How to Make a One-Piece Plaster Mold

Making a Plaster Mold

Making a plaster mold is one of the easiest and cheapest ways to create a mold for reproducing your own latex props and masks. By just following a few simple steps you can create an inexpensive plaster mold of almost anything!

In this tutorial we’re going to be making a one-piece mold of a flat-backed prop. This technique works great for props that will be sitting the ground or hanging on a wall. It also works well for half-masks. If you are making a prop that needs to be seen in 360º detail, or a full over-the-head latex mask, you will probably need to make a two-piece mold, which we’ll cover in a different tutorial.

The one major factor to consider when making this type of mold is undercuts. An undercut is a place where the sculpture overhangs or bulges out. Since the mold is rigid it might be hard to get your final piece out of the mold once it has cured if there are deep undercuts, so be careful when creating your sculpture!

Let’s take a look at what we’re molding for this tutorial! This is a sculpture for our Skinned Lab Chimp prop sculpted by the super-talented Christian Hanson.

Making a Plaster Mold - Prop Making

As you can see we placed the sculpture on a piece of plywood for this project. This is a good idea as making a mold directly on a table can be messy and can damage your table. You can also see there are some decent undercuts in this sculpture, but nothing that will be a problem when demolding. You will learn as you go what works for you and what creates problems down the line. The best way to learn is just to try it out!

This sculpture was made using our Creature Clay, which is a water-based clay that is very easy to work with and allows for the quick creation of excellent detail.

Making a Plaster Mold - Prop Making

Using a brush, we apply some Petroleum Jelly to the board around the edges of the sculpture. This will help release the mold from the project board once we’re finished. If you don’t do this you are going to have a heck of time getting that mold off, trust us! You don’t want to put the Petroleum Jelly on the actual sculpture, as it will harm the detail of your sculpt.

Supplies for making plaster molds

When using water-based clay the plaster has a tendency to bead-up or run off the sculpture. We’ve found that using a coating of Krylon Dulling Spray helps the plaster adhere to the sculpture.

Halloween prop mold making

We spray the sculpture with a coating of dulling spray to get it ready to mold.

For this mold we’ll be using Ultracal 30 Gypsum Cement. This is a type of plaster that dries extremely hard with minimal shrinkage, making it perfect for molds. Once the mold is finished and cleaned it will also wick moisture from the casting latex, which helps it dry.

Plaster for mold making

When mixing your plaster you should start with water in a plastic container. You will get a feel for how much water to plaster you need to use as you progress. You should start with less water that you think you need as the plaster is very absorbent. You might end up using a lot more plaster than you think.

Plaster for Mold making

Add plaster to the water until the mixture takes on the appearance of a dry creek bed. Once it takes on this look, it’s time to mix!

plaster stirring stick

You can mix by hand using a stir stick, or, if you are mixing large batches or just don’t like stirring, you can use a mechanical mixer and a drill. There are a wide variety of these paddle mixers available. We like this type (with a rubber disc), but any type will work.

mixing plaster

Mix the plaster thoroughly until it is smooth and creamy with no lumps. As with in a lot of things in life, lumps are bad.

coating with plaster - prop making

Once the plaster is mixed, use a disposable brush to paint the plaster onto the mold. The first coat is your detail coat, so don’t worry about how thick it is, just make sure you are getting a nice smooth coat and you are completely covering the sculpture. You don’t want to miss areas or create bubbles (those will show up as imperfections in the final mold). You may need to use a blotting motion to get the blaster into deeper crevices. Again, this is the coat that will pick up all your detail, so take your time and be careful. You must be wise and patient to be a master of the mold.

mold making

Continue to coat the sculpture with plaster until it is completely covered. Try to imagine yourself as an evil supervillain as your sculpture screams in agony.

Dry mold for prop

When you’re done with the detail coat, let it cure until it is dry to the touch (you don’t need to wait for it to dry completely, but you don’t want it to still be wet for your next coat). When plaster is dry to the touch, do your next coat of plaster. This next coat can be much thicker than the first. Since the first coat captured the detail, now you need to build up durability. Make sure you are still getting into all the crevices so you don’t create any air bubbles.

burlap for prop making

After one or two more coats, you will want to create a reinforced layer of plaster. To do this, cut some squares of burlap cloth. We used sections that were about 6″ x 6″ square, but if they are slightly rectangular or even shaped like a rhombus, this will also work.

plaster mold

Dip the burlap cloth into the plaster, making sure it is well-coated.

plaster mold making

Layer the plaster-infused burlap onto the sculpture and smooth it out. You are now treating your prized Halloween prop like it is at an expensive resort, taking a mud-bath.

coating your prop mold with plaster

Continue covering the mold in with the burlap strips until the whole sculpture is covered. Pay special attention to thinner areas of the mold (like the arms on this chimp) and take special care to make sure they are very well reinforced.

prop coating

Once the burlap layer is complete, continue adding layers, allowing the plaster to dry to the touch between layers and until the desired thickness is reached. This mold was between a 1/4″ to 1/2″ thick. The thicker the mold, the more durable it will be, but it it will also weigh more. As a general rule, the bigger your mold, the thicker you should make it.

After you are through with the final coat of plaster you will want to let it dry completely. We usually leave the mold on the sculpture overnight to dry, just to make sure it is completely cured. When the mold is fully cured you can remove the mold from the board – you might need to pry it up with a large flat screwdriver or chisel. After you flip the mold over you are ready to clean out the clay. To do this, we really recommend you only use wooden tools, so as not to damage the mold. You might find using a loop tool to remove larger chunks of clay helpful. Once most of the clay is removed, you can take the mold outside and spray it with a hose at high pressure to remove the rest of the clay.

completed prop mold

Here is the mold – fully cleaned, washed and dried. Once the mold is completely dry it is ready to cast! Using casting latex, fill the mold as much as you can. For small molds, it’s best to try and fill it completely with latex – for larger molds (like this one), it might be better to fill part of the way and brush the latex onto the walls of the mold. You will want to let the latex dwell for at least 30 minutes. You also might want to agitate the mold to make sure there are no bubbles in the latex.

Also make sure the latex gets all the way down into the deepest parts of the mold. Once the latex has dwelled you can either pour the latex back out into a bucket (for small sculptures) or scoop out the remaining latex with a small cup (for large sculptures). You will then want to put your mold in a positions that lets it drip (so that the remaining latex runs out of the mold rather than pooling). For smaller sculptures, propping the mold up in a bucket works well for this.

The latex will probably need to dry overnight for the first few castings (as the plaster continues to dry out it will wick moisture faster and thus dry latex more quickly). Once the latex is dry, you can pour in your expanding foam. We recommend using Flexible Urethane Foam (specifically FlexFoam-iT! V). We have found this foam to be the perfect mix of flexible and durable and it also adheres well to latex. You will need to experiment with the amount of foam to see what works for you. You may also want to cut a piece of plywood to the size of your mold, drill a hole in it, coat it with vaseline and hold it onto your mold after you pour the foam in. This will help the foam cure level with your mold, but you can also let the foam expand and trim it down after it has fully cured. Make sure to follow the directions on your foam carefully and mix it thoroughly!

unpainted latex prop

Once the foam is totally cured you can pull the completed cast from your mold. Work carefully and take your time to make sure you don’t damage the mold. The last thing you want to do is rush. Depending on your undercuts, the cast should remove fairly easily, as latex does not adhere to plaster.

And voila! You’re done! You’re now a genius and the greatest haunter who has ever lived. Except now it just looks sort of sickly and weird because it isn’t painted.

Now see our tutorial on Painting Latex Props for our tips on how to finish your prop or mask!

 

 

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2 Responses to How to Make a One-Piece Plaster Mold

  1. Scott says:

    This was so helpful! I’m very new to prop making and am curious on doing body parts and even heads. Do you have how to’s on 360 props?
    Great stuff! Thanks!

    • admin says:

      Hey Scott, thanks and glad you liked it! We don’t have a how-to on a 360 prop yet. Until we do, check out YouTube and search for “Making a Two Piece Plaster Mold” to find some good tutorials.




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