In-Mold Heating and Drying Technology for Pulp Molding | Process & Methods

2. In-Mold Heating and Drying

In the wet pressing process of pulp molding, in-mold drying utilizes high-temperature molds to rapidly dry and shape wet pulp preforms within a short time. This method is mainly used for cushioning packaging products and premium tableware requiring high dimensional accuracy and consistent weight, such as mobile phones, electronics, alcoholic beverages, gifts, etc. Currently, in-mold hot-press drying serves as the primary drying method for pulp tableware and high-end industrial packaging products. Most equipment adopts direct in-mold heating – when the wet preform enters the cold pressing section, it gets compressed between upper and lower molds to mechanically remove moisture and densify the structure. Subsequently, the preform is transferred to hot-press molds for thermal drying. The closing pressure, heating temperature, and duration can be precisely controlled according to product specifications, producing items with smooth surfaces and superior physical properties (density, stiffness, strength) compared to tunnel drying. This integrated process combines drying and shaping into one step.

Drying Process Stages

In pulp molding production, a pulp suspension below 3% concentration forms wet preforms (70-80% moisture content) through vacuum filtration. Final products require over 90% dryness to meet mechanical strength requirements. The in-mold hot-press drying process involves three stages:

(1) Water Extraction Stage

The high-moisture preform containing free water and bound water undergoes compression under high temperature/pressure. Softened fibers release most free water as hot liquid through mechanical pressure, reducing moisture content to approximately 35%.

(2) Drying Stage

Bound water within fiber cell walls and surface-adhered moisture require thermal energy to evaporate as steam. Initial pressure removes large inter-fiber pore water without structural changes. Subsequent temperature/pressure increases eliminate capillary water, compressing fibers to enable molecular bonding.

(3) Plasticization Stage

With moisture content below 10%, lignin melts under high temperature/pressure to create hydrogen bonds between fibers. Precise control of temperature, time, and pressure is critical: insufficient drying causes blistering and weak strength; low temperatures prolong drying; excessive heat degrades fibers and reduces whiteness.

Heating Methods

(1) Electric Heating

Electric heating elements embedded in plates transfer heat to molds. Advantages include cleanliness, reliability, and easy control, but drawbacks involve high energy consumption and technical challenges:

1. Deep drilling requirements for heating plates
2. Air gaps reducing thermal conductivity
3. Steam corrosion on electrical components
4. Slow heating response and low efficiency

Normally,it shows a vacuum-assisted electric heating system demonstrating fast heat transfer and 90% constant-rate drying phase efficiency.

(2) Steam Heating

Saturated steam circulates through mold channels. While providing good product finish, this method suffers from:

1. Pipeline corrosion risks
2. Steam entrapment reducing efficiency
3. Temperature limitations (~200°C max)
4. Significant thermal energy waste

(3) Thermal Oil Heating

Circulating thermal oil offers advantages:

• Closed-loop system with minimal heat loss
• Low-pressure operation and uniform heating
• Customizable pipeline layout for mold contours

Challenges include complex mold manufacturing and strict sealing requirements.

(4) Thermal Superconductivity Technology

This innovative approach uses high-frequency molecular friction in sealed media for ultra-fast heat transfer (7,000× faster than silver). Benefits include:

• Near-zero thermal resistance
• 20-30% energy savings vs electric heating
• Balanced surface temperature
• Extended component lifespan (100,000+ hours)

By eliminating internal/external temperature differentials, this technology addresses traditional heating limitations while improving product quality and energy efficiency.