Tiffany Haney
Creating a plaster mold from a silicone pan is a popular project for beginners. Plaster is an ideal material for molding as it is very malleable when wet and can withstand high temperatures when dry. The process involves measuring and mixing plaster with water, wetting the silicone mold, and pouring the plaster mixture on top. The silicone mold can be used multiple times and produces detailed casts. However, it is important to note that plaster molds may only be used for a single cast, and the flexibility of silicone molds can prevent the cast from getting stuck.
| Characteristics | Values |
|---|---|
| Cost | Using plaster is a cheaper option than resin. |
| Ease of use | Plaster is very malleable in its unhardened shape. |
| Durability | Plaster can withstand very high heat when dry. |
| Reusability | Silicone moulds can be used multiple times before wearing out. |
| Flexibility | Silicone is flexible enough to prevent casts from getting stuck. |
| Thickness | Thicker silicone is easier to work with than thinner silicone. |
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What You'll Learn
Plaster of Paris and wood glue casting
Casting with Plaster of Paris and wood glue is a cheaper alternative to resin kits. The best mix ratio of plaster of Paris and wood glue is 2 parts water (and plaster) to 1 part wood glue. The paint is included to ensure the glue is mixed well into the plaster and water. Adding too much paint can cause issues, so only use a drop.
When creating a mould, you should measure out the cold/room-temperature water (1 part) and then slowly sift in the plaster (2 parts). Let it sit for a minute and then stir, as heat makes it harden faster and can cause bubbles. You want a nice fluid mix. The mould should be thick and as wide as the box you are using to cast. Wet the mould and then shake off the excess water.
The casting is done when the plaster mould absorbs the water from the casting slip. The mould can be used multiple times before it wears out.
The plaster and wood glue mixture creates a stronger cast than just plaster, which can be very delicate. The mixture takes longer than normal plaster to set, so it is recommended to leave it for 12 hours in the mould.
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Preparing the silicone mould
Preparing a silicone mould is a simple process, but it requires careful planning and preparation. Here is a step-by-step guide to preparing a silicone mould:
Firstly, select a suitable material for your mould box, such as coated MDF, acrylic, or even Lego. Ensure the material is non-porous and will not react with the silicone. Seal the edges of the box with hot glue to make it watertight, and ensure it can be easily broken apart once the mould has cured.
Next, determine the part line, which is the seam where the mould will be split in two. Visualise how you will remove the object from the mould to ensure it will work. Spray the inside of the box and your object with a release agent to aid in releasing the part from the silicone later.
Now, you can prepare the silicone. Mix the silicone rubber according to the package instructions, taking care to avoid creating air bubbles. If necessary, use a vibrating device to remove any bubbles.
Slowly pour the mixed silicone into the mould box, starting at the lowest point and gradually working your way up. Ensure you pour consistently to avoid trapping air bubbles. If you need to, you can mix more silicone and pour it on top; it will cure as one.
Once the silicone has cured, break apart the mould box and remove your object. Clean any residual clay or particles, then build the mould box around the silicone again, ensuring the edges are watertight. Add a pour spout if necessary.
Finally, your silicone mould is ready for casting. You can use this mould multiple times and create multiple identical casts from it. Silicone moulds are durable, pliable, and lightweight, making them easy to work with and reuse.
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Mixing the plaster
When mixing plaster for a silicone mould, there are a few key considerations and steps to follow. Firstly, it is important to use the right type of plaster and mix it properly to ensure a smooth and lump-free consistency. The plaster should be poured slowly and carefully to avoid air bubbles, which can affect the final product.
To start, measure out cold or room-temperature water, using one part water to two parts plaster. Slowly sift in the plaster, stirring constantly, until you achieve a watery paste consistency. It is important to mix thoroughly for about 2-3 minutes to ensure all lumps are dissolved, as these lumps can weaken the final product and cause cracking. Let the mixture sit for a minute to allow any air bubbles to rise to the surface, and then give it a final stir to eliminate any remaining bubbles.
It is recommended to mix the plaster in small batches, as it sets quickly and you don't want any waste. Avoid adding too much water, as this can weaken the plaster. Before pouring the plaster into your silicone mould, wet the mould and shake off any excess water. This will help ensure your plaster cast releases from the mould easily.
If you are using a two-part silicone mould, you can fill one half with water and then pour that water into your mixing container to ensure you have the exact volume needed for your plaster mixture. Slowly pour your plaster mixture onto a slide, like a spatula, and then into your mould, being careful not to overflow. Gently shake the mould to help any remaining bubbles rise to the top.
For more complex projects, you may need to create a silicone mother mould, which is essentially a mould that produces other moulds. This can be done by 3D printing a mould design, casting it in silicone, and then using that mould to create your plaster moulds. The silicone will pick up the details from the print while still being flexible enough to release from the plaster easily.
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Casting the object
Casting an object in a silicone mould is a simple process, but it can be time-consuming and messy. It is a great way to reproduce an object with precision. Here is a step-by-step guide to casting an object using a silicone mould:
Firstly, you will need to measure out the width and height of the object you wish to cast. For the width, add at least one inch to your measurement. The height just needs to be taller than the object, with no specific amount required. Cut out four sides and a bottom piece to create a box that fits your object. You can use cardboard or foamboard for this, with the latter being a better option if possible. Grease the surface of the cardboard with Vaseline to prevent it from absorbing water.
Next, hot glue your object to the bottom piece of your box, then attach the side panels with glue. Fill all gaps with hot glue to contain the silicone. Now, prepare your silicone mixture. A 1:1 ratio is easy to work with and affordable. Pour equal parts of the mixture into two cups, then combine them in a larger container. Mix thoroughly, scraping the sides and bottom of the container to ensure an even consistency.
If you have access to a vacuum chamber, you can use it to remove air bubbles from the silicone. Otherwise, try pouring the mixture into your mould in a thin stream to release some of the bubbles. You can also use a vibrating tool, such as a sander, to force the bubbles to rise. Once the silicone has fully cured, remove your temporary box. Use a sharp blade to cut a path from the opening, being careful not to tear the silicone. Cut away any undercuts, taking care only to remove what is necessary to release your object.
To prepare your mould for casting, spray the inside with a mould release agent. Wrap your mould with blue tape, using a piece of wood on each side to distribute the minimal pressure of the tape. Do not squeeze the mould, as it may deform. Now, you are ready to cast your object.
For this example, we will use plaster, specifically Plaster of Paris, as it is a cheap and effective option for beginners. Measure out room-temperature water (1 part) and slowly sift in plaster (2 parts). Let the mixture sit for a minute, then stir it gently, as heat will cause it to harden faster and create bubbles. You are aiming for a fluid mix. Wet your silicone mould and shake off the excess water.
Now, simply pour your plaster mixture into the mould and let it sit. Once the plaster has fully cured, remove your cast just as you did with the original object. And there you have it—a perfectly reproduced object!
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Removing the object
When creating a plaster cast using a silicone mould, it is important to consider how you will remove the object from the mould once the plaster has set. Here are some detailed instructions on how to do this:
Before you begin the casting process, prepare your mould by wetting the silicone and shaking off any excess water. This will help prevent the plaster from sticking to the mould. It is also recommended to use a mould release agent, such as Vaseline, liquid hand soap, cooking spray, or vegetable oil, to create a barrier between the plaster and the mould, making it easier to separate them later.
For small or detailed moulds, a thin layer of primer can be used. Mix the primer well and pour it into the mould, swirling it around for a few seconds before dumping it back out. This can be done multiple times, and any excess primer can be guided out of the details using a small artist's paintbrush.
If you are working with a large mould, an aerosol product like MR-97 can be used to create a barrier between the plaster and the mould. This method can be expensive, but it is effective for large, open-face castings.
When using plaster, it is important to note that it can be challenging to remove, especially if it has embedded itself into the surface of the mould or has settled into crevices. In these cases, you may need to use hand tools or pads to carefully remove the excess plaster. Sandblasting is another option, but it may not be suitable if you want a hand-finished look for your piece.
For a hand-finished look, refining the plaster cast by hand is a common practice. This process can take many weeks, but it allows for a more intricate and unique finish.
Additionally, when creating plaster casts, it is important to consider the thickness of your silicone mould. Thinner silicone moulds are more flexible and can be harder to work with, while thicker moulds provide more stability and can be used to create more detailed casts.
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Frequently asked questions
Yes, you can make a plaster mold from a silicone pan. Silicone is often used as a mother mold (a mold that produces other molds) and can be used to create plaster molds.
To make a plaster mold from a silicone pan, you will need to first measure out cold or room-temperature water (1 part) and then slowly sift in the plaster (2 parts). Let the mixture sit for a minute and then stir it slightly as heat will make it harden faster and cause bubbles. Next, wet the silicone mold and shake off the excess water. Finally, pour the plaster mixture into the silicone mold and let it dry.
When making a plaster mold from a silicone pan, it is important to use a thicker silicone mold as it is easier to work with. Additionally, make sure to use a good quality plaster that will not degrade over time. You can also use clay or play dough to mask off areas and separate the form into sections to prevent overhangs.
