𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞 : 𝐓𝐫𝐚𝐧𝐬𝐟𝐞𝐫 𝐥𝐢𝐧𝐞 𝐝𝐞𝐬𝐢𝐠𝐧 𝐢𝐬 𝐨𝐟𝐭𝐞𝐧 𝐨𝐯𝐞𝐫𝐥𝐨𝐨𝐤𝐞𝐝

𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞

𝐓𝐫𝐚𝐧𝐬𝐟𝐞𝐫 𝐥𝐢𝐧𝐞 𝐝𝐞𝐬𝐢𝐠𝐧 𝐢𝐬 𝐨𝐟𝐭𝐞𝐧 𝐨𝐯𝐞𝐫𝐥𝐨𝐨𝐤𝐞𝐝 — 𝐲𝐞𝐭 𝐢𝐭 𝐢𝐬 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐭𝐥𝐲 𝐭𝐡𝐞 𝐫𝐨𝐨𝐭 𝐜𝐚𝐮𝐬𝐞 𝐨𝐟 𝐫𝐞𝐬𝐢𝐧 𝐝𝐞𝐠𝐫𝐚𝐝𝐚𝐭𝐢𝐨𝐧 𝐚𝐧𝐝 𝐠𝐞𝐥 𝐝𝐞𝐟𝐞𝐜𝐭𝐬 𝐢𝐧 𝐞𝐱𝐭𝐫𝐮𝐝𝐞𝐝 𝐟𝐢𝐥𝐦 𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬.

In extrusion, transfer lines must balance two competing demands: minimizing pressure consumption while maintaining sufficient wall shear stress to prevent resin stagnation. A pipe that is too large reduces pressure drop but allows polymer to linger at the wall, degrading over time and appearing as gels or black specks in the final product.

Key design parameters to address both concerns:

- Target wall shear stress of 20 to 30 kPa to ensure continuous material renewal at the pipe wall

- Design the transfer line as short as possible, since pressure consumption is directly proportional to pipe length

- Use clam-shell type elbows with a bend radius at least five times the pipe diameter

- Ensure all pipes and connections are the same diameter to eliminate ledges and stagnation points

Two blown film case studies demonstrate that reducing transfer line diameter to reach the recommended shear stress range eliminated persistent gel and black speck defects without imposing unmanageable pressure consumption.

Full analysis and supporting equations are available in the complete article. https://lnkd.in/euaePEDU

source : Plastics Technology