Vacuum activation and absorption coating process
1. Introduction, vacuum adsorption is one of the important production methods of automobile interior decoration parts. Now, this is how to produce middle-grade and high-grade automobile door interior decoration board, the cuticle is a polyvinyl chloride film, which vacuum adsorbs the cuticle onto the substrate. Due to the influence of material, technology and environment, the product will have some quality problems, such as cracking, skin cracking, wrinkle, bubble and so on. In view of the causes of the above quality problems, the corresponding solutions are put forward. 2. Process analysis, the main factors affecting the vacuum suction process are glue activation temperature, skin temperature, vacuum pressure, pressure cooling time 4 parameters. 2.1 the bonding performance of the activation temperature glue directly affects the bonding effect. Sika 9539 glue (Sika 9539) is chosen here. The surface tension of the PP material for the door inner decoration plate is more than 38 d by flame treatment, and then the Sika glue is evenly sprayed on the base material to make the glue point-like distribution (see Fig. 1) . According to the activation temperature of glue glue at 70 °C, that is, the best adhesion of Sika glue at 70 °C. 2.2 skin temperature PVC skin temperature is the key factor of the whole vacuum adsorption process. The optimum temperature of the PVC skin is determined by the properties of the polyvinyl chloride film and the properties of the adhesive. According to the property of PVC skin, the baking temperature of PVC skin can not be too high, higher than 160 °C, PVC skin will decompose and produce pungent smell, and also can not be too low, tOO LOW PVC skin will not have enough softening deformation", can not meet the requirements of molding, reasonable heating temperature should be close to its plasticizing temperature.". But not above the plasticizing temperature. In addition, due to the poor thermal stability of PVC skin, it is necessary to allow PVC skin to reach the deformation temperature at a faster speed during baking, to form and cool in a shorter time, which is good for the PVC skin to keep its original characteristics 3. 2.3 vacuum pressure, the higher the vacuum pressure, the better the adhesion effect between PVC skin and PP base material. The vacuum degree above 0.9 MPA, but it will fluctuate greatly because of the discontinuity of the continuous production process. In order to reduce the fluctuation of vacuum degree in continuous production, two vacuum pumps were used to pump the vacuum (see Fig. 3) and a vacuum tank was added to store the gas. 2.4 The initial adhesion force between PVC skin and PP substrate can not be obtained until the bonding time of PVC skin and PP substrate is maintained at a certain pressure for a period of time. Therefore, vacuum adsorption must be cooled by holding pressure before mould can be produced. Because this process is completed on the mold, holding pressure cooling time too long will affect the production efficiency, too short will affect the bonding effect, easy to produce skin and substrate separation defects. 3 defect analysis 3.1 opening glue, opening glue is one of the common defects of vacuum adsorption. After vacuum adsorption of PVC skin and PP base material, the internal stress of PVC skin is not released enough in the time of cooling and holding pressure, and it tends to contract during storage and use. The main reason for the defects is that PP is a kind of widely used polymer, and PVC skin is bonded with it by Kaxi Glue. PP low price, corrosion resistance, with high specific strength. It is a semi-crystalline structure in which the crystals are usually spherical. Measured by AFM, PP plastic Surface roughness are generally in the range of 7 to 15 NM. Flame treatment technology utilizes a high temperature (above 1000 °C) gas flame to ionize particles in the air. The high-energy particles in plasma, especially the electron impact on the surface of the material, cause the etching of the plastic surface, and the migration of the free radicals and impurities produced by plasma treatment cause the growth of the crystal composition on the surface, which makes the Surface roughness of the material larger (generally larger than 40nm) and increases the surface adsorption force. In addition, the air plasma produced by Dielectric barrier discharge contains a large number of energetic particles such as electrons, neutrons, and reactive groups such as N2, N + , Oh, and HO2. Among them, the electron has a very high speed in the electric field because of its small mass, which plays an important role in breaking polymer molecular chain and forming bond. In general, the average energy of the electrons in the Dielectric barrier discharge is greater than that of the C-C and C-H bonds in polypropylene, so the high energy electrons can break the molecular bonds of polypropylene and create unsaturated bonds on the surface. These unsaturated bonds not only react with active groups in plasma, but also absorb oxygen and water in air for a period of time after treatment. These will be in the polymer surface to generate a variety of oxygen polar hydrophilic groups, such as Carboxyl, hydroxyl, which greatly increased the material surface tension, so that the PVC skin more firmly bonded to the PP substrate. Because PP is a non-polar material, the adsorption is very poor, so must be flame treatment, and to use a dyne pen to test, so that its surface tension up to 38 dyne. If the surface tension is lower than 38, it is easy to produce open glue defects. 3.2 broken skin, broken skin that is in vacuum adsorption process, PVC skin suddenly broken situation, will cause parts scrap and production interruption, equipment will be locked due to vacuum negative pressure suddenly disappeared. There are two main reasons for this: First, the mold temperature is too high; second, the uneven heating of PVC skin 3.2.1 mold temperature is too high, one of the reasons for the skin cracking is the mold temperature is too high, through the heat conduction so that the temperature of the substrate is too high, the sharp corner of the PVC skin scalded, vacuum adsorption under the negative pressure, the internal stress increases, and finally in the role of internal stress around the sharp corner of the pull broken skin, this break is generally large and shrinkage into a circle, tensile side, mostly accompanied by poor adhesion. The defect can be solved by adjusting the die temperature to reduce the temperature of the substrate. 3.2.2 uneven heating of PVC skin
