Machine adjustment method when filling of each cavity is seriously unbalanced
When filling is severely uneven across cavities, the primary approach is to optimize the injection rate curve. By adjusting the injection speed in stages, the melt flow in each cavity becomes more consistent. For cavities that fill too quickly, the injection speed can be reduced during the corresponding melt flow phase, for example, from 80 mm/s to 50 mm/s, to reduce the amount of melt pre-filling within the cavity. For cavities that fill too slowly, the injection speed can be increased during the corresponding phase, from 50 mm/s to 70 mm/s, to accelerate the melt filling process. During this adjustment process, the injection molding machine’s multi-stage injection function can be used to divide the entire injection process into 3-5 stages, each corresponding to a different cavity filling area. For example, the first stage (0-20% fill) targets the cavity closest to the sprue, the second stage (20%-60%) targets the middle cavity, and the third stage (60%-100%) targets the farthest cavity. By precisely controlling the speed of each stage, the filling time difference between all cavities can be reduced to less than 0.5 seconds. At the same time, real-time pressure monitoring is required to ensure that the maximum injection pressure difference of each mold cavity does not exceed 10%, so as to avoid flash caused by excessively high pressure or insufficient filling caused by too low pressure.
Adjusting the gate size for each cavity is an effective way to address uneven filling. By varying the gate width, thickness, or length, the melt flow rate into each cavity can be adjusted. For cavities that fill too quickly, the gate size can be appropriately reduced. For example, the gate width can be reduced from 3mm to 2.5mm, and the thickness from 1mm to 0.8mm to increase the melt flow resistance into the cavity. For cavities that fill too slowly, the gate size can be increased from 3mm to 3.5mm in width and from 1mm to 1.2mm in thickness to reduce flow resistance. When adjusting the gate size, adhere to the principle of “small adjustments and gradual verification,” with each adjustment not exceeding 0.2mm to avoid causing new imbalances due to excessive adjustments. For example, in a mold with eight cavities, cavities 3 and 5 filled 1.2 seconds faster and 1 second slower than the other cavities, respectively. By reducing the gate thickness of cavity 3 from 1mm to 0.9mm and increasing the gate thickness of cavity 5 from 1mm to 1.1mm, the filling time difference was reduced to 0.3 seconds after the mold trial. Furthermore, if the mold design allows, an adjustable throttle valve can be installed at the gate. Rotating the valve stem changes the effective cross-sectional area of the gate, achieving more flexible flow control. This is particularly suitable for molds used in small-batch, high-mix production.
Adjusting the mold temperature distribution is a key method for balancing the filling conditions of each cavity. By varying the temperature of different cavity zones, the melt’s fluidity within each cavity is adjusted. For cavities that fill too slowly, the corresponding cavity temperature can be increased, for example, from 60°C to 75°C, to reduce melt viscosity and accelerate flow. For cavities that fill too quickly, the cavity temperature can be lowered from 60°C to 50°C to increase melt viscosity and slow flow. Mold temperature adjustment can be achieved through a zoned temperature control system. Each cavity is equipped with an independent heater and thermocouple, achieving a temperature control accuracy of ±1°C, ensuring that the temperature difference between cavities does not exceed 5°C. For example, in a four-cavity mold, cavity 2 was found to be filling slowly due to a low temperature of 52°C. Raising its temperature to 65°C reduced the filling time of this cavity from 8 seconds to 6.5 seconds, and the difference with other cavities decreased from 2 seconds to 0.5 seconds. At the same time, attention should be paid to the temperature balance of the hot runner system. If a hot runner is used, it is necessary to ensure that the temperature of each branch runner is consistent, and the temperature difference does not exceed 3°C to avoid differences in melt fluidity caused by uneven hot runner temperature.
Optimizing holding parameters can effectively alleviate dimensional discrepancies caused by uneven filling of each cavity. By adjusting the holding pressure and holding time, differences in filling volume between cavities can be compensated. For underfilled cavities, the corresponding holding pressure needs to be increased and the holding time extended. For example, increasing the holding pressure from 80MPa to 90MPa and extending the holding time from 5 seconds to 7 seconds allows more melt to enter the cavity to compensate for shrinkage. For overfilled cavities, reducing the holding pressure to 70MPa and shortening the holding time to 4 seconds reduces the amount of excess melt filling. Adjustment of the holding parameters needs to be combined with cavity filling feedback. This can be achieved by installing a pressure sensor on the mold to monitor the holding pressure curve of each cavity in real time. When the holding pressure of a cavity is more than 5% below the average, the holding parameters for that cavity need to be specifically increased. For example, in a six-cavity mold, the holding pressure in cavity 4 was 8% lower than average. By increasing its holding pressure from 85 MPa to 93 MPa, the part weight in this cavity increased from 25 g to 26.5 g, and the difference from the average dropped from -2 g to -0.5 g. Furthermore, the setting of the holding switch point is crucial. Ensure that all cavities reach at least 95% fill before switching to the holding stage to avoid underfill caused by premature holding.
If the above adjustments still fail to resolve uneven filling, the mold’s mechanical structure should be inspected to eliminate any potential causes of filling discrepancies, and targeted repairs should be performed. First, check the gates of each cavity for blockage or wear. If a cavity’s gate is clogged with foreign matter, clean it. If the gate is worn and enlarged, replace the gate insert to restore the size. Next, check the mold’s venting system. If filling is hindered in a cavity due to poor venting, clean the vent groove or increase the vent depth (no more than 0.03mm) to ensure smooth gas discharge. For example, in a four-cavity mold, cavity 1 filled slowly due to a clogged vent groove. Cleaning the vent groove reduced the filling time for that cavity from 9 seconds to 6 seconds. Additionally, check the fit of the guide pins and bushings. If uneven mold closing results in excessive clearance in a cavity, regrind the guide pins and bushings to ensure consistent clearance across all cavities (with a tolerance of no more than 0.02mm). By inspecting and repairing the mechanical structure, the problem of uneven filling caused by mold problems can be fundamentally solved. By optimizing the machine parameters, the filling time difference of each mold cavity can be controlled within 0.3 seconds, and the weight difference of the plastic parts cannot exceed 1%.
