Analysis on MDF powder coating process

1.

Foreign research and development of MDF powder coating technology has experienced: high temperature rapid curing coating - UV powder coating - LTCP (low temperature curing powder) coating

(1) High-temperature fast-curing coating is to pass the powder-coated wood board at a high speed through a high temperature zone above 250°C to make it level and solidify

The UV powder curing section in the equipment adopts the UV lamp of Fussen Company in the United States

LTCP coating technology is a powder coating developed recently at home and abroad using infrared heating technology to melt and solidify the surface of MDF boards

(2) Shanghai Xinxing Electrostatic Spraying Equipment Co.

Shanghai Xinxing Company began to study the medium wave infrared heating technology for MDF powder coating, and in 2002 successfully developed the first vertical medium wave infrared heating drying tunnel production line for MDF furniture board powder coating in China

At the same time, Xinxing Company has developed a medium wave infrared heating drying tunnel for laboratory powder coating, which is provided to Aksu Company and Beijing Wood Machinery Research Institute respectively

(3) In the following 10 years, due to the demand of foreign trade, Guangdong began to develop the traditional medium-wave infrared curing drying tunnel of infrared tube horizontal cloth, and adopted the popular foreign MDF board preheating coating process

The furniture manufacturing enterprises in this area have accumulated and mastered the construction points of the MDF powder coating process adapted to the humid climate in the south after years of repeated experiments and explorations on the construction process, and realized the production of finger coating

(4) After I came to Shandong Langfabo Powder Coating Company in 2011, I continued to improve the original experimental powder coating production line

2.

MDF wood-based panels are made by mixing wood fibers and adhesives under steam heating and pressure

1.

2.

3.
The shorter the residence time of MDF boards in the high temperature zone of the curing tunnel, the better, and the corresponding fast curing powder must be developed
.

3.
MDF powder coating process

3.
1 Process flow

The process flow of MDF powder electrostatic spraying at home and abroad is shown in Figure 1
.

3.
2 Pretreatment back cover

3.
2.
1 The pretreatment of MDF is completely different from the pretreatment of metal parts

The pretreatment of cold-rolled sheet metal parts adopts: degreasing, rust removal, phosphating, water washing, drying and other processes to form a rust-proof phosphating film
.
For hot-rolled metal parts, the oxide scale and rust are relatively serious, and the pre-treatment process of mechanical derusting by sandblasting and hot shot is often used
.

MDF board is made by hot pressing of wood fiber and adhesive
.
There are a lot of voids in the board and on the surface
.
Therefore, the purpose of its pretreatment is to fill and close these gaps as much as possible to obtain a dense and smooth surface similar to a metal plate, so as to obtain a smooth and smooth powder coating film
.

Another purpose is to improve the heat resistance of the MDF board, so that it can prevent the side cracking of the MDF board and the quality defects such as blistering and pinholes caused by volatile gases during the melting, leveling and curing process in the medium wave infrared heating drying tunnel
.
So its pretreatment has its own characteristics
.

3.
2.
2 Pretreatment back cover method

Powder coating back cover: This back cover method is used in Europe and the United States
.
The powder of the back cover adopts epoxy powder coating with lower melting point and curing temperature, which can close the gap and obtain a smooth back cover coating
.
Due to the use of high-quality MDF sheet, there are few volatiles and low hygroscopicity, so the powder sealing layer is not prone to cracking, blistering, pinholes and other defects during the melting and solidification process
.

This back cover process is suitable for coating special-shaped MDF boards
.
However, the price of this MDF board and epoxy powder primer is relatively expensive, so it is difficult for ordinary furniture manufacturers to accept this high-cost construction method
.

UV coating back cover: The construction method of UV liquid coating back cover has made progress in China
.
It can seal the periphery of the MDF board well, reduce the defects such as cracking of the board, ensure the appearance quality of the powder coating film on the surface of the MDF board, and has a high coating efficiency.
It is suitable for rectangular and round product back covers
.
The edge sealing construction of special-shaped MDF boards is very difficult
.
In addition, there is still a certain amount of odorous volatiles in the powder-coated MDF board with the UV liquid coating back cover
.

Water-based paint back cover: Water-based paint is developing rapidly in China.
It and powder coating are both the coatings that are popularized and applied by the country.
As the back cover material of MDF board, it has environmental protection, low price, and its process is easy to connect with the traditional spray painting process, and it is easy for workers to operate.
Mastery and other advantages
.

The selection requirements of water-based primers are relatively high.
At present, domestic epoxy-modified acrylic water-based paints and epoxy-modified polyurethane water-based paints are all available varieties, with a solid content of 40~45%
.
On request, suitable fillers and curing accelerators can be added to the paint
.
Water-based paint seals require multiple back-sealing treatments.
The paint film can be cured naturally at room temperature or quickly cured by heating
.

The advantage of this back cover method is that it can be applied to the edge sealing treatment of MDF special-shaped boards
.
For mass production enterprises, low temperature curing drying tunnel should be configured
.
The quality of MDF pretreatment back cover is an important factor determining the quality of MDF board powder coating film
.
Improper pretreatment of chemical methods in metal powder coating can affect the quality of powder coating film by up to 30%, and improper treatment before sand blasting and shot blasting can cause quality defects up to 50%
.

Improper pretreatment and back cover of the MDF board will also cause a lot of problems in demand
.
Furniture manufacturing companies must update their understanding of pre-treatment in traditional paint construction
.
When applying the MDF powder coating process, attention must be paid to the quality of the process before the back cover
.
This link is a common obstacle encountered in the current promotion and application
.

3.
3 Powder electrostatic spraying

MDF wood-based panels are poor conductors, and solving its uniform adsorption of charged powder has always been a difficult problem for early researchers
.

3.
3.
1 Preheating electrostatic spraying method

The preheating spraying method is an effective method that has been successfully applied to MDF powder electrostatic spraying in foreign research
.
Preheating the MDF board makes the moisture contained in the board escape to the surface of the board after being heated, and the surface resistance of the board drops to 105~109Ω, showing electrical conductivity
.
This ensures a good electrostatic adsorption capacity on the surface of the MDF board
.
A typical preheating process is 5~10 minutes at 90~125°C
.

In the construction practice, according to the size of the workpiece, the thickness of the MDF board and the chain speed, it is necessary to find out the appropriate preheating process parameters through experiments
.
Some domestic furniture companies use this preheating electrostatic spraying process in powder coating
.

3.
3.
2 Normal temperature electrostatic spraying method

MDF board room temperature electrostatic spraying technology is a relatively simple construction technology independently researched in China
.
Shanghai Xinxing Electrostatic Coating Equipment Co.
, Ltd.
uses this new technology to conduct powder electrostatic spraying for the products of General Electric (GE) of the United States
.

The 5% water-based conductive liquid studied by the author is sprayed on the MDF board after the water-based paint back-sealing treatment at room temperature, and it is naturally dried for a few minutes or the surface is dried with compressed air.
At this time, the surface of the MDF board will occur in the electrostatic field.
The displacement of the space charge shows the characteristics of electrostatic powder absorption similar to the metal plate
.

This state can be maintained for more than a few hours, which can meet the process time required for electrostatic powder spraying construction
.
This technology has been tested in the application of MDF board powder electrostatic coating construction for more than 10 years
.
And it is also used in the classic coating of insulating material powder
.

3.
3.
3 Process evaluation

(1) Process technology is to serve the product
.
As long as the process technology that can make qualified products has practical value
.
Through extensive application and the test of time, people will gradually recognize the advantages, disadvantages and applicability of different process technologies, and choose the best process technology for their own products
.
This has been confirmed in the development and application of powder coating technology for metal products
.

MDF powder coating technology is in the early stage of research and development and popularization and application, and it is normal for multiple process technologies to coexist
.
Through the promotion and application in the future, it will be gradually rationalized and standardized by the market and the industry
.

(2) Comparison of two coating processes

3.
4 Medium wave infrared heating drying tunnel

The hot coating method and the cold coating method of the powder classic coating process only solve the difficulty of uniform adsorption of the powder on the surface of the MDF board
.
To solve the damage to the financial department and ensure the leveling and curing of the MDF board surface coating, it is necessary to take the lead in the new irradiation technology and the supporting fast curing powder
.

3.
4.
1 Far infrared heating drying tunnel

Far-infrared radiation heating technology swept the country in the 1960s
.
The far-infrared drying tunnel heating pipes at home and abroad are arranged horizontally along the length of the drying tunnel
.
This classic horizontal arrangement has a fatal flaw, that is, the combination of the heat of the irradiation plant and the heat of the temperature field in the drying tunnel makes it difficult to ensure the uniformity of the upper and lower temperatures in the drying tunnel
.

The irradiation energy is inversely proportional to the square of the distance.
The closer the lamp is to the workpiece, the greater the irradiation energy, and the greater the difference in the irradiation energy received by the surface of the workpiece
.
Taking the distance between the headlamp and the workpiece can improve the uniformity of irradiation, but the space and heating power of the drying tunnel increase accordingly, and the advantages of irradiation heating and energy saving are lost
.
This is an important reason why far-infrared heating energy-saving technology has not been fully promoted and applied in China for many years
.

3.
4.
2 Horizontal medium wave infrared heating drying tunnel

(1) The wavelength of far infrared rays is greater than 4μm, the temperature of the radiator is 400~600°C, and the radiation energy is low
.
The wavelength of medium wave infrared is 2.
5~5μm, the peak is in the area of ​​2.
4μm, and the temperature of the radiator is 800~900°C
.
The absorption peaks of the structural genes of polymer materials are shown in Table 1
.

It can be seen from the above table that the wavelength emitted by Zhongbohong's appearance is more suitable for the absorption and heating of powder coatings, and the heating efficiency is as high as 30~50%
.
Therefore, the powder coating of MDF boards at home and abroad adopts medium wave infrared heating technology
.
It can make the powder adsorbed on the surface of the MDF board quickly absorb heat and level and solidify in a short time without operating the MDF board substrate
.

(2) The horizontal medium-wave infrared heating drying tunnel developed in China is similar in structure to the far-infrared heating drying tunnel, except that the heating element is replaced with a medium-wave infrared heating tube
.
In order to solve the uniformity of irradiation in the height direction of the drying tunnel, it also depends on increasing the distance between the heating tube and the workpiece
.
In this way, the utilization rate of the heat of the drying tunnel is reduced, and the heating power of the drying tunnel is bound to increase
.

(3) The domestic Guangzhou Institute of Electrical Appliances has done a lot of research work on the horizontal medium wave infrared heating drying tunnel
.
At the beginning of 2011, a research report was published on the Guangdong Innovation Network, and it was believed that there are still many problems in the study of the temperature uniformity of the board surface and the front and rear sides of this type of horizontal medium-wave infrared heating drying tunnel for MDF powder coating.

.

(4) The medium wave infrared tube is the most famous product of Heraeus, Germany
.
In the 1990s, Jinzhou High Infrared Research Institute took the lead in developing this type of infrared heating element
.
There are many similar products on the domestic market at present, but the quality of the products varies greatly
.

3.
4.
3 Vertical medium wave infrared heating drying tunnel

In view of the shortcomings of the horizontal medium wave infrared heating drying tunnel, the author arranges the medium wave infrared heating drying tunnel based on 50 years of powder coating research experience in China
.

The concept of equal probability inhomogeneity to achieve uniformity under specific conditions is proposed, and the vertical mid-wave infrared tube is designed to solve the problem of uniform irradiation of MDF board in the drying tunnel, and realize the reduction of the heating space for irradiation in the drying tunnel.
Improve the utilization rate of irradiation field energy and greatly reduce the heating power of the drying tunnel
.
The unevenness of equal probability achieves uniformity: the radiation intensity of each emission point of the vertically arranged mid-wave infrared lamps on its heating section is uniform, that is, the radiation intensity on a straight line perpendicular to the drying tunnel is the same.
of
.

In addition, the vertically arranged tubes have different spacings, so the irradiation intensity at each point along the X axis of the drying tunnel has obvious fluctuations.
Such a drying tunnel is not a uniformly irradiated heating drying tunnel
.

But when each vertical line on the MDF board has the same probability of receiving this uneven wave irradiation during the movement, the powder on the entire MDF board receives the same amount of irradiation
.
It is the meaning of "equi-probable unevenness to achieve uniformity" under this specific condition
.
This concept proved its correctness in practical application
.

The heating time of MDF board in the vertical medium wave infrared drying tunnel is 4~6MIN
.
After the MDF board enters the leveling section, the temperature of the powder in the 60S inner mixing surface quickly rises to 160°C, which is the rapid melting and leveling of the powder, and then the temperature rapidly falls back to 130~140°C to complete the curing of the coating film
.
The powder coating products suitable for Zhongbo infrared rapid curing are produced by Taicang Tiger Company in China, and the curing conditions are 130°C/10min
.

4.
Conclusion

MDF powder coating application is a new high-end market that is being developed
.
As the market matures, the gold content of China's powder coatings and coating technology will continue to increase
.

It is a technological leap from metal powder coating to the coating of non-metal heat-sensitive materials
.
In the course of research in this technical field, China has begun to narrow the gap with foreign countries, which is worthy of congratulations
.
But more importantly, in mass production applications, there are still many difficulties that we need to solve
.
For example: the quality of domestic MDF boards varies greatly; environmental factors, especially how to prevent the impact of humid environment on the quality of product coating,
etc.
All of these need to be solved by the joint efforts of colleagues in the industry
.

Source: Shandong Langfabo Powder Coating Company, powder debut