Research on the anti-rock resistance and protection methods of the whole vehicle

introduction

When the vehicle is driving, the stones on the road will splash on the car body, and the paint surface of the car body will be damaged to some extent

1.

Automobile manufacturers at home and abroad have requirements for the adhesion of coatings, and they are usually judged according to DIN EN ISO 2409 or GB/T 9286-2015

Taking a joint venture automobile factory as an example, a dynamic method that uses hard particles to simulate the impact of stone impact is used to evaluate the stone impact resistance of automobile surface coatings

Figure 1 Stone hit result level

Body stone protection involves body panels, coating technology, and exterior parts covering protection

In the coating process, the stone-strike resistance and the anti-corrosion performance of the vehicle are improved by selecting the coating combination and optimizing the coating parameters; for the frontal stone-strike area under the vehicle body such as the chassis and door sills, PVC coating is required to protect the surface of the vehicle body

2.

Taking into account the anti-corrosion quality assurance requirements of the whole vehicle, usually galvanized steel plates are used for the outer panels of the body, and cold-rolled plates are used for part of the inner panel area

During the drying process of electrophoretic paint, the electrophoretic thickness of the edge part is lower than the normal part due to the effect of surface tension, and the adhesion is also poor, commonly known as the sharp edge effect

Figure 2 No polishing area of ​​the car body

During the vehicle manufacturing process, the quality assurance department must work with the paint shop to inspect the body-in-white incoming vehicles, and at the same time, clearly inform the body shop of the prohibition of sanding requirements on the edge of the body shop to avoid serious sanding of the vehicle, after-sales complaints, and unnecessary losses
.

3.
Painting process

After the vehicle flows into the paint shop, the body-in-white is first subjected to electrophoretic coating, and then to PVC coating, topcoat, wax injection and other processes
.
Among them, the bottom anti-stone attack PVC coating process and paint coating process are closely related to the whole vehicle's anti-stone attack
.

3.
1 PVC coating protection at the bottom

When the vehicle is running, the chassis and wheel cover are easily impacted by gravel and sand
.
The faster the vehicle speed, the greater the impact force, and the painted surface that is hit or scratched is easy to fall off
.
The body sheet loses the protection of the coating and is easy to rust
.
Generally, the underbody coating is difficult to face the impact of gravel during driving.
At present, the method commonly used by vehicle manufacturers is to use stone-resistant polyvinyl chloride PVC coating
.
Usually after the electrophoresis process in the paint shop, the PVC material is sprayed, and then cured by baking to form a noise reduction, shock absorption, stone attack, and anticorrosion coating
.

As for the requirements for stone-strike resistance at the bottom of the vehicle body, as shown in Figure 3, the red and yellow areas are high-risk areas
.
Generally, the red area requires a coating film thickness of more than 1200 µm, mainly on the sides of the chassis, which is most vulnerable to crushed stones; the yellow area requires more than 500 µm, mainly at the lower 10 cm on both sides of the threshold and the front part of the spare tire pit
.

Figure 3 PVC coating distribution

During the manufacturing process, the quality assurance department monitors the thickness of PVC coating through the anti-corrosion inspection of the whole vehicle
.
In view of the obvious low thickness of PVC coating or missing PVC in some locations, the paint shop is required to optimize and rework defective cars
.
In view of the phenomenon of high PVC thickness and accumulation, no adjustment can be made if it does not affect the subsequent assembly
.

3.
2 Selection of paint coating

The anti-stone protection for the visible surface of the vehicle body is not directly impacted by gravel like the vehicle chassis, door sills and other areas.
It is often subjected to secondary impact, and the amount of gravel is not as large as the chassis
.
At the same time, PVC coating is not suitable for the visible area due to its ugly visual appearance, so it is often necessary to adjust the coating and process to improve the stone-strike resistance
.

Common automotive coating processes include pretreatment, electrophoresis, PVC sealing (including fine sealing, coarse sealing, bottom protection), intermediate coating (traditional coating technology includes intermediate coating; new water-based technology does not include intermediate coating), top paint (color Lacquer, varnish, the color paint of water-based process can choose whether to need pre-spraying), wax injection and other processes
.

This article selects traditional solvent-based monochromatic paint, traditional solvent-based metallic paint, water-based monochromatic paint, water-based metallic paint, water-based monochromatic paint with pre-spray and water-based metallic paint with pre-spray (the coating film thickness is controlled within 90~110µm), Carry out stone-strike experiments to compare the stone-strike resistance of different types of coatings
.
The results are shown in Table 1
.
 

Table 1 Anti-stone impact results of different coatings

From Table 1, the following three conclusions can be drawn:

1) Traditional solvent-based coatings have better stone-strike resistance than water-based coatings;

2) The stone-resistance performance of metallic paint is better than that of single-color paint;

3) After pre-spraying the water-based coating belt, the stone-strike resistance is improved
.

Solvent-based coatings have excellent stone-strike resistance because they contain intermediate coatings.
However, as the requirements of environmental protection regulations become higher and higher, they are gradually replaced by water-based coatings
.
At present, the coating process of the newly-built vehicle plant all adopts water-based process
.
For the water-based process, try to increase the pre-spraying on the exterior area of ​​the car body to improve the stone-strike resistance of the coating, especially the water-based monochromatic paint
.

4.
Outer cover protection

For the wheel cover area below the waistline of the vehicle body, the lower part of the threshold, etc.
, which are the areas where the stones first impact, the stone impact resistance can be improved by installing plastic parts such as mudguards, threshold trims, and wheel eyebrows
.

In order to improve the appearance of the door sill area, a large door sill panel is used to replace the PVC coating to improve the stone-strike resistance of the door sill, as shown in Figure 4
.
The outer surface of the wheel cover is protected by a wheel eyebrow to protect the coating, and the inside of the wheel cover is equipped with a plastic baffle to block the debris from the wheel
.

Figure 4 Door sill trim panel

In addition, after the chassis is closed in the final assembly workshop, the purchased parts such as rear axles and subframes only contain a single black electrophoretic paint, which has limited stone-strike resistance.
After the vehicle is used for a certain period of time, the surface of the electrophoretic paint is damaged and rust is prone to occur
.
At present, some high-end car chassis use a full set of guard plates to improve the vehicle's anti-stone and anti-corrosion performance
.

Figure 5 The bottom plastic shield

Concluding remarks

It introduces the whole vehicle stone-strike protection from the aspects of car body plate, PVC coating, coating process, plastic protection board and so on
.
At present, as consumers have higher and higher requirements for automotive products, the stone-strike resistance of the whole vehicle is also an important indicator, and at the same time, it is related to the anti-corrosion of the whole vehicle.
It is necessary to ensure the excellent performance of the whole vehicle
.
At present, the domestic and foreign vehicle markets are fiercely competitive, especially as the domestic market is close to saturation, and vehicle manufacturers are pursuing the goal of low cost and high added value
.
Controlling the vehicle's resistance to stone attack is still a long-term, difficult process that requires continuous improvement and exploration
.

References (omitted)

Authors | Lin Xufeng, Huang Yong, Zou Weiwei, etc.
 

(SAIC Volkswagen Automobile Co.
, Ltd.
)