Meaning
Statistical limits define the maximum and minimum values that a process variable or material property can take before it is considered out of spec. A quality control threshold is set based on the requirements of the final application and the known capability of the production equipment. These boundaries ensure that every part produced meets the necessary safety and performance standards.
Acceptable Variation
Production consistency is measured by how well the process stays within the defined range of the target value. The quality control threshold for a critical dimension might be just a few microns, while for a less important feature it could be much wider. Setting these limits correctly is a balance between ensuring part quality and avoiding the costs associated with excessive scrap or unnecessary machine downtime.
Outlier Detection
Monitoring systems use these limits to identify individual parts or batches that deviate from the norm. When a measurement crosses a quality control threshold, it triggers an alarm that alerts the operator to a potential problem in the process. This immediate notification allows for a quick investigation into the cause, whether it is a blocked gate, a heater failure or a change in the resin moisture.
Process Capability
Long term analysis of the data shows how often the production line stays within its intended boundaries. A process with a high capability index will have almost all of its measurements well within the quality control threshold. This indicates that the moulding cell is stable and that the risk of producing defective parts is very low, which is a primary requirement for suppliers in the medical and automotive industries.
Maintaining this stability over thousands of cycles is the sign of a well controlled injection moulding operation.
