molding is one of the most important processes in plastic processing, but do we still worry about "waste products" in our daily work? Maybe you have all encountered the production phenomenon of "inexplicably starting to produce scrap products, and then suddenly getting better again"? In fact, most Injection Molding defects are related to improper setting of process parameters. Continuous learning and mastering scientific process technology can’t only greatly improve the yield rate, but also serve as the basis for achieving efficient, energy-saving and intelligent manufacturing.
Today, we will do an in-depth review of the complete knowledge system from production preparation to parameter setting.
Adequate preparation is half the battle before pressing the start button of the injection molding machine. This mainly includes the following four links:
1 Plastic color matching: For products that require precise color, two methods are usually used. The first is to use color masterbatch and mix it with the raw materials at a ratio of 1%-5%; the second is to mix the raw materials with white oil (dispersant) and 1%-5% colorant. Coloring thermoset plastics is relatively simple, just mix in the pigment.
2 Plastic drying: This is a critical step! Materials such as PA, PCand ABS are easy to absorb moisture. Excessive moisture content will cause silver streaks and bubbles in the products. Be sure to fully dry according to the material characteristics so that the moisture content reaches the allowable range for molding (for example, PC requirements are less than 0.02%).
3 Insert preheating:When the product contains metal inserts, due to the large difference in shrinkage rates between plastic and metal, stress cracks are prone to occur around the inserts. For rigid materials such as PC and PS or large inserts, preheating (usually 110-130°C) is essential. The purpose of preheating is to reduce the temperature difference between the insert and the melt, reduce thermal stress, and avoid sink marks and insufficient bonding force caused by rapid cooling when the melt contacts the cold insert.
4 Selection of release agent: For products with complex structures, rational use of release agents can ensure smooth demoulding. But be careful, excessive use may affect the appearance of the product or subsequent painting.
Process parameters are direct instructions for controlling the molding process. Understanding the meaning and function of each parameter is the cornerstone of accurate machine tuning.
1 Injection pressure:the key to overcoming melt flow resistance and providing filling speed. The pressure drops gradually from the nozzle to the end of the mold cavity, and the loss may be as high as 80%. It needs to be set reasonably according to the length and structure of the flow channel.
2 Holding pressure and switching: After filling, it is used to compact the melt and compensate for shrinkage. The holding pressure is generally about 50% to 80% of the maximum injection pressure. The setting of the pressure-holding switching point is crucial - switching too early will result in insufficient feeding and shrinkage; switching too late may result in a sudden increase in pressure, causing flash or even damage to the mold. Optimizing the holding pressure curve can effectively control product shrinkage and internal stress.
3 Back pressure: the resistance encountered when the screw rotates and retreats. Appropriately increasing the back pressure can compact the melt, discharge gas, and improve the plasticizing and color mixing effects. The sensitivity of different materials to back pressure varies greatly: back pressure of heat-sensitive/low-viscosity materials such as PVC, POMand PA needs to be controlled at 0.5-3MPa, and it is easy to decompose if it exceeds 3MPa; medium-viscosity materials such as PP and ABS only require 3-8MPa, and only high-viscosity materials such as PC and glass fiber reinforcements require 8-15MPa. Excessive back pressure increases shear heat and may cause material decomposition. Glass fiber reinforced materials need to increase the back pressure (10-15MPa), and transparent parts need to accurately control the back pressure (8-12MPa or lower) to avoid product yellowing.
4 Clamping force:guarantee against melt expansion force. "Big horse-drawn carts" waste energy, while "small horse-drawn carts" lack pressure and speed. According to the formula of clamping force = cavity pressure × projected area × safety factor, scientifically select a machine with appropriate tonnage. However, it should be noted that the pressure of the melt will decrease significantly after passing through the runner. The actual required clamping force must also consider the influence of the mold runner structure and the number of cavities. Multi-cavity molds and long runner molds need to increase the clamping force appropriately.
5 Temperature control:
(1) Barrel temperature: affects plasticizing quality and melt fluidity. Segmented control is required, and precise temperature control of the feeding section, compression section, and metering section is required. At the same time, it should be noted that the nozzle temperature must be slightly lower than the maximum temperature of the barrel, otherwise it will easily lead to melt salivation.
(2) Mold temperature: directly affects cooling rate, crystallinity, appearance and dimensional stability. High mold temperature is usually beneficial to improving appearance and dimensional accuracy, especially for crystalline plastics (such as PA, POM), but it will extend the cycle. There is a need to balance appearance with productivity.
6 Speed and time: Injection speed: High-speed injection is good for reducing flow resistance and improving weld line strength, but it is easy to produce spray lines and trapped air; low-speed injection is good for exhaust and reducing flash, but may cause flow lines and insufficient filling. Multi-stage injection technology can combine the advantages of both.
7 Injection/holding/cooling time:The injection time is much shorter than the cooling time (accounting for about 10%-15% of the cycle). The holding time is based on the gate solidification as the lower limit, and the cooling time is based on the principle of ensuring that the product is fully solidified and demolded. It usually accounts for 70%-80% of the entire molding cycle and is the key to optimizing efficiency.
8 Screw related parameters:
Screw speed: affects plasticizing ability and shear heat. For heat-sensitive materials (such as PVC), low speed is appropriate.
Anti-salivation amount (loosening): Prevent the nozzle from salivating;
Amount of material (cushion): The melt reserved at the screw head plays the role of buffering and stabilizing the injection volume;
Small-diameter screws (below φ20mm) need 2-3mm and 3-5mm of padding for loosening. Large-diameter screws (φ50mm and above) need 6-12mm and 8-15mm of padding for loosening. Reasonable settings can avoid inaccurate measurement of large screws and air inclusion in small screws. In practical applications, the influence of different screw types on parameters must also be considered.
Initial settings (safety first): Start with low temperature, pressure, speed, reasonable clamping force and long time to prevent damage to the mold and equipment.
Plasticizing and mold temperature: Set according to material recommendations and check the actual temperature.
Determine the injection end point: first set it to 2/3 of the cavity volume as a safety buffer.
Screw speed and back pressure: Set to the minimum value that can complete plasticization without extending the cycle.
Injection pressure and speed: In the initial stage, the injection pressure can be gradually increased from low to high, generally starting from 50% to 70% of the system pressure, so as to observe the flow situation.
Holding pressure and time: The holding time is determined by the product wall thickness, material shrinkage rate, and gate freezing time.
Cooling and mold opening time: The cooling time is first roughly set based on the product wall thickness data. The mold opening time is generally 2-5 seconds.
Gradual filling: Gradually increase the injection volume based on the screw position, and observe the filling status through short shots.
Switch to automatic mode: ensure stable continuous process.
Optimize mold opening, closing and ejection: adopt "slow-fast-slow" mode to protect the mold and eject smoothly.
Complete filling: Adjust the injection end point to 99% full to make full use of the injection speed.
Optimize the holding pressure: Gradually increase the holding pressure until you find the lowest effective pressure that can eliminate sink marks to reduce internal stress. Find the shortest holding time: gradually shorten the holding time until the quality of the product begins to decline (sink marks appear). This critical point is the shortest effective holding time.
Find the shortest cooling time: It is necessary to ensure that the ejection has no top white, stable dimensions, and no deformation. Gradually shorten the cooling time until the product is deformed or has a high top white after demoulding, thereby determining the ultimate cooling time.
Understand the mold cavity pressure curve: comprehensively analyze the stage characteristics, pressure distribution, process parameter correlation and curve shape of the mold cavity pressure curve and find out the melt flow state during the
injection molding process.
Some products require post-processing after demoulding to improve performance:
A Annealing treatment: The product is heated in a constant temperature oven or hot liquid for a period of time and then slowly cooled. The purpose is to eliminate the internal stress generated during the molding process and prevent cracking or deformation during future use. For amorphous plastics (PC, ABS, PMMA) and products with internal stress, the processing temperature is 10~20°C higher than the product's use temperature and 10~15°C lower than the material's thermal deformation temperature, and the processing time is 1~4 hours. For example, the annealing temperature of PC products is 100~120℃and the processing time is 2~3 hours. After processing, it is slowly cooled to room temperature to avoid the generation of secondary internal stress. The specific number can be combined with the latest TDS (technical data sheet) of the material supplier.
B Humidity conditioning treatment: specially designed for nylon (PA6/66) products to eliminate internal stress and stabilize dimensions. The treatment method is to soak in hot water at a temperature of 80~90°C for 2~4 hours until the moisture absorption of the product is balanced; or use steam treatment at a temperature of 100~120°C for 1~2 hours. After treatment, it needs to be naturally cooled to room temperature, and the environmental humidity is controlled to prevent size fluctuations.
C Other surface treatments: Painted and electroplated products require surface degreasing, polishing and other treatments to remove release agent residue and surface defects.
