Brief Analysis and Prevention of Foaming Problem of Automotive Coating PVC Sealant

Huang Ping,Luo Qiu (Dongfeng Liuzhou Automobile Co.,Ltd.,Liuzhou Guangxi,545005,China)

Abstract :This paper lists several kinds of phenomena of PVC sealant foaming in the process of automobile painting. The reasons for the typical case of PVC sealant foaming are analyzed,and the targeted rectification plan is formulated to effectively improve the foaming problem of PVC sealant. The results obtained in this paper can provide solutions and theoretical basis for the foaming problem of PVC sealant in automotive painting process,and have certain engineering application value.

0 Introduction
As a kind of important auxiliary material necessary for automobile production, PVC sealant can improve automobile comfort and corrosion resistance of body welds. With the development and improvement of automobile manufacturing technology, its application is becoming more and more extensive. PVC sealant has unique functions: sealing, damping and noise reduction, fastening and loosening.
The application of PVC sealant on the car body is to improve the car body’s tightness (no water leakage, no air leakage), noise reduction and heat insulation, corrosion resistance and stone resistance. The parts that are sealed with PVC sealant in the body mainly include large welds such as the engine compartment, the front floor of the body, and the rear floor, as well as four-door two-cover hem welds. Among them, the sealing requirements of the four doors and two covers are relatively high. Because their parts are directly facing the car purchase customers, the appearance requirements are particularly strict. The sealant must be uniform in thickness and full, and the transition must be smooth and tidy, and the thickness and width must be consistent. PVC anti-stone impact adhesive is mainly applied on the surface of the body floor and the 4 wheel covers. Its main role is to resist the impact of sand and stone, protect the bottom of the vehicle, improve the anti-corrosion ability and reduce noise.

This research is aimed at the foaming problem of the glue used in the coating process. It analyzes and researches the various process links, and proposes a rectification control scheme, which has certain engineering application value.

1 Problems
The presence of air bubbles in the sealant strip will reduce the adhesion of the sealant and increase the risk of stripping off the strip, which is one of the common defects that have a large impact. During the coating process, the foaming of the sealant includes the air bubbles that come with the output of the glue gun; because of the gap in the hemming edge, when the sealant is applied and dried in the drying room after drying, the gas in the weld seam expands and the glue rises. Foam; The glue absorbs moisture in the air for a long time, and foams when baking. That is, the gas in the pipeline causes foaming, the gas in the gap between the welds foams after passing through the drying room, and the rubber stores moisture absorption and foaming. Because the construction of the welding seam sealant used for the welding of the inside and outside of the car is mostly by manual coating, the effect of the process depends on the operating skills of the operator. Therefore, improving the operating skills of the operator and rationalizing the job allocation is to improve An effective approach to sealant process quality and production efficiency.

2 Introduction and analysis of foaming problems in rubber
2.1 Bubbles generated in the pipeline of the glue.

The bubbles that come with the PVC glue when output through the glue gun are characterized by dense bubbles and uniform distribution. Once they appear, the entire rubber strip is honeycomb-shaped.
There are two reasons for air bubbles in the pipeline. The first is that the air in the main filter and each filter is not evacuated during the rubber replacement or equipment maintenance and repair process, so that the PVC glue is mixed with air and then enters the hose. Bubbles will appear after the road. When there is more air in the pipeline, the phenomenon of “blasting gun” will occur in the process of gluing, that is, when it is pressurized to a certain high pressure, the PVC glue wrapped with air will burst when it is discharged from the mouth of the gun and accompanied by a crisp sound. phenomenon. After the explosion, the rubber particles moving at high speed are likely to splash on the eyes and skin of the operator and cause injury, so the operator needs to wear protective glasses. In addition, during the manufacturing and filling process, air may be mixed in. The PVC glue mixed with air is honeycomb inside after spraying and extrusion coating, which affects the sealing and aesthetics of the weld.

2.2 Gases in the weld gap foam after baking.
2.2.1 Causes of glue foam in the gap of the weld
. The four-door, two-edging structure of the car body and the interior of the PVC glue will inevitably contain air and moisture. When baking through the drying room, these air and moisture will be heated to expand and eventually push up the sealant to form a bubble. In addition, improper coating of hemming glue, excessive glue amount, too short distance between the glue application position and the flanging distance, etc., will cause the blockage of the exhaust passage in the hemming structure. Because the heated and expanded gas cannot be smoothly discharged along the exhaust passage, it can only enter the underside of the PVC glue along the gap of the non-glued bonding surface of the inner and outer plates. With the increase of the baking time and the increase of the body temperature, The volume of gas under the PVC glue is constantly expanding and the air pressure is constantly rising, eventually causing bubbles to appear on the surface of the PVC glue, that is, foaming of the hemming edge (Figure 1, 2).

Figure 1 Foaming of sealant in edgefold
Figure 2 The sketch map of foaming of sealant in edgefold

2.2.2 Validation of blistering of rubber
(1) Flanging and edging failure verification
According to the distribution of blistering position of the trial model, a gap measuring feeler was used to measure the gap of the edging weld of the engine cover, and it was found that the gaps were in the process requirements Inside. When the flange of the outer plate is uneven or the amount of hemming is insufficient, the outer plate cannot be tightly packed after the hemming, resulting in a large gap or uneven gap at the weld. The larger the gap, the more gas exists in the gap, and the more obvious the gas expands after baking, the weaker the PVC glue will be pushed up to form convex bubbles.
(2) Verification of glue amount
When the five doors and one cover are covered, hemming glue needs to be applied between the inner and outer panels to strengthen the structure, seal the car body, replace the welding points and ensure the smooth and beautiful body. During the welding and encapsulation process, due to insufficient glue injection, the hemming glue did not fill the space between the inner and outer plates, and the gas in the cavity was heated to expand to form bubbles. Disassemble the foam sealing strip (Figure 3). As shown in Figure 3, after dismantling, it was found that the amount of hemming glue on the outer panel of the foaming area was insufficient, and in some places, hemming glue was not even available. It can be seen that insufficient injection of hemming glue is also one of the main reasons for foaming of glue. The schematic diagram of the filling state of the hemming glue is shown in Figure 4.

Figure 3 Engine covers anatomy
Figure 4 Schematic diagram of the filling state of folding edge glue

(3) Validation of the gap between the hem rubber and the rubber edge . Inspection of the foamed car body revealed that the foaming position of the weld sealant occurred mostly at the transition of the inner panel profile, and the bubble size was 3 to 7 mm. Reverse verification of the coating method of the hemming glue found that when the size of the colloidal edge was> 10 mm, no foaming occurred; when the size of the colloidal edge became smaller from the flanging size, the foaming appeared, and the difference was obvious. That is, when the distance of the rubber edge becomes narrow, the exhaust passage is easily blocked, and the gas cannot smoothly reach the rubber receiving areas on both sides to be discharged. As the temperature rises during the baking process, the gas expands to eject the colloid from the foam.

Figure 5 Edge spacing verification of folding edge glue

2.3 Glue storage, moisture absorption, foaming, and unbaked PVC glue. When exposed to air for a long time or when the air humidity is high, it absorbs more moisture. After exceeding the limit of the material itself (the PVC glue formula is specifically added for moisture absorption). When it is baked at high temperature, the surface of the glue first begins to solidify and form a film. At this time, the moisture inside the glue does not have time to evaporate completely, and then water vapor is formed to eject bubbles of different sizes, as shown in Figure 6. As shown, after storage for a certain period of time, both the baked PVC sealant and the PVC anti-stone hammering foamed.

Figure 6 Foaming problem due to moisture absorption

3 Prevention of foaming of PVC sealant
3.1 Prevention of foaming of rubber pipeline
The quality problem caused by the problem of pipeline bubbles is more serious. It is extremely important to prevent and control pipeline bubbles. First, we must do a good job of training of production and equipment personnel, standardize the normal feeding process, and try to exhaust the pressure plate and bucket. After cleaning and repairing the filter, the air in the filter must be exhausted before the PVC glue can enter the main road. Second, the supplier is required to strictly control the material manufacturing and canning process, and try to avoid canning. Air was mixed in during the process. In addition, because the site management distribution is from front to back, under normal circumstances, bubbles will appear from the first station, and then continue to the subsequent processes. When a blaster appears, it is necessary to discharge the colloid with bubbles first to prevent the colloid with bubbles from flowing into the next process. In order to reduce the impact on production efficiency, at this time, the station operation can be adjusted by other stations to minimize the impact time. If there is a bubble in the left door rubber station, the shift worker or the team leader can use the right door. The glue gun of the glue station helps the left door glue station to operate.

3.2 Foam prevention and control of hem welding seam sealant 

Foam prevention and control of hem seam sealant need to control the position and amount of hem glue, modify and improve the glue application process, the hem must be applied in the process The method, essentials, requirements and precautions of glue are clearly explained in the form of pictures and texts. When manually performing the hemming glue application, the glue gun needs to be equipped with a control device for the glue position to prevent the hemming glue from being applied irregularly. It is important to point out that: when the glued position is far from the edge of the outer panel, it will affect the bonding strength after the inner and outer panels are edged; when the distance between the glued position and the edge of the outer panel is too short, it is extremely easy As a result, the hemming glue will block the exhaust channel, and the gas in the hemming will not be discharged smoothly, which will eventually cause the sealant applied on it to foam. Therefore, it is necessary to standardize the glue application operation of the hemming glue to ensure that the operator masters the glue application method, essentials, requirements and precautions. The quality of crimping is also one of the main factors influencing the foaming of the hem sealant. When welding, the quality of crimping of the body-in-white door cover hem must be improved to ensure that the margin of the hem is within the standard range.
The foaming of the sealant of the welding seam is mainly generated after drying. The processing method can only be repaired. The general repair method is to use a blade in the next process to cut off the bubbles flush, then fill the sealant of the welding seam, smooth it, and dry it. According to the appearance requirements, grinding and repainting can basically guarantee the appearance quality of the rubber strip after repairing.

3.3 Prevention of moisture absorption and foaming
According to the analysis of the production operation conditions and reasons, it was found that the foaming of PVC sealant was mainly reflected in the inventory problem. The probability of cracking is low when the inventory time is short, otherwise it is high. To ensure the normal production, Comparative analysis of foaming of the dried adhesive board after baking in a coating and drying room (140 ℃, 20 min), the results are shown in Table 1.

Comparative analysis of moisture absorption foaming between PVC wet rubber sheet and dry rubber sheet

The results in Table 1 indicate that when the storage environment temperature is 35 ℃ and the relative humidity is 80%, the PVC welded sealant wet rubber board will not foam when the storage time does not exceed 56 h; the dry rubber board is stored at 84 h , No foaming, continue to store until 96 h, and then blisters appear. PVC anti-stone hammering, wet rubber board storage within 64 h, no foaming; dry rubber board after storage for 84 h, then blisters appear. Therefore, when the car body needs to be discontinued for more than 56 hours due to irresistible factors (especially when the air humidity is high), the stocked car body can be entered into the baking room for pre-drying, its water content is evaporated, and then the topcoat is sprayed. Operation to achieve the purpose of avoiding moisture absorption and foaming of the sealant. If the storage time is longer than 84 hours, the car body must be completely painted, even the pre-baked car body cannot be left. If the inventory after middle coating cannot be avoided due to production and other reasons, you can slow down the cooling process by extending the sealant cooling process time (such as extending the cooling channel of the topcoat baking room, closing the topcoat cooling, etc.) to reduce or avoid The purpose of blistering. In short, moisture absorption and foaming of PVC glue can be avoided through effective management of inventory.

4 Concluding remarks The
car coating sealant process is an important process in the painting workshop, which plays a vital role in the quality of the car body, and every aspect of it cannot be ignored. Foam defect is one of the common defects in the sealant process. Once it appears, it can only be repaired, and the appearance of the rubber strip is affected after repair. At the same time, the repair process will affect production efficiency. According to different types of rubber foam, it is the key to solve the problem of foaming of PVC rubber.

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