this.

Geoff

There should be no need to vent that core... it's too small of a diameter. How are you gating this cast? Do you have any pictures?

Frank

Frank,
Hmmm. I think I need to start this mould all over again. My flour-filled cores are just about impervious - certainly to what little lung-pressure I can blow after too-many years of smoking.

Over the week-end I'll try again, using your suggestions, to make a less-dense core.

I poured with the mould horizontal, in to the side at one end, with the riser at the opposite corner, and several 4mm vents along the top. It looked as though the vents filled and chilled to solid, then the gas from the core couldn't escape so it built-up in the still-molten part of the casting, if that makes sense.

As far as pictures - I was so disappointed that I just chucked the casting straight back into the melting pot. Next time I ~will~ take pictures.

Geoff