Principle
Ultrasonic thickness gauge is a thickness measurement device based on the principle of ultrasonic pulse reflection. When the ultrasonic pulse emitted by the probe travels through the couplant to the surface of the material, part of the sound wave is reflected at the interface, and the other part penetrates the material and reflects again at its bottom interface. The instrument calculates the thickness of a material by accurately measuring the time it takes for ultrasonic waves to travel back and forth within the material, combined with the known speed of sound in the material. The calculation formula is:
T = (v × t) / 2
Among them, T represents the thickness of the material, v represents the propagation speed of ultrasonic waves in the material, and T represents the propagation time of ultrasonic waves once back and forth within the material. For flexible packaging materials, such as plastic film, composite film, aluminum foil, etc., its non-metallic and isotropic characteristics make ultrasonic methods an efficient and non-destructive measurement method.
The physical properties of flexible packaging materials impose a specific set of requirements for thickness measurement. First, materials are usually thin and soft, and may have a multi-layer composite structure, which requires ultrasonic thickness gauges to have high resolution and accuracy to distinguish small thickness differences. Second, the acoustic properties of materials (such as the speed of sound) can vary depending on the material, density, and temperature, so accurate sound velocity calibration is a prerequisite for obtaining reliable data. When measuring, ensure that the probe is well acoustically coupled to the material surface with a couplant (such as a special gel or water) to eliminate signal attenuation caused by air gaps. In addition, the surface curvature, flatness, and internal uniformity of the material all have an impact on the repeatability of the measurement results.
Measurement process
To ensure the accuracy and repeatability of measurement results, it is recommended to follow a standardized operating procedure. First, according to the specific type of flexible packaging material to be tested (such as polyethylene PE, polypropylene PP, polyester PET, etc.), consult the relevant technical information or calibrate the instrument through a standard sample block to enter the accurate sound velocity value of the material. Secondly, clean the surface of the material and apply an appropriate amount of couplant evenly. Position the probe vertically and steadily at the measurement point, maintaining proper pressure to avoid material deformation. Multiple readings are taken in each measurement area and averages are recorded to counteract local fluctuations. For composite materials, attention should be paid to the measurement possibilities of their overall thickness and the thickness of each layer, which depends on the function of the instrument and the difference in acoustic impedance between layers.
Key factors
Measurement accuracy is affected by a combination of factors. The instrument's own performance, such as probe frequency, pulse width, and circuit design, determines its minimum resolution and measurement range. Environmental conditions, especially temperature, can affect the sound velocity of materials and instrument electronics, and it is recommended to operate in a temperature-controlled environment. The skill level of the operator, including couplant dosage control, probe placement skills, and reading timing, is also crucial. Regular calibration of instruments with standard thickness blocks is necessary to maintain their performance. At the same time, probe maintenance, such as keeping the wafer clean and avoiding wear, should not be neglected.
Application scenarios
Ultrasonic thickness measurement technology is widely used in the field of flexible packaging, covering production quality control, incoming material inspection, process research and development, and product durability evaluation. For example, in a blown film or casting process, film thickness uniformity is monitored online or offline; In the lamination process, check whether the total thickness of the finished product meets the design specifications. Once the thickness data is obtained, it needs to be interpreted in conjunction with product standards or internal specifications. Thickness uniformity is a key metric and is often assessed by calculating the mean, standard deviation, or extreme deviation across multiple measurement points. Thickness deviations can point to production process issues, such as uneven mold temperatures or fluctuating tensile rates.
Summary
Ultrasonic thickness gauges provide a fast, non-destructive solution for measuring the thickness of flexible packaging materials. Its successful application relies on a deep understanding of measurement principles, accurate grasp of material properties, and standardized operation and maintenance procedures. With the development of sensor technology, digital signal processing algorithms and automation integration, future equipment is expected to further improve measurement accuracy, analysis ability of ultra-thin and multi-layer materials, and intelligent data management to better meet the refined needs of the packaging industry for quality control.
References
ASTM E797 / E797M, Standard Practice Guide for Thickness Measurement Using the Contact Pulsed Echo Direct Method.
GB/T 11344, Non-destructive testing - Contact ultrasonic pulsed echo thickness measurement method.
ISO 16809, Non-Destructive Testing Ultrasonic Thickness.
"Plastic Film Thickness Determination Method" related industry technical manual.
The chapter on the acoustic properties of polymers in the Handbook of Acoustic Materials.