Analysis of common defects on the surface of electrostatic coating of powder coating

Abstract: This paper briefly introduces the coating defects that are easy to occur during the powder coating process of spin-on oil filters and diesel filters. The coating defects were analyzed and a solution was proposed.

introduction

    Spin-on oil filters and diesel filters have the characteristics of simple shape and single color coating, while powder coatings are superior to solvent-based coatings in terms of unit cost, corrosion resistance and environmental pollution. Therefore, since 2000, domestic filter manufacturers have gradually replaced the coating process with the previous spray-based coatings and powder coatings. The most widely used powder coating method is electrostatic spraying. This paper analyzes the common defects in electrostatic spraying of spin-on engine oil and diesel filter powder coating, and proposes a solution.

1 orange peel

    The so-called "orange peel" is actually a surface defect similar to the orange peel shape, and a slight regular convex and concave marking is seen under the magnifying glass. The reasons for this are:

    (1) The powder solidification speed is too fast, the self-leveling property is poor, or the curing temperature is low, and the melt flowability is poor.

    (2) When spraying parts, the spray gun is intermittent, the parts are too far away or electrostatically shielded, resulting in different thickness of the powder on the surface of the part, the coating film is too thin, and the surface of the coating is muscle wrinkles; Too thick, the film is striped orange peel;

    (3) When spraying, powder atomization is not good;

    (4) Mixing the same color powders from different manufacturers. Due to the different formulas and raw materials of the same color powders of different manufacturers, the speed and surface tension of the powder during melt leveling form orange peel or shrinkage cavities;

    (5) The defects of powder coatings mainly include: the dispersion of powder coatings is not good, that is, the dispersibility between pigments and resins and additives is not good, resulting in inconsistent leveling speed and surface tension of powder coatings during melt leveling. , forming orange peel or even shrinkage; the fineness of the coating is not enough (mainly the problem of pigment), there are some large particles, orange peel or shrinkage after curing; raw materials for the production of powder coating (resin, leveling agent, curing agent, etc.) Auxiliary) improper selection or low grade or a selection of materials for a variety of materials, may cause the skin to solidify to form orange peel, shrinkage.

2 exposed

    In most cases, the bottom is understood to mean that the parts have a low powder rate at one time, which is caused by:

    (1) The chain speed is too fast, causing the corresponding dusting time to be too short;

    (2) The amount of powder discharged is small, that is, the conveying pressure is low, causing the workpiece to be exposed, or the powder supply line is blocked;

    (3) The powder coating is charged little, mostly due to the low voltage of the electrostatic generator, and the general voltage is maintained at about 80;

    (4) The grounding of the parts is not good. Due to the repeated use of the hanging tools for a long time, the surface layer of the hanging device is too thick to be cleaned, resulting in poor electrical conductivity of the hanging device, which causes the workpiece to be exposed;

    (5) The workpiece size is too large, and the position of the spray gun or the powder fan surface is improperly adjusted, which should cause the bottom to be exposed and occur frequently;

    (6) There is also a primer, the spray coating of the powder coating is not very thin, but also within the thickness required by the process, but the cured powder coating is

3 particles

    The rework of parts caused by particles attached to the surface is a large part of the rework of the entire part. The factors that influence are:

    (1) There are particles such as dust on the surface of the pre-treated part, causing particles on the surface after the powder is solidified;

    (2) The working environment of dusting is unclean, and the workpiece is dusted and dusted after the workpiece is sprayed. The drying tunnel was not cleaned for a long time, and the particles were dropped during the powder solidification process;

    (3) The recycled powder is mixed with particles, and the vibrating screen of the powder box does not work (damage or poor sealing), causing particles mixed in the powder coating sprayed by the spray gun;

    (4) The powder coating provided by the powder manufacturer itself contains particles such as dust and large-diameter pigments;

    (5) Powder tube or venturi throat section due to long-term use, the powder in which it melts and collides with the tube wall to form individual powder skin, which is not thoroughly cleaned and sprayed on the surface of the part to form particles;

    (6) The hanging device adopts a burning method to remove the powder layer, and the residual dust impurities on the surface are not cleaned, and are attached to the powder layer on the surface of the part to form particles;

    (7) The powder is agglomerated by moisture, the powder is not uniform, and the atomization is not good, and the particles on the coating film can be formed.

4 shrinkage holes

    The reasons for the shrinkage of the surface of the part are:

    (1) After the pretreatment, the surface of the part is oily. This may be caused by the incomplete degreasing of the parts, or it may be caused by oil stains in the gloves or in the transfer box during the suspension and transfer of the parts;

    (2) The compressed air contains oil;

    (3) The powder conveying chain has just recently added lubricating grease. When the curing drying tunnel is baked at a high temperature, the volatile oil is adsorbed on the surface of the part to form shrinkage cavities in the air in the drying tunnel. There is oil in the drying tunnel. The lubricating grease of the conveying chain is condensed on the wall of the drying tunnel after baking, and is not cleaned for a long time, the oil is accumulated, volatilized under high temperature conditions, diffused in the drying tunnel, and adsorbed on the surface of the part to form shrinkage holes;

    (4) Mixing the same color powders from different manufacturers will also cause shrinkage holes;

    (5) The improper ratio of the raw materials of the powder coating itself or the insufficient grade may also cause shrinkage cavities;

    (6) The fluidized plate in the powder box is contaminated by oil and can also cause shrinkage. However, this problem is ultimately caused by oil in the compressed air.

5 poor mechanical properties

    Poor mechanical properties of powder coatings include poor adhesion, poor flexibility, poor impact strength, and poor hardness.

    (1) The cause of the poor mechanical properties of the powder coating is the insufficient curing time or temperature of the powder. Under certain conditions, the curing time of the powder is inversely proportional to the curing temperature. At present, we can understand that the powder curing conditions are: epoxy powder coating, epoxy-polyester powder coating, which can be cured at 140 ° C for 25 min. It can also be cured at 220 ° C for 5 min. Each powder coating product has its own curing conditions. The result of random shortening (prolonging) curing time or lowering (increasing) curing temperature is that the powder coating cannot be completely cured or over-baked, which will inevitably affect the powder. The adhesion of the layer, the impact strength and other properties, the ultimate damage is the image of the product in the customer's mind.

    (2) Powder coatings provided by powder manufacturers have quality problems, and most of them are problems with curing agents, resins, fillers, pigments and the like.

    (3) For the adhesion is not good, there is another reason is that the pre-treatment of the parts is not clean, and the presence of oil, dust and other substances on the surface affects the adhesion of the powder.

    (4) The storage time of the powder coating exceeds the storage period, thereby affecting the mechanical properties of the powder coating.

    (5) In addition, a fine, dense and complete phosphating film is a prerequisite for ensuring good mechanical properties after solidification of the powder. When the cause of poor mechanical properties of the powder is investigated, this reason should also be considered.

    (6) If the powder curing time is too long or the temperature is too high, the flexibility of the powder will also be lowered.

6 Lost light

    Loss of light and discoloration are put together here, mainly because in many cases, the causes of these two defects are the same.

    (1) Discoloration (loss of light) In most cases, it is caused by the curing time of the powder coating being too long or the curing temperature being too high. Since the pigments contained in the red powder, the yellow powder, and the blue powder are not resistant to high temperatures, if the high-temperature baking is beyond the range of curing conditions for a long time, the yellowing of the powder is liable to occur. Excessive high temperature baking also tends to age the resin and make the powder brittle;

    (2) If the gloss of each batch of the powder coating of the manufacturer is different, it also causes the artificial feeling of the powder to lose light; the mixing of the different gloss powders also causes the gloss of the product to be different. This may be a problem with pigments or matting agents, brighteners, fillers;

    (3) Mixing powder coatings from different manufacturers may cause discoloration and loss of light due to the different resins, pigments and additives used.

7 pinhole

    The main reasons for the pinholes in the film are:

    (1) The voltage is too high, causing the powder layer to be broken by high voltage, or the distance between the spray gun and the part is too close, causing the powder layer to be broken by high voltage. Generally, the distance between the spray gun and the parts is about 200 to 300 mm;

    (2) The phosphating film is rough and has scum, and the powder coating layer is not too thick, and the roughness of the bottom layer cannot be completely covered, and pinholes may also appear in the powder layer.

8 Frequent problems in the production process of dusting

    (1) Powder coating escape. When the electrostatic powder is sprayed, the powder escapes. One is to affect the cleaning of the coating environment and damage the physical and mental health of the operator. The second is that the powder is escaping, which easily causes pollution and interference between the light-colored powder and the dark powder, making the product easy to produce. Color point, causing repairs. Common reasons are:

    1 The distance between the spray gun and the inlet and outlet of the powder chamber is too close;

    2 The air pressure is too large, causing the flow rate of the powder to be too large;

    3 The dust chamber recovery component is not sealed;

    4The suction of the worm gear fan is small, and the excess dispersed powder can not be completely sucked into the recovery box; (When the power of the worm gear fan is relatively large and the suction force is strong, the powder is sprayed from the spray gun, and it has not yet been absorbed. Some of the workpiece is sucked into the recycling bin, which tends to cause the workpiece to be exposed. At this time, it is necessary to increase the amount of powder.)

    5 Recycling filter plugging. The reason for the blockage of the recovery filter element is that the powder is damp and cannot be vibrated after being adsorbed on the recovery filter element; the back-blowing air pressure is too low to vibrate the powder of the recovery filter element; the recovery component is not sealed, and the back-blown air cannot recover the filter element. The powder vibrates down.

    (2) When the spray gun is broken, the thickness of the powder layer on the surface of the part will be different, forming a partial orange peel or partial exposure. Common reasons are:

    1 compressed air is unstable;

    2 The powder supply pipeline is blocked, tortuous or too long, and the general powder supply pipe does not exceed 8m;

    3 The powder coating in the powder bucket is not well fluidized. The reasons for the poor fluidization of the powder are: unstable compressed air; poor gas permeability of the fluidized plate, or blockage of the fluidized plate; excessive powder in the fluidized plate, insufficient fluidization of the powder, and incomplete fluidization; The powder coating is damp and cannot be fully fluidized in the fluidized tank.

9 Conclusion

    To do a good job of electrostatic spraying of powder coatings, the most important thing is to control the voltage and air pressure of the spraying, and do a good job of daily cleaning and maintenance of the equipment. When spraying, the powder coating is always in a sufficient atomized flow state and under the action of a good electric field, most of the powder coating defects can be avoided. For common defects, as long as you are familiar with the spraying process, part characteristics and equipment condition, you can also solve it.