Raw material collection, paper pulping, additives adding, molding, drying, and hot pressing are six steps you should know in the egg tray production process.
01 Raw Material Collection & Sorting
As the first stage in the paper egg tray manufacturing process, the collection of raw materials is extremely important. In fact, except for a small number of pulp-molded products used for food packaging or high-end packaging (such as disposable tableware), most products are made from recycled paper or commercial pulp.
There are three main sources of waste paper recycling:
- Company waste paper is one of the main sources. This type of waste paper is diverse and available in large quantities, including ONP (Old Newspapers), OMG (Old Magazines), MOW (Mixed Office Waste), packaging paper, boxes, and cartons.
- Individually collected waste paper is usually sold to recycling stations. These papers are produced in smaller quantities and are generally of lower grade.
- Waste paper generated by paper mills is mostly reused internally. However, offcuts and scraps that cannot be reused from paper product factories, carton box plants, printing houses, and similar facilities are excellent raw materials. This type of recycled paper is relatively clean and offers stable quality.
Sorting
Recycled paper is a mixture of various types of recovered waste paper that have been processed and treated. To make full use of the collected waste paper, it must be graded, stored, and processed—this is what we refer to as material sorting. During the sorting process, foreign matter such as metal, wood, stones, ropes, plastics, or PE sheets must be removed to achieve higher raw material purity.
The complexity of the sorting process depends on the source of the waste paper and the specific application of the recycled pulp. For example, waste paper that has not yet entered the market, such as white paper trimmings and printing plant offcuts, can be conveyed to the waste paper recycling system after simple sorting and on-site inspection, requiring only minimal sorting work.
02 Disintegration & Pulping
The disintegration of waste paper is carried out by a hydraulic pulper. Although the hydraulic pulper is driven by an electric motor, it actually relies mainly on hydraulic action, specifically, the shearing force generated between the rotating rotor and the water.
A hydraulic pulper consists of a cylindrical tank and a concentric (sometimes eccentric) rotor impeller fitted with blades. Depending on the material being processed, hydraulic pulpers can be classified into high, medium, and low-consistency types, with consistencies of 19%, 12%, and 6%, respectively. High and medium-consistency hydraulic pulpers operate in batch mode, while low-consistency hydraulic pulpers can run continuously. The size of the rotor depends on the pulp consistency and the purification system.
When the rotor rotates, the blades mounted on it strike the waste paper. The sudden motion of the rotor and the inertial forces generate acceleration; under the action of viscous forces, the suspended materials in the water inevitably move at velocities different from that of the waste paper. These viscous forces cause friction between fibers, and the waste paper is further disintegrated by shear stress. At the same time, the rotor generates strong vortices, creating a high-speed turbulent zone around it. The pulp closer to the interior moves more slowly than the material in the turbulent zone, and the resulting velocity difference promotes the completion of the pulping process.
The advantages of a hydraulic pulper include high efficiency, low energy consumption, a simple structure, and a small footprint. In the egg tray production process, the required pulp consistency is approximately 5%–8%, with a beating degree of 28°–35° SR.
03 Additives Addition
During the production of paper egg trays, various additives may be added to the pulp in measured amounts according to the formulation. For example, sizing agents (such as rosin or emulsified wax) are mainly used to improve the liquid resistance of pulp-molded products. For non-adhesive pulp, rosin sizing equivalent to 3% of the dry fiber content must be added, while for adhesive pulp, 1.5%–2% is required.
Adding talc as a filler helps improve the yield of finished egg trays. Some manufacturers also add small amounts of oil-resistant and water-resistant agents, as well as wet-strength agents. If colored egg trays or egg cartons are required, colorants need to be added to the pulper.
04 Molding
Molding is the key step in the production of paper egg trays and plays a decisive role in product quality, breakage rate, energy consumption, and production efficiency. During the molding process, the fibers in the pulp must be evenly deposited to ensure uniform thickness throughout the final egg tray product.
Vacuum forming is a commonly used method in pulp molding, achieved through vacuum filtration technology. First, the mold is immersed in concentrated pulp with a consistency of approximately 1%. With the assistance of a vacuum system, negative pressure is generated inside the mold cavity. The fibers suspended in the pulp are uniformly deposited onto the mesh surface of the mold to form the shape. At the same time, a large amount of water is removed through vacuum suction, resulting in a semi-wet egg tray with a moisture content of 75%–80%.
To improve molding efficiency, high-quality virgin pulp, appropriate forming methods, and efficient egg tray machines are all essential.
05 Drying
In the egg tray drying section, approximately 3.5–4 kilograms of water must be removed for every kilogram of finished egg trays. Therefore, the drying stage accounts for a major portion of the production cost. Improving drying efficiency is the key to achieving better profitability.
In addition to natural drying methods such as sunlight and wind, hot air convection is another common drying method used in egg tray production lines. Whether using brick kiln drying or metal drying systems, the egg tray dryer should be equipped with effective insulation layers to reduce heat loss and ensure uniform internal temperature. The temperature, humidity, and airflow velocity of the hot air are the three main parameters that must be properly controlled.
1) The hot air temperature is generally controlled between 130–200°C. Although higher temperatures can improve drying efficiency to a certain extent, they may also cause thermal degradation of plant fibers and excessive shrinkage deformation of the egg trays.
2) Humidity has a significant impact on the drying process. Hot air with lower humidity is beneficial for improving drying efficiency. Acting as a carrier, the hot air absorbs and removes the moisture evaporated from the wet egg trays through the difference in moisture concentration between the air and the trays. By continuously supplying fresh hot air and promptly exhausting the used hot air, the overall humidity of the drying air can be maintained at a stable level.
3) During the drying process, sufficient hot air must be directed toward the egg trays. Proper control of the hot air velocity is essential and is typically maintained at around 5 m/s. If the airflow is too fast, energy consumption increases; if it is too slow, drying efficiency decreases.
06 Hot Pressing
The function of the hot press machine is to place the finished egg trays into a forming mold and apply heat and pressure to achieve better flatness and strength. This is the final step in the paper egg tray production process.
Hot pressing can eliminate mold marks formed during molding and minor deformation that occurs after drying. In addition, it can imprint necessary logos or text onto the surface of egg trays and egg cartons, or trim edge burrs to ensure the product is neat, aesthetically pleasing, and smooth inside and out.
The hot press machine is essentially a four-column press (hydraulic or pneumatic), with heating elements installed in the upper and lower molds. The working pressure is 0.4–0.6 Pa/m², and the mold temperature ranges from 180–200°C. The hot pressing time depends on the shape and thickness of the product, typically ranging from 30 to 50 seconds.