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.