Maya Ellison
Plastic is often assumed to be a material that is sensitive to heat and will melt when exposed to a heat source. However, this is not always the case, as there are many high-performance plastics that can withstand continuous high temperatures of up to 500 °F (260 °C) without losing their mechanical properties. The heat capacity of plastic materials depends on their chemical structure and varies among the different types of plastics. The actual plasticizing capacity of a given heating cylinder depends on factors such as moulding temperature, thermal conductivity, specific gravity, and specific heat of the material. Some plastics, like PTFE, are known for their impressive heat resistance and ability to withstand extreme temperatures, making them suitable for specialized applications.
| Characteristics | Values |
|---|---|
| Heat conductivity | Varies depending on the type of plastic |
| Melting point | Varies depending on the type of plastic |
| Heat capacity | Varies depending on the type of plastic |
| Continuous service temperature | Up to 500 °F for some plastics |
| Operating temperature range | -328 °F to 500 °F for PTFE |
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What You'll Learn
Heat conductivity
The heat conductivity of a material refers to its ability to conduct heat. Metals, for example, have a much higher heat conductivity than plastics due to their free electrons. This is why a metal object feels cooler to touch than a plastic one at room temperature—the metal is better able to conduct heat away from your hand.
Plastics are often viewed as sensitive materials that will melt when exposed to a heat source. While this is true of some plastics, many modern polymers have high melting points and are designed to perform in harsh conditions. These plastics are known as high-performance plastics and can be exposed to high temperatures without compromising their mechanical properties.
One example of a high-performance plastic is PTFE (polytetrafluoroethylene), more commonly known as Teflon. PTFE can withstand continuous temperatures of up to 500 °F (260 °C) without losing its mechanical properties. It is also chemically inert, meaning it can be exposed to acids, cleaning products, and corrosive agents without being damaged. These properties make PTFE a popular choice for industries that require heat resistance, such as the medical industry.
Another high-performance plastic is PEEK, which can retain its properties when exposed to continuous temperatures of up to 482 °F (250 °C). PEEK is available in two forms: virgin or containing additives such as glass, carbon, and graphite. Its excellent mechanical and chemical resistance make it a versatile material for a variety of applications.
The heat conductivity of plastics is an important factor in their processing and manufacturing. The moulding temperature, thermal conductivity, specific gravity, and specific heat of a plastic material all play a role in the heat transfer and heat-generating processes during injection moulding. As such, it is important to carefully analyse the heating capabilities of the injection-moulding heating cylinders when working with plastics, especially those with lower thermal conductivities and higher melting points.
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Heat-resistant plastics
While plastic is often viewed as a material that melts when exposed to heat, many polymers have high melting points and are designed to perform in harsh conditions. Heat-resistant plastics are used in a variety of industries, including the automotive, aerospace, semiconductor, and medical industries.
Types of Heat-Resistant Plastics
Common Heat-Resistant Plastics
- PTFE (polytetrafluoroethylene or Teflon): PTFE has an impressive heat resistance of up to 500 °F (260 °C) and can also withstand extremely low temperatures. It is chemically inert and resistant to acids and corrosive agents.
- PEEK (polyether ether ketone): PEEK is a high-performance engineering thermoplastic with excellent heat resistance. It can withstand temperatures up to 310 °C for short periods and has a melting point of over 371 °C. PEEK is widely used in medical devices, automotive components, and aircraft exterior parts.
- ULTEM® or Polyetherimide (PEI): ULTEM is a strong and flame-resistant thermoplastic with a high melting point of 219 °C. It is commonly used in circuit boards, food sterilization equipment, and aircraft parts.
Factors Affecting Heat Resistance in Plastics
The heat resistance of plastics is determined by various factors, including the glass transition temperature (Tg), melting point, and molecular structure. Amorphous plastics have a disordered molecular structure and gradually soften when heated, while semi-crystalline plastics have a semi-ordered structure and a defined melting point. The heat deflection temperature, flammability, and flame resistance are also important parameters in assessing the heat resistance of plastics.
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Thermoplastics melting rates
Thermoplastics are a type of plastic that melts when heated and can be moulded into a new shape when it cools. They have a specific melting point, and this varies depending on the type of thermoplastic. The melting point of thermoplastics can be studied using a tool called Differential Scanning Calorimetry (DSC), which heats samples at different rates and shares the results in plot form.
Thermoplastics have a wide range of melting points. Common thermosetting plastics like epoxy, phenolic, and polyester resins typically begin to degrade at temperatures between 200°C and 300°C. However, high-performance thermoplastics, or high-temperature thermoplastics, have much higher melting points, ranging from 6500 to 7250 F, which is significantly higher than standard engineering thermoplastics.
The melting point of a plastic is essential to know for manufacturers, as injection moulding at the incorrect temperature can impact the final product's appearance and strength.
Some plastics are known for their heat-resistant properties, such as PTFE or "Teflon", which can be exposed to continuous temperatures of up to 500°F (260°C) without losing its mechanical properties. PEEK is another example of a heat-resistant plastic, able to withstand temperatures of up to 482°F (250°C).
It is worth noting that the heat conductivity of plastics is generally lower than that of metals due to the presence of free electrons in metals, which is why plastic objects often feel warmer than metal objects at room temperature.
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Heat input and removal
The heat input and removal of plastics are essential considerations in various applications, especially when selecting suitable materials for specific conditions. While the assumption that plastics melt easily when exposed to heat is widespread, many plastics offer excellent heat resistance and can maintain their mechanical properties even at high temperatures.
Thermodynamics principles apply to plastic materials, and each plastic has a unique heat capacity, influencing the amount of heat required to raise its temperature. The specific heat of a material expresses its ability to absorb or release heat energy. The complexity of specific heat data in calculating heat input and removal may necessitate the use of mathematical extrapolations, such as heat content.
The heat capacity of plastics can vary significantly, and it depends on factors such as the plastic's chemical structure, crystalline melting point, and specific gravity. The actual plasticizing capacity of a heating cylinder is influenced by the moulding temperature, thermal conductivity, and specific heat of the plastic being processed. As production rates increase, the quantity of heat needed to achieve the desired melt viscosity also increases. However, the instrumentation on most machinery may not accurately reflect the temperatures within the barrel or the plastic melt.
Some plastics, such as PTFE (polytetrafluoroethylene) or Teflon, exhibit exceptional heat resistance. PTFE can withstand continuous temperatures of up to 500 °F (260 °C) without compromising its mechanical properties. This makes PTFE suitable for environments with high temperatures, chemicals, and friction. Additionally, PTFE is chemically inert, allowing it to withstand exposure to acids and corrosive agents without damage.
Another example of a heat-resistant plastic is PEEK, which can retain its properties at continuous temperatures of up to 482 °F (250 °C). PEEK is known for its mechanical and chemical resistance and is commonly used in the medical industry and other sectors requiring high-performance plastics.
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Heat capacity of plastic
The heat capacity of a material refers to its ability to absorb or release heat energy. Every material has a specific heat capacity, which is the ratio of heat needed to elevate the material by one degree in relation to the heat needed to elevate water by one degree. The heat capacity of plastics varies depending on the type of plastic. For example, the heat capacity of polystyrene is different from that of polyethylene. The heat capacity of a plastic material also depends on its specific gravity and specific heat.
Thermodynamics principles are applicable to plastic materials, and the quantity of heat required to melt various thermoplastics per unit weight can vary significantly due to differences in heat capacity. As the production rate increases, the quantity of heat needed to achieve the proper melt viscosity also increases. This is because the heater bands supply heat energy to the outside of the cylinder, but the effective heat input to the polymer will be less due to heat loss.
Some plastics, such as PTFE or "Teflon," have high heat resistance and can be continuously exposed to temperatures of up to 500 °F (260 °C) without losing their mechanical properties. These high-performance plastics are suitable for specialised applications in high-temperature environments. Other plastics, like PEEK, can retain their properties at continuous temperatures of up to 482 °F (250 °C).
Plastics are often viewed as sensitive materials that melt easily, but this is not always the case. Many polymers have high melting points and can perform well in harsh conditions without compromising their mechanical properties. The heat capacity of plastics is an important factor in their processing and recycling, as it affects the heating capabilities required for injection moulding and the ability to withstand high temperatures.
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Frequently asked questions
All materials have a specific heat capacity, which expresses their ability to absorb or release heat energy. The heat capacity of plastic varies depending on the type of plastic. For example, PTFE can withstand temperatures of up to 500 °F (260 °C), while PEEK can withstand temperatures of up to 482 °F (250 °C).
The heat capacity of plastic depends on several factors, including the moulding temperature, thermal conductivity, specific gravity, and specific heat of the material. The production rate and cylinder design can also impact the heat capacity of plastic during manufacturing.
No, this is a common misconception. Many plastics have high melting points and are designed to perform in high-heat environments. However, some plastics have limited heat resistance and may melt at lower temperatures.
PTFE (polytetrafluoroethylene), also known as Teflon, is a heat-resistant plastic that can withstand temperatures of up to 500 °F (260 °C). PEEK (polyether ether ketone) is another heat-resistant plastic that can handle temperatures up to 482 °F (250 °C). These plastics are used in various industries, including medicine and food-grade applications.
