The surface treatment and finishing of castings is the final stage in the manufacturing of cast objects, and it entails removing the moulding and core mix residues to achieve the desired surface quality and condition.
Mechanical surface treatment procedures, particularly abrasive (or shot blasting) methods, appear to be popular. Low energy demands, high quality treated surfaces, significant potential for process automation, use of shot blasting equipment built of more durable materials, work safety, and environmentally friendly features are only a few of their important advantages.
A stream of abrasive media with the requisite kinetic energy is created and driven onto the surface to be treated in shot blasting operations. Numerous investigations have showed that the cleaning agent stream, which is made up of a mixture of metal shoots, is a dynamically changeable system with a concentrated and diffused zone.
The actual proportion of specific zones in the shot stream layout has a significant impact on shot blasting efficiency. The more concentrated the zone, the more efficient the process is. Other factors that influence the effectiveness of shot treatments include:
1. meeting the criteria that guide the development of blast mechanisms, 2. qualities of the cleaning agent that must comply with the process's precise requirements, 3. the best shot stream form in terms of geometric, kinetic, and dynamic factors, 4. the direction in which the jet hits the treated field's surface.
The surface quality of shot-treated castings is determined by these parameters.
Due to the complexity of shot-blasting processes, substantial research and analytical studies are carried out to optimise the process and mechanical characteristics of system components, as well as to determine the essential phenomena involved in shot-blasting. The methodology for measuring and evaluating process parameters is very important, as it is required to assure the process's efficiency.
The efficiency of shot-blasting operations is intimately related to the construction and operational parameters of shot-blasting equipment. The circulation of an abrasive substance in the form of a heterogeneous mixture is the essential process involved in shot-blasting. This heterogeneity is linked to the following:
The geometry, kinetics of motion, and dynamic behaviour of the abrasive agent flowing through specific parts of the shot-blasting unit all contribute to the heterogeneity of the mixture circulation.
There are two stages to the abrasive agent's circulation. The first stage begins in the shot-blasting machine's hopper, which is located on the highest level. The hopper serves two purposes: it holds the appropriate amounts of abrasive agent to maintain process continuity, and it also serves as a dosing device, feeding the shots to the blasting turbine using gravity force via a piping installation. The concentration of the shot column, which controls the volume of shots and the dynamics of their passage to the turbine, is a key component in the dosing process.At that time, the abrasive combination contains both recycled abrasive agent that has been filtered to eliminate scraps from the core and sand mix, as well as new agent that has been added in stages to fill up to the appropriate amount. The use of new shot helps to grain size homogeneity, which increases process performance. Aside from the shot fractions having to be separated first in a separator, the recycled abrasive agent must be pure. A magnetic-pneumatic separator is the most common alternative. The passage of the abrasive substance to the separating rotor is the next stage of the circulation process.
The flow rate of the abrasive agent to the rotor should be properly managed in terms of operational needs so that the entire rotor volume is filled. The relationship between the circular internal cross-section of the separating rotor and the cross-section of the circular outlet opening of the feeding pipe expresses this need. The compact column of abrasive substance in the rotor is divided into smaller batches and directed to the blast blades via the hopper. Rectangular slots carved into the rotor's cylinder-shaped side wall separate the streams. The number of slots in previous rotor systems ranges from 8 to 12. The majority of experts advise opting for 8-slot alternatives.
A kinematic pair is formed by the separating rotor and an adapting sleeve. The adaptable sleeve's primary role is to distribute abrasive agent doses to the blast rotor's blades. The abrasive agent stream begins to form on the inner of the rotor, where the feeding takes place. The sleeve is made up of a cylinder with the necessary wall thickness and a rectangular slot cut into it. Its volume is determined by the slot parameters (length, width, and thickness obtained from the difference between the outside and inside radii), which determines the mass flow rate of abrasive agent fed through the separating rotor.
The best slot length, according to testing conducted by major shot blasting system manufacturers, is the sum of the widths of two slots in the separating rotor and the width of the wall between those two slots. This criterion must be met in order to maintain the abrasive agent stream between the separating rotor and the sleeve. This consistency is maintained until the abrasive agent stream comes into contact with the blade surface.
The sleeve location is critical in terms of operational requirements. The inclination angle of the slot with regard to the horizontal plane defines it. The dynamic behaviour of the abrasive agent discharged under the action of the centrifugal force is determined by the value of this angle. The best dynamic effects are achieved at a 20-degree angle, according to test data.
The blast rotor with blades is the main mechanism at the last stage of the controlled circulation line of abrasive agent. On the turbine blades, the shot produces a stream that runs the length of the blade. The flowing stream will be fairly uniform on the width and length of the blades, depending on the actual configuration of the separating rotor and the sleeve.
As soon as the stream of shot exits the blades, the wheel controls its direction, while the shape of the stream changes in width and length, providing a range of shot flow. The concentration of the shot stream changes, resulting in the production of a concentrated and scattered stream, which is arranged along the concentrated stream's outer perimeter. The flow intensity of the focused stream is related to the efficiency of the shot blasting process.
We acquire a mixture of sand and moulding mix in the second step of shot circulation. The moulding mix must first be separated from the sand in order for the sand to be recirculated. Mechanical separation methods, such as screening, as well as electromagnetic and pneumatic technologies, are used to accomplish this. When such strategies are combined, they produce excellent results.
The separation system entails the following steps:
The following are key measures of separation efficiency:
The separation (shot cleaning) procedure is crucial. The abrasive agent's homogeneity and purity have an impact on:
Regardless of the machine design and process settings, the type of abrasive medium used has a significant impact. The quality and efficiency of treatment procedures, as well as the wear and tear of machine components, are directly influenced by shot form and material.
Abrasive media in the form of round-shaped or sharp-edged metal shots of variable grain size, abrasion-resistant, extremely elastic, and of specified hardness are used in most shot blasting systems in foundry engineering. The type of shot to use is determined by the casting method, the required surface quality, and the number of castings to be treated.