Production status and Development Countermeasures

2022-08-06
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Production status and Development Countermeasures of pulp molded products in China

1 Introduction

pulp molded products is a product made of pulp with a certain concentration, adding an appropriate amount of chemical additives in the molding machine to evenly distribute the fibers on the mold surface through vacuum or pressure, so as to form a wet paper mold blank with the proposed geometry and size, and then further dehydration, demoulding, drying and shaping. The product has abundant raw materials, pollution-free production and use, light weight, high compressive strength, good cushioning performance and good recyclability. As a packaging product, it has a wide range of applications. According to relevant data, at present, the output of pulp molded products in the world has reached more than 400000 tons. With the approaching of economic integration between China and other countries in the world, there is no doubt that the popularization and application of pulp molded products has great potential

at the end of 1991, the 12 European Community countries promulgated the mandatory implementation standard "new packaging rules", which clearly announced that since 1992, the use of expanded polystyrene materials in transport packaging and sales packaging has been prohibited and replaced by pulp molded products. Later, the Japanese trade circle also designated some Chinese electrical appliance export manufacturers to Japan to use paper mold products instead of expanded polystyrene packaging. Some regions in China, such as Liaoning, Jiangsu, Guangdong, Zhejiang and Fujian, have made rapid response to this major change in the market of inter container packaging. Although the development of China's pulp molded products began in the 1980s and has formed a certain production scale so far, the current production and operation situation is not very optimistic, and there are still many areas worthy of in-depth study and improvement. Now, the relevant issues are summarized and analyzed as follows

2. Current production process and equipment of pulp molded products

2.1 Production process of pulp molded products

the production process of paper molded products includes waste paper sorting, shredded paper beating, slurry preparation, pulp sizing, slurry (concentration), product molding, product cold extrusion, product drying and product finishing

waste paper is one of the main raw materials for the production of injection molding products. It should be carefully classified by hydraulic grinder and sorter (waste newspaper, corner waste of corrugated board packaging, etc.) and all kinds of impurities should be removed. The grinding of waste paper can be realized on either intermittent beaters or continuous mills and fine mills. After separation and purification, the waste paper is grinded and beaten into uniform pulp. In the grinding process, several raw materials can be added into the slurry quantitatively according to the designed slurry ratio. The purpose of pulp sizing is to improve the liquid penetration resistance of injection molded parts. For paper mold packaging products, the sizing materials are generally pine long-term, paraffin emulsion or rosin paraffin adhesive. The results showed that in order to obtain high quality products, rosin gum equivalent to 3% of the dry fiber mass of the pulp must be added to the non bonded waste paper product pulp; For the bonded waste paper product pulp, 1.5% - 2% rosin gum should be added

the mixing concentration is to fill soda or caustic soda into an open water bath heater with a stirrer and dissolve it, then slowly add rosin blocks of appropriate size, heat the solution to a temperature of 100-105 ℃, and keep stirring for hours until carbon dioxide no longer escapes. The glue should be uniform and free from clots. When checking by visual inspection, pick up the glue with a small wooden shovel and the glue will flow down in pieces. After boiling, the boiled glue shall be vibrated in another intermediate container to cool down slowly, and then diluted to the appropriate concentration with water. In the process of self-made pulp, the diluted glue solution is added to the pulp in the beater or mixing tank. In order to make the glue easy to adhere to the pulp fiber, an appropriate amount of aluminum sulfate solution with a concentration of 10% should be added. The degree to which the rubber is attached to the pulp fiber depends on the pH value of the medium, which should be 4 0 If necessary, the paper pulp shall be given a single color, because there are various printing contents on the waste paper, so the injection molded parts cannot get a single color. Unlike cardboard, which continues to unswervingly support the Hong Kong Special Administrative Region to develop democracy step by step according to law and ultimately achieve the goal of universal suffrage, the Pulp Sizing concentration for the production of molded products should be high, about 1%-2%

there are three methods for molding molded products: vacuum molding, hydraulic molding and compressed air molding. In practice, one of the above methods or combined methods can be used as far as possible to improve production efficiency

after the pulp is filled with the volume of the model and the upper part of the shell, the water will be discharged through the metal wall of the model, and the fiber will adhere to the surface of the model. After forming, lift the model with the help of hydraulic cylinder, take out the product after compaction and transfer it to the drying process. The compaction of the workpiece is realized in a special profiler organized by the upper mold and the lower mold. The structure of the profiler shall ensure that the workpiece will not be stretched during compaction, otherwise the wall of the workpiece will be broken. This equipment works according to the principle that the workpiece is only compressed during the whole pressing process. In order to discharge water from the pressed part during extrusion, a vacuum pumping equipment must be provided. Due to different degrees of extrusion, the moisture content of the workpiece is 50%-75%. After the drying process, the moisture content should be reduced to 10%-12%, and in some cases, it can be reduced to 3%-4%. The drying of products is carried out in the drying furnace with periodic or continuous operation. The product is put on the anti shrinkage model and sent to the drying furnace. The product is dried with hot air. The initial temperature is ℃ (the water content of the product is reduced by 50% within seconds), and then the temperature is reduced to ℃. There is a conveyor in the continuous drying furnace. The parts pass through different temperature zones in the furnace and finally dry to about 10%-12% moisture content. The paper mold production process has been accompanied by dehydration, from gravity dehydration, vacuum dehydration, pressing dehydration to the final thermal dehydration, namely drying. Vacuum dehydration and press dehydration remove most of the water in the product, and the remaining water must be dehydrated by heating and drying

after drying, it is more compact to use the extrusion method on the hot post forming machine, which is necessary for the uniform thickness of the wall of injection packaging products, the smooth and flat outer surface and the improvement of the strength of the wall. Paper molded products can be used after drying, but sometimes they need special treatment, such as calendering, coating baking or printing patterns and words on the surface

there are differences in the process of pulp molded products used in the production of fast tableware and packaging products. For example, the disposable fast food tableware shall be waterproof and oil proof, meet the rigidity requirements, meet the health standards, and the surface shall be printed with trademarks, with smooth surface and clean inside and outside. However, the general shape of packaging products is large and complex, which requires bearing capacity, buffer function and positioning function. Based on these special requirements, the process characteristics of paper mold packaging products are determined as follows: no matter how complex the product shape is, the wall thickness of the same product must be the same. Only in this way can the requirements of forming and finalizing (or drying) of the product in the production process be met. Sometimes, industrial product packaging does not require too high the accuracy of size and shape, and requires certain strength and cushioning. Therefore, the industrial packaging of pulp molded products rarely requires finalization. It is a one-time drying process after coming down from the molding machine. This drying method changes the shape and size of the product due to the precipitation of moisture from the wet paper blank, in this respect "There was a large gap in plastic raw materials in China every year before. The accumulated experience and data can be used as the basis for mold design in the future.

2.2. production equipment for pulp molded products

according to the process requirements, the main equipment used for the production of paper molded products are hydraulic crusher, mixing tank, molding machine, dryer and sizing machine, etc.

the hydraulic pulper is used to break up the waste cardboard and turn it into pulp again. It can be separated and Horizontal type. Horizontal hydraulic pulper is commonly used in the production line of industrial packaging products, which has the advantage of only dispersing the paper fiber without cutting off; High shredding efficiency, short shredding time, less power consumption, simple structure, and can handle waste paper containing large sundries and metal sundries. The machine is mainly composed of a horizontal tank body and a working impeller. There is a feeding port above it and a slurry and slag discharge port below it. The working impeller is fixed on the shaft end extending into the tank body from one end. The impeller diameter is generally 1/2 or 1/3 of the tank diameter, and the linear speed of impeller rotation is generally within 7 m/s. When working, the impeller rotates to produce high-speed rotating water flow, and the cardboard begins to break under the action of high-speed water flow and centrifugal force. The shredded pulp can be discharged from the orifice outside the impeller through the discharge port. Under the action of centrifugal force, the sundries that cannot be broken are attached to the tank wall for rotation. During shutdown, these sundries are deposited at the slag discharge port on the lower wall of the tank. Open the slag discharge port to discharge the slag. The beaten thick slurry is discharged into the slurry mixing tank, where it is diluted to working strength. At the same time, some process and functional additives are added. The slurry tank is equipped with a propeller to keep the slurry rotating and circulating in the working system to avoid fiber sedimentation

the forming machine is a machine that can ensure various operation processes such as pulp feeding, dehydration and removal of wet paper blanks. Its main structure includes: frame, pulp box, lifting cylinder, guide column, upper and lower formworks and chlorine chamber fixed on the formworks (although connected with vacuum system and air compressor). During operation, the punch of the forming mold is fixed on the lower mold. A large number of small holes are uniformly distributed on the mold to connect with the air chamber. The punch is lined with a mold that fully fits the irregular punch surface. The lower mold is pushed by the chlorine cylinder to carry the punch into the slurry of the slurry tank. At this time, the chlorine chamber of the lower mold is under negative pressure by the vacuum pump. The water in the slurry flows through the mold flow inlet chamber, is filtered through the air chamber and pipeline, and the fiber is intercepted on the mold to form a wet paper blank; Then the lower mold starts to rise, so after the slurry level rises, the wet paper blank on the mold continues to dehydrate under the action of vacuum adsorption. At this time, the lower mold continues to go up and close with the concave mold fixed on the upper template for mechanical pressing and dehydration. Then, the vacuum in the lower mold air chamber is cut off and enters the compressed air. At the same time, the vacuum in the upper mold air chamber is connected. The negative pressure is transmitted to the wet paper blank through the small hole on the concave mold. The wet paper blank is transmitted from the lower mold to the upper mold under the action of "pushing" and "pulling", and the upper mold starts to go down again. Manually extend the tray below the upper mold, touch the pneumatic reversing valve in the chlorine chamber of the upper mold, cut off the vacuum in the air chamber of the upper mold and enter the compressed air, and the wet paper blank is blown down on the tray. Manually place the tray and the wet paper blank on the conveyor belt of the drying channel, which is a complete working section of the molding machine. Although there are many forms of pulp molding machines, there are two kinds of molding processes: quantitative grouting in the mold and adsorption molding outside the mold. The principle is to make the pulp flow through the mold, filter the water, and intercept the fiber on the mold to form a wet paper blank. The method of in mold grouting is to ensure the product thickness through a certain amount of pulp, while the method of out mold adsorption is to control the pulp smoothness and adsorption time to ensure the product thickness. Paper mold industrial packaging products are mostly formed by reciprocating adsorption outside the mold. This forming machine can produce products with a large area. It has strong adaptability and can form an automatic production line

it is recommended to use oil fired furnace for heating the dryer. Its drying structure includes: head, tail, transition section, intermediate heating section, burner, induced draft fan, conveyor belt and automatic control box. During operation, the combustion furnace atomizes the diesel oil and sprays it into the combustion chamber for combustion

if the shape and size requirements of the product are very strict, the wet paper blank needs to be shaped and dried to one

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