Articles from June 2017

The measurement of extruded width in plastic and rubber products can have hidden benefits to process control.  With most modern extruders the material is forced into the die with a constant displacement screw or pump.  With a constant material feed into the die and proper die temperature control, there are several factors that can contribute to finished product thickness.  The most significant factor is generally the takeaway tension pulling the product away from the output of the extruder.  With greater takeaway tension the product will neck down on both the width and thickness of the extrusion.  When the extrusion material feed is constant and the die temperature is tightly controlled, the thickness of the extrusion can be precisely controlled by maintaining a constant width.  In many operations where the product dimension is much larger than the thickness, it is possible to control thickness to very tight tolerances by controlling the width using only take away tension as a control variable.

In the plastics industry, it is important to meet minimum thickness requirements.  In many extruded sheet and film products a variation of a thousandth of an inch can be very significant.  Considerable money must be invested to continuously measure the precise thickness of the extruded products using optical or radiation based measurement systems.  This tiny deviation from the ideal can easily amount to 10% or more of the raw material used in the extrusion. With a continuous process, this can add up to many thousands of dollars in as little as one month.

Scan-A-Line, from Harris-Instrument Corporation, offers a very simple and cost effective solution to controlling extrusion thickness by measuring the extruded width.  To determine the feasibility of this type of control we generally take a process capability survey for the extruder operation.  A width measurement system is first installed to record any width variation in the extrusion. While observing the width readings we watch for maximums and minimums to occur.  When we observe a maximum or minimum width reading, we mark the extrusion with a plus or minus sign.  After the process is finished, we locate our marked locations and then carefully measure the thickness of the product at these locations.  In nearly every case we can see a direct correlation between the maximum and minimum width and the maximum and minimum thickness.

By observing the period and magnitude of the variation in width, we can usually identify what is causing the width and thickness variation.  Typical sources are a bad bearing or bent roller in the takeaway transportation system, or a lack of good synchronization between the drive systems and the extruder feed.  The recorded strip chart of Scan-A-Line width measurements often serves to identify, help correct, and fine tune the line for optimum performance.  This can easily shave several percentage points off of the required raw material while maintaining in-spec thickness in every case.

A sudden step change in extruded width should not always be used to adjust tension.  This sort of sudden excess error may indicate foreign material caught in the die, a defective temperature control, or other problems that will affect the extrusion profile.  By understanding when there is likely to be a problem, the extrusion can be checked and the problem cleared long before the product reaches the quality assurance checkpoint at the end of the process.

In my recent venture out for a service call, I discovered that many of our faithful customers are unaware of recent developments in our measurement and control technology. A prime example is one of our original customers who is controlling his accumulator loop with one of our Loop Control Processing Units (LCPU) and a 10X-HD40 sensor. We had encountered a problem with a very long analog line back to the PLC that controlled the drive system for the line. The problem was quickly solved and then I began to ask some questions.

Why were they not guiding the line with Scan-A-Line? Had they ever tried to measure strip width on this line? Could there be any cost savings for using either of these applications here.?

As it turns out, the material they were pickling was a rather heavy gage material with very wavy edges. For any guide system to work, it needed to be independent of the strip pass line because of the constantly changing vertical edge positions. When I asked about using our PLI (Pass Line Independent) technology, they were totally unaware of this development.

What is Scan-A-Line’s PLI all about? As the name suggests, PLI enables edge position and width measurements that are independent of the vertical position of the strip. Our Measurement Processing Unit is using a formula from geometry to calculate the intersection of two lines of sight from two separate receivers. Because the positions for each of the receivers, and the positions along the Scan-A-Line LED array are both known, it is possible to find the view line intercepts to calculate the vertical and horizontal position of an edge. This enables a Scan-A-Line system to compensate for changes in pass line of an edge while accurately determining its “X” position across the line. This has implications for Edge and Center Guiding as well as Width Measurement of strips with poor edge shape or a constantly changing pass line.

Our original Pass Line Independent System required a special Scan-A-Line Emitter, with dual video processing circuitry. Although these systems worked very well, they were sometimes difficult to set up because of the LED balance as viewed from two different receivers. The receiver balance problem made them much more susceptible to troubles from misalignment and normal dirt buildup on the lenses.

Two years ago, we undertook a project to solve these problems and improve the PLI system performance. The project was called “The External BR” (binocular receiver) project. This led to two significant improvements to the Scan-A-Line PLI Systems, The first improvement was a method to permit a single video processing circuit to be used with both receivers in a time share arrangement. This makes it possible to use any standard Scan-A-Line Emitter for PLI service. The next improvement was in a program and circuit to permit separate storage of LED balance information for each of the receivers in the system. This improvement eliminated the problems with misalignment and stray lens contamination making the new External BR Systems as easy to install and operate as our original Scan-A-Line systems.

Rather than purchase a whole new sensor system to have PLI, it is now possible to have your existing 10XAS-series sensor upgraded to work as part of a PLI system. With the addition of a second receiver, cable, and a small external circuit to multiplex the receivers, we can turn your old Scan-A-Line sensor into the latest PLI system. The upgraded Emitter will still function as a single 10XAS-series sensor if ever needed as a spare.