Pipeline process for blasting pretreatment on steel plate pretreatment lines
In the domestic shipbuilding, container and large machinery industries, three pretreatment methods are commonly used for steel: shot blasting abrasive treatment, shot blasting abrasive treatment and pickling. Of these, only shot blast cleaning is currently performed in order to obtain a high quality automatic flow operation.
Shot blasting is the use of centrifugal force generated by the impeller of a shot blasting machine to blast abrasives (steel shot, steel wire cross-section, angle grit, etc.) onto the steel surface at a very high speed. The effect of being treated to produce blowing and grinding is to remove oxidation and rust from the steel surface, exposing the steel surface to the original color of the metal and presenting a certain roughness, which facilitates the adhesion of the macaroon paint (lacquer).
Usually, there are two types of shot blasting pretreatment lines: one is a steel plate pretreatment line and the other is a section steel pretreatment line. The working principle of both is the same. Current shipyards, containers and large port machinery plants use the method, and the method is used for general steel construction plants, the purpose of both methods is the same, to ensure the quality of painting.
Steel plate shot blasting pretreatment line process flow chart
(I) Steel plate leveling
During the transportation of steel plates or after long-term stacking, deformation will occur. When producing the workpiece, it will affect the processing accuracy, so the steel plate should be leveled. After leveling, the loose scale on the surface of the steel plate can be peeled off, which is of great benefit to the subsequent shot blasting process. Meanwhile, the leveling machine has different functions, generally 5mm-30mm is appropriate.
In addition, the steel plate straightening machine is also located behind the assembly line, which is a test for the performance of the shop primer.
(II) Steel plate transportation
After loading the steel plates, the roller table completes the transportation of each process.
The roller table is usually cylindrical with bearing seats at both ends and the distance between the roller tables is 500?750mm. After the painting work is completed, there are special requirements for the structure of the roller table in order to prevent damage to the paint on the side in contact with the roller table. Therefore, in general, the contact area between the roller table and the steel plate will not cause damage to large areas of shop primer and the effect will be minimal.
In order to ensure that the steel plate is not deformed during the blasting process, the distance between the rollers in the blasting machine should not be less than 500mm.
(iii) Warm-up
Preheating means heating the steel plate before the shot blasting process to remove surface moisture and some oil stains, and then raising the steel plate to dry after painting.
The general preheating equipment includes wire heating, medium frequency induction heating, liquefied petroleum heating and water heating.
No matter which heating method is used, the temperature of the steel plate should be maintained at about 40°C. Too low a temperature is not conducive to removing water and oil stains. Subsequent spraying primer drying too high, which will consume more energy and lead to paint drying process blistering.
(D) blast cleaning device
The blast cleaning machine consists of impeller, shroud, directional sleeve, granulating wheel, bearing housing and motor. The impeller is driven by an electric motor that rotates at high speed (2200-2600 rpm) to produce a strong centrifugal force. When it is on the wheel, it is thrown at a speed of 60?80 m/s by the centrifugal force in the direction of the blade length. The thrown abrasive has a fan-shaped flow rate to remove oxidation and rust from the surface of the steel plate.
Usually 4 to 16 heads are thrown off and the head is about 300?400mm away from the steel plate, and the angle and direction of the workpiece to the nozzle must be adjusted to control the number of each nozzle.
Flat steel and steel sections below 6mm are not suitable for steel plate pretreatment because they are too thin and will be deformed.
Control the speed of sandblasting.
Select the appropriate blasting steel shot and angle sand.  E) Paint spraying
The surface of shot blasted steel needs to be immediately sprayed with shop primer.
Spraying equipment is 9 ℃ of the Yangtze River domestic high-pressure airless paint spraying pump, pressure ratio of 1:32
The nozzle is usually around 19?350, 19?400, 17?350, 17?400.
When spraying the primer in the workshop, the mixer keeps stirring slowly until it is used up.
The distance between the spray gun and the steel plate is usually about 300mm, and the distance of the lower gun should be slightly less than the distance of the upper gun.
The paint coverage rate is 50%.
(F) Drying
After spraying, it should enter the drying oven to promote the rapid drying of the paint film for quick handling and stacking.
(G) Abrasives
Abrasives in sandblasting can be divided into two types: metallic and non-metallic (mineral) or slag. Metals such as steel shot, slag and steel wire segments are mostly disposable abrasives, but blasting abrasives include iron shot, steel shot, angular sand and steel wire segments. They are ideal for abrasive and mixed abrasives with a diameter of about 0.8. 1.2mm is suitable.  (Roughness of 40?75mm)
(H) roughness
The roughness of the steel surface after sandblasting is the depth of the steel sand traces left on the steel plate during the sandblasting process. Roughness is too small, the surface of the steel plate is smooth, poor adhesion of the coating, roughness is large, the paint can not cover the peak of roughness. It is easy to form pitting and will increase the consumption of paint. Usually, the roughness of inorganic zinc silicate shop primer is 40?75μm. It is necessary to control the following aspects.
Properly maintain the speed of the impeller, do not let the blasting speed of steel sand too fast or too slow, and control the amount of steel sand blasting.
Adjust the ratio of steel shot and angular sand.
Steel shot and angle sand diameter, abrasive grain size
Control the angle and distance between the head and the steel plate.
Roughness picture
(9) Measurement method of dry film of pretreatment workshop primer
Workshop primer is made on sandblasted ISO sa2.5 steel plate with roughness of R240-75 microns. The thickness of the workshop primer itself is usually 15-20 microns, so the film thickness cannot be measured directly on the primer. Iron plates. The standard test method is.
After sandblasting, a smooth test plate is fixed to the steel plate with tape, usually placing three pieces (as shown in the figure)
First calibrate the electronic thickness gauge. After spraying and drying, remove the test panel for measurement. Normally, the dry film thickness measured after sandblasting should be greater than the specified value on the smooth test panel.