Alexei Volkov
Measuring the thickness of plastic film is important for a variety of industries, including construction, horticulture, and agriculture. There are several methods and tools available to determine plastic film thickness, including micrometers, spectroscopic ellipsometry, and x-ray reflectivity. ASTM standards provide guidelines for measuring plastic film thickness, with specific methods depending on the thickness and characteristics of the film. Micrometers, for instance, are suitable for thicker films, while spectroscopic ellipsometry is a preferred method for thin films. The choice of measurement technique depends on factors such as accuracy, speed, and ease of use, with each method presenting its own advantages and limitations.
| Characteristics | Values |
|---|---|
| ASTM Standards | E252, D6988, D8136 |
| E252 Method | Calculating average film thickness using weight, density, length, and width |
| D6988 Method | Using a micrometer for films over 2 mils thick |
| D8136 Method | Using a non-contact capacitance thickness gauge with higher precision than D6988 |
| Micrometer Precision | +/- 0.05 mils or +/- 5% of material thickness |
| Thickness Measurement Tools | Thickness gauge, micrometer, digital caliper |
| Thickness Units | Mils, microns, inches, millimeters |
| Thickness Measurement Points | Thickest point, cross-section of string lacing |
| Other Methods | Microscope, spectrometry, SEM, TEM, AFM, confocal microscopy, ellipsometry, x-ray reflectivity |
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What You'll Learn
Using a micrometer
Measuring the thickness of plastic film is critical in the plastic industry to ensure product quality, comply with regulations, and optimise production processes. While there are several methods available, using a micrometer is a common approach.
A micrometer is a precision instrument used for contact measurement, where the device physically touches the plastic surface. This method provides accurate and precise measurements, but it has some limitations and requires a skilled operator.
When using a micrometer, it is important to note that plastic films are often very thin, typically under 1 mil (or 25 microns) thick. This thinness presents challenges in achieving accurate readings with a micrometer. A good-quality micrometer has a precision of +/- 0.05 mils, which means that for thinner films, the accuracy may only be +/- 5% of the material thickness. As the film gets even thinner, the precision worsens on a percentage basis.
There are also practical issues to consider when using a micrometer. For instance, dirt can accumulate on the probe, affecting the accuracy of readings. Additionally, there may be issues with the speed of measurements and poor reproducibility from operator to operator.
Despite these challenges, micrometers are relatively inexpensive and easy to use. They are suitable for thicker plastic films and can provide reasonably accurate measurements in those cases. However, for thinner films, non-contact gauging methods, such as those offered by SolveTech, are often preferred as they can provide greater accuracy and precision.
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ASTM standards
The ASTM, formerly known as the American Society for Testing and Materials, has developed a set of standards for measuring plastic film thickness to improve the final product, maintain industry integrity, and facilitate the manufacturing process.
There are three main ASTM standards for measuring plastic film thickness:
E252 – Test Method for Thickness of Foil, Thin Sheet, and Film by Mass Measurement
This method is often referred to as gauge by weight, and it is commonly used in the United States. The average film thickness can be calculated using the material's weight, density, length, and width. However, this method has limitations, as it is time-consuming and challenging to use to obtain the variability of the film.
D6988 – Guide for Determination of Thickness of Plastic Film Test Specimens
D6988 is a global ASTM international standard method for determining the thickness of plastic film test specimens. This standard is suitable for films over 2 mils thick that do not require a comprehensive profile. It provides recommendations and good practices to determine precise dimensions when necessary for the calculation of properties. Micrometers are commonly used to measure thickness according to this standard, but they have limitations in terms of precision and bias, especially for thinner films.
D8136 – Standard Test Method for Determining Plastic Film Thickness and Thickness Variability Using a Non-Contact Capacitance Thickness Gauge
D8136 is the latest ASTM standard and applies more advanced technology, providing detailed profile scans that determine if the film meets average thickness and variability requirements. This method eliminates issues with dirt on the probe, measurement speed, and reproducibility. It provides precise dimensions necessary for the calculation of properties and characterises the variability of material thickness in different directions. This standard is useful for communication between plastic film buyers and sellers, as it provides a precise and reliable test method that defines film variability profiling.
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Thickness uniformity
There are several methods and instruments available to measure thickness uniformity in plastic films. One common approach is the use of a micrometer, which involves physically touching the plastic surface with a mechanical device. Micrometers are precision instruments that provide accurate and precise thickness measurements. However, they have limitations when used with very thin plastic films, as their precision decreases as the film thickness decreases. For example, a micrometer with a precision of +/- 0.05 mils may only be accurate to +/- 5% of the material thickness for films under 1 mil (25 microns) thick. Additionally, micrometers may face issues such as dirt accumulation on the probe, variability in measurement speed, and operator-dependent reproducibility.
Another method for assessing thickness uniformity is mechanical scanning using a digital deadweight micrometer. This technique, described in ASTM standard D6988, helps determine the thickness uniformity across a film sample. It is particularly useful for thicker films, providing a rough average thickness and quick spot measurements. However, it may not be as accurate for thinner films.
To overcome some of the limitations of micrometers, non-contact thickness gauging systems have been developed. These systems, such as those offered by SolveTech, use sensors or lasers to measure the distance between the sensor and the plastic surface without physically touching it. The BF200 Blown Film Gauging System and the Precision Profiler by SolveTech are highly accurate and precise tools for blown film thickness measurement. These non-contact methods are advantageous as they do not compromise the product during or after production. The Precision Profiler, for instance, is an off-line system used in laboratories for quality control, providing thickness readings at intervals of 0.125" to generate a comprehensive profile.
Ultrasonic measurement is another non-destructive technique that employs sound waves to measure plastic film thickness. This method is well-suited for online quality control as it can be used on moving materials. It provides accurate and efficient thickness measurements, contributing to process optimization and ensuring consistent product quality.
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Digital deadweight micrometers
The digital display of these micrometers provides users with quick and precise thickness measurements. This feature enhances the efficiency of the measurement process, making it particularly useful when dealing with a large number of samples or time-sensitive tasks. The motor-driven mechanism ensures consistent and reproducible results, reducing potential variations between different operators.
One notable advantage of digital deadweight micrometers is their versatility. They can be configured to meet various international specifications and test standards, making them applicable across different industries and materials. This versatility allows users to measure the thickness of not only plastic films but also paper, corrugated materials, textiles, and other sheet-like substrates.
When using a digital deadweight micrometer, it is important to follow certain guidelines to ensure accurate measurements. For instance, plastic film should be measured at its thickest point. Additionally, when dealing with string-reinforced plastic sheeting, it is recommended to take measurements from the cross-section of the string lacing.
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X-ray reflectivity
To measure the thickness of a thin film, X-ray reflectivity uses the phenomenon of reflected light interference. This is observed when light is reflected on thin films, and can be used to study the optical properties of the film. This technique is known as reflectometric spectroscopy or thin-film reflectometry.
The experimental setup for thin-film reflectometry involves a probe for measuring reflectance, a light source, and a spectrometer. The light reflected by the thin film is measured, and the resulting data is used to determine the thickness of the film.
In one example, a Thunder Optics spectrometer and a Czerny-Turner spectrometer were used to measure the thickness of a plastic film and a thin sheet of Mica. The plastic film was found to have a thickness of 10 μm, while the plastic sheet was 0.1 mm thick.
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Frequently asked questions
There are three main ASTM standards for measuring plastic film thickness: E252, D6988, and D8136. The thickness of a plastic film can also be determined using a micrometer, a microscope, or a plastic film thickness gauge.
ASTM standards for measuring plastic film thickness include test methods for thickness variability and average thickness. E252 is a test method for determining the thickness of foil, thin sheet, and film by mass measurement. D6988 is a guide for determining the thickness of plastic film test specimens. D8136 is a standard test method for determining plastic film thickness and thickness variability using a non-contact capacitance thickness gauge.
D8136 provides detailed profile scans that determine if the film meets average thickness and variability requirements, while D6988 is used for rough average thickness information and quick spot measurements. D8136 also eliminates issues with dirt on the probe, measurement speed, and reproducibility.
