I've been trying to identify the industry standard approach to OQ of plastic extrusion processes, and am instead finding a wide range of opinions and practices among medical device OEMs and extrusion suppliers. It's a complicated process with a high number of inputs and a significant noise factor in the resin material property (melt flow index), so some confusion is to be expected. Some individuals recognize the importance of extrusion process validation, while others are strongly disdainful of its engineering or business value or even feasibility. Others grudgingly acknowledge the reality of interpretation and enforcement: extrusion is similar enough to injection molding that most auditors regard validation as mandatory under 21 CFR 820 / MDSAP. However, the implementation can differ significantly from an OQ of a simple process with just one set of time, temperature, and pressure variables.
Practitioners I've encountered generally understand that they have to identify the most sensitive inputs as CTQ and focus the OQ on those. Some use final (as close to the die as possible) melt temperature and haul-off (puller) speed, others may also use screw speed. Some use final melt pressure instead of temperature to compensate for variation in resin MFI. Some of these answers may be better than others, but perhaps the best answer may be different for various combinations of equipment and material.
The compromises begin to creep in when interpreting OQ limits relative to process control limits. Even simpler processes are not immune to misinterpretation. I've encountered scenarios for a variety of process types in which:
A less extreme but still debatable approach involves OQ test runs that only vary one CTQ control parameter at a time. While it's a significant improvement over the previous school of thought, it's still far less rigorous than running at the worst-case high/low input combinations. (Imagine proposing this method for a sterile pouch sealing process.) As before, the defense is typically that noise factors make it impossible to run worst-case combinations and get conforming output. I.e., the process can't really be validated at the level of control the equipment provides, the level of precision & accuracy required in the output, and the level of noise that must be compensated.
What have you observed in practice? What are the most capable and effective organizations doing? Are they treating extrusion OQ the same as other processes and running worst-case combinations of the CTQ controls? Are they making compromises relative to validation of other processes? If so, how do they rationalize them?
Practitioners I've encountered generally understand that they have to identify the most sensitive inputs as CTQ and focus the OQ on those. Some use final (as close to the die as possible) melt temperature and haul-off (puller) speed, others may also use screw speed. Some use final melt pressure instead of temperature to compensate for variation in resin MFI. Some of these answers may be better than others, but perhaps the best answer may be different for various combinations of equipment and material.
The compromises begin to creep in when interpreting OQ limits relative to process control limits. Even simpler processes are not immune to misinterpretation. I've encountered scenarios for a variety of process types in which:
- OQ is regarded as unnecessary because the process has just one "set point" for each CTQ control variable
- Calibration tolerances of the gauges are not taken into account when determining OQ limits relative to process control limits
A less extreme but still debatable approach involves OQ test runs that only vary one CTQ control parameter at a time. While it's a significant improvement over the previous school of thought, it's still far less rigorous than running at the worst-case high/low input combinations. (Imagine proposing this method for a sterile pouch sealing process.) As before, the defense is typically that noise factors make it impossible to run worst-case combinations and get conforming output. I.e., the process can't really be validated at the level of control the equipment provides, the level of precision & accuracy required in the output, and the level of noise that must be compensated.
What have you observed in practice? What are the most capable and effective organizations doing? Are they treating extrusion OQ the same as other processes and running worst-case combinations of the CTQ controls? Are they making compromises relative to validation of other processes? If so, how do they rationalize them?
