How to Make a Mold Casting Resin

A Skillfully designed mold
A Skillfully designed mold (Image:

Resin casting can be a cheap and easy way to duplicate treasured figurines, make your own jewelry, or create an encasement to show off (yet permanently protect) your keepsakes. Making a mold can be difficult the first time around, but with a few simple techniques and tricks you will be casting in resin like a pro in no time! Molds used in resin casting are typically made of RTV rubber, silicone, or latex, and the ones bought in craft stores to be used in making jewelry are typically made out of polystyrene or similar hard plastics. The hard plastic molds make removal of the resin cast simple, since the resin will not stick to the mold. However, these molds are typically made in plain geometric shapes only useful for making jewelry or magnets. If you want to cast your own item, be it a sculpture, figurine, toy, etc., you will have to make your own mold, to your own size and specifications. Latex molds are the best for beginners because latex is an inexpensive and easy material to work with, and is more forgiving and less toxic than silicone.

Things You'll Need

  • Object to duplicate (flat or 2 sided)
  • Liquid latex (available at most craft stores)
  • Mold release agent (also available at craft store)
  • Clay (super sculpey, available at craft or art supply stores)
  • Plaster (available at craft, home improvement or art supply stores)
  • Legos (optional), or cardboard and packing tape
  • Disposable brushes
  • Disposable gloves

Since latex is relatively thin and runny before it sets, it helps to make a box to contain the object, and the latex, within. Here's where the Legos come into play: Legos (or similar building blocks) can be used to form a box that will give your mold a shape and hold the mold making substance in place as it dries. Legos can be built into just about any rectangular size, they pull away clean from the latex mold, and they can be washed and reused as many times as you need. If you don't want to use Legos, you can use anything from a cardboard box and packing tape to the bottom half of a plastic milk jug (cleaned out and allowed to dry), Popsicle sticks glued together, any sort of disposable plastic container that is square or rectangle in shape can be used. Just keep in mind that whatever box you decide to use will have to allow for a little bit of space around the object being duplicated, so the mold will have a thick, reinforced edge.

Once you have decided upon a box, you will have to prepare it by lining the bottom with an even, flat layer of clay. It helps to use a box without a bottom, or one which is easy to cut the bottom out of, but this is only necessary if you are casting an object with two sides. If you are casting a one-sided object, or an object with a flat back or on which the back doesn't matter, then you will need to do nothing else but lay down the bottom layer of clay and center the object on top of the clay, pressing it into the clay slightly to give it a tight connection/seal with the clay. If you are casting a 2-sided/3 dimensional object you will need to shore the clay up around the edge of the object, forming a low wall about halfway up the side of the object; also attach a clay cylinder to one end of your object, which will serve as the tube or path through which you will pour the resin into the mold. This clay division will form the basis for the shape of the second half of the mold, and the cylinder should remain attached to the end of your object throughout the mold making process. Make sure that the clay walls are pressed into the clay base securely, and that the object is not going to move about. If it still moves around, then you will want to use clay under the downward/bottom edges of the object to secure its connection to the base, or build up a base thick enough that you can simply press the object in a bit. Once you have your box prepared and object secured, slip on your disposable gloves and begin.

Open the container with the latex and allow it to breathe for about ten minutes, so the ammonia fumes built up in the container can escape, making the latex smoother and less bubbly. Use your disposable brush to dab on a sloppy layer, not too thick, covering the entire surface of the object being cast and the surrounding clay. Latex won't stick to anything but itself, so you can be quite sloppy with this first layer, as long as you make sure all the bubbles are popped and the overall thickness and placement are even across the surface. Allow this layer to dry completely before laying on a slightly thicker second layer. You will be able to tell that the first layer is dry because it is clear and darker in color than when it was brushed on, and the surface will be slightly tacky but not sticky. It will take anywhere from 6 to 10 layers/coats of latex to make each half of the mold, and each coat will have to dry completely before the next coat is applied, which can take hours at a time. As you apply the coats be sure to lay the latex in thick in the space surrounding the object, so a sturdy lip is formed around the edge of the mold, and the mold won't sag around the edges. Once you have applied enough coats to form a sturdy, thick walled top-half of the mold and the mold is completely dry and no longer tacky, you are ready to reinforce your mold.

Mix up some plaster of paris, approximating how much you will need without making too little, then pour the plaster in to the box atop the latex mold, filling the box while leaving a bit of room on the top. Try to keep the top of the plaster as flat as possible, so it will sit flat on the table when turned over. Once the plaster has been allowed to set (about an hour), you are ready to turn the box over and get to work on the second half. If your object is flat on the back and only one side of the mold will be needed, then you can carefully cut the box away from the plaster and latex. Once you have removed the mold from the box, you are ready cast your object in resin!

If you need to do two sides of the object, turn over the mold and set it securely on the table. You will need to carefully remove the clay base and the wall from the original object, making sure that the object stays seated securely within the first half of the mold. Once the clay has all been removed from the surface of the latex, spray it down with a mold release agent, so that the two halves of the mold won't stick to each other. Let the mold release agent dry/soak in thoroughly before you begin brushing on the latex for the second half. Follow steps three and four, starting with a thin yet sloppy layer, then brushing on a thicker layer, making sure to keep the sides nice and thick with latex. Once you have applied enough coats to form a strong, thick-walled mold, you are ready to pour the plaster and reinforce the second half of the mold. Depending on the box you used for the first half, you will need to adjust the box so the walls will hold the plaster in place for the second half. Keep in mind that this plaster reinforcement doesn't need to be extremely thick, just thick enough to ensure the plaster is durable and won't crack after being handled, and to ensure that the mold doesn't sag when casting. Once this second half of plaster has been allowed to dry, remove the box once again and hang on to the top half so it doesn't fall and crack. Carefully pull the top half away, and don't worry if the latex mold comes loose from the plaster base -it can be easily set back in place, and glued in place if desired. If you plan ahead, you can make sure that the two plaster base halves are as thin as possible, so you can easily bind them together before pouring your resin casting medium in.

Now that you have the two halves of your mold constructed, and you have removed the original object and clay cylinder, you will want to press the two halves together, securing them tightly with either rubber bands or packing tape. Make sure that the tube/path is clear and not obstructed, then use a funnel to pour in your resin casting medium. Information on resin casting can be easily found online. Your latex mold can be used dozens of times, and the life of your mold can be prolonged by using mold conditioning/release agent on the surface. Make sure you keep the mold in a cool, dry place. The two latex halves of the mold should also be sprayed with release agent to prevent them from sticking to each other when pressed together. Happy casting!

Tips & Warnings

  • Depending on the size of the mold and the object being cast, you can opt to use clay instead of plaster to reinforce the mold from behind. Just make sure that the bottom of the clay lies flat on the table, and that the entire back side of the mold is reinforced/secured by the clay.

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