Molding sand is more than just sand. Typically it is a fine grade of sand (mine is 110 grit sand blasting sand), clay binder and something to moisten it. There are two types of molding sand of interest to the casting hobby.
Water bonded sand or Green Sand has been around forever and is the traditional molding sand. It is made with fine sand, Bentonite Clay and water.
NB: Bentonite clay is not the only binder available for making green sand. There are other 'flour' types available, though for the home/hobby foundry Bentonite is probably the most common.
This is a LOT different. Commercially sold as Petrobond and homemade as K-bond. Fine sand is combined with Bentone Clay, oil and a Catalyst. If You're in a hurry to get casting/ want excellent results / have the budget, use Petrobond.
Petrobond is available in at least two forms - Pre-mixed and binder / catalyst to which you mix your own sand.
Petrobond is re-usable, HOWEVER, you will need to obtain or construct (self constructed being the far cheaper option) sand muller. A sand muller is basically a mixing machine that 'squeezes' all the sand thoroughly so that all grains of sand are evenly coated with the binder.
Petrobond is very 'sticky' and will hold shape very well, provided sufficient draft is allowed. Having said that, I have found that slightly less draft is required for oil bonded sand. Wear disposable gloves when working with oil bonded, as it WILL stain your hands, as well as pretty much everything it touches.
Another very good property is the very detailed surface finish oil bonded sand will give you. I would estimate a 1000% improvement over green sand.
The difference is in the clay. Bentonite is common and cheap and absorbs water. Bentone is a little pricier and absorbs oil.
Don't make the mistake of using kitty litter to make green sand. It makes a mess. Some report success, but many others have had to bury it.
Bentonite is the base clay that Bentone
is made from. Bentone is
treated to bond with oil. Bentonite naturally bonds with water. Ray Brandes
Aluminum has a lot of
surface tension so when you pour more head
(tall sprue) is better than short if you have thin sections. Brass and
bronze alloys behave differently. Some pour like lead and others
develop a heavy skin and need to be poured hotter. Berilium-copper
added to the melt just before pouring makes the metal flow
For bronze, use Everdure, a silicon bronze. I get it from Atlas in Colorado. If you order from them get some berilium-copper shot too. You can also use berilium-copper welding rod cut into small bits. Ray Brandes