Abstract

In-line ultrasonic measurement of wall thickness has been for many years a common quality-monitoring tool in medical tubing extrusion, most commonly for medical applications where documented quality is essential. Recently this measurement technique has been applied to wire and cable jacketing, and the significant material savings achieved demonstrate it can provide an attractive return on investment. This valuable tool can also be applied to many of the key constructions in fibre optics technology, including tight-buffer, loose-tube, and cable jacketing. This article reviews the fundamentals of this measurement technique, the benefits to be expected from its use, and various extrusion applications.

Introduction

Ultrasonic wall thickness measurement uniquely permits measuring wall thickness at the beginning of the extrusion line, which allows for optimum die centring by an operator. The real-time display of the wall about the core immediately shows the results of the centring effort, so that subsequent changes can be made as needed to further optimise wall balance. Without this capability, the manufacturer is dependent on sample checking at the end of the line. Quite often, a manual change to the die effects an over-correction and begins a process of multiple changes which, though time-consuming, still yield less than optimum result. Understandably, the operator may eventually decide to “leave well enough alone”, to the detriment of quality.

Beyond optimum die centring, measurement at the die also promotes a much faster control response when the system is set to control average or minimum wall thickness. Of course, continuous wall thickness measurement also permits detailed quality documentation as to other parameters.

Ultrasonic measurement basics

The basics of ultrasonic measurement (Figure 1) are readily understood. An ultrasonic transducer contains a piezo-electric crystal which, when hit with a voltage pulse of very short duration, causes a sound wave to be created at the face of the transducer. Depending on the shape of the transducer lens, a shaped sound wave emanates from the face of the transducer, through water, to the surface of the product to be measured. A change in the speed of sound in water occurs when a material with a different acoustic impedance from water is encountered. This change causes a reflection back towards the transducer, resulting in an interface “echo.” A portion of the sound energy continues on into the material. When a subsequent change in speed is encountered, a second echo is reflected back to the transducer.

Figure 1: Ultrasonic measurement technique

An ultrasonic processor looks for these reflections (echoes) and correlates the distance (t) between them to a wall thickness. The quality or clarity of the echo is a function of the strength of the sound wave to begin with, the degree of difference in acoustic impedance between the two materials, and the alignment of the transducer to the reflecting surface. Transducers are generally very rugged, relatively low-cost devices with long life expectancy. Their low cost facilitates using multiple transducers for applications, which require multiple points of wall thickness measurement. A wide range of transducer constructions is readily available for customisation to the product application as to wall thickness, resolution required, and target (product) size.

Use of this technology calls for product immersion in water. Unlike the low-frequency ultrasonic transducers used for camera distance measurement, these applications require frequencies in the 10 to 30MHz range which dissipate in air. They therefore require water as a transmission medium. The range of transducer types in routine use allows measurement of thickness from 0.02 to 50mm and beyond. Repeatability is on the order of ±0.001mm.

Applications

1. Tight-buffer
Application of ultrasonic wall thickness measurement to a tight-buffer product is quite straightforward (Figure 2). The relatively low line speed, line stability, and PVC-coated fibre interface make for a very stable installation. Measurement is done as close to the crosshead as mechanically practical (Figure 4). Great precision can be achieved in measurement of a 0.325mm wall thickness. A small, four-point sensor assembly, similar to that shown in Figure 3, would be used, often with guides to assure product centring within the four-point measurement field.

Figure 2: Tight-buffer

Figure 3: Four-point ultrasonic sensor assembly

The sensor assembly shown has provision to raise and lower the assembly as needed to facilitate product centring. As with all ultrasonic measurement, the sensor requires submersion to a point approximately 25.4mm above the tight-buffer.

Figure 4: Instrumentation on tight-buffer line

2. Loose-tube
Loose-tube production presents more of a challenge for ultrasonic measurement. First, the wall interface between the PBT tube material and the gel filling does not provide as strong an interface as with other materials. This necessitates taking some care initially to assure that the processor is appropriately set-up to distinguish this interface. Secondly, it is common practice for the product here to be cooled gradually.

Figure 5: Loose-tube construction

The first 10 to 20 feet of “cooling” trough may in fact employ a hot-water quench of approximately 57°C. Subsequent cooling trough sections will have progressively cooler water. Standard ultrasonic transducers are rated at 49°C and therefore are not appropriate for continuous submersion in this hotter water. This can be circumvented by positioning the ultrasonic assembly in the second section of the cooling trough. While not ideal from the standpoint of ease in die centring, it is still a reasonable trade-off. Alternatively, high-temperature transducers may be used that are suitable for continuous immersion in the hotter water.

Figure 6: Instrumentation on a loose-tube line

3. Jacketing
Fibre optic jacketings cover a broad range of sizes and applications but all generally lend themselves to wall thickness measurement. The outer jacket material (Figure 7) may be applied over a host of surfaces, including metal and other blocking materials.

Figure 7: Jacketed cable

The size range of the cable to be produced will dictate the size of the sensor assembly employed and the number of measurement points: four, six, or eight (Figure 8). A fixed sensor assembly as depicted in Figure 5 is positioned just after the crosshead in the first cooling trough.

Figure 8: Four-point cable sensor assembly

Figure 9: Instrumentation on a jacketing line

Again, provision must be made for assuring water submersion to a point just above the face of the lowest transducer. Note that, in this application, the measurement field for each transducer is quite wide, enabling product position freedom without compromising the measurement. This sensor assembly (Figure 10) can be mounted in an existing cooling trough, or the trough modified to accommodate it. Alternatively, a custom trough section with sensor assembly may be supplied for positioning at the entry to an existing trough.

Figure 10: Custom tank with sensor assembly

Benefits

Once in-line wall thickness measurement is employed in these applications, all the traditional benefits of gauging will follow. Typically the wall measurement function is added to a more common host processor that provides for the primary instrumentation, including diameter measurement and fault detection. Wall measurements and corresponding data collection simply become integrated with these more common measurements. SPC control charts, SQC quality records, and data archiving are among the reporting capabilities. Standard trending and tolerance alarms are also available to the user. Possibly the chief benefit is more uniform die centring. What was once a tedious chore becomes an almost effortless accomplishment. Thickness control of average or minimum wall thicknesses can be implemented, and significant material savings readily achieved for jacketing applications.

Conclusion

Ultrasonic wall thickness measurement has proven itself in hundreds of applications in both the tubing extrusion and wire and cable markets. It can also contribute importantly to the quality production and reporting needs of the fibre optic industry, while at the same time delivering very attractive returns on investment.

Zumbach Electronic AG
P.O. Box - CH-2552 Orpund
Switzerland
Fax: Int’l +41 32 356 04 30
E-mail: [email protected]
Website: www.zumbach.com