Advantages and weaknesses of pulp molding products

Modern packaging generally uses paper, plastic, glass, ceramics, metal as packaging materials, because paper and plastic are cheap and easy to process, and the source of raw materials is wide, so paper and plastic packaging materials are widely used in daily life.

However, because plastic packaging materials can not degrade themselves, it will cause environmental pollution and other problems, and the white pollution problem in China is the most typical example.

With the development of society and the requirements of global economic integration, green packaging has gradually become the consensus of society, and has become another bright spot in the development of China's packaging industry. According to the quantitative evaluation of international authoritative experts, only paper is the most promising green packaging material. In recent years, China's paper molding products equipment has developed very fast, and gradually mature. With the continuous promotion of the application range of pulp molding products, the requirements of paper molding products continue to improve, and the design of pulp molding production process and mold need to be constantly improved and improved.

In this paper, according to the process characteristics of pulp molding products, the present situation of pulp molding products is analyzed, and some elements of mold design of pulp molding products are studied.

First, the process analysis of pulp molding products

1. Process process Under normal circumstances, the production process can be changed according to the different requirements of customers for products, mainly including slurry preparation, molding, hot press setting, setting, molding and packaging and other auxiliary processes.

2. Raw material selection of pulp molded products The sources of pulp molded products are mainly divided into: Pulp, straw pulp and reed pulp, generally bleached commercial pulp (for fast-food boxes) or waste paper (for industrial packaging), will not produce secondary pollution in the production and use process, the product has the advantages of light weight, pressure resistance, earthquake resistance, good buffer performance, anti-static, strong air permeability and good recyclability. As a packaging product, it can replace polystyrene foam (EPS) for disposable fast food boxes, various electronic products, handicrafts, medical devices, mechanical equipment parts, poultry eggs, fruits, flowers, glass products, ceramic products and other fragile products.

Pulp molding products are generally divided into fast food boxes and industrial inner packaging, the production method is basically the same, but the raw materials used for the production of fast food boxes are bleached commercial pulp, quantitative and specification indicators are more strict, and sterilization, waterproof and oil-proof treatment, equipment and pipelines are mostly stainless steel and plastic materials, storage pool wall lined with tiles, The plant and operator health requirements are also higher.

3. Pulping Pulping is the first process in the production of pulp molded products. The pulping process includes: the pulp board is broken into fiber pulp with a hydraulic pulper, the broken fiber pulp is screened, sand removal and purification treatment, and then the fiber is properly spun by the pulper to improve the binding force between the fibers.

In the pulping process, strict control of the beating degree of various slurps is very important for the product molding and product quality of the following process.

The beating degree is too high, when the product is vacuum adsorption molding, it will produce water filtration difficulties, and after drying by the drying channel, the product will produce a large shrinkage deformation (except for mold drying).

If the beating is too low, the combination strength of the product is low, and the stiffness and compressive strength of the product will be greatly reduced, which will affect the performance of the product. Therefore, the corresponding beating degree should be determined according to the selected slurry, the proportion of the slurry, and the amount of beating required.

The equipment used in the pulping process of paper pulp molding products mainly includes hydraulic pulper, fiber desanding machine, cone desander, beater (or pulper) and pulp pool and other general equipment in the paper industry. For relatively simple and small scale production processes, only the use of hydraulic pulper and slurry tank can also meet its general production requirements.

4. Matching pulp is to add fillers, pigments, and functional additives such as waterproof and oil repellent to the purified and treated slurry, and to prepare the slurry suitable for the quality requirements of the product, and sometimes to control the pulp concentration. In order to ensure the continuous production process and the evenness of the product, it is necessary to control the liquid level height in the slurry storage tank and the slurry distribution tank, and to add fillers, water and ingredients regularly and quantitatively.

Pulp molding products according to the requirements of enterprises can choose to add waterproof, oil-proof and other functional additives.

At present, the water proofing agents used in paper pulp molding products are mainly rosin, iron sulfate and cationic starch, which are attached to the surface of paper fiber ions and strengthen the adhesion between ions.

5. Molded pulp molding products mainly use vacuum adsorption forming method, forming is to prepare the fiber slurry on the mold with metal mesh by vacuum adsorption forming (product shape), and remove a lot of water. According to the mold sizing method can be divided into grouting type and suction type two categories, grouting molding machine is the earliest used for the production of pulp molding tableware, and suction molding machine is the future development direction of pulp molding packaging container molding equipment.

The working procedure is: punch down to the slurry pool → vacuum suction → punch up → punch up → die closure pressurization and dewatering → die transfer billet moving up and blowing → mold release.

The forming principle is: the slurry is filled in the slurry pool, and the net die is fixed on the concave die. Through the flow of the slurry in the slurry tank and the movement of the die in the slurry tank, the slurry is stirred to make the slurry uniform. When the die is moved down to the liquid level of the slurry tank, the slurry is deposited on the mesh mold through vacuum suction, and then it is moved out of the slurry tank and closed with the punch, and the billet with higher dryness is obtained by pressurization and dehydration.

The thickness of the molded product can be controlled by controlling the suction time during the molding process. Since the strength of the vacuum adsorption in each direction is the same, the thickness of the resulting pulp molded product should theoretically be uniform.

However, during the suction process, due to the flow rate of the slurry and gravity, there may be uneven thickness of the molded product.

6. Shaping refers to the process of heating, pressurizing and dewatering the wet paper blank directly after forming it. The wet paper blank is dried and set in the mold by shaping. The product processed by the mold has smooth and beautiful surface, accurate size, solid and good rigidity. Pulp molding disposable fast food utensils are made of this mold.

In the process of industrial packaging, some small, precise, large number of small items are packaged in a multi-layer stacked way, and the packaging products used for positioning between each layer need to be shaped if the pulp molding products are used to manufacture. The general industrial packaging products are single-sided work without shaping, wet paper billet directly into the dryer to dry.

7. Due to the filtration resistance of the fiber layer, the dryness of the wet pulp molded by vacuum adsorption can usually only reach 20% to 30%, and the wet blank can only meet the requirements after heating and drying, and the final dryness of the general pulp molded products should reach more than 90%.

Because the wet paper blank will have different degrees of deformation after drying or drying, it must be rounded when the appearance, shape and precision of the product are high. The water content of paper molding products before finishing is 20% ~ 30%, but it is difficult to accurately control the water content of paper molding products in production applications. If the water content is too high, the molding process will consume high energy and take a long time, and if the water content is too low, the molding will become very difficult.

In the process of molding, the general hot pressing temperature is controlled between 150 and 180 ° C, which can be moderately adjusted according to the size of the product, but the hot pressing temperature can not be less than 135 ° C.

8. Auxiliary process Auxiliary processes mainly include integral trimming, sterilization, product packaging, inspection, etc. Shaping and trimming is to correct the shape of the dried products, calender, remove the net marks left by the mold mesh molding, so that the internal and external surfaces are smooth and smooth, and can be pressed on some words and patterns according to different customers and use requirements, while cutting the burrs on the edge of the product, pressing the indentation that is easy to fold and open the lid.

Sterilization is one of the processes that distinguish food packaging containers (such as egg cartons) from ordinary pulp molding products. At present, there are three main methods for the sterilization of finished products: First, the tunnel type ozone sterilization machine is used to carry out final sterilization of finished products; The second is to sterilize the finished product with ultraviolet light; The third is the use of high temperature drying channel sterilization.

After the dried or finished products are inspected for quality, they are collected, stacked, counted, and packaged in the required quantity, thus completing the entire production process of pulp molded products. The production of pulp molding products, after forming and drying, has more raw edges, irregular appearance and beautiful appearance, and sometimes it is necessary to cut edges to ensure that the product appearance is neat and beautiful.

Second, the structural design of pulp molding products

1. The structural characteristics of paper molded products Traditional polyethylene foamed plastics mainly rely on the elastic modulus of the material to play a role in seismic buffering, while pulp molded products mainly rely on their own structural seismic. Therefore, the structural design of pulp molding products is very important.

Pulp molded products have the strength, stiffness and toughness of general paper products. Products of various complex shapes can be made according to the functional requirements of use. When used as a disposable fast food appliance, the shape is relatively simple and the strength requirements are not high.

When used as an industrial inner packaging, its shape is very complex, must meet the three major requirements of industrial inner packaging: First, positioning requirements, paper products must ensure the fixed position of the packed items in the packaging box, requiring the shape of the paper products and the packaged items to be consistent, appropriate contact, accurate size.

The second is the buffering requirements, the paper mold products as a whole should have a certain toughness and flexibility, the surface should also be some soft, and the contact friction with the packaged goods will not damage the shiny surface of the packaged goods, and can eliminate the impact of shock and vibration during transportation and loading and unloading.

The third is the bearing requirements, the paper mold products must have a certain strength and stiffness to support the packaged goods and external pressure.

2. Structure design of paper molded products Pulp molded products as a new material used in buffer packaging time is short. At present, its structural design is still in the experience design stage, the design of pulp mold products should be based on the appearance size of the packaged product, the size of the outer container to be reasonably determined.

Pulp molded products in each side must consider its damping effect, so the convex to achieve the purpose of buffering, so that it on each surface of the section line are broken lines, to improve the strength of the paper die. The strength of the paper die is inversely proportional to its buffering performance. The more convex ribs are added, the greater the strength is, but at the same time its buffering performance decreases.

Pulp mold products have a smooth surface (with the surface of the molding mold) and a rough surface, the shape and size of the smooth surface to be packaged products are consistent with the surface; The rough surface should be able to implement good support and fixation in the outer packaging of the product (such as cartons, etc.).

Pulp molding products are formed by vacuum adsorption process, and the absolute thickness and thickness difference of each part are limited by the forming conditions. Under normal circumstances, the absolute thickness of the pulp mold products is 2 ~ 5mm, which requires us to ensure the overall dynamic buffer performance by setting the cavity on the appropriate part, and set a certain number of reinforcement to ensure its overall strength, although the thickness of each part of the pulp mold products can be controlled by adjusting the opening rate of the corresponding part of the molding mold. But too large thickness difference in the molding and drying process is difficult to ensure the quality of pulp molding products. When forming, the dryness of the wet paper mold blank is about 10%, and the slurry still has a certain fluidity, which is easy to produce fiber flow in the vertical direction, causing local fiber flocculation phenomenon, affecting the molding quality, especially for the pulp mold products with larger vertical size, this situation is more serious. Therefore, it is necessary to design a certain slope in the vertical direction of the pulp mold product, which is also conducive to the demudding of the wet paper mold blank. Under normal circumstances, its slope design is 2o ~ 5o. At the surface corner of the pulp molding product, it is necessary to avoid right Angle or acute Angle contact, otherwise, it is easy to produce stress concentration phenomenon here, and the reasonable treatment method is to use rounded corner transition.

In the structural design of pulp molding products, the edge structure should be properly considered, and the edge structure of paper molding products has a boundless structure, a flanging structure and a double edge structure. The boundless structure is suitable for disposable tableware and some industrial internal packaging products with light weight and low strength requirements; Flanging structure and double edge structure are mostly suitable for industrial inner packaging products, which can improve the overall strength of paper mold products, buffer performance and beautify the appearance of products.

The mold design of the pulp molding products is the key to the entire production process

First of all, it must meet the structural requirements of the pulp molded products and the above molding process conditions (that is, the impact of pulp molding on the molding mold, dehydration and drying process), and then it must meet the requirements of mold design and manufacturing.

The molding process of paper molding products mainly includes vacuum adsorption molding method and grouting molding method. Vacuum adsorption molding method is the most commonly used molding method in the production of paper pulp molding products. In this process, the final paper pulp molding products can become qualified products after many times entering and leaving the mold. The molds used in pulp molding include adsorption molding molds, finishing molds and die-cutting molds.

1. Adsorption molding mold The structure of adsorption molding mold is composed of punch, concave die, mesh die, mold base, mold back cavity and gas chamber, etc. The mesh mold is the main body of the mold. Because the mesh mold is woven from a diameter of 0.15mm wire mesh (metal or plastic), the mesh number is generally between 40 and 65 mesh. It cannot be independent of the molding mold and must be attached to the surface of the mold to form. The mold back cavity is a mold that maintains a certain thickness with the mold working face and is completely synchronized with the shape of the mold working face. The cavity formed between the back part and the mold seat, that is, the convex and concave die, is a shell with a certain wall thickness. The working face of the die and the back cavity are connected by a uniform small hole. The die is mounted on the formwork of the forming machine by the die holder. The other side of the template is equipped with an air chamber, which is communicated with the back chamber, and the wall of the air chamber is provided with two channels for compressed air or vacuum extraction.

The pulp is formed after the pulp is broken through the molding mold to adsorb the pulp on the convex mold, and the thickness of the product is mainly controlled by adjusting the adsorption time. The thickness is usually 2 ~ 5mm, and the thickness is determined according to the size and strength requirements of the paper mold product.

① The size of the adsorption molding mold to determine the reserved amount of the adsorption molding mold is the key to mold design. Molded products from the wet paper blank to the finished product, due to the elimination of water shrinkage, and the same product at different locations of the shrinkage rate is not the same, and there is no law, resulting in the product in the molding process of multi-directional, changeable shrinkage changes, to the mold design to determine the reservation caused by difficult, this is mainly through experience. The size of the hot pressing die is mainly determined by the size of the integral die.

The adsorption molding mold is currently made of atomic ash, curing agent, resin and aluminum alloy. When it is used for industrial internal packaging products with low precision requirements, it can be directly manufactured by casting method, and those with high precision requirements must be manufactured by mechanical cutting processing. The lower convex mold of the molding mold is mainly made of aluminum alloy after processing, and the upper concave mold can be directly poured out from the corresponding lower convex mold using resin to ensure its manufacturing accuracy.

2. Mold mold structure includes punch die, concave die, mesh die and heating element. The punch or concave die with a mesh mold is provided with a drainage vent. When working, the wet paper blank is first squeezed in the forming mold, and 20% of the water is squeezed out. At this time, the moisture content in the wet paper blank is 50% ~ 55%, and then the wet paper blank is heated in the mold, and the remaining water is heated and vaporized.

In the process of pressing and drying the wet paper blank with the molding mold, the mesh mold in the molding mold will cause the surface of the product to produce mesh marks and the mesh mold will be deformed or even damaged quickly in frequent extrusion. In order to solve this problem, a new type of netless mold has been developed. The life of the non-mesh mold is 10 times that of the general mesh mold, and the cost is reduced by 50%. The paper mold products made with it are of high precision, and the internal and external surfaces are smooth and without mesh marks.

3. The wet paper blank will be deformed to different degrees after drying. When the outer surface shape of the paper mold products requires high precision, the paper mold products should go through an integral process. The mold used in the integral process is called the integral mold.

The size of the punch under the mold is obtained according to the outline size of the packaged product, the size of the punch under the mold is the outline size of the product, and the size of the concave mold is the sum of the size of the lower punch mold and the thickness of the paper and plastic product.

The mold is generally made of brass, aluminum alloy and stainless steel.

4. Die cutting mold according to the needs of enterprises and products to decide whether to die cutting, die cutting tool with alloy steel material, die specific use of ordinary carbon steel material.

Due to the deviation of mold design and manufacturing accuracy, mold wear after use and other reasons, the edges of paper mold products are often uneven. In order to make the products clean and beautiful, the paper mold products with higher appearance requirements are equipped with a cutting process. Die cutting die is used to trim the rough edges of paper mold products. However, the cutting process of paper molding products is unreasonable, it wastes resources, increases the cost of products, and destroys the edge fiber structure of products. At present, high-precision molds have been developed, and products produced using such molds do not need cutting processes.

Four, the advantages and weaknesses of pulp molding products

Advantages of Pulp Molded products Pulp molded parts have a number of significant advantages over EPS buffer parts:

The raw materials of pulp molding packaging products are waste paper and various fiber materials, which are widely sourced and easy to collect, and will not cause obstacles to the environment and recycling, which is conducive to the export of products. As foamed plastics come from petroleum products, resource depletion and price problems will become more and more serious.

(2) Pulp molded packaging products have a wide range of applications, and the products that use foam as lining can basically be replaced by pulp molded products. Pulp molding products can be stacked for storage, which accounts for 1/2 ~ 2/5 less storage space than foamed plastics, and the freight and storage costs are smaller.

③ Paper products have good anti-static property, foam plastic friction is easy to produce static electricity, which has a better protective effect on electronic products and home appliances.

When packaging plastic products, food moisture is not volatile, easy to mold and rot, and paper products have permeable gas performance, so as to achieve anti-corrosion effect.

2. The weakness of pulp molding products:

① The production process of pulp molding products itself is weak. The production quality of pulp molding products is easily affected by various production factors, such as: if the quality of raw materials is unstable, the shrinkage ratio of products after drying is not consistent with the original design process, and the improper ratio of beater slurry will cause high product breakage rate; The same set of molds are produced on different types of molding machines, and there are certain differences between the products, which leads to the increase of scrap rate during the molding process; If the surface of the hot pressing molding mold is not smooth, there are more sand holes, or when the molding mold is installed, the upper and lower molds are not parallel, the friction phenomenon is serious during the molding, and it is easy to wipe the product. The humidity of the formed wet product is not well controlled during drying, resulting in high scrap rate. If the deformation of the dried product is too large, the overall quality will be affected. If a large number of products after drying do not run in time, long-term air drying leads to low moisture content of the product, excessive shrinkage, and high scrap rate when finishing.

② Raw materials are still relatively single, and it is urgent to develop new sources of raw materials. Pulp molding products need to consume a lot of natural forest resources, which is not conducive to ecological balance.

(3) The production equipment of pulp molding products is not advanced and automated enough. The advanced nature of pulp molding equipment is relatively backward compared with the foam plastic production equipment, such as the automatic molding machine of pulp molding, whether it is the production efficiency or the stability of the equipment is not as good as the foam plastic production equipment, and the degree of automation between the various process equipment on the whole production line is low. The artificial dependence of the production line on transportation, operation, scheduling and monitoring is still relatively serious.

④ The cost performance ratio of pulp molding products is high. Compared with foamed plastic products, the cost of paper molding products is high, mainly because of: high raw material cost, high cost of chemical additives, low degree of automation. In order to achieve the purpose of waterproof and oil proof, paper mold packaging products also need to be coated with plastic or add various additives, which will inevitably increase the difficulty of recycling and increase the cost, and also affect its inherent degradation performance.

⑤ The structural strength and buffering properties of pulp molded products are poor.