Analysis and treatment of construction faults in t

2022-08-14
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Analysis and treatment of construction faults in powder spraying

powder coating, like other coatings, will inevitably have some faults in the construction process. First of all, we should analyze the causes of coating defects, so as to effectively deal with the fault

factors affecting coating quality

1 poor quality of powder coating

unqualified raw materials: powder coating quality problems are easy to cause various defects of the coating. We must start from the quality of the coating, select the coating with good quality, and improve the technical inspection of the coating entering the factory

poor formula design: the formula design should not only meet the high-grade requirements of the product, but also have market competitiveness, but also make the formula stable and continuous. Imperfect formulation design will reduce the performance of powder coating and affect the performance of coating

nonstandard production process: the production process of powder coating is a physical mixing process, and the production equipment must operate normally and have standard operating procedures

poor recycled powder: impurities in recycled powder, unreasonable dosage ratio of new powder to recycled powder, etc. will affect the coating quality. The proportion of recycled powder: new powder should be controlled to be 1:1 or 2:3

powder mixing: if the leveling agent in the two powders is different or the powder contains silicone oil additives, the two powders are incompatible. Mixing the two powders will reduce the gloss of the coating or produce shrinkage cavities

poor storage of materials: the powder coating cannot be heated, moisture absorption, extrusion, etc., so as to avoid caking and reduce the quality. At the same time, it cannot be stored in the powder supply bucket for a long time, otherwise the fluidization is not good

the quality inspection system is not perfect: the technical indicators of powder coatings should be carefully sampled before leaving the factory

2 the spraying process is not strictly implemented

poor pretreatment: the quality of oil removal, rust removal and phosphating is poor, and there are residues on the surface, causing defects such as orange peel, shrinkage cavity and particles of the coating and reducing the adhesion of the coating. The workpiece is not completely dried, so that the coating bubbles without noise; There is no oil pollution

process conditions are out of control: the compressed air is not clean and the air pressure is insufficient, which is easy to produce defects such as poor powder leveling and powder spitting. It is better to control the air pressure to be 9.8-20.6kpa

the voltage is too high or too low, and the voltage is unstable, so that the concave part of the workpiece cannot be powdered or the powdering rate is reduced, and the coating thickness is uneven. The voltage shall be controlled at 60-90 kV

the curing temperature is too low or too high, the impact strength and hardness of the coating are low, and the appearance and gloss are poor. There are orange peel, etc

improper operation: the position or direction of the spray gun mouth is incorrect, and the powder feeding rate is low. Adjust the direction of the nozzle of the spray gun so that the powder cloud directly sprays into the concave area of the workpiece. Too close spray distance will produce strong anti ionization phenomenon, leading to electrostatic breakdown, powder combustion and ignition, or the formation of lines or metal flash effect is not good; Spraying too far will reduce the amount of powder. The spraying distance should be kept at 10-20cm. The spray gun contacts the powder spraying cabinet or metal objects, which makes the workpiece poorly grounded and causes powder combustion and ignition. One of the reasons for pinholes and bubbles in the coating is that the spray gun stays on the workpiece for a long time and produces electrostatic breakdown

poor grounding of a workpiece: the joint between the grounding wire and the workpiece is sprayed with paint, which reduces the conductivity of the wire, reduces the powder rate, uneven coating thickness, orange peel, pinholes, wrinkling, sagging, poor adhesion, etc. The movable shielding tube can be sleeved at the joint. When loading, only move the shielding tube to expose the joint of the wire, connect the binding wire of the workpiece, and then cover the joint with the shielding tube to ensure that the workpiece is well grounded

3 poor equipment and auxiliary accessories

microporous plate blocked: caused by oil stains and impurities in ultra-fine powder or compressed air, resulting in poor powder fluidization

powder deposition and scaling in the powder conveying pipe: due to the too fast spraying speed, the powder collides and melts and scales on the pipe wall. In addition, the regular cleaning and replacement are not enough, which eventually leads to powder blockage. At this time, the pressure of powder spraying should be reduced

the powder spraying process does not meet the requirements: the internal resistance is low, resulting in excessive current and high voltage; Secondly, the powder is not atomized and dispersed well, resulting in powder spitting, which affects the appearance quality of the coating

the ventilation pipe is blocked, causing the powder to fly

the powder feeder is accumulated or blocked, resulting in uneven powder output

the venturi is seriously worn, resulting in reduced or irregular powder output, and the coating is scratched. Replace the nozzle at this time

improper use of powder conveying pipe: the use of reinforced plastic pipe is easy to form coating spots (spots different from the coating color). The solution is to replace the powder conveying pipe; The long powder conveying pipe leads to uneven powder spraying. The corresponding solution is to shorten the powder conveying pipe and minimize the distance between the powder feeder and the spray gun

the abnormal operation of the powder recovery device causes the powder to fly. Check the recycling device at this time

4 the production environment is not clean

there are scattered objects polluting the coating site in the spraying workshop: at this time, the hard attachments of workpiece hangers, recovery equipment, catenary, drying channel and other equipment should be removed, and the air circulation and make-up air in the spraying workshop should be filtered to maintain a certain positive pressure inside and outside the workshop. In addition, do not walk around the air inlet and polish the repaired workpiece. Install a filter (60-70 mesh stainless steel wire) at the air outlet of the air duct to filter the circulating hot air in the oven. Use towels instead of cotton yarn when maintaining the equipment

the oven is polluted: some substances with molding shrinkage of 0.3-0.8% in the powder volatilize into gas at the curing temperature, and become loose solids when they are cooled at the curing oven mouth. When they accumulate to a certain thickness, they loosen and fall off on the powdered workpiece, causing sand collection. Therefore, it is necessary to remove or flush the volatiles at the oven mouth regularly

powder room is polluted: causing coating shrinkage. The pollutants should be removed from the site. It is better to seal and isolate the powder room, or stop using the contaminated powder

common troubleshooting

1 poor powder fluidization

insufficient air pressure: the air filter must be cleaned and the pressure should be increased appropriately

powder caking: the powder needs to be loosely filtered manually

too much powder loading in the fluidized bed will also affect the fluidization of powder. At this time, the powder loading in the fluidized bed should be reduced. Generally, the powder dosage is 2/3 of the volume of the fluidized bed

2 powder clogging gun

poor fluidization of powder: the powder loading of the fluidized bed should be reduced, and the powder dosage should be controlled to 2/3 of the fluidized bed volume

the content of ultra-fine powder in the powder is high: it is necessary to reduce the proportion of recycled powder added to the raw powder and check the particle size distribution of the raw powder

3 when powder is sprayed on the workpiece, there is powder mass.

compressed air contains water or oil: it is necessary to check the purification of air source and improve the purification equipment of compressed air

the content of ultra-fine powder in the powder is high: it is necessary to reduce the proportion of recycled powder added to the raw powder and check the particle size distribution of the raw powder

too much powder output: it is necessary to appropriately reduce the powder conveying pressure and control the powder supply to 150-200g/min

poor fluidization of powder: reduce the amount of powder in the fluidized bed. Generally, the amount of powder added is 2/3 of the volume of the fluidized bed

4 low powder loading rate

the static voltage of the spray gun is insufficient: the static voltage should be checked and adjusted appropriately; In addition, the electrode of the gun head can be cleaned

the powder ejection speed is too fast: it is necessary to reduce the powder conveying pressure appropriately

the content of ultra-fine powder in the powder is too high: the proportion of recycled powder added to the raw powder should be reduced, and the particle size distribution of the raw powder should be checked

the exhaust volume of the powder spraying room is too large: the exhaust volume should be reduced

the particle size of the powder is too coarse or the density is too large: the particle size distribution of the powder and the specific gravity of the coating should be well controlled during powder preparation

5 powder combustion ignition

excessive powder concentration: reduce the output air pressure and check the recovery device

the humidity in the powder spraying room is too high: the humidity should be kept ≤ 80 ③ the clip details are generally excellent medium carbon steel and alloy structural steel%

6 poor coating adhesion

poor pretreatment, incomplete rust removal of the workpiece or residues in the seam, when the workpiece is baked at high temperature, these substances will seep out and flow to the lower end of the workpiece. At this time, the spray angle should be adjusted to reduce the drying temperature of phosphating film and coating

unreasonable process hole design (round): it is necessary to change the process hole to waist shape and open it at the lower end of the workpiece

insufficient curing of the coating: the baking temperature should be increased and the curing time should be extended to meet the requirements of the curing specification

insufficient output voltage: the output voltage should be increased

the resistivity of powder is too high or too low: the resistivity of powder should be adjusted

7 low hardness or poor wear resistance of the coating

insufficient curing of the coating: the baking temperature should be increased and the curing time should be extended to make it meet the requirements of the curing specification

the embrittlement hardness of the cross-linked structure of the coating decreases due to the excessive curing of the coating. The baking temperature should be properly reduced to shorten the curing time

the curing temperature rise rate is too slow. During the slow melt flow process, some leveling AIDS migrate and float up to the coating surface. The rate of temperature rise dbj/t01 (5) 0 (2) 002 "technical specification for construction of external wall external insulation flexible water-resistant putty" should be increased

coating is too thick or too thin: the coating thickness can be controlled by adjusting the spraying equipment and spraying parameters

the curing temperature of the coating is too low: adjust the baking temperature to the range required by the process

the heating rate of the oven is too fast: the heating rate should be appropriately slowed down. Generally, the time required for the workpiece to rise from normal temperature to curing temperature should be controlled within 4-5 minutes

strong anti ionization effect: reduce the voltage and increase the distance between the spray gun and the workpiece

the phosphating film is too rough, which hinders the leveling of powder. At this time, the compactness and fineness of phosphating film should be improved

poor powder atomization: voltage and wind pressure can be adjusted

damp and caking of powder: the powder should be prevented from being damp

9 the gloss of high gloss powder is low after curing.

a large number of micro pinholes are formed by bubbles escaping during the curing process of the coating. First, check whether the substrate has pores or is affected by moisture, and dry the moisture of the workpiece before coating

the volatile components of raw powder and recovered powder exceed the standard and the humidity of compressed air is too high. At this time, the powder coating must be subject to factory sampling inspection to reduce the humidity of compressed air

excessive curing: it is necessary to control the curing conditions of the powder

10 low gloss powder has high gloss after curing

incomplete curing: it is necessary to check the heating system to ensure the curing temperature and prolong the residence time of the workpiece in the oven

the heating rate of the oven is too slow: it is necessary to improve the heating rate of the oven

11 pinholes and bubbles

there are pores on the surface of the workpiece: using the preheating method, bake for 20 minutes under the condition of 20% higher than the powder curing temperature to eliminate tiny bubbles. Antifoaming agents can also be added to the paint to help eliminate bubbles. If the oil and rust removal of the workpiece is not complete or the water washing after phosphating is not clean, the pretreatment quality must be improved. When the moisture is not completely dried after surface treatment, it is necessary to appropriately increase the preheating temperature

the compressed air is not clean: check the purification of the air source and improve the compressed air purification equipment

the coating is too thick: reduce the spraying times

voltage too high: adjust the voltage

spray distance is too close (electrostatic breakdown): increase the spray distance, and pay attention that the spray gun should not stay on the workpiece for too long

12 shrinkage

compressed air contains a small amount of oil: check the purification of the air source and improve the compressed air purification equipment

uneven powder discharge: eliminate the blockage of the powder supply system

poor surface quality of the workpiece: if the surface of the workpiece is not degreased, the pretreatment process needs to be strictly controlled. If there are significant uneven holes on the workpiece surface, it is necessary to improve the flatness of the workpiece surface

mix other types of powder and impurities: strengthen the cleaning work

13 coating

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