Center ground: Why the next generation of automatic die centering technology could help to minimize die drool
Graham Engineering Corporation explains why the next generation of automatic die centering technology could help to minimize die drool.
Graham Engineering Corporation has completed testing of its American Kuhne Automatic Die Centering (ADC) technology with PA12 and Pebax polymers. Previously ADC has been used successfully in the field with high viscosity polymers such as polyolefins, thermoplastic elastomers and bioresorbables such as PLLA.
In addition, ADC is now available on XC300 Navigator, the company’s new PC-based extrusion touchscreen control system. Navigator leverages the multi-core PC computing power without the need for a PLC. This allows Graham Engineering to cost-effectively retrofit ADC technology into existing tubing extrusion lines that include an ultrasonic wall thickness measurement system.
How it works
ADC allows for touchscreen based manual or automatic adjustment of tubing concentricity as shown on Figure 1. The die head includes four low wattage pin heaters located in the rear mandrel of the die head where they can be heated in an uneven pattern to ‘flex’ the die pin position relative to the die bushing. The integrated touchscreen system allows the operator the ability to graphically view the current centering positions, make manual adjustments and control automatic operation. Once the line is running the operator hits a button to bring the die to center. The system reacts to adjust to the power levels of pin heaters in the mandrel to flex the mandrel and precisely adjust the pin location relative to the die. The system is integrated with an on-line ultrasonic gauge for full closed-loop control of concentricity (wall thickness variation). This involves capturing the data from the gauge controller into the PC to perform the process control. Concentricity levels of 97% and higher can be quickly reached, levels that cannot be achieved by the traditional and time-consuming method of manually adjusting four die bolts. The system also allows the operator the ability to make adjustments to compensate for the tubing twisting as it moves through the ultrasonic wall gauge.
The importance of uniform wall thickness
ADC technology is ideal for stringent micro tubing applications where the physical properties of the polymers are being pushed to the limits such as thin-walled PA12 and PA12/Pebax blend tubing for angioplasty and stent delivery balloons, and bioresorbable PLLA tubing for stent scaffold applications. Thin wall PA12 balloon tubing with pressure requirements up to 30atm (450 psi) often require concentricity levels of > 95% across the length of the balloon tubing. Any thin spots in the wall thickness result in mechanical failure points. For bioresorbable stent applications ultra-uniform wall thickness control is critical because the tubing lumen accommodates fluid flow and any variations in wall thickness (particularly on the higher side) can cause increasingly turbulent flow that can lead to adverse clinical events such as thrombosis in vascular applications.
Long runs plagued by die drool
ADC technology is also beneficial for long runs of single-lumen medical tubing from PE, TPE, and TPU for fluid transfer applications. In the production environment, lot-to-lot variations in the raw material do occur. Most modern extrusion lines compensate for drifts in the process by incorporating sophisticated inline gauging systems that include closed loop control of OD tolerances and in some cases wall thickness tolerances. Today’s gauging systems include high measurement rates (4000+/second) with up to 4 axis OD micrometers and up to 8 point wall measurement systems. ADC adds a new dimension to extrusion technology by enabling tight control of tubing concentricity. However, one of the limiting factors related to long tubing runs is an interruption in production from die drool.
Die drool occurs during the extrusion of polyolefins, polyamides, thermoplastic elastomer (TPE), thermoplastic polyurethane (TPU), and most radiopaque filled polymers resulting in an unwanted build-up of low molecular weight fragments on the exit face of the die. One of the compounds where die drool is recognized as a chronic problem is aromatic TPU with 20% barium sulfate in the 85A to 55D durometer range. In some cases, a savvy extrusion technician can scrape the accumulated material from the die exit region. Die drool can also be found on the inside surface of the tubing as shown on Figure 2, which makes manually scrapping the drool mass from the die virtually impossible. The die deposit (internal or external) will eventually release from the die face and affect the quality and appearance of the extruded tubing. Invariably the extrusion line has to be shut down to clean the die deposit, which is costly and time consuming. In order to fully take advantage of ADC for long run tubing applications, further work must be done to minimize the frequency of die drool.
Die drool is a complex problem with many interrelated mechanisms, but there are a few extrusion parameters that can be studied in an attempt to minimize build up on the outer die surface. These include pressure fluctuations (surging) in the screw, moisture levels, die tooling coatings, and die exit angles.
Effect of die swell and screw surging
For unfilled polymers, die swell has a large influence on die drool build-up. Both are proportional to extruder output rate or the velocity of the melted polymer through the die. Die swell is the release of normal stresses at the exit. Figure 3 shows where the swell of the polymer melt is excessive and impinges on the outer die face. With increasing output rates, there is a corresponding increase in shear stress at the die exit land as shown on Figure 4 that enhances die swell and die drool build-up. This problem is exacerbated if there are pressure fluctuations (surging) in the screw, as shown on Figure 3, which can originate from lot-to-lot variations in the raw material. When a severe surge occurs the swell can make the extrudate expand so rapidly that it contacts the exterior of the die face leaving a buildup. The amount of die swell is largest with zero land length die tooling. Increasing the die land length as shown on Figure 5 reduces die swell and, as a result, die drool; however, the drawback is increased die head pressures.
Some experiments have been performed that show reduced die drool as the surface energy between the die-wall and the polymer decreases. The addition of fluoropolymer or silicone slip additives compounded with the base polymer can help by depositing a thin layer of the low friction additive to the internal die surfaces during processing, thereby reducing the surface energy between the additive and the polymer interface. However, this may not be a practical solution for many processors. Coating the die surfaces with fluoropolymers such as PTFE with low-friction properties has shown promise, but they do not last very long during tooling handling and cleaning. Ongoing studies will include the use of low friction metallic coatings on the die land length that are well-known in extrusion for their ability to eliminate stick-slip.
Modified die exit geometries
Some experiments have been performed in academia to analyze the effect of die exit angles on die drool intensity during extrusion. This includes chamfering, flaring, and sharp edges. These designs in addition to increasing die land lengths will be studied as part of the comprehensive understand the role of die design on die drool phenomenon
Video link: https://www.youtube.com/watch?v=ysg56dz1zc0&t=1s