About the vacuum coating process of PP plastic
About the vacuum coating process of PP plastic: In the vacuum coating process, PP plastic is limited by its physical properties, such as non-porous surface, poor tension, low density, extremely small polarity, etc. The general vacuum coating primer is difficult to directly adhere to the PP plastic substrate, resulting in unstable vacuum coating adhesion, and the coating is easy to fall off. At present, there are mainly two methods for solving this problem in the industry: one is to carry out flame treatment to the material before spraying primer or spraying PP treatment agent to improve the adhesion of vacuum coating;
1. Flame treatment:
Flame treatment can be used to adjust the surface properties of plastics. The macromolecules on the plastic surface undergo oxidation reactions at high temperatures, resulting in polar groups. In addition, it also affects the structure and morphology of surface molecules, allowing the water-based coating film to adhere to the plastic surface. The function of flame treatment is to generate hydrophilic components on the plastic surface, so that the primer coating of vacuum plating can better combine with the PP plastic substrate;
During flame treatment, a specific blowtorch is used to burn a certain composition and proportion of gas and air to form an oxidizing flame with a temperature of up to 1100~1800 °C, so as to achieve the purpose of instantly changing the surface properties of PP plastic. The flame treatment acts quickly on the surface of the material, changing its properties, thereby greatly enhancing the material's ability to bond with the vacuum coating primer coating. In the actual treatment process, the temperature of the flame, the distance between the flame and the film, and the treatment time are several key factors that affect the treatment effect. A common method of flame handling is to use a robot with a flame sprinkler mounted on the forearm of the manipulator. The manipulator moves on the surface of the plastic part according to a specific trajectory. The flame treatment head is covered 20 cm away from the plastic surface by switching the different axes of the manipulator. This distance allows the extension of the flame to contact the plastic surface, making it instantly reach a high temperature of 1000 degrees Celsius, otherwise under-burning will occur due to poor contact. Manipulator movements must cover all plastic surfaces and must not be repetitive. Otherwise, the plastic surface will be burnt, resulting in overburning.
The vacuum coating process of PP flame treatment process is as follows:
Plastic parts→surface flame treatment→primer spraying→IR leveling→UV curing→vacuum coating→topcoat spraying→IR leveling→UV curing→finished product.
The choice of flame treatment has strong economic benefits, is environmentally friendly, and makes the entire processing process more environmentally friendly. However, special attention should be paid to controlling the fire safety hazards caused by flame treatment to ensure production under safe and controllable conditions.
2. Spray PP treatment agent:
The surface tension of PP material is not good. Applying a layer of PP treatment agent (bottom water) before the vacuum electroplating primer can solve the problem of adhesion between the electroplating layer and the PP plastic surface. The process flow is usually as follows: cleaning the material surface → spraying PP vacuum plating treatment agent → 3-5um → (recommended baking 3-5 minutes * 60 ℃) → spraying UV primer → baking curing → vacuum coating → spraying UV topcoat → Baking and curing → finished product is offline.
Of course, neither of the above two methods is foolproof. Note that other basic general preprocessing procedures are still quite important and necessary. The surface of PP plastic parts must be cleaned well, and there should be no oily substance residue, nor oily mold release agent used in the injection molding process, so as to ensure the cleanliness of the substrate surface. Using either of the above two methods, vacuum coating can achieve ideal and stable adhesion properties.
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