Polymer melt filter optimisation 

Melt purity is non-negotiable in polymer processing, but achieving it consistently requires more than durable polymer melt filters or advanced filtration materials. As discussed in part 1 of this series, stainless-steel media provides a strong foundation for removing common contaminants in high-temperature, high-flow environments.  

Beyond material choice, process engineers must also consider how pore size, porosity and dwell time affect overall filter performance, especially in systems with varying viscosities, contaminant types and flow or velocity profiles. 

With application-specific configurations, precision-engineered filters optimise flow, contaminant capture and ease of cleaning. This article explores how specific design parameters help polymer manufacturers strike the right balance between performance, reliability and throughput. 

Key factors in filter media selection

Filter performance requirements vary depending on the industry, whether pharmaceuticals, chemicals, food and beverage or polymers, but the core selection criteria remain consistent. Effective polymer melt filtration requires selecting the right material and understanding how media characteristics impact performance. 

While stainless-steel mesh provides a robust foundation, engineers must also evaluate a range of performance parameters: 

• Filtration efficiency – Ability to remove a percentage of particles at specific sizes 
• Pressure drop (ΔP) – Resistance to flow through the polymer melt filter 
• Flow rate – Volume of melt processed over time 
• Dirt-holding capacity – Amount of contamination retained before clogging 
• Mechanical strength – Ability to withstand operating pressures and thermal stress 

These characteristics vary depending on the filtration material and construction, and must be carefully balanced to support throughput, product quality and filter longevity. 

The importance of pore size and porosity

Pore size directly impacts a filter’s ability to capture contaminants. Smaller pores catch finer particles but can create higher pressure drops, while larger pores preserve flow but may allow finer contaminants to pass. 

High porosity, meaning more pores per unit area, offers several advantages: 

• Enables higher flow rates with lower resistance 
• Reduces the risk of filter clogging 
• Helps maintain stable melt flow, even with challenging contaminants 

In high-temperature polymer applications, optimal pore size must account for the behaviour of both rigid and flexible particles under heat and shear, especially within a polymer melt filter, where pressure drop and flow rate are key criteria for system design. Selecting the right size ensures effective separation without deforming contaminants or compromising polymer melt quality. It also avoids dead space and, by homogeneous flow, ensures full usage of all installed filtration surface, resulting in the longest online time and constant high quality. 

Comparing stainless-steel filter media types

Different stainless-steel formats meet the specific filtration and processing demands of polymer manufacturing. Some common options include: 

• Wire mesh laminates: 
  • Constructed from multiple sintered mesh layers, with the porosity determined by the finest filtration layer 
  • Self-supporting design suitable for a wide range of temperature applications, often without perforated supports 
  • Rigid but highly porous structure can be processed like sheet metal, though not ideal for pleated formats due to stiffness and stress to the finest layer during forming 
  • Filtration behaviour similar to wire mesh, but overall pressure drop increases with layer count 
  • Potentially usable for screening or shearing or filled polymer melt 

Metal fibre fleece structure 

• Metal fibre fleece: 
  • Formed from sintered stainless-steel fibres with diameters ranging from 1 μm to 80 μm and lengths of several millimetres 
  • Randomly layered to create depth filtration and trap black spots and other particles within the media 
  • Funnel-shaped pore structure helps retain flexible particles (gels) without fragmentation at limited shearing 
  • Very high porosity (70–80%) creates a dirt holding capacity within the porous structure 
  • Requires support layers on both sites to ensure high pressure stability and drainage 
  • Can be laminated with wire mesh on one or both sides, depending on application requirements 
  • Cleanable for re-use  

The choice of media depends on factors such as contaminant profile, flow volume, process temperature and expected filter life. These stainless-steel media types fit both disc and candle filter formats, providing high surface area for contaminant capture and enabling continuous melt filtration. 

John Crane’s Seebach® leaf discs 

Dwell time and melt flow simulation

Dwell time, the duration of the melt's contact with the filter media, directly influences filtration efficiency and quality consistency. Longer dwell times typically improve particle retention but can increase the risk of polymer degradation. The potential risk of dead space requires special attention. 

Computational Fluid Dynamics (CFD) simulations allow engineers to analyse melt flow patterns within the polymer melt filter, helping to optimise geometry and internal structure. Simulations can be used to: 

• Eliminate dead zones, short cuts or irregular flow  
• Adjust media layering to match shear sensitivity 
• Support predictive modelling of filter clogging and lifetime 

Using CFD tools during filter development allows for fine-tuning performance for each polymer grade and process. It helps assess the advantages of different filter media, element shapes and size ratios, as well as various valve setups and service concepts, to identify the best configuration for continuous or non-continuous processes. 

Engineering filter solutions for polymer producers

Polymer processing requires more than off-the-shelf filtration. Tailored filter designs help balance throughput, contaminant retention and long-term use. John Crane’s stainless-steel filter elements are custom-engineered to meet these needs, offering: 

• Flat or pleated metal fibre fleece media: Optimised for surface area and process requirements 
• Secure welded construction with custom support cores: Built to withstand repeated cleaning cycles and high-pressure operation 
• Long service life: Designed for multiple cleaning cycles without compromising structural integrity 
• Cylindrical or pleated (polymer candle filter) and disc formats: Suitable for a range of system geometries 

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

Polymer manufacturers can ensure melt purity and minimise production inconsistency by combining application-specific design with advanced simulation and high-performance materials. These configurations help ensure that each polymer melt filter performs reliably under variable flow, process conditions and contamination loads. 

Filtration strategy that maximises uptime and reduces cost

In high-throughput polymer processes, success depends not only on contaminant capture but also on system uptime. A properly engineered filter system can extend the time between cleaning cycles, enabling longer production runs without interruption.  

Duplex systems are one solution, where one vessel can remain operational while the other is cleaned. This setup is often used with polymer candle filters, allowing one filter set of elements to stay online while the other is cleaned or replaced. Ideally, the filter’s onstream life exceeds cleaning time, allowing continuous processing with minimal downtime. 

In this context, downtime translates directly to lost revenue, so filtration becomes not just a quality control measure but a cost control strategy. When combined with tailored pore size, optimised porosity and dwell time control, the result is a high-performing system that maintains melt purity while reducing process waste and improving operational efficiency. 

Rely on proven expertise for precision filtration

Choosing the right filter type and media is only part of the equation. Effective filtration performance comes from precision engineering that tailors pore size, media construction and dwell time for your specific process. These elements must align with the application, whether using a disc or polymer candle filter within your polymer melt filtration system. 

By working with experienced partners, polymer manufacturers can achieve consistent melt purity, extend filter life and reduce downtime. Visit our website to explore how our filtration expertise can support your polymer processing goals: johncrane.com/en/industries/polymers