What are the main factors affecting the uniformity of coating? How to control?
There are many factors affecting the uniformity of coating, including human, machine, material, method, and ring, but the basic factors are several conditions directly related to the coating process: coating substrate, adhesive, coating Steel roller / rubber roller and compound machine etc.
1) Coated substrate, mainly material, surface characteristics, thickness and uniformity.
2) Adhesive, mainly its working viscosity, affinity and adhesion to the substrate surface.
3) Coated steel roller, which is not only the direct carrier of the adhesive, but also the support base of the coated substrate and the rubber roller, so it is the core of the entire coating mechanism. Its shape and position tolerance, rigidity, dynamic and static balance quality, surface quality, temperature uniformity and thermal deformation conditions all affect the uniformity of coating.
4) Coated rubber roller, rubber roller refers to an important variable of coating quality, its material (such as the life of the rubber layer), hardness, shape and position tolerance, rigidity, dynamic and static balance quality, surface quality, thermal deformation, etc. are also affected To the uniformity of the coating.
5) The laminating machine is the basic platform for coating. In addition to the precision and sensitivity of the pressing mechanism of the coating steel roller and rubber roller, it also includes the maximum speed of design operation and the overall stability of the machine.
The influencing factors of coating uniformity in the horizontal and vertical directions are different, so the control measures are also different accordingly. In the specific implementation, it is related not only to the design and manufacture of the machine, but also to the operation and process control.
1. The lateral uniformity of the coating and its control
The uniformity of the coating width direction is mainly affected by the following factors:
1) The geometric tolerance of steel roller and rubber roller.
2) Relative to the contact pressure between the steel pipe and the substrate.
3) The adhesive is distributed in the lateral direction, here mainly refers to the viscosity of the adhesive and the corresponding transfer rate. It is mainly determined by the first two factors and its working viscosity.
The first reference for coating accuracy guarantee is the shape and position tolerance of the steel roller and its rigidity. During the operation, the shape and position tolerance, rigidity, balance accuracy, assembly accuracy and built-in heating and insulation system of the steel roller directly affect the state of the adhesive, thereby affecting the transfer rate and coating uniformity. Therefore, the design and selection of high-precision, wear-resistant steel rollers is a basic task.
The rubber roller, especially the coating rubber roller, is the first influencing factor or variable of the lateral uniformity of the coating. The material, shape tolerance, rigidity, hardness of the rubber layer, dynamic balance of the rubber roller, rubber roller support structure, pressure adjustment, etc. will obviously affect the uniformity of the coating. As a slender shaft, the rigidity of the rubber roller is the most obvious factor. From the perspective of use, the cleanliness of the rubber roller surface is also an important variable in the production process that affects the uniformity of the rubber roller coating. Therefore, the quality and maintenance of rubber rollers need to be carefully controlled.
From the above analysis, it can be seen that the wider the substrate or coating width, the more difficult it is to guarantee the uniformity of coating. When the coating width reaches a certain value, the uniformity of coating will become the first major challenge of the accuracy of the entire solvent-free compound machine.
2. The longitudinal uniformity of the coating and its control
The longitudinal uniformity of the coating is the uniformity along the substrate running direction. It depends on many factors, including:
Substrate type, surface quality and consistency;
The running speed of the machine, the number and frequency of machine speeds;
The viscosity status of the adhesive, especially the consistency of viscosity during long time running;
The quality of the rubber roller, especially the elasticity, wear resistance and heat resistance of the rubber roller;
Control mechanism and structure of the amount of glue applied in the solvent-free compound machine;
Structure of gluing system of compound machine, etc .;
Environmental factors, etc.
Under normal circumstances, when the machine is running at a steady speed, the transfer rate of the adhesive is consistent, so the uniformity of the coating is basically the same. However, when the machine speeds up or down, the transfer rate of the adhesive will change slightly, sometimes even significantly, thereby affecting the uniformity of the coating. Therefore, the machine must be pre-processed on the designed program to reduce the influence of frequent start-up or speed increase and decrease on the coating uniformity.
The pressure of steel roller and rubber roller will not only produce bending deformation and extrusion deformation, but also deform after being heated. These deformations usually affect the pressure condition and the contact width of the contact area, which will affect the transfer rate and the uniformity of the coating to a certain extent. The influence of the rubber roller material and hardness on the uniformity of coating is sometimes very obvious. Therefore, the influence of these deformations must be fully considered when designing the machine, especially the high-quality and durable rubber rollers must be designed and selected.
The length of downtime in the middle will also affect the viscosity of the adhesive in the rubber tank and the steel roller of the coating rubber roller, so there may also be the possibility of affecting the uniformity of the coating. Therefore, we must pay attention to control downtime to prevent the viscosity of the adhesive from being too high.
The control mechanism and structure of the amount of glue applied by the solventless laminator is a very important but often overlooked factor affecting the uniformity of longitudinal glue application. The method of adjusting the amount of glue will seriously affect the accuracy of the amount of glue and the speed of its response. For example, modern solvent-free laminating machines generally use a multi-roller transfer coating method to control the amount of glue applied through the speed difference. However, some machines use the adjustment of the metering interval to control the amount of glue. The reaction speed of this system is usually very slow, and the coating accuracy and uniformity are very low.
The glue application system of some solvent-free compound machines cannot set the glue width continuously or arbitrarily, or the glue application width and the glue application width are inconsistent, so that there is a certain glue dead angle in the glue tank (the area near the glue block) . After running for a long time, the adhesive concentration will increase or whitish in the area near the rubber block, thereby reducing the local (edge) adhesive transfer rate and directly affecting the uniformity of the transfer. As a result, the amount of glue applied locally is low, usually the edge is lower than the amount of glue applied in the middle area. This kind of unevenness is very special. The appearance is mainly horizontal unevenness, but it is caused by continuous operation, and it will repeat according to the time of glue adding. Therefore, it is necessary to pay attention to the selection of the compounding machine with reasonable control of the coating amount and the width of the glue can be arbitrarily set.
The influence of environmental factors will be more obvious in the long-term continuous production. The change of relative humidity and temperature will affect the application of the adhesive to a certain extent.
In short, the overall uniformity of the coating can only be achieved if both the horizontal and vertical uniformity are guaranteed. Relatively speaking, the uniformity of the coating width direction is usually affected by the machine itself. However, in long-term or mass production, the uniformity in the length direction will be affected by more factors, and its uniformity is more difficult to control than the width direction.
Of course, from the short-term, medium-term and long-term observation periods, the main factors affecting the unevenness of the coating will be different, and corresponding measures must be taken according to different situations.
(This article was contributed by Chairman Zuo Guangshen of Guangzhou Tongze Machinery Co., Ltd.)
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