Metal processed products use processing oils and mold release agents in the manufacturing process. When there are subsequent processes such as coating or plating, these oils can cause defects, making a cleaning process to remove the oil essential. There are various methods for confirming this cleanliness evaluation, and this article introduces those evaluation methods and principles.
Various oils are used during metal processing. For example, lubricating oil for processing machines, processing oil and rust preventive oil applied to metal products - oil adheres to metal processed items for various reasons and purposes.
When there are subsequent processes such as coating application or plating, the oil on the metal surface repels these treatments and causes defects. Therefore, cleaning oil from metal processing is essential. This process of cleaning oil is called "degreasing."
There are two main degreasing methods. The first is physical degreasing. Specifically, this includes wiping with cloth, and the baking method where oil is oxidized and burned off by heating in a high-temperature furnace for several hours.
The second method is chemical degreasing. Previously, chlorinated solvents and fluorocarbon solvents with high cleaning power were used, but their use has been restricted due to concerns about human toxicity and environmental impact. For these reasons, there has currently been a shift to
leaning agents with lower environmental impact, such as hydrocarbon cleaning liquids and alkaline cleaning liquids.
In manufacturing sites, it is necessary to confirm the cleanliness after degreasing to manage the quality of processed products. If confirmation is insufficient, oil residue on products cannot be detected, leading to defects in the next process and potentially requiring time for process improvement.
Therefore, from the next section onwards, we will introduce 6 types of oil residue tests actually used in manufacturing sites, from simple confirmation methods that are easy to use in manufacturing sites to more detailed and quantitative evaluation methods.
This is a simple method performed as degreasing cleanliness confirmation. After degreasing and drying the sample, ink is applied or the sample is immersed, and the repellency of ink on the sample surface is confirmed. If degreasing has been performed properly, the ink will wet the sample surface uniformly, whereas if insufficient, areas where oil remains will repel the ink.
Dyne pens are available as a method to confirm wettability somewhat more quantitatively than ink. Dyne pens are products for checking wettability and simply evaluating surface tension (dyne value). Multiple dyne pens with different surface tensions are used. The evaluation method involves applying a dyne pen with a certain dyne value to the sample, leaving it for 2 seconds, and evaluating the degree of repellency. For example, when using a dyne pen with a dyne value of 40 mN/m, if the sample repels the dyne pen ink after standing, it can be determined that the sample's dyne value is 40 mN/m or less. From there, the dyne value is lowered to 38, 36... mN/m, and the value at which the ink is no longer repelled can be identified as the sample's dyne value.
As a simple method to confirm oil residue at manufacturing sites, there is a method of visually inspecting samples under blacklight. The oils used in metal processing often contain fluorescent substances, and when blacklight is irradiated, the residual oil on the sample emits light and can be visualized.
Like blacklight, adhesion evaluation exists as a simple evaluation method. Tape such as cellophane tape is applied to the sample after degreasing, constant pressure is applied, and a peel test is performed. This is an evaluation method where if degreasing is sufficient, peeling does not occur, and if degreasing is insufficient, peeling occurs. When evaluating, it is necessary to consider the type of tape used and peeling conditions.
As a method to measure wettability more quantitatively, there is a method of measuring "contact angle" using a contact angle meter. One water droplet is placed on the sample surface, and the "contact angle" is measured by measuring the shape of the water droplet. When liquid is dropped onto the sample surface, it becomes round due to the surface tension of the liquid. The angle formed between the tangent of the rounded water droplet and the sample surface is called the "contact angle." If this contact angle is small, wettability is good and sufficient degreasing has been achieved; conversely, if the contact angle is large, wettability is poor and degreasing is insufficient.
Furthermore, by knowing this "contact angle," it is possible to measure "surface energy." This is done by dropping a water droplet with known surface energy onto the sample surface and using Young's equation to calculate the sample's surface energy. In other words, it is also possible to measure the degree of degreasing cleanliness as surface energy.
There is also a method to quantitatively measure oil on the sample surface using infrared spectrophotometry. The evaluation principle is to measure the absorbance of extracted oil and calculate the amount of oil using a calibration curve evaluated in advance.
The absorbance that can be measured by infrared spectrophotometry can be expressed by the Lambert-Beer law. Since the Lambert-Beer law can be expressed by the following equation, it can be said that there is a proportional relationship between the concentration of residual oil and absorbance. In other words, by measuring the absorbance of liquids containing oil of known concentrations at several points, a calibration curve can be created. By extracting the oil on the sample surface, measuring the absorbance, and comparing it with this calibration curve, the quantitative amount of oil residue on the sample surface can be calculated.
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