Compression Set: Definition, How It Works, and Understanding the Graph
Performing the test and understanding the graph
Compression set testing helps you choose the right material for applications where constant compression and recovery are required. If a material has a high compression set, it may lose its shape and fail over time, leading to problems like leaks in seals or gaskets. By testing the compression set, you can avoid choosing materials that won’t hold up over time, saving you from costly failures and maintenance issues. Let’s look at exactly what compression set is and how to do the test.
What is a Compression Set?
In materials science, a compression set measures how much deformation or height loss a material experiences after being subjected to a particular compressive force for a given time. It tells you how well the material can return to its original shape once the compressive force is removed. This is especially important for materials used in products that are constantly being compressed and need to regain their shape. In simple terms, it's the amount of permanent deformation that stays after the material is compressed. A common test procedure for calculating the compression sets of rubber and rubber-like materials is ASTM D395.
Different polymers have different abilities to recover from compression. If a material has a high compression set, it’s less resilient and will be more likely to stay deformed under pressure. This makes it unsuitable for applications that need long-term compression resistance. If its compression set is lower, it’ll be more resilient and recover better, making it a good choice for applications where the material will repeatedly be compressed, such as in seals, O-rings, seals, and gaskets.
Higher temperatures tend to cause more deformation and less recovery. Also, the longer the material is compressed, and the more stress or strain it endures, the more deformation it could experience. A material’s compression set can be improved with fillers or by adjusting the curing process.
We’ve put some common manufacturing materials in the table below with their approximate compression sets, as well as their suitable applications.
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:
- Look for the key material being studied, in this case, silicone rubber.
- Keep in mind that it differs from other elastomers in that its elastic behavior only modifies marginally with temperature change.
- The y-axis, which represents the percentage of the compression set, should be noted.
- Pay attention to the x-axis, which represents the range of temperatures in Celsius.
- 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.
- Keep in mind that the elastic behavior of silicone rubber reduces as the temperature rises because its compression set also rises—though slightly.
- 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.
- 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.
| Material | Approx. Compression Set | Applications |
|---|---|---|
Material Silicone Rubber | Approx. Compression Set 20–40% | Applications Gaskets, seals, O-rings, medical devices, automotive parts |
Material Nitrile Rubber | Approx. Compression Set 25–50% | Applications Seals, gaskets, fuel hoses, O-rings, automotive and industrial applications |
Material Polyurethane | Approx. Compression Set 20–40% | Applications Seals, gaskets, cushioning materials, wheels, industrial equipment |
Material Neoprene | Approx. Compression Set 15–30% | Applications Gaskets, seals, wet suits, electrical insulation, automotive parts |
Material Polyethylene | Approx. Compression Set 5–15% | Applications Packaging, containers, piping, insulation, toys |
Material Polypropylene | Approx. Compression Set 5–15% | Applications Car parts, containers, packaging, textiles |
Material Polyvinyl Chloride (PVC) | Approx. Compression Set 5–20% | Applications Pipes, plumbing, electrical insulation, flooring, medical products |
Material Acrylic (PMMA) | Approx. Compression Set 1–5% | Applications Optical applications, display cases, lighting, car components |
Common Materials and Their Compression Sets
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.
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?
There are two common types of tests to measure compression set, typically referred to as Compression Sets A and B.
Keys
- H0 = Original height of material
- H1 = Height/thickness of material after a specified compression and time
- H2 = Height/thickness of material after specified compression and time, and subsequent recovery period
- Hn = Material’s height during testing
1. Compression Set A
This test is typically used to compare different materials’ resistance to permanent deformation. The material is compressed under a specific force (usually 1.8 kN) for a set amount of time. After the compression, it’s given 30 minutes to recover. The recovered thickness is measured, and the compression set is calculated using the formula below.
Compression Set A = [(H0-H1)/H0] * 100%
Example
If a material started at 20 mm thick and, after compression, it only recovered to 18 mm, you could calculate the compression set percentage as follows:
[(20 mm - 18 mm) / 20 mm] x 100% = 10%
2. Compression Set B
This test is used when you need to know how a material behaves when compressed to a specific thickness. The material is compressed to 75% of its original height for a specific time and then allowed to recover for 30 minutes. The recovered thickness is then measured, and the compression set is calculated. The formula for compression set B is below:
Compression Set B = [(H0-H2)/H0-Hn] x 100%
Example
If a rubber sample starts at 20 mm and is compressed to 75% (15 mm), then after recovery, it’s measured to be 18.5 mm, the calculation would be:
[(20 mm - 18.5 mm) / (20 mm - 15 mm)] x 100% = 30%
What Factors Affect Compression Sets?
Several factors can affect the compression set of material, including:
- Polymer Type: Different polymers have different levels of resilience and resistance to compression.
- Fillers: Adding fillers to a polymer matrix can affect its compressive properties and deformation recovery.
- Temperature: Higher temperatures can cause greater deformation and lower recovery, leading to a higher compression set.
- Time: Longer compression periods can cause greater deformation and lower recovery, leading to a higher compression set.
- 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.
FAQs on 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)?
Compression set refers to the permanent deformation of a material after compression, while creep is the gradual, time-dependent deformation that happens when a material is under constant stress for a long period. Compression set is a one-time deformation, while creep happens over time.
How Xometry Can Help
If you have any more questions about compression set or need help choosing the right material for your project, just reach out to one of our representatives, who would be happy to assist. Xometry also offers so many related manufacturing services like CNC machining, laser cutting, and 3D printing. Get started today by requesting your free, no-obligation quote.
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