Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? | MachineMFG

# Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem?

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At present, shot blasting technology is the most commonly used economic and reliable surface treatment method.

Related reading: Shot Blasting vs Sand Blasting

An automobile company purchased a hook type shot blasting machine for the surface shot blasting of CVT pulley and pulley shaft products.

## 1. Product status and shot blasting requirements

The products that undergo shot blasting are the driving and driven pulleys as well as the pulley shaft of the CVT stepless transmission.

The workpiece material used is 20CrMoH steel (GB/T 5216-2014), and the workpiece hardness ranges from 58 to 64HRC.

Before the operation, the workpiece has undergone carburization, quenching, and tempering. A small amount of quenching oil remains in the central hole of the pulley shaft.

The requirements after shot blasting are as follows: the surface must be clean and uniform in color, free from any damage, oil stains, water stains, and oxide skin. Steel shot residue is not allowed in the central hole, and the coverage rate of the shot-blasted surface must be ≥ 98%.

Beat requirement: 35s/piece (two shift system, 10.5h/shift, 300 days/year).

## 2. Shot blasting equipment information

The equipment model is hook shot blasting machine SHBX-III-AL, and the technical parameters of the equipment are shown in Table 1.

Table 1 Main Technical Parameters of Equipment

## 3. Main characteristics of equipment

1）The hook’s rotary and self-rotating devices are of the friction disc type. Two stations rotate 180° and have a three-point stop function. The maximum lifting weight is 100kg per hook.

2）The loading area of the shot blasting machine is open. While one station is performing shot blasting, the other station can manually load and unload the workpiece.

3）The shot blasting chamber adopts high wear-resistant hard protection, including high manganese steel wear-resistant guard plates (SMn80) and cast Cr15 at the direct shot position of the shot. The inner side adopts wear-resistant rubber guard plates, which effectively prevent the abrasion of the shot on the chamber shell.

4）The machine has a compact structure, good sealing, low running noise, and is suitable for batch production.

## 4. Description of equipment problems

After shot blasting the workpiece, there is no rust or watermark on the outer wall or cone surface of the inner cavity of the driven pulley. However, there are 1 to 4 light-colored watermarks on the bottom side of the wall of 3 to 4 hydraulic cylinders for each workpiece hanging on the upper layer.

Additionally, when inspecting the large end deep hole of some driving pulley shafts with a light, watermarks were found. These watermarks can be removed by extending the shot blasting time to 1800 seconds, but this results in production delays that do not meet the original set requirements.

Workpieces with watermarks after shot blasting are shown in Fig. 1 and Fig. 2.

Fig. 1 Residual Watermark of Driven Wheel Product

Fig. 2 Mark in deep hole of spindle product

Currently, customers have increasingly stringent requirements for product surface quality. Products with visible defects in appearance will be rejected, leaving a negative impression on customers and potentially impacting product support share. As a result, surface quality issues must be taken seriously and addressed appropriately.

## 5. Analysis of the Causes of Watermarking

There are various factors that impact the surface quality of shot blasting.

We conduct an analysis of the potential reasons affecting the surface quality of shot blasted workpieces. We examine factors such as the quality of the workpiece’s surface before polishing, process parameters, tooling, and the shot blasting machine itself. We then propose corresponding countermeasures and perform process tests to verify our approach.

### 5.1 Surface quality of workpiece before shot blasting

(1) The workpiece is rusted before shot blasting

Based on our observations and analysis, the factory halted production after the Spring Festival in 2020 due to the pandemic situation. Consequently, the workpieces were left in storage for an extended period of time.

Furthermore, the high levels of air humidity have resulted in the corrosion of the workpieces, as depicted in Figures 3 and 4.

Fig. 3 Rust on pulley before shot blasting

Fig. 4 Rust on pulley shaft before shot blasting

To enhance the corrosion resistance of the workpieces, we have taken several measures. Apart from implementing a stringent production plan and minimizing the storage duration, we have also applied rust prevention treatments and packaging to the workpieces in stock.

(2) Before shot blasting, the silver gray substrate surface presents irregular yellow mottling

Figure 5 shows that the macula primarily appears after tempering, and it has been analyzed that this may be due to residue from the cleaning solution.

The cleaning process after quenching involves four steps: oil drainage, degreasing (including soaking and spraying), cleaning (spraying), and high-temperature drying.

Since there is only one cleaning process involved, it is easy for the degreasing solution to mix into the cleaning tank, causing contamination of the tank.

If the cleaning tank solution is not entirely drained of contamination before the workpiece dries, a watermark may result.

Fig. 5 Surface macula after cleaning

To test the influence of cleaning agent concentration on the presence of macula on the workpiece surface, we conducted tests using cleaning solutions with different concentrations of cleaning agent: 2.5%, 2.0%, and 1.5%.

However, it was discovered that the macula could not be eliminated by simply changing the concentration of the cleaning solution.

Fig. 6 shows that methanol cleaning can effectively eliminate the macula.

Since there is no available space in the production line to add a cleaning station, we propose adding a water blowing process after cleaning, changing the cleaning agent model, and selecting a residue-free cleaning agent to eliminate this potential surface quality hazard caused by the degreasing cleaning solution.

Fig. 6 No yellow spot on the surface cleaned with methanol

(3) After tempering, the surface of the workpiece remains greasy dirt, which is difficult to remove by shot blasting

After analyzing the situation, it was found that due to the perpendicular position of the exhaust pipe of the reheating furnace to the furnace, there is a presence of residual oil stains at the interface when the pipe is disassembled for inspection.

Moreover, when the oil stains flow back to the reheating furnace directly, they get stirred by the fan and fall onto the surface of the workpiece, resulting in the formation of oil stains as depicted in Fig. 7 and Fig. 8.

Fig. 7 Residual Oil Spots on the Surface after Tempering

Fig. 8 Residual Oil in Exhaust Pipe

To address this issue, we recommend reconstructing the pipeline and incorporating oil receiving and draining devices at its bottom to prevent oil contamination from entering the tempering furnace. These devices should be connected to the flue to allow for efficient drainage of oil stains, as illustrated in Fig. 9 and Fig. 10.

Fig. 9 Schematic diagram of oil drainage device

Fig. 10 Oil drain device of flue

### 5.2 Analysis of influence of process parameters on shot blasting surface quality

The process parameters include steel shot material, diameter, velocity, flow rate, shot angle, distance, time, coverage, etc. Any changes to these parameters will have varying effects on the shot blasting process.

To determine the optimal shot blasting parameters, hang the tooling filled with workpieces and conduct process tests under normal processing conditions. Adjust the flow, hardness, size of steel shot, shot angle, time, position, orientation, quantity of workpieces, etc. Compare the differences in workpiece cleaning effects under various conditions to find the parameters and time required for qualified shot blasting. This will provide a basis for proposing rectification plans and improving the surface quality of workpieces.

Based on on-site verification of the test machine, the process conditions were re-determined after discussion and research. The driving and driven pulley shafts were changed from 9/hang to 15/hang and from 20/hang to 28/hang. The steel shot was changed from φ0.6 mm to φ0.8 mm, and the hardness was increased from 45-52 HRC to 58-63 HRC. The results of the re-process verification are shown in Table 2, and the qualified cleaning time was found to be between 1260-1500 s.

Table 2 Effect of Process Parameters on Shot Blasting Surface Quality

## 6. Countermeasures for shortening the production rhythm

Despite replacing the tooling and testing various process parameters, the surface quality of the workpieces has not significantly improved within the given production beat time. Additionally, due to equipment restrictions, the shot blasting flow cannot be increased, and extending the shot blasting time would negatively impact production requirements. After conducting analysis and research, it has been decided to reform the equipment.

The following steps will be taken:

1）Replace the two 7.5kW motors of the shot blasting machine with two 11kW motors, along with two frequency converters, belt wheels, electrical parts, and other necessary accessories. This will shorten the shot blasting cleaning time to meet the required beat time range.

2）Remake the shot blasting test plate based on the impact marks of the steel shot on the tooling hanger’s surface to determine the effective shot blasting area of the polishing head.

3）Reduce the diameter of the hanger and adjust the tilt angle of the shaft suspension. When hanging the parts, the opening face will be outward so that the steel shot can hit the inner cavity and cone directly.

4）Reduce the number of workpieces on the driving pulley and driven pulley from 28/hanging to 24/hanging, with one piece removed from each layer to prevent workpieces from covering each other and affecting the shot blasting effect.

The tooling on the third floor’s driving pulley and driven pulley shaft will be changed from 15 pieces/hanger to 16 pieces/hanger on the fourth floor.

See Table 3 for technical parameters of equipment after transformation.

Table 3 Technical Parameters after Equipment Renovation

## 7. Final process validation results

After the equipment transformation was completed, a small batch process validation was conducted for approximately 1000 pieces of 4 different workpieces. During this validation, the process parameters, such as the angle, frequency, and current of the polishing head, were re-optimized to ensure optimal performance.

See Table 4 for the results.

Table 4 Final Process Validation Results

The driving pulley shaft undergoes shot blasting for 525 seconds, resulting in workpiece surfaces that are free of rust and watermarks. However, there may be some small imprints in the deep holes of certain workpieces’ large ends.

After 120 seconds of shot blasting in the opposite direction, the imprints were mostly removed. However, the single piece beat time was 40.3 seconds, which exceeded the required setting of 35 seconds.

Since there is one set consisting of four types of workpieces, the total beat time for each set must be 140 seconds.

Currently, the actual production time for one set of workpieces is 116.875 seconds, and the total beat time is within the required range.

## 8. Conclusion

The validation of the shot blasting process is a critical step.

To identify the optimal combination of factors that yield superior surface quality, samples should be collected under varying process parameters, shot strength, and coverage.

The amount of shot blasted is typically directly proportional to the shot blasting current.

During calibration, the upper and lower shot blasters’ current values are typically adjusted to be the same. However, it’s worth noting that the lower shot blaster is used to throw the steel shot upward.

Despite carrying the same load, the lower shot blaster throws fewer steel shots than the upper shot blaster.

To ensure consistent surface quality between the upper and lower workpieces, we initially adjusted the current value of the lower shot blaster to be 5% higher than the upper shot blaster. However, during the process validation, it was discovered that the watermark on the upper layer was more stubborn than the lower layer’s. Consequently, we increased the current value of the upper shot blasting device by 10% over the lower one. This adjustment ensured that the surface quality of the upper and lower workpieces was nearly identical.

During the process validation, we realized that the maximum adjustment of the tooling and process parameters, including the current and shot blasting flow, did not meet the surface quality and beat requirements. After careful consideration, we replaced the equipment with a higher power motor. The process validation showed that the modified equipment could meet the product quality and beat requirements, and the issue of residual watermark on the workpiece’s surface after shot blasting was resolved.

After four months of operation, the modified equipment has produced over 220,000 pieces, meeting the quality standards and proving successful.

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