Injection molding humidity control
Injection molding humidity conditioning is a post-processing technique tailored to the specific characteristics of certain plastic materials. Its primary purpose is to adjust the performance of plastic parts, eliminate internal stresses, stabilize dimensional accuracy, and improve reliability by controlling ambient humidity and temperature. This treatment is particularly suitable for highly hygroscopic plastics such as polyamide (nylon), as these plastics absorb moisture from the air after molding, causing dimensional changes. Furthermore, dry plastic parts are often brittle and prone to fracture when subjected to stress. By allowing plastic parts to absorb an appropriate amount of moisture in a specific temperature and humidity environment, humidity conditioning improves their toughness and accelerates dimensional changes, preventing deformation or performance degradation during use due to moisture absorption. This is a crucial step in ensuring the quality of these plastic parts.
The fundamental principle of humidity conditioning during injection molding is closely related to the hygroscopic properties of plastics. Plastics like polyamide contain polar groups in their molecular structures, which have a strong affinity for water molecules. This makes them susceptible to moisture absorption and leads to volume expansion, a phenomenon known as “hygroscopic expansion.” After injection molding, plastic parts are dry, with tightly packed molecular chains and high brittleness. When moisture is absorbed, water molecules enter between the molecular chains, weakening the intermolecular forces and enhancing their mobility, thereby improving the part’s toughness and impact strength. This hygroscopic expansion also causes dimensional changes in the plastic part. If humidity conditioning is not performed before use, these dimensional changes in the natural environment can affect assembly performance. Humidity conditioning, by artificially controlling the moisture absorption process, allows the plastic part to reach a stable hygroscopic equilibrium quickly, enabling dimensional adjustments to be completed in advance and ensuring dimensional stability during use.
The process parameters for humidity conditioning treatment directly impact the treatment results, primarily including treatment temperature, humidity, time, and medium. The treatment temperature is typically determined by the type of plastic and the intended use of the part. For polyamide 6 and polyamide 66, hot water or steam at 80-100°C is generally used. Higher temperatures accelerate the diffusion of water molecules and shorten the treatment time. The treatment humidity must be maintained at a high level, typically using saturated steam or deionized water, to ensure sufficient moisture absorption by the part and avoid uneven treatment due to insufficient humidity. The treatment time is determined by the wall thickness and dimensions of the part. Thicker-walled parts require longer treatment times to ensure that moisture penetrates the interior, typically 2-24 hours. For example, for polyamide parts with 5mm walls, treatment in 90°C hot water for 8-10 hours achieves ideal humidity conditioning results; for thinner-walled parts, the treatment time can be shortened to 2-4 hours. Furthermore, contact between the part and metal must be avoided during the treatment process to prevent corrosion or contamination that could affect the part’s appearance.
In practical applications, humidity conditioning treatment requires targeted adjustments based on the characteristics of the plastic part and the environment in which it is used to achieve optimal results. For plastic parts that need to withstand impact loads (such as gears and bearings), humidity conditioning treatment can significantly improve their impact toughness and reduce the risk of breakage during use. For example, after humidity conditioning treatment, the impact strength of polyamide gears in automobile engines can be increased by over 30%, significantly extending their service life. For plastic parts with high dimensional accuracy requirements, humidity conditioning treatment can effectively stabilize their dimensions and prevent sticking or loosening due to moisture absorption and expansion after assembly. For example, polyamide connectors in electronic equipment can be subjected to strict humidity conditioning treatment to control dimensional changes to within 0.1%, ensuring precise coordination with other components. Furthermore, for plastic parts used in low-temperature environments, humidity conditioning treatment can also improve their low-temperature brittleness and enhance their weather resistance.
While humidity conditioning can effectively improve the performance of plastic parts, there are some precautions that require careful consideration during actual operation. First, treated plastic parts should avoid prolonged exposure to dry environments, as moisture loss may cause performance reversal. If necessary, they can be sealed or coated with a moisture-proof coating. Second, for reinforced polyamide plastic parts containing reinforcing fibers (such as glass fiber), humidity conditioning may reduce the bonding strength between the fiber and matrix interface, affecting the strength of the plastic part. Therefore, the treatment time and temperature must be controlled to avoid excessive treatment. Finally, the surface of the plastic part may show water stains or discoloration after humidity conditioning treatment, requiring appropriate post-treatment (such as cleaning and drying) to ensure the appearance quality. With the development of the plastics industry, humidity conditioning equipment is also constantly being upgraded. For example, the use of constant temperature and humidity chambers for precise control can achieve more uniform and stable treatment results, further improving the quality and stability of the plastic parts.
