About some hardware machining
By shumijia | 27 November 2021 | 0 Comments

Surface degreasing of mechanical parts and treatment quality control

    The purpose of grease removal is to remove grease and stains from the surface of the material, commonly used cleaning agents are organic solvents, emulsions, alkaline washing solutions, water-based cleaning agents and so on. Cleaning methods are wiping method, dipping method, spraying method, steam method, electrolysis method, ultrasonic method, roller method, baking method, etc.

    Usually the quality of the surface pre-treatment refers to the surface cleanliness, which is obtained after degreasing and rust removal, even including finishing treatment to obtain the required surface cleanliness. The general requirements for surface cleanliness are: thorough removal of surface oil and dirt so that it becomes hydrophilic from hydrophobic or partially hydrophobic; thorough removal of metal surface debris so that it reveals the original colour of the metal. The level of surface cleanliness is an indicator of the quality of the pretreatment of the surface. Oil removal quality assessment methods are as follows.

(1) Water film method assessment method: after cleaning the specimen (or workpiece) immersed in clean water, take out immediately after inspection, its surface should be with a continuous layer of water film, such as water film rupture, indicating that the oil has not been removed. Features: The water film method is one of the most common methods used in industry and is simple and intuitive. If there is residue on the surface, it will affect the judgement of the results. The water film test can be carried out again after dipping and washing in a weak acid. When cleaning with surfactants, if the rinse is not clean, it may also affect the results of the judgment.

(2) Wipe method assessment method: after cleaning the specimen (or workpiece) with a white cloth or white paper wipe, white cloth or white paper if there are stains, indicating poor cleaning. Features: wipe method is a qualitative method, simple and intuitive.
(3) Test paper method assessment method: take standard G polar solution about 0.1mL, drop on the inspected surface, unfold about 20mm × 4mm, with A-type oil paper (white slightly with yellow) close to its about 1mm, remove the test paper inspection, if the display of uniform, continuous red-brown, the cleaning qualified. If the red-brown colour is not uniform and continuous, it indicates that the oil removal is not clean. Features: The test paper method detects a residual oil content of no more than 0.12g/m² on the surface, and is suitable for checking the degree of oil removal before painting.
(4) Weighing method: The specimen is washed with ether and then weighed, then dabbed with oil, then degreased with cleaning agent, washed and dried and then weighed. Before and after the difference between the two quality, that is, the amount of oil residue. The more residue, the worse the cleaning effect. Features: Weighing method can be quantitative, but can only use the test piece to do the test.
(5) copper plating method of assessment: after cleaning the specimen into the aqueous solution containing copper sulfate 15g / L, sulfuric acid 0.9g / L, at room temperature for 20 s. The clean surface of the specimen will be chemically deposited a layer of copper, while there is residual oil part is no copper deposition. Features: copper plating method is intuitive, but only for steel parts.

(6) Spray method assessment method: The specimen is cleaned, acid dipped, washed and dried, then placed vertically in a blue solution mist, and the spray is stopped when the droplets are about to drip off the surface of the specimen. The specimen is placed flat and heated slightly so that the surface state is fixed. Areas not covered in blue indicate residual oil contamination. The proportion of uncovered blue areas is assessed using a transparent evaluation plate with a grid. Features: The spray method expresses the degree of cleanliness quantitatively and with high sensitivity, but cannot be tested directly with the workpiece.
(7) Fluorometric assessment method: The specimen is coated with oil containing fluorescent dye, cleaned and then examined under UV light. A transparent evaluation plate with a grid is used to assess the percentage of the residual fluorescent area. Features: The fluorescence method can be quantified, but requires the preparation of artificial oil stains and the use of specimens for testing only. It is less sensitive than the water film method and the spray method.


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