Screen Changer for BOPP Film Extrusion: Process Requirements and the Continuous Filtration Standard

BOPP — Biaxially Oriented Polypropylene — film is produced by extruding polypropylene through a flat die, then stretching the solidified film sequentially in the machine direction (MDO) and the transverse direction (TDO). Typical stretching ratios reach approximately 5× in MD and up to 9× in TD.

This biaxial orientation process dramatically amplifies any defect present in the melt. A contamination particle or gel that would be invisible in a thick section becomes a pinhole, fisheye, or tear-initiation point once the film is stretched to 15–60 µm. Oxidized particles, gels and foreign inclusions are the primary quality failure modes in oriented film, and all of them are linked to melt filtration performance.

Beyond particle removal, BOPP imposes a second and equally critical filtration requirement: pressure stability. The flat die, the casting section, and the entire downstream stretching train operate within tightly controlled process windows. Any melt pressure variation upstream — including the transient generated by a manual screen change — will disturb this balance with consequences that are difficult or impossible to correct downstream.

BOPP lines are typically co-extrusion systems, with multiple extruders feeding a common flat die. Each co-extruder — handling the core layer or a skin layer — operates at its own throughput. The screen changer on each co-extruder must be sized for this throughput and must meet four specific requirements.

Screen cleaning must occur without stopping extrusion and with minimal pressure disturbance. Every manual screen change requires a line stop, during which polypropylene residence time accumulates and gel formation risk increases. A continuous and self-cleaning screen changer removes these stops: while one cartridge keeps filtering, the other is cleaned by backflush, and melt flow is maintained.

Standard woven wire mesh provides adequate filtration for many applications. In BOPP, where gel removal is critical for optical quality, metal nonwoven screens offer significantly higher gel capture efficiency. However, nonwoven media generate higher back-pressure per unit area. A screen changer with a small filtration surface cannot accommodate nonwoven screens without exceeding operating pressure limits. A large filtration area makes nonwoven screens viable — and with them, the level of melt cleanliness that BOPP quality standards demand.

Any hold-up volume in the screen changer body is a zone where polypropylene can stagnate, overheat, and degrade. In a PP process already operating at or near oxidation limits, this risk is not theoretical — it is a direct source of the gels and black specks that filtration is designed to intercept. Screen changer geometry matters as much as filtration fineness.

The cleaning cycle must occur with pressure variation contained within the process tolerance of the line. In practice, the AP Series keeps pressure disturbance minimal during the cleaning cycle.

Brückner Maschinenbau, headquartered in Siegsdorf, Germany, is one of the world’s leading manufacturers of biaxial stretching lines for plastic film. Their BOPP lines are used by film producers globally and are recognised as a benchmark for high-output, precision-oriented film production.

Brückner BOPP lines are multi-layer co-extrusion systems. Each co-extruder — handling core or skin layers — operates at the throughput of a typical co-extruder position. At this output, a pressure transient during screen change propagates through the entire coextrusion stack and into the stretching section. The consequences are gauge deviation across layers, optical defects in the finished film, and — in the worst case — web breaks that require full line restart.

The combination of requirements — fine gel removal, continuous operation, minimal pressure variation, and no stagnant zones for a thermally sensitive polymer — defined a demanding specification for the filtration position on each co-extruder.

Brückner specified the COFIT AP08 — a model from the AP Series of continuous and self-cleaning double cartridge screen changers — for the co-extruder filtration positions on their BOPP lines. While one cartridge filters, the other is cleaned by backflush, activated by the operator with a key, without interrupting melt flow.

The AP Series’ large filtration area makes metal nonwoven screen media viable. This is a distinguishing feature: screen changers with smaller filtration surfaces would generate excessive back-pressure with nonwoven media and are usually limited to woven mesh. Nonwoven screens provide the gel removal performance that BOPP film quality requires.

Over more than 20 years, Brückner has integrated more than 600 AP08 units across their BOPP line production. This scale of adoption reflects a filtration solution that consistently meets the process stability and quality requirements that BOPP film producers expect from a Brückner line.

For Cofit, the Brückner relationship represents the most significant independent validation of the AP Series in demanding biaxially oriented film applications.

The COFIT AP Series is a continuous and self-cleaning double cartridge screen changer designed for quality-sensitive extrusion applications operating with clean or semi-clean polymer melts. BOPP is one of its primary target applications — alongside cast film, stretch film, fiber, and extrusion coating.

In the AP Series, while one cartridge filters, the other is cleaned by backflush. The large filtration area on each cartridge enables the use of metal nonwoven filter media, providing gel removal performance that standard woven mesh cannot match. Filtration fineness extends down to 3 µm, depending on the screen media selected. Other key AP Series data: over 500 bar maximum operating pressure, up to 350 °C, 5–2,000 kg/h, no hydraulic control unit, and up to 90% downtime reduction vs manual screen changers on lines running more than 3 changes per shift.

For BOPP co-extruder positions, the AP08 model provides the appropriate combination of filtration area, throughput capacity, and dimensional integration with the extruder output flange. The same AP Series architecture scales across a range of extruder sizes for co-extruder positions on BOPP and BOPET lines.

The validation provided by more than 600 units installed on Brückner BOPP lines over more than 20 years is the most direct indicator of fit for this application.

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