In powder coating, there are two reasons for poor film curing: one is that the formula design of powder coating is unreasonable, and the film performance can not meet the requirements; the other is that the performance of powder coating itself is no problem, but in powder coating, the coating process control is unreasonable, resulting in the film performance can not meet the technical indicators.
Considering powder coatings, if the matching between resin and curing agent is unreasonable in formula design, for example, the selection of reactive activity of resin or the type and dosage of curing agent is unreasonable;
The physical and mechanical properties of the film will be affected by the mass percentage or volume concentration of fillers.
Considering from the powder coating process, the reasons that affect the film curing are as follows.
1. The baking temperature of baking oven can not reach the baking temperature required by powder coating.
Although the control temperature of the oven is easy to reach the design temperature or the baking temperature required by the powder coating when there is no load, sometimes it may not reach the design temperature or the baking temperature required by the powder coating due to the lack of total heat supply under full load.
Especially in the northern area, the temperature of the workshop is low in winter. If the thermal insulation condition of the oven is poor, the temperature of the coating before entering the oven is low, and the wall thickness and heat capacity of the coating material are large, this problem is more likely to occur.
When the temperature of the oven can not reach the curing temperature of the powder coating, this problem is more likely to occur.
When the oven temperature can not reach the curing temperature of powder coating, on the one hand, it can be solved by increasing the heating power; on the other hand, it can also be solved by extending the curing time.
2. In the baking oven, the curing time can not reach the baking time required by powder coating.
In the baking conditions of powder coatings, baking time refers to the baking time calculated after the coating reaches the baking temperature, rather than the time calculated after the coating is put into the baking oven.
The thicker the coating material is, the more pieces are hung, the greater the heat capacity of the coating, and the longer it takes for the coating to rise to the control temperature.
In a certain length of baking oven, when the speed of the conveyor chain is constant, the effective baking time is shorter. If the effective baking time is not enough, the film curing is not complete, and the physical and mechanical properties of the film can not meet the requirements.
Therefore, in powder coating, it is better to measure the temperature change of the coating in the baking oven under the full load condition with the furnace temperature tracking tester, so as to determine the reasonable baking curing temperature and time, so as to ensure the complete curing of the film and the physical and mechanical properties of the film.
3. The surface of the coating is not well treated.
The oil on the surface of the coating is not cleaned, the oxide scale or rust is not removed, or the quality of the phosphating film is not good, and many phosphating liquid residues are smeared on the surface.
In this case, the adhesion between the film and the substrate is not good after powder coating, and the impact resistance of the film is also not good. This problem is solved by improving the surface treatment quality of the coating.
4. In terms of film performance inspection, it should be carried out in strict accordance with the test temperature and film thickness specified in the product standard.
Generally speaking, the film thickness is thicker than the specified thickness; when the laboratory temperature is lower than the specified temperature, the impact strength, flexibility, cupping test and adhesion of the film are poor, and it is not easy to pass;
However, when the film thickness is thin and the test temperature is higher than the specified temperature, these physical and mechanical properties are easy to pass. If you want to truly and accurately determine the film performance, it is better to test according to the conditions specified in the standard or close to the conditions.
In order to ensure the quality of coating products, we should try our best to test the performance index of coating film under relatively harsh conditions.
5. The curing temperature is too high
When the baking time is too long, the color of the film changes due to the thermal aging of the film, and the physical and mechanical properties of the film such as flexibility and impact strength decrease. Therefore, too high curing temperature and too long curing time are also unfavorable to the physical and mechanical properties of the film.
In powder spraying, the curing temperature and time of powder coating play a decisive role in the curing performance of coating. Therefore, we must go through many tests, reasonably classify the sprayed objects, determine the number and arrangement of hangers, and the corresponding baking temperature and time, so as to ensure the quality of coating products.
