How to Make a Simple Mold & Core (Casting Plaster)

In this instructable, you will make a simple pencil holder. You will learn how to make a draw, cut, and tape a simple mold & core and make plaster to pour into the mold, how to cut open the mold and take out the cores and much more.

For this step, all you need to do is to simply get a piece of polypropylene, a ruler, scissors, and a pencil or pen. And for later on you will need an Exacto knife and sandpaper.

Using your pencil or pen draw out the shapes that you will be cutting out later. In this step, you will be making the rectangular mold, which has a length of 12cm, a width of 6cm, and a height of 6cm. To make the mold you will have to draw out three 6cm by 12cm polypropylene pieces and two 6cm by 6cm pieces.

In this step, you will be making the square core. The square core is a cube. For the square core, you simply just have to draw out six 3cm by 3cm pieces of polypropylene on the sheet.

In this step, you will be making the rectangular triangle core which has a length of 6cm, a width of 3cm, and a height of 3cm. For this shape, you will have to draw out two 6cm by 3cm pieces and a single 3cm by 3cm piece which should then get cut in half diagonally, giving you 2 triangles. The closing piece of the triangle should be a single 6cm by 4.1cm piece of polypropylene. To make sure what the right length of the rectangular triangle is you will have to measure the length of the hypotenuse and that is the length of the next piece you will be cutting out. Whatever the length of your hypotenuse is, whether it is 4 or 4.1 or 4.2cm make sure the piece you cut out is the side which is the same length as the hypotenuse, then make sure the other side is 6cm. Now with all your pieces drawn out, you are ready to cut.

For this step, you will simply just cut out all the pieces that you have already drawn. Once you have cut out all the pieces go onto the next step where I will tell you which pieces get taped together.

In this step, you will just simply be taping the pieces together at that, is it! The rectangle mold has 5 pieces with three of them being 6cm by 12cm and two being 6cm by 6cm pieces. The square core is simply just six 3cm by 3cm pieces all taped together to make a simple square shape. Lastly, the rectangular-triangle is just the two 6cm by 3cm pieces and a single 3cm by 3cm piece that you should have cut in half to give you two triangles, and the last piece is the 6cm by 4.1cm piece. Make sure that you are tapping the pieces well together and leaving no gaps for the plaster to leak through.

In this step, you will pour the plaster into the mold. Make sure to not our to much into the mold since it might overflow but also because you will be putting in the cores which will make the plaster rise higher. Once you have poured in the plaster it's time for you to put in the cores.

Once the plaster is in the mold you will quickly have to put in the two cores before the plaster drys. The rectangular triangle core gets put into the mold with the 6cm by 4.1cm side facing upwards and you put it in the mold on the far left-hand side of the mold like in the image above. Make sure there is just a tiny gap between the left-hand side piece of the mold and the core. For the square core, you just put it halfway between the right-hand side of the mold and the other core. Hold the two cores in place for 2-3 minutes and then it will be partially solid and in place. Once it is partially hard leave it somewhere overnight for it to further dry and once it is dry after the night has passed you can move onto the next step. Now that you have both cores in the plaster go to the next step.

In this step, all you will need to do is use an Exacto Knife to cut out the two cores and separate them from the mold without damaging the mold. Once you have cut out the two cores you can remove the polypropylene pieces and throw them into the bin.

In this step, you will simply use sandpaper to sand the mold to the preference that you want it. Once you have sanded the mold you can sand the insides where the cores were to smooth out the edges.