Treatment of Abnormal Falling Off of Electrostatic Powder Coating on Small Steel Wheel Hub

introduction

Wheel hub, also known as wheel rim, plays a role in supporting the body load, and is an important functional safety component that connects the car's axles and tires.
Wheel hubs can be divided into two categories according to their main materials and uses: the first category is aluminum alloy wheels, referred to as aluminum rims, which are currently mostly used in passenger cars such as cars; the second category is steel hubs, referred to as steel rims, which are mainly used Commercial vehicles, such as trucks and buses, the latter is also the product category to be analyzed in this article.

For the anti-corrosion and aesthetics of the wheel hub, surface treatment is required during the product manufacturing process.

Coatingol.

(1) Pre-treatment for painting, that is, degreasing, rusting and phosphating on the surface of steel wheels;

(2) Cathodic electrophoretic primer coating;

(3) Spray liquid baking varnish or electrostatic spray powder as the top coat.

Painting is the last process in the manufacture of steel wheels, and its effect directly determines the appearance and durability of the product.

Painting is also a link where there are many poor quality.

1 Description of abnormal coating quality

An auto parts factory (hereinafter referred to as Y factory) built a new rim production line in October last year, mainly producing small steel wheels under 16 inches for light trucks, as shown in Figure 1.

Figure 1 Small steel wheels

One month after the production of the new wheel hub line in the Y factory, one day in November, the quality control personnel routinely inspected a batch of painted products, the model is 16X5J, and the white powder coating appeared when the two steel rims accidentally collided.

The phenomenon of primer peeling.
Subsequently, an iron rod tied with a soft cloth was used to fully inspect hundreds of mass-produced finished products to be packaged and put into storage.

Figure 2 The phenomenon of powder coating falling off the electrophoretic layer

According to statistics, the part where the powder coating of steel wheels is peeled off is not fixed, and there are spokes and rims, especially the edges of products appear more frequently.

The phenomenon of abnormal peeling of the powder coating on the product seriously affects the appearance of the coating and the performance of the product.

Since the Y factory just opened the new line within the first month, no such abnormal peeling of the powder coating was found, the auto parts factory attached great importance to it, notified the wheel hub line to stop production for rectification, and applied for technical assistance to our company.

The author was entrusted To assist in dealing with this abnormal coating quality problem.

2 Troubleshoot the cause of the powder coating peeling off

After understanding the situation with the on-site painting technicians, the following information was obtained:

After the occurrence of the abnormal quality of electrostatic powder spraying, the technicians have also conducted a self-examination and analysis of the powder spraying coating line in accordance with the requirements of total quality management from the five aspects of "human-machine material law": in terms of human factors, workers are all After training and standardizing operations, improper operation was initially ruled out.

In terms of machinery and equipment, the spraying equipment is brand new.
The compressed air is automatically degreasing and dewatering through the freeze dryer.

Visually inspect the surface of the cathodic electrophoretic coating of the steel ring for smoothness and no shrinkage, and use a magnetic thickness gauge to detect that the thickness of the electrophoretic coating is within 15~25µm to meet the requirements; according to the GB/T9286-1998 standard, the electrophoretic coating adhesion test reaches level 1 It also meets the product requirements; spraying operation process: after hanging up, polishing, wiping, blowing soot-electrostatic spraying-170 ℃ baking for half an hour, in line with the operating instructions and process requirements, testing the total thickness of the electrophoretic coating and the powder spraying composite coating at 70 ~100um, not exceeding the standard; the working environment of the new powder spraying line is relatively clean on site, and 5S work inspections are done every day; after the visit, I learned that there is no injection molding factory within 200m of the Y factory, and there will be no mold release agent pollution in the air.

In order to find out the true cause of the powder coating falling off the electrophoretic layer, the following experiments were done:

2.

1 Powder coating curing test

(1) Temperature tracking test of powder coating baking oven

Take a painted 16X5J steel ring in the powder spraying line for baking operation and follow-up furnace temperature tracking test.

The results show that the baking operation after electrostatic spraying: the temperature of the workpiece is 170 ℃ for 30 minutes, which basically meets the requirements of the powder coating manufacturer The curing conditions.

(2) Evaluation of solvent resistance wipe

Take a 16X5J steel ring that has been coated with white powder before, first check the appearance of the coating, and visually compare it with the standard color palette, and there is no obvious color difference with the naked eye.

Then, do a complete curing test in accordance with GB/T23989-2009 "Testing Method for Solvent Resistance of Paints" in the place where there is no fall off.
Specific operation: Wear disposable gloves with both hands, dip absorbent cotton in acetone solvent to a wet state (squeeze by hand without dripping).

Use your thumb and the rest of your fingers to pinch the remaining part of the absorbent cotton, place your index finger at a 45° angle with the surface of the test coating, and wipe it in the test area of ​​the steel ring powder coating 120mm×25mm with a pressure of about 1~2kg.
Wipe forward, then wipe backward.
One wipe forward and backward is counted as one reciprocating wipe, and one reciprocating wipe is controlled at about 1s.
On the surface of the coating film, a total of 25 reciprocating wipes were performed.
The result of wiping is slight discoloration, which is within the allowable range.

The above two tests show that the baking conditions of the wheel hub powder spray set by the Y factory process basically meet the curing requirements, and there is no under-baking or over-baking of the powder coating, that is to say, improper curing conditions are not the cause of the powder coating shedding .

2.
2 Coating comparison test of different factories

View the entire coating process of steel wheels in Y factory.
All 3 lines are conveyed by catenary chains.
The hangers are 1 hanger 4, and 2 small wheels are hanged symmetrically.
All "immersion" refers to the product "swimming immersion" , All "spraying" refers to the "moving spray" of the product along with the catenary; the curing furnace is heated by hot air circulation, and the temperature refers to the air temperature.

In order to investigate the reasons for the powder coating of steel wheels, the author asked Y factory to make 24 small steel wheels of 16X5J according to the original process.
After assembly, they were divided into two groups and the following tests were continued:

The first group of 12 pieces were first shipped to our company's small steel ring coating line for pre-treatment-cathodic electrophoresis, of which 6 were sprayed with powder by our company, and the other 6 were returned to the wheel coating line of Y factory for powder spraying and baking.
In the percussion test, none of the 12 small circles showed powder coating shedding, and the adhesion test met the requirements at 0~1 level.

The test showed that the powder spraying line of the Y factory coating line is not abnormal, and the factors that cause the powder spraying coating to fall off are likely to be abnormal in the "pickling line-cathodic electrophoresis line" section.

The second group of 12 units are painted in the order of "pickling line-electrophoresis line-powder spraying line" in the hub line of the Y factory, but all the spraying tanks used for the "pickling line-electrophoresis line" are all discharged.
Wash the bath with tap water twice, rebuild the bath with a new bath; drain two-thirds of the ultrafiltrate to make up pure water; use four large baths for immersion, except that the electrophoresis bath is temporarily unchanged due to cost reasons, and the rest is acid Washing, neutralization and passivation are all rebuilt with new bath solution.

Test results: The appearance, film thickness, adhesion, hardness and other data of the 12 small wheels after electrophoretic coating and baking are in line with the indicators.
However, after the electrostatic spraying and baking, there are still 2 powder coatings from the percussion test.
The electrophoretic layer falls off.

Tests have shown that the surface of the electrophoretic coating from the hub electrophoresis line of Y factory contains substances that cause the powder coating to fall off.
Based on this, based on personal work experience, it is inferred that the electrophoresis bath has been slightly contaminated by one or several oils.
The most urgent thing is to remove the electrophoresis bath.
At the same time, find the source and composition of the oil pollution and cut off the pollution source.

2.
3 Investigation of the source of oil pollution in electrophoresis tank

(1) The lubricating oil drips on the electrophoretic catenary chain

Improper installation of the chain refueling machine of the new painting line caused slight oil leakage.

(2) Incomplete removal of grease on the surface of the wheel hub

Chemicals are used in many places in the manufacturing process of steel wheels: high-temperature lubricating fluid is used for rim rolling, lubricating oil is used for rim flaring to prevent cracking and burrs from straining, anti-wear hydraulic oil is used for spoke pressure type, and anti-splashing is used for assembly welding.
etc.

After visiting the front-end process staff and consulting with the supplier, I learned that: due to the large amount of high-temperature lubricating fluid used for roll type, recycling and reuse often mixes with equipment leakage, while lubricating oil for flaring and anti-wear hydraulic pressure for pressure type All oils contain grease, especially when spraying a layer of anti-splashing agent before CO2 protective welding in Y factory contains silicone or silicone oil, which is quite different from our water-based non-silicone oil anti-splashing agent.
The pre-treatment of the workpiece in Y factory is not clean, and the silicone oil is mixed into the electrophoresis tank.
After the wheel hub electrophoresis is completed, the silicone oil migrates to the surface of the electrophoretic layer and then sprayed with powder coating will seriously affect the adhesion between the bottom and top coatings.
It is reasonable to infer This is the main reason that the powder coating falls off the electrophoretic coating this time.

On-site observation of the degreasing operation process: The electrophoresis line hanger is 1 hanger 4, and 2 small wheels are symmetrically hanged on the left and right sides.
The hanger moves forward with the catenary and passes through the pre-degreasing tank and the nozzles on the left and right sides above the degreasing tank to spray normal temperature and alkaline.
Degreasing fluid, so that the grease removal on the surface of the workpiece is definitely not complete.
There are three reasons: small wheels use lubricating substances in the multiple processes before painting, and the surface oil is heavy; the hanger design is symmetrical on the left and right sides of the two steel rims.
The degreasing spraying operation just blocks the inner side of the wheel, including the welding seam.
It is completely sprayed into the degreasing liquid; the component of the on-site degreasing liquid is alkaline degreasing powder aqueous solution, the principle of degreasing is saponification reaction, and the process is normal temperature.

3 Solution

Since it has been found that the main reason for the powder coating falling off the primer is the contamination of the surface of the cathodic electrophoretic coating, it can be addressed in a targeted manner.
On-site correction method, stop production of the electrophoresis tank to remove oil, the specific method is to filter the electrophoresis tank liquid and install a special oil removal filter bag to circulate the filter tank liquid.
Replace the new oil removal filter bag every 24 hours.
Continuous 3d filtration basically removes the oil floating in the electrophoresis tank.
To be on the safe side, draw out one third of the electrophoresis tank solution (a small amount of it can be added and used up during the next 10 days), and add the same amount and concentration of the new electrophoresis paint solution into the tank and mix well.

3.
1 Replace the welding anti-spatter agent

The original types of welding anti-spatter agents containing silicone oil were discontinued and replaced with water-based non-silicone oil welding anti-spatter agents to cut off the source of silicone oil pollution.

3.
2 Add a heating cleaning process before the pickling line

Regarding whether the pre-coating treatment line should be pickled or degreasing first, because of the rationality of the equipment layout and process, there are many different opinions in the coating industry.
The process principle of degreasing more first.

Due to heavy oil pollution in the production process of small wheels below 16 inches, the coating line has been finalized in the Y factory.
It is recommended to add a pretreatment and degreasing process on site [3].
The specific method is: purchase a heating cleaning machine, which is specially used for small wheels.
The cleaning of the surface grease after the ring assembly, the operating process: use a liquid surfactant-based degreasing solution to heat to 55 ℃, spray and degreasing in all directions-then spray with 70 ℃ hot water to clean it-under 120 ℃ baking After the line, switch to the pickling line.

3.
3 Design change of the small circle hanger

The original electrophoresis line has a small loop hanger with 1 hanger 4 and two hubs on each side.
Due to the depth of the boat-shaped electrophoresis tank, the design is changed to a single side 1 hanger to ensure that the cleaning fluid can be fully sprayed to each part of the hub during degreasing operation.
There are no dead ends.

3.
4 Strengthen on-site process and equipment management

(1) Strengthen the management of process parameters of each tank liquid

Technicians test each tank every day to replenish the liquid in time, clean and replace the tank when it is aging, and prepare a new tank for heavy oil stains and easy aging of the wheel hub.
The sides of the pre-degreasing tank and the main degreasing tank are equipped with oil scraping devices.

(2) Product protection after electrophoretic primer is completed

The electrophoretic primer coating should be as consistent as possible with the powder spraying operation time.
Do not leave the electrophoresis finished in the workshop for too long.
The electrophoretic steel ring that is primed and sprayed within a week can be protected by plastic wrap film; use a special protective cover for more than a week Shelter and protect, store more than 50m away from the pickling line.

(3) Specification for maintenance of catenary wire lubrication

Both electrophoresis and powder spraying catenary conveyor lines need to be cleaned regularly, and the lubricating oil shall be supervised by special engineering equipment managers to prevent the lubricating oil from leaking into the electrophoresis tank and the electrophoretic coating surface.

(4) Oil specification for mechanical processing before painting

The amount of spoke pressure type anti-wear hydraulic oil and lubricating oil for flaring is specified as the minimum amount; the amount of lubricating fluid used for rim rolling type is large, and the recycling and use prevents the equipment from leaking and mixing.
The reclaimed fluid should filter the slick oil.

Concluding remarks

Through test investigation and analysis, it was found that the reason why the powder coating of the small steel wheel hub of Y factory fell off from the electrophoretic layer was closely related to various factors such as the previous production process and chemical auxiliary materials, coating hangers, degreasing agent varieties, and on-site production process management.
In response to the above reasons, specific control measures are proposed: reduce the amount of grease used in the previous process, replace the welding anti-splash agent to use non-silicone oil varieties, add a surfactant to heat and degrease before the pickling line, improve the process, and strengthen the coating site production The standardization of process management and equipment maintenance management cuts off the source of grease pollution on the surface of the electrophoretic coating, and smoothly solves the problem of abnormal powder spraying quality of small steel wheels in the Y factory.

References (omitted)

Author | Zhang Guanhao

(Xiamen Rishang Group Co.
, Ltd.
Technology Center)