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Wall Thickness Gaging in the Blow Molding Industry

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 cut 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. 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. 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). The gage measures the transmit time of a pulse of sound through a material (see Fig. 2), and 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 in.) 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 material 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 GagingTheory: 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 that no couplant is used, there is no velocity variation with temperature or other material properties, and 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 can measure up to approximately 10 mm (0.400 in.). 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 with thick walls are to be measured, the preferred method is ultrasonic. When small, thin wall (less than 0.100 in or 2.5 mm.) parts with tight corners are to be measured, Hall Effect gages such as the OlympusMagna-Mike 8600 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.

For ultrasonic measurements, any of the Olympus precision thickness gages can be used. These include models Model 38DL PLUS and 45MG with Single Element software. Both can store multiple velocity and transducer setups in the gage, making gaging of a variety of materials a simple process. M116, M208, or V260 Sonopen transducer are commonly recommended for thin walled parts. For thick walled parts use the same gages 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 high temperature delay line transducer.

Summary: It is possible to calibrate either type of gage quickly with a few simple steps. 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, eliminating the possibility if transcription errors.

Olympus IMS

Products used for this application


The 38DL PLUS is an advanced ultrasonic thickness gage. Uses dual element transducer for internal corrosion applications, and has features that include THRU-COAT technology and echo-to-echo. Uses single element transducers for very precise thickness measurements of thin, very thick, or multilayer materials.


The handheld 45MG ultrasonic thickness gage is packed with measurement features and software options. This unique instrument is compatible with the complete range of Olympus dual element and single element transducers, making this gage an all-in-one solution for virtually every thickness gage application.

Magna-Mike 8600

The Magna-Mike is a Hall effect thickness gage that uses a magnetic probe to perform accurate measurements on nonferrous and thin materials such as plastic bottles.
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