Egg trays are one of the most common pulp molding applications. The raw materials for pulp molding can come from various types of waste paper, which has great environmental value. This article mainly introduces the manufacturing process of egg trays, which can be learned in detail through the video in the article.
Material collection and classification
Collection
As the first stage of the pulp molding process, the collection of raw materials is very important. In fact, except for a few molded pulp products used for food or high-end packaging (such as disposable tableware), most are made of recycled paper or commodity pulp.
There are three main sources of waste paper recycling:
1. From recycling companies As one of the main sources, this type of waste paper is of various types and large quantities, coming from old newsprint, old magazines, mixed office waste paper, wrapping paper, boxes, cartons, containers in stores or supermarkets.
2. From individuals This type is collected by individuals, including waste paper swept by units, and is usually sold to salvage stations due to its small quantity. Generally speaking, it is of lower grade.
3. From paper mills Waste paper is inevitably generated in any kind of papermaking process, and most of these waste papers are utilized and digested by paper mills themselves. For scraps that cannot or should not be used from paper products factories, cardboard box factories, printing factories, etc., they are sent to recycling plants for processing. This kind of recycled paper is relatively clean, of stable quality, and easy to recycle.
Classification
In most cases, recycled paper is not a single type, but a mixture of multiple types. In order to make full use of it, they need to be graded, stored and processed separately, which is what we call “material classification”. During the sorting process, foreign matter such as metal, wood, stone, rope, plastic or PE sheet must be removed to obtain a better raw material purity quotient.
Considering the wide range of materials, manual sorting will be the best choice. Although magnetic separators help to sort metal or heavy foreign matter, it still needs to be done manually. Usually, grading uses an inclined conveyor with a variable speed mechanism. Several people stand beside the conveyor belt to pick out foreign matter that is harmful to production.
The amount of physical labor depends on the source of the waste paper and the specific application of the recycled pulp. For example, waste paper that has not yet been put on the market, including white paper scraps and printing factory scraps, can be transported to the waste paper recycling system after simple sorting and on-site inspection, and the amount of sorting required is low.
Decomposition and pulping
The traditional decomposition of recycled paper is done by a hydraulic pulper. Although the hydraulic pulper is driven by an electric motor, its main function is actually hydraulic action, which is a shearing action generated when the rotor rotates.
The hydraulic pulper consists of a cylindrical tank and a concentric (sometimes eccentric) rotor impeller equipped with blades. According to the different materials processed, there are three main types of hydraulic pulpers: high, medium and low concentrations, with densities of 19%, 12% and 6% respectively.
High-density and medium-density hydraulic pulpers work intermittently, while low-density hydraulic pulpers operate continuously. The size of the rotor depends on the pulp density and purification system of the hydraulic pulper.
When the rotor rotates, the blades on it come into violent contact with the paper material. The sudden movement of the rotor and the inertial force cause acceleration, and under the action of viscous force, the suspended matter in the water must have a different speed from the waste paper chips. Viscous force causes friction between fibers, and finally the chips are further decomposed due to shear stress.
At the same time, due to the strong vortex generated by the rotor, a high-speed turbulent zone is formed around the rotor, and the speed of the recycled pulp near the inside is lower than that of the turbulent zone, and pulping is completed due to the speed difference.
The pulp density in the egg tray production process is about 5%~8%. The advantages of the hydraulic pulper are high efficiency, low energy consumption, simple structure and small coverage area. It plays the advantages of fiber dispersion without shearing.
The disintegrated pulp falls into the storage tank, and the prepared pulp is pumped into the pulping. The general beating degree is 28°~35°SR.
Adding additives
During the egg tray production process, a variety of quantitative ingredients may be added to the pulp pool according to the formula, and sizing and functional agents are added. For example, sizing agents (such as rosin and emulsified wax) are used for anti-seepage liquids for pulp molded products. For non-sticky pulp, rosin glue equivalent to 3% of dry fiber must be added, and for sticky pulp, 1.5%-2% must be added.
By adding talcum powder as a filler, it helps to achieve a higher retention rate. Some manufacturers also add a small amount of oil/waterproofing agent, wet strength agent and/or retention aid. If you want a colored egg box or paper box, you need to add a colorant to the beater.
Molding
During the molding process, all parts of the fiber product are deposited evenly. That is, no matter how complex the shape of the product is, the thickness is required to be uniform everywhere.
Molding is a key step in the egg tray production process and has a decisive influence on the quality, breakage rate, energy consumption and production efficiency of the final product.
As a common method in the pulp molding industry, vacuum molding is achieved through vacuum filtration technology. The mold (also called the mold) is placed in a concentrated pulp solution of about 1%, and a negative pressure is generated in the mold cavity with the help of vacuum. The fibers contained in the pulp are evenly deposited on the mesh surface of the mold, and eventually a large amount of water is discharged while vacuum suction.
When the product reaches the required thickness, it is transferred by the pulp molding mold for extrusion dehydration until the moisture content of the wet product reaches 75%~80%, and then the wet product falls off under the action of compressed air in the mold cavity.
In order to improve the molding quality, not only good virgin pulp and reasonable molding methods are required, but also good egg tray machine design is required to ensure the safe supply of pulp. Phenomena such as return and exhaustion need to be avoided. The design and processing of the mold should have good filtering performance and uniform dehydration performance.
Drying
During the entire egg box manufacturing process, 3.5~4 kg of moisture is dehydrated per kilogram of finished product, reducing it to 10%~12%. Therefore, the drying process accounts for the main proportion of production costs, and improving drying efficiency is a key measure to obtain benefits.
In addition to utilizing natural conditions such as sunlight and wind, the use of hot air convection is another common drying method in pulp molding plants. To improve drying efficiency, the most effective way is to improve the conditions of drying equipment and reasonably control the drying process.
Whether it is a brick cabinet or a steel cabinet, the egg tray drying line should have a good insulation layer to reduce heat loss and ensure uniform internal temperature. The temperature, humidity and flow rate (flow rate) of hot air are the three main parameters that must be logically controlled.
1)
Usually, the hot air temperature is controlled at 130~200°C. Too high a temperature may improve the drying efficiency to a certain extent, but it will cause thermal degradation of plant fibers and lead to excessive shrinkage strain of molded pulp products.
2)
Humidity level has a great influence on the drying process. As a carrier of moisture, hot air absorbs and discharges the evaporated moisture in the wet product through the difference in moisture concentration between hot air and wet product. Keeping low humidity in hot air is conducive to improving drying efficiency. By continuously supplying fresh hot air and timely discharging waste hot air, the overall stability of hot air humidity can be basically achieved.
3)
During the drying process, there needs to be enough hot air blowing to the molded pulp products. It is important to reasonably control the flow rate of hot air entering the drying equipment. It should usually be maintained at 5m/s. Too fast speed will increase energy consumption, and too low speed will reduce efficiency.
Hot Pressing
The hot press is to put the wet molded product dried to a certain content into the molding mold and pressurize and heat it to obtain better parallelism and strength. This is the last process of pulp molding and is crucial to the final quality.
In order to eliminate the line marks and drying deformation during molding, print the necessary logo or text on the surface of the molded pulp product, or remove the edge burrs, hot pressing and trimming are required to ensure that the product is neat and beautiful, with accurate size and shape, and smooth inside and outside.
The hot press is actually a four-column press (hydraulic or pneumatic), the difference is that the heating elements are assembled in the upper and lower molds.
The working pressure is 0.4~0.6 Pa/㎡, and the temperature in the mold is 180~200°C. The hot pressing time is determined by the shape and thickness of the product, generally 30~50 seconds. In actual operation, the final thickness is about 70% of the mold cavity.
