What Do You Mix With Silicone To Make Molds?

Silicone molding is a process used to create reusable molds out of silicone rubber. It involves pouring a liquid silicone material into a mold cavity or impression, letting it cure, and then removing the solid silicone mold. The silicone mold can then be used to cast multiple copies of an original model or design out of various materials, like plastic, concrete, wax, chocolate, or soap.

Silicone is ideal for mold making because it is flexible, durable, and able to capture fine details. The material is also heat resistant and non-stick, allowing easy release of cast parts. The basic materials needed for silicone molding include liquid silicone rubber, a mold release agent, silicone thinner if desired, and colorants or pigments for tinting the silicone if wanted.

The process involves preparing the original model, building a containment fixture, measuring and mixing the silicone, degassing the material, pouring it over the model, letting it cure, and then carefully removing the finished mold. Proper preparation and following silicone best practices will ensure the mold captures all the desired details. This guide will walk through the entire silicone mold making process step-by-step.

Why Silicone Molds

Silicone is a popular material for creating molds due to its many advantages compared to other mold materials like rubber, plastic, or metal. Some key pros of silicone molds include:

Flexibility – Silicone is extremely flexible and elastic, allowing you to demold intricate shapes cleanly. It also allows for easy release of baked goods or chocolates.

Heat resistance – Silicone has a very high heat tolerance, often over 500°F. It won’t melt or warp from exposure to high temperatures. This makes silicone ideal for baking applications.

Durability – Silicone molds are very durable and tear resistant. They can last for thousands of repeated castings if cared for properly.

Non-stick surface – Food releases easily from silicone without the need for oils or cooking sprays. Many silicone molds have a glossy surface that makes demolding even easier.

Some disadvantages of silicone molds include susceptibility to staining from strongly colored foods and a higher upfront cost than materials like plastic or rubber.

Overall, the unique properties of silicone like heat resistance, flexibility, and non-stick behavior make it an exceptional material choice for detailed and reusable molds.

Types of Silicone

The two main types of silicone used for making molds are tin cure and platinum cure silicones. The key difference between them is the catalyst used to cure the silicone from a liquid to a solid.

Tin cure silicones use a tin catalyst and cure through a condensation reaction with moisture in the air. The curing process produces a tin salt byproduct. Tin cure silicones generally have a shorter pot life and demold time compared to platinum cure, but may suffer from inhibition issues if the cured mold comes into contact with certain materials like sulfur-cured rubber.

Platinum cure silicones use a platinum catalyst and cure through an addition reaction that does not produce any byproducts. They have a longer pot life and demold time than tin cure silicones. Platinum silicones are also more chemically resistant than tin cure, with no risk of inhibition.

Sources:

https://www.smooth-on.com/support/faq/184/

https://www.specialtyresin.com/2017/04/06/tin-platinum-cured-rtv-silicone-rubber/

Choosing a Silicone

When choosing a silicone for mold making, there are several key factors to consider:

Shore Hardness – This refers to the firmness of the cured silicone, rated on the Shore A scale. A shore hardness of 10-20A is good for molding detailed objects, while 30-40A works better for molding multiple castings (SiliconeTop). The higher the shore number, the firmer the silicone.

Cure Time – How much time you need for the silicone to fully cure depends on your project. Faster curing silicones (30 minutes – 2 hours) allow you to work more quickly but give less time to mix and pour. Slower cure times (4-16 hours) provide more working time (MouldLife).

Cost – Platinum-cure silicones usually cost more but have better tear strength and stability. Tin-cure silicones are more affordable but can have issues with inhibition and shrinkage (MouldLife). Consider your budget and project needs.

Overall, examine the details of each product to find the best balance of hardness, cure time, and cost for your particular mold making application.

Silicone Thinners

Silicone thinners are commonly used when making silicone molds to lower the viscosity and improve flow of the silicone rubber. Thinners allow the silicone to spread more easily into intricate details and reach all areas of the mold frame. This helps reduce air bubbles for a smooth finish.

Thinners are especially useful when working with high durometer silicones or adding fillers, which can thicken the silicone mixture and inhibit flow. Adding silicone thinner ensures the silicone will fully fill the mold frame. Most manufacturers recommend using no more than 10% thinner by weight.

Common silicone thinners for mold making include TAP Silicone Thinner and Castin’ Craft Silicone Thinner. These thinners are designed to be compatible with platinum-cure silicones. Using other thinners or solvents can inhibit curing.

Only use thinners when necessary, as they lower durometer and tensile strength. Test on a small batch first to determine the ideal ratio of thinner to silicone.

Colorants

One of the best features of silicone is that it can be easily colored to achieve the exact hue you desire for your mold. By adding pigments and dyes, the silicone takes on vibrant, opaque colors. There are specially formulated colorants made for adding color to silicone rubbers.

Silicone pigments are concentrated color particles that disperse throughout the silicone base. They come in powder or paste form and result in intense, opaque color. Some common silicone pigment colors are white, black, red, yellow, blue and flesh tones. Silicone dyes are liquid colors that produce transparent and translucent hues. The combination of pigments and dyes allows for an endless spectrum of possibilities.

When adding colorants, it’s recommended to thoroughly mix in .1 to .5% of the total weight. Adding too much can inhibit curing. Measure carefully and add colorants slowly until the desired shade is achieved. Always test a small amount before fully coloring your silicone base. The color will appear slightly different in the final cured rubber. Knowing the correct ratios for the colorants and getting the color right takes some trial and error.

With the right colorants, you can produce medical prosthetics, colorful toys, realistic food items and anything else you can imagine. The creative possibilities are endless with colored silicone molds.

For more information, see:
https://www.alumilite.com/dyes-colors/silicone-color-pigments-25-lb/

Mixing the Silicone

Properly measuring and mixing the silicone is crucial to creating a high quality mold. Silicone rubbers consist of two parts, Part A and Part B, that must be mixed together in the right ratio. This ratio is typically 1:1 or 10:1, so be sure to carefully follow the manufacturer’s instructions. Measure both parts by volume not weight for the most accurate ratio.

Use a clean, dry container made of plastic or metal. Avoid using containers that could interact with the silicone chemistry. Thoroughly mix Part A first to disperse any fillers, then add Part B. Mix slowly to minimize air entrapment. Use a stiff, flat mixing stick and be sure to scrape the sides and bottom of the container several times. Continue mixing for 2-3 minutes until the silicone is a uniform color with no streaks.

Avoid vigorous mixing as this can whip air bubbles into the silicone. Let the mixture sit for a few minutes so any bubbles can rise to the surface before pouring. Always mix and pour new batches of silicone as the working time is limited. Make sure everything is ready before starting.

Pouring the Mold

When pouring the silicone into the mold frame, it’s important to do so gently in a slow, thin stream. Aim for the lowest point of the containment box to minimize splashing and air bubbles. Pour steadily and deliberately, and avoid pouring from too high above the mold. The stream should be about 1/8 inch thick. As the silicone fills up the mold, you can pour a bit faster but should still take care.

Bubbles can be an issue when pouring silicone molds, so try to pour in a way that minimizes this. Tapping the sides of the mold frame gently as you pour can help release air bubbles to the surface. Some people also recommend using a toothpick, skewer, or straw to poke bubbles after pouring. Let the mold sit for 5-10 minutes before doing this to allow it to level out.

Be sure to pour an even amount across the mold, so you end up with a consistent thickness and the silicone fully covers the object. Pour until you’ve added about 1/4 inch of silicone above the highest point of the object. Going too thin risks tearing the mold when demolding, while too thick uses excess silicone. The ideal thickness is typically 1/4 to 1/2 inch.

Sources:

https://formlabs.com/blog/how-to-make-silicone-molds/

https://entropyresins.com/blog/2023/06/11/how-to-make-your-own-silicone-mold/

Curing the Mold

Once you have poured your silicone and the mold has set, you will need to allow adequate time for curing before the mold can be safely removed and used. Curing refers to the chemical process by which the silicone hardens and becomes solid.

Most platinum-cure silicones take 16-24 hours to fully cure at room temperature. You’ll want to avoid handling or demolding until the silicone has had time to fully harden. Demolding too early can result in tears or deformities in your finished mold.[1]

As a general rule, silicone molds should cure for at least 16-24 hours before attempting to demold. The exact curing time depends on factors like silicone type, room temperature, humidity, and thickness of the mold walls. Check the product instructions for your specific silicone to determine the recommended cure time.

One way to test if your silicone mold is ready for demolding is the fingerprint test. Press your finger gently into the surface of the mold. If your fingerprint remains, it needs more time to cure. If the indentation pops back up, the silicone has likely hardened enough to be safely demolded.[1]

Be patient during the curing process. Rushing the cure time risks permanent damage to your finished mold. Allowing adequate curing produces the most durable, long-lasting molds for repeated casting and demolding.

Using the Finished Mold

Silicone molds should be handled with care to maximize their lifespan. According to a YouTube video tutorial How To Use Silicone Molds with Epoxy, silicone can tear if pulled or stressed too much. Avoid using sharp objects or abrasive materials on the mold’s surface. When storing the silicone mold, keep it away from extreme temperatures and direct sunlight.

Properly releasing castings from the mold is also key for longevity. Using mold releases like vaseline, olive oil, or soap can help prevent tearing and damage when removing castings. The YouTube channel My Virgin Kitchen recommends lightly greasing the silicone mold with vegetable oil before each use. Letting castings fully cure before removal is also advised.

With care and proper storage, silicone molds can yield dozens of detailed castings over many uses before needing replacement.