Although the puller unit is located at the very end of an extrusion line, it has a tremendous impact on the stability, consistency, and quality of extruded products. Pullers, and more specifically their drive motors, play a primary role in regulating the speed at which the extrusion moves and this, in turn, affects sizing, surface finish, wall thickness, cooling, and mechanical properties.
As recently as 10 years ago, variable-speed puller drives were considered quite precise and accurate if they could regulate drive speeds within ± 1 to 2 percent. And, with all else equal, it’s still true that pullers that run at the most stable speeds tend to run a more uniform and close-tolerance product.
Typical pullers today feature a choice of vector or servo drives. Both are brushless AC drives, and both deliver a far greater degree of speed regulation and precision than the drives of even a few years back. Today, speed regulation accuracy for brushless AC motor drives is 10 to 100 times better, ranging from ± 0.1% for high-quality vector drives to ± 0.01% for quality servo drives.
In the past, it was common to choose a vector drive for “run of the mill” extrusion applications because vector drives cost substantially less and the added precision available from a servo drive did not always return enough to justify the added cost. So, servo drives were more often used only in high-value applications, such as medical tube extrusion. The situation is considerably different today, where the continued development of servo drive technology has pushed its price downward. Now, with vector and servo drives available at very similar prices, it makes sense for processors to invest in pullers with servo drives to take advantage of the added degree of precision — whether or not it is required in a particular application. This is one reason why servo drives have become the drive of choice for most new pullers.
The second reason that servo puller drives make sense today is that improved drive controls now make it much easier for extruders to actually use the added precision that the pullers offer. Not so long ago, drive controls consisted of push buttons, a potentiometer (dial), and a digital rate readout. Setting and managing very precise speeds using a dial and readout on these older controls was possible, but relatively difficult, no matter what puller drive technology you had selected.
In contrast, today’s touchscreen HMI controls enable pullers to program and manage very precise speeds with ease. These machines also offer a range of other benefits, including alarms, diagnostics, and programmable, product-specific production recipes as well as the ability to communicate with other devices, such as cutters, extruders, or plant computers to better manage production.
Still another reason why servo drives continue to gain in popularity today is that they operate over a much wider range of speeds. Compared to vector drives that offer a maximum-to-minimum speed range of about 100:1, servo drives offer a much larger range, typically in the range of 1000:1, as well as constant torque and speed regulation.
If you’ve got questions—about extrusion applications or puller drives —Conair has answers. Just send us a message or give us a call.