Modern pulp packaging generally uses materials such as paper, plastic, glass, ceramics, and metal. Among these, paper and plastic are widely used due to their low cost, ease of processing, and abundant raw material sources. However, plastic packaging materials are not biodegradable and often cause environmental pollution—China’s white pollution problem is a typical example.
With the development of society and the demands of global economic integration, green packaging has gradually become a social consensus and a new highlight in the development of China’s packaging industry. According to evaluations by international authoritative experts, paper is the most promising green packaging material. In recent years, the development of pulp molding equipment in China has been rapid and is gradually becoming more mature. As the application of pulp molded products continues to expand, the requirements for these products are increasing. This calls for ongoing improvements in the pulp molding process and mold design. This article analyzes the current state of pulp molded products based on their process characteristics and explores key elements of mold design for pulp molding.
Analysis of the Pulp Molding Process
1. Process Flow
The production process of pulp molded products varies depending on customer requirements but generally includes slurry preparation, forming, hot pressing, shaping, trimming, and packaging.
2. Raw Material Selection
Pulp moulded products are typically made from wood pulp, grass pulp, or reed pulp. Bleached commercial pulp is used for fast-food containers, while recycled paper is used for industrial packaging. These products do not cause secondary pollution during production or use and offer advantages such as lightweight, compressive strength, shock resistance, anti-static properties, breathability, and recyclability. As packaging materials, they can replace EPS foam and are used for disposable tableware, electronics, crafts, medical instruments, mechanical parts, eggs, fruits, flowers, glassware, ceramics, and other fragile items.
Pulp packaging is generally classified into fast-food containers and industrial packaging. Though the production methods are similar, fast-food containers use bleached pulp and have stricter specifications. Sterilization, waterproofing, and oil-proofing are required. Equipment and pipes are usually made of stainless steel or plastic, and high hygiene standards are expected in factories and among workers.
3. Pulping
This is the first step and includes breaking down pulp boards into fiber slurry using a hydropulper, followed by screening, sand removal, and refining to improve fiber bonding strength. The degree of refining greatly affects molding and pulp moulded product quality. Over-refined pulp causes difficulties in water filtration and shrinkage, while under-refined pulp weakens bonding and structural strength.
Main equipment includes hydropulper, deflaker, cleaner, refiner, and pulp tanks. Small-scale production may only require a hydropulper and tank.
4. Slurry Preparation
In this step, functional additives such as fillers, pigments, waterproofing, and oil-proofing agents are added. Water level and additive ratios in storage and mixing tanks must be carefully controlled. Common waterproofing agents include rosin, ferric sulfate, and cationic starch.
5. Forming
Forming is typically done via vacuum suction, where fiber slurry is shaped on a mesh mold and dewatered. There are two methods: injection and suction. Injection is often used for tableware, while suction molding is becoming mainstream for packaging.
Process: mold descends into slurry → vacuum draws slurry → mold lifts → compression dewatering → transfer and air blow → demolding.
Controlling suction time affects product thickness. Ideally, thickness should be uniform, but differences in slurry flow and gravity can cause variation.
6. Shaping (Hot Pressing)
This step involves heating, pressing, and drying wet pulp to achieve final shape. Shaping molds provide a smooth surface, accurate dimensions, and solid rigidity—commonly used in disposable tableware. Some industrial packaging with complex shapes requires shaping; simpler products are directly dried.
7. Trimming
After drying, molded products may deform. For items needing high shape precision, trimming is essential. Moisture content must be controlled (20–30%). High moisture increases energy/time consumption; low moisture makes shaping difficult. Hot pressing is done at 150–180°C, not below 135°C.
8. Auxiliary Processes
Includes trimming, sterilization, packaging, and inspection. Trimming removes burrs and imprints logos, text, or folding lines. Sterilization for food packaging is done via ozone tunnels, UV light, or high-temperature drying. After inspection, products are stacked, counted, and packaged.
Structural Design of Pulp Molded Products
Structural Features
Unlike traditional foam plastics that rely on elasticity for cushioning, pulp molded products depend on structure. Therefore, structural design is critical.
They have strength, rigidity, and toughness, and can be shaped to fit various needs. Simple shapes are sufficient for tableware, while industrial packaging demands complex designs that meet:
- Load-bearing: Must support the product and external pressure.
- Positioning: Securely fix items in place.
- Cushioning: Prevent damage during transport via soft contact surfaces and flexibility.
Design Considerations
Design is currently based on experience and must consider product shape, packaging dimensions, and cushioning needs. Ribs and raised patterns help strengthen structure but may reduce cushioning.
One side of the molded product (contacting the mold) is smooth, and the other is rough. Smooth sides must fit the product’s surface; rough sides must support packaging. Typical thickness is 2–5mm. Uniform fiber flow is essential to prevent fiber clumping during forming, especially for deep vertical designs, which require a 2–5° draft angle and rounded corners to avoid stress concentration.
Edge structures—plain, flanged, or double-layered—also affect performance. Flanged or double edges are used to improve strength and aesthetics in industrial packaging.
As a green, recyclable, and biodegradable packaging material, pulp molded products are becoming increasingly important in the packaging industry. With the continuous expansion of applications, the performance requirements for these products are also rising, placing higher demands on pulp molding technology and mold design.
Through better understanding of the pulp molding process and the key factors in mold design, manufacturers can produce higher-quality, more efficient, and environmentally friendly products. Future developments in mold materials, design automation, and intelligent manufacturing will further improve the competitiveness of pulp molded packaging in both domestic and international markets.
