Blast cleaning, a methodology used by W. Granowski , is the most widely used process for the surface preparation of steel and the abrasive is the most important tool. Abrasive blast cleaning is a mechanical process whereby many abrasive particles, travelling at high velocity, bombard a surface with the object by removing a contaminant or preparing surface for subsequent processes such as painting, metal spraying or powder coating.
The most important part of any blast process is the abrasive and careful consideration should be given to selecting the correct one. Not only should it remove surface contamination and create a surface roughness on the type and thickness of the abrasive, but it must achieve this under the most economic conditions that is provide a suitable output with the low wear and tear on the blasting equipment and the lowest abrasive consumption per unit shot blasted.
Choosing the correct abrasive, with the right shape, size hardness, and maintaining the optimum operating mix are the most important elements in controlling costs. There are, however, other factors that must be taken into account if maximum efficiency is to be achieved.
It is very rare to find industries using different blasting machines for different applications. In the main, cleaning surface preparation has to be made in the same blasting equipment. In this case, the abrasive choice is difficult and it is necessary to find the optimum balance.
The main factors to be taken into consideration are chemical composition, shape, size, density, hardness, price and wear life. Together with the application and surface profile required.
Speed of cleaning- To achieve the maximum production output per tonne of abrasive, it is necessary to throw the maximum amount of abrasive. To achieve this, in this case of wheel machine, the electric motor used should be operated at the rated full loads amp. A drop of one amp indicated in the ammeter is a loss of about 12kg abrasive per minute.
Abrasive life- The abrasive breakdown has a strong influence on the blast cleaning operation. The most obvious function is that it determines the abrasive consumption, cost, particle shape, size in the operating mix, parts wear and maintenance costs. The higher the breakdown rate, the greater amount of abrasive consumed. The other factors to be considered while determining the abrasive consumption are the abrasive flow rate and the amount of abrasive lost due to carry out, leakage or poor housekeeping.
Maintenance costs- It is important to choose the abrasive which will achieve the required results with the lowest wear and tear on the blasting equipment. It is equally important to define the extent of cleaning required in each particular case as over blasting will not enhance the end result, but will add the costs. Parts wear rate and maintenance costs increase dramatically due to the presence of contaminants, which can be presented in varying amounts of the abrasive mix. It is essential to keep adequate records of all machine parts replacements, based on the wheel hour system so that costs can be controlled.
Efficient use of abrasive- The success of a blast cleaning operation depends on three factors: the abrasive, the machine and the component (work piece). For optimum efficiency, all three should be of high quality. The highest quality abrasive will not clean efficiently in poorly adjusted or worn out machine. A machine that is badly adjusted will wear faster; clean slower and cost more to run. Therefore, it is essential to plan the maintenance.