How to Measure Car Paint Coating Thickness?
The thickness of the paint film is directly related to its performance and quality. To accurately assess the thickness of the paint coating, we should use professional techniques and instruments for testing.
How to measure the thickness of plastic parts paint film, layered paint film, and the advantages and disadvantages of various measurement methods.
This article introduces the common methods and application standards of paint coating thickness measurement in different industries.
Since the paint film thickness is an important indicator of the paint film, which not only affects the materials, labor, and costs when making the film, but also affects the use of the paint film.Therefore,we should test the paint thickness to check its quality.
There are many test methods for paint film thickness, which can be divided into the weighing method, measuring tool method, microscope method, non-destructive instrument measurement method, etc.
The characteristics of paint film can be divided into dry film thickness measurement and wet film thickness measurement.
Each method has its own characteristics and application. Commonly used thickness test standards include:
- ISO2808 “Paints and Varnishes – Determination of Paint Film Thickness”;
- ASTMD1400-81 “Non-magnetic Metal Substrates Paint, Varnish, Spray Paint and Related Determination of dry film thickness of non-metallic coatings of products”;
- ASTME376-69 “Determination of coating thickness by magnetic method or eddy current method (electric field method)”; ASTMD2691-70 “Microscopic determination of dry film thickness of wood product surfaces”;
- ASTMG12- 83 “Non-destructive Measurement of Steel Pipe Coating Thickness”;
- ASTMD1212-79 “Determination of Wet Film Thickness of Organic Coatings,” etc.
The paint thickness tester has a color scale in microns. With a magnetic tip, it can stick to any curved surface or a hard-to-reach place. The thickness of the varnish and putty is measured by touching the tip of the tester with the magnet of the surface being tested. It can quickly and precisely check whether the vehicle was painted in the past.
Coating thickness measurement method
Weighing method
It is used in a place where the paint film is too soft and cannot be measured by instrumentation. For example, the thickness of silane film is generally expressed by film weight.
Characteristics: the measurement is not accurate enough, but it can estimate the average paint film thickness of the specified range, and the tested paint film is non-destructive.
Measuring tools
It is used on occasions where the substrate is flat.
Characteristic: The paint film must be hard enough to withstand the pressure of precision contact with the micrometer. The accuracy is ±2μm, and the paint film is damaged during the test.
Thickness gauge method
Using a thickness gauge to measure the coating film thickness is the most intuitive and conventional method. All kinds of instruments have a certain range of substrate adaptation and are non-destructive, fast, and simple.
a. Magnetic thickness measurement method: It is suitable for thickness measurement of non-magnetic layers on the magnetic conductive material. Magnetic materials are generally: steel, iron, silver, and nickel. This method has high measurement accuracy.
b. Eddy current thickness measurement method: It is suitable for non-conductive layer thickness measurement on conductive metal; this method has lower accuracy than the magnetic thickness measurement method.
c. Ultrasonic thickness measurement method: It is suitable for measuring the thickness of multi-layer coatings or the occasions where neither of the above two methods can be measured, but it is generally expensive, the measurement accuracy is not high, and there are few practical applications.
d. Electrolytic thickness measurement method differs from the above three methods. It does not belong to non-destructive testing. It needs to destroy the coating layer. The general accuracy is not high and is more troublesome to measure than other types.
e. Radiation thickness measurement method: It is very expensive and used for special occasions.
Determination of wet film thickness
It is mainly used for freshly painted wet films. Since the thickness of the wet film changes as it dries, which can give only approximate thicknesses.
How to measure the thickness of paint in the automotive
Automotive coatings can generally be divided into three categories: primers, middle coats, and color paints. The primer is electrophoretic paint, which can be anti-corrosion, anti-rust, anti-strike, and anti-aging. The middle coat is used for UV resistance, stone-chip resistance, and the base of the color paint. The color paint of a car is one of the important factors for customers to choose a car. The main functions are as follows:
Protective effect: As an outdoor vehicle, automobiles are eroded by moisture, oxygen, industrial atmosphere, carbon dioxide, and other media in the atmosphere for a long time.
The car paint forms a firmly attached continuous film layer on the car, which can avoid the chemical or electrochemical reaction between its surface and the surrounding medium and prevent or delay the occurrence and development of car damage.
Decorative effect: The car’s surface is painted with various brightly colored paints, which look elegant and show the vehicle’s personality.
Signing function: The color of the car paint can mark the type and function of the car, such as white for ambulances, red for fire trucks, etc.
Other functions: Some automotive coatings can improve the comfort and sealing of the car and prevent noise caused by vibration.
Determination Method: magnetic and eddy current thickness gauge
YNB-100 Digital Car Paint Thickness Gauge is a portable coating thickness gauge, which can measure the thickness of non-metallic coating(such as paint, film, etc.) on metal substrates quickly, nondestructively, and accurately. It is mainly used to measure the thickness of the vehicle’s surface paint and other coating measurements.
Magnetic and eddy current thickness gauges are commonly used in car paint coating inspection.
The magnetic thickness gauge is a non-destructive measurement method for coating thickness, which uses the principle of magnetic induction, the thickness of the coating is determined by the size of the magnetic flux flowing from the magnetic head through the non-ferrous coating into the ferromagnetic substrate.
The instrument contains a magnet. When the probe containing the magnet is in contact with the coating, the probe and the magnetic metal substrate form a closed magnetic circuit. Due to the existence of the non-magnetic covering layer, the magnetic resistance of the magnetic circuit changes. The smaller the pass, the thickness of the cover layer can be calculated by measuring its change.
Magnetic thickness gauges are used to measure the thickness of non-magnetic coatings on magnetic metal substrates. Magnetic materials mainly refer to substances that can directly or indirectly generate magnetism from excessive elements such as iron, cobalt, nickel, and their alloys, such as steel, iron, alloys, and hard magnetic steel. Enamel, rubber, paint, etc., are non-magnetic overlays.
Therefore, we can use the magnetic thickness gauge to measure the paint layer, plastic, and rubber coating on the steel surface, as well as the non-ferrous metal plating layer of nickel-chromium metal. It is widely used in measuring the coating thickness of steel components in engineering.
Eddy current thickness gauge is also a measurement method for non-destructive testing of coating thickness. The instrument contains an electromagnet, which uses a variable frequency alternating current to generate an electromagnetic field in the coil.
When the probe is in contact with the coating, an eddy current is generated on the metal substrate, and the coil in the head produces a feedback effect.
The closer the probe is to the conductor, the greater the eddy current. The thickness of the coating can be determined by measuring the size of the feedback effect.
Eddy current thickness gauges are used to measure the thickness of non-conductive coatings on conductive metal substrates. Copper, aluminum, zinc, tin, etc., are non-magnetic metal substrates, and rubber, paint, and plastic are non-conductive coating layers.
Therefore, the eddy current method is often used to measure the thickness of paint, plastic coating, and anodized film on the surface of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products.
The main difference between the magnetic and the eddy current method is the probe, and the relationship between the frequency, signal, and scale of the two is different.
In order to facilitate the detection, the instrument manufacturers are equipped with two probes on one device to fit various detection situations and provide convenience for the detection.
How to measure the thickness of paint in the Transportation industry
Metal components are widely used in Transportation engineering. As we all know, corrosion is the main reason for the failure of metal components. Therefore, various measures are taken to coat the surface of metal components to improve the safety and reliability of components, aesthetics, adaptability to the environment, and prolong service life.
Only effective and high-quality coatings can protect components from loss, so the quality inspection of coatings is particularly important, and coating thickness is an important indicator of metal component coatings.
In Transportation engineering testing, bridge expansion devices, bridge steel structures, signs, guardrails, and isolation barriers of traffic safety facilities all need to measure the coating thickness.
Common coatings:
metal coatings, such as zinc coating, aluminum coating, zinc-aluminum alloy, aluminum-zinc alloy, etc.,
non-metallic coatings: such as plastic coating, electroplating, etc.;
composite coatings, such as metal and non-metallic composite coating composed of epoxy zinc-based polyester composite coating, zinc complex coating, powder galvanized coating, etc.
Among them, the composite coatings composed of metal and non-metal include galvanized polyester, galvanized dipped plastic, aluminized polyester and aluminum dipped plastic, etc. The inner coating is a metal coating, and the outer coating is a non-metallic coating.
Commonly used in Transportation engineering are galvanized layers, aluminized layers, polymer coating, and metal and polymer composite coating.
We should clarify the type of coating and related parameters before testing to facilitate the selection of suitable testing methods.
The galvanized layer is generally silver-gray, dark in color, soft, and easy to scratch. The aluminized layer is generally silver-white, lighter in color than the galvanized layer, and harder, and it is easy to fall off in lumps after hammering.
The color of zinc-aluminum coating is generally silver-gray, the coating hardness is moderate, and the anti-corrosion is better than that of single zinc and aluminum coatings.
Determination Method: magnetic and eddy current thickness gauge
Detection of single-coating thickness
Measurement point selection:
Select three parts on the sample for measurement; each part is about a reference surface of 1cm2, and take 3 to 5 scattered measurement points in this square reference surface with a side length of 1cm. The test results are calculated by the arithmetic mean of the measured values.
Measurement steps:
Preparatory work:
- Select the probe according to the test piece, install and tighten it;
- Press the power button; after the normal power is on, it will display the measured value before the last shutdown;
if the battery voltage is insufficient, the low-voltage indication will be displayed for about 1 second, and the power will be turned off automatically, and the battery should be replaced immediately and then powered on again.
Calibration:
A)Zero point calibration: measure on the calibration foil or substrate, press the zero key, repeat the measurement to obtain a more accurate zero point, and the zero point calibration is completed.
B)Two-point calibration: two-point calibration is divided into one test piece method and two test piece methods.
One test piece method: It is suitable for high-precision measurement and measurement of small workpieces.
First, perform zero calibration, and then measure on a standard piece whose thickness is roughly equal to the expected thickness of the test piece, and adjust the reading to make it reach the standard value.
Two-piece method: It is suitable for rough sandblasted surfaces and high-precision measurement. The thickness of the two standard pieces is at least three times different, and the thickness of the coating to be measured is between the two calibration values.
The difference between the two-piece and one-piece methods is that they should measure and adjust the values on the thinner and thicker two standard pieces, respectively.
Measurement:
After the calibration is completed, Quickly touch the probe vertically with the test surface and press the probe positioning sleeve lightly. With a beep, the screen displays the measured value and lifts the probe to do the next measurement.
Measurement of composite coating thickness
When the delamination method is used for the thickness of the inner and outer layers of the composite coating, the following steps are used to measure:
1) Prepare test equipment: plate thickness micrometer or ultrasonic thickness gauge with a resolution of 1µm, magnetic thickness gauge, scalpel, crossbar, bamboo sheet or hard plastic sheet, magnifying glass, marker, deplasticizer, anhydrous Alcohol, cleaning wipes;
2) Measure the total thickness of the coating: mark the sample with a marker, measure the total thickness (diameter) at the mark with a thickness micrometer or an ultrasonic thickness gauge three times, and take the arithmetic mean of the three times as the total thickness of this point as T;
3) Demolding: first, remove the thermoplastic outer coating by heating or mechanical method, or remove the thermosetting outer coating with a deplasticizing agent, and then use bamboo or hard plastic sheets, anhydrous Alcohol, and cleaning rags to clean the exposed inner coating properly cleaned.
4) Measure the thickness of the inner coating: use a thickness gauge to measure the thickness of the exposed inner coating three times, and take the arithmetic mean of the three times as the thickness of the inner coating at this point, as T1;
5) Calculate the thickness of the outer coating (single side) T2:T2=T-T1.
Judgment Rules
There are two judgment rules: “85-15” and “90-10”.
The “85-15” rule allows 15% of the readings to be lower than the specified value, but each reading must not be lower than 85%.
The “90-10” rule allows 10% of the readings to be lower than the specified value, but each reading must not be lower than 90%.
The determination of coating thickness should be determined by the entrusting party;
For the main structure’s outer surface, the “90-10” rule can be used for determination. The maximum value of the coating thickness measurement point should not exceed three times the design thickness.
Factors Affecting the Accuracy of Measuring Coating Thickness
1) The thickness measurement by the magnetic method is affected by the magnetic change of the base metal. When calibrating the instrument, it should use the standard sheet or the test piece to be coated with the same properties as the base metal of the test piece for calibration.
2) The electrical conductivity of the base metal has an influence on the thickness measurement by the eddy current method, and the electrical conductivity of the base metal is related to its material composition and heat treatment method and should be calibrated using a standard sheet with the same properties as the base metal of the test piece.
3) Each instrument has a measurement range of the base metal, and the measurement should be done within the available range.
4) The electromagnetic method and the eddy current method are more sensitive to the abrupt change of the sample’s surface shape, and the area’s edge or corner should be avoided during measurement.
5) The curvature of the specimen influences the measurement, and the influence increases significantly with the decrease of the curvature radius, which is unsuitable for the surface measurement of the curved specimen.
6) The surface roughness of the base metal and the covering layer influences the measurement, and the influence increases with the roughness.
Rough surfaces will cause systematic and accidental errors, and the number of measured volumes should be increased at different positions for each measurement to overcome such accidental errors.
If the base metal is rough, it should take several positions on the uncoated base metal specimen with similar roughness to calibrate the zero point of the instrument;
7) The strong magnetic field generated by various electrical equipment around will seriously interfere with the thickness measurement by the magnetic method. Therefore, the detection should be done in an environment without a strong magnetic field.
8) The instrument is more sensitive to the adherent substances that prevent the probe from being in close contact with the covering layer. The adhered substances should be removed during detection to ensure that the instrument’s probe and the test piece’s surface are in direct contact.
9) The pressure applied by the probe on the test piece affects the measurement results. During calibration and measurement, the pressure should be kept constant.
10) The measurement angle of the probe impacts the measurement results. The probe and the sample’s surface should be kept vertical during the calibration and measurement process.
Other Pain thickness measurement tools
Ultrasonic Coating Thickness Gauge
Ultrasonic Coating Thickness Gauge can measure paint on plastic, metal, wood, glass, and ceramics. It can measure the thickness of the layered film and measure the thickness of 5 layers non-destructively at one time In the thickness of plastic coating.
It can also measure the thickness of thin Substrate thickness through the coating.
The working principle of the ultrasonic coating thickness gauge is based on ultrasonic reflection. After the ultrasonic pulse sent by the intelligent probe enters the coating, when the pulse is transmitted to the bonding surface of the coating or the critical surface with the substrate, a part of the pulse is reflected back. After the sensor receives reflected ultrasonic pulses, the thickness of the coating is calculated by the speed of sound wave in the medium.
Another difficulty in ultrasonic coating thickness measurement is measuring the thickness of coatings with similar characteristics.
The difference in impedance values of these coatings is not enough to distinguish the ultrasonic feedback pulses.
New technical innovations can solve this problem, weaker pulse signals can be clearly distinguished, and complex measurement tasks can be completed reliably and accurately.
White light interference film thickness tester
The film’s surface is irradiated with an optional wide-spectrum light source from deep ultraviolet to near-infrared, and the probe receives the reflected light at the same location.
According to the reflected interference light, the thickness of the thin film is quickly calculated by a repeated calibration algorithm. The measurement range is 1nm-3mm, and the test of multi-layer film thickness can be completed at the same time. For films above 100nm, the n and k valuescan also be measured, which is widely used for various substrates and coatings such as 3C, automobiles, and medical devices.
Conclusion:
We can select different thickness gauges according to actual requirements. Magnetic and eddy current thickness gauges generally measure 0-5 mm thicknesses. These types of instruments are divided into:
- probe and host integrated type
- probe and host-separated type
The former operation is more convenient; the latter is suitable for measuring non-planar shapes. If you want to measure thicker dense materials thickness, you should use an ultrasonic thickness gauge, and the measured thickness can reach 0.7-250 mm. The electrolytic thickness gauge is suitable for measuring the thickness of gold, silver, and other metals plated on very thin wires.
