Differences Between Silicone, Latex, and Urethane Molding Rubber

Learning the differences between silicone, latex, and urethane molding rubbers can help you determine which material is best for your next project. Regardless of the material you choose, they are all flexible and capable of producing highly detailed, durable molds. Their unique features can make one better for a certain application compared to another.

What Is Silicone Rubber?

Silicone is a synthetic rubber used to make brush-on or pour molds. It is a two-component material, and these parts are combined to cause a chemical reaction that converts the liquid into an elastomer.

Its main structure is composed of siloxane chains (silicon and oxygen). The manufacturer can adjust these chains to enhance the rubber’s properties for a desired project.

It is the most expensive rubber molding material. There are two common catalysts used to cure silicone:

Platinum-based

This type of silicone is also known as addition curing and uses a platinum catalyst. It’s a premium molding rubber that lasts longer than tin-based silicone but is also the most expensive.

This material can be formulated for various uses, including brush or pour application, skin-safe application, or food-safe projects when needed. Little shrinkage occurs when making molds, plus the material has great temperature stability and chemical resistance.

Tin-based

This type of silicone is also known as condensation curing and uses a tin catalyst. The tin catalyst makes it less expensive than platinum silicone.

It is used for general mold-making and prototyping, but it is not as long-lasting as other materials. As it ages, it will wear, lose elasticity, and become brittle.

Unlike platinum-based silicone, it is not prone to cure inhibition and can be used with most materials. It can create fine and intricate details for polyester, epoxy, gypsum, wax, concrete, and plaster casting. However, it is not suitable for skin application, even after curing.

It is prone to shrinking over time and as it cures. Its shrinkage partially comes from the release of methanol during the reaction. So, the bigger the mold, the less stable it will be. Fillers can make it more stable, but it will lose tear strength and become heavier.

Benefits of Silicone Rubber

• Its most appealing feature is that it does not require a release agent, so it demolds easily from most materials. It will also be able to withstand several casts without wearing significantly.
• Silicone can have varying properties, including Shore hardness, elasticity, working times, and curing times. This versatility makes it easier to choose one that meets your needs.
• Silicone is mostly formulated on the Shore A hardness scale. A lower hardness and ability to replicate details make them a great choice for historic restoration. There is a reduced chance of damage to the original or master.
• Regardless of where you need to make your molds, silicone works well for molding on or off-site. Various formulas offer fast cure times for easier in-field molding.
• It is not sensitive to moisture, so you can work in various conditions without worry of bubbles forming in your mold.
• It is desirable as a brush-grade material because it gets thicker better than urethane. Brushable formulas also replicate complex designs for specialty projects more easily. It is best for creating what’s known as a skin or glove mold. These molds will need wood or plaster backing.
• If you need your mold to withstand high temperatures, silicone works better in these environments.

Cons of Silicone Rubber

A downside to using silicone molds is that with repeated use, they will not last very long. Depending on the mold, they start to disintegrate or tear. They also lose their detail over time.

Because of cost, silicone molds are usually made with mold boxes or additional support, which will take added time and experience to develop.

These molds have poor abrasion resistance, so they’re not always recommended for casting concrete or materials like it. If you cast concrete, they work for low-volume production.

What Is Latex Rubber?

Latex is naturally sourced from rubber trees and was the first widely used rubber. It’s typically composed of rubber particles, water, proteins, and sugars, which all work together to provide a unique set of properties.

Compared to other molding rubbers, it is a thin material that can be brushed onto surfaces or sold as a sheet good. A brush-on mold can be desirable compared to solid molds as it saves on material costs.

Unlike other room-temperature vulcanizing rubbers, latex cures by evaporating the water and ammonia in its formula. Once cured, it leaves a highly elastic, durable mold that replicates perfect detail.

Benefits of Latex Rubber

• It’s the most affordable molding rubber and is extremely durable, so you benefit from a more cost-effective, long-lasting product.
• It has good strength, tear resistance, and flexibility, which all contribute to its longevity. It will last a long time in production if maintained and stored properly.
• In terms of ease of use, as a single-component material, it does not require measuring and mixing. Like silicone, little or no release agent is required to prepare the molding surface. It’s also water-based, so cleanup is easier and faster.
• Due to its abrasion resistance, it is good for casting abrasive materials like concrete and cement as well as wax and plaster. It is perfect for making flat molds for ornamental concrete projects like stone veneer and two-dimensional architecture. It can also be used for limited resin casting.
• Like silicone, it makes a good thin-walled glove mold but cannot be applied to the skin during or after application. By applying it with a brush, you can create molds with irregular shapes or very intricate molds with fine details.
• It is biodegradable, which makes it more environmentally friendly. This also means that, over time, when exposed to the elements, it can rot, degrade, and crumble.

Cons of Latex

Latex has a strong ammonia smell, which is undesirable during the extensive application process. When it’s not in use, it has to be stored at room temperature. If it freezes, it is unusable and must be disposed of.

While latex is strong, it has a high shrinkage rate as it cures. It can shrink as much as 10-20%, which causes it to lose fine details. It’s generally not recommended for casting urethane, polyester, or epoxy resins.

Because this material is used to make thin-walled glove molds, it requires rigid backing or a support mold. An unsupported latex mold will deform and stretch out of shape while casting.

Like silicone, certain materials can inhibit its cure like Vaseline and sulfur-based clay. Touching the surface of one latex layer before you add another can prevent it from curing. It’s also known to cause allergic reactions in some people upon skin contact.

Even though it is the cheapest molding rubber option, its multiple thin layer application method increases labor costs. Since only one layer can be applied at a time, and many coats are needed, it may take several days or weeks to make a single mold.

What Is Urethane Rubber?

Urethane rubber is another synthetic molding material for use in a variety of molding and casting applications. Like silicone, it is 2 part rubber that must be measured and mixed accurately. The chemical reaction occurs between an isocyanate and a poly. It is best for concrete casting.

Polyurethane is an affordable molding material. It is also more versatile because it can be formulated into different materials like plastics, foams, and coatings. It ranges between the A and D Shore hardness scale to accommodate various application needs.

Benefits of Urethane Rubber

• It has excellent physical properties that make it tougher than other molding materials. High properties include elongation and tensile strength for extended wear and tear, which are also customizable to meet specific performance requirements.
• Urethane is more dimensionally stable than silicone or latex, which means low shrinkage when making molds. It also maintains stability after substantial pressure is applied for consistent castings.
• It is more versatile than silicone or latex. Due to its range of property options, it’s used for a wide variety of projects. It can be formulated harder than other rubbers, which is better for tooling projects like stamps and formliners.
• Because it is highly durable, it can be reused over and over, especially in abrasive applications. Its reusability makes it the best material for high-volume production.
• Its ability to transfer pigment is better than other molds, which makes the color of the casting more vivid. It’s also more friendly to in-mold coatings and other modifications like painting. This ability is important to enhance the look of casting materials.
• Since urethane molds are thicker and solid, they typically don’t need rigid backing like silicone and latex glove molds. However, wood backing can be beneficial for large, flat molds to prevent shrinking in extreme temperatures.
• While release is required to prevent urethane from sticking to other surfaces, VFI offers a line of urethane rubbers that demold more easily. They have best-in-class release characteristics and are proven to lower breakage rates by 80%. This can be especially beneficial when demolding from delicate undercuts and details.

Cons of Urethane

The biggest downside of urethane rubber is that it requires a release agent before casting. It is highly adhesive and will bond to any surface. When making the mold, porous surfaces, including wood and natural stone, also have to be sealed properly.

Another downside of urethane is that it is moisture-sensitive in its liquid form. This can make it hard to work in warm and humid environments. If moisture finds its way into the mixture, it causes bubbles to form, which may affect your final mold surface.

Which Molding Rubber Should I Use?

What material you choose depends on several factors, including:

1. What is your budget?
2. What is your model or master made of?
3. What properties do you require? (i.e., high tear strength, high hardness)
4. What level of detail do you require? (i.e., extreme vs simple detail)
5. What material are you casting into the mold?
6. How many castings do you need to produce, or how long do you need your mold to last?

Molding Material Comparison Chart

	Urethane	Silicone	Latex
Application	Pourable	Pourable and brushable	Brushable (needs 12-20 coats)
Cost	Cost-effective for high-volume production	Highest upfront cost, depends on catalyst	Most affordable
Composition	Two-component	Two-component	Single component
Cure time	16-24 hours	4-24 hours	1 hour per coat; 24-72 hours after final coat
Ease of use	Mixing can be difficult, and release is required	Can be difficult to mix, but release is not required	Time-consuming to apply, but release is not typically required
Wear resistance	Very durable and reusable for abrasive casting	Durable but wears over time when used with abrasive materials	Most durable but can degrade over time
Casting materials	Concrete, plaster, wax, and other abrasive castings	Concrete, plaster, wax, urethane and polyester resin castings	Concrete, cement, wax, and plaster castings
Storage Stability	Does Not Freeze	Does Not Freeze	Will Freeze