What conditions should be paid attention to in injection molding
by：Vowin Rapid Prototyping2023-02-12
What conditions should be paid attention to in injection molding processing? Shanghai Mulan Injection Molding Factory said that even if PC plastic encounters very low moisture, it will be hydrolyzed, resulting in bond breaking, molecular weight reduction and physical strength reduction. Therefore, before molding and injection molding, the water content of polycarbonate should be strictly controlled below 0.02% to avoid the reduction of the mechanical strength of the molded product or the abnormal appearance of bubbles and silver streaks on the surface. In order to avoid abnormal conditions caused by moisture, polycarbonate should be dried by a hot air dryer for more than three to five hours before processing. The air should have a dew point of -30°C at the inlet of the funnel.
To meet the needs of various injection molding processes, polycarbonate is available in different melt index specifications. Usually the melt index is between 5 and 25g/10min and is suitable for injection molding. However, the injection molding processing conditions are quite different depending on the type of injection molding machine, the shape of the molded product, and the specification of polycarbonate, and should be adjusted according to the actual situation.
1. Injection molding machine selection points
Clamping pressure: 0.47 to 0.78 tons per square centimeter of projected area of the finished product (or 3 to 5 tons per square inch).
Machine size: The weight of the finished product is about 40 to 60% of the capacity of the injection molding machine. If the capacity of the machine is represented by polystyrene, it needs to be reduced by 10%. 1 ounce = 28.3 grams.
Screw: The length of the screw should be 15 diameters long, and its L/D is 20:1. The compression ratio is preferably 1.5:1 to 30:1. The stop valve at the front end of the screw should be a sliding ring type, and the resin flow gap should be 3.2MM.
Nozzle: The opening should be 4.5MM. If the weight of the finished product is more than 5.5KG, the diameter of the nozzle should be more than 9.5MM. In addition, the opening needs to be 0.5 to 1MM less than the diameter of the gate, and the shorter the section, the better, about 5MM.
2. Key points of molding conditions
Melting temperature and mold temperature: The molding temperature setting is related to many factors, such as the size of the injection molding machine, screw configuration, design of the mold and molded products, and molding cycle time. Generally speaking, in order to melt the plastic gradually, Shanghai Mulan Injection Molding Factory sets a lower temperature in the rear section of the material tube/feeding area, and a higher temperature in the front section of the material tube. However, if the screw design is improper or the L/D value is too small, reverse temperature setting is also acceptable.
In terms of mold temperature, high-temperature molds can provide better surface appearance, and the residual stress will be smaller, and it can also be filled for thinner or longer molded products. The low mold temperature can shorten the molding cycle.
Screw rotation speed: 40 to 70rpm is recommended, but it needs to be adjusted depending on the design of the machine and screw.
Injection molding pressure: In order to fill the mold as quickly as possible, the higher the injection molding pressure, the better, generally about 850 to 1400KG/CM², up to 2400KG/CM².
Back pressure: Generally, the lower the setting, the better, but in order to ensure uniform feeding, it is recommended to use 3 to 14KG/CM².
Injection molding processing speed: The injection speed has a great relationship with the gate design. When using a direct gate or an edge gate, in order to prevent the phenomenon of sunlight and wave flow marks, a slower injection speed should be used. In addition, if the thickness of the finished product is more than 5MM, in order to avoid air bubbles or depressions, slow injection will help. Generally speaking, the principle of rate of fire is that the thin ones are fast, and the thick ones are slow.
What conditions should be paid attention to in injection molding processing? Shanghai Mulan Injection Molding Factory has switched from injection molding to pressure holding. The holding pressure should be as low as possible to avoid residual stress in molded products. The residual stress can be removed or alleviated by annealing; the condition is about thirty minutes to one hour at 120 to 130°C.
(1) At the molding temperature of polycarbonate, add cleaning material and continuously shoot twenty to thirty times.
(2) Move the machine back, and continue to inject the cleaning material until the injected cleaning material starts to expand and foam.
(3) Reset the barrel temperature to 200 to 230°C.
(4) Continue to spray the cleaning material until the temperature of the cleaning material melt reaches 260°C and the surface looks clean.