Process Principle and Workflow Characteristics
The vacuum thermoforming process relies primarily on negative pressure adsorption. Stock concentration is typically maintained at around 1%. Deviations from this concentration, whether higher or lower, can affect fiber distribution uniformity.
After the mold is immersed in the slurry, the vacuum system activates, creating a negative pressure environment within the mold cavity. Fibers migrate and accumulate on the forming wire surface due to the pressure differential. Stable control of the vacuum degree, generally maintained at around 520 Pa, is crucial during this phase.
The dewatering process involves two main stages: initial vacuum dewatering and subsequent mechanical pressing. Once the fiber deposition reaches the predetermined thickness, the mold is removed from the vat. The upper and lower molds close for pressing dewatering. At this stage, compressed air pressure of approximately 0.2 MPa is applied to reduce the product's moisture content to the range of 65%-75%. Finally, the demolding operation is completed using the pneumatic system.
1--Lifting cylinder
2--Mobile platform
3--Upper mold
4--Lower mold
5--Air tube
6--Overflow tank
7--Molded paper pulp storage tank
8--Bottom plate
9--Upper plate
10--Moving guidepost
Key Control Parameters
Fiber deposition speed is influenced by multiple factors. These include the type of fiber raw material, the degree of refining, the level of vacuum, and the stock temperature.
Production cycle times vary significantly based on product structure. Shallow, thin-walled products like egg trays or fruit pads typically require only 10-15 seconds forming time.
Deep-drawn, thick-walled products necessitate correspondingly longer forming cycles. The specific required time must be determined through process testing.
Stock temperature directly impacts fiber dispersion. It is usually controlled within the range of 35-45°C.
Vacuum stability plays a decisive role in achieving product uniformity.
Equipment Operational Characteristics
The vacuum system must maintain a consistently stable negative pressure environment. In practice, vacuum pump sets often use a parallel configuration. This ensures production continues uninterrupted if a single unit fails.
The air pressure system primarily handles two functions: mold clamping/pressing and demolding. Pressure settings need precise calculation based on the product's projected area.
In mold design, the mesh count selection for the forming wire is critical. Excessively dense mesh impedes dewatering efficiency, while overly sparse mesh leads to fiber loss. Typically, forming wires in the range of 80-120 mesh are selected based on the fiber length distribution.
Product Applicability Analysis
This method is particularly suitable for manufacturing products with relatively shallow depths and uniform wall thickness. Examples include everyday item packaging like egg trays, fruit cushioning pads, and plate liners. These products share common characteristics: simple structure, large draft angles, and no need for secondary processing.
For deep-cavity structural products, special attention must be paid to fiber deposition uniformity. In practice, methods like intermittent vacuum application or adjusting stock flow rate are often used to improve fiber coverage in corner areas.
Quality Control Focus Points
Product thickness uniformity is a core quality indicator. Online inspection often uses non-contact thickness gauges for real-time monitoring.
Moisture content control directly affects subsequent drying energy consumption. It is generally managed by adjusting pressing pressure and dwell time.
Structural integrity must be ensured during product demolding. For complex structures, increasing compressed air pressure to around 0.25 MPa might be appropriate. Maintaining the mold surface finish is also important, requiring regular acid cleaning to remove fiber scale.
Production Efficiency Optimization
Forming time is a major factor affecting production capacity. Optimizing the vacuum pipeline layout can reduce vacuum drawing time by approximately 15%. Adopting dual-station alternating operations can effectively improve equipment utilization.
An automatic stock concentration compensation system helps maintain consistent concentration during production. Currently, more advanced equipment utilizes PLC-based full-automation control. Operators only need to set the process parameters to complete the entire production cycle.