Relative comparison of seam strength for Sheertex Classic tights vs competitors
Dave Cox
Senior Staff Technologist
Amanda Fleury, PhD
Senior Data Scientist
In order to better understand seam strength and failure, testing was conducted on two types of seams in the toe and the reinforced gusset area. Sheertex Classic tights proved to be more than twice as strong in both the toe and reinforced gusset area, when compared to close competitors.
Most industrial knitting machines used for hosiery man- ufacturing knit the garment legs separately, requiring seams to bind them together. While necessary, seams puncture and stretch the fabric to add new threads and structure, which will sometimes make the seam weaker than an equivalent area of fabric without a seam. Most hosiery is constructed by sewing a gusset between the two legs to allow for body shape and movement. The gusset area can be subjected to many forces during movement and is therefore a common point of seam failure.
Seams, specifically at the gusset and toe, on circular knit tights are areas commonly prone to damage. There are many important factors during and after manufacturing which affect seam strength - the type of seam used, the tuning of the machine parameters to the fabric being sewn, wear on the threads over time, and the strain to which the threads will be subject during the garment’s lifetime. Often, more material is knit into the leg fabric near the gusset seam to address these issues.
To determine strength of both the gusset and toe seams, two different test methods were employed. The first was a pressurized air balloon burst test for the toe seams in order to best emulate the way that stresses are placed on a toe seam (eg. a foot pushing forward with force through the seam). The second was a tension test for the reinforced gusset seams, to emulate the direction of tension experienced while being worn on the body.
In the case of the reinforced gusset seams, strength was calculated by testing the overall tensile strength of a section of reinforced gusset fabric containing a seam.
We believe that Sheertex toe seams are twice as strong as competitors. We believe that Sheertex tights’ yield at the seams is at least twice that of similar non-control-top competitors.
A tension test device was built which uses a calibrated load cell in combination with a towing winch to stretch samples under test. Fabric clamps were also built which use a capstan mechanism to ensure proper clamping and fabric handling, rather than a more traditional C-grips which promote early breakage at the clamping point of the fabric. The tension apparatus can withstand loads up to 10kN, has a constant extension rate of 2058 mm/min, and an overall working distance of 57.5 cm. This apparatus was used to test the overall strength of the gusset seams.
Inflation Apparatus
An inflation test device was designed and built allowing for a sample of closed-toe pantyhose to be mounted to a clamp with an air bladder interior. The air bladder can be inflated, applying pressure evenly to all areas of the sample until fabric failure.
Reinforced Gusset Strength Measurement
A 10cm wide section of the reinforced gusset area was loaded into the clamps of the tension apparatus. The seam was centered in the apparatus so as to be within the section of the fabric under tension during testing. The fabric was then pulled until it tore in two.
Toe Seam Strength Measurement
Firstly, the toe area of the sample garment was loaded onto the upper clamp of the apparatus and held in place using magnets. The rubber air bladder was placed onto the base, and the upper clamp was then lowered in place. The clamp was then tightened onto the base, forming an airtight seal. The bladder was then inflated until fabric rupture, and the pressure at failure noted.
When examining the non-control-top styles in fig. 1, we can see similar trends to other strength measurements of Brand 1 (Sheertex Classic Tights) vs. this composite competitor. The average with-seam strength (41.1kgf) is 1.8 times stronger than the average competitor (21.9kgf), as well as 3.2 times stronger than the weakest competitor (Brand 2, 12.9kgf). The strength of Brand 4 (33.8kgf) comes closest to Sheertex Classic Tights, but it is worth noting that the fabric at the gusset seam is actually quite opaque in this style due to it being a control top, which Sheertex Classic Tights are not. The thicker fabric lends more strength, but at the expense of sheerness in this area.
Fig. 1. Yield strength for reinforced gusset sample containing a seam.
Fig. 2. Yield PSI for toe seam by brand.
Finally, when examining the strength of the toe seam, we find that Sheertex (35.0psi) held 2.3 times more pressure than the average competitor (15.3 psi). Compared to the next strongest toe seam (Brand 2, 18.0 psi), Sheertex held 1.9 times more pressure, and 3.5 times more pressure than Brand 5 (disposable tights, 10.0 psi).
When examining all of the results, we can clearly see that the Sheertex Classic tights are able to withstand more force around seam areas. Especially when it comes to frustrating rips around the toe and reinforced gusset areas, we believe that this additional raw strength contributes to longer-lasting tights that can be relied upon for many more wears than traditional tights.