What is sand blasting

What is sand blasting?

Sand blasting process

Dry sand blasting process conditions for different parts

Part type	Degree of quartz sand (mm)	Compressed air pressure (MPA)
Large steel parts with thickness over 3 mm.	2.5~3.5	0.3~0.5
Medium sized steel parts with thickness of 1 ~ 3mm.	1.0~2.0	0.2~0.4
Small thin brass parts.	0.5~1.0	0.15~0.25
Steel sheet metal parts and aluminum alloy parts with thickness less than 1 mm.	0.5 ~ down	0.1~0.15

There are two kinds of dry sand blasting: mechanical sand blasting and air pressure sand blasting. Each method is divided into manual, non automatic or continuous automatic modes. Manual air pressure sand blasting machine is widely used in China, and it also uses various complex shapes of small and medium-sized parts.

Wet spray Sand technology

The abrasive used in wet sand blasting is the same as that of dry sand blasting. The abrasive can be mixed with water to form mortar. The abrasive is usually 20% - 35% and stirred continuously to prevent sinking. Compressed air is used to press into the nozzle and sprayed to the parts. Sand and water can be put into the tank respectively and mixed before flowing to the nozzle before spraying to the parts. In order to prevent corrosion of steel parts, sodium nitrite and other corrosion inhibitors must be added in water, Sand should be dried before next use.

There are many kinds of equipment for wet sand blasting machine, and the most commonly used one is sand blasting chamber.

Spray Pill Technology

(1) General On

Shot peening is similar to sand blasting, but steel shot or glass shot is used instead of abrasive for sand blasting. Shot peening can produce compressive stress on parts, and there is no dust pollution containing silicon. This process is mainly used for:

• (1) The fatigue strength and stress corrosion resistance of the parts are improved by compressive stress.
• (2) Straighten the twisted thin-walled parts.
• (3) Instead of the general cold and hot forming process, the large thin-walled aluminum parts are processed, which can avoid the high residual on the surface of the parts Tensile stress And the favorable compressive stress is formed.
• It should be noted that the service temperature of the parts after shot peening should not be too high, otherwise, the compressive stress will be eliminated and the expected effect will be lost. The limit of using temperature depends on the material of the parts. For example, the temperature of steel parts is about 260 ~ 290 ℃, and that of aluminum parts is about 170 ℃.

(2) Types of pills

• 1. Cast steel shot: its hardness is generally 40-50HRC. When processing hardware metal, the hardness can be increased to 57-62HRC. Cast steel shot has good toughness and is widely used. Its service life is several times that of cast iron shot.
• 2. Cast iron shot: its hardness is 58-65HRC. It is brittle and easy to be broken. It has short life and is not widely used. It is mainly used in occasions with high strength without shot peening.
• 3. Glass pill: hardness is lower than the former two, mainly used in stainless steel. Titanium, aluminum, magnesium and other materials that do not allow iron contamination can also be used for secondary processing after shot peening to remove iron contamination and reduce surface roughness of parts.

(3) Shot blasting technology

For shot peening, which requires compressive stress on the surface of parts, it must have enough coverage, which is difficult.

In addition, it is difficult to make quantitative judgment. Therefore, the method of controlling shot peening strength is often adopted, that is, the method requiring a certain sandblasting degree is required, that is, the required stress value can be obtained after reaching a certain sandblasting strength. The shot peening strength is mostly in accordance with the American SAE standard J442.

Almen arc altimeter was used for measurement

Almen arc height meter uses 1070 cold rolled steel with thickness of 0.8 mm, 1.3 mm or 2.4 mm. Compared with 70 carbon steel in China, its hardness is 44-50 HRC. The shot peening strength can be obtained by measuring the arc height of bent steel bar. The shot peening strength is mainly affected by the following factors:

• (1) Shot size: the larger the shot peening, the greater the impact energy, the greater the shot peening intensity, and the shot peening coverage. Therefore, on the premise of producing the required shot peening strength, it is beneficial to reduce the shot size as much as possible.
• In addition, the selection of shot peening size is also limited by the shape of shot peening parts, and its diameter should not exceed half of the radius of the inner circle of the groove. The particle size of shot peening is generally 2.00-0.30mm
• (2) Hardness of Shot Peening: when the hardness is greater than the hardness of parts, the change of shot peening hardness value does not affect the shot peening strength, otherwise, the decrease of shot peening hardness will reduce the shot peening strength.
• (3) Shot peening speed: when the shot peening speed is increased, the shot peening intensity will be increased, but if the shot peening speed is too high, the crushing amount will increase.
• (4) Spray angle: when it is vertical, the shot peening intensity is the highest. Therefore, shot peening is generally carried out in this state. If it is necessary to peen at a small angle due to the shape of the parts, the size and speed of shot peening shall be appropriately increased.
• (5) Shot peening fragmentation: the shot peening strength of crushing shot blasting is low, so the shot peening should be removed frequently to ensure that the integrity of shot peening is not less than 85%. In addition, the shot with sharp angle will scratch the parts.

Function and application of sand blasting

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