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Analysis And Countermeasures Of The Contamination Of Sheet Metal By The Door Sealing Strip

1 Introduction

The door sealing strip is a rubber piece installed on the door assembly. It is installed on the door assembly by clipping or bonding. It is extruded and matched with the body to form the sealing structure of the whole vehicle. It is waterproof, dustproof, noiseproof and The role of decoration can also buffer the impact of closing the door and the driving process of the vehicle.During the use of the vehicle, the door seal is closely attached to the body, which is prone to problems such as discoloration of the paint surface, blackening of the sealing strip, paint wear, and adhesion of the sealing strip. The phenomenon of discoloration of the paint surface and blackening of the sealing strip is called the sealing strip. Contaminated sheet metal[1]. Since the weather strip contaminates the sheet metal frequently, and the contamination is located on the B-level surface of the vehicle, it is very easy to be perceived by customers, which is one of the key problems for car companies to solve.

2 Analysis of sheet metal contamination by sealing strip

The door of a certain model adopts a rolled window frame, the door sealing strip (hereinafter referred to as the sealing strip) is clamped on the window frame, and the sealing structure section is borrowed.The company’s mature models have an interference amount of 4.3mm for the sealing bubble tube and 2mm for the outer lip. The material of the sealing strip is EPDM rubber with good elasticity, repeated compression resistance and environmental resistance.[2], of which the water-stop edge and the bottom of the sealing foam tube are made of dense glue, and the rest are made of styrofoam, and the coating film thickness is 4~10μm. The matching relationship between the sealing strip, the window frame and the side wall outer panel is shown in Figure 1. In the early stage of production, black pollution occurred in the contact parts of the outer panel of the side wall of the vehicle, the outer lip of the sealing strip and the sealing bulb for about 5 days after closing the door at room temperature, as shown in Figure 2.

2.1 Types of sheet metal contamination by sealing strips

a. Chemical reaction pollution: high activity in the rubber formulation systemThe pollution caused by the additives (such as vulcanization accelerators, antioxidants, softeners, etc.) migrated to the paint and reacted with the paint. Usually, the stains are light yellow, which is a permanent change.

b. Pollution caused by physical changes: a kind of pollution formed by the adhesion of the sealing strip coating, rubber body, post-treatment grease and other substances to the surface of the sealing fittings, or a kind of pollution formed by mixing with the gray layer and rainwater. Usually, the stains are black. eliminate.

In this case, the appearance of the sealing strip has no obvious defects such as broken rubber and poor spraying, and the contaminated parts of the sheet metal are gray-black, which can be removed by wiping with cotton cloth. The type of sheet metal pollution in this case is determined to be physical change pollution.

2.2 Factors that cause the sealing strip to contaminate the sheet metal

2.2.1 Analysis of sealing gap

When analyzing the sealing gap, firstly, the appearance of the door window frame and the side wall outer panel is checked. The results show that the sealing surface of the window frame and the side wall outer panel is smooth, and there are no uneven welding gaps or fold deformation, and the real vehicle meets the design requirements. Then, the sealing gaps of sheet metal polluted vehicles and uncontaminated vehicles were tested and compared. The measurement positions are shown in Figure 3, and the measurement results are shown in Table 1. It can be seen from the measurement results that the actual sealing gap at the rear corner of the window frame is 1.7mm smaller than the theoretical sealing gap, which is one of the reasons that the sealing strip contaminates the sheet metal.

2.2.2 Coating properties of sealing strip

In order to verify the coating performance of the weather strips of the failed vehicle models, samples of the seal strips of other brands and problem models were selected respectively. Standard pressure (9N)and friction factor, a specific type of wear cloth was used to reciprocate 5000 times at a certain frequency on the surface of the sponge rubber coating. The verification results show that the wear resistance of the seal strip of the problem model is comparable to that of other brands of vehicles, as shown in Table 2.

2.2.3 Influence of coating thickness of sealing strip

According to the test requirements of the enterprise, samples of normal sprayed parts and abnormal sprayed parts (coating thickness ≤3μm) were cut and wiped repeatedly 20 times with toluene-resistant white-headed cotton swabs dipped in toluene. The test results showed that the thicker the coating, the better the wear resistance. OK, as shown in Figure 4.

2.2.4 Consistency test of coating film thickness of sealing strip

Cut the functional surface of the sealing strip of the problem vehicle into a spline with a width of 1 mm and a length of 1-2 cm, and place it on an electron microscope slide to measure the film thickness.It is only 1~2μm, which is lower than the design minimum value, which is one of the reasons for the sealing strip to contaminate the sheet metal.

2.2.5 Extrusion precision inspection of door weather strip

The outline dimension of the door sealing strip is detected, and the outline dimension deviation of the sealing bubble tube and the outer lip is within 0.2mm, which meets the design requirements.

2.2.6 Analysis of sheet metal paint quality

The drying, hardness, and adhesion tests of the paint used on the car body of the problem vehicle were carried out. The results showed that the paint performance met the quality requirements. The test results are shown in Table 3.

According to the above analysis, it can be concluded that the small sealing gap of the door and the insufficient coating film thickness at the edge of the sealing strip are the main reasons that cause the sealing strip to contaminate the sheet metal.

3 Countermeasure implementation and verification

3.1 Improvement of sealing strip spraying process

After communicating with the sealing strip supplier, we learned that the sealing strip of this model is sprayed online with solvent-based paint. The production process is: feeding → extrusion → preheating tank → microwave tank → surface treatment → online spraying → oven 1 → oven 2 → Cooling water tank → cut to length.

Because the microwave tank is at a high temperature of about 500 °C, the surface of the product is not cooled enough during spraying, and the temperature is still as high as 210~230 °C, which is very easy to cause the solvent to evaporate too quickly, especially the ridges and corners that are difficult to hang. The thickness is seriously insufficient and the appearance quality is poor. In order to reduce the surface temperature of the rubber during online spraying, without affecting the production efficiency, while increasing the viscosity of the coating, the oven 1 is placed after the microwave tank process, and a cooling water tank process is added. The adjusted production process is: feeding →Extrusion→Preheating Tank→Microwave Tank→Oven 1→Cooling water tank 1→surface treatment→on-line spraying→baking 2→cooling water tank 2→cut to length.

After measuring the coating film thickness of the trial production samples, the film thickness of the sealed bubble tube reached 8 μm~10 μm after the process improvement, and the film thickness of the ridge line and sharp corners reached 6~8 μm, which was significantly increased compared with that before the improvement.

3.2 Sealing gap improvement

In order to improve the sealing gap of the front door and window frame parts, the following measures have been taken:

a. Check the window frame sub-assembly on the inspection tool and know that the detection result of the connecting plate of the inner panel of the window frame is close to the lower limit of the tolerance, and fine-tune the fixture of the window frame assembly. The direction is adjusted to 0.2~0.5mm;

b. Adjust the hinge nut plate on the front door to the Y direction by 0.5mm;

Adjust the sealing gap of the rear window frame to meet the size requirements of the design value of 12±1mm

3.3 Result Verification

A total of 50 vehicles were produced in the workshop after continuous tracking. After 5 days of compression, there was no contamination of the sheet metal by the sealing strip, and the problem was solved perfectly.

4 Conclusion

Weather strip contamination of sheet metal is one of the common problems during automotive development. In this paper, the types and influencing factors of the contamination of the sheet metal by the door sealing strip are analyzed. Through measurement analysis and experimental verification, it is shown that the small sealing gap and the insufficient coating film thickness at the edge of the sealing strip are the main reasons for the physical pollution of the sheet metal. In view of the pollution reasons, measures were taken to improve the sealing gap, and the production process of the sealing strip was adjusted, and the problem of sheet metal pollution was effectively solved after the actual vehicle verification.


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