When to Use Silicone vs Urethane Rubber for Decorative Concrete

Knowing which liquid rubber is right for your decorative concrete project can be tricky since there are many factors to consider. Silicone and urethane are the best rubbers for these applications because of their durability, minimal shrinkage, and easy processing. Plus, they reproduce extreme details and textures, which many other molding materials are not capable of doing.

What Is Liquid Rubber?

Liquid rubbers are synthetic materials that typically come in two parts that will cure at room temperature. After curing, the rubbers will have a specific set of properties based on their formula.

• Polyurethane – There are two types of urethanes for mold making: TDI and MDI. TDI rubbers are the industry go-to, but MDI rubbers can also be useful for certain projects. Urethane comes in many forms (plastic, coatings, rubbers, foams) and has high adhesion to most surfaces, which is why it will require a release agent in all molding applications. VFI has developed a line of urethane rubbers with best-in-class release properties that can be used in place of silicone. You will still need release, but it will release much cleaner, providing better parts and extended mold life.
• Silicone – There are two types of silicones for mold making: platinum-based and tin-based. Platinum-based is preferred due to its dimensional stability, durability, and release properties. The reason you should not use tin-based rubbers is that they will shrink over time and lose tear strength. Since concrete castings need to be consistent in shape and size, this material won’t work well in the long term. The main use of tin-based silicone molds is making thin texture mats where you do not need dimensional stability.

What Is Decorative Concrete?

Non-structural concrete elements that have intricate designs and patterns are considered decorative.

Rubber molds are used to create castings such as pavers, ornaments, statues, fireplace accents, modular outdoor kitchens, and other architectural features. While metal, wood, and plastic molds can come in handy for some precast projects, they are generally not great for decorative concrete.

Factors to Consider When Choosing a Liquid Rubber

What Properties Do You Require?

1. Hardness

Shore hardness is a determining factor in whether a concrete caster will use silicone or urethane rubber. Most rubbers are measured on the Shore A scale for hardness. Urethane rubbers range from 20-90 A on the Shore A scale, while most silicone rubbers are between 10-60 A. A general rule of thumb is the softer the rubber, the easier it will release from complex shapes. However, a softer rubber will not be as durable. Hardness will also contribute to the level of detail you can get.

The following rubbers are recommended for projects that require the following hardnesses:

• 10-30 A – Silicone rubber is usually the best option if a softer, flexible mold is crucial. Its flexibility generates easy release from intricate details and undercuts without tearing the mold or breaking the casting. You can also use it on delicate architectural restoration projects without damaging the original piece. There are brushable and pourable options.
• 30-60 A – TDI urethane rubber is recommended for these projects because it works better than silicone at these hardnesses and is more cost-effective. TDI is recommended over MDI rubbers because it is less sensitive to moisture, making it easier to use in various environments. These hardnesses are a good middle ground for cast stone, manufactured stone, and advanced detail formliner projects.
• 70-90 A – A combination of TDI and MDI rubbers can be used when an extremely rigid, durable mold is required. MDI can be great for simple formliners with rigid backing because of its increased adhesion and tear resistance. These materials also work well for concrete stamps and rollers.

Note: If you’re using any material above 90 A, it will always be an MDI urethane. This is also where urethane will get rigid and feel more like plastic. Urethane at harder durometers can be great for making master models to protect fragile originals.

2. Tear strength

Most silicone rubbers are lower in hardness, so tear strength is crucial for durability. They generally will have higher tear strength than urethane at the same durometers. This allows them to remain intact when demolding from intricate details.

Urethane rubbers tend to have better tear strength at higher hardnesses, making them extremely durable for large projects. If you’re making massive forms, they are better at holding up to the weight of the concrete.

3. Tensile strength

This property is also similar for both urethane and silicone. Silicone will have better tensile strength at lower durometers, while urethane will have better at higher durometers. This is another property that’s important when you’re working on more detailed, fragile work. On the other hand, it is also useful when making large molds that need to hold up in abrasive situations.

4. Elongation

The opposite effect happens with elongation. At lower durometers, silicone rubbers will have poor elongation, while urethane’s elongation will be better. Elongation is lower at higher durometers because the molds have lower flexibility without the ability to stretch. Demolding urethanes with higher elongation is easier because they will also have increased flexibility.

Conclusion: silicone has better properties at lower durometers, while urethane has better properties at higher durometers. Which you choose will still be very dependent on your unique project.

How Long Do You Need the Mold to Last?

The durability of the mold, how frequently you use it, and how well you maintain it will all factor into how long it will last. A reusable mold is desirable, but it is not always necessary. Silicone molds are great for low-volume and unique concrete work, but their poor abrasion resistance makes them less suitable for high-volume concrete projects.

If you need a highly reusable mold for high-volume production, urethane should be your go-to. It is the best choice for most concrete casting projects because it offers superior abrasion resistance. This allows you to get up to 100 casts from a single mold.

What Is the Budget of Your Project?

At surface level, silicone tends to be more costly than urethane rubber. You must account for how much material you need and how much waste you might generate in making the mold. Waste will also coincide with the reusability of the mold. It may not be worth it to spend more on silicone if you need to cast a larger quantity of concrete.

A way to save money is by using rigid or semi-rigid foam to back your urethane mold. This will also make your mold more lightweight. Lightweight molds for concrete casting projects can be beneficial if you move them around often because thick rubber can get heavy.

What Are Your Project Deadlines?

How long it takes to make the mold can have an influence on the molding material you choose.

Certain materials can be brushed on a model or poured into a mold box or form. Pouring the material is easier and saves you time, labor, and materials. Brush-on molds must be layered on and require a supportive mold shell, which can take longer to make. You will need to have the skills to make a skin mold as well.

For processing times, most urethane molding rubbers have a standard demold time of 16 hours. Some MDI formulas are as short as 4 hours for faster turnaround on part production. Silicone rubbers tend to have demold times between 16-24 hours.

Alternatives for Decorative Concrete

Urethane Form Coating – Another way polyurethane can be used is by spraying it onto an EPS form before casting concrete into it. The coating creates a releasable surface that works well for decorative formwork.

The form hard coat isn’t as permanent as rubber, but it’s a great option for 1-5 uses. This spray-applied method also allows for a faster turnaround on custom pieces, especially since post-work is not required.

Urethane Hard Coat – Alternatively, VFI offers high-pressure and Qwik Spray hardcoats for architectural shapes and forms. You can shape the foam and encapsulate it with a durable urethane hard coat before painting it to look like natural materials.

Epoxy Form Coat – Additionally, if you need to roll a form coat onto metal, wood, or EPS forms, we recommend using our VFI-4385 82 D Form Coating Epoxy.

Contact VFI if you have questions about whether urethane or silicone rubber is a better option.