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

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 used for shot blasting are the driving and driven pulley and pulley shaft of CVT stepless transmission.

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

Before operation, it has been carburized, quenched and tempered, and a small amount of quenching oil remains in the central hole of the pulley shaft.

Requirements after shot blasting: the surface shall be clean, uniform in color, free of damage, no oil stain, water stain and oxide skin are allowed, no steel shot remains in the central hole, and the coverage rate of shot blasting surface shall 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

Project

Contents of a project

Device parameters

Remarks

Main specification

Shot blasting machine Y-30

2×7.5kW×4P

Variable frequency motor

Processing range of products/mm

 

 

φ700×1000

 

Steel shot

φ0.66mm Stainless Steel Shot

45~52HRC

Maximum lifting weight of hook/kg

 

100

Including tooling

Power/kW

22

 

Workbench

 

Hook revolution speed/rmin-1

 

About 3.2

 

Hook revolution turntable motor

 

0.4kW×4P

With three-point stop function

Hook rotation speed/rmin-1

 

About 14

 

Hook turntable motor

0.2kW×4P

Speed reducer

3. Main characteristics of equipment

1) The rotary and self rotating devices of the hook are friction disc type, two stations rotate 180 °, with three-point stop function, and the maximum lifting weight is 100kg/hook.

2) The loading area of the shot blasting machine is open.

When one station is performing shot blasting, the other station can manually load and unload the workpiece.

3) High wear resistance hard protection is adopted in the shot blasting chamber: high manganese steel wear-resistant guard plate (SMn80) and cast Cr15 are adopted at the direct shot position of the shot, and wear-resistant rubber guard plate is adopted at the inner side, which can effectively prevent the abrasion of the shot on the chamber shell.

4) Compact structure, good sealing, low running noise, suitable for batch production.

4. Description of equipment problems

After the workpiece is shot blasted, there is no rust or watermark on the outer wall and cone surface of the inner cavity of the driven pulley, but there are 1~4 light colored watermarks on the bottom side of the wall of 3~4 hydraulic cylinders for each workpiece hanging on the upper layer;

In addition, in the large end deep hole of some driving pulley shafts, light was used to illuminate and inspect, and water marks were found.

The watermark can be removed when the shot blasting time is extended to 1800s, but the production beat cannot meet the original set requirements after the shot blasting time is extended.

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

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 1

Fig. 1 Residual Watermark of Driven Wheel Product

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 2

Fig. 2 Mark in deep hole of spindle product

At present, customers have more and more strict requirements on the surface quality of products.

The products with visual appearance quality defects will be rejected and will leave a bad impression on customers, which may affect the share of product support.

Therefore, it must be taken seriously and solved properly.

5. Analysis of the Causes of Watermarking

There are many factors that affect the surface quality of shot blasting.

We analyze the possible reasons that affect the surface quality of shot blasting workpiece from the factors such as the surface quality of workpiece before polishing, process parameters, tooling and shot blasting machine itself, and propose countermeasures and conduct process test verification.

5.1 Surface quality of workpiece before shot blasting

(1) The workpiece is rusted before shot blasting

According to the observation and analysis, due to the epidemic situation in 2020, the factory will stop production after the Spring Festival, and the work pieces will be stored for a long time.

In addition, the high air humidity will lead to corrosion of the work pieces, as shown in Fig. 3 and Fig. 4.

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 3

Fig. 3 Rust on pulley before shot blasting

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 4

Fig. 4 Rust on pulley shaft before shot blasting

In order to improve the corrosion condition of workpieces, in addition to strictly controlling the production plan and shortening the storage time as much as possible, the workpieces in stock were treated with rust prevention and packaging.

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

It was observed that the macula mainly appeared after tempering (see Fig. 5), and it was analyzed that it might be caused by the residue of cleaning solution.

The cleaning process after quenching is: oil drainage → degreasing (including soaking+spraying) → cleaning (spraying) → high temperature drying.

As there is only one cleaning process in the process, it is easy to mix the degreasing solution into the cleaning tank, which will lead to the pollution of the cleaning tank.

If the contaminated cleaning tank solution is not completely drained before the drying of the workpiece, it may produce a watermark.

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 5

Fig. 5 Surface macula after cleaning

In order to test the influence of the concentration of cleaning agent on the macula on the workpiece surface, we used 2.5%, 2.0% and 1.5% cleaning solutions with different concentrations of cleaning agent for test verification.

It was found that the macula could not be eliminated by changing the concentration.

The macula can be obviously eliminated by methanol cleaning, as shown in Fig. 6.

Since there is no place in the production line to add a cleaning station, in order to eliminate the impact of degreasing cleaning solution on the workpiece surface as much as possible, it is proposed to add a water blowing process after cleaning, change the cleaning agent model, and select a residue free cleaning agent to eliminate this potential surface quality hazard.

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 6

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

According to the analysis, since the exhaust pipe of the reheating furnace is perpendicular to the furnace, it is found that there is residual oil stain at the interface when the pipe is disassembled for inspection.

When the oil stain directly flows back to the reheating furnace, it falls to the surface of the workpiece after being stirred by the fan to form an oil stain, as shown in Fig. 7 and Fig. 8.

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 7

Fig. 7 Residual Oil Spots on the Surface after Tempering

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 8

Fig. 8 Residual Oil in Exhaust Pipe

Improvement measures: reconstruct the original pipeline, add oil receiving and oil draining devices at the bottom of the pipeline to prevent oil contamination from entering the tempering furnace, and connect oil draining devices to the flue as shown in Fig. 9 and Fig. 10.

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 9

Fig. 9 Schematic diagram of oil drainage device

Watermarks Left on the Workpiece Surface After Shot Blasting: How to Solve This Problem? 10

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 and coverage, etc.

Any change of these parameters will affect the shot blasting effect to varying degrees.

Hang the tooling full of workpieces, according to the normal processing conditions, conduct process tests by adjusting the flow, hardness, size of steel shot, different shot angle and time, and adjusting the position, orientation, quantity, etc. of workpieces, compare the difference of workpiece cleaning effects under various working conditions, find the parameters and time required for qualified shot blasting, and provide a basis for further proposing rectification plans and improving the surface quality of workpieces.

According to the 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, the driving and driven pulley shafts were changed from 20/hang to 28/hang, the steel shot was changed from φ0.6 mm to φ0.8 mm, 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 1260~1500 s.

Table 2 Effect of Process Parameters on Shot Blasting Surface Quality

Process ParametersForecast reasonValidation ProtocolValidation effect
Shot flowInsufficient shot supply and insufficient shot strengthThe shot blasting speed is set to high speed, and the current of the shot blasting head is adjusted to 15~16A.
The best load current of the shot blasting machine is found by comparing the difference of the cleaning effect of different steel shot flows on the workpiece.
There is no obvious improvement in surface quality.
It is verified by different shot blasting machine frequencies of 55Hz, 65Hz, 75Hz and 85Hz.
The shot blasting verification results show that high frequency is not better than low frequency.
Projection angle and distanceThe projection area is not aligned with the workpieceBy adjusting the angle of the two throwing heads, changing the tooling hanger, changing the hanging method, increasing the angle of oblique setting, and adjusting the hanging height, opening orientation and angle of the workpiece, the steel shot shall strike the inner cavity and cone of the workpiece as far as possibleThrough the test and comparison of different shot blasting angles and different loading methods, the surface quality of the pulley shaft has been improved to a certain extent, but the pulley has not been significantly improved, and there are still obvious watermarks in the inner cavity and cone surface of each layer of workpiece.
Hardness of steel shotImproper hardness of steel shotUse steel shot with hardness of 58~63HRC to replace steel shot with hardness of 45~52HRC, compare the difference of cleaning effect of steel shot with different hardness on the workpiece, find the steel shot with similar hardness (58~64 HRC), and find the qualified time of shot blasting.1) During the beat time, the belt wheel shot blasting effect is blocked by the inner wall of the hydraulic cylinder, and the watermark cannot be removed.
2) In the beat time, the shot blasting effect of the pulley shaft has not been significantly improved, and the watermark can be removed only after a longer shot blasting time.
Shot diameterImproper shot sizeUse φ0.8mm steel shot to replace φ0.6mm steel shot. The mass and size of a single steel shot are about 2.3 times of the original.
Compare the difference of cleaning effect of different sizes of steel shot on the workpiece.
φ0.8mm steel shot is stronger than φ0.6mm steel shot, but under the condition of constant current, the number of particles projected at a single time is small, the coverage is affected, and the shot blasting effect is not significantly improved.
Ejection timeInappropriate ejection timeContinuously adjust the ejection time and increase the coverage until the surface cleaning quality meets the requirements.When the ejection time is extended to 1500s, the cleaning effect can be achieved, but the beat still exceeds the requirements, which cannot meet the production needs.

6. Countermeasures for shortening the production rhythm

After the replacement of tooling and the test verification of various process parameters, the surface quality has not improved significantly within the beat time.

Due to the restriction of equipment, the current and shot blasting flow cannot be adjusted higher.

If the shot blasting time is extended, the production beat cannot meet the set requirements.

Therefore, after analysis and research, it is decided to reform the equipment.

1) Replace two 7.5kW motors of the shot blasting machine with two 11kW motors, replace two frequency converters, replace belt wheels, electrical parts and other accessories, so that the shot blasting cleaning time can be shortened to the range required by the beat.

2) The shot blasting test plate shall be remade according to the impact marks of steel shot on the surface of the tooling hanger to verify the effective shot blasting area of the steel shot of the polishing head.

3) Reduce the diameter of the hanger, adjust the tilt angle of the shaft suspension, and make the opening face outward when the parts are hung, so that the shot steel shot can hit the inner cavity and cone directly.

4) The number of workpieces of the driving pulley and driven pulley is changed from 28/hanging to 24/hanging, and one piece is reduced for each layer to prevent the workpieces from covering each other and affecting the shot blasting effect.

The tooling of the driving pulley shaft and driven pulley shaft is changed from 15 pieces/hanger on the third floor 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

Contents of a projectOriginal equipment parametersParameters after transformation
Shot blasting machine Y-30(Variable frequency motor)2×7.5kW×4P2×11kW×4P
Processing range of products/mmφ700×1000invariant
Hardness of φ0.6mm stainless steel shot HRC45~5258~63
Maximum lifting weight of hook (including product hanger)/kg100invariant
Power/kW2229

7. Final process validation results

After the completion of the equipment transformation, the small batch process validation of about 1000 pieces of 4 kinds of workpieces was carried out by re optimizing the process parameters such as the angle, frequency and current of the polishing head.

See Table 4 for the results.

Table 4 Final Process Validation Results

Workpiece

Process Parameters

Number and beat of each pendant

Validation results

Driving pulley

1) Current value:

NO.1:22A

NO.2:20A

2) Frequency: 55Hz

3) Throwing angle:

Up 45 °, down 55 °

4) Stop at three o’clock:

Stop before 175s

Medium stop for 175s

Stop after 175s

24 pieces/hang

Shot blasting 525s, beat 21.875s/piece

1) No rust or watermark on the workpiece surface

2) The beat meets the requirements

3) Qualified

Driven pulley

24 pieces/hang

Shot blasting 525s, beat 21.875s/piece

Driven pulley shaft

16 pieces/hang

Shot blasting 525s, the beat is 32.8125s/piece

Driving pulley shaft

16 pieces/hang

Shot blasting for 525s, there is still a small amount of imprint in the deep hole at the big end of the workpiece, and it will be removed after another 120s in the opposite direction, with the beat of 40.3125s/piece

1) No rust or watermark on the workpiece surface

2) The beat exceeds the set requirements, acceptable

The driving pulley shaft is shot blasted for 525s, and the workpiece surface is free of rust and watermark, but there is still a small amount of imprint in the large end deep hole of some workpieces.

After 120 seconds of shot blasting in the opposite direction, the impression was basically removed, but the single piece beat was 40.3 s/piece, exceeding the setting requirement of 35 s.

Considering that there is one set of four kinds of workpieces, the total beat time of each set of workpieces is required to be 140s.

Now, the actual production time of one set of workpieces is 116.875s, and the total beat time is within the required range.

8. Conclusion

Shot blasting process validation is an important process.

Samples shall be taken under different process parameters, shot strength and coverage to determine which combination of factors can obtain better surface quality.

Generally, the shot blasting current directly reflects the shot blasting amount.

Generally, the current value of the upper and lower shot blasters will be adjusted to be the same during calibration.

But in fact, the lower shot blaster is used to throw the steel shot upward.

Under the same load, the steel shot thrown is less than the upper shot blaster.

Therefore, in order to make the surface quality of the upper and lower workpieces more consistent, we initially adjusted the current value of the lower shot blaster to be about 5% higher than the upper shot blaster.

In the process of actual process validation, it was found that the watermark on the surface of the upper layer of the workpiece was more difficult to remove than that on the lower layer.

Therefore, the current value of the upper shot blasting device was adjusted to be about 10% higher than that of the lower shot blasting device, so that the surface quality of the upper and lower workpieces tended to be consistent.

In the process of process validation, it was found that by adjusting the tooling and process parameters, the current and shot blasting flow were adjusted to the maximum, and the surface quality and beat of the workpiece did not meet the requirements.

Finally, the equipment was modified and a higher power motor was replaced.

The process validation showed that the equipment could meet the requirements of product quality and beat, and the problem of residual watermark on the surface of the workpiece after shot blasting was solved.

The equipment has been put into operation for 4 months after the transformation, and the cumulative production has exceeded 220000 pieces, with good results.

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