1.Design Strategies for Different "Structures"
Single-layer vs. Multi-layer Co-extrusion:
Single-layer: The runner design is relatively simple, with the core goal of ensuring uniform material distribution.
Multi-layer (such as A/B/A structure): Multiple sets of independent runner systems need to be designed to converge within the die head. The key lies in precisely controlling the flow rate matching of each layer's runner and the stability of the convergence interface to prevent interlayer disturbance.
Arrangement and Density of Ribs (Hollow Columns):
This is the essence of hollow board mold design. The distribution plate at the end of the runner and the sizing mold jointly determine the rib structure.
When ribs are dense or the structure is complex, it requires more balanced melt pressure and better fluidity in the flow to this area. It may be necessary to perform pre-compensation in the upstream runner or install regulating valves (throttle blocks) in the manifold before the die lip to fine-tune the local flow rate.
2.Design Strategies for Different "Specifications"
Sheet Width:
For wider sheets, the design of the runner (especially the hanger-shaped section) spread angle and straight section (calibration section) length becomes more critical to ensure consistent pressure drop and flow rate from the center to both ends of the melt. Wide-width molds often adopt a "dual-inlet" or "multi-inlet" design to shorten the melt flow path.
Sheet Thickness:
An increase in thickness is mainly achieved by increasing the die lip gap. However, the overall depth of the runner and the length of the calibration section also need to be adjusted accordingly to maintain appropriate extrusion pressure and shear rate, avoiding uneven discharge or thermal decomposition.
3.General Design Guidelines and Considerations
Streamlined Transitions: The inner walls of the runner must be smooth without any dead corners to prevent melt retention and decomposition.
Symmetrical Design: Symmetrical runners with central feeding (such as T-shaped and hanger-shaped) are usually adopted, which is the basis for achieving uniform distribution.
Pressure and Temperature Balance: The design goal is to ensure that the pressure, temperature, and flow rate of the melt are as consistent as possible at each point when it reaches the die lip.
Adjustability: High-quality molds are equipped with precision fine-tuning bolts (die lip adjustment blocks) at the die lip, which are used to micro-adjust the discharge gap online and correct minor thickness unevenness.
Cooling and Calibration Matching: The runner design must be closely coordinated with subsequent sizing molds (cooling plates, rollers, vacuum calibration tables). A stable slab is the foundation, but the final sheet thickness, flatness, and rib fullness are determined by cooling and calibration.