One normally assumes that products offered by different vendors in a given field are more or less the same and differ only in details. This is definitely not true in the fields of tubing and profile extrusion. Harrel offers a completely different product!
The difference is CONTROL! Harrel is the only extruder systems manufacturer that was not originally a machine shop. We came out of the controls field, and, although we today offer a very complete line of mechanical products, we freely admit that we are still control freaks!
This has profound implications in the practical application of extrusion systems. Consider some examples of how different lines are started up:
Result:In 3-5 minutes, the tubing will be on spec -- OD, ID, and length at the designated set point values -- and all without any further intervention by the operator. Furthermore, it will stay in spec hour after hour, day after day without operator intervention, even as the extruder warms up, the polymer changes, line voltage varies, etc.
OTHER SYSTEMS:With other systems, the operator does not enter the final dimensions; he enters assumed values of extrusion parameters -- line speed, screw speed, air pressure (or vacuum) and other machine parameters. He then makes a sample tube and measures it. Then he enters new values of the extruder parameters, makes another sample, measures it, enters new values of extrusion parameters, makes a new sample, etc., etc., etc. until the tubing is finally in spec. It typically takes an hour or so, if the operator is skilled. And of course, as the machine warms up, or the polymer changes, or line voltage varies, or whatever, he has it all to do over again.
To be fair, some systems do bring the OD under closed loop control. But what of the ID? It's just as important. If you pound the table hard enough, some vendors will furnish an ultrasonic gauge, but this is difficult to use, and it is inherently of far lower accuracy than the Harrel TUBETROL system.
Harrel systems: No essential difference, except that here you have to set into the DIGIPANEL two values of OD and two values of ID -- one set for the bump and the other for the stem. Push a button, and in parhaps 7-8 minutes the tubing is on spec -- both OD's and both ID's, plus the length!
Other systems:Here's where things really get an Alice-in-Wonderland touch. Several vendors offer very attractive consoles, complete with color displays and elaborate controls. What is not stated, however, is that these controls are simply very elaborate switching devices. There is no closed loop control of OD and ID. They switch line speed at specified intervals. In some cases, air is also switched, as it should be. In other cases it is ignored, although it is is really just as important as line speed.
In using these systems, the operator must set in and execute a trial-and-error procedure for not just two but four parameters -- line speed for bump and stem and air for bump and stem. All of these are heavily interacting, for line speed affects both OD and ID, and so does air pressure. Further, there is nothing to keep the air in the bump from leaking into the stem and vice versa. It can typically take as long as 8 hours for even a skilled operator to sort it all out and get on target. And of course, every time any conditions change, he has it all to do over again!
Harrel's principle of Total Control is very simple:
And furthermore...
Full color schematic displays -- easy to use, even for inexperienced operators!
