In modern plants of terry towel are used automated sewing machines with high speed heads. For that reason the set needle, tissue and thread has a dynamic which can not be neglected. The forces that needles can to be submitted reach easily 5.000cN and in these circumstances there is a big probability to broken the thread and to become the needle blunt. If that happen going to lose quality, maybe a rejection, but also or more important going to lose production because it will be necessary amend the thread, replace the needle and tune the head. The University of Minho have studied these theme in particular Hélder Carvalho, researcher of textile department which developed a equipment to measure the forces built-in.
While production manager of terry towel plant I paid more attention to this theme in 2013, it was then that I meet Hélder Carvalho to talk about the particular case of terry towel produced in automated cutting and crossing hemmer units like Texpa and Magetron. We did a first test using a machine developed by Hélder Carvalho, we wanted to know the order of magnitude of the needle penetration force. Ready in this test were exceeded 3.000cN, value never achieved in previous studies.
A fact verified by whom work with automated crossing hemmer units are the big differences of productivity between different towel designs. Why? What to do reduce the differences? Where these questions that we wanted to see answered. To the team joined Manuel Carvalho, finalist of master degree in textile engineering and answer that questions was the theme of your these. The master degree was guided by Hélder Carvalho.
I gave them the sample towels, and for each one of them I controlled the production conditions and the ambient humidity. We controlled three different patterns of towels, we wanted conclusions in heavy and lighter fabrics. All the sample towels where tested by Manuel Carvalho to measure the penetration force of needle. Other tests were made to characterize the fabrics: linear mass of the yarns, thickness of fabrics, mass per unit area, texture, coefficient of friction, crease recovery angle, bending rigidity and conventional humidity. It was not easy remove conclusions because the variable are many and the combination between them there are many more, so it was necessary to go through what seemed more obvious.
First they concluded that the penetration force of needle is dependent of characteristics of fabric, if it has higher texture the force is greater. They think it's obvious because if the fabrics have more passages for centimetre, the gaps between yarns are smaller and so needle have less space to drill. On the other hand the penetration force is also related to the mass of the yarns, so if the yarns have more mass they take up more space and the gap between yarns is less. The researcher Hélder Carvalho thinks that the needle when crossing the fabric "search" the gaps and so if the gaps are smaller it have more difficulty to cross.
In the process of penetration, the type of needle is very important and so they tested two types of needle beak and two sizes of diameter. Also tested two types of covering. In theory the needle FFG or ball beak have the capacity to search better the gaps and crossing between yarns. In the other side the RG needle have a conical beak and maybe can pass by the core of yarn, but not that the RG is not a complete conical beak, in fact it is a middle way between R (complete conical beak) and FFG. We not selected a R needle because I was afraid that needle could destroy the yarns and the needles themselves can be destroyed when touching in the needle guard, also for this reason I do not use these needles in production.
Manuel Carvalho and Hélder Carvalho concluded that the force decreases with the decrement of the needle diameter, between a 120 and 110 diameter they obtained a 40% of difference. They also concluded that the RG needle have more facility to cross the fabric than FFG. The process of penetration have three important moments, the moment of entry into the fabric, the moment of penetration and the moment when the needle returns to leave the fabric. The researchers think that, due the tip shape FFG, in the first moment there is a greater opposition to entrance of needle which is maintained during the penetration, but in these two moments, the yarns maybe deformed and consequently the process of leave has less opposition, is a sign that there was damage into the fabric. So the RG needle has less opposition by the fabric.
The researchers also discovered that the needles with hardener covering have more resistance to the entrance in the fabric than normal ones.
These results were always tested in normal conditions of production and I concluded the normal needles damaged very quickly. I think that, although they have less resistance to the penetration, they do not have structure sufficiently strong for the involved forces. One the other hand, the minimum diameter that we consider acceptable in real production was 110. In fact we change our production from 120 to 110 and we have results in the reduction of needle consumption.
As a general conclusions, the repeatability of the parameters which forming fabrics, like linear mass of the yarns, thickness of fabrics, mass per unit area, texture, coefficient of friction, crease recovery angle, bending rigidity and conventional humidity it's very important because contribute to the constancy of the forces. In this situation the maintenance mechanic and operators will not always have to tune the machine.
I believe that a mill of terry towels that want the best performance of your sewing units need to control the parameters that influence sewability, beginning soon in the design of the towels. So when developing a new article, if they want it to be great consumption it is advisable that the texture in the hemmers be as small as possible, for example if it is necessary to differentiate the article it will be better to invest in adding a bar next to the hem. Soon after monitor the parameters of fabric and keep them as constant as possible, for this it is necessary to make regular checks and communicate the results. Finally I believe that in the future the sewing machine should be equipped with systems to measure the parameters of process sewing, the penetration force of needle, thread tension, and needle bar pressure, so it will be possible for maintenance mechanical know the parameters and tuning the machines to the measured values.
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