Vacuum Forming in Pulp Molded Products

Vacuum forming is a method commonly used in the production of pulp molded products. During the production process, attention should be paid to the following issues.

The influence of the main process parameters of the suction filtration process on product performance indicators

The main process parameters of the suction filtration process are suction filtration time, suction filtration vacuum and backwash stirring water time, which affect the main performance indicators (water removal rate) of paper mold products to varying degrees.

(1) The influence of suction filtration time on water removal rate

Before the suction filtration process, grouting is injected into the grouting tank and stirring water is injected at the same time. When the grouting and stirring water reach a certain level, vacuum suction filtration is started. During the entire process of suction filtration, because the amount of grouting will directly affect the thickness of the formed wet paper mold, it is necessary to ensure that the two parameters of suction filtration vacuum and backwash stirring water time remain unchanged. When the grouting amount is constant, the influence of suction filtration time on water removal rate is considered separately. When the grouting amount is constant and the backwashing water time is constant, if the suction filtration time is too long, the production efficiency will be reduced, resulting in a decrease in the overall vacuum degree. In addition, if the suction filtration time is too long, a lot of energy will be wasted. After a certain degree of suction filtration, the moisture content of the wet paper mold will no longer decrease, and the moisture content must be further reduced in the subsequent production process. If the suction filtration time is too short, the moisture content of the wet paper mold will be too high, demoulding will be difficult, and insufficient vacuum will cause the wet paper mold to fail to demould, or even cause the wet paper mold to break. To ensure that the quality and appearance of the paper mold product meet the requirements, it is necessary to extend the hot pressing time and increase the hot pressing temperature, which will increase energy consumption.

(2) The effect of vacuum forming on water removal rate

At the end of grouting, the inner cavity of the molding mold is vacuumed to create a pressure difference between the inner cavity of the molding mold and the outside of the mold, so that the fibers in the pulp are evenly deposited and attached to the surface of the molding mesh to form a wet paper mold. A large amount of water (filtrate) is removed through the suction filtration pipe at the bottom of the mold during the vacuum suction process. The effect of vacuum on the final water removal rate is considered by keeping the suction filtration time and backwashing time unchanged. If the vacuum is too low, the internal and external pressure difference decreases, and there is not enough pressure difference to suck the forming mold, resulting in a large amount of water remaining in the formed wet paper mold at the end of the suction filtration. In this way, corresponding measures must be taken during the hot pressing process to ensure product quality, otherwise the breakage rate will increase, but energy consumption will increase, making the outer surface rough, with indentations, poor pressure resistance, poor surface strength, and increased scrap rate. If the vacuum is too high, the internal and external pressure difference becomes larger. At this time, although there is enough pressure to suck the water in the formed wet paper mold, the water content in the formed wet paper mold is too low, which affects the hot pressing process, making the product light and weak in strength after hot pressing. If the temperature is high, the surface of the paper mold product will be charred, making the product color irregular and becoming scrap, reducing production efficiency.

(3) Effect of backwashing time on water removal rate

The main role of backwashing water in the suction filtration process is to stir the slurry, and it also plays a role in diluting the slurry. The purpose of stirring is to prevent uneven precipitation of the slurry during the suction filtration process after the grouting is completed, which will lead to uneven thickness of the adsorbed fiber layer on the surface of the molding die, and finally make the product uneven in thickness. The purpose of dilution is to make the slurry reach the required concentration, to prevent the slurry concentration from being too high and making the adsorbed fiber layer on the surface of the suction filtration molding die too thick, so that the product does not meet the requirements. It should be said that the longer the backwashing time, the more uniform the stirring of the slurry. However, if the backwashing time is too long, on the one hand, the slurry concentration will become too low, resulting in too thin thickness of the suction filtration molding die surface, and on the other hand, it will affect the suction filtration effect, so that the water content in the molding wet paper mold will increase relatively, and even the demolding of the subsequent process cannot be carried out normally, so that the wet paper mold will be damaged from semi-finished products to waste products during the demolding process. If the backwashing time is too short, the slurry in the grouting tank cannot be stirred well, and the uneven slurry leads to uneven surface of the molding die. After suction at a fixed time and pressure difference, the product is dry, which reduces the qualified rate of finished products.

(4) Effect of beating degree and compression force on water removal rate

The degree to which paper fibers with different beating degrees are affected by compression force is different. The higher the beating degree, the finer the paper fibers, the smaller the pores between the fibers, and the lower the permeability of the wet paper mold. Due to the influence of fiber size and compression force, the pores in the wet paper mold are not all through pores for fluid flow. A considerable number of them are blind pores or pores blocked by bubbles. The finer the paper fibers (higher beating degree), the greater the probability of pore blockage, and the lower the suction and filtration water removal efficiency. At the same time, the finer the fiber, the greater the relative thickness of the surface hydration film, and the stronger the bond with the fiber. Since the fluidity of the hydration film is very poor, it occupies part of the pore space, which also reduces the porosity of the wet paper mold, thereby reducing the water removal rate. However, in order to ensure the uniform wall thickness and density of the paper mold product, the paper fiber used cannot be too coarse, and a certain beating degree must be guaranteed. Generally speaking, the beating degree of pulp in production should be 28~30°SR.

(5) Effect of pulp viscosity on water removal rate

The higher the slurry viscosity, the worse its fluidity, and the water removal rate is inversely proportional to the liquid viscosity. In addition, the greater the slurry viscosity, the finer the pulp fibers, the thicker the hydration film on the fiber surface, and the stronger the bond. Due to the application characteristics of paper mold catering utensils, waterproof and oil-proof agents must be added to the slurry used, which increases the viscosity of the slurry. Moreover, the finer the fibers (the higher the beating degree), the greater the added dosage, the greater the impact on the viscosity, affecting the water removal rate of the wet paper mold.

(6) Effect of pulp concentration on water removal rate

Slurry concentration has an impact on filtration resistance. The lower the slurry concentration, the more fully the fine fibers fill the gaps in the wet paper mold, and the higher the dehydration resistance of the wet paper mold. Considering that pulp molding is different from a simple filtration process, in addition to considering filtration efficiency, the uniformity of product wall thickness must also be considered. Therefore, the pulp slurry concentration used in paper mold molding is relatively low, generally around 1%.

Factors affecting the quality of pulp molded products during vacuum filtration

(1) Holes

Sometimes, some holes are generated during the production process of pulp molded products, making them waste products. The forms and causes of these holes are mainly as follows:

① A sheet-like honeycomb-shaped hole

A sheet-like honeycomb-shaped hole is generated in a certain part of the paper mold product, accompanied by uneven slurry distribution.

(2) Uniformity

The uniformity of pulp molded products, that is, the uniform distribution of pulp in each part of the paper mold product and the consistency of thickness. The main reasons for the unevenness of paper mold products are as follows:

① The influence of pulp concentration on uniformity

During the production process of paper mold products, the pulp concentration entering the molding slurry tank has a great influence on the uniformity of paper mold products. During the production process, the concentration is generally kept constant and cannot fluctuate. Generally, the pulp concentration entering the molding slurry tank is about 1%. If the pulp concentration is too high, the thickness of the product will be uneven, that is, the phenomenon of thin top and thick bottom will occur.

② The depth of the mold sinking into the slurry

The slurry concentration in the molding slurry tank during the production of pulp molded products is related to the shape and height of the product and the depth of the mold sinking into the slurry. As shown in Figure 4-6, when the depth H of the mold sinking into the slurry is equal to or greater than the height h of the paper mold product, the slurry distribution is uniform and the thickness is consistent when the paper mold product is molded.

③ The opening rate of the molding mold

The molding process of pulp molded products is a three-dimensional papermaking process, which makes the top angles, edges and surfaces of the molding mold have different heights and inclinations. In order to make the slurry deposition and distribution uniform, the opening rates of the dehydration holes in each part of the mold will be different. Generally, the opening rate of the top angle is about 70%, the opening rate of the edge is about 60%, and the opening rate of the upper top surface is about 40%. The opening rate on the inclined surface is generally variable, that is, the upper opening rate is high, the lower opening rate is low, generally 50%~35%, and the opening rate at the bottom is the lowest, about 20%, so that the slurry deposition and distribution can be more uniform. There are also some other reasons that affect the uniformity of pulp molded products, such as filter mesh clogging, failure to clean the filter mesh in time, low vacuum degree, etc., mold filter mesh.

(3) Cracks

① Errors in the production and installation of the filter mesh. As shown in Figure 4-7, the production and installation errors of the filter mesh refer to the fact that during the production and installation of the filter mesh, the filter mesh is not tightly adhered to the surface of the molding mold at all places, making the expanded length of the filter mesh less than the expanded length of the mold. During pulp molding, the filter mesh is stretched at the grooves and turning points of the mold to produce a tight gap, so that the wet paper mold formed during the molding of the pulp molded product is lifted up, so that the expanded length of the wet paper mold is reduced, causing the wet paper mold to be stretched and broken in the molding and heating and drying molds, forming cracks on the paper molded product, and producing waste. At the same time, the filter mesh of the molding mold is installed too tightly, and the filter mesh is also easily cracked during cold pressing, causing pulp loss during molding and large holes on the paper molded product to form waste. The solution is to fully mold the filter screen on the molding mold at the groove and turning point during production and installation, and to make it slightly loose during installation.

② Manufacturing error of mold. The manufacturing error of mold refers to the geometric shape and size errors of the molding mold, heating and drying mold, and the punch and die during the manufacturing process. This error has a certain impact on the quality of pulp molded products. If there is an error between the punch and die, the gaps at various locations of the mold will be different after the mold is closed. The turning point with a small gap may squeeze or shear the paper mold product, resulting in cracks, thereby producing waste products.

(4) Irregular periphery

During the production process of pulp molded products, peripheral asymmetry, uneven periphery or burrs may occur, which reduces the product quality of the paper mold products. The main reasons for these problems are as follows:

① Peripheral asymmetry

The reason for the peripheral asymmetry of pulp molded products is that the gaps at various locations of the heating mold are inconsistent, which makes the thermal shrinkage of the paper mold product inconsistent during the heating and drying process, thus making the periphery of the paper mold product asymmetric. The solution is to adjust the gap between the upper and lower molds when the mold is closed.

② Uneven periphery

The reasons for the uneven periphery of pulp molded products are: first, the moisture content of the wet paper mold is high, and part of the blank is squeezed out during the squeeze drying, causing uneven periphery of the product. The solution is to increase the drainage time of the molding process and reduce the moisture content of the wet paper mold; second, the mold closing speed is too fast during the squeeze drying, splashing part of the blank. The solution is to reduce the mold closing speed at the moment of mold closing.

③ Burrs

When the automatic production equipment of pulp molded products produces products without trimming, the wet paper mold is in the process of switching between the upper and lower molds in the mold. Due to the change of blowing pressure and the length of blowing time, the product will have burrs. The solution is to control the pressure of the compressed air in the blowing circuit to about 0.4MPa, the blowing time to within 2s, and the blowing generally starts 1s before the mold is separated.