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   Effective Use and Maintenance of Shot Blasting Machines (4/4)
Effective Use and Maintenance of Shot-Blasting Machines
By. Mr. P. A. PATEL
A - 2/510, G.I.D.C., Makarpura, VADODARA - 390 010.
E-mail : / sales@pshotblast.comĀ    website : www.pshotblast.comĀ    M. : +91 9737061333
allow abrasives to accumulate in the bottom section of the elevator, causing elevator to jam and increase wear on boot pulley.
Separator (Fig. 8) : Separator is another important part of any shot blasting machine. The function of the separator is to remove sand and contaminants from abrasives so that good clean abrasives of proper size are fed to the wheel. It also controls the size of the abrasive mix. This gives good cleaning and improves productivity. From the examples given in the figures one can see that the setting of the separator is very important in controlling all phases of operation.
What does usually go wrong with the separator? The drop of air pressure on separator can be too great, causing the removal of good shot with sand. This can be corrected by adjusting the blast gate sliding baffle and swing baffle.
If the separator is functioning properly, sand will continue to come out of dribble valve when shot blasting machine is running. As the effective operation of the separator depends on the airflow, please ensure that bag type dust collector is operating efficiently.
Dust Collector (Fig. 9)
The fabric filter dust collector commonly used in foundry is very simple in construction and in maintenance. The dust laden air or gas passes into the dust hopper where the heavy particles are deflected downwards and settle into the hopper. The fine dust passes up into the filter tube where it is trapped inside, and clean air passes through filter cloth. It is necessary to shut off the air at regular interval and operate the shaking mechanism to remove the foundry dust from inside the filter tube into the dust hopper and remove for disposal.
This will improve efficiency of shot blasting machine and dust collector. The efficiency of shot blasting wheel will depend upon the type of abrasives, quantity of abrasives thrown and percentage of sand in the shot. It is observed that 1 to 2 % sand reduces blade life by 80 to 100 hours. Therefore it is essential to maintain all parts of the machine at maximum efficiency.
Shots & Abrasives
The economy and performance of blast cleaning depends upon the abrasives used. The proper size and type of abrasives is most important. The following three factors should be considered while selecting the abrasives
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.
Concluding Remarks
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.