Extrusion product knowledge shows that the color of the extruded product has spots or yellowing
Q: Can you describe the most common sources for extruded product visual defects, such as black specks or gels, and what can be done to address them?
A: Degradation happens in slow-moving areas of the head and die, especially if the die is left full and hot for a long period of time. Resin with less thermal stability (less antioxidant) will degrade sooner. Thermal stability can be tested but is seldom part of a purchase spec. It is reduced if substantial regrind is used in the mix, but more antioxidant can be added as a concentrate, like colorants.
Drool on the external lip edges may be a cause, too, especially with blown film, where the die extrudes upward and the surface is horizontal. Process aids and air directed at the emerging line will reduce drool.
Contaminated feed can introduce just about anything, but much can be caught on the screens, depending on the mesh used. For very fine filtration, sintered metal fiber screens are used. Screening will help with anything coming out of the screw, but not with what forms in the die.
True gels are cross-linked polymers in an early stage of overheat reaction—clear if the product is clear but maybe yellowed, and insoluble in solvents that dissolve the unreacted polymer. They may form anywhere, even in the screw, and get through the screens, where they may break up into gel "showers."
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