I’ve been in this business 30-plus years and continue to be amazed at the pace at which tubing extrusion technology is advancing in several areas: the ability to run extremely stable extrusion lines that offer not only much tighter tolerances, but also faster production rates, and greatly improved operator productivity. It wasn’t so many years ago that a processor needed one operator for every one or two extrusion lines. Today, we see processors running as many as 12 lines with just one operator.
How is this possible? What accounts for these huge differences in extruder productivity? The short answer, beyond the extruder itself, is that downstream extrusion equipment, measurement devices, and controls are dramatically more precise, accurate, and capable than ever before—up to 100 times better than just a decade ago. The digital control and feedback loops that link these elements enable them to interact with amazing speed and precision, starting as the extrudate leaves the extruder, continuing through gaging, cooling tanks, and quality measurements on down the line. At every point in the process, process adjustments that used to require significant operator attention and interaction are now are made automatically to keep high-quality, end-product running. Here are a few examples:
Controlling tubing diameter. One typical control loop involves a multi-axis laser gauge that continuously measures the diameter of a hollow profile and compares it to a process setpoint. If the diameter is undersize, the controller can signal the vacuum tank to slightly increase the vacuum level so that the profile expands to the correct diameter (and vice versa for oversized tubing.) While this is the same type of correction that extruders have always made, digital technology makes it vastly more controllable. On older vacuum tanks, processors struggled to hold vacuum levels within ½ inch of mercury. Today’s digital controls can hold vacuum levels to within 1/10th an inch of mercury – roughly a 68x improvement.
Controlling wall thickness. To track wall thickness, processors may use an ultrasonic gauge that continuously bounces a measurement signal off the OD and ID of the profile, computing the wall thickness. If the wall is thinning, the gauge can signal the puller to slow down slightly. Again, this is the same sort of correction that extruders have made for years. But today’s digitally controlled servo drives make far more subtle and precise speed changes than old transducer-driven drives. Today’s servo drives achieve speed regulation accuracy up to ± 0.01% – a 100x improvement over what was possible even 10 years ago.
Regulating speed and distance. Precise digital logic, together with the speed and accuracy of servo drives, also allows extrusion lines unprecedented precision in speed and distance measurement. Let’s say that an ultrasonic measuring head is positioned down the line, perhaps 30 feet from the vacuum tank. If your line is running at 60 ft. per minute, and that measuring head has just signaled the puller for a line-speed adjustment to correct wall thickness, your system’s speed/distance logic is going to know to wait about 30 seconds to see the impact of the correction before attempting any further adjustment. Meanwhile, wall-thickness measurement continues so that if a length of profile somehow goes out of spec, it will be tracked and signals sent to a downstream cutter, which will cut out and reject that length. Accuracy like this was flat-out impossible with old analog-controlled drives.
The big idea behind all of these control improvements is greater long-term process stability: More and more automated quality adjustments are made downstream, more subtly and accurately than ever, without the need to touch the most sensitive element of the process—the extruder itself. As any line setup crew knows, that’s the first element to “get right” and the last one that anyone should be adjusting during the production process.
Advances like these, driven further by the ingenuity of processors, are why extrusion lines are running faster, delivering more and better quality product, and requiring less operator intervention than ever before. And, if the past is any guide, we’ll be joking a few years from now about how these improvements pale in comparison to the advances that lie ahead.