In general, we can assess whether the conditions of the injection molding machine, process, and mold have reached a state capable of consistently producing qualified products through six key indicators. These six key indicators involve filling time, switching pressure, residual material, screw recovery time, cycle time, and product surface temperature.
For most operations, the filling time must remain constant and not vary by more than ±0.04 seconds. It should be the same for each batch, each machine, and across different seasons, with no noticeable changes. To achieve consistent filling time, the machine’s pressure should be set higher than the pressure required by the process to prevent pressure limitations.
If the filling time is inconsistent, it may be necessary to investigate the machine as a potential source of the problem. When transferring a mold from one machine to another, theoretically, the filling time should stabilize within 10 shots (usually 3 to 5 shots). If it doesn’t stabilize, it may be necessary to identify the root cause of the problem. And that could include issues like restricted injection pressure, unreasonable residual material settings, or inefficient barrel utilization, etc.
Switching pressure refers to the peak pressure when the injection molding machine transitions from the first stage to the second stage. Before the machine reaches a stable state, there will be some fluctuations in the switching pressure within a certain range, indicating that the entire process has not yet reached stability.
When does it stabilize? What is considered a reasonable range of fluctuations? Specific values need to be determined based on factors such as the machine, product, mold, and material.
A reasonable approach is to observe it in batches of 20 shots and check if the switching pressure is continuously rising or falling. If it is, it means the machine is not yet stable. If there is only a certain range of fluctuations, it is considered normal machine variation, indicating that the process is already in a stable state.
The stability of residual material is crucial for process stability. It is generally considered stable when the fluctuation range of residual material is within ±1-1.5 mm.
If the fluctuation range of residual material is too large, the primary consideration is the possibility of wear and leakage issues in the check ring, which may require inspection or replacement of the three major components.
Ideally, the difference in screw recovery time between cycles should be within a 2% range.
If the difference is too large, the primary consideration should be whether the temperature setting at the feed throat is within the recommended range for the material.
Without a doubt, the prerequisite for process stability is the stability of the cycle time. There are two specific allowable ranges for cycle time fluctuations.
(1) For processes with a frozen gate (meaning the gate has solidified after the removal of pack and hold pressure), the allowable range for cycle time fluctuations is ±1-1.5%.
(2) For processes with a non-frozen gate (meaning the gate has not solidified after the removal of pack and hold pressure, typically done to reduce internal stress), the allowable range for cycle time fluctuations is 0%.
Here, the product surface temperature refers to the temperature measured with an infrared thermometer after the product has been removed from the mold.
When the process is stable, the product’s surface temperature should also be stable. It is generally considered acceptable to have a fluctuation range of ±5ºC or less, with smaller fluctuations being preferable. If it exceeds this range, it indicates that the process is not yet stable.
The stability of the production process can be determined through six key indicators: filling time, switching pressure, residual material, screw recovery time, cycle time, and product surface temperature. The recommended fluctuation ranges for these indicators are as follows:
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