The general process of automotive painting is as follows: pre-treatment → electrophoresis → gluing → sanding → medium coating → grinding inspection → top coating → finishing → off-line. In the whole process of automotive painting, PVC coatings are widely used in the automotive gluing process in order to improve the sealing, sound insulation, shock resistance and stone impact resistance of the car.
1 Introduction to PVC coatings
PVC coatings are mainly composed of PVC resin, mixed with pigments, plasticisers and fillers to produce a high solids coating with a solids content of more than 90%, which is a white viscous liquid with sealing, sound insulation, anti-vibration and anti-stone impact properties. The weld sealant has good flexibility, stretchability, hardness and abrasion resistance, which can play a good role in sealing, sound insulation and water leakage prevention; the underbody paint is mainly sprayed on the bottom of the car and the four wheel covers, which has high strength, hardness and high adhesion, and can play a role in shockproofing, stone impact resistance and protection of the car body.
2 PVC coating construction process
2.1 Weld sealant construction process
The weld sealant is mainly applied to the weld seam of the body by means of a glue gun. As the internal structure of the body is complex, the structure and shape of the weld seam of different models are also varied. In our company, for example, the nozzles currently in use are round guns, flat guns, snap-on flat guns, 7-hook guns, etc. Different models and different parts of the weld seam use different types of nozzles. In the past, our company used all the round gun nozzles on the glue, encounter the body weld seam is large, the quality of the wrap is not good, the weld seam is irregular, there is easy to card gun nozzle, slide gun, jump gun, the final glue strip easy to accumulate, glue coating hollow, uneven glue strip, on the thick and thin, not beautiful and other problems, the skills of the staff requirements are also high, new employees have to train for a long time before they can independently start operating. Later on, there was a gradual transition to the use of a flat gun for gluing. The big advantage of the snap-on flat gun is that there is a positioning bar, through which the gun nozzle can be guaranteed to travel along the weld and not deviate from the weld, while the glue strip of the snap-on flat gun is more uniform and the contact surface with the weld is larger, and the aesthetics are higher, with relatively low requirements for the operator's skills. These parts are hidden and difficult to glue, so it is impossible to glue them with an ordinary gun nozzle, but with the 7-hook gun you can easily finish the glue application on these parts, forming them in one go and the glue strip will not be interrupted. In the process of welding sealant construction, the equipment used is mainly GRACO's pneumatic pump, filter, connecting the main glue pipeline and glue reducing valve, and then connected through the hose and glue gun, so as to form a complete glue delivery system. The pressure reducing valve used is a pressure reducing valve with a pressure stabilising device, through which the size of the glue output can be adjusted to make the glue output even, stable and continuous, thus improving the aesthetics of the glue strip, and also avoiding the accumulation of glue strip when shooting and closing the gun, reducing the production of residual glue, reducing the operation of the staff scraping glue, saving operating time and improving the production speed.
2.2 Underbody coating construction process
Underbody coating is mainly applied by spraying the PVC coating evenly on the underbody and wheel cover, the gun distance should be suitable, the speed of the gun should be even, no flow hanging, no leakage, no accumulation, no spraying on the cab and the outside of the car, the PVC coating surface should be even and flat, the spraying must reach a certain thickness, the company requires the PVC coating thickness to be between 500~1 000 μm. In order to guarantee the thickness of the coating, the production line is checked at regular intervals and the corresponding records are kept to ensure the quality of the coating. We use a high pressure airless spray gun from GRACO, which has a stable glue output, a suitable spray width and a glue pressure between 10 and 20 MPa, making it ideal for use on the production line.
3 Common problems during the construction of PVC coatings
3.1 Glue application hollowness
The glue application cavity (Fig. 1) mainly refers to the body weld where the glue application is not sufficient and leaks, which will affect the sealing and sound insulation of the glue strip, and will seriously leak water and wind. The main reasons for this are: the gun nozzle deviates from the weld during operation and the glue application area is incorrect; the bodywork weld is too large and exceeds the relevant standards for the weld. When this problem occurs, the first thing to do is to check the size of the weld here, and then improve the way the staff apply glue, so that the gun nozzle fits the weld, the speed is even, the flow of paint is evenly controlled, and the amount of glue applied is increased when the weld is large, so that similar problems can be avoided.
3.2 Cracked glue application
Cracking of the coating (Figure 2) is mainly caused by the PVC coating being subjected to body stress during the baking process, resulting in cracking of the adhesive strip and the iron bond. The main reasons for this are: (1) the sealant baking time is not sufficient and the adhesive strip is not fully cured. The company's PVC coating baking requirements for 140 ℃ × 20 min, in this condition the adhesive strip can be completely cured, the performance can meet the requirements of product design. If the baking temperature is too low, the baking time is not enough, the adhesive strip is not fully cured, its mechanical properties are poor, the adhesion to the body is poor, under the action of greater stress, the adhesive strip is prone to cracking; (2) the impact of the body weld: the body weld is large, as well as the weld at the inner and outer steel plate wrapping is not sufficient, easy to cause the weld at the steel plate thermal expansion and contraction of the stress becomes large, resulting in the part of the sealant excessive expansion or contraction, cracking (3) glue effect: glue strip as far as possible to completely cover the surface of the weld, without affecting the appearance of the situation, the glue strip to maintain a certain thickness, if the position is offset, brush thickness is too thin, the glue strip shear resistance is reduced, but also easy to cause cracking.
Fig. 1 Gluing hollows from large body welds
Fig. 2 Cracked tailgate glue application
3.3 Blistering of glue strips
If the paint is mixed with air, the adhesive strip in the baking process, the temperature suddenly rises, the air from the adhesive strip discharge process is easy to produce bubbles, the sealant surface is covered with raised small lump-like material, the naked eye looks like particles, but after observation under the microscope found to be bubbles (Figure 3), resulting in the adhesive strip surface is not smooth, uneven, poor aesthetics, and will affect the mechanical properties of the adhesive strip, reduce adhesion, the adhesive strip easy The surface of the tape is not smooth and flat, and the aesthetics are poor. The main reasons for this problem are: (1) the manufacturer of the sealant does not vacuum the product in the production process, resulting in air residue in the sealant. Generally PVC coating manufacturers have strict vacuum measures, the probability of the raw material itself containing air is relatively small, but it is not completely guaranteed that each batch of material is 100% qualified. As the sealant cannot be adjusted at the construction site, all that can be done when the problem arises is to switch to a new batch of sealant and spend a long time discharging the air from the pipeline; (2) during the construction process, air enters the sealant: the sealant pump pressure plate is poorly sealed and leaks, resulting in air and sealant mixing into the rubber pipeline; secondly, when cleaning the pipeline and filter, the residual air in the pipeline and filter is not completely discharged. The residual air inside the pipes and filters is not completely drained; thirdly, when the painting workshop operators change the materials, the time for draining the glue is too short and the air is not drained, resulting in the air remaining in the glue barrel and finally entering the sealant.
Figure 3 Air bubbles on the sealant surface
3.4 The small amount of glue output during on-site construction affects the operation
The old painting workshop of our company is an open workshop, and there is no temperature control system inside the workshop, resulting in large temperature fluctuations inside the workshop, and the temperature difference between winter and summer is especially obvious. In winter, when the temperature suddenly decreases, the amount of glue will become small and affect the construction. As the viscosity of sealant cannot be adjusted at the construction site, the supplier should pay attention to the weather changes in the coming period when producing, and ensure that the construction site can have a variety of different viscosity of sealant, and selectively distribute the suitable viscosity of sealant for users according to the change of temperature; (3) the site users should choose the suitable viscosity of sealant according to the actual situation at the site, to avoid the viscosity is too large and the sealant is too thick to affect the effect of glue; (4) to carry out The pipeline cleaning, general painting workshop rubber pipeline and pressure reducing device should be cleaned regularly, especially after a long period of shutdown, to avoid PVC paint clumping in the pipeline, blocking the pipeline, affecting the amount of glue.
3.5 " Blowing up the gun"
When air is mixed into the underbody anti-stone coating, the air and the PVC coating will be mixed from the high-pressure spray gun and there will be an obvious "gun blowing" phenomenon, at this time the PVC coating will be splashed around and accompanied by an ear-piercing sound, it is easy to splash into people's body and eyes, which is very harmful to people and should be avoided. The "gun blowing" phenomenon should be avoided. The reason for this problem is that the glue discharge time is usually too short, resulting in air remaining in the glue barrel. The solution: stop the line immediately to discharge the glue until the air in the pipe and glue barrel has been exhausted before you can start spraying.
4 Conclusion
The current competition in the automotive industry is getting more and more
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