What is the dimensional stability of plastics?
During the process of plastic parts processing, assembly, and product application, a series of external conditions such as processing and environment will affect whether the product functions well from materials to parts throughout the entire life cycle. The most commonly affected by dimensional stability is the size of the part, which causes the material to expand or contract due to excessive humidity or extreme temperature environments.
Even if the part is not exposed to particularly harsh operating environments, when machining parts to close tolerances, the machining process will be subject to a lot of mechanical stress! So, it is important to consider the dimensional stability of the material!
What environmental factors affect dimensional stability?
The dimensional stability of plastics is an important performance indicator, especially in applications that require precise fit and stable dimensions. As shown in the following figure:
In addition to the green card assembly fit, metal screws and plastic parts must also be considered to maintain consistent expansion coefficients under thermal expansion and contraction conditions.
How to evaluate the dimensional stability of engineering plastics?
Coefficient of Thermal Expansion (CTE):
The coefficient of thermal expansion indicates how much a material expands in volume when its temperature changes. A low CTE means that the material changes less in size when the temperature changes. Due to their chemical structure, plastic materials expand and contract to varying degrees when heated and cooled. This change in material size can cause fit and clearance issues and can weaken the material’s mechanical properties over time.
For precision plastic parts, it is particularly important to maintain consistent dimensions at different temperatures. In engineering applications involving extreme temperatures and/or temperature fluctuations, plastic materials must have sufficient thermal stability to prevent expansion and contraction. The lower the CTE of an engineering plastic, the better its dimensional stability.
Water absorption:
Engineering plastics swell when they absorb water, which causes dimensional changes. Different plastics have different water absorption capacities. Plastics with low water absorption have less dimensional changes in a humid environment, so they have better dimensional stability. In addition, water absorption can also weaken other properties of the material, such as electrical conductivity and mechanical strength.
Whether facing ambient humidity, or dealing with applications such as long-term immersion, wet or steam, and exposure to the natural environment, it is necessary to consider the time that plastic parts are exposed to water during their life cycle, as well as the concentration and temperature of the water. For example, nylon PA6 and PA66 have high water absorption. They are hard when injected, but they become soft after boiling water for a few days! They are easily hydrolyzed. Materials like PPO, PEEK, and LCP have low water absorption. They are so hard when they are punched out, and they are still hard when they are put in water!
Internal stress:
The internal stress generated during the injection molding or compression molding process will affect the dimensional stability of the material, causing the material to deform and crack in severe cases. Some parts may be fine after injection molding, but crack the next day. Some parts may even crack after delivery. Internal stress may cause the material to deform or warp during the cooling process. This is because the plastic molecules cool from the outside to the inside. Appropriate processing techniques (such as annealing) can reduce internal stress and thus improve dimensional stability.
Crystallinity:
The dimensional stability of semi-crystalline plastics such as PP, PA66, and POM is affected by their crystallinity. High crystallinity usually leads to a lower coefficient of thermal expansion and higher dimensional stability. Amorphous plastics such as PC, PMMA, PPE, and PEI usually have better dimensional stability because of their lower coefficient of thermal expansion.
Fillers and modifiers:
Adding fillers such as glass fiber and carbon fiber can significantly improve the dimensional stability of the material. Modifiers such as lubricants, heat stabilizers, etc. can also affect the high-temperature stability of the material. Appropriate toughening can improve the stress cracking of the material.
Processing conditions:
Temperature, pressure and cooling rate during the injection molding or molding process all affect the dimensional stability of the final product. Optimizing processing parameters can reduce internal stress and improve dimensional stability.
Environmental conditions:
in different temperatures, humidity, outdoor, oily, alcohol and other environmental factors. For the physical and chemical properties of materials working under extreme conditions, engineering plastics with corrosion resistance, high and low temperature resistance, hydrolysis resistance and other properties should be selected for application.
How to detect the dimensional stability of plastics?
- Dimensional change test: measure the dimensional change of materials under different temperature and humidity conditions.
- Thermomechanical analysis (TMA): determine the dimensional change of materials during heating.
- Differential scanning calorimetry (DSC): used to evaluate the crystallinity and melting point of materials, indirectly reflecting dimensional stability.
- Water absorption test: evaluate the water absorption rate and dimensional change of materials under different humidity conditions.
Only by understanding the specific application and environment of plastic components can we decide how to choose materials. For example, sealing components require extremely stable material dimensions, because changes in the size of the seal may cause leakage. For some parts that need to be cleaned regularly with steam, chemicals or high-temperature liquids, will they deform due to water absorption and corrosion resistance? Only with sophisticated equipment, process technology, and engineering plastics with dimensional stability can complex parts with small sizes and strict tolerances, such as electrical connectors and test sockets, be made.
