Abrasives for Dry Blast Cleaning

The materials used in dry abrasive blast cleaning can be categorized as metallic grit, metallic shot, sand, glass, and miscellaneous. Hardness, density, size, and shape are important considerations in choosing an abrasive for a specific application. The selection of the type and size of the blast cleaning material will depend on the size and shape of the parts to be cleaned, the finish desired, and the treatment or operation that may follow blast cleaning. The success of blast cleaning operations depends primarily on judicious selection of method and abrasive medium. The surfaces, especially ferrous surfaces, tend to be very active following abrasive cleaning, and any subsequent operation such as plating or painting should be performed as soon as possible after abrasive cleaning.

Metallic abrasive media consist of grit, shot, and cut wire.

Grit consists of angular metallic particles with high cutting power. Grit is usually made of crushed, hardened cast steel shot, which may be tempered, or of chilled white cast iron shot, which may be malleabilized. Size specifications for cast grit are shown in Table 3. In general, three hardnesses are offered in steel grit: 45, 56, and 65 HRC. The screen distribution and the velocity of the grit impacting on the part surfaces control the finish. Usually, grit blast produces a brighter finish than shot blast. Applications for grit include removal of heavy forging and heat-treat scale, removal of rust, and controlled profiling of workpieces before bonding or coating. Hard grit is also used to provide a gripping surface on steel mill rolls.

Table 3 Size specifications for cast grit (SAE J444)

Size No.

Screen tolerances(a)

Screen opening

mm

in.

G10

All pass No. 7

2.82

0.1110

80% min on No. 10

2.00

0.0787

90% min on No. 12

1.68

0.0661

G12

All pass No. 8

2.38

0.0937

80% min on No. 12

1.68

0.0661

90% min on No. 14

1.41

0.0555

G14

All pass No. 10

2.00

0.787

80% min on No. 14

1.41

0.0555

90% min on No. 16

1.19

0.0469

G16

All pass No. 12

1.68

0.0661

75% min on No. 16

1.19

0.0469

85% min on No. 18

1.00

0.0394

G18

All pass No. 14

1.41

0.0555

75% min on No. 18

1.00

0.0394

85% min on No. 25

0.711

0.0280

G25

All pass No. 16

1.19

0.0469

70% min on No. 25

0.711

0.0280

80% min on No. 40

0.419

0.0165

G40

All pass No. 18

1.00

0.0394

70% mm on No. 40

0.419

0.0165

80% mm on No. 50

0.297

0.0117

G50

All pass No. 25

0.711

0.0280

65% min on No. 50

0.297

0.0117

75% mm on No. 80

0.18

0.0070

G80

All pass No. 40

0.419

0.0165

65% mm on No. 80

0.18

0.0070

75% mm on No. 120

0.12

0.0049

G120

All pass No. 50

0.297

0.0117

60% mm on No. 120

0.12

0.0049

70% mm on No. 200

0.074

0.0029

G200

All pass No. 80

0.18

0.0070

55% mm on No. 200

0.074

0.0029

65% min on No. 325

0.043

0.0017

G325

All pass No. 120

0.12

0.0049

20% mm on No. 325

0.043

0.0017

(a) Minimum cumulative percentages (by weight) allowed on screens of numbers and opening sizes as indicated

(a) Minimum cumulative percentages (by weight) allowed on screens of numbers and opening sizes as indicated

Shot, normally made of the same materials as grit, is usually in the form of spherical particles. Shot removes scale, sand, and other surface contaminants by impact. Size specifications for cast shot are indicated in Table 4. Steel shot is the most widely used metallic abrasive medium and is least destructive to the components of the abrasive blast system. The matte finish produced by steel shot on metal surfaces can be controlled by the screen distribution of the operating mix and the velocity of shot impacting on part surfaces.

Table 4 Cast shot size specifications for shot peening or blast cleaning (SAE)

Screen No.

Screen size

Screen opening(a)

Passing(a), %

mm

in.

7

2.82

0.111

780

All pass

8

2.38

0.0937

660

All pass

10

2.00

0.0787

780

85 min

550

All pass

460

All pass

12

1.67

0.0661

780

97 min

660

85 min

460

5 max

390

All pass

14

1.41

0.0555

660

97 min

550

85 min

390

5 max

330

All pass

16

1.19

0.0469

550

97 min

460

85 min

330

5 max

280

All pass

18

1.00

0.0394

460

96 min

390

85 min

28G

5 max

23G

All pass

2G

G.841

G.G331

39G

96 min

33G

85 min

23G

1G min

17G

All pass

25

G.711

G.G28G

33G

96 min

28G

85 min

17G

All pass

3G

G.59G

G.232

28G

96 min

23G

85 min

llG

All pass

35

G.5GG

G.G197

23G

97 min

llG

1G max

4G

G.419

G.G165

17G

85 min

7G

All pass

45

G.351

G.G138

17G

97 min

7G

1G max

5G

G.297

G.G117

11G

8G min

8G

G.18

G.GG7

11G

9G min

7G

8G min

120

0.124

0.0049

70

90 min

(a) Screen opening sizes and screen numbers with maximum and minimum cumulative percentages allowed on corresponding screens

Cut wire is available from aluminum, zinc, steel, or stainless steel primary metal. Cut wire deforms into rounded particles during usage or conditioning processes prior to sale; it is used frequently in the same manner as cast shot. Table 5 shows the specifications relating standard size numbers for cut steel wire shot to diameter and minimum hardness.

Table 5 Specifications for cut steel wire shot (SAE J441)

Size No.

Diameter of wire

Minimum hardness, HRC

mm

in.

CW-ó2

1.59±0.05

0.0ó25±0.002

36

CW-54

1.4±0.05

0.054±0.002

39

CW-47

1.2±0.05

0.047±0.002

41

CW-41

1.0±0.05

0.041±0.002

42

CW-S5

0.89±0.03

O.OSSiO.OOl

44

CW-S2

O.SliO.OS

0.032±0.001

45

CW-2S

0.71±0.0S

0.02S±0.001

46

CW-2S

o.ssío.os

0.023±0.001

48

Nonmetallic abrasive media include sand, glass, agricultural products, and plastic and nylon. Table 6 lists physical properties and comparative characteristics of a variety of nonmetallic abrasives.

Table 6 Physical pro

perties and comparative characteristics of nonmetallic abrasives

Description

Glass beads(a)

Coarse mineral abrasives(b)

Fine angular mineral abrasives(c)

Organic soft grit d abrasives(d)

Plastic abrasives

Physical properties

Shape

Spherical

Granular

Angular

Irregular

Cylindrical (diameter/length = 1)

Color

Clear

Tan

Brown/white

Brown/tan

Nylon: white, polycarbonate: orange

Specific gravity

2.45-2.50

2.4-2.7

2.4-4.0

1.3-1.4

Nylon: 1.15-1.17, polycarbonate: 1.2-1.65

Free silica content

None

100%

<1%

None

None

Free iron content

<1%

<1%

<1%

None

None

Hardness (Mohs)

5.5

7.5

9.0

1.0

R-110 to R-120

Media comparisons

Toxicity

None

High

Low

Low/none

None

Metal removal

Low/none

High

High

None

Deburring only

Cleaning speed

Medium/high

High

High

Low

Low

Peening ability

High

None

None

None

None

Finish achieved

Range (various matte)

Rough anchor

Various matte

Smooth

Smooth

Surface contamination

None

Medium

Medium

Medium/high

Low to none

Suitability for wet blasting

High

Low

Low

Low

Low

Suitability for dry blasting

High

High

High

High

High

Standard size ranges

20-325

8-200

80-325

60-325

0.76 by 0.76 mm (0.030 by 0.030 in.)

U.S. mesh

U.S. mesh

U.S. mesh

U.S. mesh

1.1 by 1.1 mm (0.045 by 0.045 in.) 1.5 by 1.5 mm (0.060 by 0.060 in.)

Consumption rate

Low

High

Medium

High

Very low

Cost comparison

Medium

Low

High/medium

High/medium

High/medium

(a) Glass beads are used for cleaning, finishing, light-to-medium peening, and deburring.

(b) Coarse mineral abrasives such as sand are used where metal removal and surface contamination are not considered.

(c) Fine angular mineral abrasives such as aluminum oxide are used in cleaning when smooth finish and surface contamination are not important.

(d) Organic soft grit abrasives, for example, walnut shells, are used in light deburring and cleaning of fragile items.

(e) (e)Plastic abrasives such as nylon and polycarbonate are used to deflash thermoset plastic parts and deburr finished machine parts.

Sand. This term is applied to diverse nonmetallic abrasives, in addition to ordinary silica sand. These materials are used when it is necessary to protect the surface of the workpiece from metallic contamination. They may be either natural materials, such as garnet, novaculite, dolomite, pumice, and flint quartz, or manufactured materials such as aluminum oxide, silicon carbide, and slag. The natural materials are lowest in initial cost; the manufactured materials, although somewhat more expensive than natural sands, cost less than metallic abrasives.

Glass is available as angular particles (ground glass) or spherical particles (glass beads) ranging in size from approximately 1 mm (0.039 in.) to less than 40 ^m (0.0016 in.). The particles are usually made from soda-lime-silica glass with a hardness of about 500 HK (100 g load), equivalent to 46 to 50 HRC. Ground glass is effective for deburring and relatively aggressive finishing. Glass beads produce a fine matte appearance and clean without removing base metal. Size and roundness specifications for glass beads are given in Table 7.

Table 7 Standard size and roundness specifications for glass beads (MIL-G-9954A)

standard screen

Bead size

Roundness, minimum, %

Passing, %

10

1

60

100

12

1

60

95-100

2

60

100

14

1

60

0-15

2

60

95-100

3

65

100

20

1

60

0-5

2

60

0-15

3

65

95-1GG

4

7G

1GG

3G

2

6G

G-5

3

65

G-15

4

7G

95-1GG

5

7G

1GG

4G

3

65

G-5

4

7G

G-15

5

7G

95-1GG

6

8G

1GG

5G

4

7G

G-5

5

7G

G-15

6

8G

95-1GG

7

8G

1GG

6G

5

7G

G-5

7

8G

95-1GG

8

8G

1GG

7G

6

8G

G-15

8

8G

95-1GG

9

8G

1GG

8G

6

8G

G-5

7

8G

G-15

9

8G

95-1GG

1G

9G

1GG

1GG

7

8G

G-5

8

8G

G-15

1G

9G

95-1GG

11

9G

1GG

12G

8

8G

G-5

9

8G

G-15

11

9G

95-1GG

12

9G

1GG

14G

9

8G

G-5

12

9G

95-1GG

13

95

1GG

17G

1G

9G

G-15

13

95

95-1GG

2GG

1G

9G

G-5

11

9G

G-15

23G

11

9G

G-5

12

9G

G-15

325

12

9G

G-5

13

95

0-15

400

13

95

0-5

Miscellaneous Materials. Mild abrasive action is provided by the use of such agricultural products as crushed walnut or pecan shells, rice hulls, rye husks, corncobs, and sawdust. Plastic and nylon media are also widely used for special purposes. Recent developments include the use of more environmentally attractive media such as CO2 pellets and wheat starch.

Selection of abrasive for a specific application is influenced by the type of surface contamination to be removed, size and shape of the workpiece, surface finish specified, type and efficiency of cleaning equipment, and required production rate. Type, size, and hardness of metallic abrasives recommended for some typical applications are given in Table 8. In addition to the recommendations in this table, the following general observations relating to the performance of abrasive particles may be helpful:

• The smaller the abrasive particle, the finer the surface finish and the faster the surface coverage.

• The larger the abrasive particle, the greater the impact.

• In general, the harder the abrasive particle, the faster its cleaning action.

• The larger or harder the abrasive, the greater the wear on the equipment.

Table 8 Selection of abrasive

Application

Recommended abrasive(a)

Type

Nominal diameter

Hardness HRC

mm

in.

Blasting of ferrous metals

Removal of light scale®

Shot or grit

0.2-0.71

0.007-0.028

30-66(c)

Removal of heavy scale®

Shot or grit

0.71-2.0

0.028-0.078

45-66

Cleaning of castings

Shot or grit

0.43-2.0

0.017-0.078

30-66(c)

Blasting of nonferrous metals

Frosted appearance only

Grit

0.1-0.43

0.005-0.017

50-66

Preparation for other surface finishes

Shot or grit

0.2-0.71

0.007-0.028

30-66(c)

(a) Cast iron or cast steel.

(b) For phosphating or painting.

(c) Cast steel abrasive usually is not available in a hardness of less than 40 HRC.

Replacement of Abrasive. Production of a uniformly abraded surface depends on maintaining a uniform working mix of abrasive in the machine at all times. Metal surfaces treated with hard grit are more sensitive to a change in working mix than those treated with a soft grit or with shot. A program that includes testing and continuous gradual replacement of the abrasive is recommended. The working mix contains more abrasive fines than new abrasive. Examples of screen analyses of new and used abrasives are given in Table 9.

Table 9 Screen analysis of three metal abrasives

Screen No.

Screen size

Abrasive remaining on screen, %

mm

in.

New abrasive

Working mix

G25 cast steel grit

16

1.19

0.0469

Trace

None

20

0.841

0.0331

82

23

30

0.589

0.0232

16

33

40

0.419

0.0165

1

19

50

0.297

0.0117

Trace

17

70

0.21

0.0083

Trace

6

Pan

Trace

2

G40 cast steel grit

16

1.19

0.0469

None

None

20

0.841

0.0331

19

None

30

0.589

0.0232

76

18

40

0.419

0.0165

4

51

50

0.297

0.0117

Trace

26

70

0.21

0.0083

Trace

5

Pan

Trace

Trace

S280 cast steel shot

16

1.19

0.0469

None

None

20

0.841

0.0331

40

27

30

0.589

0.0232

58

56

40

0.419

0.0165

1

11

50

0.297

0.0117

1

5

70

0.21

0.0083

None

1

Pan

None

None

A practical method of maintaining a reasonable degree of consistency of the working mix is to keep a uniform level of abrasive in the supply tank, bin, or hopper. This is accomplished by adding new abrasive periodically, usually at least once every 2 h. If surface finish requirements are more critical, hourly additions of new abrasive may be needed. The total tonnage of abrasive required to fill the machine to operating capacity also may affect the frequency of additions. When surface finish requirements are critical, the use of an automatic abrasive replenisher is indicated. This device maintains the abrasive level in the blasting machine by automatically feeding abrasive from a supply hopper. Consumption is gradual; therefore, replenishment should be equally gradual.

For control purposes, a representative sample of the abrasive used in the machine should be analyzed periodically. The frequency of these tests depends largely on surface finish and production requirements. Dust fines should be removed from the mix by an air separating system; proper performance of a separating device depends on a uniform flow of air through the separator and on maintenance of uniform abrasive sizes and quantities.

Control of Contaminants. After the abrasive medium makes contact with the workpiece, it is returned to a storage hopper for reuse. Coarse and fine contaminants picked up in the process are removed as the medium is returned to the hopper. Coarse contaminants include tramp metal, fins, core wire, core nails, slag, sand lumps, large flakes of rust and scale, and flash. These are usually removed by screening the abrasive mix through wire mesh, perforated plate, or expanded metal. Automatic rotary screens are preferred over stationary tray-type screens.

Fine contaminants include sand, fine mill scale and rust particles, metallic dusts, and disintegrated abrasive particles. Buildup of fine contaminants reduces blast efficiency, and a high sand content results in excessive wear of centrifugal blast wheel parts. Fine contaminants are removed from the mix by a current of air. In an expansion chamber, the heavier fine particles resist an upward turn of the air current and drop into the settling area to be discharged as refuse. Lighter particles remain suspended in the air current and are carried out through the ventilating lines to the dust collector or exhaust.

In many blast cleaning operations, small metallic particles resulting from the wearing or breaking down of abrasives assist in scouring small crevices or valleys. If such fines are to be retained, separators must be adjusted accordingly.

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