Wet Blasting

Wet blasting differs from dry blasting in that the abrasive particles used are usually much finer and are suspended in chemically treated water to form a slurry. The slurry, pumped and continually agitated to prevent settling, is forced by compressed air through one or more nozzles, which are directed at the work.

Applications

In further contrast to dry blasting, wet blasting is not intended for the gross removal of heavy scale, coarse burrs, or soil, but is intended to produce only relatively slight effects on the workpiece surface. Wet blasting is most commonly used for:

• Removing minute burrs on precision parts

• Producing satin or matte finishes

• Inspection finish ground, hardened parts

• Removing fine tool marks from hard parts

• Removing light mill scale or machining marks in preparation for plating

• Removing surface oxide in preparation for soldering of electronic components and printed circuits

• Removing welding scale

Many small parts, including hypodermic needles and electronic components, are deburred by wet blasting. The application of wet blasting to large parts is limited to the cleaning and finishing of forging dies from which a minimum of metal removal is desired; dies weighing up to 90 tonnes (100 tons) have been wet blasted. Many cutting tools are wet blasted after final grinding.

Precleaning. In most instances, precleaning must precede wet blasting to prevent contamination of the recirculating slurry. Grease, protective coatings, and heavy oils may be removed by conventional degreasing methods. Heavy rust and dry soils may be removed by dry blasting. A very light layer of rust may be removed from machined parts by wet blasting without precleaning. However, in general, the finer the abrasive used in the wet blast slurry, the greater is the need for precleaning to prevent contamination of the slurry.

Abrasives for Wet Blasting. Many different kinds and sizes of abrasives can be used in wet blasting. Sizes range from 20-mesh (very coarse) to 5000-mesh (much finer than face powder). Among the types of abrasives used are organic or agricultural materials such as walnut shells, baking soda, and peach pits; novaculite, silica, quartz, garnet, and aluminum oxide; other refractory abrasives; and glass beads.

The organic or agricultural materials are used for mild blasting only. Novaculite, a soft type of silicon dioxide (99.46% silica; Mohs hardness, 6 to 6.5), is used to remove very light burrs and produces a fine matte finish. The silicas are slightly more aggressive than novaculite and are used to remove larger burrs and scale. Quartz is still more aggressive than silica and lasts longer; it is used for the rapid removal of tenacious burrs and scale.

Garnet abrasives are highly aggressive and have long life; for a comparable mesh size, garnet will produce a rougher surface finish than will silicon dioxide. Aluminum oxide, silicon carbide, and other artificial abrasives are the most aggressive and erosive. Round glass shot (beads) composed of 72% silicon dioxide, 15% sodium monoxide, 9% calcium oxide, and 1% alumina produce a brighter finish and provide the most peening action. The high ricocheting property of glass beads is of value in blasting areas that are hard to reach. Table 10 describes various abrasives used in wet blasting and lists typical applications.

Table 10 Characteristics and typical applications of abrasives used in wet blasting

Abrasive

Mesh size

Characteristics and applications

Silica

40-80

Fast-cutting. Used for deburring steel and cast iron, removing oxides from steel. Close tolerances cannot be held

Silica

80

Fast-cutting. Used for deburring steel and cast iron, roughening surfaces for plastic bonding or rough plating. Has peening action. Tolerances cannot be held

Quartz (ground)

80

Very fast-cutting. Used for removing heavy burrs, light or medium scale, excessive rust. Can be used on nickel alloy steels. Tolerances cannot be held

Novaculite

100

Fast-cutting. Used for cleaning carbon from piston and valve heads: deburring brass, bronze, and copper. Can be used on crankshafts. Tolerances cannot be held

Quartz (ground)

100, 140

Fast-cutting. Used for blending-in preliminary grind lines on steel, brass and die castings; removing medium-hard carbon deposits; blasting radii of 0.1 to 0.3 mm (0.005 to 0.010 in.)

Silica

140

Used for removing small burrs from steel, copper, aluminum, and die castings; rough cleaning of dies and tools, removing metal. Tolerances cannot be held

Novaculite

325

Slow-cutting. Used in first stage for cleaning master rods and glass, and in second stage for cleaning aluminum pistons, crankshafts, impellers, valves. Holds tolerances to 0.06 mm (0.0025 in.)

Aluminum oxide

400

Fast-cutting. Used on stainless steel and on zinc and aluminum die castings. Excellent for oil-contaminated surfaces

Novaculite

1250

Used in second stage for cleaning crankshafts, impellers, rods, pistons, valves, gears and bearings. Also for polishing metals, tools, dies and die castings. Tolerances can be held

Novaculite

5000

Used for obtaining extra-fine surfaces on parts

Glass beads

20-400

Used for removing scale or discoloration after heat treating, removing oxide from jet-engine and electronic components. Produces peening effect

Liquid Carriers. The liquids most commonly used to carry the abrasive particles are water-containing additives such as rust inhibitors, wetting agents, and anticlogging and antisettling compounds. In a few applications, such as in the manufacture of spark plugs, petroleum distillates have been used as abrasive carriers for the removal of oil residues and fine chips and burrs. In these instances, the use of water would create difficulties. Petroleum distillates, however, can be used only with specially designed wet blasting units because of the fire hazard.

The proportion of abrasive to liquid in the wet abrasive slurry can be varied over a wide range, although certain limitations apply. The use of a very small percentage of abrasive results in slight cleaning action, while too large a percentage of abrasive might result in the formation of a paste that could not be properly circulated. Proportions should be fixed at a predetermined level for each application, to ensure uniform cleaning action and production of a uniform finish. A range of 20 to 35% volume abrasive is satisfactory for most applications but may be modified because of particle size, surface tension, specific gravity, agitation, or the desired effect on the workpiece. Figure 9 illustrates the effects of three variables on metal removal during wet blasting of 1010 steel. Reclamation of wet abrasive is usually impractical. The mixture generally is used until its cleaning action becomes unsatisfactory and is then discarded.

Weight of metal removed, g

Weight of metal removed, g

Weight of metat removed, g

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Weight of metal removed, oí

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Weight of metal removed, g

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Weight of metal removed, o?

Weight of meta! removed, oz

Fig. 9 Effect of variables on amount of metal removed during wet blasting of 1010 steel

Equipment. Although equipment for wet blasting is often of special design for a particular application, several basic types of machines (Fig. 10) have been developed for general use, including:

• Cabinet-type machines

• Horizontal-plane turntable machines with tables of various diameters

• Vertical wheel-type machines

• Chain or belt conveyor machines

• Shuttle-type cabinets with cars and rail extensions

• Car-mounted, self-contained turning mechanisms for shafts or tubular parts

These basic types may be equipped with strippers, takeoff conveyors, and wash-rinse-dry stations. The blast guns may be mounted in a stationary position or be made to oscillate, depending on the application.

Fig. 10 Wet blasting machines

Combination blasting-tumbling cabinet-type machines also are available. These machines are made with an extension stand provided with rails so that a car-mounted tumbling barrel can be moved into the cabinet. The barrel, a self-contained unit, is provided with a driving mechanism for turning the barrel and oscillating the blast gun or guns inside the barrel. Barrels range in diameter from 305 to 660 mm (12 to 26 in.) and are perforated to allow the slurry to escape and recirculate.

Nozzles for wet blasting vary in form, shape, length, and diameter according to the type of abrasive used and the size of the parts to be blasted. The most common form is a cylindrical nozzle about 25 mm (1 in.) long, 13 mm (2 in.) inside

diameter, and 19 mm (4 in.) outside diameter. It is made of low-carbon steel and is used in hand-operated cabinets for general cleaning purposes. This nozzle uses silicates or quartz in mesh sizes ranging from 100 to 300 and at blasting pressures of 550 to 620 kPa (80 to 90 psi), and it has an average life of about 40 h.

Special nozzles, such as fan-shaped types, usually are made of an alloy cast iron chilled to a high hardness, although some special nozzles are made of rubber.

Carbide nozzles are used in mechanized units that provide surface preparation for processes such as anodizing and phosphating. The higher cost of cylindrical carbide nozzles is justifiable in terms of long life and low maintenance. For special shapes, however, the cost of carbide is likely to be prohibitive. When used with fine abrasives, a cylindrical carbide nozzle has a life of several thousand hours. In one application, a carbide nozzle exhibited no measurable wear after 1000 h of service with 140-mesh quartz at a pressure of 620 kPa (90 psi).

A nozzle for wet blasting is considered to be worn out when its wall thickness has been so reduced as to be potentially damaging to the gun, or the blast pattern provides inadequate coverage of the workpieces.

The amount of air required for wet blasting depends on the diameter of the air-jet nozzle and the operating pressure.

Figure 11 illustrates the cubic feet of air needed to operate air-jet nozzles 2.4 to 8 mm (— to — in.) in diameter at pressures from 70 to 690 kPa (10 to 100 psi). The data are based on operating a wet blasting gun with a slurry containing 40 vol% abrasive of 140-mesh.

Fig. 11 Amount of air required for various sizes of air-jet nozzles at different operating pressures

Maintenance. Typical maintenance schedules for hand-operated wet blasting cabinets are as follows:

• Weekly: Wash filter and pump-intake strainer, making sure all foreign particles are removed; grease the pump follower.

• Monthly: Check pump packing gland for leaks and tighten if necessary.

• Semiannually: Remove wear plate from pump and inspect impeller and housing for wear; grease fan pillow-block bearings.

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