Equipment for Dry Blast Cleaning

Dry blast cleaning is probably the most efficient and environmentally effective method for abrasive cleaning and finishing. Proper ventilation helps maintain a clean work area. No settling ponds or chemical treatment are required. Dust collectors provide dust disposal that is clean and simple, using sealed containers. Dry-blast systems need only be kept dry and can be started and stopped with minimum startup or shutdown operations. Several types of equipment are available for dry blast cleaning, and equipment selection is primarily based on the type of parts to be blasted and the relative throughput required.

Cabinet Machines. A high percentage of dry blast cleaning is performed using cabinet machines. A cabinet houses the abrasive-propelling mechanism, such as a centrifugal wheel or compressed air nozzle(s), holds the work in position, and confines flying abrasive particles and dust. Cabinets are available in a wide range of sizes, shapes, and types to meet various cleaning, production, and materials handling requirements. Cabinet machines may be designed for manual, semiautomatic, or completely automated operation to provide single-piece, batch, or continuous-flow blast cleaning.

The table-type machine (Fig. 4) contains a power-driven rotating worktable. Within the cabinet, the blast stream is confined to approximately half the table area. The unit shown is self-contained and mounted on the floor. The work is positioned on the slowly rotating table, and the abrasive particles are propelled by an overhead centrifugal wheel. When the doors are closed, blast cleaning continues for a predetermined time. Some table-type machines are designed with one or more openings in the cabinet. These openings are shielded by curtains and permit continuous loading and unloading or movement of parts during the blast cycle.

Fig. 4 Table-type blast cleaning machine. The centrifugal wheel propels the abrasive particles.

Removal of the contaminants and fines is performed with an airwash separator, as shown in Fig. 5. Spent abrasive and contaminants are fed by a belt and bucket elevator to the helicoid conveyor. The abrasive is screened in the rotary screen, falls in a vertical curtain, and passes under a swinging baffle. The abrasive is then subjected to a controlled cross-flow of air, which cleans it and removes foreign contaminants and fines. Finally, the abrasive gravitates to a storage hopper and is ready for reuse, while contaminants are routed to disposal.

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Fig. 5 Airwash separator

Continuous-flow machines equipped with proper supporting and conveying devices are used for continuous blast cleaning of steel strip, coil, and wire. These machines are also used to clean castings and forgings at a high production rate, making use of flat face or skew rolls, monorails, and other continuous work-handling mechanisms.

A continuous centrifugal blast cleaning machine, equipped with a monorail, is shown in Fig. 6. In operation, the work is loaded outside the blast cabinet and is conveyed into it through a curtained vestibule, which can be designed with 90° turns to reduce the escape of flying abrasive particles. The conveyor indexes the work to the center of each blast station and rotates it for complete blast coverage. If the workpiece contains intricate pockets, it may be indexed to an off-center position and be slowly conveyed past the blast in a manner that most effectively exposes the pockets to the abrasive stream. To minimize cycle time, the work is moved at an accelerated rate between blast stations. As it is conveyed and rotated on a return passageway that follows along the back of the cabinet, the work is exposed to additional cleaning and acts as a barrier to protect the cabinet walls from wear. Continuous-flow machines incorporate abrasive recycling facilities and an exhaust system for removing dust and fines.

Fig. 6 Continuous centrifugal blast cleaning machine

Blasting-tumbling machines (Fig. 7) consist of an enclosed endless conveyor, a blast-propelling device or devices, and an abrasive recycling system. These machines simultaneously tumble and blast the work. They are made in various sizes to accommodate work loads from 0.03 to 2.8 m3 (1 to 100 ft3). The work usually is loaded into the conveyor by means of a skip-bucket loader. As the conveyor moves, it gently tumbles the work and exposes all workpiece surfaces to the abrasive blast. At the end of the cleaning cycle, the conveyor is reversed and the work is automatically discharged from the machine.

Bucket belt elevator to abrasive satiaisiüi'

Fig. 7 Blasting-tumbling machine

Blasting-tumbling machines are used for cleaning unmachined castings, forgings, and weldments whose size, shape, and material permit them to be tumbled without damage. This equipment is not used for cleaning parts after machining, because tumbling damages machined surfaces. Blasting-tumbling machines remove dry contaminants such as sand, rust, scale, and welding flux, and they provide surface preparation for enameling, rubber bonding, electroplating, or etching before galvanizing. Blasting-tumbling machines can be integrated into automatic systems for high production rates. An example is the presort and tote box loader work-handling system shown in Fig. 8. Automatic vibrating feeder conveyors are also available to feed single or multiple machines in lieu of skip-bucket loaders.

Blasting-tumbling machines

Blasting-tumbling machines

Fig. 8 Presort and tote box loader work-handling system. The sorter operates the tote box shuttle to and from the blasting-tumbling machine.

Portable Equipment. When parts to be cleaned are too large to be placed in blasting machines, portable equipment, such as air blast equipment, can be brought to the workpiece. A low-cost sand usually is used, because it is difficult to reclaim or recirculate the abrasive with portable equipment. Also, it is necessary to prevent random scatter of flying particles.

Portable recycling equipment is a new development in air pressure blasting. This equipment uses a pressurized media hose contained within a larger, evacuated hose. After impact, the media are returned through the outer hose to the central unit for reclaiming and recycling. A brush baffle prevents escape of media at the part surface. With this equipment, large external jobs may be done with specialized media without environmental problems.

Microabrasive blasting is another portable air blasting method. Both the abrasive particle size and nozzle opening are very small. Particle sizes are normally 10 to 100 ^m (0.4 to 4 mils) and nozzle openings are 0.4 to 1.2 mm (0.015 to 0.045 in.) in diameter. The tungsten carbide nozzle tips are usually screwed into a pencil-shaped handpiece. Microabrasive blasting is normally a handheld operation for precision deburring, cleaning, or surface preparation. The design of the handpiece and the size of the abrasive particle allow a large degree of control in pointing the blast at the work surface. This is advantageous in deburring or cleaning blind orifices, intersecting slots, or internal bores with irregular surfaces. Microabrasive blasting is not effective for gross material removal or for covering large areas. Dryness and uniformity of particle classification are very critical, and abrasives cannot be reused. Because of the small nozzle size and the types of applications, abrasive usage is not excessive and nonreclamation is reasonable. In continuous-duty operation, 0.2 to 0.5 kg

(1 to 1 lb) of abrasive is consumed per hour.

Ventilation. To ensure adequate ventilation of abrasive blast cabinets, a fabric filter dust collector is generally used with properly designed duct work. The fabric filters are generally equipped with exhaust fans on the clean-air side of the dust collector. This location is preferred because it eliminates erosion of the exhaust fan parts.

Two primary styles of fabric filter collectors are used. The first and oldest is the mechanical shaker type, in which an eccentric style drive activates pivoting racks from which the filters are suspended, providing periodic filter cleaning. A second type is the pulse jet, which uses tubular filter bags made of natural fibers or synthetic felt with an internal support cage of heavy wire and a venturi. Dust and foreign material accumulate on the outer surfaces of the bag and are removed by a short-time, high-pressure pulse of compressed air into the top opening of the venturi. Both types of fabric filters can be designed for light or heavy dust loadings and have throughput capacities from 2.83 m 3/s (100 ft3/min) to several thousand cubic meters per second. The newest type of filter system uses a cartridge in lieu of a fabric tube. The cartridge is either paper- or fabric-based and is also cleaned by compressed-air pulsing.

Maintenance. Abrasive blasting machines are essentially self-destructive, and every effort must be made to protect components from the violent action of the abrasive. Machine interiors should be protected with wear-resistant cast or alloy metal liners or with heavy rubber mats or sheets to prevent erosion of metallic surfaces. High-velocity particles usually bounce from the rubber without damage to either the rubber or the abrasive. If the rubber receives the full impact of the blast, it will require periodic replacement.

Following are typical maintenance schedules that have proved satisfactory for the principal types of abrasive blasting machines:

Centrifugal wheel machines: weekly

• Check blades and wheel for wear. An unbalanced wheel can cause bearing wear and shaft bending. Install new blades if needed, check wheel balance, and test for cleaning pattern.

• Check for loose buckets on elevator belt; loose buckets may catch on the elevator shaft.

• Check sprocket at top and bottom of elevator shaft for wear and broken teeth.

• Check for wear on top plates of machine, rubber table tops, and table rings.

• Check for leaks in ventilation ducts.

• Check entire machine for possible wear holes through which abrasive might escape.

• Check rubber flaps at opening of machine for wear and escaping abrasives.

Automatic air blast machines: daily

• Check all nozzles and air jets for wear and proper flow.

• Check media and air hoses for leaks.

• Check table plates and rubber table tops for wear.

• Check suction lines for leaks.

• Check belts and chain for wear or slippage.

• Check shear pin; replace if necessary.

Hand air blast machines: daily

• Check nozzles and air jets for wear and proper flow.

• Check the following for leaks: media and air hoses, door gaskets, roof bellows and gauntlets, suction lines.

• Check gun bodies for uneven wear.

• Check suction lines for leaks.

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