The spraying effect of electrostatic powder spray gun needs to be evaluated from multiple dimensions, including appearance quality, coating performance, uniformity, etc., combined with specific detection methods and standards. The following are detailed viewing methods and evaluation points:
1. Appearance quality inspection
The appearance is the most intuitive reflection of the spraying effect. It needs to be observed under natural light or standard light source (such as D65 light source), focusing on the following aspects:
- Coating uniformity
Observe whether the coating has uneven thickness, such as sagging and powder accumulation caused by local excessive thickness, or bottom exposure and missed spraying caused by too thinness
Check whether corners, gaps and other complex parts are covered with powder to avoid "dead corner missed spraying" caused by electric field shielding
- Surface defects
Particles/impurities: Are there raised particles on the surface (may be caused by powder impurities, spray gun pollution or environmental dust).
Pinholes/bubbles: Are there small holes or bubbles after the coating dries (may be related to the powder particle size and the oil and water content of compressed air during spraying).
Orange peel/shrinkage: Does the surface present an uneven texture similar to orange peel, or a depression formed by local shrinkage (may be related to the curing temperature and powder formula).
Color difference: Compare with the standard sample to check whether the coating color is consistent, whether there is local darker, lighter or abnormal color difference (affected by powder batch and spraying thickness).
2. Coating performance test
After the appearance is qualified, the physical properties of the coating need to be tested by tools or experiments to ensure that it meets the use requirements:
- Coating thickness
Use a coating thickness gauge (such as a magnetic thickness gauge, eddy current thickness gauge) to measure the thickness at different positions (at least 5 points) of the workpiece, and calculate the average value and deviation.
Usually, the thickness range of electrostatic powder coating is 60-120μm (adjusted according to industry requirements), and the deviation needs to be controlled within ±10%.
- Adhesion
Use the cross-cut method (ISO 2409 or ASTM D3359 standard): Use a utility knife to make a cross-cut (such as a 1mm×1mm square) on the coating, deep enough to reach the substrate, apply the tape and quickly tear it off to observe whether the coating falls off.
Grade judgment: Grade 0 (no falling off) is the best, and grade 5 (severe falling off) is unqualified.
- Hardness and wear resistance
Use a pencil hardness tester to test the hardness: Use pencils of different hardness (such as HB, 2H) to scratch the coating, observe whether there are scratches, and judge whether the coating hardness meets the standard (usually ≥2H is required).
Abrasion resistance can be tested by a wear tester (such as Taber abrasion tester), and record the weight loss or wear degree after a certain number of frictions.
- Corrosion resistance
Salt spray test: Put the workpiece into a salt spray chamber (5% sodium chloride solution, temperature 35℃), and observe whether the coating rusts, bubbles or peels according to the required test time (such as 240 hours).
Wet heat test: Place the coating in a high temperature and high humidity environment (such as 40℃, 95% humidity) for a certain period of time to check the stability of the coating.
3. Evaluation of electrostatic adsorption effect
The core of electrostatic powder spraying is to use electrostatic adsorption of powder. It is necessary to check whether the electrostatic effect is normal:
- Powder coating rate
Estimated by weighing method: weigh the weight of powder before spraying and the weight of recovered powder, and calculate the powder coating rate (powder coating rate = weight of powder attached to the workpiece / total spray powder weight × 100%). The powder coating rate of high-quality spray guns is usually above 85%.
Low powder coating rate may be caused by insufficient electrostatic voltage of the spray gun, abnormal powder resistivity or poor grounding of the workpiece.
- Corner coverage ability
Focus on checking the corners and sharp edges of the workpiece to observe whether the coating is fully covered. If the corners are exposed or the coating is too thin, it may be that the electrostatic field of the spray gun is unevenly distributed, and the spray gun distance or voltage needs to be adjusted.
4. Equipment parameters and process verification
When the spraying effect is not good, it is necessary to reverse the problem in combination with the equipment parameters. Common adjustment directions:
Electrostatic parameters: Check whether the voltage (usually 60-100kV) and current of the spray gun are stable. Too high voltage may cause the "Faraday cage effect" (leaking spray at the corners), and too low voltage will result in low powder application rate.
Spraying distance: The distance between the spray gun and the workpiece is usually 150-300mm. If it is too close, powder will accumulate easily, and if it is too far, the powder application will be insufficient.
Powder flow rate: The flow rate is adjusted by the powder pump of the spray gun. If the flow rate is too large, it will easily cause sagging, and if it is too small, the coverage will be uneven.
5. Comparison with standard samples
Compare the sprayed workpiece with the standard sample (approved qualified sample) in all aspects, including appearance, color, gloss (measured by gloss meter, unit GU), feel, etc., to ensure consistency.
Through the above steps, the spraying effect of the electrostatic powder spray gun can be fully evaluated, and a basis for process optimization can be provided at the same time. In actual operation, it is necessary to adjust the testing items and indicators in combination with specific industry standards (such as home appliances, automobiles, building materials, etc.).
