Injection Molding Precautions for POM Materials
POM (Polyoxymethylene), also known as acetal, is a high-crystallinity engineering plastic characterized by high rigidity, excellent mechanical strength, good flowability, significant shrinkage, and thermal instability. When exposed to excessive temperatures during processing, POM undergoes thermal decomposition, releasing corrosive formaldehyde gas and causing product defects such as yellowing, brittleness, and burning. Strict control of processing parameters and adherence to safety guidelines are essential to ensure product quality, production stability, and equipment longevity. This article provides a comprehensive overview of the key precautions for POM injection molding, covering material handling, process settings, mold design, defect prevention, and safety measures.
Material Handling and Drying
POM has an extremely low water absorption rate, typically around 0.25%, so it does not require drying under normal storage conditions. However, if the material has been exposed to moisture, such as in a humid environment or after prolonged storage, it must be dried before processing to prevent surface defects like silver streaks and bubbles. The recommended drying temperature is 80–90℃, with a drying time of 2–3 hours. It is crucial to avoid high-temperature, long-time drying, as this can cause oxidation and degradation of the material, leading to reduced mechanical properties and color changes. Before production, the hopper and barrel should be thoroughly cleaned to remove any residual materials or impurities, which can cause contamination, color inconsistency, or processing instability.
Barrel Temperature Settings
The barrel temperature is a critical parameter in POM injection molding, as it directly affects melt flowability and thermal stability. The recommended barrel temperature range is 180–205℃, and it must never exceed 215℃. Exceeding this temperature threshold will trigger thermal decomposition, resulting in the release of irritating formaldehyde gas, product yellowing, brittleness, and burning. The temperature should be set in a gradient, gradually increasing from the feed zone to the front zone to ensure uniform melting. The nozzle temperature should be slightly lower than the front zone temperature, typically by 5–10℃, to prevent drooling and material decomposition at the nozzle. When starting up or changing materials, the barrel should be purged with PE or PP to remove any residual POM and prevent overheating during idle operation.
Mold Temperature Control
Mold temperature has a significant impact on POM’s crystallinity, dimensional stability, surface finish, and warpage. The recommended mold temperature range is 60–90℃. Higher mold temperatures (80–90℃) promote complete crystallization, resulting in improved surface finish, reduced internal stress, and better dimensional stability, making them suitable for products with high appearance requirements or thick walls. Lower mold temperatures (50–60℃) can be used for thin-walled products to shorten cycle times, but temperatures below 50℃ should be avoided, as they can lead to incomplete crystallization, surface haze, and reduced mechanical strength. The cooling system must be designed to ensure uniform cooling across the mold, as uneven cooling can cause warpage, shrinkage, and dimensional instability.
Injection and Pressure Holding Control
POM has good flowability, so the injection speed should be set to medium-to-fast to ensure complete filling of the cavity without causing flow marks or burning. A slow injection speed can lead to cold flow marks and incomplete filling, while an excessively fast speed can cause turbulence, air entrapment, and burning at the weld lines. The injection pressure is generally set to 60–100 MPa, depending on the product complexity and wall thickness. Excessively high injection pressure can result in high internal stress, flashing, and post-molding cracking. The holding pressure should be 30–50% of the injection pressure, and the holding time should be optimized to prevent shrinkage and sink marks without introducing excessive internal stress. Multi-stage injection and holding are recommended to achieve stable filling and minimize defects.
Back Pressure and Screw Speed
Back pressure should be kept low, typically between 3–8 MPa, to avoid excessive shear heat generation, which can cause material degradation and color changes. Higher back pressure can improve melt homogeneity but increases the risk of overheating. The screw speed should be set to a moderate level, generally 50–100 r/min, to prevent excessive shear and heat generation. The metering stroke should be stable to ensure consistent shot volume and prevent overflow and drooling.
Mold Design and Protection
POM decomposition produces corrosive formaldehyde gas, so the mold cavity and core should be made from corrosion-resistant steels such as S136 or NAK80 to prevent rust and corrosion. The mold must be designed with sufficient venting, especially at weld lines and last-filled areas, to allow trapped air to escape and prevent burning and incomplete filling. A sufficient draft angle (1.5–3°) and large ejection area are essential to ensure smooth demolding and prevent sticking, cracking, and deformation. After production, the mold should be thoroughly cleaned to remove any residual POM and gas, and a rust inhibitor should be applied to protect against corrosion.
Defect Prevention
Common POM injection molding defects and their solutions include: cracking and brittleness, which are often caused by high internal stress and can be addressed by reducing injection pressure, increasing mold temperature, and extending cooling time; shrinkage and sink marks, which are typically due to insufficient holding pressure and can be resolved by optimizing holding parameters and increasing mold and material temperatures; burning and flow marks, which are caused by poor venting or excessive injection speed and can be fixed by improving venting, reducing injection speed, and lowering material temperatures; weld lines, which can be minimized by increasing material and mold temperatures, improving venting, and adjusting gate position; and warpage, which is often due to uneven cooling and can be prevented by stabilizing mold temperature, extending cooling time, and optimizing the cooling system layout.
Safety and Equipment Protection
Formaldehyde gas released during POM decomposition is irritating to the eyes, nose, and throat, so the production area must be well-ventilated to ensure operator safety. If a strong odor is detected during production, the material temperature should be immediately reduced, and the barrel should be purged to check for overheating. Before shutdown, the barrel must be thoroughly purged with PE or PP to remove any residual POM, preventing decomposition and corrosion of the screw and barrel during idle periods. Long-term high-temperature idle operation should be avoided to prevent equipment damage and safety hazards.
Post-Treatment
For precision components and thick-walled products, annealing treatment is recommended to eliminate internal stress and improve dimensional stability. The recommended annealing temperature is 100–120℃, with a holding time of 2–4 hours, followed by slow cooling to room temperature. This process helps reduce the risk of post-molding cracking and warpage. After annealing, the products should be handled carefully to avoid rapid cooling and forced assembly, which can reintroduce internal stress and cause defects.
