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Thickness Measurement of Multilayer Plastic Food and Beverage Containers

Application: Measurement of individual layers and total thickness in multilayer plastic food and beverage containers.

Background: Many containers for packaged food products and many carbonated beverage bottles contain a gas barrier layer between two layers of structural plastic such as polyvinyl choride, polycarbonate, or polyethylene. The thin barrier layer, which is usually made of ethylene vinyl alcohol (EVOH), polyvinylidence choride (PVDC), polyester, acrylic copolymer, or similar material, is designed to preserve freshness and lengthen shelf life by preventing the migration of gas in or out of the package, for example to keep oxygen out of a frozen food package or to keep carbon dioxide inside a beer bottle.

Equipment: Because the barrier layers in finished containers are very thin, it is generally necessary to make this measurement with a high frequency pulser/receiver system that is capable of working at frequencies of 100 MHz or higher. Minimum measurable thicknesses will always depend on the acoustic properties of specific materials, but barrier layers down to at least 0.001 in. (0.025 mm) in thin-wall containers can typically be measured with this type of system. High frequency delay line transducers in the range 50 MHz to 225 MHz are typically recommended for this type of test.

Typical Procedure: The sample waveform below shows a 100 MHz measurement of a three-layer microwave dinner tray comprising a 0.167 mm (0.006 in.) outer structural layer, a 0.054 mm (0.002 in.) barrier layer, and a 0.271 mm (0. 01 in.) inner structural layer. This test was performed with a 100 MHz V2012 transducer driven by a pulser/receiver.

The minimum measurable thickness in a given application will be determined by the highest frequency that is transmitted by the plastic in question. Some plastics are highly attenuating to high frequency sound waves, and thus very thin barriers embedded in thick structural plastic may not be measurable because of low pass filtering effects. For a given product, the measurable thickness range will typically be determined by experimentation with representative samples.

The reflection ratio at the boundary between any two materials is determined by the relative acoustic impedances of those materials. Because virgin and regrind plastics of a given type have essentially identical acoustic impedances, it is not possible to separately measure regrind layers. Also, adhesive layers adjacent to barrier layers are generally too thin and/or too closely impedance matched to measure with ultrasonic techniques and usually cannot be resolved.

As with any ultrasonic thickness measurement, accuracy is dependent on proper sound velocity calibration. Velocity calibration must be performed for each material being measured, on samples of known thickness.

Olympus IMS

O 38DL PLUS é um medidor de espessura por ultrassom avançado. Ele utiliza um transdutor de elemento duplo à inspeção de corrosão interna e inclui THRU-COAT e Echo-to-echo. Os transdutores de elemento simples são utilizados à medição precisa de espessuras de materiais finos, espessos ou multicamadas.
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