Hydropower Project Group: Steps for Amateur Sand Casting
We gave a primitive overview of the casting process in our first blog post, but to further clarify the technique to readers, we decided to prove an in-depth guide. Online resources for sand casting are hard to come by, and those that exist are mainly semi-professional—far beyond our skill level. Hopefully, this sand casting guide can help students like us learn from our triumphs – and (many, many) mistakes – and try sand casting for themselves.
- The object that you want to cast
Must have a simple shape. Any cavities must exist on the same plane; for instance, a standard coffee mug cannot be casted because the cup’s main cavity runs perpendicular to that of the handle’s.
- Casting flask
Make a box with walls and a bottom, and another with only four walls. The top and bottom boxes are called the “cope” and the “drag,” respectively.
The size of the box will differ according to the size of the object being cast—allow at least two inches on all sides of the object to allow for the heat of the aluminum pour.
The cope and the drag should be the same length and width, although height can vary. Must be of a sturdy material. The reasons for this will become clear in the step-by-step instructions.
- Something that can be used to tightly pack sand into the mold, a “rammer”
The sand must be packed very tightly into each part for the mold to stay in place. While we used our hands to press the sand in early one-part molds, we found that the mold maintained its shape much better when we had a solid object to physically pack the sand into the box with. For the two-part mold, working without a rammer is virtually impossible.
- Fine silica sand
Playground sand, available at low prices, works well. Silica sand is chosen because of its relatively low porosity; sand made from other rocks can absorb moisture and explode when it comes into contact with molten aluminum.
- Fire clay
This may be slightly more difficult to obtain. It must be able to withstand the high melting point of aluminum.
- A mesh sheet to sift sand through
Anything with holes small enough to allow through grains of sand but not small pebbles should work. We have found that a kitchen sifter with fine, double-layer mesh serves the purpose well.
Making a Sand-Clay Mixture
- Make a 1:9 ratio mixture of clay and sand. Mixing in small quantities is recommended to ensure that the materials are mixed together well. When you think you’re done, mix again; inconsistent mixtures are more likely to crumble apart in a two-part mold. Each time that you reuse a sand-clay mixture, make sure you sift the material well to make sure that you are not working with any clumps.
- Mix water into the mixture. Make sure that the added water is thoroughly mixed into your sand before adding more. For the amount of sand-clay we generally deal with, we add our water in roughly tablespoon increments. Use the test below to determine when your sand is ready.
- Squeeze (hard!) a handful of sand into a hotdog shape. You should be able to pick up the shape and break it in half without crumbling. However, when loose, the mixture should flow through your fingers similar to dry sand. Adding too much water during this step will lead to bubbling during the cast.
Here, the mold-making method will differ slightly according to the object being cast.
- Once the mixture is ready, fill the drag to the brim with the mixture. Pack the mixture very tightly into the drag using the rammer. The mixture should not move in the mold when the box is jostled. The sand will compress; continue to fill and ram until the entire drag is filled.
- Level the sand, using a rod to scrape off excess sand.
- Pack sand tightly into any of the object’s cavities and scrape off excess sand, so that any cavity or valley in the object is filled and leveled.
- Place the object cavity-side down on the flat sand surface of the drag.At this point, it is wise to lightly tap the object and make sure that you can raise it without taking the sand with it—just as if you were building a sandcastle.
- Sprinkle a bit of dry sand on the drag’s surface. This will help keep the two parts of the mold from adhering to one another.
- Place the cope on top of the drag and situate them so that they are exactly on top of one another. Aligning them correctly at this stage is crucial, as you will have to realign them later. Slippage will result in a skewed cast.
- Pack sand into the cope. When sprinkling the firstlayer of the sand-clay ixture into the cope,becareful not to disturb the dry sand layer.
- When adding sand to the ope, include at least half an inch of extra sand-clay mixture above the top of the object. This will depend on the size of the object being cast. Using the rammer, pack the mixture tightly into the cope.
- Once the surface of he mixture in the cope has been rammed smooth, identify where the highest point on one end of
yourobject is in the sand and poke a hole here, using a pointed tool such as a pen.Make it pinky-sized—this is the hole that you will pour your molten aluminum into.
- Find the object’s highest point on the opposite side and also open a hole here. This will be used to allow steam to escape from the sand as molten aluminum flows into your mold.
- Gently gently, making sure not to shift it horizontally, lift the cope straight up off of the drag.
- Gently gently put the cope down on a smooth surface, being careful to not to jostle it. Unnecessary movement here may cause your entire mold to fall through.
- That process should have left your drag and object looking just as it was before you added the top half of the mode.
- Lift the object straight off of the drag, being careful not to let any of the mold mixture move out of place.
- Replace the top mold to exactly where it was before removal of the object.