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Main methods for controlling melt degradation during injection molding
2025-08-27
In the Injection Molding process, melt degradation is the key factor that causes various defects such as burn marks, black spots, silver streaks, strength loss, uneven color, abnormal odor, etc. Controlling degradation is one of the core goals of process parameter setting.
The following are the main methods to control melt degradation:
1. Precise control of melt temperature:
Follow the recommended material temperatures: Set the barrel temperature (back section, middle section, front section) and nozzle temperature strictly within the recommended processing temperature range provided by the plastic material supplier. Avoid excessive temperatures.
Avoid hot spots: Ensure that the heating coils in each section of the barrel are working properly and that the thermocouples are measuring temperature accurately to prevent local overheating (hot spots). Regularly calibrate the temperature control system.
Optimize nozzle temperature: The nozzle temperature is usually set slightly lower than or equivalent to the front section temperature to avoid overheating and degradation of the material at the nozzle due to thermal inertia.
2. Minimize the residence time of the melt in the barrel:
Optimize the molding cycle: Under the premise of ensuring sufficient filling, pressure holding, cooling and other links, try to shorten the entire molding cycle and reduce the residence time of the melt in the high-temperature barrel.
Avoid machine idling/pauses: During production interruptions (e.g., mold changes, machine downtime due to malfunctions), if the expected downtime exceeds the material's allowable thermal stability time (usually several to ten minutes), the molten material in the barrel must be drained (the machine must be purged) or cooled to a safe temperature (e.g., below 150°C). Prolonged exposure to high temperatures is strictly prohibited.
Choose the right machine capacity: Avoid using a machine with an injection volume significantly larger than the part weight to produce small parts. The machine's single shot volume (barrel capacity) should be 20%-80% (ideally 35%-65%) of the part weight plus the runner aggregate weight. An oversized machine means the material will remain in the barrel for an extended period of time.
3. Control shear rate and shear heat:
Optimize screw speed: Excessively high screw speeds can generate significant shear heat, leading to localized temperature increases and degradation. While ensuring uniform plasticization and plasticization time, use a low screw speed whenever possible. This is especially important for high-viscosity or shear-sensitive materials.
Optimize injection speed: Excessively high injection speeds can generate extremely high shear rates and shear heat at the gate and runner constrictions, leading to localized material degradation. Mold flow analysis or process testing should be conducted to find an injection speed profile that quickly fills the cavity without generating excessive shear heat. Multi-stage injection is often used, with appropriate speed reduction near the gate.
Optimizing back pressure: Appropriate, stable back pressure helps compact the melt, remove gas, and stabilize plasticization. However, excessive back pressure increases screw rotational resistance and significantly increases shear heat. While ensuring plasticization quality, use the lowest possible back pressure.
4. Ensure the material is fully dry:
Strictly adhere to drying requirements: For hygroscopic plastics (such as PA, PET, PC, ABS, etc.), moisture can cause hydrolysis and degradation at high temperatures. Thorough drying must be performed in strict accordance with material specifications (temperature, time, dew point).
Use a drying hopper: Continuously use a drying hopper during the injection molding process(https://www.xmrex-tech.com/customized-mold-manufacturing-and-plastic-injection-product/) to prevent the dried material from absorbing moisture again.
Check the drying system: regularly maintain the drying equipment to ensure the drying effect, the air dew point meets the standard, and there are no leaks in the pipeline.
5. Good equipment maintenance and cleaning:
Prevent molten material from stagnating: Regularly inspect and clean the barrel, screw, check ring, nozzle, and other parts. A worn screw/barrel, damaged check ring, clogged nozzle, or improper nozzle design (such as an overly long tapered nozzle) can all create stagnant areas, causing the molten material to decompose under prolonged heat and produce black spots.
Clean the hot runner system: If there are dead corners or improper design inside the hot runner system's flow channel or nozzle, molten material may be easily retained, leading to degradation. Regular cleaning and maintenance are required.
Remove degradation residues: When changing colors or materials during production, or when signs of degradation are found in the product, the machine must be thoroughly cleaned to remove all residues in the barrel.
6. Optimize mold design(https://www.xmrex-tech.com/services/) (reduce shear and overheating):
Reasonable gate size and location: A gate that is too small will force the melt to pass through at a very high speed, causing severe shear. The gate location should facilitate smooth filling of the melt and avoid weld lines appearing in high-stress areas.
Good venting: Poor mold venting can lead to trapped air, which is then compressed violently and generates high temperatures (the "diesel engine effect"), causing localized scorching and degradation of the melt. Sufficient and well-placed venting slots are essential.
Flow channel design: avoid sudden changes in flow channel cross-section, keep the flow channel smooth, and reduce flow resistance.
7. Use high quality and stable raw materials:
Avoid impurities and contaminants: Impurities can be the starting point of degradation. Ensure the cleanliness of raw materials and prevent contamination during transportation, storage, and use.
Control the proportion and quality of recycled materials: Recycled materials undergo multiple thermal processes, which can reduce their stability. The proportion of recycled materials added should be strictly controlled (especially for products with high quality requirements), and the recycled materials should be clean, dry, and not excessively degraded.
Pay attention to masterbatches/additives: Certain pigments or additives may reduce the thermal stability of the material. Select masterbatches and additives that have good compatibility with the base resin and high thermal stability.
Therefore, the key to controlling the degradation of injection molding melt lies in "low temperature, fast speed and cleanliness":
Low temperature: Strictly control the melt temperature (barrel, nozzle) to avoid local overheating.
Fast: Under the premise of ensuring quality, try to shorten the residence time of the melt at high temperature (optimize the cycle, avoid downtime and retention) and reduce the high shear time (optimize the screw speed and injection speed).
Cleanliness: Keep the raw materials fully dry and ensure that there are no dead corners or degradation residues in the equipment (barrel, screw, nozzle, hot runner).