Rheological Variance and Gel Contamination in House Blended Compounds
House blending recycled resins causes melt viscosity drift and crosslinked gel contamination that disrupts mold filling and degrades part mechanical strength.
Screen Pack Filtration
Meaning
The mechanical separation of solid contaminants from a molten polymer stream is accomplished by forcing the melt through a series of wire mesh screens before it enters the die. Implementing screen pack filtration prevents unmelted resin pellets, degraded gel particles, and foreign debris from ruining the surface finish of extruded films or clogging fine die orifices. This filtration assembly sits inside the extruder barrel just before the breaker plate, which provides mechanical support for the flexible wire meshes.
The configuration of the screen pack can be adjusted by combining screens of different mesh sizes to target specific particle diameters. This custom arrangement allows the extruder to run smoothly even when processing highly contaminated materials.
Extrusion Protection
Coarse screens protect the finer downstream meshes from being overwhelmed by large debris, while the finest screen determines the ultimate filtration level. Utilizing screen pack filtration is necessary when running recycled resins that contain high levels of paper, wood, or metal fragments. Without this protective barrier, foreign particles would scratch the highly polished surfaces of the extrusion die, resulting in permanent die lines on the extruded sheet or film.
Pressure Dynamics
As contaminants accumulate on the screens, the resistance to melt flow increases, causing a steady rise in head pressure. This pressure increase across the screen pack filtration unit must be monitored to prevent mechanical damage to the extruder components. High pressures can also cause the polymer melt to overheat through viscous shear heating, leading to thermal degradation of the resin.
Operators must change the screen pack before the pressure reaches a critical limit.
Contaminant Separation
Choosing the right mesh sequence requires balancing the required cleanliness of the melt against the acceptable cycle time between screen changes. Fine filtration using high-mesh screens captures tiny gel particles that would otherwise cause cosmetic defects in thin films. However, these fine meshes clog quickly, requiring frequent production interruptions or the use of expensive continuous screen changers.
The selected configuration must match the contaminant load of the incoming raw material.