Research on production technology and quality realization of PVD primer powder
Abstract: in this paper, the material characteristics of the base powder, the production process of the production equipment and production environment transformation, production equipment parameter Settings, so that the appearance of the media powder products, product quality standards and stability.
The powder coating is used as the base powder to replace the medium paint. This process can improve the quality of dielectric powder and promote its application in vacuum plating layer.
PVD (PhysicalVaporDeposition, PVD) physical vapor deposition technology, since it has good adaptability to materials, process, low temperature, fast film forming, coating density, adhesive strength with substrate, compared with the traditional process due to no longer use strong acid, strong alkali and cyanide plating, chromium anhydride, poisonous and harmful chemicals such as the environmental impact is small, compared to traditional electroplating process of pure physics, no pollution, etc.
PVD plating technology mainly includes vacuum evaporation, splash plating and ion plating, which are widely used in the fields of machinery, electronics, communication and automobile. Vacuum plating has become a new trend of environmental protection.
In the field of automobile industry, vacuum electroplating technology, instead of traditional electroplating technology, is directly applied to the electroplating of metal and non-metal parts, such as wheel hub, car headlights, fog lamps, car labels and decorative strips of chrome-plated parts.
Due to the poor leveling of metal parts, manufacturing surface defects, direct coating on the substrate will have problems such as unstable luster, poor adhesion, weak salt fog resistance, poor water resistance, appearance defects.
So in these metal parts and electroplating layer between the need for a layer of medium layer to undertake, to meet the requirements of vacuum electroplating film on the bottom of the luster, full, flat and various performance requirements, so that PVD coating plays an aesthetic decorative role, and can meet the performance standard requirements of automotive parts.
At present, the main media layer used solvent-based media paint and solid media powder, solvent-based media paint construction is simple, widely used in automotive after-sales parts, the usual production process is "substrate + solvent-based primer + solvent-based media paint + coating + solvent varnish".
But the VOC emission in its products does not adapt to the increasingly strict national environmental protection policy, pollution of the environment, and solvent-based medium paint as the medium layer.
After electroplating, the adhesion of electroplating coating, heat shock resistance, corrosion resistance, fuel oil resistance and other difficult to meet the performance requirements of the current automotive parts performance standards, the major automotive metal parts vacuum electroplating manufacturers have turned to zero-voc emission powder coatings for solutions, raising a wave of paint powder craze.
As the medium layer, powder coating has the advantages of high gloss, high fullness, high yield and adhesion, heat shock resistance, corrosion resistance, fuel oil resistance and other better performance.
This powder coating is known as dielectric powder, powder using medium vacuum plating process is "metal substrate + base + medium powder + plating + transparent powder", medium vacuum chrome powder as the dielectric layer, it is easier to meet the standard of performance of auto parts, the medium powder mass adoption in the field of automobile, market share is more and more big.
However, the production and manufacture of medium powder is difficult. Because vacuum electroplating requires zero defects in the coating film of medium layer, zero tolerance for shrinkage holes, pinholes, particles and other defects in the coating film, and high requirements for the gloss and fullness of the coating film of product batches.
Therefore, compared with the production of ordinary powder coating, the production process of medium powder has many problems, such as high production difficulty, low production efficiency and low product yield.
This study based on the current medium powder production technology, powder upgrades to improve production technology, media has been studied through study on characteristics of PVD powder materials, production process of the production equipment and production environment of transformation, production equipment parameters, such as research, lowered the medium powder production difficulty, improve production efficiency, high yield, product quality is stable.
1. Basic formula of medium powder and brief introduction of PVD vacuum plating process
1.1 basic formula of medium powder (see table 1)
1.2 General technical specifications for media powders
1.2.1 introduction to PVD vacuum plating process
Powder coating as vacuum plating medium layer, the common plating process is "metal substrate + base powder + medium powder + electroplating + transparent powder".
Take the vacuum electroplating process of the wheel hub as an example, first paint the wheel hub base powder according to the normal hub base powder spraying process, and then paint the base powder coating film according to the medium powder coating process, to generate "glass surface ";
Then, the sprayed hub parts are put into the vacuum metal spraying chamber containing a small amount of argon. The target materials of the selected materials, such as nickel, chromium and aluminum, are mainly plated onto the parts by means of evaporation or splash plating, with a thickness of about 1.0×10-7m. Finally, transparent acrylic powder is used as the protective layer of electroplating.
As a dielectric layer, powder coating is most important to form a "glassy surface" with characteristics of high leveling, high gloss and high fullness, so as to achieve a fog-free mirror effect. The basic parameters of coating film of the dielectric layer are shown in table 2:
1.2.2 powder coating
No mechanical impurities, loose and unagglomerated (visual measurement), powder coating density: 1.1±0.5g/ml, moisture content: ≤1%.
1.2.3 Coating (technical indicators are shown in table 2)
2.1 raw materials and equipment
Dow epoxy: commercially available; Phenolic curing agent: on the market; General additives for powder coating (levelling agent, benzoin), imported; Double screw extruder: kebeilong machinery co., LTD. ACM2PSR mill: HOSOKAWA MIKROPUL GmbH, mixer.
2.2 production process and preparation of media powder
The production process of medium powder is similar to that of other powder coatings, through the following steps: batching → mixing → melting extrusion → pressing → crushing → screening. (see figure 1)
Each component was weighed according to the formula (table 1). After premixing, it was fused and mixed by a twin-screw extruder. The cooling roller was used to press the tablet, and then it was grinded and crushed by a pulverizer.
2.3 quality testing
After the coating particle size reaches the standard, the coating quality is evaluated through a series of tests.
As a vacuum electroplating medium layer, the coating film of the medium layer needs to achieve the appearance of zero defects, including film thickness, particles, powder point, shrinkage hole, pinhole, gloss, flow equality test, but also including the compatibility between batches, compatibility between each box, coating adhesion, coating bending, impact, surface hardness, cup, etc.. Table 3 shows the test results.
2.3.1 testing environment
The appearance of the coating film of the dielectric layer should be free from defects, and the interference factors of misjudgment of the detection environment should be eliminated, including fiber particles, powder points, electrostatic holes, etc. The main approach is to close the spraying test room, and the parameters of the spray gun voltage, current, powder output should not be too large.
2.3.2 film thickness
The film thickness was measured by a film thickness meter.
2.3.3 coating appearance
When the film thickness is in accordance with the test, the film shall be visually tested, and there shall be no defects such as particles, powder points, shrinkage holes and pinholes on the film.
2.3.4 coating gloss, adhesion, bending and flow equality test
Under the condition that the film thickness is consistent, gloss meter, bagel meter and shaft and bar bending meter are used for measurement, and the leveling test needs to be compared with the comparison plate for judgment.
2.3.5 compatibility test
Media powder needs to be tested for compatibility. After the test powder is mixed in a 50:50 ratio, it will be solidified by spraying plate, and the gloss instrument and visual test will be used to determine that it cannot lose light and fog. Compatibility test should be conducted for the first, middle and last box Numbers and multiple batches of each batch.
2.3.6 problems and defects of coating film
After curing, defects with high probability are particles, low DOI value and luster, substandard leveling, shrinkage cavity, pinhole, etc.
3.Results and discussion
3.1 influence of material characteristics on production difficulty and quality
Since medium powder requires leveling (R)≥9, gloss ≥ 110%, and no shrinkage cavity, particle, pinhole and other defects can be found on the coating surface, excellent compatibility between resin and curing agent molecular groups is required, and the basic characteristics are close, so as to have good mechanical processing performance.
The resin and curing agent are fully and evenly mixed, so as to achieve the uniformity of reaction in the curing process of the whole coating film and provide guarantee for the product performance and reliability.
However, under the condition of equal flow performance, the basic characteristics of the selected low-molecular weight epoxy resin and phenolic curing agent, such as softening point and viscosity, have some differences, which leads to the general effect of extrusion mixing can not meet the high requirements of medium powder coating film.
The basic characteristics of resin and curing agent are shown in table 4. The fixed production process is adopted, and the test results obtained by the combination are shown in table 5.
The combination of type 2 epoxy and curing agent 1 can get the best results. It can be seen that the viscosity of epoxy and curing agent plays a decisive role in the final product quality.
3.2 influence of production environment on powder quality
Medium powder is easily polluted by the external environment, resulting in shrinkage cavity, particle and other defects of the coating film, so the requirements for the cleanliness of production equipment, production environment is very high, take the establishment of independent production workshop, production environment air isolation filtration, air particle cleanliness to maintain 500,000 level.
3.3 influence of extruder screw on coating quality
The extruder screw is composed of feeding section, melting section and homogenizing section (see figure 2). The mixing effect of the extruder will affect the product luster and leveling.
The power of extruder, length-diameter ratio of screw, cross-sectional area, thread depth and rotation speed have influence on the uniform dispersion of materials.
Screw with a changjing ratio of 24:1 was selected for the study and test. The influence of screw with different specifications and extruder on the gloss and leveling of the coating film is shown in table 6.
The results show that increasing screw length and thread depth can improve the appearance of medium powder. The compatibility problem between batches can be solved by using secondary repeat extrusion.
4, the conclusion
Through the analysis of the characteristics of raw materials, it is found that the softening point and viscosity of raw materials used are quite different, and the coating film of medium powder requires zero defects. As a result, the production process of medium powder is more difficult to produce, less efficient, and less finished product.
Research through the production environment to ensure the cleanliness, the selection of extruder screw, secondary extrusion, to ensure the full uniform dispersion and mixing, the elimination of impurities in the powder.
The inspection method and environment can ensure the timely discovery and feedback of quality problems, so that the quality of medium powder can be improved, more efficient productivity can be obtained, yield rate can be improved, and appearance qualification and quality stability can be guaranteed.
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