A rapid cure paper tray wet press forming process

Abstract

The application discloses a kind of quick solidification paper tray wet compression molding process, belong to paper pulp molding technical field.It uses 55~65% plant fiber matrix and 35~45% core-sheath structure bicomponent hot melt fiber as core raw material, and adds dispersing aid and waterproofing additive, and is formed by multi-station continuous vacuum adsorption, and wet embryo is fixed in the whole process without transfer in forming lower die;Through three-stage gradient microwave preheating tunnel, the wet embryo is uniformly pre-stored energy, and then through high-frequency induction and heat conduction double-frequency cooperative hot pressing, the wet embryo is super-fast solidified in 10~20s, and finally, the wet embryo is demolded without trace through online cold pressing setting.The application completely solves the problems of long curing time in traditional process, dependence on external spraying to solve the problem of hair and dust, and deformation of wet embryo transfer, realizes the continuous and automatic production of process, and the paper tray has excellent mechanical properties, wear resistance and dimensional stability.

Description

Technical Field

[0001] This invention relates to the field of pulp molding technology, specifically a rapid curing paper tray wet pressing process. Background Technology

[0002] Paper trays are environmentally friendly packaging materials made from plant fibers. With their advantages of complete biodegradability, excellent cushioning performance, and renewable raw materials, they have gradually replaced plastic packaging and are widely used in electronics, cosmetics, food, and other fields. Among them, wet-pressed paper trays, with their smooth surface and high dimensional accuracy, have become the mainstream solution for inner packaging of high-end products.

[0003] Existing wet-pressed paper tray forming processes still have the following problems: First, the curing efficiency is extremely low. Traditional processes rely on heat conduction through the mold to dry and cure the wet preform. Heat is transferred from the surface to the interior, which easily leads to surface crusting and difficulty in removing internal moisture. The curing time for a single station is generally 50-120 seconds, resulting in extremely low production line efficiency and an inability to meet the needs of large-scale mass production. Second, the problem of lint and dust shedding is difficult to eradicate. Free plant fibers on the surface of wet-pressed paper trays are easily detached during friction, contaminating the packaging contents and even causing electronic product malfunctions. Existing solutions, such as the technology disclosed in CN106555365A, adopt... The problem can be solved by spraying a protective coating onto the wet blank, but this adds an extra spraying step, resulting in high production costs and defects such as VOC emissions and easy coating peeling, which are not environmentally friendly and stable enough. Finally, the molding accuracy and yield are low. Existing processes, such as the technology disclosed in CN119711257A, require the transfer of the wet blank from the molding mold to the hot pressing mold. The wet blank has a high moisture content and low strength, and it is very easy to deform and break during the transfer process, resulting in low product yield. In addition, this process limits the fixed pulp ratio of 35% sugarcane pulp and 65% bamboo pulp, resulting in poor raw material compatibility. The hot pressing time still needs to be 60-70 seconds, and the efficiency improvement is limited. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a rapid curing paper tray wet pressing process, which solves the problems of long curing time, reliance on external spraying to solve lint and dust, and easy deformation during wet transfer in traditional processes. This process enables continuous and automated mass production, and the resulting paper trays have excellent mechanical properties, wear resistance, and dimensional stability.

[0005] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: a rapid curing paper tray wet pressing process, comprising slurry preparation, vacuum adsorption molding, hot pressing curing, and demolding steps, specifically including the following steps: Step 1: Preparation of hot-melt fiber composite functional slurry: Based on oven-dry weight, use 55-65% plant fiber matrix and 35-45% core-sheath structure bicomponent hot-melt fiber as the main body, add dispersing agent and waterproofing agent, and adjust the slurry concentration to 0.8-1.2% after dispersion and control the pH value at 7.0-7.5; the sheath melting point of the core-sheath structure bicomponent hot-melt fiber is 110℃, and the core melting point is 250℃; Step 2, Multi-station continuous vacuum adsorption molding: A rotary multi-station continuous molding machine is used. The wet preform is produced by adsorbing the slurry in the slurry tank at a vacuum degree of -0.06 to -0.08 MPa through the lower molding die. The wet preform is fixed in the lower molding die throughout the process without any transfer operation. The moisture content of the wet preform is controlled at 70% to 75%. Step 3, Gradient Microwave Preheating: The molding die with the wet preform continuously passes through a three-section microwave preheating tunnel, maintaining a micro-vacuum state of -0.02MPa throughout. The wet preform is uniformly heated to 80-85℃ through gradient heating, and the heating temperature is lower than the melting point of the skin layer of the bicomponent hot melt fiber. The total preheating time is 10-14s. After preheating, the moisture content of the wet preform drops to 60-65%. Step 4, Dual-frequency hot pressing and co-curing: The preheated wet blank mold directly enters the hot pressing station. The mold is closed using a dual-system hot pressing mold that combines high-frequency induction heating and oil circulation temperature control. The upper and lower molds are uniformly controlled at 130-140℃, the mold closing pressure is 3-6MPa, and the curing time is 10-20s. During the hot pressing process, the lower mold maintains a vacuum of -0.05MPa to remove vaporized water vapor, resulting in a paper tray semi-finished product with a moisture content of 4-6%. Step 5, Online cold pressing and demolding: The paper tray semi-finished product is directly transferred to the cold pressing station along with the forming lower mold. The cold pressing mold with circulating cooling water at 10-15℃ is used to close the mold and hold the pressure for 5-8 seconds to quickly cool the paper tray to below 60℃. Then, the low-pressure back-blowing of the forming lower mold is used to achieve traceless demolding.

[0006] Further, the plant fiber matrix in step 1 is composed of 40 wt% recycled hardwood pulp, 35 wt% wheat straw pulp and 25 wt% flax fiber, and the beating degree of the plant fiber matrix is ​​35-40°SR.

[0007] Furthermore, the length of the bicomponent thermoplastic fiber in the core-sheath structure described in step 1 is 3-6 mm, and the denier is 2.0D.

[0008] Further, in step 1, the amount of the dispersing agent added is 0.3-0.5% based on the total mass of the oven-dry materials, and the amount of the waterproofing agent added is 0.8-1.2%; the dispersing agent is nonionic polyacrylamide, and the waterproofing agent is alkyl ketene dimer.

[0009] Furthermore, in step 2, the lower mold is made of porous sintered titanium plate with a pore size of 15-25 μm and an opening rate of 12-18%; the upper mold of the molding machine is a non-porous silicone press plate.

[0010] Furthermore, after the slurry is sucked in step 2, the upper mold is gently pressed and scraped off the excess slurry with a pressure of 0.2 MPa, and the uniformity deviation of the moisture content of the wet mold is ≤ ±2%.

[0011] Furthermore, the process parameters for the three-section microwave preheating tunnel described in step 3 are as follows: microwave power density of the first preheating cavity is 0.3–0.4 W / cm². 3 The wet embryo is heated to 40–50°C and held for 3–5 seconds; the microwave power density of the second-stage preheating cavity is 0.5–0.6 W / cm³. 3 The wet embryo is heated to 70–85℃ and held for 4–6 seconds; the microwave power density of the three-section insulation cavity is 0.2–0.3 W / cm³. 3 The wet embryo is kept at a constant temperature of 80-85℃ for 2-3 seconds.

[0012] Furthermore, the high-frequency induction heating in step 4 has a frequency of 40 kHz and a power density of 0.8 W / cm². 3 The surface roughness Ra of the working surface of the hot pressing mold is ≤0.2μm.

[0013] Furthermore, the closing pressure of the cold pressing mold in step 5 is 2 to 4 MPa.

[0014] Furthermore, the static bending strength of the paper tray is ≥18MPa, the dimensional shrinkage rate after 72 hours at room temperature is ≤0.2%, and there is no powdering, lint shedding or chipping after 1500 cycles of rubbing against white paper under a 4-pound load.

[0015] (III) Beneficial Effects Compared with the prior art, the present invention has the following significant advantages: 1. By using a three-stage gradient heating method, the wet preform is uniformly preheated to the critical value of the melting point of the hot melt fiber to complete the energy storage. Then, combined with high-frequency induction and heat conduction dual-frequency synergistic hot pressing, the wet preform is simultaneously heated and cured inside and out, which shortens the hot pressing curing time of a single station and improves production efficiency.

[0016] 2. By adding a core-sheath structure of bicomponent hot-melt fibers, the sheath melts and forms a physical weld with the plant fibers during the hot pressing process, constructing a three-dimensional network structure. This locks in the free fibers from the root of fiber bonding, resulting in a product that does not shed powder, lint, or flakes.

[0017] 3. The multi-station continuous molding process is adopted, and the wet blank is fixed in the lower mold throughout the process. There is no transfer operation from molding, preheating, hot pressing to demolding, which completely solves the industry pain point of wet blank transfer deformation and damage. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the following examples and comparative examples, all raw materials used are commercially available industrial-grade products. The core-sheath bicomponent hot-melt fiber is a commercially available PET / PE core-sheath composite fiber with a sheath melting point of 110°C and a core melting point of 250°C. The recycled hardwood pulp, wheat straw pulp, and flax fiber are all conventional commercial pulp boards used in the papermaking industry. The nonionic polyacrylamide and alkyl ketene dimer (AKD) are both common additives used in the papermaking industry. All equipment used is mature industrial equipment, and those skilled in the art can repeat the implementation based on the description.

[0021] Example 1 This embodiment is the optimal implementation of the present invention, and the specific process steps are as follows: 1. Preparation of hot-melt fiber composite functional pulp: Based on 100 parts of oven-dry total mass, weigh 60 parts of plant fiber matrix, including 40 wt% recycled hardwood pulp, 35 wt% wheat straw pulp, and 25 wt% flax fiber, and mix and beat to a freeness of 38°SR. Add 40 parts of core-sheath structure bicomponent hot-melt fiber with a length of 4 mm and a denier of 2.0D. Add 0.4 parts of nonionic polyacrylamide dispersant and 1.0 part of AKD waterproofing agent. When preparing the pulp, first mix the plant fiber with water to adjust the concentration to 2.0%, add the dispersant and stir for 5 minutes, then add the hot-melt fiber and stir at a low speed of 300 rpm for 8 minutes to avoid fiber breakage and agglomeration. Add the AKD waterproofing agent and stir for 5 minutes. Finally, add water to adjust the pulp concentration to 1.0% and adjust the pH value to 7.2 to complete the pulp preparation.

[0022] 2. Multi-station continuous vacuum adsorption molding: An 8-station rotary continuous molding machine is used. The lower mold is made of porous sintered titanium plate with a pore diameter of 20μm and an open area ratio of 15%. The upper mold is a silicone press plate without open areas. The lower mold is immersed in the slurry pool, the vacuum degree is turned on to -0.07MPa, the slurry adsorption time is 5s, and the moisture content of the wet blank is 72%. After the lower mold is rotated out of the slurry pool, the upper mold is lightly pressed with a pressure of 0.2MPa, and the excess slurry at the edge is scraped off. The uniformity deviation of the moisture content of the wet blank is ≤±1.5%. The wet blank is fixed in the lower mold throughout the process without any transfer operation.

[0023] 3. Gradient microwave preheating: The forming mold with wet blank continuously passes through a three-section microwave preheating tunnel along the production line, maintaining a micro-vacuum state of -0.02MPa throughout; one section of the preheating cavity has a microwave power density of 0.35W / cm³. 3 The wet embryo is heated to 45℃ and held for 4 seconds; the microwave power density of the two-stage preheating cavity is 0.55W / cm³. 3 The wet embryo is heated to 82℃ and held for 5 seconds; the microwave power density of the three-section insulation cavity is 0.25W / cm³. 3 The wet embryo was kept at a constant temperature of 82℃ for 2 seconds; the total preheating time was 11 seconds. After preheating, the moisture content of the wet embryo dropped to 62%, the temperature difference between the inside and outside of the wet embryo was ≤±2℃, and there was no local overheating.

[0024] 4. Dual-frequency hot pressing and co-curing: The preheated wet blank mold directly enters the hot pressing station. A dual-system hot pressing mold combining high-frequency induction heating and oil circulation temperature control is used for mold closing. The upper and lower molds are uniformly temperature-controlled at 135℃. The high-frequency induction heating frequency is 40kHz, and the power density is 0.8W / cm³. 3 The mold closing pressure is 4MPa and the curing time is 15s. During the hot pressing process, the lower mold maintains a vacuum of -0.05MPa throughout the process to continuously pump out the vaporized water vapor, resulting in a paper tray semi-finished product with a moisture content of 4.8%.

[0025] 5. Online cold pressing and demolding: The paper tray semi-finished product is directly transferred to the cold pressing station along with the forming lower mold. The cold pressing mold is circulated with 12℃ cooling water, the mold closing pressure is 3MPa, and the pressure is held for 6s to quickly cool the paper tray to 55℃. Then the vacuum of the forming lower mold is switched to low-pressure back-blowing to achieve traceless demolding and complete the paper tray preparation.

[0026] Example 2 In this embodiment, the ratio of hot-melt fiber to plant fiber is adjusted, while the remaining process parameters are completely consistent with those in Example 1, as detailed below: In step 1, based on a total dry mass of 100 parts, there are 65 parts of plant fiber matrix and 35 parts of core-sheath bicomponent hot melt fiber. The proportions of the remaining raw materials and pulping parameters are the same as in Example 1. All process parameters from step 1 to step 5 are completely consistent with those in Example 1.

[0027] Example 3 In this embodiment, the hot-pressing curing time is adjusted, while the remaining process parameters are completely consistent with those in Example 1, as detailed below: In step 4, the curing time of dual-frequency hot pressing synergistic curing is adjusted to 10s, and the remaining hot pressing parameters and all process parameters of the remaining steps are completely consistent with those in Example 1.

[0028] Example 4 In this embodiment, the gradient microwave preheating parameters are adjusted, while the remaining process parameters are completely consistent with those in Embodiment 1, as detailed below: In step 3, the process parameters for the three-section microwave preheating tunnel are adjusted as follows: microwave power density of the first preheating cavity is 0.4 W / cm². 3 The wet embryo is heated to 50℃ and held for 5 seconds; the microwave power density of the two-stage preheating cavity is 0.6W / cm³. 3 The wet embryo is heated to 85℃ and held for 6 seconds; the microwave power density of the three-section insulation cavity is 0.3W / cm³. 3 The wet embryo was kept at a constant temperature of 85°C for 3 seconds; the total preheating time was 14 seconds. All other parameters and process parameters for the remaining steps were completely consistent with those in Example 1.

[0029] Comparative Example 1 This comparative example uses a traditional, conventional wet pressing molding process, as detailed below: Pulp preparation: 100% recycled hardwood pulp is used, with a beating degree of 38°SR. 1.0 part of AKD waterproofing agent is added to adjust the pulp concentration to 1.0% and the pH value to 7.2. Vacuum adsorption molding: Using the same molding mold as in Example 1, with a vacuum degree of -0.07MPa and a slurry adsorption time of 5s, a wet preform with a moisture content of 72% was obtained; Hot pressing curing: The wet blank is manually transferred to the hot pressing mold, the upper and lower mold temperatures are 150℃, the mold closing pressure is 4MPa, and the curing time is 90s, to obtain a paper tray semi-finished product with a moisture content of 4.8%. Demolding: Demold after natural cooling to below 60℃.

[0030] Comparative Example 2 This comparative example omits the gradient microwave pre-energy storage and preheating step, while the remaining parameters are completely identical to those of Example 1, as detailed below: After obtaining the wet preform in step 2, it directly proceeds to step 4, dual-frequency hot pressing and curing, without the gradient microwave preheating process in step 3. The hot pressing and curing time is 15 seconds, and all other process parameters are completely consistent with those in Example 1.

[0031] Comparative Example 3 This comparative example omits the bicomponent hot melt fiber, but all other parameters are completely identical to those of Example 1, as detailed below: In step 1, 100% plant fiber matrix is ​​used, without hot melt fiber, and the proportions of other raw materials and pulping parameters are the same as in Example 1; all process parameters in steps 2 to 5 are completely the same as in Example 1.

[0032] Performance Testing and Comparative Analysis The paper trays prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to performance tests according to a unified standard. The test standards and methods are as follows: Hot pressing curing time: Record the total duration of the hot pressing process at a single workstation; Abrasion resistance and lint prevention performance: Using a 4-pound load and a friction frequency of 43 times / min, the test sample was rubbed against white paper, observed at a distance of 250mm and a 45° angle, and the number of frictions at which lint and lint first appeared was recorded. At the same time, the state after 1500 frictions was also recorded. Dimensional shrinkage rate: The rate of change of the product's key dimensions was tested after placing the product in an environment of 25℃ and 50%RH for 72 hours. Static bending strength: tested according to GB / T28210-2011 "Molded Pulp Tableware" standard; 24-hour water absorption rate: tested according to GB / T28210-2011 standard; Production yield: The percentage of qualified products out of 10,000 units produced continuously.

[0033] Test Result Analysis Examples 1-4 all achieved an ultra-short hot-press curing time of 10-15 seconds, which is more than 70% higher than the traditional process, i.e., Comparative Example 1. At the same time, the wear resistance, dimensional stability and mechanical properties all achieved a qualitative leap, verifying the core technical effect of the process of the present invention.

[0034] In Comparative Example 2, after removing gradient microwave preheating, although the hot pressing time remained at 15s, the product size shrinkage rate increased significantly, and uneven curing led to excessive internal stress and a decrease in yield. This verified the key role of gradient microwave preheating in the product size stability and curing uniformity.

[0035] In Comparative Example 3, even after removing the hot-melt fiber, the problem of lint and dust could not be solved even with microwave preheating and rapid hot pressing. The wear resistance was not significantly different from that of the traditional process, which verified the core role of in-situ welding of hot-melt fiber in preventing lint.

[0036] In summary, the process of this invention, through the synergistic effect of its core innovations, simultaneously achieves ultra-fast curing, spray-free anti-shedding, high-precision and high-yield production, and can be implemented industrially.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A rapid-curing paper tray wet pressing process, comprising slurry preparation, vacuum adsorption molding, hot pressing curing, and demolding steps, characterized in that, Specifically, the following steps are included: Step 1: Preparation of hot-melt fiber composite functional slurry: Based on oven-dry weight, use 55-65% plant fiber matrix and 35-45% core-sheath structure bicomponent hot-melt fiber as the main body, add dispersing agent and waterproofing agent, and adjust the slurry concentration to 0.8-1.2% after dispersion and control the pH value at 7.0-7.5; the sheath melting point of the core-sheath structure bicomponent hot-melt fiber is 110℃, and the core melting point is 250℃; Step 2, Multi-station continuous vacuum adsorption molding: A rotary multi-station continuous molding machine is used. The wet preform is produced by adsorbing the slurry in the slurry tank at a vacuum degree of -0.06 to -0.08 MPa through the lower molding die. The wet preform is fixed in the lower molding die throughout the process without any transfer operation. The moisture content of the wet preform is controlled at 70% to 75%. Step 3, Gradient Microwave Preheating: The molding die with the wet preform continuously passes through a three-section microwave preheating tunnel, maintaining a micro-vacuum state of -0.02MPa throughout. The wet preform is uniformly heated to 80-85℃ through gradient heating, and the heating temperature is lower than the melting point of the skin layer of the bicomponent hot melt fiber. The total preheating time is 10-14s. After preheating, the moisture content of the wet preform drops to 60-65%. Step 4, Dual-frequency hot pressing and co-curing: The preheated wet blank mold directly enters the hot pressing station. The mold is closed using a dual-system hot pressing mold that combines high-frequency induction heating and oil circulation temperature control. The upper and lower molds are uniformly controlled at 130-140℃, the mold closing pressure is 3-6MPa, and the curing time is 10-20s. During the hot pressing process, the lower mold maintains a vacuum of -0.05MPa to remove vaporized water vapor, resulting in a paper tray semi-finished product with a moisture content of 4-6%. Step 5, Online cold pressing and demolding: The paper tray semi-finished product is directly transferred to the cold pressing station along with the forming lower mold. The cold pressing mold with circulating cooling water at 10-15℃ is used to close the mold and hold the pressure for 5-8 seconds to quickly cool the paper tray to below 60℃. Then, the low-pressure back-blowing of the forming lower mold is used to achieve traceless demolding.

2. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: The plant fiber matrix in step 1 is composed of 40 wt% recycled hardwood pulp, 35 wt% wheat straw pulp and 25 wt% flax fiber, and the beating degree of the plant cellulose matrix is ​​35-40°SR.

3. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: The length of the core-sheath structure bicomponent hot-melt fiber described in step 1 is 3-6 mm, and the denier is 2.0D.

4. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: In step 1, based on the total mass of the oven-dry material, the amount of the dispersing agent added is 0.3-0.5%, and the amount of the waterproofing agent added is 0.8-1.2%; the dispersing agent is nonionic polyacrylamide, and the waterproofing agent is alkyl ketene dimer.

5. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: In step 2, the lower mold is made of porous sintered titanium plate with a pore size of 15-25 μm and an opening rate of 12-18%; the upper mold of the molding machine is a silicone press plate without openings.

6. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: After the slurry is sucked in step 2, the upper mold is gently pressed and scraped off the excess slurry with a pressure of 0.2 MPa. The uniformity deviation of the moisture content of the wet mold is ≤ ±2%.

7. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: The process parameters for the staged microwave preheating tunnel described in step 3 are as follows: microwave power density of one preheating cavity is 0.3–0.4 W / cm². 3 The wet embryo is heated to 40–50°C and held for 3–5 seconds; the microwave power density of the second-stage preheating cavity is 0.5–0.6 W / cm³. 3 The wet embryo is heated to 70–85℃ and held for 4–6 seconds; the microwave power density of the three-section insulation cavity is 0.2–0.3 W / cm³. 3 The wet embryo is kept at a constant temperature of 80-85℃ for 2-3 seconds.

8. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: The high-frequency induction heating in step 4 has a frequency of 40 kHz and a power density of 0.8 W / cm². 3 The surface roughness Ra of the working surface of the hot pressing mold is ≤0.2μm.

9. The rapid curing paper tray wet pressing process according to claim 1, characterized in that: The closing pressure of the cold pressing mold in step 5 is 2-4 MPa.

10. The paper tray produced by the rapid curing wet pressing process according to any one of claims 1-9, characterized in that, The paper tray has a static bending strength ≥18MPa, a dimensional shrinkage rate ≤0.2% after being placed at room temperature for 72 hours, and exhibits no powdering, lint shedding, or chipping after 1500 cycles of rubbing against white paper under a 4-pound load.

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