Failures can occur on filter bags for many reasons but determining the root cause of the failure almost always begins with close examination in two areas: the yarns that make up the fabric or the thread that binds the seams. The section below provides some general guidance on how to differentiate between failures of each component and what design changes may be appropriate to eliminate the problem. Examples of these types of failure modes in garments (the textile product that everyone is most familiar with) are provided for context.
Yarn Failures in the Fabric
When a failure occurs away from the sewn seams and exclusively in the weave/body of the fabric, this a yarn failure or fabric failure. The failure may be a rip or tear due to mechanical abrasion (wearing) of the fabric or due to tensile overload which occurs when the fabric is stretched to the point of breaking.
Mechanical abrasion can be seen in the knee and elbow portions of well-worn – sometimes called ‘threadbare’ – pants and shirts, while a common example of fabric overload would be when loose clothing is caught on a sharp corner and tears.
Even if a fabric does not break while being stretched, the filtration properties of the fabric will change as the pore sizes are elongated. Furthermore, repeated stretch and relaxation cycles will alter the mechanical properties of the yarns that make up the fabric.
Solutions to abrasion and mechanical overload are generally straight forward:
- Eliminate or manage sharp edges and relative motion between the filter bag and adjacent surfaces.
- Support the fabric with a load-bearing system. Perforated baskets are commonly used.
- Use a stronger fabric; however, changing the fabric will alter the filtration characteristics, so altering the fabric may require some ‘tuning’ to achieve the desired filtration performance.
- Reinforcing the filter bag with straps or ‘webbing.’
- Reducing the size of the filter bag but increasing the quantity so that the laden weight of each bag is lower and the available filtration area is greater.
As noted elsewhere, yarns can also fail because of operating temperature and chemical incompatibilities. Some chemicals will cause common filtration fabrics to become brittle or soft or even dissolve. Grossly exceeding the temperature range of a fabric will result in melting or burning the fabric. However, repeated thermal cycling at or near the peak acceptable temperature of a fabric can also reduce the material properties over time. These types of chemical and thermal incompatibilities must be addressed before any further design changes are warranted.
Thread Failure at the Seam
A failure at the sewn seam encompasses all failure that occurs where two or more pieces of fabric are stitched together. These failures involve both the thread and fabric. Three common failures at the seam are thread breakage, seam grinning and seam slippage.
Thread breakage is exactly as it sounds: The thread that is used to create the sewn seam (rather than the yarn that makes up the fabric) ruptures and the seam opens. If the thread did not fail because of a chemical or thermal incompatibility in the application, this type of failure can be remedied by one or more of the following changes: choosing a stronger thread, shortening stitch length, changing stitch configuration, or adding additional rows of stitches.
Seam grinning occurs when two pieces of fabric are pulled apart and the sewn seam appears to stretch apart, resulting in a gap forming between the two pieces of fabric. In this failure mode, neither the thread nor the fabric fails. Grinning occurs most often when the seam is loaded perpendicular to the direction of the stitch. This failure is common on the outer leg seam of pants. It is especially easy to spot on jeans where contrasting thread color makes the failure more obvious. This problem could be caused by improper thread tension or stitch length, or seam overload. Solutions to grinning for overloaded seams are the same as for thread breakage.
Seam slippage is the most difficult seam failure to solve. It is evident when the thread remains mostly intact, but the fabric itself comes unwoven. You can identify this by the long, fringe-like edge of the exposed yarns of the fabric where the seam has come undone.
In clothing, this is common with silk and satin garments – silk fibers are slippery because they are long, smooth, and hard with a continuous surface. Silk fibers have very little surface texture to help them interlock with adjacent fibers. These same traits are common with synthetic monofilaments used to make filtration fabrics. While these traits are highly desirable for durable and precise filtration textiles, they make the fabric far more prone to seam slippage.
To resolve this failure mode, stitch length may need to be adjusted (shortened or elongated) depending on the fabric. Thread selection for the sewn seam and stitch configuration can also help, as can reducing the seam loading, as described above.
If you have bag filtration application that is experiencing any of these types of failures, please use the form below to share as many of the details about your application as possible. We look forward to working with you to refine your filtration process and eliminate these failure modes.
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