|
 Introduction: For many years, quality control for blow molded parts involved cutting them up with utility knives in order to make thickness measurement with calipers. There are a number of problems with this traditional method of testing. When a part is ct open, a burr is generally left at the cut edge. If the operator makes a measurement over the burr, it is not a
true wall measurement. Assuming that the operator is careful and avoids distorted edges, there are still limitations as to where measurements can be made with mechanical devices. Often, the part's geometry won't permit access to tight corners or handle areas on bottles. Once a part is destroyed for thickness measurements, it can't be used for most other testing. Variation in operator technique is frequently a problem. Calipers can cause errors when they are held at an
angle to the part, and when calipers are used on materials that can be compressed by jaw pressure, thickness readings will vary from one operator to another. There is a potential safety problem as well. Operators are required to section parts with utility knives several times a shift, which creates a constant possibility of serious injuries.
Two electronic methods which can reduce or eliminate all of these problems are available: ultrasonic gaging and Hall Effect gaging. Both of these methods are now commonly used in blow molding quality control. The selection of a measuring method is generally dependent on the product to be tested, and the factors involved in choosing a method is generally dependent on the product to be tested. And the factors involved in choosing a method are discussed at the end of this
note.
Ultrasonic Gaging Theory: Ultrasonic thickness gages provide an accurate, reliable, repeatable means of nondestructively measuring wall thickness from one side of the part. They work by the means of measuring the time it takes for an ultrasonic sound wave to travel through the part. An ultrasonic gage sends timed pulses of electrical energy to a piezo-electric crystal within a small probe called a transducer. When a pulse of energy hits the crystal it begins to
vibrate in the ultrasonic frequency range. A person with exceptional hearing might hear a high pitched tone at 18,000 Hz (18KHz), but the frequencies which are used for this test method are much higher, generally ranging from 1 million Hz (1 MHz) to (20 MHz). The phenomenon that causes the electrical energy to be converted to mechanical energy, or sound, is called the Piezo-Electric Effect (see key word index 1). The transducer is placed on the surface of the part to be measured and
acoustically coupled to the part using a fluid, usually glycerine, propylene glycol, or water. Air is not a good transmitter of sound at ultrasonic frequencies, so a coupling medium is required. The pulse of sound travels from the contact surface to the opposite surface, and bounces back to the transducer as an echo (see Fig. 1).
When it reaches the transducer again, the sound pulse is converted through the piezo-electric effect. The gage measures the transmit time of a pulse of sound through a material (see Fig. 2). Using the velocity of sound in the material being measured, the gage calculates the thickness of the material by the following equation.
Where, D is the thickness of the material, t is the pulse transmit time and V is the velocity if sound in the material. Since the transit time is for a round trip, the product is divided by 2. The speed of sound in most places will range from approximately 2.0 to 2.8 mm (.0800 to .1100 inches) per second.
Fig.1- The transducer is placed on the part. Sound from the transducer makes a round trip between the contact surface and the back surface.
Fig. 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: Ultrasonic gages are extremely accurate if the conditions that cause errors are understood and a few simple precautions are taken. If the gage has been properly calibrated, it will display an accurate wall thickness. The calibration process requires materials samples of known thickness. Typically, the gage will be set up on samples representing the maximum and minimum material thickness to be measured. Material sound velocity and zero offset (a
transducer-related parameter) are set by performing a simple keypad operation involving entering the known thickness of reference standards while coupled to the material. The gage uses the known thickness to calculate a sound velocity and zero offset for that material and transducer, respectively. When the gage is making thickness measurements, it uses the calibrated velocity to calculate the thickness of the product.
Advantages and Limitations: A primary advantage of ultrasonic gaging is that thickness measurements require access to only one side of the test material, permitting measurement of closed containers, large sheets, and other geometries where across access to both sides is difficult or impossible. Gages are generally hand-held and easy to use. A potential limitation is that the accuracy of measurement is only as good as the accuracy to which material and sound velocity
is known, and is there for subject to inaccuracies if material sound velocity changes unpredictably. Velocity can be affected by changes in the material's properties, which include substantial temperature shifts or variations in density. Most plastics exhibit noticeable velocity shifts as the temperature changes by more than 5º C (10º F). The easiest way to avoid temperature induced errors is to calibrate and measure at ambient temperature. If that is not possible, calibration
and measurement should be made at a known, constant position in the manufacturing process. As most standard transducers will be damaged by contact with parts hotter than approximately 50ºC (125ºF), testing at elevated temperatures is not recommended unless special transducers are used. Heavy wall products, in which the inside of the part stays hot while the outer surface cools may have large temperature variations from the outside of the part to the inside. These temperature
variations can cause substantial velocity changes through the wall of the part which in turn can introduce measurement uncertainties.
Hall Effect Gaging
Theory: The other electronic gaging method employs a phenomenon known as the Hall Effect. The Hall Effect uses a magnetic field applied at right angles to a conductor carrying a current. This combination includes a voltage in another direction. If a ferromagnetic target such as a steel ball of known mass is placed in the magnetic field and hence the induced voltage is changed. As the target is moved away from the magnet, the magnetic field and hence the induced
voltage are changed in a predictable manner. If these changes in the induced voltage are plotted, a curve can be generated which compares induced voltage to the distance of the target from the probe (see Fig. 3).
To make measurement, a hall probe is simply placed on one side of the product to be measured and a ferromagnetic target, usually a small steel target ball, is placed on the other side of the product. The gage displays the distance between the target and the probe, which is wall thickness.
Fig. 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.
Calibration: The instrument is calibrated by placing a series of shims of known thickness on the probe, placing a ball over the shims, and keying into the instrument each known thickness. The information that is keyed into the instrument during the calibration allows the gage to build a lookup table, in effect plotting a curve of voltage changes. The gage checks the measured values against the lookup table and displays thickness on a digital readout. While all of
this sounds complicated, operators only need to key in known values during calibration and let the gage do the comparing and calculating. When Hall Effect gages are used, it is not necessary operator anything about the physics that enables the measurements. The calibration process is automatic.
Advantages and Limitations: The advantages to this system are (1) that no couplant is used, (2) there is no velocity variation with temperature or other material properties, and (3) wall thickness in tightly radiused areas and in extremely thin samples can be measured. Additionally, it is often easy to scan the probe around the part to quickly verify thickness at a number of points or look for the minimum thickness in an area. The only potential limitation in blow
molded plastic applications is that it is necessary to place a target ball inside the part being measured, preventing use on closed containers (which can, however, be measured ultrasonically). The system guides the operator through the calibration process, which means that the operator does not need to know what the Hall Effect is. The system can measure up to approximately 10mm (400"). It can measure compressible materials, but the ball can compress the material and the
smallest ball possible should be used, when making these measurements. In production use an operator is able to scan an entire part within a few seconds, while storing several readings or scanning for a minimum wall. Frequently this type of unit is placed in a production area, where it is used by the molding equipment operators. This approach permits true Statistical Process Control.
Selecting a Gaging Method: There are no hard and fast rules for choosing between the two methods. In general, if large rigid parts are to be measured, the preferred method is ultrasonic. When small, thin wall (less than .100") parts with tight corners are to be measured, Hall Effect gages such as the
Panametrics - NDT Magna-Mike® 8500 are preferred. The majority of blow molding applications favor Hall Effect Gages. Most blow molders have parts with complex shapes, relatively thin, flexible walls, and corners that are difficult to measure with mechanical or ultrasonic gages. Hall Effect gages are not well suited to double wall blow molded parts. In general, it is difficult to use the target ball on double wall parts. For these applications the preferred method is
ultrasonic gaging. The latest models of ultrasonic gages, such as
Panametrics - NDT Model 35DL, make it possible to store multiple velocity and transducer setups in the gage, making gaging of a variety of materials a simple process.
Equipment: Hall Effect
Magna-Mike Model 8500
Ultrasonic
35, 35DL, or 25DL Plus with an M116 transducer is recommended for thin walled parts. For thick walled parts use the models 25 of 25DL with a lower frequency
contact transducer (M112, M110, or M109). For thickness measurements on hot plastics at temperatures in excess of 120ºF or 50ºC use a
delay line transducer.
Summary: It is possible to calibrate either type of gage quickly with a few simple steps. In the case of the Hall Effect gage the operator is guided through the calibration process by the gage. The ultrasonic gage need only be calibrated once, although periodic checks are generally recommended for in-plant operating procedures. Once calibrated, either type of gage will produce accurate, repeatable results/ Users have found that operator technique is less of a factor
with these methods than with mechanical gaging. Calibration data is stored with logged readings and provides a check of the operator's work. Both the Ultrasonic and Hall Effect gages provide datalogging capabilities. The ability to store data along with calibration information eliminates the possibility if transcription errors. As more buyers of blow molded parts require Statistical Process Control programs and ISO 9000 certification of their vendors, electronic gaging methods
that are combined with internal dataloggers will become more popular.
For further information on both ultrasonic and Hall Effect instruments,
contact Olympus NDT.
|
