Understanding the Properties of Urethane Plastic

In the part making and prototyping world, understanding the properties of urethane plastic is essential for choosing the best material for a specific application. If you’re new to using thermoset polymers like urethane, you may not know which properties are most important for making plastic parts. The answer will vary based on how you need your parts to perform. This comprehensive guide will lead you through all the essential properties and why you should know them.

What Physical Properties Are Important?

The physical properties of urethane pertain to the material when it is in a cured state. These properties will tell a urethane user how their cast parts will hold up in their desired application. They are tested by manufacturers and third-party laboratories using standard methods from the American Society for Testing and Materials (ASTM). The following are the most popular properties that manufacturers list on their technical data sheets:

Shore Hardness

Test method: ASTM D2240

Definition: Shore hardness indicates a material’s resistance to indentation or deformation. Materials are compared using different scales based on similar characteristics. The higher the number on the scale, the greater the plastic can resist indentation, meaning the part is increasingly harder.

Importance: Shore hardness is a factor that indicates the durability, flexibility, and machinability of urethane plastic. High performance plastics are measured on the D scale from medium hard to extra hard (20 D – 90 D).

A rigid material is typically more durable, abrasion-resistant, and can bear more weight than softer, flexible materials. Many high performance plastics are rated 60 D and above, indicating that they are very tough and resistant to tears with limited flexibility, if any.

These hard plastics are a great choice for making parts that will experience a lot of wear and tear, such as production parts and equipment housings. They are highly suitable for applications where rigidity, wear resistance, and dimensional stability after cure are crucial.

Note: When looking at VFI urethane plastics, 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-4170 70 D Fast Casting Plastic).

Tensile Strength

Test method: ASTM D638

Definition: Tensile strength is the maximum load a material can support before it breaks or fractures when stretched. It has also been called a tension or pull test and is expressed in pounds per square inch (psi).

Importance: Parts are often subjected to various types of stress (tension, compression, bending, etc.), so tensile strength is important for determining if a urethane part has the structural integrity and durability to resist tension forces. The ability to sustain higher stress than other materials is one of polyurethane ’s greatest qualities.

A part with high tensile strength can withstand significant loads without breaking. Applications where the material will be subjected to constant stress, such as industrial parts, rotocast parts, and tooling, can benefit from high tensile strength. These parts are less likely to fail, so you can count on them to perform in tough situations. Materials with higher tensile strength also tend to be rated higher on the shore hardness scale.

Tensile Modulus

Test method: ASTM D638

Definition: Tensile modulus is closely associated with tensile strength as it relates to the ratio of tensile stress to strain when a material undergoes deformation when stretched. It is a calculated number that uses the same test method and measurement (psi) as tensile strength since both test tension (pulling) force.

Importance: Tensile modulus is a property that helps you determine how stiff or rigid a urethane plastic part will be under tension. It will also tell how much the part will deform or elongate under that force. A high tensile modulus is great for applications that require high stiffness and minimal deformation to increase the longevity of the part.

When a part has a high tensile modulus, it means it will resist stretching. It has great dimensional stability to maintain its shape under consistent loads. Like tensile strength, it is often associated with hardness. Harder parts may have a higher tensile modulus, though some custom formulas can break this standard.

Flexural Strength

Test method: ASTM D790

Definition: Flexural strength measures the maximum force needed to bend a material until it breaks. Like tensile strength and modulus, it is expressed in pounds per square inch (psi).

Importance: Many applications subject plastic parts to bending forces in everyday use. A part with good flexural strength can handle this stress without snapping, breaking, or warping. Knowing flexural strength will make it much easier to identify if a material is strong enough to resist deformation. High flexural strength parts are durable and will keep their shape and function better for longer. If a material’s flexural strength is on the lower end of the spectrum, it might mean that the material is more brittle. Plastics with higher flexural strength will usually be rated higher on the hardness scale, which helps them resist the bending force.

Flexural Modulus

Test method: ASTM D790

Definition: Flexural modulus measures a material’s stiffness or resistance to a bending action. It uses the same test as flexural strength and is expressed the same (psi). It is different from tensile modulus because it tests a material’s resistance to deformation when bent, whereas tensile modulus tests this resistance when stretched. Both properties, however, indicate a material’s rigidity.

Importance: Flexural modulus is important because many part makers require a certain degree of stiffness in their plastic parts to withstand bending forces. A higher modulus means the material is stiffer and will bend less under a specific load. Understanding this property and how it affects the performance of plastic parts makes it easier to create quality components for high-stress situations.
Parts with a high flexural modulus are also strong and rigid, so they can provide support where needed. Rigid parts are used for applications like automotive parts, gears, furniture pieces, etc.

Shrinkage Linear

Test method: ASTM D2566

Definition: Linear Shrinkage is the change in the length of a material along linear dimensions (length, width, and height) as it cures. The contraction is usually expressed as inches per inch (in/in) and tests a 12 x ½ x ½ sample.

Importance: Part makers should know that urethane plastic shrinks as it cures, which could affect a part’s final dimensions. Most shrinkage will occur when the part is cooling in the mold, and a small amount can occur after demolding.

The rate of shrinkage is dependent on several factors. Some plastic materials might not have thermal properties to withstand shrinkage. Processing conditions like temperature, pressure, and flow rate can increase the shrinkage of a part as it cures. The part size, if you are using a lot of material to make large parts, can also cause shrinkage.

When you know how to work around linear shrinkage, you can maintain consistent part dimensions throughout production runs. If a part is undersized, it might not be able to perform its intended function, which can be a waste of time and materials.

Izod Impact

Test method: ASTM D256

Definition: Izod impact is a test method that measures a material’s toughness or resistance to impact. It is used in the polymer manufacturing industry to determine the toughness of plastics. It is typically measured as the amount of work required (ft-lb) to break the material a certain amount (in). The two ways Izod is tested are:

• Notched: It tests impact resistance from a swinging pendulum with energy focused on a notch. This measures the material’s ability to absorb impact despite having a flaw (the notch). Calculated values are typically lower because a notch is much easier to continue breaking and simulates real-world situations better.
• Unnotched: It tests impact resistance from a swinging pendulum with energy focused on the entire test piece so force is distributed more evenly. There is no premade notch because it measures the material’s overall toughness. Calculated values are typically higher and might not represent real-world situations where imperfections are likely.

Importance: Good impact resistance is essential for plastic parts that might experience sudden force, shock, or blows. It allows part makers to determine which materials will have the necessary toughness needed to create durable, long-lasting parts. Understanding impact resistance can also help prolong the life of your part and prevent it from failing in high-impact scenarios.

A material’s impact resistance can vary based on the type of material, how thick it is, and external conditions. Urethane plastic can be customized with different properties and has the benefit of being as soft or hard as you want it to be. Softer urethane plastics are going to be better for impact resistance.

However, Izod impact is just one property that can tell a user how tough a material is. Other factors that affect the strength of a material include tensile and flexural strength, which may be more important depending on the application.

Heat Deflection Temperature

Test method: ASTM D648-18

Definition: Heat deflection temperature (HDT), also called heat distortion temperature, is a thermal property unique to polymers and plastics. It tests the maximum temperature at which a material deforms under a given load. Depending on the material, it will be tested at 66 psi or 264 psi.

Importance: HDT determines if a plastic part can remain rigid and keep its shape under high temperatures and constant loads. When a part reaches its HDT for an extended time, it will lose its load-bearing capability and may fail. This is why you must look at the HDT if you know you’re making a part that will be exposed to elevated temperatures.

A material’s HDT can be affected by several factors. The longer the material is exposed to heat, the more likely it will deform. If the direct or indirect temperature rises slowly, the material might have a higher HDT. The size and shape of the part can also change the HDT, as the thicker the part, the higher the heat transfer rate.

When urethane plastic has a high HDT, it’s better at retaining its shape and functionality in high-heat applications. Some industries where HDT would be important include packaging components, automotive parts, electronic pieces, and medical devices.

Note: VFI also has plastics with fire-retardant in their formula for part-making in a variety of industries, such as medical, automotive, and aerospace.

Cured Color

Definition: Cured color is the color that a material cures to.

Importance: Cured color can affect your plans when making a urethane plastic part, as it is not the same for every product. Some products have a manufacturer’s standard color, but most come neutral, white, or even UV-stable clear to make them customizable to the user. Neutral-colored plastics allow part makers to paint the cured surface for a production-quality finish. However, painting can be avoided by using dyes and pigments to alter the color. Pigments are added to the polyol (B side) before mixing for more time to achieve the desired color before the pot life begins.

However, cured color may not always be important for an application. If your part is hidden from sight or used as an internal component, you probably don’t need to be concerned about this property or UV stability. Most plastic materials on the market are aromatic, and that is okay for most users. If you need a UV-stable part, you should look for products marketed as UV-stable.

What Liquid Properties Are Important?

While not all companies separate physical properties from liquid properties, VFI does, so you can identify which applies to a material when it’s a liquid and when it’s a solid. The following properties are important to know when working with liquid urethane plastic:

Mix Ratio

Definition: Mix ratio is a property that establishes the exact amount of material to combine to produce a solid final product, such as urethane plastic. This property is reported as a ratio for materials with two or more liquid components that must be mixed together. A mix ratio can be expressed in two ways:

• By volume: Using equal-sized containers or dispensing equipment to measure the exact proportions of two or more components that must be combined. It is not dependent on weight.
• By weight: Using an accurate scale to measure the exact proportions of two or more components that must be combined. It is not dependent on the volume.

Importance: Following the mix ratio of any urethane plastic is crucial because it directly affects the chemical reaction that allows the plastic to cure. The combination of materials will ensure that the plastic will develop strong cross-links that lead to a durable, high-strength part.

If these materials are mixed incorrectly, it can result in an incomplete cure. The plastic might be weak, soft, or sticky and won’t function as needed. In some cases, it may never develop full physical properties. Use the proper mix ratio to be sure your parts produce consistent results from batch to batch and reduce material waste.

Viscosity

Definition: Viscosity is a measure of a fluid’s resistance to flow at room temperature (77°F). Many manufacturers list several viscosities for their materials if they include more than one component (i.e., A side, B side, and mixed). It is expressed in centipoise (cps). For a better understanding of the viscosity of urethane plastic, here is how it typically compares to some household items:

Material	Viscosity
Water	1-5 cps
Corn syrup	50-100 cps
Maple syrup	150-200 cps
Castor oil	250-500 cps
Urethane plastic	80-1500 cps
Honey	2000-3000 cps
Molasses	5000-10000 cps
Chocolate syrup	10000-25000 cps

 

Importance: Viscosity is an important property that impacts how you work with a material and affects some of the cured material’s final characteristics. It helps determine how easy it is to pour the urethane into a mold. A low-viscosity material will have a better time flowing into intricate molds with tight corners for detail reproduction.

Low viscosity urethanes are also less likely to trap air bubbles while curing. As the material flows into a mold, the bubbles dissipate more easily to minimize voids and imperfections in the cured plastic. Not only does the surface finish look better, but fewer voids help the material exhibit better performance characteristics.

For quality assurance, you can vacuum degas and pressure pot materials to reduce air entrapment further. These techniques are essential if you are making clear plastic parts.

Pot Life/Work Time

Definition: Pot life or work time is the time it takes for a material’s viscosity to thicken to a state where it is deemed unworkable. More simply, it is the amount of time you can work with the material before it becomes too thick and begins to cure. This can be expressed in minutes or hours, depending on how fast the material sets at room temperature (77°F).

Importance: Since urethanes are customizable, the pot life can be varied depending on a user’s processing needs. There are unique plastic materials that are extremely fast and can only be worked with for 2 minutes, and there are some materials that have a working time of 2 hours or more.

Pot life can always be affected by the temperature of your material, environment, and mold. If you are working above room temperature, your pot life is guaranteed to decrease, which means you’ll have even less time to work with the material. However, some people are okay with increasing the temperature as this can also decrease the time it takes for the part to be ready to demold.

Depending on how big of a part you are making, make sure the material you choose provides enough time to mix, degas, and pour the material. For a material that has an extremely fast pot life, a different casting method is recommended. You will typically not have enough time to mix and pour the liquid before it sets, so injection molding is the processing method that would utilize fast resins.

Gel time

Definition: Gel time is the time it takes a material to stop flowing or become gel-like. It is typically measured in minutes or hours, as it comes shortly after the pot life ends. Like pot life, it is assessed at room temperature (77°F).

Importance: While gel time might not be as important as pot life, it can help you plan your project so you know your workable window. It will also help you avoid mistakes caused by exceeding the workable timeframe and prevent you from wasting material.

Demold time

Definition: Demold time is the time a casting should be allowed to cure before removing it from a mold. It is usually expressed in minutes or hours, depending on factors like pot life and gel time.

Importance: Your material’s demold time is important for part production. Like pot life, demold time can vary depending on the formula. If you are processing parts that must be ready quickly, you’ll want to find a material with a short demold time. If you need to pour large parts, you’ll need a material with a longer working time, but this will result in a longer demold time.

Something to note about demold time is even if your part is ready to be demolded, it will not have full physical properties. Some plastics are unique and require post-curing in an oven to achieve full physical properties. Heat can help any material obtain its properties faster; otherwise, your parts will take a few extra days to obtain the properties listed on a technical data sheet.

Where to Find Material Properties?

It is very important that material properties are correctly listed so that users can trust the material will perform as expected. Our on-site lab uses various standard ASTM test methods to determine the unique properties of each material. Once they have been reviewed thoroughly, we post them on all our product pages and the technical data sheet for each product. Technical data sheets can be found on any product page under resources.