Polymer melt filter optimisation

In polymer processing, purity is everything. Even small amounts of contamination in the melt can cause defects, nonuniform mechanical properties or early equipment wear, whether you're producing fibres, films or moulded products. 

Achieving consistent melt purity requires a robust polymer melt filter. It must effectively remove particles like inorganic matter, such as abraded metal and dust, false agglomerates and unreacted process chemicals that can form gels. The filter must also capture degraded polymer from process dead zones, without affecting flow rate or thermal stability. 

For decades, polymer manufacturers have used stainless-steel media as their filtration media. Its mechanical integrity, high temperature resistance and cleanability deliver long-term value and support operational efficiency. We explain why stainless-steel media is the industry standard and show how different filtration formats maintain melt consistency in challenging conditions. 

Understanding contaminant challenges in polymer melt

Contaminants such as abrasion from extruders, residual catalyst, degraded polymers, other foreign particles and unreacted materials are common in polymer melts. These particles can compromise both product and process by: 

• Reducing optical clarity and surface finish in films and fibres 
• Causing irregular tensile strength spots, which affect stretching 
• Leading to unplanned downtime and equipment maintenance 

However, waste takes many forms throughout the polymer production process, from converting raw material to creating end products such as films, fibres or specialist compounds. 

Process waste includes by-products or residues generated during chemical and mechanical processing, such as: 

• Only partially polymerised chains and even unreacted monomers (e.g., caprolactam in nylon production) 
• Side reaction products (e.g., oligomers, tars) 
• Spent catalysts or solvents 

If the system doesn't efficiently remove these through proper filter media and design, they contribute to further inefficiencies, most notably, off-spec batches that must be reprocessed or discarded. As a result, operators use more raw material, consume more energy and lose valuable production time. 

In high-temperature applications, melt temperatures can reach over 400 °C. When flow volumes are substantial, filters must offer thermal stability and sufficient porosity to maintain throughput without excessive pressure drop, while protecting the process from unnecessary waste. 

Filtration’s role in process efficiency

In polymer production, waste isn’t limited to contaminants in the melt. Unreacted monomers, side products and spent catalysts all contribute to process inefficiencies. But beyond material waste, downtime itself is a significant cost driver.  

Every minute the system isn’t running means lost productivity, often outweighing routine utility or disposal costs. A well-designed polymer melt filter becomes a key enabler of uptime, helping extend the time between cleanings and reduce the risk of off-spec batches. 

Why stainless-steel media is the industry standard

Stainless-steel media offers a unique blend of performance properties that make it ideal for demanding polymer filtration applications: 

• Mechanical strength: Withstands high pressures and retains structural integrity under stress 
• Corrosion and temperature resistance: Performs reliably in chemically aggressive and high-heat environments 
• Fine particle filtration: Customised pore sizes to filter a wide range of contaminants 
• Long service life: Can be cleaned and restored multiple times, reducing waste and cost 
• Easy maintenance: Smooth surface simplifies cleaning procedures 
• Environmental and economic value: Long service life and lower replacement frequency 

Because a polymer melt filter's soft impurity retention efficiency depends heavily on minimising pressure drop while maintaining high throughput, only highly porous and consistent filter media can deliver the necessary performance. Stainless steel meets these demands better than alternatives. 

Types of stainless-steel filter media: surface filtration options

Surface filtration is the primary mechanism in stainless-steel wire media, where particles are retained on the filter’s surface rather than penetrating it. Polymer melt filters commonly use these stainless-steel mesh types, especially in cylindrical designs known as polymer candle filters. These filters provide a clear cutoff and controlled shearing, including screening capabilities for any additives that tend to agglomerate.  

Different media structures are suited to different applications: 

• Square media: Uses identical warp and weft wire diameters. A basic over-under weave pattern makes this a simple, reliable format for general-purpose filtration. 
• Dutch twilled weave: Features smaller diameter weft wires and combination of different diameters and weaving types and a denser pattern. This design allows finer  pore sizes with increased stability and improved flow rates, which is ideal for finer filtration. 
• Special media: Combines different wire diameters and geometries to optimise flow and mechanical strengths in specific applications. 

These media types typically capture hard particles larger than the media openings. However, they have limitations when it comes to flexible or shearing-sensitive contaminants. As melt flows through the media, high shear forces may deform or fragment soft and brittle contaminants, allowing them to pass through the filter media, even if they are technically larger than the nominal pore size. Reagglomeration of sheared particles has been observed in multiple applications. 

This makes it essential to select the appropriate polymer melt filter media design based on the physical properties of the contaminants and the operating requirements of the polymer process. 

Partnering with the right experts

Maintaining melt purity is essential for consistent product quality and process reliability. The stainless-steel media used in a polymer melt filter provides the durability and thermal resistance needed in high-flow, high-temperature polymer environments, but material alone isn’t enough. Whether using a flat disc or a polymer candle filter, selecting the right design, filter media combination, vessel setup and pore structure is key to maintaining performance.

John Crane’s Seebach® large area filter, replaceable filter element; metal fibre fleece media 

That’s where expert support matters. From identifying the ideal pore size and media structure to optimising filter life and simplifying filter cleaning, John Crane can help ensure your filtration system delivers long-term performance and minimal waste. 

Read part two of this blog series to explore how to tailor media performance through pore size, lamination and dwell time. Learn more about our polymer filtration expertise at: 
https://www.johncrane.com/en/industries/polymers