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.

How sturdy do you need the cast do be? Would something like candle wax work as a casting material or is it too soft for your purposes? Do you need any nails or tacks to hold in the cast? Does it have to be water resistant? It would really help if you could describe the actual process and what the cast needs to withstand. — Elmy, Aug 04 '22 at 08:58
@Elmy - Updated the post with details about sturdiness, texture and colour. To answer you here: it has to be sturdy enough only to survive a single pressing on a wet leather (so that I make the leather form some shape). This is also why the colour, texture, and fine details are not important, as wet-forming leather shapes don't capture these details. — caveman, Aug 04 '22 at 09:05
Your requirement for "fast" is subjective, but also lacks available tools. A 3D printer owner could build a mold to fit your requirements with a print that takes 3 hours. A CNC owner could build a mold in less than that time. High durometer silicone molding cures in a very short time, but can be more expensive than softer silicone molding material. "Cheapest" is also a subjective term. — fred_dot_u, Aug 04 '22 at 13:25
@fred_dot_u - Question is not about the mold, but the cast. You're free to pick whatever molding suitable for your approach as stated in the question. — caveman, Aug 04 '22 at 14:08
for you to create the casting, but it would be ready to use in a few minutes or hours. 3. Casting implies that you will use something as a mold. Do you need to create that (is that part of the "fast" requirement), or will you use items on-hand as the mold? 4. If you will use items on-hand, are there any limitations on the casting material? e.g., if you will use plastic molds or containers, you couldn't use a hot casting material. 5. It would help to understand the nature of the shape, which could make a big difference in the recommendations. Can you add an example image or brief description? — fixer1234, Aug 04 '22 at 17:40
@fixer1234 - 1. Looks like a bowl with wiggles, Or a valley. Then I pour cast in it (no requirement to put a cap on it). 2. Mainly labor time. 3. The mold must be made fast as well. E.g. one thought I had is to use clay as the mold, then pour cast in it. 5. The shape is similar to the bottom half of a sphere (so it's a convex), except that it has unique wiggles on its surface. — caveman, Aug 04 '22 at 17:56
I'd suggest any sort of casting plaster – like Hydrocal ( What is Hydrocal?) – it's cheap, easy and pleasant to work with, and despite being very hard, is able to be refined after casting (compared to, say, cement). — spring, Aug 06 '22 at 22:35

fixer1234 · Answer 1 · 2022-08-04T20:41:38.903

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.

Carving wax was my first instinct on this one too, so I'd second that option. It's a particularly hard wax used for casting purposes; very easy to build up, and can be readily reused for the next mold. — Allison C, Aug 05 '22 at 13:47

score 4 · Answer 2 · answered Aug 04 '22 at 17:32

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.

With a powerful ethnographic enough blender you could get a few uses out of the same HPDE, but eventually it won't reform well enough — Chris H, Aug 04 '22 at 17:48
How does anthropology fit into chopping plastic with a blender? Ethnographic - pertaining to ethnography. Ethnography is a branch of anthropology and the systematic study of individual cultures. From a search. — fred_dot_u, Aug 04 '22 at 18:17
SwiftKey fail: I'm having issues with deleted autocorrect failures (ethnographic) on my phone reappearing when I try again (enough), in addition to the correct word. I don't always catch them,though I did spot at least one other in that comment before submitting it — Chris H, Aug 04 '22 at 19:13
That's one of the more interesting results of automated text entry I've seen. — fred_dot_u, Aug 05 '22 at 08:08

score 2 · Answer 3 · answered Aug 04 '22 at 22:19

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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).

answered Aug 04 '22 at 22:19

RobynStarveling

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Welcome to Arts & Crafts. Plaster softens and crumbles when it gets wet, and the use here involves long exposure, probably with some pressure, for wet-forming leather. I'd worry more about the casting being compromised than it leaving a film on the leather. However, it could probably be sealed well enough to survive the planned single use (might need something better than hair spray for this application unless you have a big supply). Could you expand your answer a little to address that? Thanks. – fixer1234 Aug 04 '22 at 23:46
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A cheap idea would be to cover the plaster with cling film to protect against moisture. However, that would leave behind a pile of discarded casts that will probably never be used a second time. Other materials mentioned in other answers can be reused and therefore end up cheaper. – Elmy Aug 05 '22 at 05:42

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

3 Answers3