3 defects caused by poor curing

Good curing conditions are an important factor in ensuring coating quality. Curing conditions include curing temperature and curing time. In the preheating stage, if the temperature is too high or the preheating time is too short, the coating surface will be dried too quickly and a cured film will be formed on the surface, which will hinder the further curing of the inner layer, and the curing rate will be greatly reduced, resulting in incomplete curing. And "pinhole" phenomenon, boiled water or boiled acid test, there will be bubble peeling or cracking phenomenon. In the constant temperature curing phase, if the temperature is too high, “over-bake” phenomenon will occur, the coating will become discolored and brittle, the mechanical processing performance will be reduced, the adhesion will be reduced, and the impact resistance will be poor; when the temperature is lower than the curing requirement, the crucible will be cured; Or boiled acid test white, corrosion resistance is poor, mechanical processing or high temperature sterilization will produce coating cracking, shedding phenomenon. In the production process, due to the structure of the oven, wind speed and other reasons, there may be a phenomenon that the temperature difference between the same plate is too large. Although the set temperature meets the requirement, there may still be partial curing in the low temperature area.

Before the production, the curing time must be set according to the nature of the coating, the amount of coating, and the characteristics of the oven. The time of the iron skin in the constant temperature curing zone must be within a certain range. The time is too long, and as the temperature is too high, it will produce "over-baking" phenomenon, and the coating will become discolored and brittle. If the time is lower than the necessary curing time, the curing may be incomplete when the temperature is lower than the curing temperature.

During the solidification and cooling phase, pay attention to exhaust emissions and sufficient cooling time and cooling temperature. Cooling time is too short, the cooling temperature is too high, the coating surface is not completely hardened, showing a "melting soft" state. After the iron skins are collected together, the iron skin and the iron skin will be bonded together. This phenomenon is known as "anti-sticking." This phenomenon is very easy to occur in the varnish coating. After the “anti-adhesion” of the iron sheet, the coating will lose its luster and its adhesion will decrease. This is actually an incomplete phenomenon caused by the cooling phase.

In order to avoid the above defects, when setting the curing temperature, the difference between the same plate temperature at the curing temperature, the difference between the surface temperature and the actual furnace temperature, and the fluctuation range of the furnace temperature should be measured to ensure that the low temperature zone of the iron sheet should be Above the minimum curing temperature. At the same time, the temperature curve of the oven at the solid temperature should be regularly tested. In the high temperature zone, the temperature of the oven should be basically stable and cannot have large fluctuations. Generally, the temperature should not exceed 5°C. When the furnace temperature cannot reach the set temperature, check the heating system and ventilation system of the oven, such as oil, flame, hearth, fan and so on. When the furnace temperature fluctuates greatly, check whether the production speed is too fast and whether the furnace temperature monitoring and control system is abnormal.

4 defects caused by poor operation of oven equipment

The defects caused by poor operation of the oven equipment are mainly manifested as “bracket injuries”. The support scratches the coating layer, and the iron will be severely generated. The conductive value of the coating will increase and the coating will lose its protective effect. There are many reasons for the “bracket injuries”, such as the damage of the bracket itself, the bending deformation of the bracket running rails, uneven wear and deformation of the guide wheels, and the mechanical causes of the jitter on the brackets. This is the main reason for the “bracket injury”. When the iron sheet just enters the bracket, the running of the iron sheet and the bracket is not synchronized, and the iron sheet has a large collision force with the bracket, destroying the coating, and thus also causing a “bracket injury”. In short, if there is a collision force or relative motion between the stent and the sheet metal, or if there is a large adsorption force between the stent and the stent, “bracket injury” will occur.

Another defect caused by poor operation of the oven equipment is the "scratching" of the iron skin. Mainly because the speed of the conveyor belt of the outlet collecting device is not enough, so that the initial speed of the iron sheet is too low, and the iron sheet cannot be transported to the front end of the collecting box, but falls on the front sheet metal sheet, resulting in a "scratch." Another reason is that the upward blowing air pressure in the collecting device is too small to effectively lift the iron skin so that the front end of the iron skin cannot reach the front end of the collecting frame and the front end falls down and scratches the front sheet metal. In addition, at the exit of the oven, due to the synchronization of the position of the bracket and the conveyor belt, when the iron sheet is not completely detached from the bracket, the front end portion has fallen on the belt and is dragged by the belt, causing “dragging” between the iron skin and the bracket. ".

5 Concluding remarks

Impression of iron coating defects in a variety of manifestations, some performance defects; some performance is incomplete curing, adhesion is reduced, boiling boiled acid white, in the high temperature cooking and sterilization process cracking, shedding; Some showed brittle cracking and shedding of the coating; some showed more dew points or insufficient denseness, increased electrical conductivity, and poor corrosion resistance. There are many factors that cause coating defects. Inadequate factors such as raw materials, process formulation, equipment condition, operation level, and production environment all cause coating defects. When defects in the coating are found in the production process, the cause of the defect should be determined based on the nature of the defect and be resolved to avoid causing greater losses.

(Author / Huang Laijun, Wu Zhenxin)

"Chinese and Foreign Packaging and Printing"