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Knittability specially in synthetic yarns such as polyester and nylon is an essential index
of the performance in ensuring the proper conversion of yarn to a knitted fabric. This can
be affected by various conditions of the yarn manufacturing, yarn storage, knitting
machine, knitting pattern and the environment. In footwear knitting industry,
Thermoplastic Polyurethane synthetic yarn is being used due to its higher heat resistance
and colour fastness properties compared to normal melt spun yarns. One of the major
concerns in the process of knitting is the continuous yarn burst, which occurred
significantly in thermoplastic polyurethane yarn. Occurrence of this burst appeared to be
random and emerged in every yarn knitted areas. This research aimed to identify the
critical factors affecting the yarn burst. After identifying the most critical factors through
literature review, temporary and permanent countermeasures were developed. Initially,
Prisma approach was applied to extract the relevant information from literature. Then,
parameters were categorized into three based on factors related to: yarn compound,
storage environment, and machine and knitting pattern. Under each category the critical
parameters relevant to each factor was identified after analyzing the significance of each
quantitatively and qualitatively. In terms of yarn aging, previous literature explains three
types of tests namely natural exposure, field aging and artificial acceleration. According
to literature, the strength of Thermoplastic Polyurethane yarn deteriorates once it is
subjected to extreme conditions. In this study, the relationship between the Thermoplastic
Polyurethane polymer manufactured date, the yarn lot manufactured date and the yarn
burst was experimentally tested. The correlation with knittability and the yarn
manufacturing dates were found to be not significant. In terms of yarn compound related
factors, yarn unevenness was identified as the most critical parameter. An experimental
set up was developed to measure the unevenness of black and white colourway samples.
These colourways were selected as the defect rate of white colour was the highest while
the black colour was the lowest. Through the statistical analysis of “t” distribution, it was
identified that the statistical variance of the data set was high between the two
colourways. This proves that the factor of yarn unevenness is directly correlated with defect rates. Related to machine and method related factors, the programming structures
were less researched in the literature. According to the relevant footwear application,
initial all needle knit structure is changed to 1*1, 1*2, and 2*2 structures. All 1*1, 1*2
and 2*2 structures prove better knittability compared to initial all needle structure.
Depending on the aesthetic factors, 1*1 structure is considered as the optimum one for
the relevant application. This study was limited to few parameters and the dependence of
other parameters such as temperature, time and moisture absorption has to be further
investigated in future works. Further, the needle knit structure is also an interesting
avenue for further research. |
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