Measuring Wall Thickness in Blow Molding

By AZoM

Table of Contents

Introduction
Ultrasonic Gaging and Hall Effect Gaging
Ultrasonic Gaging
Calibration of Ultrasonic Gages
Merits and Demerits of Ultrasonic Gages
Hall Effect Gaging
Calibration of Hall Effect Gages
Merits and Demerits of Hall Effect Gages
Conclusion
About Olympus NDT

Introduction

During quality control of blow molded parts, utility knives are generally used to cut them up so that thickness measurement can be made with calipers. This conventional technique of testing presents a number of issues because when a part is cut, a burr is usually left at the edge of the cut. When measurement is made over the burr, it does not provide the actual wall measurement. In addition, when calipers are held at a particular angle to the part, they can induce errors. Also, thickness readings differ from one operator to another when calipers are utilized on compressible materials. Safety is also a constant issue as operators have to use utility knives to cut the molded parts.

Ultrasonic Gaging and Hall Effect Gaging

Ultrasonic gaging and Hall Effect gaging are two electronic techniques, which help in minimizing or eliminating all these issues. These techniques are now employed in quality control for blow molding.

Ultrasonic Gaging

Ultrasonic gaging is a nondestructive method and offers a precise and reliable way of determining wall thickness from one section of the part. Ultrasonic thickness gage is one such instrument that determines the time taken for an ultrasonic sound wave to traverse the part. Using water, glycerine, or propylene glycol, the transducer is joined to the surface of the part to be determined. The sound pulse passes from the contact surface to the opposite surface and again travels back to the transducer in the form of echo.

Figure 1. The transducer is placed on the part. Sound from the transducer makes a round trip between the contact surface and the back surface.

The gage determines the transmit time taken by a pulse of sound to travel via a material and then uses the sound velocity in the material being measured to calculate its thickness.

Figure 2. The initial pulse represents sound entering the part. The backwall echo represents sound returning from the opposite surface. "t" is the time of flight if the pulse of sound. Mode 1 refers to the measurement method which used the initial pulse and the backwall echo to determine thickness.

Calibration of Ultrasonic Gages

Upon proper calibration, the ultrasonic gage shows a precise wall thickness. Material samples of known thickness are required for the calibration process. Generally, the gage is placed on samples denoting the maximum and minimum material thickness to be determined.

Merits and Demerits of Ultrasonic Gages

Generally, gages are portable and user-friendly. The main benefit of ultrasonic gaging is that thickness measurements need access to just one part of the test material. This enables measurement of large sheets and closed container where access to both sides is quite difficult. However, one major drawback is that correct measurement can be obtained as long as the accuracy of the material and sound velocity is known, and may result in inaccuracies if material sound velocity changes randomly. Variations in the material's properties can affect the velocity; for instance, significant temperature changes in density. One technique to eliminate errors caused by temperature is to calibrate and calculate at ambient temperature. In heavy wall products, the interior surface remains hot while the exterior surface stays cools. Such parts may exhibit huge temperature changes from the exterior surface to the interior part. These temperature changes can result in significant velocity variations across the wall of the part which in turn can cause measurement ambiguities.

Hall Effect Gaging

The second technique uses a phenomenon dubbed as the Hall Effect gaging. In this technique, a magnetic field is applied at right angles to a conductor that carries a current and this comprises a voltage in another direction. When a steel ball of known mass is positioned in the magnetic field, the induced voltage is altered. When the target is shifted from the magnet, the magnetic field and the induced voltage are altered in an expected manner. By plotting these variations in the induced voltage, a curve can be produced which in turn makes a comparison between the induced voltage and the distance of the target from the probe.

Figure 3. A target ball is placed on one side of a part to be measured. The probe is placed on the opposite of the part and the ball is attracted to the probe.

In order to make measurement, a hall probe is positioned on one section of the product that is to be measured, and a tiny steel target ball is positioned on the other part of the product. The distance between the probe and the target is displayed by the gage.

Calibration of Hall Effect Gages

During calibration, operators have to simply key in the known values and the gage will do the comparison and calculation. When Hall Effect gages are employed, the calibration process is done automatically and makes it easy for operators to perform the measurements.

Merits and Demerits of Hall Effect Gages

In this system, couplant is not employed and there is no velocity change with respect to material properties or temperature. Wall thickness in very thin samples can be determined. For measuring parts that have small and thin wall below 2.5 mm, Hall Effect gages such as the Olympus NDT Magna-Mike 8600 and Magna-Mike 8500 are an ideal choice. However, ultrasonic is the preferred technique for measuring huge parts with thick walls. Olympus NDT precision thickness gages include Model 45MG with Single Element software, 38DL PLUS and 35DL are suitable for this purpose. In most blow molding applications, Hall Effect gages are generally employed.

Conclusion

The Ultrasonic and Hall Effect gages can be calibrated with a few simple steps. After calibration, these gages generate precise and repeatable results. They have datalogging capabilities and thus remove potential transcription errors.

About Olympus NDT

Olympus NDT is a world-leading manufacturer of innovative nondestructive testing instruments that are used in industrial and research applications ranging from aerospace, power generation, petrochemical, civil infrastructure and automotive to consumer products.

Leading edge testing technologies include:

  • Ultrasound
  • Ultrasound phased array
  • Eddy current
  • Eddy current arra
  • and X-ray fluorescence.

Olympus NDT products include:

  • Flaw detectors
  • Thickness gages
  • In-line systems
  • Automated systems
  • Industrial scanners
  • Pulser-receivers
  • Probes
  • Transducers
  • and various accessories.

Olympus NDT is also a distributor of remote visual inspection instruments and high speed video cameras in the Americas.

This information has been sourced, reviewed and adapted from materials provided by Olympus NDT.

For more information on this source, please visit Olympus NDT.

Date Added: Dec 27, 2012 | Updated: Jun 11, 2013
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