How to select appropriate abrasive, then can better save overall cost of metal cleaning

The value of any abrasive is determined by the quantity, quality and cost of the workpieces processed per unit time (including abrasive, wearing parts and labor costs).

At present, some high-hardness abrasives (iron shots, iron sands) on the market can also be quickly cleaned (or strengthened), and the price of abrasives is also low, but these abrasives are high consumption and fragile. Premature wear of the wearing parts of the shot blasting equipment (causing Replacing equipment wearing parts requires machine halt and increased maintenance costs. There is a very high operating cost that is not used. On the contrary, some other abrasives (such as low-hardness abrasives) have a low consumption rate and low prices, but they have long cleaning cycles, low productivity, wasted labor and working time, and these abrasives cannot be used.

High-quality abrasives should meet: 1. Long service life; 2. Quick cleaning (or strengthening); 3. Low maintenance cost; 4. Stable effect

The crushing characteristics of the abrasive will directly affect the abrasive cost, maintenance cost and cleaning effect.

The fracture of the abrasive is important in determining the quality and cost of shot cast blasting. It has an extremely important impact on the cost of abrasives, wear of wearing parts and maintenance costs.

Each type of abrasive has its own inherent fatigue characteristics due to its obtained different fundamental metallurgical structure of its chemical, metallurgical and production processes, which determines its breaking characteristics in shot blasting equipment.

The wear of the abrasive particles is small, it is not a factor in the reduction of the size of the abrasive, and the reduction in the size of the abrasive is caused by the fatigue of the abrasive structure, which eventually breaks it into smaller particles.

In the shot blasting equipment, the abrasive particles are ejected from the high-speed rotating blade wheel (or air nozzle) at a very high speed, impacting on the surface of the workpiece, completing the cleaning and strengthening tasks, and the shot impacts the surface of the workpiece each time. At the same time, the pellet itself is consumed by the impact, and at the same time causes wear and tear on the equipment. From continuous circulation and repeated impact, from large particles to small particles, from small particles to smaller particles, and finally because the particles are too small (less than 0.2 mm) It cannot be used, and removed by the dust removal system of shot blasting equipment.

The wear rate of the parts of the shot blasting equipment affects the maintenance cost. The iron sand, the ordinary cutting wire shot (overproof length, more bevel angle) or  large proportion with steel shots added directly into the equipment, and the wear rate data of the parts can be quickly obtained. , directly reduce component life by 50% – 80%.

Iron sand can destructively reduce component life because the breaking rate of the abrasive determines the amount of new sharp corner particles formed during operation. Ordinary cutting wire shot has a long service life due to its own characteristics, but the shape defect resistance is large, causing the abrasive circulation to flow poorly. The oblique angle is easy to produce scratches, which will cause unnecessary damage to the equipment components and the useless work of the cutting wire shot on the workpiece because of its transverse action, all those are main determinant of relevant parts life, so the use of long-life, high-quality abrasives provides additional benefits.

The hardness of the abrasive affects the cleaning speed and quality – the hardness of the high hardness abrasive under long life affects the cleaning speed, resilience and the smoothness of the workpiece.

When the abrasive particles move at a high speed, they impact the surface of the workpiece, and the impact kinetic energy applied to them by the blades will be consumed in many aspects. In fact, all the kinetic energy of the moving pellets will be transferred to the surface of the impacted workpiece to achieve sand cleaning and rust removal, and the removal of other deposits such as scales, to complete the effective effect. In reinforced applications, energy is used to remove compressive stress created on subsurface . At the moment of impact, the force is so great that it produces the excess strength of abrasive. In this way, this part of the energy is always consumed in the deformation of the abrasive, and this energy is consumed, and the useful effect is not completed. The energy consumed is largely determined by the hardness of the abrasive. The so-called hardness, defined as the ability to resist deformation, the harder the bigger resistance to deformation, the greater percentage of energy transferred to the surface of workpiece then will be.

Under normal circumstances, the ideal hardness range of the abrasive is between 40 and 50 HRC. It is too hard to be broken, too soft and easy to deform.

The size of the abrasive determines the quality of the cleaning (smoothness). When working with abrasives initially, it is best to use the estimated minimum size of abrasive until larger size abrasive is added when appropriate size range set up. Coverage is the most important for cleaning speed. Under the premise of cleaning the workpiece, the small size will be faster and better than the larger size. The reason is that there is a large coverage (more abrasive droppoint of the unit area of the workpiece being cleaned per unit time), and the coverage is the most important for cleaning speed.

When changing from one type of abrasive to another, the mixture is sieved to determine the appropriate working size range to achieve the same cleaning effect. Due to the different abrasive properties, when changing from cut wire shot to steel shot or from iron shot to steel shot, the size of the abrasive must be reduced to the next or next size.

 

Analysis of the causes of abrasive crushing by the technical department:

The pre-crushing of steel pellets is caused by the macroscopic defects of steel pellets. Macroscopic defects refer to pores, looseness, inclusions and cracks, and late crushing is caused by metallographic defects of steel pellets. Tissue defects refer to coarse, reticulated carbides, large amounts of retained austenite, and some metamorphic products. The best metallographic structure should be tempered martensite, tempered troostite, and uniformly finely distributed carbides.

Measures to prevent abrasive loss: 1. Adjust the shot blasting equipment; Make the abrasive effectively work on the workpiece while avoiding the discharge of useful abrasives in the shot wheel or dust collector; 2. Retrieving the abrasive leaking outside device; 3. Preventing abrasives Take away with the workpiece; 4. Always check the device for loopholes.

Mixed abrasives determine the overall cost of shot blasting:

The steel shot has small working resistance, suitable hardness and high resilience. The cleaning degree is high when the working surface is continuously sprayed, but it is not easy to throw out a certain roughness. For the workpiece with the required coating standard, the surface roughness has a great influence on adhesion of paint film , corrosion resistance and protection life. The appropriate roughness of the surface of the workpiece is beneficial to the improvement of the protective performance of the paint film. The roughness is too small, which is not conducive to the improvement of the adhesion of the paint film. The roughness is too large. If the amount of the paint film is constant, the thickness distribution of the paint film will be caused Inhomogeneity, especially at the peak, the film thickness is insufficient and lower than the design requirements, causing early rust. In addition, the roughness is too large, and the bubbles are often trapped in the deep trough pits, which will become the roof of paint film blistering.

For common coatings, a suitable roughness range of 39-75 um is preferred. Therefore, the mixed abrasive must be used reasonably and the steel sand should be added in an appropriate amount. The specific ratio is matched according to the coating standard requirements, so that it can quickly clean (improve work efficiency) and minimize the wear of the shot blasting equipment (control equipment maintenance cost) and effectively meet the coating requirements (reduced paint waste) , thereby reducing the overall cost.


Post time: Sep-18-2018
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