As the first stage of paper pulp molding process, collection of raw material is so important. Actually apart from a few of molded pulp products that are used for food or high-grade packaging (such as disposable tableware), most are made from recycled paper or market pulp.
There are three main sources of wastepaper recycling.
1）From recycling company
As one of main source, this kind of scrap paper is wide in variety and in large quantities, comes from ONP (old newsprint), OMG（old magzines), MOW (mixed office wastepaper), wrappers, boxes, cartons, containers from shops or supermarkets.
This kind is collected by individuals, including waste paper from a sweep-up in a unit, usually sells to salvage station due to small quantity. Generally it have lower grade.
3）From paper mill
It’s inevitable to cause scrap during any kind of paper-making, most of these scrap paper is utilized and digested by paper mill itself. For those scraps which can not or unsuitable for utilization, such as from paper products plant, hard paper case factory or printing house, are sent to reclamation factory for processing. This kind of recovered paper is comparatively clean, have steady quality and easy for recycling.
In most cases, recycled paper is not a sole sort, but a mixture of varieties. To make the best use of it, it’s necessary to rate, store and handle them separately, that’s what we called “material classification”. During classification, you have to remove foreign substance such as metals, wood, stones, ropes, plastic or PE pieces for better raw material purity quotient.
Manual classification would be the best choice considering a wide rage of materials. Even though a magnetic separator helps to sort out metals or heavy foreign matters, work done by hand is still needed. Usually classification use an inclined conveyor with a speed-change mechanism. Several people stand by the conveyor and pick out foreign substances that are harmful to production.
Amount of manual labor depends on the source of waste paper and application of reclaimed pulp. For example, pre-consumer recovered paper including white paper edges and leftovers from printing house, can be released to waste paper recycling system after simple classification and check on site.
Manual classification of recovered paper can be divided into active and negative. Active method refers to not only pick out non paper element, but also separate them based on quality. This method can produce cleaner recycled pulp, however, productivity is rather low. While negative method lead to lower quality but higher efficiency.
Traditional disintegration of recovered paper is fulfilled by a hydropulper. Although driven by a motor, hydraulic action of a hydropulper is actually the main effect , which is a kind of shearing action generated by rotor when rotating.
A hydropulper is consist of a cylindrical tank and a concentric (sometimes eccentric) rotor impeller assembled with blades. High, medium and low concentration hydropulper are three main types in terms of different material processing, and their density are 19%, 12% and 6% accordingly. High and medium density hydropulpers work intermittently, while low concentration one runs contiguously. The dimension of rotor is up to pulp density and purification system of hydropulper.
When rotor rotates, blades on it shatter harshly contact paper material. The sudden movement of a rotor and inertia force cause a acceleration, under the action of viscosity force, suspended solid in the water surely have a speed which is different from the pace of waste paper chips. Viscosity force lead to friction among fibers, eventually chips are disintegrated further due to shearing stress. Meanwhile, because of powerful vortex produced by rotor, a high speed turbulent region is formed around rotor, and speed of recycled pulp close to inner is lower that of turbulent region, pulping is finalized as a result of speed difference。
Pulp density during egg tray production process is 5% ~ 8% approximately. Benefits of a hydropulper are efficient, low energy consuming, simple structure with small covering area. It take advantage of fiber dispersion instead of cutting.
After disintegration, pulp falls into storage tank, formulated pulp is pumped for pulping. Generally, the beating degree is 28° ~ 35° SR.
During egg tray manufacturing process, several kinds of quantitative ingredients may be added in pulp pool according to your formulation, and proceed with sizing and addition of functional agent. For example, sizing agent ( such as rosin, emulsified wax) is applied for liquid penetration-resistance of molded pulp products. For non tacky pulp, rosin size equivalent to 3% of qualitative properties of bone dry fiber must to be added, and 1.5%-2% for tacky pulp.
By adding talcum powder as filling in to pulp helps to reach higher retention. Some manufacturers also add a small quantity of oil/water resistance agent, wet strength agent and/or retention aid. Coloring agent need to be added into pulping machine If you want colored egg boxes or cartons.
Molding is a key step in egg tray making process, and have decisive effects on quality, broken rate, energy consuming and production efficiency of final products.
As an common method in pulp molding industry, vacuum forming is achieved by vacuum filtration technology. Place dies (also called molds) into approximate 1% concentrated pulp solution, and make sure negative pressure is generated inside the cavity with the help of vacuum. Fiber contained in pulp slurry deposits evenly on the surface of mesh of molds, and finally huge amounts of moisture content is drained off while vacuum suctioning. When required thickness of product is fulfilled, forming mold transfer from pulp and proceed with squeezing and dehydration until moisture of wet product reaches at 75%~80%, and then wet product falls off by the action of compressed air inside cavity of mold.
To enhance molding quality, good virgin pulp and rational molding method is not only required, a good design of egg tray machine is also expected for safe pulp supply. Phenomenon such as return and run out need to be avoided. Designing and machining of the mold are supposed to be with good filtration performance and uniform dehydration property.
During all egg carton manufacturing process, 3.5~4 kg moisture content is dehydrated from every kilogram of finished product and lowered to 10%~12%. Therefore, drying process covers the major ratio in production cost and improvement of dry efficiency is the key measure to benefit.
Apart from taking advantage of natural condition such as sunshine and wind, adoption of hot air convection is another common drying method in pulp molding factory. To increase drying efficiency, the most effective method are improvement of drying facility condition and rational control of drying technology. No matter brick or steel cabinet, egg tray drying line is supposed to have good insulating layer to reduce thermal losses, and make sure uniform temperature inside. Temperature, humidity and flow velocity (flow rate) of hot air are three main parameters, which must be controlled logically.
1）Usually, temperature of hot air is controlled at 130~200°C, over-high temperature may improve drying efficiency to some extent, but lead to thermal degradation of plant fiber, and cause excessive shrinkage strain of molded pulp products
2）Humidity level exerts a tremendous influence on drying procedure. As the moisture carrier, hot air absorb and drain off moisture evaporated from wet products by the action of concentration difference of moisture between hot air and wet products. It’s favorable for improvement of drying efficiency by maintaining lower humidity in hot air. Generally stable humidity in hot air can be basically achieved by continuous fresh hot air supply and timely discharge of waste hot air.
3）During drying process, enough hot air is required to blow toward molded pulp products. It’s important to control flow speed of hot air rationally entered into drying facility. Usually 5 m/s should be maintained, and over-high speed will increase energy consuming while over-low decrease efficiency.
Hot press is to put wet molded product which is dried to a certain content into the shaping die, pressurize and heat up for better parallelism and strength. It’s the last process of pulp molding and is critical to the final quality.
To eliminate wire marks when molding and deformation caused by drying, stamp necessary logo or text on the surface of molded pulp product, or to remove burs on edge, it’s needed to proceed with hot press and trimming, ensure products are neat and artistic, have a accurate dimension and shape, smooth inside and outside.
Hot press machine in fact, is a four column press (hydraulic or pneumatic), the difference is that heater element is assembled inside upper and lower dies. Working pressure is 0.4~0.6 Pa/㎡ and temperature inside die is 180~200°C. Hot pressing time is decided by shape and thickness of product, generally is 30~50 seconds. During practical operation, the final thickness is approximately 70% of die cavity.