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Shot blastingWHAT IS SHOT BLASTING

08/03/20220

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Shot, or abrasive, Blasting is a surface treatment process using high-velocity steel abrasive. It is a method through which it is possible to obtain excellent cleaning and surface preparation prior to secondary finishing operations, like painting or powder coating. Shot blasting is commonly used for:

  • The cleaning of iron, steel, castings, forgings, etc.
  • Mechanical cleaning of sheets, rods, coils, wire, etc.
  • Shot peening to alter mechanical properties (increasing resistance to fatigue for springs, gears, etc. )
  • Preparing surfaces to be painted, coated,

In general, shot blasting concentrates abrasive particles at high speed (150–200 mph) in a controlled manner as the material, thereby removing surface contaminates due to the abrasive impact. Initially in the 1930s the shot blasting process used compressed air for propelling the steel shot. This method remains in use today for cleaning metal frames and weldments.

Shot blast production lines, both manual and automated systems, became possible with the introduction of centrifugal wheel blast machines. The system of shot blasting by centrifugal wheel is more productive than by compressed air and achieves a better more uniform surface finish. The criteria used for selecting the type of shot blasting system depends on the size and shape of the parts, the condition of the surface to be cleaned, final surface finish specification, and overall process required.

Multi Shape Blast Machine 1 1200x786 1

 

  1. Abrasive delivery method:
    1. By Compressed Air
    2. By Centrifugal Wheel Blast
  2. Abrasive recovery and cleaning
  3. Dust collection
  4. Blast Cabinet
  5. Part movement and support system
  6. Controls and instrumentation

 

Abrasive Delivery Method

There are two ways of accelerating the steel abrasive:

  1. By compressed air:

This system is suitable for lower production applications where maximum flexibility is needed. These systems are very flexible in that the abrasive can be delivered horizontally through a rubber hose and nozzle assembly. This enables uses in finishing operations of steel frames and weldments thereby replacing hand tools. Because of this, an air blasting machine for a production line is expensive compared to the centrifugal wheel blasting machine.

 

  1. By Centrifugal wheel blasting:Blastec wheel

Centrifugal wheel blasting is the more common blast cleaning technique, as well as, the most economical and environmentally friendly method. The turbine (Wheel) delivers abrasive by centrifugal force in a specific and controlled direction, speed and quantity. The function of the Wheel is similar to that of a fan or centrifugal pump. Shot blasting machines may use one or a multitude of Wheels positioned in such a way that the abrasive blast pattern covers the entire surface of the material to be shot cleaned. The shape and size of the parts determine the number of Wheels used in a machine. The power of the wheel motor is based on the degree of cleaning needed and the throughput speed of the material.

Abrasive Recovery and Cleaning System

Recirculation and cleaning the abrasive shot are required to maintain a consistent cleaning operation. In conventional shot blasting equipment, after the shot hits the part, the abrasive falls into a collection hopper under the machine. The shot is then carried by gravity or screw conveyor to a bucket elevator. The elevator carries the shot, oxides that have been removed from the product, and other contaminates to an air wash separator located in the upper level of the machine. The air wash separator removes the fine contaminants that are mixed in with the abrasive. The cleaned abrasive is contained in an upper hopper and is subsequently fed into the shot wheel by gravity. The recirculating and cleaning capacity of the abrasive in each machine is related to the shot blasting power (total HP) used for the wheels. An incorrectly sized system will cause premature wear to the machine and decrease overall shot blasting effectiveness and shot consumption.

Blast Cabinet

The machine cabinet contains dust and abrasive. A dust collector creates airflow through the machine, thereby preventing dust from escaping into the shop environment. Material access openings in the entrance and exit of the shot blaster must be designed and protected to prevent abrasive spillage, generally by incorporating brushes and rubber or urethane seals. Cabinets are generally built of abrasive resistant materials including high-strength alloy plates.

In the areas that are subject to direct high-velocity shot, alloy steel plates are used which have much more abrasion resistance than other more commonly used materials like manganese steel. Blastec® blast chamber cabinets are made using 11-14% Hadfield manganese steel, in addition to incorporating wear-resistant cast liners on the interior of the cabinet.

Dust Collector System

Dust produced during shot blasting is removed from the blast machine’s cabinet by a dust collector. The typical dust collector design uses cartridges. The dust collector not only evacuates dust within the machine but also keeps the surrounding area clean and dust free. Changes in airflow will reduce collector efficiency and therefore result in lower dust extraction, loss of the cleaning power, and contribute to dust in the immediate production area. A properly designed and sized dust collector is critical to the ongoing performance of the shot blasting system.

 

System for Holding and Transporting Parts for Shot Blasting

Handling and transporting products through the descaling process depends on several factors, such as dimensions, weight, etc. For small parts in large quantities tumble blast machines are used. For larger and heavier pieces, (i.e. Motor blocks, large weldments, propane tanks, etc.) spinner hanger machines are used. For steel plates or large structural fabrications, roller conveyors, overhead monorails, or drag chain systems are used. For the shot peening of gears and other special components, table machines or multi-table machines are used. For cleaning pipes, bars, wire, and steel strips, continuous machines are used.

Controls and Instrumentation

The central console houses the system that provides the controls and instructions for the starting and stopping all machine functions including (but not limited to) elevators, dust collectors, wheels, material handling systems, ammeters and wheel time. The control panel is designed with sequential startup to assure the different systems are energized in the proper order. All systems can be automated for continuous processing utilizing PLC (Programmable Logic Control) technology that will increase production, reduce operator interaction and consistently maintain a particular surface specification.

There have been many modernizations to shot blast equipment over the years and it is important that when selecting a descaling system for the application that you consider the design of the blast wheels, separators, abrasive reclamation system and work handling system, dust collection system, and abrasives. These components all play a part in meeting the cleaning performance, production, and maintenance requirements, as well as, impacting the overall costs associated with the system. Properly maintained blast systems minimize the cost of successive cleaning and finishing operations.

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Source: BLASTEC® Catalog

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