Stringent product quality and process capability requirements confront producers of precision medical tubing used in medical device applications. This includes final tubing physical properties, such as burst strength, elongation, and lubricity. Medical tubing must also be free from surface quality imperfections such as gels, knit lines, and melt fracture (rough surface finish). While most of these requirements can be controlled with proper process parameters, tooling design, and clean product contact surface finishes; they cannot necessarily be controlled in a closed-loop fashion. The final tubing dimensions, such as outside diameter, wall thickness, and concentricity must also comply with the specification of the tube.

Fig. 1 – Fixed center cross head die head with pin heaters located in the back of the mandrel.

Fortunately, with the use of diameter and wall thickness measurement control systems, the diameter and average wall thickness are often automatically controlled. For instance, the diameter of the tube can be controlled by automatically adjusting air pressure in the die and the average wall thickness can be controlled by automatically adjusting the puller (haul-off) speed. This leaves concentricity as the dimension has not been controlled automatically.

Typical adjustable center crosshead die heads that are used for small medical applications incorporate four die adjustment bolts that are manually “finetuned” to adjust concentricity of the tubing to avoid thin and thick sides of the tubing cross section. In many cases, an on-line ultrasonic gauge is used to display real-time tubing wall thickness and concentricity. The majority of the manual concentricity adjustments occur during “set-up” operations. This procedure can be very time consuming, and is difficult to repeat. Often, as the tubing is being produced, the die center will need to be manually adjusted over time to compensate for uncontrolled variables, such as die buildup.

Designing Automatic Die Centering

American Kuhne leveraged the die centering technology previously developed by its partner Graham Engineering, under patent number 5,674,440, for parisan side wall adjustment on blow molding mac hines, and successfully implemented automatic die centering technology for small medical tubing applications. The company is working to develop the Graham Engineering technology a step further by incorporating closed-loop control of tubing wall thickness concentricity.

The new die centering technology allows automatic die centering for uniform wall thickness distribution for crosshead dies. It includes a fixed center crosshead die head that is used to eliminate manual die centering. Instead, the die pin center location relative to the die bushing is precisely adjusted by touchscreen control of four die pin heaters. These heaters are located in the back of the mandrel of the die head where they can be heated in an uneven pattern to “flex” the die pin to control its position. (See Figure 1)

Fig. 2 – Screen shot from touchscreen control of concentricity
The pattern will then expand the mandrel on one side and move the die pin stem and pin accordingly. The die centering system is integrated with an on-line ultrasonic gauge for full closedloop control of concentricity. This involves capturing the data from the gauge controller and executing an algorithm in the programmable logic controller (PLC) to perform the process control. The operator has the ability to view graphically the current centering positions, make manual adjustments, and control automatic operation. Once the line is running, the operator merely has to flip a switch to bring the die to center. (See Figure 2)

This system has been successfully tested in American Kuhne’s laboratory both in manual and automatic closed-loop control modes running medical tubing from Pebax®, nylon, and polyurethane resins. In one experiment with 50 percent die pin heater power on one side, a medical tube with a 0.13 mm (0.005 inch) concentric wall thickness changed to 0.05 mm (0.002 inch) wall thickness on one side and 0.23 mm (0.009 inch) wall thickness on the other side. Concentricity changed from 100 percent to 25 percent in approximately one minute. The tip flexing has a very quick reaction to power inputs, but the control system is designed for stability and to react to longer term changes in concentricity so as not to over-react to short term gauge reading fluctuations, which can be caused by air bubbles or tube location within the gauge head. (See Figure 3)

Fig. 3 – Tubes with a reasonably centered wall (left), and with partial power on one side of the tip (right).
Significant benefits can be realized with this technology, such as reduced product changeover times by eliminating operator manual adjustment of die centering during set-up, which can be time consuming and difficult to accurately reproduce. Improved product quality and process capability (Cpk) is also achieved during production by maintaining concentricity with automatic closed-loop control. These benefits will result in reduced start-up times, lower defect rates, and improved overall process repeatability.

This article was written by Steve Maxson VP, Extrusion Systems, American Kuhne, Ashaway, RI. For more information, Click Here.