In the application of engineering plastics, dimensional stability is a crucial performance indicator. It not only relates to the function and performance of the product but also directly affects the quality and lifespan. This article will comprehensively interpret the dimensional stability of engineering plastics from aspects such as definition, influencing factors, evaluation methods, and practical applications.
I. What is Dimensional Stability in Engineering Plastics?
Dimensional stability refers to the ability of a material to maintain its external dimensions unchanged under mechanical forces, heat, or other external conditions. For engineering plastics, dimensional stability refers to the ability of engineered plastic products to maintain consistent dimensions during use or storage. Since the long-chain molecules of polymers are stretched and frozen during the processing, when the activation energy of the molecules increases, the chain segments curl to some extent, leading to some degree of dimensional change in the product.
The dimensional stability of engineering plastics is usually represented by the dimensional change rate, which is the ratio of the change in material dimension to the original dimension under certain conditions. The smaller the dimensional change rate, the better the dimensional stability of the material.
II. What Factors Affect the Dimensional Stability of Engineering Plastics?
The dimensional stability of engineering plastics is influenced by various factors, mainly including the following:
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Temperature:
Temperature is a significant factor affecting the dimensional stability of engineering plastics. As the temperature rises, the movement of polymer chain segments intensifies, causing the material to expand; conversely, a decrease in temperature leads to material contraction. This thermal expansion and contraction phenomenon are particularly evident in extreme temperature environments. Therefore, for engineering plastics that need to work in high or low-temperature environments, their dimensional stability is particularly important. -
Humidity:
Humidity is also an important factor affecting the dimensional stability of engineering plastics. Engineering plastics expand when absorbing moisture, leading to dimensional changes. This expansion phenomenon is especially significant in high-humidity environments. Therefore, for engineering plastics that need to work in humid environments, their water absorption rate and dimensional stability need special attention. -
Mechanical Stress:
During processing and use, engineering plastics are subjected to various mechanical stresses, such as tension, compression, bending, etc. These mechanical stresses can cause the molecular chains inside the material to rearrange and orient, thereby affecting the dimensional stability. Especially when precision machining is carried out, the machining process is also affected by a lot of mechanical stress. -
Processing Conditions:
Processing conditions such as temperature, pressure, and cooling rate during injection molding or molding affect the dimensional stability of the final product. Optimizing processing parameters can reduce internal stresses and improve dimensional stability. For example, increasing the injection molding temperature can reduce the viscosity of the material, which is conducive to the flow and filling of the melt; extending the cooling time allows the product to solidify better, reducing shrinkage and deformation. -
Material Properties:
The dimensional stability of engineering plastics is also related to their intrinsic properties. For example, amorphous plastics such as PC, PMMA, PPE, and PEI generally have lower coefficients of thermal expansion (CTE) and thus better dimensional stability; whereas semicrystalline plastics such as PP, PA, and POM have higher CTEs and relatively poorer dimensional stability. Additionally, factors such as the crystallinity of the material, the addition of fillers and modifiers, etc., also affect its dimensional stability.
III. How to Evaluate the Dimensional Stability of Engineering Plastics?
Evaluating the dimensional stability of engineering plastics typically requires considering the following aspects:
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Coefficient of Thermal Expansion (CTE):
CTE is a key parameter for measuring the volume expansion of a material when the temperature changes. A low CTE means that the material undergoes less dimensional change when the temperature changes, thus having better dimensional stability. By measuring the length change rate of the material at different temperatures, its CTE value can be calculated. -
Water Absorption Rate:
The water absorption rate refers to the ability of a material to absorb moisture under certain conditions. Plastics with low water absorption rates undergo less dimensional change in humid environments, thus having better dimensional stability. By measuring the mass change rate of the material under certain humidity conditions, its water absorption rate can be calculated. -
Internal Stress:
Internal stress generated during injection molding or molding processes affects the dimensional stability of the material. Reducing internal stress through appropriate processing techniques (such as annealing) can improve dimensional stability. Internal stress can be measured using equipment such as stress meters. -
Crystallinity:
For semicrystalline plastics, their crystallinity is an important factor affecting dimensional stability. High crystallinity typically results in a lower CTE and higher dimensional stability. The crystallinity of the material can be measured using methods such as X-ray diffraction (XRD). -
Thermal Shock Testing:
Thermal shock testing is a commonly used method to evaluate the dimensional stability of materials. By setting a certain temperature change rate and cycle number, while detecting the remaining length of the specimen at a specific temperature after each temperature shock, the dimensional change of the material can be clearly detected. This test simulates the temperature change environment of the material during actual use, thereby evaluating its dimensional stability. -
Long-term Aging Testing:
Long-term aging testing is an effective method to evaluate the dimensional stability of materials over extended periods of use. By placing the material under specific conditions (such as high temperature, high humidity, light exposure, etc.) and periodically measuring its dimensional change rate, its long-term dimensional stability can be assessed.
IV. Practical Applications of Dimensional Stability in Engineering Plastics
The dimensional stability of engineering plastics has widespread applications in multiple fields. Here are some typical application cases:
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Automotive Manufacturing:
In automotive manufacturing, engineering plastics are widely used in the manufacture of various components. For example, engine compartment covers, bumpers, door trim panels, and other parts require good dimensional stability to ensure they do not deform or crack during use. -
Electronic Devices:
In electronic devices, engineering plastics are used to manufacture various casings, brackets, connectors, and other components. These components require good dimensional stability to ensure precision and reliability in their fit with other components. For example, mobile phone casings need to have good dimensional stability to ensure they do not deform or loosen during use. -
Aerospace:
In the aerospace industry, engineering plastics are used to manufacture various structural and connecting components. These components need to withstand extreme environments such as extremely high temperatures and pressures, thus requiring extremely high dimensional stability. For example, the skins and frames of aircraft wings require good dimensional stability to ensure they do not deform or crack during use. -
Medical Devices:
In medical devices, engineering plastics are used to manufacture various surgical instruments, implants, and medical device casings. These components require good dimensional stability and biocompatibility to ensure they do not cause any harm to patients during use.
