I must first say that lot-to-lot MFI and viscosity variation with most melt-processible polyurethane is a universal issue for extrusion engineers, making validation of an extrusion process nearly impossible and impractical. To best approach this issue, I think we need to understand it as having three parts: lot-to-lot variability in the resin manufacturer’s process, reading the extrusion process and designing tools that work.

First, the resin manufacturer’s lot-to-lot consistency is dependent upon the consistency of the ingredients that make up the resins. It is my understanding that manufacturing the ingredients of PUR resins is inherently variable, so the resin manufacturer has the challenge of monitoring the variability and doing his best in using those materials to produce consistent resins. Supplying a technical analysis of the resin with each lot is extremely helpful to the extruder and should not be ignored.

Second, reading the extrusion process is more of an art than a science, and understanding what one is working with is fundamental to producing a good tube. Therefore, as stated above, the engineer or technician first needs to read the analysis sheet, which should indicate MFI and viscosity. With any medical extrusion there will be a variety of process variables to control. Documenting these variables and learning to read the extrusion process is what extrusion companies consider to be a large segment of their proprietary knowledge. Because there is generally a degree of artistry in producing a PUR medical tube, validating or defining exact process conditions to produce an exact tube is probably impractical. However, you can allow for artistry by specifying a range of extrusion heats as well as tolerances for the other process variables. With a good technician or engineer and proper documentation, everybody learns how to read the variability of the resin and the process; in the end, tubing gets made more consistently.

Finally, the die and pin design is critical to making a good tube. However, the design of the die and pin is also dependent upon the design of the extrusion head and the design of the screw. It is important to recognize that what works for one multilumen tube may not work for another. This area of extrusion technology is highly proprietary. Someone who intuitively understands the melt flow characteristics of the resin used in the extrusion process and how to design tooling for the process is worth his or her weight in gold.