4 Tips for Making a Prototype/Model for a Slip Casting Mold

mold making slip casting Apr 28, 2020


Before we even talk about making slip casting molds, we need to discuss something important, your prototype. This is what you will make your mold from (some people call it a model)

Why is your prototype so important? It dictates what your cast product will be. If you have marks, rough spots, or unintentional lines on your prototype, it will show up in your cast. If you don’t make your prototype right, it won’t release from the mold.

In my several years of mold making, I have learned along the way some helpful tips to making successful prototypes. So, I’ve decided to share these with you today!

#1. Use a smooth, white clay

Using a white porcelain or very smooth white stoneware clay is better than using a grog red or brown clay. The porcelain clay is easier to get perfectly smooth. If you use a grog clay, those little tiny beads of grit are hard to smooth out and can show in the plaster mold, and in your cast piece. 

Using a white clay will make cleaning up the inside of the mold easier than using a coloured clay. Having a red or brown residue inside your mold can transfer onto your first cast. 

#2. Take your time 

Take your time to make it perfect. Don’t rush it. If you have any little lines from trimming, finger marks or unwanted imperfections, they will all show up in your mold and transfer to your cast. 

This is why it’s important to pay careful attention to detail in this process and take your time to perfect. Use a rib to smooth out all imperfections. 

#3. Use it when leather hard

The timing of using your prototype to make your mold is also important. You don’t want to use it when it is still wet or else you will deform and finger mark your piece. You also don’t want to wait till it is too dry and it could break under the pressure of the plaster or be difficult to get out of the mold. Use your prototype when it is leather hard (like cheddar cheese).

#4. Remember undercuts

This is a crucial one. If you don’t follow this your cast may not ever come out of the mold. You want to be sure that the rim of your prototype is wider than any other part of the form to ensure it slides out of your one piece mold. If you have undercuts, ensure you account for them when planning out what type of mold you will make. 

Remember, undercuts may be invisible. Use a straight edge (like a ruler) and stand it up against your piece to see if there are spots that bulge out that need to be trimmed down to create a perfect form.

I hope that these four tips will help you create better prototypes to make your slip casting molds. Since the prototype is an important part of the mold, I want you to create successful ones to make high quality slip casting molds!


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