Views: 4 Author: Site Editor Publish Time: 2024-11-15 Origin: Site
Printing Techniques for Plant Fiber Molded Products
Significant advancements have been made in the printing of plant fiber molded products, especially in complex printing applications. For instance, in the past, colored printing on egg cartons often relied on label application, but now there are manufacturers who have developed direct printing methods. The same goes for the direct printing on cosmetic boxes and more. As plant fiber molded products penetration ate into the daily-use and fast-moving consumer goods sector, the demand for product aesthetics and the need to attract consumer attention will continue to rise. Consequently, the demand for printing will also increase. Additionally, for egg carton products, there will be a growing need for inline printing, production capacity, and requirements such as unique coding for each item.
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Plant fiber molding (pulp molding, straw molding) is a production process that directly shapes plant fibers into a slurry using a water medium and then forms the product using molds. The products have diverse structures, with countless variations and a multitude of convex-concave changes. Moreover, they feature small printable areas, which are challenging to accommodate with traditional flatbed printing methods. More flexible printing techniques should be considered, such as pad printing, inkjet printing, screen printing, and letterpress, to achieve beautiful multicolor printing on the concave-convex surfaces of paper mold packaging products.
Thi article shares knowledge related to the printing processes of plant fiber molding, and the content is provided for reference by friends:
The picture comes from Baidu Pictures
Plant fiber molding (pulp molding) is a three-dimensional papermaking technology that involves adding a certain concentration of pulp with an appropriate amount of chemical additives into a forming machine. Through vacuum or pressure, the fibers are evenly distributed on the surface of the mold, thus creating a wet paper mold blank. The blank is then further dehydrated and demolded, dried, and shaped to produce pulp mold products. Products made from different components of waste paper exhibit various properties.
01 Molding processStep 1: Molding Formation: A pulp with a purity of 1.4% is continuously stirred with water until it is uniformly mixed. Then, a part of the mold is immersed in the pulp, and due to the vacuum action of the mold, the pulp adheres to the mold surface until the thickness of the pulp on the mold surface reaches 2-3mm (0.79-0.115in).
Image from Consciousness Folder
Step 2: Transfer: Similarly, through the action of vacuum, the pulp on the mold surface is transferred to the inner wall of another mold.
Image from Consciousness Folder
Step 3: Demolding: After demolding, the product is transferred by a conveyor belt to an oven for heating/drying for 15 minutes. The process for high-quality wet pressed products is different, as it involves drying directly within the mold.
Image from Consciousness Folder
Screen printing involves placing a thick ink on a screen plate, then using a squeegee to make contact with the screen plate at a certain angle while applying pressure and moving it, allowing the ink to pass through the screen and be transferred onto the substrate.
In manual screen printing, a single squeegee is used to both apply the ink and create the print. Mechanical screen printing requires two separate squeegees: one for applying the ink and another for scraping it back. These two squeegees operate in an alternating back-and-forth motion, with the printing squeegee pressing the ink onto the substrate during the forward stroke and lifting off the screen during the return stroke. To ensure there is enough ink for the next print, the ink-scraping squeegee returns the ink to its original position, facilitating continuous printing.
The main advantages of screen printing are its simple plate-making and printing process, low equipment investment, and cost-effectiveness, making it suitable for small batch production. It is not limited by the type, size, or shape of the substrate, allowing for printing on curved, spherical, and fragile surfaces. It is also well-suited for printing on paper mold packaging products with complex structural shapes. Additionally, screen printing produces prints with thick ink layers, vibrant colors, strong three-dimensional effects, and good coverage. Therefore, small-scale screen printing equipment can be used to print on the surfaces of paper mold packaging products.
Paper mold packaging products have a variety of convex-concave changes in their structure, and inkjet printing can also be used to complete the decorative printing on their surfaces. Inkjet printing is a non-contact, plateless imaging printing method that applies computer-stored information, without the need to store graphic information on a printing plate. The print head does not come into contact with any part of the substrate, allowing for reliable printing of clear text and images on the various convex-concave surfaces of paper mold packaging products under a wide range of operating conditions.
The principle of inkjet printing involves injection ting tiny droplets of ink from one or more print heads through a nozzle onto the surface of each paper mold product on the production line, forming text and images composed of a dot matrix. The printing machine uses a microprocessor to control the required text and graphics, and through the action of sensors, it synchronizes with the speed of the production line to ensure that each paper mold product is correctly printed with the desired text and images.
The inkjet process is carried out by pressure or electrostatic force, allowing printing from bottom to top, top to bottom, or at any angle, which is particularly suitable for surface printing on paper mold packaging products with varied and complex structural shapes.
Based on the printability of the paper mold material and the structural characteristics of the paper mold products, pad printing technology can be used to decorate the surface with graphics. Pad printing is an indirect printing method that uses a silicone rubber pad shaped like a hemisphere to press against a copper or steel intaglio plate, picking up the ink from the plate’s graphics, and then transferring it to the substrate to complete the printing. It can achieve beautiful multicolor printing on irregular convex-concave surfaces. Additionally, processes such as hot stamping gold and silver have also demonstrated excellent effects on Samsung’s product series.
Other printing methods, such as water transfer printing, etc., are also documented in literature, but actual products have not yet been obtained.
As the application scenarios for plant fiber products become more widespread, the demand for printing will also increase, leading to the emergence of more printing solutions alongside the industry’s development.
Plant fiber molded products and their mold design
01 The Basic Production Process of Pulp (Plant Fiber) Molded Products
The base material and additives for pulp (plant fiber) molded products are similar to those used in paper, and the process is quite alike, with the only difference being that pulp (plant fiber) molded products are shaped on a mesh mold. As the pulp (plant fiber) passes through the mesh mold, the fibers are retained by the mold while the water is filtered out, forming the shape according to the mold’s design. The pulp fibers (plant fibers) bond together through hydrogen bonds between the fibers to form a wet paper blank. The wet blank is then dried or heat-set to finally become a pulp (plant fiber) molded product. This is the basic process of shaping pulp (plant fiber) molded products.
2 Pulp (Plant Fiber) Molding Mold Features
Pulp (plant fiber) molding involves using molds to shape paper products in the pulp state, making the mold a key component of pulp (plant fiber) molding technology. When producing disposable fast-food containers using pulp (plant fiber) molding, the products are limited to boxes, bowls, and plates, with relatively simple shapes and a degree of versatility. However, when pulp molding is used for industrial inner packaging and other applications, the products tend to be larger and have very complex shapes. Each style is tailored to fit specific items, with no versatility, requiring mold design and manufacturing to be an ongoing process.
The shaping of pulp (plant fiber) molded products is quite different from the molding of rubber and plastic products in a mold cavity. The pulp used for shaping is a water solution containing only about 10% fiber, and it is necessary to ensure that most of the water drains away, so the molds for pulp (plant fiber) molding must be mesh molds.
The shaping of pulp (plant fiber) molded products is a continuous process of removing a large amount of water from the pulp. The paper products form to a certain thickness on the mesh mold through a deposition process. Therefore, the shaping must occur on the surface of a single convex or concave mesh mold. The shaping process of pulp (plant fiber) molded products is also a filtering process; as the thickness increases, the filtering performance decreases, and the rate at which the thickness forms slows down. Thus, the wall thickness of the product is formed simultaneously across the piece. No matter how complex the shape of the product, the wall thickness is consistent throughout.
After shaping, the pulp (plant fiber) molded products are wet blanks with a water content of 70-75%, as the force pushing the pulp onto the mesh mold is vacuum, and the maximum force does not exceed one atmosphere. The newly shaped wet blanks, with low wet strength, adhere to the mesh mold and must be placed in the mold cavity after a corresponding convex or concave mold is closed. Air pressure is then used to transfer them. Pulp molded products are shaped on a single convex or concave mold and then transferred within the mold cavity after the molds are closed. Due to these specific characteristics of pulp molding, the complexity of product design and mold design is increased.
3、Design of Pulp (Plant Fiber) Molded Products
Pulp (Plant Fiber) Molding Product Design involves more than just the design of structural equipment; many of the product’s properties are determined by the raw materials, additives, fillers, and production process. Therefore, the design of pulp (plant fiber) molding products is a comprehensive technology that combines process and structure.
Some Basic Requirements
In addition to the general physical strength requirements of paper products, the following properties should be emphasized in terms of process and structural design to meet specific usage requirements, depending on the different purposes of the products.
(1) Absorbency: The hydrophobic properties of pulp molding products, including the absorption properties for liquids such as water, oil, and ink, are particularly important for paper tableware.
(2) Appearance Quality: Defects in the appearance of the product that can be observed with the naked eye should be minimized. These defects include dust, holes, wrinkles, ridges, screen printing marks, spots, pulp lumps, fish scale spots, cracks, rolled edges, depressions, and color inconsistency.
(3) Precision: This refers to the error between the size, shape, and position of the product and the design requirements. The precision of pulp molding products, given the current manufacturing methods, can only reach the national standard grade “&” or below.
Key Points of Structural Design
Pulp (plant fiber) molding products have the strength and toughness of paper products and can be made into various complex shapes based on functional requirements. For disposable fast-food containers, the shapes are relatively simple, and the strength requirements are not very high. For industrial inner packaging, the shapes are complex and must meet the three main requirements of industrial inner packaging:
First, positioning requirements. The packaging product must ensure that the packaged item has a fixed position within the packaging box, requiring the packaging product to be shaped in sync with the packaged item, fit closely, and have accurate dimensions.
Second, cushioning requirements. The product as a whole should have a certain degree of toughness and flexibility, and the surface should be somewhat soft to avoid damaging the surface luster of the packaged item during contact and friction, and to absorb the impact caused by collisions and vibrations during handling.
Third, load-bearing requirements. The product should have sufficient strength and rigidity to support the packaged item and withstand external pressure. In addition to selecting appropriate raw materials, additives, and production processes, the most important aspect is structural design.
(1) Functional Structure: The shaping process characteristics of pulp (plant fiber) molding products dictate that the same product can only have one thickness. To adapt to functional requirements, the shape of the cavity and vertical ribs can be used to adjust the strength and cushioning. The cavity can increase the overall elasticity and flexibility of the product, while vertical ribs can increase the product’s strength and rigidity. In areas where higher load-bearing capacity is required, such as larger planes, the design can incorporate corrugated or honeycomb shapes.
(2) Process-Oriented Structure: Since the initial stage of pulp molding products is a wet paper blank, the water must be gradually removed during the production process, leading to significant changes in the shape, position, and size of the product. The total shrinkage varies at different positions on the same plane and in the same direction, causing the product to warp or twist. To adjust this deformation, only structural shapes that balance the shrinkage can be used. This structural shape used to adjust shrinkage deformation is called the product’s process-oriented structure.
(3) Principles for Determining Structure-Related Parameters: Draft Angle: Since the wet paper blank of pulp molding products tightly adheres to the mold and the pulp fibers may become embedded in the mesh holes of the mold, a reasonable draft angle should be provided on the product surfaces parallel to the demolding direction to facilitate the transfer of the wet paper blank. Too small a draft angle will make demolding difficult, leading to surface scratches or breakage; too large a draft angle will reduce the precision of the product size, affecting the packaging function.
Wall Thickness: The wall thickness of the product is determined based on the use conditions and type of pulp, and it is an important factor affecting the strength of the product. Increasing the wall thickness is not just about increasing the consumption of raw materials; it also affects the production efficiency during adsorption shaping, and it may cause quality defects such as concavities, shrinkage holes, and delamination. Therefore, the wall thickness should be minimized while still meeting the product strength requirements. For adsorption shaping, the wall thickness range is between 0.5-6mm, and for compression shaping, it is between 3-20mm.
Transition Radius: The corners of the inner and outer surfaces of pulp molding products, the connections between vertical ribs and the main body, and the ends of the vertical ribs must all be rounded to avoid sharp angles. A rounded transition is beneficial for mold manufacturing and mesh attachment, facilitates demolding during the transfer of the wet paper blank, aids the flow of pulp during
4、Pulp (Plant Fiber) Molding Mold Design
The design of pulp molding molds must meet the structural requirements of the pulp molding products, and while the structure of the pulp molding products must comply with the usage and molding process conditions, it must also meet the requirements of mold design and manufacturing. Therefore, before designing the pulp molding mold, a thorough analysis and study of the structure of the pulp molding product must be conducted to ensure that the economic rationality and technical feasibility of the product design and mold design are harmonized and unified.
Types of Pulp Molding Molds
The formation process of pulp molding products requires multiple entries into the mold to complete the manufacturing process. As mentioned earlier, pulp molding products are formed on a single convex or concave mold and then transferred to the mold cavity after the convex and concave molds are closed. The wet paper blank is then formed in the mold cavity under pressure and heat in the shaping mold. Therefore, pulp molding molds are divided into forming molds, shaping molds, finishing molds, and cutting molds, etc. Forming processes can be divided into two types: adsorption forming and compression forming, with compression forming molds being somewhat similar to rubber and plastic molds, while adsorption forming molds are more specialized and complex.
(1) Adsorption Forming Mold: The structure of an adsorption forming mold consists of a convex mold, a concave mold, a screen mold, a mold seat, a mold back cavity, and an air chamber. The screen mold is the main part of the mold, as it is made of a mesh of 0.15mm diameter wire (metal or plastic) and cannot form independently; it must be attached to the mold surface to function. The mold back cavity is the cavity formed by the mold back, which is in complete synchronization with the mold working surface and is kept at a certain distance from the mold seat. This cavity, also known as a shell with a certain wall thickness, is connected to the mold working surface by evenly distributed small holes.
The mold is installed on the template of the forming machine through the mold seat, with the other side of the template equipped with an air chamber that communicates with the back cavity. The air chamber also has channels for compressed air and vacuum.
The reserved amount of an adsorption forming mold is a key aspect of mold design.
As the pulp molding product transitions from a wet blank to a finished product, it undergoes shrinkage due to the evaporation of water. Different parts of the same product have different and unpredictable shrinkage rates, causing the product to undergo multi-directional and variable shrinkage changes during the molding process, which makes it difficult to determine the reserved amount during mold design. However, since the packaging industry’s requirements for the manufacturing precision of pulp molding products are not very high, empirical design methods can still meet the requirements. To achieve higher manufacturing precision, improve mold design methods, and summarize a calculation formula for the reserved amount of the mold with universal application value and high precision through long-term testing and theoretical derivation (which cannot be published for understandable reasons). This formula takes into account not only the structure and shape constraints but also the coefficients of the pulp type, forming conditions, and drying methods.
(2) Pulp (Plant Fiber) Molding Shaping Mold: The shaping mold is used after the wet blank is formed to directly enter a mold with heating, pressing, and dewatering functions. The shaping mold is designed to dry and shape the wet blank inside the mold, producing products with a smooth surface, accurate dimensions, solidity, and good rigidity. Disposable fast-food containers are all produced using this type of mold. For industrial inner packaging, small, precise, and large-quantity small items are often stacked layer by layer, with each layer serving as a positioning packaging product. If using pulp molding products, the shaping mold must be used to produce them. Generally, industrial inner packaging products work on a single side and do not require heat setting; they can be dried directly. The shaping mold structure includes a convex mold, a concave mold, a screen mold, and heating elements, with drainage and exhaust holes on the convex or concave mold with the screen mold. When working, the wet blank first undergoes pressing in the shaping mold, with 20% of the water being squeezed out. At this point, the moisture content in the wet blank is 50-55%. The remaining water is then evaporated and removed after the wet blank is heated in the mold, forming the product after being pressed, dried, and shaped.
The screen mold in the shaping mold will leave web marks on the product surface and can quickly break down due to frequent pressing. To address this issue, a web-free shaping mold has been designed, which is made of copper-based spherical powder metallurgy. Over the past two years, through multiple structural improvements and the selection of appropriate powder particle sizes, the web-free shaping mold has a lifespan 10 times that of the web mold, with a cost reduction of 50%. The paper products produced using this mold have high precision and a smooth surface on both the inside and outside.
(3) Pulp Molding Finishing Mold: After the wet blank is dried, it may undergo deformation, with some parts severely deformed or when high precision is required for the product’s external shape, the product undergoes a shaping process using a finishing mold. This mold also requires heating elements but does not necessarily need a screen mold. The products that require shaping should retain a moisture content of 25-30% during drying to facilitate the shaping process. In practical production, it is difficult to control the moisture content, making it challenging for the products to meet quality standards. I have designed a spray finishing mold with spray holes corresponding to the areas that need shaping. When in operation, the product is placed in the finishing mold after thorough drying, and during closing of the mold, the spray holes on the mold spray hot mist onto the product, similar to the spray iron used in the garment industry.
(4) Cutting Mold: The cutting mold is used to trim the rough edges of the product, also known as the edge trimming mold.
Pulp (plant fiber) moldable mold material
The cutting dies, excluding the cutting blades, are typically made of ordinary carbon steel; the shaping molds and forming molds are generally made of brass, aluminum alloy, and stainless steel.
Forming molds are currently manufactured using brass, aluminum alloy, and resin. When used for industrial packaging products with low precision requirements, they can be directly produced by casting methods, while for molds with high precision requirements, mechanical cutting and machining methods must be used. The transfer mold in the forming mold is best made of resin, directly inverted from the corresponding male or female mold, offering high precision. If there is a paper sample available, a low-melting-point alloy can be used to directly flip the mold using the sample as the master mold.
The screen mold is a key mold in pulp moldmaking, directly affecting the quality of the products and production efficiency.
Screen molds use screens which can be either metal or plastic. Metal screens are further divided into brass screens, stainless steel screens, and chrome-plated screens, with wire diameters ranging from 0.15 to 0.25mm. According to the weaving method, screens are categorized into single warp, three warp, twisted weave, and double-layer weave, etc. The geometric shapes of the screen holes include square, rectangular, and hexagonal. The mesh count is generally between 40 to 65 threads per inch.
Mold transition materials. Transition materials are used to make the master mold for initial shaping or for flipping molds using samples. Transition materials include wood, molding sand, paraffin, silicone rubber, gypsum, resin, and others.
5 Technological advancements in mold making have greatly expanded the applications of pulp molding——paper seedling trays
A type of cardboard with regular grooves can be used in a machine that can be called an “indoor desktop seeder.” In the grooves of the cardboard, plant seeds are sown, and then fertilizer and herbicide are added. Another type of paper is used to laminate it, creating a seed planting paper with a certain width. Before use, this planting paper is cut into strips, which are then buried in the soil in rows. Regular and appropriate watering is applied, and the water permeates through the soil to moisten the planting paper. The water dissolves the fertilizer and herbicide, allowing them to seep into the soil. With moisture, the seeds begin to germinate, and the young sprouts penetrate the moistened planting paper to emerge from the soil. The remaining planting paper degrades in the soil, becoming fertilizer. After the planting paper is buried in the soil, there is nothing left to do but wait for the harvest.
