As a mainstream representative of green packaging, pulp molding is favored by brand owners. In the production process of pulp molded products, molds, as core components, have high technical requirements, large investment, long cycle and high risk in research and development design. So, what are the key points and precautions in the design of pulp molding molds? The following will share experience from the perspective of packaging structure design, taking you to explore the design logic of pulp molding molds.

I. Forming Molds
Structural Composition: Consists of punch, die, screen mold, mold base, mold back cavity and air chamber, among which the screen mold is the main body of the mold. Since the screen mold is woven from metal or plastic wires with a diameter of 0.15-0.25mm, it cannot be formed independently and needs to be attached to the surface of the mold for collaborative work.

Back Cavity Design: Relative to the mold base, the mold back cavity is a cavity with a specific thickness and shape that is completely synchronized with the working surface of the mold. The punch and die are shells with a certain wall thickness, and the working surface of the mold is connected to the back cavity through uniformly distributed small holes.

Installation and Working Principle: The mold is installed on the template of the molding machine through the mold base, and an air chamber is installed on the other side of the template. The air chamber is connected to the back cavity and is provided with two channels for compressed air and vacuum.

II. Shaping Molds
Shaping molds are molds that directly process the formed wet paper blanks and have the functions of heating, pressurizing and dehydrating. Products manufactured with shaping molds have smooth surfaces, precise dimensions, firmness and good rigidity. Disposable tableware is mostly produced with this kind of mold; in industrial packaging, if pulp molded products are used for some small, precise and multi-layered positioning packaging parts of small items, they also need to be manufactured through shaping molds.

Structure and Process: Shaping molds include punch, die, screen mold and heating elements. The punch or die with screen mold is provided with drainage and exhaust holes. During operation, the wet paper blank is first pressed inside the shaping mold to squeeze out 20% of the water (at this time, the water content of the wet paper blank is 50-55%), and then the remaining water is vaporized and discharged after heating inside the mold. The wet paper blank is pressed, dried and shaped to form the product.

Innovative Design: The screen mold of the traditional shaping mold is easy to be damaged during frequent extrusion and will leave screen marks on the surface of the product. Designers have developed screen-free molds through copper-based spherical powder metallurgy technology. After two years of structural optimization and powder particle size screening, the service life of the produced screen-free shaping molds is 10 times that of the screen molds, the cost is reduced by 50%, and the produced paper products have high precision and smooth inner and outer surfaces.

III. Hot Pressing Molds
If the wet paper blank is deformed after drying, or the appearance precision of the product is required to be high, it needs to be shaped by hot pressing molds. Although this kind of mold needs heating elements, it does not need screen molds. The products to be shaped should retain a moisture content of 25-30% during drying to facilitate shaping.

Technical Optimization: In actual production, the moisture content is difficult to control, so a manufacturer has designed a spray hot pressing mold, and spray holes are arranged on the mold corresponding to the parts that need shaping. After the dried products are put into the shaping mold, spray hot pressing is carried out on the products through the spray holes, which is similar to the spray iron in the clothing industry in principle.

IV. Transfer Molds
Transfer molds are the last workstation of the entire process, and their main function is to safely transfer the products from the integral auxiliary mold to the receiving tray. For transfer molds, their structural design should be as simple as possible, with evenly distributed suction holes to ensure that the products can be smoothly adsorbed on the mold surface.

V. Trimming Molds
In order to make pulp molded products clean and beautiful, products with high appearance requirements are provided with trimming processes. Trimming molds are used to trim the rough edges of pulp molded products, also known as edge cutting molds.

Summary of Design Core Key Points

Material Selection: Select wear-resistant and good thermal conductivity materials (such as copper-based alloys, stainless steel) according to the use of products. The powder metallurgy technology of screen-free molds can improve the service life.

Structural Optimization: Pay attention to the connection design between the working surface of the mold and the back cavity, the drainage and exhaust efficiency. Hot pressing molds need to balance the heating uniformity and moisture content control.

Process Adaptation: Shaping and hot pressing molds need to match the drying process parameters. Transfer molds need to ensure the balance of adsorption force, and trimming molds need to ensure the accuracy of the cutting edge.

Cost Control: Reduce the development and maintenance costs of molds through modular design and material substitution (such as powder metallurgy).

The design of pulp molding molds needs to balance functionality, durability and economy, and innovative technologies such as screen-free molds and spray hot pressing are promoting the industry to develop towards high efficiency and precision.