Cheapest casting material for a single use of wet-forming leather?

Question

Suppose that we have a mold, made of whatever material that we are happy with. This mold is usually used to be filled by some casting material.

Many casting materials exist. Usually, this material is supposed to capture details in the mold, and hold the little details. It is also supposed to be durable.

However, my casting requirement is different:

• I do not care about the details.
• I do not care about the durability.

Instead, my requirements are:

• The cast must be durable enough only to be used as a template for a single leather work, after which the cast is thrown away. So I don't care about long time durability.

• Sturdiness should be enough to allow for using the cast to press it on a wet leather (to wet-form/shape leather) for a single time. After this single time, the cast is thrown away.

• The casting will be used to wet-form the leather into a 3D shape, such as a bowl.

• Surface details are not important, as the leather won't capture the details anyway. There can be a few chips at the edges, no problem.

• The texture and colour of the cast is irrelevant, since it is not used in the final product (it is used only to press some wet leather to form some shape).

• The process is repeated for many pieces. So it has to be fast in terms of my time involvement (but it's fine if the casting needs to harden for a few days after I make it). This eliminates slow techniques, such as crafting a template using wood that is chipped until it forms the required shape.

• The mold for the casting will be custom made for that use, but with some fast process, such as shaping it from clay.

I know that there are different casting materials, such as various types of plasters, chemicals to mix to form various plastics, etc, but I don't know much about these, as I have never did any molding nor casting.

For this requirement, my question is: What is the cheapest casting material?

The casting material and technique may depend on the molding material. In case it does, feel free to pick the molding material and technique suitable for your answer.

Answer 1

Based on your clarifications in the comments, if the idea is to quickly create the mold from clay, the casting material would need to be something workable at room temperature. You could use cement, which is cheap. As long as it is just sand and cement, or contains very small aggregate, it will pick up fine enough details for your purpose. But any design that protrudes from the surface that is thin or comes to a sharp edge will be brittle. If you don't want to mix your own, a concrete topper or leveler would work. There is finer, stronger cement often used for casting, like CementAll, but it's more expensive.

You can mold and shape green sand as a reusable, high-temperature mold material. Green sand is a mixture of sand, clay, and water, and sometimes other additives, commonly used for casting metal. There are metals that melt at low temperatures that can be cast with household stuff (but pay attention to proper ventilation, fire precautions, and PPE). You can remelt the metal to make new castings.

If you mold a positive out of clay, you can use that to cast a mold out of concrete, plaster, or green sand, which can be used to mold other materials that would soften clay. Those other materials can be reusable, so the net cost is low. For example: hard wax, hot-melt glue (build up in layers to reduce shrinkage), or hot-water-moldable plastic. I would use this technique with green sand only for casting metal, as the other casting materials are likely to hold onto some of the sand, which would be a problem for reusing it, and some could crumble the sand getting it in.

You could use this technique with concrete, and probably plaster, with fred_dot_u's suggestion of recycled HDPE. But note that HDPE requires keeping it under some pressure in the mold until it is cool because it really warps, especially if not cooled evenly and slowly. The mold would need to be reinforced, like contained in a sturdy wooden box, to prevent the pressure from breaking the mold

The casting doesn't necessarily need to be solid. The casting can be a shell containing the detail, which you support by filling from behind with a cheap, or reusable material. This can save you some cost if you are using a casting material with a consistency like putty that you can press against the wall of the mold to create a shell. In addition to what's been mentioned, there are some cement products, like SculptCrete, that can be molded like clay by mixing the appropriate amount of water. They're more expensive, but you don't need much to mold a shell of it, which you can then backfill with cheap material.

Answer 2

One material to consider, in quite good supply, with substantial strength and fast working time is HDPE. It is recycling symbol 2 and is used to make various bottles and other products. Durability is high, although you've noted that it is a single use item.

One chops up bottles into easily melted pieces until the material will flow, although it is viscous. Obviously, the mold must be able to handle the temperatures used to melt the plastic, 131 °C / 268 °F.

One can create walls on the mold sufficiently to provide for appropriate thickness of the cast, increasing the strength from that of the ordinary bottle from which the plastic has been melted.

My experience is that a piece of this melted material, when cooled, of approximately 10 mm thickness would withstand wet-leather forming.

Cheapest? Perhaps yes, if you can collect from recycling bins or from your own use. The only cost becomes the toaster oven necessary to melt it, although I've seen videos of electric griddles being used as well.

Peter Brown's video shows that using a griddle did not work as well as he liked, and moving to a toaster oven apparently worked for him. I should note that he used a blender to "pulverize" the plastic, in his words and it does seem to do a great/fast job. I hope to remember that for my next HDPE project. Baking paper is also a requirement.

Answer 3

Plaster of paris (a.k.a "drywall mudding") is probably your best bet for cheap and strong; [almost] every hardware store has it. The only drawback is a possible film (on the underside of the leather ifit is wet) - several variables can affect this. If it's a problem, you can use a poor-man's fixative: Hair spray (cheap and quick-drying).