Chloe Zhang
The thermal expansion of plastics is a critical property that determines their suitability for various applications, especially in the semiconductor industry. Plastics generally exhibit a higher thermal expansion than metallic materials, with polymeric plastics expanding about 10 times more than metals. This property is quantified using the Coefficient of Linear Thermal Expansion (CLTE) or Linear Coefficient of Thermal Expansion, which describes the rate at which a material expands or contracts with temperature changes. ASTM standards, such as ASTM D696 and ASTM E228, provide test methods to determine the CLTE of plastics using dilatometers or push-rod dilatometers within specific temperature ranges. The thermal expansion of plastics can be influenced by factors such as molecular orientation, cooling time during processing, and the presence of fillers. Understanding and calculating the thermal expansion of plastics are crucial for designing applications, predicting shrinkage, and ensuring the structural integrity of parts made from plastics, especially when used in conjunction with other materials like metals.
| Characteristics | Values |
|---|---|
| Thermal expansion of plastics | About 10 times greater than metallic materials |
| Coefficient of Linear Thermal Expansion (CLTE) | Defined as the rate a material expands or shrinks as a function of temperature |
| CLTE values | Generally, the lower the better |
| Factors affecting CLTE | Molecular orientation, cooling time during processing, and the degree of anisotropy of the filler and the filler orientation |
| Techniques to measure CLTE | ASTM D696, ASTM E831, ASTM E228, ISO 11359, ASTM E289 |
| ASTM D696 temperature range | -30 to 30°C (-22 to 86°F) |
| ASTM E289 temperature range | -150 to 700°C (-240 to 1290°F) |
| ASTM E228 temperature range | Outside of -30 to 30°C |
| ASTM E289 applicability | Materials with low or negative CTE (5 x 10-6/K or 2.8 x 10-6/°F) |
| ASTM D696 dilatometer type | Vitreous silica |
| ASTM E228 dilatometer type | Push-rod |
| ASTM E289 measurement method | Interferometry |
| ASTM D696 coefficient of expansion | Greater than 1 µm/(m.°C) |
| Thermoplastics CTE range | 0.6 x 10-4 to 2.3 x 10-4 K-1 |
| Thermosets CTE range | 0.2 x 10-4 to 0.6 x 10-4 K-1 |
| Mitigation techniques for plastic piping | Expansion loops, offsets, changes in direction, solvent-cemented joints, and expansion calculators |
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What You'll Learn
Coefficient of Linear Thermal Expansion (CLTE)
The Coefficient of Linear Thermal Expansion (CLTE) is a critical factor in the selection and use of plastics. CLTE is defined as the rate at which a material expands or shrinks due to temperature changes. This property is especially important in designing applications exposed to higher temperatures or used over a wide temperature range.
CLTE is a crucial factor in determining the dimensional stability of a material under thermal conditions. When a material is subjected to temperature variations, it undergoes thermal expansion or contraction, leading to changes in its dimensions and physical properties. This effect is notable in solids, including plastics and polymers. As the temperature rises, not only does the material grow, but critical features machined on the parts also change in size and location.
The linear coefficient of thermal expansion, also known as "α" for plastic and polymer materials, can be calculated using the formula: α = ΔL / (L0 * ΔT), where ΔL represents the linear expansion per unit length, L0 is the initial length, and ΔT is the change in temperature. It is important to specify the temperature ranges when reporting the mean coefficient of thermal expansion, as α can vary with temperature and is not always linear.
Understanding CLTE is essential for both economic and functional considerations in production. It helps determine the dimensional behaviour of machined parts, thermal stresses that can occur, and the selection of materials for quality and function. ASTM D696, ASTM E831, ASTM E228, and ISO 11359 are widely used standards for measuring the coefficient of linear thermal expansion in plastics. These standards employ techniques such as heating specimens in a furnace and using push rods or dilatometers to measure displacement.
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ASTM standards for testing
ASTM, formerly known as the American Society for Testing and Materials, has developed standards for testing the thermal expansion of plastics. While I cannot find specific ASTM standards for testing the thermal expansion of plastics, there are several methods outlined that can be used to calculate it. One critical property that relates to plastic selection within the semiconductor industry is the Coefficient of Linear Thermal Expansion (CLTE). CLTE is defined as the rate at which a material expands or contracts as a function of temperature. CLTE is a critical factor in designing applications exposed to higher temperatures or testing over a wide temperature range. Materials with low CLTE values are often selected to minimise expansion and contraction.
One method to measure the thermal expansion of plastics is to use a device called a dilatometer. Dilatometers are used to evaluate linear thermal expansion. This is important when considering the thermal properties of parts made by 3D printing with polymeric materials. Another method involves using an optical microscope to measure the length of a test sample in the form of an Archimedean spiral. The arithmetic mean of the lengths of the two side surfaces of the profile can be calculated to determine the average fibre length. Additionally, finite element modelling can be used to analyse the temperature differences and corresponding thermal expansions or contractions in different areas of the test sample.
It is important to note that the thermal expansion of plastics can vary significantly from that of metallic or ceramic materials. Polymeric plastics, for example, have an expansion rate about 10 times greater than that of metallic materials. This highlights the importance of understanding the thermal properties of plastics, especially when used in applications where temperature changes may occur.
By following ASTM standards and utilising appropriate testing methods, such as those mentioned above, the thermal expansion of plastics can be accurately evaluated, ensuring the selection of suitable materials for specific applications and preventing potential issues arising from thermal expansion or contraction.
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Thermal expansion in plastic pipes
When designing and installing a plumbing system, it is important to consider the impact of thermal expansion and contraction on plastic pipes. Physical changes to the length of the pipe can occur when the fluid and environmental temperatures fluctuate above the ambient temperature of 73.4 °F (23 °C). Higher temperatures can result in linear growth of the pipe, which increases the length of the run, while lower temperatures will return the pipe run to its original length. These changes are more pronounced in longer pipes and must be considered in all plumbing systems, particularly those with elevated temperatures.
The temperature differential, or delta T, is a crucial factor when calculating the expansion and contraction of a plastic pipe system. This is the difference between the installed temperature and the operating temperature. The pipe will always be shorter at the time of installation compared to when it is in service with hot water running through it. For example, FlowGuard Gold CPVC pipe will experience a thermal expansion of about 1 inch per 50 feet of straight pipe for every 50 °F increase in temperature. Plastics will expand and contract four to five times more than metallic systems.
To address thermal expansion and contraction in plastic pipes, several methods can be employed:
- Use expansion loops or offsets: Build expansion offsets or loops into the system to accommodate the movement of the pipe.
- Leave space between elbows and hangers/restraints: Ensure that there is sufficient space between these components to allow for the pipe's lateral movement without abrasion.
- Use changes in direction as deflection mechanisms: Utilise bends, drops, and turns in the piping installation to act as natural "expansion loops".
- Secure the piping at regular intervals: For vertical stacks in multi-story applications, secure the aboveground vertical piping at close intervals to maintain proper alignment and support the weight of the piping and its contents.
- Use double-contained restraint fittings: For double-contained piping systems, use fittings like Dogbones® to anchor the inner and outer pipes simultaneously, minimising the need for expansion/contraction loops.
- Solvent-cement the pipe and fittings: Instead of using threaded connections, solvent-cement the pipe and fittings together to allow for thermal movement.
By considering the temperature differential and utilising appropriate methods to accommodate thermal expansion and contraction, designers and installers can ensure the proper functioning and longevity of plastic pipe systems.
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Thermal expansion of plastics used for 3D printing
The thermal expansion of plastics used for 3D printing is an important area of study. With the increasing use of 3D-printed polymer parts, it is crucial to understand the thermal properties of these materials, including their ability to expand or contract with temperature changes. This phenomenon is known as the Coefficient of Linear Thermal Expansion (CLTE) and is defined as the rate at which a material expands or shrinks as a function of temperature. CLTE plays a critical role in designing applications exposed to higher temperatures or wide temperature ranges, as the expansion or contraction of the material can affect the placement of features such as micro holes.
Several experimental methods have been employed to investigate the thermal expansion of plastics used for 3D printing. One common approach involves the use of spiral test samples made from different polymeric materials, such as styrene-butadiene acrylonitrile, polyethylene terephthalate, thermoplastic polyurethane, and polylactic acid. These spiral samples are placed on an aluminium alloy plate with a spiral groove, and the thermal expansion or contraction is observed. Finite element modelling has revealed that different areas of the plate and the spiral sample may have different temperatures, resulting in varying degrees of thermal expansion or contraction in the spiral regions.
Another method utilised in the study of thermal expansion is the use of dilatometers, which are devices specifically designed to evaluate linear thermal expansion. Additionally, techniques such as thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis can assess the thermal expansion capacity of materials used in 3D printing processes. The time variation of thermal expansion can be recorded using a stopwatch, and the temperature of the specimen can be measured using an infrared thermometer.
The input factors in the 3D printing process, such as nozzle hole diameter, relative movement speed, material density, and temperature settings, can also influence the thermal expansion properties of the printed parts. For example, the density and distribution of the material within the printed part can vary, affecting its coefficient of expansion. Therefore, it is essential to consider these factors when studying the thermal expansion of plastics used for 3D printing and designing parts for specific applications.
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Factors influencing thermal expansion
Several factors influence the thermal expansion of plastics. One of the most critical factors is the Coefficient of Linear Thermal Expansion (CLTE), which is the rate at which a material expands or contracts as a function of temperature. The CLTE is crucial when designing applications exposed to higher temperatures or used over a wide temperature range, as the material and its critical features will grow and move. For example, micro holes in the array pattern of test sockets will change in size and location as temperatures shift. Therefore, materials with low CLTE values are often selected.
The type of plastic is another important factor. For instance, Nylon and Acetal exhibit a slight increase in their expansion rate when temperatures exceed 60°C. Specifically, a 100mm nylon6 rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease. As a result, the expansion becomes more pronounced as the diameter and temperature increase.
The polymeric structure of plastics also plays a role in thermal expansion. Polymeric plastics, such as those used in 3D printing, have an expansion rate about 10 times greater than metallic materials and significantly higher than ceramic materials. This makes it crucial to understand the thermal properties of polymers used in 3D printing, especially when creating parts through additive manufacturing technologies like vat photopolymerization and material extrusion.
Additionally, the initial temperature of the plastic can influence its thermal expansion behaviour. Experimental results have shown that polymeric materials may exhibit different levels of negative thermal expansion based on their initial temperatures. This highlights the importance of considering the entire temperature range a material will endure when designing parts using plastics.
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Frequently asked questions
CLTE is defined as the rate at which a material expands or shrinks with changes in temperature. It is a critical factor to consider when designing applications exposed to higher temperatures.
The most widely used technique is to heat a specimen in a furnace and transmit the displacement of the ends of the specimen to a sensor using a push rod. ASTM D696 is a standard test method that uses a vitreous silica dilatometer to measure the coefficient of linear thermal expansion for plastic materials. ASTM E228 is another test method that uses a push-rod dilatometer to measure the linear thermal expansion of solid materials at temperatures outside the range of -30°C to 30°C. ASTM E289 provides a standard method for measuring the linear thermal expansion of rigid solids with interferometry over a wider temperature range.
Calculating the thermal expansion of plastics is important in various industries, such as 3D printing and plumbing systems. In 3D printing, it is crucial to understand the thermal properties of polymers used for parts manufacturing. In plumbing systems, the thermal expansion and contraction of plastic pipes must be considered to prevent issues such as pipe swelling or breakage.
