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 Application: Continuous single-point or multi-point thickness gaging of extruded plastic products such as pipe, tubing, profiles, sheet, wire insulation, and cable jacketing.
Problem: In order to insure product quality, extruder operators must check plastic thickness frequently during machine setup, and periodically during a product run. In the case of tubular extrusions such as pipe and cable jacket it is particularly important to verify thickness uniformity around the circumference. If a product is too thin it may fail in use, and if it is too thick the manufacturer will be wasting money on excess plastic resin. Traditionally it has
been necessary to cut samples out of the cooled product well downline from the extruder in order to mechanically measure wall thickness. Aside from the destructive nature of this type of test, it often provides information on out-of-tolerance conditions only after many feet of defective product have been run.
Online ultrasonic gaging, on the other hand, can provide continuous information on plastic wall thickness much closer to the extruder. During setup it can provide the operator with immediate information on the effects of changes in extruder settings or puller speed, and during normal operation it can provide immediate warning of out-of-tolerance conditions.
Equipment: The Panametrics-NDT Model 25MX PLUS thickness gage and MX8 Multiplexer are designed for multiple channel online measurement, accommodating up to eight multiplexed channels. The Model 35DL is also available for single-point measurement systems. A separate gage is generally recommended for each extrusion line to be measured. In addition to a digital display of measured thickness, the 25-series gages can provide High/Low alarms and RS-232 data outputs for
continuous thickness recording or process control interfacing.
Online gaging is usually done with immersion transducers, which focus a sound beam through a water column to avoid the need for direct contact with soft hot plastic. Transducers are available from 1 MHz to 30 MHz, and are selected on the basis of specific application requirements. In general, 2.25 or 5 MHz transducers are used for most plastic pipe applications with wall thickness in the 0.1 - 0.5 inch (2.5 - 12.5mm) range, and 10 MHz transducers are used for thin walled
tubing or cable jacket under 0.1 inch/2.5mm.
Some type of fixturing is always needed to hold the transducer with respect to the product being measured, and in most cases bubblers or squirters will be used to create the necessary water column between the transducers and the part. For single-point measurement, a simple bubbler or squirter will usually be mounted in proximity to the product. For multi-point measurement of tubular products such as pipe and cable jacket, we can supply a variety of radial fixtures that support
four transducers at 90-degree intervals around the circumference.
Procedure: The transducer fixture must be mounted at a point along the extrusion line where the product is free from horizontal and vertical vibration. In some cases it will be necessary to use rollers to stabilize the product. For proper signal reflection it is necessary that the sound beam enter the product normal to its surface to within approximately two-degrees, and vibration or other random motion can disrupt this alignment. The transducer fixture may be
either free standing or mounted in a cooling tank; in the former case squirters rather than bubblers will be used and some type of water recycling system will usually be preferred. Our standard squirters each require about 2 gallons (8 liters) of water per minute, while bubblers typically use less than 1 quart (1 liter) per minute.
The ultrasonic gage is normally set up to make a mode 2 (interface to first backwall echo) measurement, as seen in Figure 1. Because of the relative acoustic impedance of plastic and water, the backwall echo will always be phase reversed (inverted) with respect to the interface echo. The normal measurement will be from the positive lobe of the interface echo to the negative lobe of the backwall echo, unless curvature causes phase distortion. In some cases hot plastics will
have acoustic impedance very close to that of water, and thus the interface echo will be much smaller in amplitude than the backwall echo. In these cases, careful adjustment of the gage's automatic gain control (AGC) circuitry is necessary to obtain clear signals such as those in Figure 1.
Figure 1: Mode 2 Measurement with Adjusted AGC
For any ultrasonic thickness measurement, accuracy is dependent on proper calibration for sound velocity in the test material. Online measurement of extruded plastic is complicated by the fact that all plastics exhibit a change in sound velocity with temperature, often on the order of 10-20% per 100° F (55° C) until they approach their melting point. While 25-series gages used in extrusion applications can be calibrated for accuracies to ±0.001 inch or ±0.01mm, the actual
accuracy in a given application will depend on the care with which the velocity calibration in done. Typically, an accuracy of ±0.001 inch to ±0.005 inch or ±0.01mm to ±0.10mm may be expected.
To calibrate velocity, make a mechanical measurement of thickness at a convenient point downline from the transducer fixture, and without changing any process variables perform a standard gage velocity calibration in accordance with the procedure described in the gage-operating manual. It may be advisable to repeat this procedure several times, each time obtaining a closer approximation of the actual sound velocity at the point of measurement. Note that if different plastic
compositions are used, or if process temperatures vary by more than 10° F or 5° C , it may be necessary to recalibrate for a new sound velocity.
The data outputs of the 25MX PLUS may be used in several ways. The RS-232 thickness output may be used with an appropriately programmed computer or a variety of external dataloggers for continuous thickness recording and statistical process control. In some cases, it may also be possible to use gage outputs to direct an extruder controller for closed-loop thickness control. The High/Low alarm may also be used to call the attention of an operator to an out-of-tolerance
condition.
Note: For detailed information or specific online plastic applications, please consult us. Information is available on thickness and accuracy ranges that would be expected in specific situations, as well as recommendations on gage, transducer, and fixture selection. We can also advise potential customers on interfacing 25-series gage data outputs with particular computers or controllers.
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