(58) Unstable injection volume Injection Molding Process Defect Handling Manual (65 types)

(1) What is unstable injection volume
It refers to the deviation between the molded products obtained from each mold.
The instability of molded products in terms of size, appearance, weight, etc. is caused by differences in molding conditions.
(1) Injection pressure
(2) Mold temperature
(3) Measurement
(4) Exhaust
The instability of the above molding conditions is the main reason.
(2) Reasons for unstable injection volume
(2-1) Insufficient pressure
The general injection molding process is injection → pressure maintaining → cooling (metering). The injection to pressure maintaining stage should be a process of pressing the melted resin through pressure. When the pressure is low, the amount of resin pressed in easily becomes unstable. There are various reasons for this lack of pressure, as follows:
• Low resin temperature
• Low mold temperature
• Injection speed is too slow
• Low pressure retaining
• Pressure holding time is too short
• VP switching position too early
• The channel parts of the main flow path, shunt path, gate, etc. are too thin, resulting in poor pressure transmission
• The resin has poor fluidity, resulting in a large pressure loss.
• There are particularly thick portions of the thickness.
(2-2) Unstable mold temperature control
When the temperature control of the mold is unstable, it is particularly prone to accompanying dimensional eccentricities or deviations. Depending on the specific situation of the mold, it is sometimes difficult to adjust the local temperature of plastic hole bolts, etc., which exacerbates the deviation.
(2-3) Measurement instability
If the metering is not stable, the amount of resin injected is also unstable. This increases the likelihood of deviations between injections. For details, please refer to the Poor Measurement section.
(2-4) Poor exhaust
When the exhaust port is weak and the exhaust is not smooth, the filling amount sometimes becomes unstable.
(3) Countermeasures for unstable injection volume
(3-1) Fully apply pressure retaining
Due to some (even local) reason, there is a possibility of instability in the actual holding pressure. Therefore, the following countermeasures should be taken. Due to the fact that the average size may be too large, it is necessary to set different standards for process management.
★ Molding conditions
• Increase resin temperature
• Increase holding pressure
• Increase mold temperature
• Extend pressure holding time
• Increase injection speed
• Delay VP switching position
★ Mold
• Enlarge the main flow path, shunt path, gate, etc
• Unify the wall thickness as much as possible. Thickness standard is 2-4
★ Materials
• Switch to materials with good fluidity
(3-2) Check the mold temperature machine
During water temperature control, please check whether the temperature control channel near the problem location is unblocked. In particular, the temperature at places such as plastic hole bolts can easily rise, so it should be controlled as much as possible. If it is an electric heater, please check the location of the heater.
(3-3) Keep metering stable
Please refer to the measurement defect countermeasures here and implement them.
(3-4) Improve exhaust conditions
Occasionally, there are situations where the exhaust port is not smooth and the size is unstable. At this point, it is necessary to reduce the injection speed or strengthen the exhaust port to ensure smooth exhaust.