Mechanical Cleaning and Finishing

Mechanical cleaning and finishing methods most commonly employed for processing iron castings include abrasive blast cleaning (the most commonly used method for cleaning cast irons), abrasive waterjet cleaning and finishing, vibratory finishing, barrel finishing, and shot peening.

Blast cleaning of castings is a process in which abrasive particles are propelled at high velocity to impact the casting surface and thereby forcefully remove surface contaminants. The contaminants are usually adhering mold sand, burned-in sand, heat treat scale, and the like.

The usual methods of imparting high velocity to abrasive particles are by the use of either centrifugal wheels (Fig. 2) or compressed air nozzles. Centrifugal wheels are the most widely used method because of their ability to propel large volumes of abrasive efficiently. For example, a 56 kW (75 hp) centrifugal wheel can accelerate steel shot to 73 m/s (240

ft/s) at 55,800 kg/h (123,000 lb/h) flow. To do the same with 13 mm (-2 in.) direct pressure venturi nozzles at 45 kg (100

lb)/min per nozzle would require approximately 20 nozzles and an air flow of 0.120 m3/s/nozzle (260 ft3/min/nozzle) x 20, or a total of 2.45 m3/s (5200 ft3/min) at 550 kPa (80 psi). Approximately 700 kW (940 hp) at the air compressor would be required to supply this amount of air, which gives a 700 kW/56 kW = 12.5 to 1 (940 hp/75 hp = 12.5 to 1) efficiency advantage for the centrifugal wheel.

Fig. 2 Front view of a typical rotating hanger blast cleaning assembly. The workpiece is rotated 360° as it is held in position on the monorail track to provide full coverage by the blast pattern produced by the centrifugal wheels.

Even though the nozzle blast is not as efficient overall as the wheel blast, in some applications it may be more efficient because the blast stream can be more efficiently applied, when, for example, blasting into small holes to clean the interior areas of a casting. Other reasons for using a nozzle blast are requirements for:

Low production Portability

• Suitability for very hard abrasives, such as aluminum oxide

More detailed information on the selection and design of blast cleaning equipment can be found in Ref 1 and the article "Mechanical Cleaning Systems" in this Volume.

In the past, chilled iron grit and malleable abrasives were used. Today, however, practically all the shot and grit used is high-carbon cast steel that is heat treated and drawn to give a desired tempered martensite microstructure and hardness. The hardness range of the most commonly used shot and grit is HRC 40 to 50. Harder shot and grit are also produced, with a range of HRC 55 to 65. For faster cleaning or special surface finish requirements, hard abrasive is not often used; but when it is, the wear on machine parts is high, and the abrasive breakdown rate is more rapid. This is especially true of hard grit. Table 3 lists commercially available shot and grit sizes.

Table 3 Society of Automotive Engineers shot and grit size specifications for abrasive blast cleaning

High-limit screen

Nominal screen

Low-limit screen

Maximum

retained

Screen number and aperture (in.)

Maximum

retained

Screen number and aperture (in.)

Minimum

retained

Screen number and aperture (in.)

Minimum

retained

Screen number and aperture (in.)

Shot number

780

1

7 (0.111)

85

10 (0.0787)

97

12 (0.0661)

660

1

8 (0.0937)

85

12 (0.0661)

97

14 (0.0555)

550

1

10 (0.0787)

85

14 (0.0555)

97

16 (0.0469)

460

1

10 (0.0787)

5

12 (0.0661)

85

16 (0.0469)

96

18 (0.0394)

390

1

12 (0.0661)

5

14 (0.0555)

85

18 (0.0394)

96

20 (0.0331)

330

1

14 (0.0555)

5

16 (0.0469)

85

20 (0.0331)

96

25 (0.0280)

280

1

16 (0.0469)

5

18 (0.0394)

85

25 (0.0280)

96

30 (0.0232)

230

1

18 (0.0394)

10

20 (0.0331)

85

30 (0.0232)

97

35 (0.0197)

S-170

1

20 (0.0331)

10

25 (0.0280)

85

40 (0.0165)

97

45 (0.0138)

S-110

All pass

30 (0.0232)

10

35 (0.0197)

80

50 (0.0117)

90

80 (0.0070)

S-70

All pass

40 (0.0165)

10

45 (0.0138)

80

80 (0.0070)

90

120 (0.0049)

Grit number

G-10

1

7 (0.111)

80

10 (0.0787)

90

12 (0.0661)

G-12

1

8 (0.0937)

80

12 (0.0661)

90

14 (0.0555)

G-14

1

10 (0.0787)

80

14 (0.0555)

90

16 (0.0469)

G-16

1

12 (0.0661)

75

16 (0.0469)

85

18 (0.0394)

G-18

1

14 (0.0555)

75

18 (0.0394)

85

25 (0.0280)

G-25

1

16 (0.0469)

70

25 (0.0280)

80

40 (0.0165)

G-40

1

18 (0.0394)

70

40 (0.0165)

80

50 (0.0117)

G-50

1

25 (0.0280)

65

50 (0.0117)

75

80 (0.0070)

G-80

All pass

40 (0.0165)

65

80 (0.0070)

75

120 (0.0049)

G-120

All pass

50 (0.0117)

60

120 (0.0049)

70

200 (0.0029)

200

All pass

80 (0.0070)

55

200 (0.0029)

65

325 (0.0017)

Making a choice between shot and grit depends on the surface contaminants or the surface texture required. Grit is used when a chiselling action is required, for example, when removing rust, or perhaps to help remove burned-in sand or provide a good bonding surface for painting, plating, or enameling. Hard grit is used to clean the surface of bathtubs prior to enameling, where a definite tooth is required on the casting surface to provide for better adhesion of the enamel. In some cleaning applications, a shot and grit mixture may be used.

Abrasive Waterjet Cleaning. Although most often used as a cutting or machining process, the abrasive waterjet process has been tested for its use in degating and defining castings, as well as burn-in removal from castings. High-pressure waterjets (without abrasives) have also been tested for cleaning hydraulic passageways in castings. Using this process, coherent fluid jet is formed by forcing high-pressure 200 to 400 abrasive-laden water through a tiny sapphire orifice. The accelerated jet exiting the nozzle travels at more than twice the speed of sound and impinges on the workpiece. Results from a study from a foundry producing castings used in lawn, garden, and farm equipment are given in Ref 2. Detailed information on the use of abrasive waterjets for cutting metals and nonmetals can be found in Ref 3 and 4.

A vibratory finishing machine is an open-topped tub or bowl mounted on springs, usually lined with polyurethane. Parts and media are loaded in a fashion similar to that of a tumbling barrel (see the discussion on barrel finishing which immediately follows). With a vibratory machine, the container can be almost completely filled. Vibratory action is created either by a vibratory motor attached to the bottom of the container, by a shaft or shafts with eccentric loads driven by a standard motor, or by a system of electromagnets operating at 50 or 60 Hz. The action of media against components takes place throughout the load, so that process cycles are substantially shorter than conventional tumbling in barrels.

Vibratory finishing is used to clean the internal passages of cast iron cylinder heads and engine blocks. The internal passages of such components are rather intricate and it is usually difficult to reach all surface areas from any externally propelled form of cleaning. High pressure water, air blast, and shot blasting all would clean the internal surface areas that were located near an external opening; however, most of the internal passageways are hidden from this method of cleaning. Therefore, the vibratory media cleaner was introduced to clean these passages (Ref 5).

The media enters the internal passageways of the castings under the vibration of the vibratory machine and literally scrubs the surface walls of the internal cavities. The in-and-out movement of the media also carries core wash and sand from the internal passages of the castings, allowing removal of the material and providing a clean casting. Additional information on vibratory finishing can be found in the article "Mechanical Cleaning Systems" in this Volume.

Barrel Finishing. The rotary barrel, or tumbling barrel, utilizes the sliding movement of an upper layer of workload in the tumbling barrel, as shown in Fig. 3. The barrel is normally loaded about 60% full with a mixture of parts, media, compound, and water. As the barrel rotates, the load moves upward to a turnover point; then the force of gravity overcomes the tendency of the mass to stick together, and the top layer slides toward the lower area of the barrel. The rotation of the barrel causes the abrasive medium to scour the casting surfaces. Scale, sand, and even fins can be affectively removed during tumbling. Additional information on barrel finishing can be found in the article "Mechanical Cleaning Systems" in this Volume.

Shot peening is a method of cold working in which compressive stresses are induced in the exposed surface layers of cast iron parts by the impingement of a stream of steel shot, directed at the surface at high velocity under controlled conditions. It differs from blast cleaning in primary purpose and in the extent to which it is controlled to yield accurate and reproducible results. Although shot peening cleans the surface being peened, this function is incidental. The major purpose of shot peening is to increase fatigue strength.

Both the benefits in improved fatigue strength and increased surface hardness have been shown in studies on shot peened austempered ductile iron gears (Ref 6). In cases where severe grinding operations are performed on a gear, resultant surface tensile stresses can have a negative impact on part endurance. Peening after the grinding operation can increase the endurance limit even above the original gentle grind design condition, as shown in Fig. 4.

Sliding layer of medium ai^d parts

Fig. 3 Action of media and parts within a rotating barrel

Sliding layer of medium ai^d parts

Fig. 3 Action of media and parts within a rotating barrel

Fig. 4 Effect of shot peening on the fatigue strength of a ground component. A part designed for a gentle grinding operation could be salvaged by shot peening after a severe grinding operation. Source: Ref 6

When fatigue problems occur, one of the solutions is to eliminate stress risers in the part by polishing or other surface refining process. Though the stress riser in the material tends to be reduced, any subsequent scarring of the surface in application can greatly reduce any benefits of the polishing operation. Costs to produce a surface finish finer than a 125 rms will usually increase substantially.

Instead of polishing the surface, shot peening will produce a compressed stress layer below the surface of the material that will prevent crack propagation and increase fatigue strength. Not only can this be used as a salvage technique, but it can be used as a cost reducer by eliminating the need for the added polishing costs.

In cases where uniform casting texturing is required, the controls used in the peening process can impart a finish that is homogenous in appearance. In this situation, the benefits of the compressive stress to produce improved fatigue strength may not be critical, and surface texture selection will most likely determine shot size and application intensity.

To retain the fatigue strength benefits produced by peening, no more than 10% of the depth of compression can be removed by the subsequent machining operation. Shot peening should be performed after heat treating so that the compressive stresses and reduction of fatigue strength are not dissipated as heat treating temperature approaches stress relieving temperature.

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