Automated In-Mould Ultrasonic Wall-Thickness Measurement (IMM)

© Rolf Diederichs


Abstract

Strict safety regulations demand extensive wall thickness measurement on plastic containers for dangerous goods and automotive gasoline tanks. At the same time increasing costs pressures are calling for rationalization efforts. Automated ultrasonic wall thickness measurement within the blow mould (IMM) is opening up new possibilities for meeting both requirements. In addition, it offers ideal preconditions for improved control of the blow moulding progress, and the possibility of optimizing the wall thickness. Details of the ultrasonic method with regard to coupling and transmission of ultrasonic signals temperatures and the digital evaluation of the measurement signal, are here described. A multi channel system , PC-based and thus with many features, makes the method user friendly and efficient. Initially the IMM blow mould application was applied . Later it was also applied to the thermoforming process, and gas-assist injection process method.

Forward from the author

This article revues already published articles [References] and allows new readers access to this, in my opinion it is excellent method. The method was investigated 1979 by the institute of plastic IKV Aachen [Bergweiler E.] investigated, and a first commercialization started in 1988. This was improved in 1991[Diederichs R.] with a new PC-based multi channel system, thus the method spread. Despit this it's still not widely used but perhaps through the Internet readers it will become more popular.

Herford, 13. Dec.1995 © Rolf Diederichs, Email: rd@ndt.net

Table of Contents


IMM overview

IMM overview


Objective

The plastic industry produces a variety of large containers for which special safety regulations apply. Among these are canisters and drums for dangerous goods, shock-proof drums, and especially motor-vehicle fuel tanks. A minimum of wall thickness is laid down in all acceptance specifications for these containers. Thus thickness measurement is an important element of the production process and also in quality testing and control.

The first introduction of an ultrasonic measurement technique which has the target to carry out the measurement in a the earliest possible time on the production line, was made in 1979 by Bergweiler Institut für Kunststoffverarbeitung (IKV) Aachen. The ideal location for making the measurement is within the blowing mould itself. The data obtained should be available early enough in order to influence the blow moulding process. However, high temperatures at the point of measurements, and the associated problem of coupling ultrasonic signals in the hot hose, made the use of ultrasonic methods with conventional devices impossible.

It took several years until in 1988 the first system of measurement was developed which delivered reproducible results under these difficult conditions. This first system was tested by a manufacturer and results [refer article Buchscheidt, Walker and Roppel] of 0,1 mm measurement accuracy were reported. Gradually a number of others adapted this measurement technique. Of course use widely was limited. Thus less expensive channel upgrade, easy operation and more data recording within a computerized solution were demanded. In response to all the requested functions a new system [Diederichs] was introduced in 1991 and has increased application regard to blow mould parts within the few years. [top]


Manual thickness measurement with ultrasound

Ultrasound methods of thickness measurements have a good record over many years. They make it possible to measure the thickness of walls that are accessible from one side only. Only with this method large scale non-destructive control of containers wall thickness could be realized. To carry out a measurement, a transducer is simply placed in contact with the point of measurement via a coupling medium [ Image right: Measurements principles].A computing device shows the digital value of the wall thickness. The ultrasonic method of measurements is based on obtaining the exact time required to transmit a pulse within the object being measured.For a further introduction to the basics of ultrasonics refer to this www-site [ transducer, electronic, calibration ].Thickness standards, made from the material to be measured, are used to calibrate the apparatus. Operation of such devices is simple. Their obtained results are reliable. But since the measuring procedure is carried out manually, great reliance is necessary with regard to operator. [top]


Thickness measurement in the blow mould

Dry coupling at high temperature

To transfer sound from the transducer to the object, a coupling medium, for example water, oil or silicon oil is needed, because sound energy will pass from the transducer into the object without large loss only if there is a acoustic matching. Temperatures of up to 240°C occur during blow moulding; thus water is excluded as a coupling medium. Oil, silicones, grease and so on interfere with the production process, since the polymer surface must be kept clean. For this reason, efforts were made to simplify the coupling process and if possible to work without a coupling medium. It is possible to pass sound pulses into polymeric materials in the plastic or molten condition by applying sufficient pressure between the transducer and the plastic surface. The quality of sound transfer is critically dependent on having the correct combination of materials for the transducer outlet and the workpiece product to be measured.

As a result of development efforts there are special transducers available for mounting into the blow mould. They meet the requirement for adequate transfer of sound [image right]to the hot plastic material without the need for a special coupling medium, and are able to withstand temperatures up to 250°C at the contact surface. Furthermore they can withstand a continuous maximum pressure of about 8 bar on the contact surface.

The contact surface of the transducer is resistant to the wearing effects due to pressure and detachment of the polymer hose. The diameter of the transducer is small enough for measurements to be made at radii of the container, particularly in critical regions, and to obtain small diameter drilled holes for transducer mounting. Since removal for service and dismantling of a tool is costly both in time and effort, the service life of a probe must be long.

In-mould transducers have reached their standardised form after a variety of experimental types and prototypes; they work reliable under extreme conditions of use. Probes like this, installed in fuel-tank moulds, have so far survived the manufacture of more than 150,000 blow-moulded containers. They show no evidence of wear, and remain fully functional. [top]

Mounting the transducer in the blow mould

The transducers are positioned to check the critical points of the container, and at the same time to generate a coordinated system from which the adjustment signals for the wall thickness control system can be guided. The probes are mounted in bores in the mould so that their faces are uniform to the inner surface of the tool. Small depressions, visible on the surface of the tank, show the measuring position.

It is advantageous to incorporate the mounting positions for the transducer and the installation channels early in the development status of the blowing mould. These positions can then be taken into consideration in the cooling system. [top]

Calibration

The ultrasonic method requires the apparatus to be calibrated at a defined sound velocity. In plastics it is depend on two principal factors - the material properties and the temperature. During continuous production the material properties are constant, because the density of the material, for example, and the kinds and amounts of fillers and pigments used are not changed. Long-term observations showed that during blow moulding on a modern line with careful control of raw materials, changes in sound velocity relating to the materials was below the 1% limit.

This is true in particular when material from one type and one supplier only was used, and when the machine had a closed recycling system for re-using waste material.

The temperature at the point of measurement changes continuously as the hose cools in the mould. Extensive series of measurements were carried out to investigate whether, during blow-moulding operations, the curve of temperature change are essentially constant from cycle to cycle. The measurements were carried out in a 70 l fuel-tank mould, first on an old blow moulding machine, and then on a microprocessor-controlled machine.

It turns out, with a machine operator under steady working conditions, that variations in the temperature cycle at the point of measurements are negligible. Even over extended periods of operation on both machines, only small temperature variations - +/- 5°C maximum - have been noted.

Under constant production conditions, (i.e. constant cycle times), one can count on an effectively constant temperature. These results permit the expectation that the variations in ultrasonically measured values, caused by the effect of temperature changes on the speed of sound, will be less than +/-2.5%.

The temperature measurements under production conditions there fore show, provided steady repetition of a blowing cycle, reproducible conditions for in-mould ultrasonic measurement. Therefore it is possible to calibrate the ultrasonic measurement [image right] system at a fixed point in the cycle time and to carry out absolute measurement of thickness.

It is certainly the case that temperatures and corresponding sound velocities differ from measuring point to measuring point. Each measuring channel therefore is required to be calibrated individually. The most favorable point in the cycle at which to make a measurement is shortly after the tool is closed. This is when the still-plastic hose makes the best contact to the probe, and ultrasonic signals are obtained with maximum amplitude. [top]


The system

A PC compatible multi-channel ultrasonic-thickness measurement board was developed, and due to this an economical system could be be realized. The system can be extended up to 32 channels due to cascading the boards into the PC slots. A powerful blow moulding application software allows an easy to use operation, including - recipe handling, trend display, part counter, automatic measurement timing, and presents this with a color graphic screen. The demands of today's ISO 9000 are met. By means of a printer record, and also a by a measurement saving on the hard disk. Further post processing can be carried out.

The main mask [gif] displays the left and right part of the mould and indicates the measurement values at their individual positions. Further more the thickness alarms indicated clearly due to red and +/- indications on the screen and alarm output.

The calibration procedure is simplified due to automatic calculation of the sound velocity by means of the manual measured wall thickness. [top]


Outlook

The advantages of this method are proven by many installations. But still there can be an increased application of the methods capabilities to assist further techniques:

- Coex blow moulding

Ultrasonics has the advantages of being able to measure also individual layers, the precondition being a minimum layer thickness and other conditions. For more details the suppliers should be approached for each individual case. Further explanations and echo pictures can be accessed on this WWW-site's chapter "pipe co-extrusion and ultrasonic". A special application is to measure the coex layers within the extrusion die.

- Feedback for machine regulations

This is also a high demand but it is still not practical to realize this in real terms.

- Thermoforming/Vacuumforming

The IKV Aachen [Poller S.] has undertaken extensive investigation with ultrasonics and also presents automatic wall thickness regulations due to temperature control of the heating devices. It shows which fundamental approach can be selected for a process simulation of the thermoforming and blow moulding process in order to permit the influence of different parameters on the quality of wall thickness distribution to be calculated in advanced. The quality of thermoformed articles is determined primarily by the wall thickness distribution. Studies conducted on the thermoforming process supply information on the correlation between temperature distribution and wall thickness distribution.

- Gas assist injection moulding process

This new injection technique in addition requires sometimes also a wall thickness measurement in a very early process stage. Also this technique can be supported by ultrasonic in mould measurement. Special transducers with the required properties (i.e. 250 bar) were developed and the first trials took place at an automotive manufacturer's plant.


There are, from a technique point of view enough arguments and solutions for IMM, even for more applications. The limitations are commercial reasons, today there is always the machine price argument. Machine makers are not interested in investigating in such techniques, because it can not be promoted as their own "home-made" technology. Machine prices are already high enough and buyers cut their budgets to include only the necessary items. [top]


References


NDTnet
© Rolf Diederichs 13. December 1995, info@ndt.net
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