Omar Khalid
Like most materials, plastic expands as temperature increases. However, plastics have a much lower thermal conductivity than metals, and they also have a lower melting point. When heated, plastics tend to soften and lose their stiffness, and if heated for long enough, they will distort. This is because plastics have a relatively unfavourable configuration, with long polymer chains that are nicely oriented by rapid cooling, and heat disrupts this orientation. The rate of expansion and contraction of plastics like nylon can be up to ten times that of metals like steel.
| Characteristics | Values |
|---|---|
| Plastic expands with heat | Yes, but at a lower rate than metals |
| Thermal Conductivity | Plastic has a much lower thermal conductivity than metals |
| Heat Distortion Temperature (HDT) | Most thermoplastics have an HDT of less than 500 degrees F |
| Effect of temperature on thermoplastics | Occurs at high heat levels, but low temperatures can also have an impact |
| Effect of high heat on thermoplastics | Softening and loss of stiffness, distortion if heated beyond operational temperature range |
| Nylon's expansion rate | Expands and contracts at ten times the rate of steel |
| Acrylic's expansion rate | Can expand up to 80mm on a hot day |
| UHMWPE or HDPE's expansion rate | Can grow up to 9mm for a 15-degree temperature change |
| Negative Thermal Expansion (NTE) | Some plastics exhibit NTE, shrinking upon heating and expanding when cooled |
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What You'll Learn
- Plastic expands with heat, but at a lower rate than metal
- Nylon expands and contracts at a rate 10 times greater than steel
- Plastic has a lower thermal conductivity than metal
- Plastic's low melting point means it softens easily with heat
- Plastic's coefficient of thermal expansion (CTE) must be considered when mating with metal
Plastic expands with heat, but at a lower rate than metal
Like most materials, plastic expands as temperature increases. This is known as the coefficient of thermal expansion (CTE). However, plastics have a much lower thermal conductivity than metals, and therefore expand at a lower rate. For example, nylon expands and contracts at around ten times the rate of steel. A 3-metre strip of UHMWPE or HDPE can grow by approximately 9mm for a 15°C temperature change, while nylon and acetal can grow by up to ten times that amount.
The reason for the difference in expansion rates between plastics and metals lies in the atomic structure of the materials. In plastics, the total volume expands when heated, but the material reconfigures from a thin sheet to a thick blob, giving the appearance of shrinking. This is due to the lower melting point of plastics, which means they soften and lose stiffness more easily when heated. The heat disrupts the long polymer chains that are formed when the material is cooled during production.
Metals, on the other hand, have a higher melting point and do not experience the same surface tension effects as plastics when heated. Instead, the heat causes the bridging oxygen atoms in the metal to oscillate more, pulling their neighbouring metal atoms closer together and causing a contraction along the M-O-M direction.
The difference in thermal expansion rates between plastics and metals must be considered when mating the two materials. If the dimensional change is obstructed, excessive stress loads can be induced in the plastic part, leading to unexpected failure.
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Nylon expands and contracts at a rate 10 times greater than steel
Like most materials, plastic expands as temperature increases. However, plastics generally have a much lower thermal conductivity than metals. This means that plastics are excellent replacement materials when thermal insulation is important.
Nylon, for example, has been found to expand and contract at a rate 10 times greater than steel. This means that nylon will expand and contract much more than steel when exposed to the same change in temperature.
The exact rate of expansion and contraction of nylon depends on the geometry and thickness of the material. For instance, a 100mm nylon rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease. This is calculated by multiplying the temperature change in °C by the length of the nylon rod and 0.00012. So, for a 20°C change in temperature, the same rod would expand or contract by 0.24mm.
It is important to consider the rate at which nylon expands and contracts when it is mated with another material, such as steel, that may have conflicting thermal expansion rates. If the dimensional change is obstructed, excessive stress loads can be induced in the nylon, which could result in unexpected failure.
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Plastic has a lower thermal conductivity than metal
Plastic, like most materials, expands as its temperature increases. However, it has a much lower thermal conductivity than metal. This means that plastic is a good insulator, whereas metal is a good conductor.
When an ice cube is placed on both a metal and a plastic block, the ice cube on the metal block melts much faster than the one on the plastic block. This is because metal is a better conductor, so energy is transferred more quickly to the ice cube. On the other hand, plastics are good insulators, so even if the plastic is at a lower temperature than your fingers, little energy conducts to the plastic, and it feels warm.
The difference in thermal conductivity between plastic and metal has important implications in various applications. For example, when plastic is mated with another material, such as metal, that has a different thermal expansion rate, it can cause stress and unexpected failure in the plastic part due to excessive tensile, shear, or compressive stress loads.
Additionally, the thermal properties of plastic can affect its performance in specific applications. For instance, in a microwave, high heat can cause plastic to soften and lose its stiffness. If heated for long enough or beyond its operational temperature range, it will begin to distort.
The thermal conductivity and expansion rate of different plastics can vary. For example, Nylon expands and contracts at approximately ten times the rate of steel. Similarly, a 3-meter strip of UHMWPE or HDPE can expand by up to 9mm for a 15°C temperature change.
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Plastic's low melting point means it softens easily with heat
Plastic materials generally have a much lower thermal conductivity than metals. This means that plastics are excellent replacement materials when thermal insulation is important. However, like most materials, plastic expands as temperature increases. This is known as the coefficient of thermal expansion (CTE).
The thermal expansion of plastics can be problematic when they are mated with another material, such as metal, which has a different rate of thermal expansion. For example, nylon expands and contracts at approximately ten times the rate of steel. If the dimensional change of the plastic is obstructed by the metal, excessive stress loads can be induced in the plastic part, leading to unexpected failure.
The effects of temperature on plastic generally occur at high heat levels, although excessively low temperatures can also have an impact. Mechanical properties, chemical resistance, electrical conductivity, material fatigue, and many other attributes can be affected by increased temperatures. Exceeding the heat distortion temperature (HDT) of a plastic can cause it to distort and soften.
The HDT of most thermoplastic materials is less than 500 degrees Fahrenheit. As the temperature increases, the material stiffness (flexural modulus) will decrease. This can be observed in everyday life by microwaving food in a plastic container. As the plastic container increases in temperature, it begins to soften and lose its stiffness. If heated for long enough or beyond its operational temperature range, it will begin to distort.
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Plastic's coefficient of thermal expansion (CTE) must be considered when mating with metal
The coefficient of thermal expansion (CTE) is a crucial consideration when mating plastics with metals. Plastics, like most materials, expand with increasing temperatures, and their CTE values can vary significantly depending on the specific plastic formulation. For example, Nylon, a commonly used plastic, has a coefficient of thermal expansion of around 90-95, which is approximately ten times higher than that of steel. This means that when exposed to heat, Nylon expands and contracts at a much faster rate than steel.
When plastics are mated with metals, the difference in CTE between the two materials can lead to significant issues. Metals typically have lower CTE values than plastics, which means they expand and contract at slower rates. If the dimensional change of the plastic is obstructed by the slower-expanding metal, excessive stresses can be induced in the plastic part. These stresses can lead to unexpected failure or distortion of the plastic component.
To mitigate these issues, it is essential to carefully select the plastic material based on its critical properties, including thermal expansion characteristics. Different plastic formulations have different CTE values, and some plastics exhibit good thermal expansion properties. For instance, Acetal (POM) offers excellent dimensional stability and has a CTE of 80-120. Nylon, despite its high CTE, is often used in applications requiring food contact due to its high melting point and strength.
Additionally, during the product development process, it is crucial to consider the environmental temperature range the part will be exposed to, the dimensional and stiffness tolerances required at different temperature levels, and the expected loads or forces on the part at high temperatures. By carefully considering these factors, potential issues arising from the mating of plastics with metals can be avoided.
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Frequently asked questions
Yes, plastic does expand with heat, but it also has a low melting point and softens easily, which is why it appears to shrink. Plastic has a much lower thermal conductivity than metals.
Plastic has a low melting point and softens easily, so it quickly becomes more liquid than most other solids. Due to surface tension, it pulls back into a minimum surface shape, which is usually a blob or ball.
No, not all plastics expand with heat. Some plastics exhibit negative thermal expansion (NTE), which means they contract upon heating and expand when cooled. However, they do not expand back to their original size when cooled.
