Background
The production of optical polymer films requires stringent control of melt cleanliness due to the high sensitivity of these materials to localized defects. Gel-related imperfections, originating from crosslinked or highly entangled polymer domains, unmelted resin fragments, or foreign contamination, are particularly critical as they can degrade optical performance and product yield. When such inclusions are exposed to elevated shear stresses in downstream extrusion zones, they may undergo deformation and elongation, resulting in so-called gel-shearing defects that extend in the machine direction and are more visually disruptive than isolated gel particles. While gel formation mechanisms in polymer processing are well described, the influence of melt filter media material characteristics on the occurrence and morphology of gel-shearing defects has received limited attention. In high-throughput, high-temperature film extrusion, the filter media represents a key interface controlling both particle retention and the mechanical integrity of retained gel populations prior to exposure to high shear. This is of particular importance for the development of thinner films (e.g. ≤4 µm).

Aim
The aim of this study is to evaluate the impact of melt filter media material and structural design on the mitigation of gel-shearing defects in optical polymer film extrusion. The work focuses on correlating filter media properties such as material composition, pore structure, and mechanical stability with gel retention performance, pressure drop behavior, and downstream film defect characteristics under representative processing conditions.

Method
Filter media panels were designed and manufactured with stainless steel (316L) fibers and evaluated. Filtration trials were conducted on real filter production equipment designed for polymer film extrusion (polymer family: bopet) at melt temperatures up to 280 °C and pressure differentials up to 60-80 bar. Differential pressure across the filter media was continuously monitored to assess fouling behavior, filtration stability, mechanical robustness under sustained load and cleanability up to 10 times.

Film quality downstream of the filtration unit was assessed using optical inspection, and inline defect detection technique, with specific attention to gel frequency (how many gel numbers in 100 m²), gel size distribution and gel shearing. Filter media deformation resistance and structural integrity were evaluated after sustained operation and, where applicable, repeated filtration and cleaning cycles. Performance was benchmarked against industry standard filter media (e.g. made by powder and fiber) tested under identical operating conditions.

Main Results
The paper will present quantitative results describing the relationship between filter media material properties and gel-shearing defect occurrence. Key outcomes will include comparative pressure drop stability, gel retention efficiency, reduction of shear‑-elongated defects in the optical film, and filter media durability under prolonged high‑-shear‑ operation. The influence of filter media structure on the balance between defect mitigation and operational stability is discussed. The cleanability up to 10 times was also assessed.