by PA Patel, Patel Furnace & Forging Pvt. Ltd.
Shot Blast Cleaning Equipment are used for surface preparation in Foundry, Forging, Shot Peening, Strip De-scaling, Plate, Billet, Wire Coil Cleaning, Stainless Steel Strip Cleaning and many more applications. It has now become more efficient and sophisticated. Shot blasting minimizes operation cost, maintenance cost. Therefore there is need for better understanding of the basic factors which controls the process and sub assemblies, which makes up a whole Shot Blast Machine.
Two Systems of Blasting
Pressure Blast System: In the Pressure Blasting System the blast media is stores in pressurized vessel, which is mixed with compressed air. The air flow accelerates the media, through a blast gun on to the surface.
Blast Wheel Blasting System: In the Blast Wheel System, the blast media is fed to the center of the blast wheel, which is rotating at high speed. The media is rapidly accelerated by centrifugal force and is directed on the surface to be blasted.
This comparison of different blasting systems clearly shows that the blast wheel blasting system has the lowest specified energy consumption. Blast wheels are driven by electric motors, whereas the air required by the air blasting system has to be generated by a compressor which is to be purchased and maintained in addition to blast machine itself and the additional power cost.
The Machine: For efficient shot blasting operation we should understand the basic concept of the process and the machine. The shot blasting machine consists of six basic parts:
1. Blast Wheel 2. Cabinet 3. Work Handling Mechanism
4. Elevator 5. Separator 6. Dust Collector.
Let’s take a close look at these six parts and see what role each plays in the shot blasting process.
1. Blast Wheel:
Abrasive particles are projected by centrifugal force from various kinds of turbine wheels. The number of wheels installed in the machine depends on the type of jobs to be shot blasted and the rate of work. The wheel is the heart of every centrifugal shot-blasting machine. Efficiency and cleaning effect depend to a great extend on the quality of the wheel and its components.
As high speed of abrasive particles (50-100 m/s) is involved the articles to be shot-blasted have to be treated in closed, vibration free booth or cabinet made of strong steel, lined with wear resistant alloy liners. For maintenance purpose proper inspection door and ventilation are essential.
3. Work Handling:
Various systems exist for conveying the parts depending on the type, size and quality of the items to be treated.
4. Abrasive Recovery System and Elevator:
The abrasives are recovered at the bottom of the cabinet by means of a screw conveyor and delivered to the base of the elevator, which then carries these to the separator.
Before abrasives enters into the blast wheel for reuse, these have to be cleaned of all contaminants. This is the role of the air separator.
6. Dust Collector:
The last essential feature is the filtration, which retrieves dust laden air from the separator and cabinet ventilation system and discharges clean air into atmosphere for pollution free environment.
Tumblast Type Machine: This design employs endless conveyor belt made of steel link and flats or rubber belt, which does not damage the job during tumbling.
Table Type Machine: In this type of machine parts being cleaned must be positioned or repositioned on the table to assure complete cleaning of the full surface.
Monorail Hanger Type Machine: In this type of machine, parts are suspended on trees hung on hanger or special fixtures and are carried into the abrasive stream.
Roller & Belt Conveyor Machine: This picture shows plate, sheet or structural shape de-scaling at high speed. Both upper and lower surfaces are shot blasted simultaneously.
Door Hanger Machine: In this machine jobs are hung on the door. The parts are carried into abrasive stream after closing the door. The parts are mounted in front of the wheel. On completion of shot blasting, door is opened and second door with job hung enters the cabinet. In this type of machine, one may get 30 to 40% more production.
Compare Old M Type Wheel (Fig 4) New High Power Wheels: With the need for increased output it has become apparent that ‘Shot Blast Cycle’ on the machine must be reduced in effort to give increased output. We have introduced New RLM (Radialok) wheel, which throw more abrasive and consume less power. In this way one machine being able to do the work formerly required by two machines. The new wheel has large control cage and impeller making it possible to throw more abrasive and better and faster cleaning.
Wheel Unit: With this increased amount of abrasive being thrown it has become necessary to have wheel, which are capable of lasting a great deal, longer than their predecessor. This has been made possible by introduction of new Hard Alloy Wheels.
In addition it has been necessary to increase the life of Blades, Impeller, Control Cage, Liners and other parts by using High Chrome Alloy material. This has been possible due establishment of our new Foundry with Induction Furnace, Shell Moulding Machine and Heat Treatment Facility. Very few equipment manufacturers understand important of wheel and its parts.
Wheel Unit Liners: The blast wheel is usually enclosed within a housing serving as a safety guard and abrasive seal around rapidly rotating wheel to minimize wear on the housing, a series of special alloy cast liners are installed inside the housing.
Older wheel consisted of 21 liners where abrasive bounces in uncontrolled manner. In our new RLM wheel only nine liners are installed inside the housing. High chrome liners are approx. 25mm thick with labyrinth type joints providing and abrasive tight seal between the liners.
New Abrasive Resistant Lining: Again due to the fact that more abrasive is being thrown, it has become necessary to completely lined machines with abrasive resistant material. This has been made possible by the use of special high chrome alloy material. These are made in various sizes to provide complete interlocking lining for most types of shot blasting machines.
Alloy Perforated Plates: One of the biggest problem in the foundry industry has been that wire, chaplets and other material use for giving support the mould gets in the elevator and separator, as the perforated plates are not proper. We manufacture and supply in High Chrome Alloy Steel with the result there is not wear or holes for wire to pass.
Tumblast Machine: (2, 5, 7, 14, 28, 34 Cubic Feet) The new machines are more automated has increased blast power and has been completely lined with abrasion resistant material to give longer life to all parts. They are available 50 Kgs to 2500 Kgs. Capacity.
Swing Table Machine: (900, 1200, 1800, 2400 Dia). The new swing table shot blasting machine have Alloy Table Top instead of rubber, Perforated Grating are of Alloy Steel instead of mild steel. They are available 300Kgs to 4500Kgs weight capacity.
Monorail Shot Blasting: (500, 1500 to 5000 Kgs): In monorail we have developed
A) Door Hanger B) ‘Y’ Type Hanger C) Continuous Monorail.
The machine consists of a cabinet on to which mounted blast wheels to cover the size and complexity of the casting to be cleaned. There is Single Track, ‘Y’ Track or Continuous Monorail. It can be manual or motorized. The castings are loaded on to hanger, moves into blast position in the machine and hanger with casting rotates in front of blast wheel to cover all the faces. In one operation various configuration and models are available to get continuous production.
Abrasive Flow Control Valve (Soundabrator): The abrasive flow to each wheel is regulated to match the mechanical capacity of the abrasive reclaiming system and must stay within the Amperage capacity of Electric Motor. Enclosed air operated Soundabrator reduces noise level, which is below 85 dB and prevents leaking of shots.
Shot Peening: Shot peening is one of the application of shot blasting for increasing the fatigue life of various components like Coil Springs, Leaf Springs, Gears, Rack & Pinion, Torsion Bar, Connecting Rod, Crank Shaft etc., for widely used in auto industries. We see on road many Car or Truck brokedown, this is due to fatigue failure of axle shaft or spring. Sometime results in serious accidents. This fatigue can be reduced by adoption of Shot Peening Process.
Shot & Abrasive: The impact coverage of steel abrasive is governed by its mass and velocity in accordance with equation of kinetic energy.
Ke = ½ MV2 (Where Ke = Kinetic Energy M=Mass V=Velocity).
The impact force delivered to the work piece will change only if the mass factor (i.e. the abrasive size) is altered. The relationship of abrasive size to both impact power and coverage is shown in figure:
The economy and performance of blast cleaning depends upon the abrasive used. The proper size and type of abrasive is most important. The following three factors should be considered while selecting the abrasive.
a. Area of job surface these can clean.
b. Quality of work produced.
c. Cost of the shots.
Chilled Iron Shots: The shots are manufactured by quenching the stream of molten gray iron over high pressure water jet. Its hardness is about 60 Rockwell C. Due to this hardness and quenched structure it fractures and disintegrates very quickly on impact with the work surface. Due to greater amount of dust produced in fracturing process, the part cleaned by chilled iron shot does not have very good finish. Also the maintenance cost is very high. This led to the introduction of malleable iron cast steel, cut wire shots and heat treated steel shots.
Heat-Treated Steel Shots: The heat treated steel shots are having hardness of 45 Rockwell C (424 Brinell hardness). These are the most suitable for the shot blasting machine.
Stainless Steel Shot & Cut Wire Shots: Die casted components manufactured from Aluminium Zink & Magnesium Alloys required to be blasted for removing burr and also give aesthetic value with aim to obtain even, shiny finish on all sides. Many users prefer stainless steel shot as the consumption is one sixth of steel shot, carbon steel shot give dull grey finish while stainless steel imparts matt silver finish pleasing to eye. Many auto units like Honda, Bajaj, CRP India uses stainless steel shots.
Shot blasting is no more an optional process in a foundry. It provides faster and better cleaning. It saves power, labour, space and also saves on cutting tools and permits better inspection, thus minimizing the rejection of castings. Apart from foundry it is also widely used in many other industries. Proper understanding about the machine, its construction and use enables us to improve its efficiency and utility.
Author: PA Patel
Patel Furnace & Forging Pvt. Ltd.,
A-2/510, GIDC Makarpura,
Baroda – 390 010, Gujarat, India
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by PA Patel, Patel Furnace & Forging Pvt. Ltd.