Device makers and medical tube extruders both strive for the tightest tolerances possible. Determining what practical tolerances are and defining how to verify that the tolerances have been met is key to successfully launching a new device.

The method of verifying dimensional acceptability can get in the way of moving a new product forward to commercialisation if all the parties are not using the same method in assessing quality and process capability. The root of this problem often stems from the producer (extruder) of a part using AQL (average quality level) procedures in determining what is possible and the user of the extruded part applying statistical methods and Cpk to the same results and obtaining unacceptable findings. This happens frequently and can be avoided with effective communication.

For example, in the development of an extruded 7 French (0.092 in.) tube, an extruder finds that he or she can repeatedly produce that tube holding dimensional tolerances of ±0.002 in. (0.05 mm). The extruder produces a new lot of the tube. The inspection report indicates that although the process drifted during the extrusion run, the tube dimensions stayed within the specified dimensional tolerance. The tube is accepted by the quality assurance department and sent on to the next user, who performs an incoming inspection using Cpk as the criteria for acceptance. That user also finds no nonconforming parts, but the statistical result is that the Cpk value is only 0.98, and the incoming lot of tubing is rejected. What has happened here is that two methods will invariably yield different and often unacceptable results. Good communication can avoid this problem. How?

During development, the extruder and the user need to communicate and share data. In this case they both understand that ±0.002 in. yields an acceptable product. They must also develop inspection procedures that produce the same results. Having agreed that ±0.002 in. works but does not meet the Cpk criteria, a potential impasse exists. However, the parties find that by opening up the incoming inspection tolerance to ±0.003 in., the Cpk standard is met! So the extruder agrees to a new Cpk specification, understanding that they must continue to hold ±0.002 in. at extrusion. By working together, the extruder and user successfully move the product and their relationship forward.