Compression Set: Definition, How It Works, and Understanding the Graph

In materials science, a compression set refers to an important parameter that describes how much deformation or height loss a material experiences after being subjected to a particular compressive force for a given time. It is a measurement of how well a material can recover to its initial shape. A compression set is a key factor in the choice of printing materials in 3D printing. 

To determine the compression set, the material is typically subjected to a specific compressive load for a predetermined amount of time. The resulting deformation or loss of height is then measured. A compression set graph offers insight into the material's propensity to hold its shape under compression over time. This article will discuss what a compression set is, examine how it works, and provide an understanding of the compression set graph.

What Is Compression Set?

A compression set is a measurement of how permanently a material is altered after being compressed for some time and then released. The amount of the material's original thickness or height that is still compressed after a certain amount of time is expressed as a percentage. A common test procedure for calculating the compression sets of rubber and rubber-like materials is ASTM D395. 

Knowing the compression set is crucial in applications where the material is continuously compressed, such as gaskets, seals, and O-rings. A compression set is also relevant to situations where the material is expected to regain its original form following compression, such as in springs and shock absorbers. A high compression set indicates that the material will not be able to maintain its shape and functionality over time, which can lead to failure or decreased performance.

How Does a Compression Set Work?

In a compression test, a sample of the material is compressed and held for a predetermined amount of time while being subjected to a specific load. The percentage of deformation that remains is calculated after the material has been allowed to recover for a predetermined amount of time. A compression set is a measurement that determines whether a material is suitable for uses that need reliable and consistent compression resistance.

How To Understand the Compression Set Graph?

The following (example) guidelines should be followed in order to understand the compression set graph:

1. Look for the key material being studied, in this case, silicone rubber.
2. Keep in mind that it differs from other elastomers in that its elastic behavior only modifies marginally with temperature change.
3. The y-axis, which represents the percentage of the compression set, should be noted.
4. Pay attention to the x-axis, which represents the range of temperatures in Celsius.
5. Pay close attention to the different graph lines that show the silicone rubber's compression set after a 24-hour period of compression at various temperatures. By selecting a point on the x-axis that corresponds to a particular temperature and projecting a line to the curve, these lines can be created. A line from the curve needs to be projected to the y-axis to obtain the compression set.
6. Keep in mind that the elastic behavior of silicone rubber reduces as the temperature rises because its compression set also rises—though slightly.
7. The rate of change in the compression set as the temperature rises can be seen if the slope at different points on the graph is drawn.
8. Examine the various lines on the graph to determine how temperature impacts the silicone rubber's compression set.

To ensure the material will function as expected over time, for instance, if silicone rubber is being used in a high-temperature environment, the compression set may need to be considered. Figure 1 below is an example of a compression set graph:

How Does the Compression Set Relate to 3D Printing?

In 3D printing, the compression set is a critical factor, especially for materials used in applications that most often compress and recover. A 3D-printed part may experience permanent deformation or a loss of its original shape if it is compressed over time. the compression set properties of 3D printing materials helps in choosing the best material for a given application and guaranteeing that the printed parts retain their integrity over time. For more information, see our guide on 3D Printing.

What Is the Importance of Compression Set Testing?

Compression set testing is essential because it establishes a material's capacity to regain its original shape after being compressed over an extended time. This test is especially important when choosing materials for gaskets, O-rings, and seals, among other applications where compression is commonly observed. A high compression set means the material will not keep its shape and could fail in other ways, such as by leaking. Compression set testing ensures that the chosen materials will perform as expected in their intended applications and prevent expensive failures and maintenance problems.

How Do They Measure the Compression Set of a Material?

The compression set of the material is measured by the following test procedures:

1. Compression Set A

Compression Set A involves compressing a sample of the material for a specified amount of time at a set temperature under a constant force of 1.8 kN. The specimen is then given 30 minutes to recover under normal circumstances before having its thickness measured. The compression set A value is then calculated using the equation:

Compression Set A = [(H0-H1)/H0] * 100%

Where:

H0 is the original height/thickness of the material

H1 is the height/thickness of the material after a specified compression and time. 

The Compression Set A test can be used to compare the compression strength of different materials. Figure 2 shows the Compression Set A test setup:

In this setup, the specimen is compressed using a press with a constant force of 1.8 kN for a set time and temperature. After the compression, the specimen is allowed to recover in normal conditions, and its thickness is measured using a micrometer.

Example: The original height of a rubber material is 20 mm, and after compression of 10 mm for 24 hrs, the height of the material recovers to 18 mm. 

Then, the Compression Set A would be:

Compression Set A = [(20-18)/20] * 100% = 10%

2. Compression Set B

Compression Set B is determined by subjecting a specimen to a constant deflection in the air (75% of its original height) for a set time and temperature. The percentage of the specimen's original thickness that it retains after being released from the compression is then measured.

To determine the compression strength of a material using Compression Set B, a sample of the material is first subjected to a specified compressive force for a specified amount of time and temperature, which results in a predetermined Compression Set B value. The specimen is then removed from the compression and allowed to rest for 30 mins, after which the thickness of the specimen is measured. Figure 3 shows a diagram of the Compression Set B test setup:

In this test setup, the specimen is compressed to 75% of its original height. The spacer thickness or the specimen thickness during the test (tn) is measured, and the compressed specimen is held for a specified amount of time and at a known temperature. After the test is complete, the specimen is removed from the compression and allowed to rest for 30 minutes. The thickness of the specimen after the rest period (ti) is measured, and the Compression Set B value is calculated using the formula:

Compression Set B = [(H0-H2)/H0-Hn] * 100%

Where: 

H0 is the original height of the specimen

Hn is the height of the specimen during the test

H2 is the height/thickness of the material after a specified compression and time and a subsequent recovery period.

Example: If the original height of a rubber material is 20 mm, after experiencing a 75% compression for 24 hrs, the height of the material recovers to 18.5 mm. 

Taking into account the spacer or specimen thickness during the test (75% of 20 mm = 15 mm), the compression set B would be:

Compression set B = [(20-18.5)/20-15] * 100% = 30%

What Factors Affect Compression Sets?

Several factors can affect the compression set of material, including:

1. Polymer Type: Different polymers have different levels of resilience and resistance to compression.
2. Fillers: Adding fillers to a polymer matrix can affect its compressive properties and deformation recovery.
3. Temperature: Higher temperatures can cause greater deformation and lower recovery, leading to a higher compression set.
4. Time: Longer compression periods can cause greater deformation and lower recovery, leading to a higher compression set.
5. Stress/Strain: Higher stress and strain levels can cause greater deformation and lower recovery, leading to a higher compression set.

What Device Is Used in Measuring a Compression Set?

The compression set of materials is measured using a compression set apparatus. A compression platen and a set of weights are typically used to apply a specific load to the sample. The sample is compressed for a set amount of time before being given time to recover. The compression set is calculated by measuring the sample's thickness before and after the compression and recovery period. The sample is placed between the platens and subjected to the weights for the required amount of time to use the apparatus. The weights are taken off after the compression period, and the sample is given time to recover. A micrometer or other measuring tool is used to measure the sample's thickness to determine the compression set.

What Does a Higher Percentage Compression Set Mean?

A higher percentage of compression set indicates that a material is less resilient and has a lower ability to recover from deformation under compressive stress. Furthermore, this means that the material has a lower compression strength and is more likely to deform or fail under compression.

What Does a Lower Percentage Compression Set Mean?

A lower percentage of compression set shows that the material is more resilient and has a better chance to recover from deformation under compressive stress. The material has a higher compression strength and is less likely to deform or fail under compression.

Which Is Better, a Higher or Lower Compression Set?

A lower compression set is considered better as it indicates that the material will retain its shape and properties after being subjected to compression over time. On the other hand, a higher compression set implies that the material may experience permanent deformation, resulting in a loss of mechanical properties. A lower compression set is preferred in applications where long-term resilience and durability are critical, such as in seals and gaskets.

Does the Rubber Have a Low or High Compression Set?

The type of rubber being considered will determine whether it has a low or high compression set. Compression set varies among various rubber types.

What Is the Compression Set Percentage of Silicone Rubber?

Silicone rubber has a different compression set percentage depending on the particular type and grade of the material. However, silicone rubbers are generally known to have low compression set values, typically ranging from 20–40%. They are therefore perfect for applications requiring repeated compression and recovery cycles, such as many seals and gaskets satisfy.

Can You Improve the Compression Set of a Material?

Yes, there are a variety of ways to enhance a material's compression set. Adding premium fillers, changing the polymer's structure, or altering the curing procedure are a few techniques. The particular strategy depends on the kind of material and the goal. In many applications, an improved compression set can lead to higher reliability and a longer life span.

What Is the Difference Between Compression Set and Creep (Deformation)?

The primary difference between compression set and creep is that compression set is a one-time deformation, while creep deformation is a continuous deformation over time. The permanent deformation that takes place when a material is compressed is referred to as a compression set. On the other hand, creep deformation describes the gradual, time-dependent deformation that takes place when a load is constant. 

Summary

This article presented compression sets, explained what they are, and discussed the importance of testing and how to understand relevant graphs. To learn more about compression sets, contact a Xometry representative.

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