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Short sprues and air vents on sand molds

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Pete W.:
Hi there, Ironman,

That's an interesting demonstration, as said above, 'the proof of the pudding etc.'

But one ingredient of the 'pudding' is the moulding sand - please tell us more about the sand you use.

Could it be that some folks in other parts of the world use sand that is less porous than yours and so needs more feed head and venting?  Maybe because of the additives they use to make their sand cohesive?  Or constituents that are in the sand because of the geology of its source? 

I guess foundry videos would get a bit boring if there's too much ramming of the sand shown but some other makers' videos seem to ram harder and longer than you do - could this also be a result of your sand being naturally more cohesive?

I've never done any casting in my life but I have watched a lot of videos - I'm not challenging your results, I just ask these questions to prompt discussion about this aspect. 

ironman:
Pete W.

 The sand I use is a waste product that is left over after all various grades of sand are sifted out. It is very fine sand with a lot of dust and other contaminants.

It is very cheap, $10 for a trailer load. I have been told by foundry sand experts that it is a poor quality foundry sand. It works for me so I do not need to buy expensive graded sand.

 This sand has poor porosity so moisture content cannot exceed 5%. It works equally well with cast iron and aluminium for the size castings I need.

So if someone is brave enough make two molds with the same pattern and gating but make one with a short sprue and the other with a tall sprue and see if both molds fill up. This is how I learnt because I was not afraid to experiment.

Beone:
The pattern also has a lot to do with it.  I cast a lot of small thin pieces such as medals with a lot of small raised text and riser height can be significant. The difference in a 2 inch versus 2 1/4 inch can mean the difference between 50% and 90% turn out.  I usually have 8 to 10 medals per sprue.
Dave

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