Understanding the Properties of Liquid Urethane Rubber

It is always essential to look at the properties of liquid urethane rubber when choosing a material for any project. If you’re new to urethane, you may not know what properties are most important to look at. The answer will vary based on what you plan to do with the material. This is a comprehensive guide to help you understand which ones are important to know and why.

What Physical Properties Are Important?

The physical properties of urethane molding rubber will tell you a lot about what the material is capable of. You’ll want to understand these properties to decide if the material you’re looking at is best for your project. These properties are tested using various methods from the American Society for Testing and Materials (ASTM). The most prominent physical properties listed for molding rubbers include the following:

Shore Hardness

Test method: ASTM D2240

Definition: Shore hardness uses a standard testing tool called a durometer to analyze a material’s resistance to localized deformation or indentation. The durometer determines the hardness of a material relative to materials with similar qualities on unitless scales.

Importance: This is one of the first properties users look at when determining if a material is suitable for their application. Hardness is a good indicator of properties, and generally speaking, the harder the material, the greater the properties.

Hardness factors into how easily a mold will demold from a model or casting. A softer material is much better when creating molds of delicate or detailed originals. Rubbers within the 20-40 A range are great for cast stone, while the 30-60 A range is great for manufactured stone. These types of applications use small and delicate original pieces that have the potential to break during the molding process, so softer molds will help demold more easily. Lower hardness is also recommended for architectural restoration projects as well.

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On the other hand, harder rubbers are better when you’re making large, flat, and simple molds. Their hardness makes them more abrasion resistant and capable of handling heavier loads, which helps lengthen the mold’s life. Rubbers within the 50-70 A range are great for detailed concrete formliners, while the 70-90 A range is great for simple formliners and concrete stamps. They also provide more strength so they can be used on casting beds.

Note: When looking at VFI urethane molding rubbers, you will notice that they all include the Shore hardness in their names to make it easier to find what you’re looking for (i.e., VFI-2143 45 A TDI Molding Rubber).

Tensile Strength

Test method: ASTM D412

Definition: Tensile strength uses a standard test to determine the maximum load or amount of stretching force a material can withstand before it breaks. The higher the tensile strength, the more force it can withstand. It is measured using pound-force per square inch (psi).

Importance: Tensile strength is one of the properties that tell you something about the durability of urethane. Generally, it will increase as hardness increases, but there are exceptions when custom formulas are created to achieve other higher properties at a lower hardness. With high tensile strength, the material can withstand loads, forces, and impacts without breaking or warping.

For mold making, the material should withstand the stretching forces exerted by the casting material. Having good strength is essential if you are making large molds that need to hold up to abrasive materials and large pours. Since urethane molds are meant to be reused, having good tensile strength allows them to endure repeated stress without failing. When demolding, the mold may be pulled, bent, and twisted to release the cast part, so this property helps it handle these forces and produce successful castings in high-volume production runs.

Elongation

Test method: ASTM D412

Definition: Elongation is tested to determine the maximum length a material can be stretched before it breaks. It is measured using a percentage of the final length compared to the original length of a tested material. It is also an inverse relationship with tensile strength as it uses the same test method.

Importance: While elongation isn’t a main concern for mold making, it’s still one of the physical properties users should know. If you require your molding material to have more stretch and flexibility, you’ll want to look at elongation. A mold with higher elongation will have a lower tensile strength, allowing you to demold from the master or the cast piece easier. Harder materials have lower elongation because they’re less flexible and have greater tensile strength, meaning they don’t have the same ability to stretch.

Tear Strength

Test method: ASTM D624 (Die C)

Definition: Tear strength, also called tear resistance, tests the maximum amount of force required to initiate a tear in the material. It is measured using pounds per linear inch (pli). Die C is the most common test type for urethane, and the test specimen is not nicked. The force acts parallel to the tab ends of the test specimen or at 45° to the 90° center.

Importance: Tear strength is considered one of the most important physical properties of urethane molding rubber. The reason for this is due to rough handling in the demold process. When removing castings from molds, tears or punctures can occur due to the pulling force when trying to break the tension between the mold and the cast part. Higher tear strength keeps the mold from tearing for easier part removal and a longer-lasting mold.

Tear strength is an important property and is loosely tied to the tensile strength of the material. Usually, the higher the tensile strength, the higher the tear strength. This means that when you have a high elongation, you will have a lower tear strength.

Dimensional Stability

Test method: ASTM D2566

Definition: Dimensional stability is tested to find a material’s ability to maintain its dimensions (size and shape) when it cures. It measures linear shrinkage expressed in inches per inch (in/in) between the cured material and the mold box or cavity it was molded from when cured at room temperature (77°F).

Importance: All two component urethane rubbers experience an exothermic reaction that generates heat as they cure, which causes the material to shrink. The degree to which this shrinkage occurs depends on the material, amount of exotherm, thickness, and geometry of the piece or mold. If more material is used to make large molds, you may notice a difference in the mold’s dimensions from the mold box or form it came from due to a larger exothermic reaction. This can also be an issue when casting thicker mold walls because it will generate more heat, leading to greater shrinkage.

If a mold material has good dimensional stability, it will retain its shape through the casting process. If the mold shrinks too much, cast pieces may be different sizes and may not fit together, especially in the case of manufactured stone.

Cast the rubber on a rigid backing material like wood to combat shrinkage when making larger molds. If you pour it around the lip of the backing material, it will create a better grip on the surface edge, making it harder for the material to shrink. Also, work at a consistent temperature so your environment doesn’t affect the material’s dimensional stability.

What Liquid Properties Are Important?

Not all companies split their properties into separate sections, but VFI specifies properties for urethane in a liquid state and when it is in its cured solid state. The following properties pertain to the unmixed and mixed liquid components:

Specific Volume

Definition: Specific volume is a property that relates to the volume of matter divided by the amount of matter or the reciprocal of its density to determine how much material is needed to occupy a given space. It is measured in inches cubed per pound (in3/lb). Simplified, this means that you can use the specific volume to determine the weight of product needed for your project.

Importance: Specific volume is an important property to know to calculate the amount of material you need to make a mold. First, the volumes (length x height x width) of your mold box and master must be calculated and subtracted from each other. Once you know the volume needed to fill the remaining space, you can divide it by the specific volume to convert it into weight. This number will be the total weight of urethane needed (Part A + Part B). Calculating this number ahead of time saves you from the risk of mixing extra material and generating waste or not having enough during the molding process.

Liquid Density

Definition: Liquid density is the weight of a material in a specified volume. It equals the mass of the liquid divided by its volume and is commonly measured in pounds per gallon (lb/gal).

Importance: Looking at a liquid density can tell you if there are useless fillers in the mix to increase the bulk and reduce costs. These fillers will notably change the weight of the material. As a standard, the liquid density of urethane rubber is around 8.5-9.5 pounds per gallon. If filler is included, you may see the liquid density range between 11-13 pounds per gallon.

Mix Ratio

Definition: The mix ratio is a property for liquid materials with several components that must be mixed to create a final product. It tells you how much of each part needs to go into the final mixture to produce the proper chemical reaction. It can appear in two ways:

• By volume: Mix ratio by volume is expressed as a ratio (Ex: 1A:1B) and is the exact proportions of Part A and B that must be combined and is not dependent on the weight. It is measured using equal-sized containers and is mostly used when processing through automated dispense equipment.
• By weight: Mix ratio by weight is expressed as a ratio (Ex: 51.50A:100B) and is the exact proportions of Part A and B that must be combined and is not dependent on volume. It is measured using an accurate scale, but if the user does not have one, selecting a material with a convenient mix ratio by volume is more desirable.

Importance: Most urethane rubbers are two-part systems (resin and hardener), and when mixed, they cure at room temperature. Staying on ratio for both parts ensures the material will cure successfully and achieve the desired properties.

If the mix ratio is not followed correctly, it can impact the final product. If you mix it with too little resin (B side), the material may become brittle. If you mix it with too little hardener, the material may become soft, tacky, or gooey to the touch. In some cases, not adhering to precise mix ratios can inhibit the cure, and the mold will never develop full physical properties, and it will be unusable.

Viscosity

Definition: Viscosity is the measure of a liquid’s resistance to flow. It will generally show up three times on a property sheet as Part A, Part B, and the mixed liquid viscosity. It is measured in centipoise (cps). For reference, below is a list of common household items and their viscosities so you are better equipped to understand the viscosity of urethane.

Material	Viscosity
Water	1-5 cps
Corn syrup	50-100 cps
Maple syrup	150-200 cps
Castor oil	250-500 cps
Pourable urethane rubber	200-3000 cps
Honey	2000-3000 cps
Molasses	5000-10000 cps
Chocolate syrup	10000-25000 cps

 

Importance: Urethane can come in a wide range of viscosities depending on processing needs, but pourable urethane will typically be between 200-3,000 cps. This property affects how easily the rubber can be mixed and poured into a mold. Less viscous rubber is easier to mix and pour, especially when using more complex molds. It will flow more readily into details, corners, and pockets. Rubbers with a lower viscosity are also less likely to trap air bubbles in the finished molds.

On the other hand, high viscosity materials are thicker and have a greater resistance to flow. There is a higher chance that the rubber will cure with air bubbles that create imperfections on a mold’s surface. A great way to combat air bubbles in thicker rubbers is to vacuum degas the material before pouring.

Having material components with similar viscosities that must be combined is also important to maintain a uniform mix. This varies when trying to incorporate a thinner material into a much thicker material because it requires a longer mixing time than if they had similar viscosities.

Pot Life/Work Time

Definition: Pot life or work time refers to the time it takes for the material to reach a viscosity where it is deemed too difficult to work with. For molding rubber, it is essentially the time after you start mixing that the material is pourable.

Importance: Understanding a material’s pot life is essential for having control and flexibility in the mold making process. It will tell you exactly how much time you have to mix, vacuum degas, and pour the material into a mold.

Depending on certain working conditions, the pot life may be shortened. Temperature is the number one factor that affects this. If the material, the environment, or the mold temperature is increased, pot life will decrease. It’s important to work at room temperature (77°F) or cooler temperatures. However, working in warmer temperatures will have the benefit of reducing the cure time if you need to process your molds faster.

The amount of material you use at one time can also affect the pot life. When you use more material, the extra mass causes more heat to be generated through the exothermic reaction. This heat then causes the rubber to become more viscous quicker. Heat is also a problem for thick-walled molds. More material concentrated in one area will cure much faster than thin sections.

Pot life is even more important if you’re making large molds or form liners. You’ll want to find a material with a long enough pot life that will allow you to mix and pour into the mold before it starts to solidify. Most urethane users will use dispensing equipment for this exact reason.

Demold Time

Definition: Demold time indicates the amount of time a material should cure before being removed from a mold, mold box, or form. Depending on the cure speed of the material, it can vary from minutes to hours.

Importance: You must wait until the material has solidified enough before demolding; otherwise, it may cause distortion or deformation. All cure times are based on room temperature (77°F) unless otherwise stated, and temperature will play a major role in demolding. The mold can be left in an area warmer than room temperature to speed up its demold time.

When the mold is removed from the mold box or form, it does not mean that it is ready for use. Polyurethane rubbers need up to 7 days to obtain final physical properties; however, they can be used for casting about 72 hours after they have sat at room temperature. Using the mold before this time can also result in deformation from casting pressure or increased difficulty when demolding.

Place Into Service

Definition: Place into service is the amount of time before a material is ready for use. It is measured in hours or days.

Importance: Not every manufacturer lists the place into service time for their urethane materials. However, it is a good property to know because it provides a time frame for when you can start using the mold for casting.
Urethane can usually take several days to develop desired properties, so a newly made mold can’t be used right after its initial cure. After sitting outside of a mold box or form at room temperature for a few extra days (72 hours minimum), the mold should have enough properties for casting.

Where to Find Material Properties?

Volatile Free, Inc. and most manufacturers list physical and liquid properties on product pages and technical data sheets or bulletins. We know the value of expressing these material properties accurately, so you can trust that the product you’re using will perform to specification. We use various standard ASTM test methods to determine a product’s properties and only publish them once they have gone through multiple reviews. Technical data sheets can be found on any product page under the resources tab.