Experimental Investigation of Vertical Density Proﬁle of Medium Density Fiberboard in Hot Press

: This research investigates the performance of medium density ﬁberboard (MDF) with respect to hot press parameters. The performance of the board, type of glue, and production efﬁciency determine the optimum temperature and pressure for hot pressing. The actual temperature of the hot press inside the MDF board determines the properties of the ﬁnal product. Hence, the optimal hot press parameters for the desired product are experimentally obtained. Moreover, MDF is experimentally investigated in terms of its vertical density proﬁle, bending, and internal bonding under the various input parameters of temperature, pressure, cycle time, and moisture content during the manufacturing process. The experimental study is carried out by varying the temperature, pressure, cycle time, and moisture content in the ranges of 200–220 ◦ C, 145–155 bar, 260–275 s, and 8–10%, respectively. Consequently, the optimum input parameters of a hot-pressing temperature of 220 ◦ C, pressure of 155 bar, cycle time of 256 s, and moisture content of 8% are identiﬁed for the required internal bonding (0.64 N/mm 2 ), bending (32 N/mm 2 ), and increase in both the core and peak density of the vertical density proﬁle as per the ASTM standard.


Introduction
Medium density fiberboard is a wood board manufactured under optimal hot press conditions using wood fibers and applying the urea formaldehyde resin.MDF boards have several indoor and outdoor applications but are mainly used in the furniture industry as a replacement for solid wood [1].There are three types of production lines in MDF manufacturing,

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Continues line press.
Figure 1 shows an overview of all the major processes involved in MDF manufacturing.This includes raw material collection, chipping, defibrating, forming, pressing, product finalizing, and shipment to the market [2].With the introduction of industrial wood-based panel manufacturing, most researchers initially focused on the technical and economical optimization of both processes and products [3].Nowadays, customer demands for sheets are steadily increasing.There are two ways to meet such needs: either by performing various experiments on MDF boards or by developing model simulation [4].The variation in density at different positions in the fiber mat is of major concern because of the consequent variations in mechanical properties.Input temperature, pressure, cycle time, and moisture content in hot pressing determine the physical and mechanical properties of the manufactured MDF board [5].A large number of boards (nearly 10-15%) or about one hundred thousand cubic meter per year are rejected in the market due to defects, such as weak internal fiber arrangement, low bending and internal bonding, sides looseness, delamination problems, and rough surfaces [6].The major reason for these defects is the lack of appropriate understanding of the inter-relationship during the hotpress process among the initial process parameters, such as temperature, moisture content, platen pressure, and its impact on the properties of the board [7].Consequently, the process optimization of MDF is important to overcome all of these issues.A viable solution is to develop a relationship between the hot press process parameters and its output results [8].A number of researchers have examined the parameters of hot pressing during operation, while our study focusses on the parameters in the hot press only.In this research, the experiments are performed on 16 mm MDF board [9].To perform the experiments, a long piece of 8 × 4 feet MDF board is taken, and it is further divided into smaller pieces according to the requirements of lab tests.Furthermore, the vertical density profile (VDP) is a critical factor that determines the strength and quality of MDF panels [10].The concept of the ratio of the modulus of bending of one layer to the sum of the modulus of bending of all layers in the previous time step, as previously given by Suo and Bowyer [11], is redefined according to the latest published findings.The equation given by Carvalho et al. is used to calculate the modulus of elasticity of different layers of the fiber cake.This model gives an improved relationship between the peak and core densities at a low hot press platen temperature at 160 • C but with an increase in hot platen temperature to 198 • C, where the rise in peak density is comparatively high [12].The results of laboratory studies indicate that the vertical density profile of MDF is made from a combination of processes that occur both during compaction and also after the press has reached its final position [13].Medium density fiberboard's strength, hardness, and other important properties are determined by its vertical density profile, which is one of the most important factors that is determined in laboratory tests of MDF performance.This factor is the combination of the results of many process parameters [14].During hot pressing, the internal condition of the sheets, temperature, and moisture content continuously change with time [15].A one-dimensional computer model depends on fundamental analysis, which shows the relationship between process parameters and density profile arrangement.Such parameters are important board characteristics that correlate with the properties of the MDF board [16].MDF boards with appropriate VDPs are created with careful determination and accurate pressing plan.Existing MDF in the wood market faces issues in regard to moisture, dampness, primary execution, and toughness [17].Producing quality MDF products that satisfy customer requirements will definitely result in a notable increase in profit for the industry as well as enhancement of the credibility from the perspective of customers [18].In the last few decades, many researchers have worked on MDF board properties, i.e., bending, thickness swelling, internal bonding, and occurring chemical changes, but only a few studies have examined the vertical density profile of MDF board mathematically.The aims of this study are to increase the bending and internal bonding strength of MDF board, to experimentally study its vertical density profile under varying hot press parameters, and to determine how its peak and core densities are affected by various hot press parameters.

Materials and Methods
The raw material used for the production of MDF is wood and glue.Generally, three types of wood are used with different percentages: popular (Populus caspica) with 60%, Ghaz (Tamarix aphylla) with 20%, and lachi (eucalyptus) with 20%.
For the required binding of the board, gluing is a crucial step in the manufacturing of MDF board.Other factors that depend on the glue content are board looseness, area roughness, bending, and internal bonding [19].The glue used is procured from various certified suppliers, and it is comprised of urea and a formaldehyde mixture.The glue is applied through nozzles with diameters from 2 mm to 3 mm with respect to the ratio of fiber/pulp; i.e., if the pulp is transferring in the blow line of 25 kg, then 15 kg glue is applied.
The process of hot pressing in MDF involves applying pressure in a specified pattern with respect to time, with the platens being heated up to the optimal temperature [20].A pressure versus time graph in hot pressing is shown in Figure 2.

Materials and Methods
The raw material used for the production of MDF is wood and glue.Generally, three types of wood are used with different percentages: popular (Populus caspica) with 60%, Ghaz (Tamarix aphylla) with 20%, and lachi (eucalyptus) with 20%.
For the required binding of the board, gluing is a crucial step in the manufacturing of MDF board.Other factors that depend on the glue content are board looseness, area roughness, bending, and internal bonding [19].The glue used is procured from various certified suppliers, and it is comprised of urea and a formaldehyde mixture.The glue is applied through nozzles with diameters from 2 mm to 3 mm with respect to the ratio of fiber/pulp; i.e., if the pulp is transferring in the blow line of 25 kg, then 15 kg glue is applied.
The process of hot pressing in MDF involves applying pressure in a specified pattern with respect to time, with the platens being heated up to the optimal temperature [20].A pressure versus time graph in hot pressing is shown in Figure 2. Experiments were carried out on 16 mm medium density fiberboard with variations in hot press parameters the ranges specified in Table 1.Experiments were carried out on 16 mm medium density fiberboard with variations in hot press parameters the ranges specified in Table 1.Afterwards, the density profile test, bending test, internal bonding test, and vertical density profile or X-ray graph test were performed on the sample of the 16 mm board.

Density Profile Test
The density profile test illustrates changes in thickness, reflecting the density of the panel.Equal distribution leads to a better quality of medium density fiberboard.The experiment was performed using twelve sample pieces, with six pieces with dimensions of 100 × 200 mm at the left side and six pieces with dimensions of 100 × 200 mm at the right side.The weight and thickness of each work piece were measured to obtain the density profile.

Bending and Internal Bonding Test
This test measures the bending of MDF boards.Five pieces with dimensions of 50 × 400 mm were used in the wood testing machine for the internal bonding and bending test.The maximum applied uniform load with a consistent speed toward the point of convergence of MDF prior to failure measured the bending and internal bonding [21].For internal bonding critical tests, five work pieces with dimensions of 50 × 50 mm were used.Figure 3 shows the wood testing machine used for the bending test, while Figure 4 depicts the internal bonding test.

Vertical Density Profile or X-ray Graph
The vertical density profile is generated as result of the input process parameters in hot pressing.Applying pressure on the fiber mat in the presence of a high temperature and the required moisture content creates a vertical density profile.This VDP across the board thickness relates to the bending, internal bonding, and other MDF mechanical properties [22].An electronic wood X-ray machine as shown in Figure 5 was used to generate x-ray profile of samples used in the experiment.Five work pieces' samples with dimensions of 50 × 50 mm was considered for the X-ray graph.The X-ray graph represents the point-to-point density as well as the density at the edges and center of the samples [23].

Vertical Density Profile or X-ray Graph
The vertical density profile is generated as result of the input process parameters in hot pressing.Applying pressure on the fiber mat in the presence of a high temperature and the required moisture content creates a vertical density profile.This VDP across the board thickness relates to the bending, internal bonding, and other MDF mechanical properties [22].An electronic wood X-ray machine as shown in Figure 5 was used to generate X-ray profile of samples used in the experiment.Five work pieces' samples with dimensions of 50 × 50 mm was considered for the X-ray graph.The X-ray graph represents the point-to-point density as well as the density at the edges and center of the samples [23].

Vertical Density Profile or X-ray Graph
The vertical density profile is generated as result of the input process parameters in hot pressing.Applying pressure on the fiber mat in the presence of a high temperature and the required moisture content creates a vertical density profile.This VDP across the board thickness relates to the bending, internal bonding, and other MDF mechanical properties [22].An electronic wood X-ray machine as shown in Figure 5 was used to generate x-ray profile of samples used in the experiment.Five work pieces' samples with dimensions of 50 × 50 mm was considered for the X-ray graph.The X-ray graph represents the point-to-point density as well as the density at the edges and center of the samples [23].

ISO and ASTM Standards for Statistical Testing Methods
ISO 16895:2016 (en) is international standards used for fiberboard, dry fiber, woodbased panels, medium density fiberboard, and dry-process fiberboard testing.According to ISO/IEC directives, this document is listed as an international standard of fiberboard testing (ISO 16895:2016 (en) ISO/TC 89 array).In addition, another standard for fiberboard testing marked as ASTM D-1037-12(2020) is listed as a standard for methods of evaluating properties of wood-based fiber and the material density of fiberboards.

Results and Discussions
Several tests were performed on MDF board by varying the hot press parameters, i.e., temperature, pressure, cycle time, and moisture content.Temperature was varied from 200 to 220 • C, pressure from 145 to 155 bar, cycle time from 260 to 275 s, and moisture content from 8 to 10%.The experimental results are shown in Table 2. From the experimental data presented in Table 2, the input values of pressure, temperature, cycle time, and moisture content are determined based on the output of bending and internal bonding.According to the experimental data, the maximum value of bending is 32 N/mm 2 , corresponding to the maximum internal bonding of 0.64 N/mm 2 .These are achieved by applying a pressure of 155 bar with a temperature of 220 • C, cycle time of 265 s, and moisture content of 8%.
Figure 6 demonstrates the effect of variations in process parameters on bending and internal bonding through a 3D graph of experimental data.Figure 7 shows the percentage representation of mediumdensity fiberboard parameters.Some of the data are selected from the experimental data in Table 3 for further analysis.To obtain the peak and center densities of the specimen, vertical density profiles are generated.Figure 7 shows the percentage representation of mediumdensity fiberboard parameters.Some of the data are selected from the experimental data in Table 3 for further analysis.To obtain the peak and center densities of the specimen, vertical density profiles are generated.Figure 7 shows the percentage representation of mediumdensity fiberboard parameters.Some of the data are selected from the experimental data in Table 3 for further analysis.To obtain the peak and center densities of the specimen, vertical density profiles are generated.

Experiment No. 2
Figure 9 shows the vertical density profile of x-ray graphs in 2nd specimen, whereas Tables 8-11 illustrate the detailed experimental data at temperature = 205 °C, cycle time = 270 s, pressure = 155 bar, moisture content = 10.00%.Five samples of medium density fiber board were produced at various temperatures, pressures, cycle times, and moisture contents.In the first sample, the temperature was 200 °C with a pressure of 145 bar, cycle time of 260 s, and moisture of 8.00%.In this experiment, all of the parameters were taken at the minimum conditions, but the result of bending and internal bonding were not good (compared to standards).
In the second sample, a minor increase in all process parameters was observed, with a temperature of 205 °C, a pressure of 155 bar, cycle time of 270 s, and moisture of 10.00%, and, as a result, the bending and internal bonding decreased.In third sample, we observed a further increase in temperature (210 °C) with pressure (150 bar) and cycle time(265 s) and a decrease in moisture (8.50%) with respect to the previous experiments' moisture content.The results of bending and internal bonding were notably better than those of the previous experiments.Five samples of medium density fiber board were produced at various temperatures, pressures, cycle times, and moisture contents.In the first sample, the temperature was 200 • C with a pressure of 145 bar, cycle time of 260 s, and moisture of 8.00%.In this experiment, all of the parameters were taken at the minimum conditions, but the result of bending and internal bonding were not good (compared to standards).
In the second sample, a minor increase in all process parameters was observed, with a temperature of 205 • C, a pressure of 155 bar, cycle time of 270 s, and moisture of 10.00%, and, as a result, the bending and internal bonding decreased.In third sample, we observed a further increase in temperature (210 • C) with pressure (150 bar) and cycle time(265 s) and a decrease in moisture (8.50%) with respect to the previous experiments' moisture content.The results of bending and internal bonding were notably better than those of the previous experiments.
In the fourth sample, we observed an increase in temperature (215 • C) with pressure (155 bar) and cycle time (275 s) as well as an increase in moisture (9.00%) with respect to the results of the previous experiments.The bending and internal bonding also increased.In the fifth sample, the highest value of temperature at 220 • C and a pressure of 155 bar were observed with normal cycle time of 265 s and low moisture content of 8.00%.The results were in accordance with our expectations, with bending and internal bonding values reaching the requirements of the standards.

Vertical Density Profile
A temperamental, one-dimensional PC model dependent on central examination, which can assist with understanding the connection between handling boundaries and density profile arrangements, was created in the current study].The vertical density profile, representing board density, has for quite some time been distinguished as one of the significant load-up qualities that relate well with internal bonding, bending, and other properties of MDF sheets [24].
Starting with the initial X-ray graphs, and after optimization, it can be observed that the graphs do not follow the standards.The board is hard at the center but loose at the sides.In the second and third graphs, the same situation can be observed in the sheets, as they contain loose sides and only their center is hard and has high density, while the density of their center is normal.After optimization, the graphs present the standards of X-ray graphs.Rendering the sheet results in the density being the same across the board, which means that by increasing the temperature to optimize the pressure, proper use of a wood recipe and maintenance of the dampness content will result in reaching the level of an MDF manufacturing plant.

Density Profile
The most significant test in the industry is the density profile test, which allows one to examine the combination of density and thickness.From this test, the density profile and weight distribution of a fiber mat can easily be determined.The test before optimization shows that the density in the right side of the board is low, resulting in it not meeting the requirements.In the same case, the second graph shows that the density of the left side is also low due to the low-density range, high weight, and high density of the sheet.In the third graph, both the sides are low, and this must be determined by examining the forming section [25].
After optimization, the density of the fiber mat arrangement in the board whose sides are equal in the sheet is in the desired range.If the density is in not in this range, then various issues can occur.Notably, with the help of the profile test, this problem can be resolved with mat arrangement in the mat or board.The use of a vacuum suction blower can help in suctioning the mat from the pen duster, thereby ensuring the correct fiber mat arrangement in the board.

Internal Bonding and Strength of the Sheet
The use of tests allows for the strength of sheets and the ways by which they resist external force to be determined.Multiple industries perform bonding tests to standardize final sheets.Prior to optimization, we used an iron pattern, but this consumed a lot of time in the heating process and in conducting the test.Now, however, we use a steel pattern, which speeds up the heating process and is less time consuming.
Several authors have worked on medium density fiberboard properties in many ways, but this study experimentally investigates the properties of MDF board by examining the effect that changing the various process parameters has on bending, internal bonding, and the vertical density profile.IN a previous study, Arun Gupta et al. worked on the modeling of the development of the vertical density profile of MDF during hot pressing [26].Paul M. Winistorfer et al. worked on modeling and provided a comparison of vertical density profiles.There is various MDF manufacturing plant types, such as single daylight opening plants, multi-opening plants, and continuous production plants [27].
The results show that the internal bonding and strength of the sample before optimization are lower than the standards, which means that the sheet can easily be rupture at the center and also from the side.In contrast, by setting the press parameters of pressure, temperature time, and dampness content, internal bonding reaches its highest level of quality.

Conclusions
This research illustrates the effect of hot press input parameters on the characteristics of manufactured medium density fiberboard.Optimum MDF with maximum bending and internal bonding as well as the desired vertical density profile is achieved through careful experimentation and its validation with standards.Five experiments are successfully conducted, where the last experiment provides the desired results.The hot-pressing process with a temperature of 220 • C, a bar pressure of 155, a cycle time of 265 s, and a moisture content of 8%produces the required MDF output.Increasing the hot-pressing temperature improves the bending stiffness and internal bonding and allows for the optimum values of 32 N/mm 2 and 0.64 N/mm 2 , respectively, to be achieved.
For future work, the quality of MDF board can be improved by considering different suitable combinations of raw wood.Similarly, the used resins can be changed, and the manufactured MDF can be analyzed in terms of its bending, internal bonding, and vertical density profile.

Figure 8
Figure8shows the vertical density profile of X-ray graphs in 1st specimen, whereas Tables 4-7 illustrate the detailed experimental data at temperature = 200 • C, cycle time = 260 s, pressure = 145 bar, moisture content = 8.00%.

Figure 9
Figure 9 shows the vertical density profile of X-ray graphs in 2nd specimen, whereas Tables 8-11 illustrate the detailed experimental data at temperature = 205 • C, cycle time = 270 s, pressure = 155 bar, moisture content = 10.00%.

Figure 11 shows
Figure 11 shows the vertical density profile of X-ray graphs in 4th specimen, whereas Tables 16-19 illustrate the detailed experimental data at temperature = 215 • C, cycle time = 275 s, pressure = 155 bar, moisture content = 9.00%.

Table 1 .
Process parameters for hot pressing.

Table 2 .
Experimental data results.

Table 4 .
Density profile across thickness.

Table 7 .
Experimental data of 1st sample.

Table 8 .
Density profile across thickness.

Table 6 .
Internal bonding test results.

Table 7 .
Experimental data of 1st sample.

Table 10 .
Internal bonding test results.

Table 8 .
Density profile across thickness.

Table 10 .
Internal bonding test results.

Table 14 .
Internal bonding test results.

Table 16 .
Density profile across thickness.

Table 18 .
Internal bonding test results.

Table 22 .
Internal bonding test results.