Sunday, 15 March 2009
1. Problems of stitch formation
It gives rise to poor seam appearance and performance
- Slipped stitches
- Staggered stitching
- Unbalanced stitches
- Variable stitch density
- Needle, bobbin or loops thread breakage
a. Slipped Stitches
Arise from the hook or loopes in the machines not picking up the loop in the needle thread.
b. Staggered Stitches
Can be caused by yarns in the fabric deflecting the needle away from a straight line of stitching, giving a poor appearance.
c. Unbalanced stitches
It can reduce the potential of stretch in a seam in a knitted fabric and may lead to seam cracking.
d. Variable stitch density
It arises from insufficient foot pressure in a drop feed system, causing uneven feeding of the fabric through the machine.
PROBLEMS OF PUCKER
Pucker is a wrinkled appearance along a seam in an otherwise smooth fabric. It generally appears as if there is too much fabric and not enough thread in the seam.
Causes of Pucker
a. Seam pucker due to differential fabric stretch
Remember that the upper fabric would tend to move forward by an amount always less than the movement of lower one. This is due to the fact that the lower layer is positively gripped by the feed dog and upper layer is driven by the friction by the lower layer.
b. Differential pucker caused by fabric dimensional instability
The essential feature causing differential pucker is the relative change in dimensions of upper and lower fabric after the seam has been made.
Differential pucker due to dimensional instability may be suspected when the two fabrics being joined are markedly different or when one shows noticeably more pucker than the other.
c. Seam pucker due to extension in the sewing threads.
All sewing threads have some extensiblity and they are extended by the action of the tension devices and pass into seam in an extended state. When removed from the machine they will tend to contract.
When thread extension is proved to be the cause of puckered seam, consideration must be given to the type of thread being used and to the tensiton settings on individual machines.
d. Seam pucker due to sewing thread shrinkage
Cotton sewing threads increase in diameter and shrink in length when wet and these distortions may cause pucker in sensitive fabrics. Synthetic sewing threads have negligible wet shrinkage and should always be used for such fabrics.
e. Seam pucker due to structural jamming
The presence of the seam itself may introduce a distortion. It is in no way dependent on the action of the sewing machine, but it invariably appears as soon as the seam is formed.
As soon as a woven fabric has been constructed so as to be close to the practical weaving limit, that is very less space left between the yarsn either warp or weft ways, it may be extremely difficult to force in any more threads in either direction.
The term 'structural jamming' is given to this type of pucker because it results directly from the act of jamming extra threads into a structure which is already too closely set to accommodate them.
Seam pucker due to mismatched patterns
This is due to the discrepency between the lengths of the stitching lines on the pattern pieces that go together in the seam. Thus there is a difference in the lengths of the cut parts which the machinist is sewing together.
Problems of damage to the fabric along the stitch line
a) Mechanical damage
1. Needles can strike and break fabric yarns and burst the loops in knitted fabrics. For this appropriate set and ball point needles are necessary.
2. Needles should always be as small as possible.
3. Sometimes the combination of the machine speed and nature of the fabric prevents the yarns from moving out of the way of the needle sufficiently fast to avoid damage .To solve the problems either reduce the speed - which means lesser production or ensure that the fabric is adequately lubricated. It calls for having resin finish on the fabric.
- All sample lengths of the fabric should be tested for sewability and the necessary finishes should be specified before the bulk fabric is ordered and bulk fabric should be tested before production to ensure that finishing treatment has been effective.
b) Needle Heating Damage
Needles heating occurs as a result of friction between the needle and the fabric being sewn.
In high speed sewing of dense material, temperatures as high as 300 deg or even 350 deg can be reached.
At this temperature it is possible that the needle may suffer damage and lose its hardness.
Natural fibres in a fabric or thread can withstand these temperatures for a short time.
With synthetic fibres, the position is more critical since the fibres melt at around 100 deg C, polyamide and polyester soften at about 230 deg C and polyacrylics will only withstand temperatures upto 280 deg C.
Overheated needes can
- Soften the synthetic fibres
- Weaken them
- Produce rough seam with
- harsh stitch holes
Melted fibres stick to the surface of the needes
- Increase its friction
- cog the eye and the groove
- No sew
- Skipped stitches
Reduction of Friction
- Reduce the sewing speed
- Changing the shape or surface of the neede
- long seams will ensure more heat build up in the needle
- Jet of compressed air.
- User spun or corespun yarns.