How to Control GSM in a Single Jersey Knit Fabric
This question was asked by one of my readers. To answer it properly, we should begin from the first principles of knitting, because GSM in a knitted fabric is not controlled by one single setting. It is the combined result of yarn count, loop length, stitch density, machine gauge, feeder setting, relaxation, dyeing, finishing and compaction.
Single jersey is one of the most common weft knitted fabrics. It is used widely in T-shirts, vests, leggings, casual tops and many lightweight knitted garments. The simplicity of the structure sometimes gives the impression that GSM control is also simple, but in actual production it requires careful coordination between yarn, machine and finishing.
Table of Contents
- What is GSM?
- Basic Principle of GSM Control
- Knitting, Weft Knitting and Single Jersey
- Role of Yarn Count
- Role of Stitch Length or Loop Length
- Role of Stitches Per Inch
- Role of Machine Gauge
- Role of IRO or Positive Yarn Feeder
- Role of Cylinder and Dial Setting
- GSM Before and After Processing
- Practical Example
- Shop-Floor Checks for GSM Control
- Common Mistakes in GSM Control
- Merchandiser’s Note
- Simple Summary
- Related Reading
- General Disclaimer
What is GSM?
GSM means grams per square metre. It tells us the weight of one square metre of fabric. In simple terms, GSM is a measure of fabric heaviness. A higher GSM means a heavier, denser or more compact fabric, while a lower GSM means a lighter, more open or softer fabric.
In knitted fabrics, GSM should not be understood only as a weighing result. It is also a reflection of how much yarn has been placed within a given fabric area. Therefore, the same yarn can produce different GSM values if the loop length, stitch density or finishing conditions are changed.
A simple way of understanding GSM is:
\[ \text{GSM} = \frac{\text{Fabric weight in grams}}{\text{Fabric area in square metres}} \]
For knitted fabric, another practical way to think about it is:
\[ \text{GSM} \propto \text{Loop density} \times \text{Yarn weight per loop} \]
Basic Principle of GSM Control
GSM in single jersey fabric can be controlled mainly in three ways. It can be controlled by changing the yarn count, by changing the stitch density, or by changing the loop length. In actual production, all three are connected, but loop length is usually the most practical day-to-day control available to the knitter.
If a coarser yarn is used, the fabric becomes heavier and GSM increases. If a finer yarn is used, the fabric becomes lighter and GSM decreases. For the same yarn count, GSM can be increased by increasing the number of stitches per inch or by reducing the loop length.
In simple terms:
\[ \text{Coarser yarn} \Rightarrow \text{Higher GSM} \]
\[ \text{Finer yarn} \Rightarrow \text{Lower GSM} \]
\[ \text{Shorter loop length} \Rightarrow \text{Higher GSM} \]
\[ \text{Longer loop length} \Rightarrow \text{Lower GSM} \]
Knitting, Weft Knitting and Single Jersey
Knitting is a process of fabric formation by the intermeshing of loops of yarn. When one loop is drawn through another loop, a knitted stitch is formed. This looped structure gives knitted fabric its stretch, softness and flexibility.
Weft knitting is a method of fabric formation in which the loops are made horizontally from a single yarn. The intermeshing of loops takes place across the width of the fabric, either in circular form or on a flat machine. Single jersey is a weft knitted fabric produced with one set of needles. It is also called plain knitted fabric.
Because single jersey is made from one set of needles, it has a face side and a back side. The face side shows vertical wales, while the back side shows semi-circular loops. This structure is simple, but it is very sensitive to yarn quality, loop length, machine setting and finishing treatment.
Role of Yarn Count
Yarn count indicates the fineness or coarseness of yarn. In the English cotton count system, a higher count means a finer yarn. For example, 40s Ne is finer than 30s Ne, and 30s Ne is finer than 20s Ne.
If the yarn becomes coarser, more fibre mass is present in a given length of yarn. Therefore, when it is knitted into fabric, the resulting fabric generally becomes heavier. If the yarn becomes finer, less fibre mass is present in the same length of yarn, and the fabric generally becomes lighter.
| Yarn or Fabric Change | Likely Effect on GSM |
|---|---|
| Coarser yarn | GSM increases |
| Finer yarn | GSM decreases |
| Same yarn with shorter loop length | GSM increases |
| Same yarn with longer loop length | GSM decreases |
However, yarn count cannot be changed casually after fabric quality has been approved. A change in yarn count can affect fabric appearance, hand feel, bursting strength, shrinkage, opacity, dyeing behaviour and cost. Therefore, for regular production correction, loop length adjustment is usually more practical than changing yarn count.
Role of Stitch Length or Loop Length
Stitch length, also called loop length, is the length of yarn required to form one complete knitted loop. This is one of the most important parameters in knitted fabric engineering because it directly affects GSM, width, extensibility, compactness, shrinkage and handle.
If the loop length is increased, each loop becomes larger. The fabric becomes more open, more extensible and generally lighter per unit area. Therefore, GSM tends to decrease. If the loop length is decreased, each loop becomes smaller and tighter. The fabric becomes more compact and heavier per unit area. Therefore, GSM tends to increase.
This relationship may be expressed practically as:
\[ \text{Loop length} \uparrow \Rightarrow \text{GSM} \downarrow \]
\[ \text{Loop length} \downarrow \Rightarrow \text{GSM} \uparrow \]
This is why loop length is one of the first settings checked when the GSM of a single jersey fabric is not matching the required specification. A small change in loop length can produce a noticeable change in fabric weight and width.
Role of Stitches Per Inch
GSM also depends on the number of loops present in a given area. In knitted fabric, this is studied through courses per inch and wales per inch. Courses are the horizontal rows of loops, while wales are the vertical columns of loops.
If the number of courses and wales per inch increases, more loops are packed into the same area, and GSM increases. If the number of loops per inch decreases, the fabric becomes more open, and GSM decreases.
The relationship may be understood as:
\[ \text{Higher stitch density} \Rightarrow \text{Higher GSM} \]
\[ \text{Lower stitch density} \Rightarrow \text{Lower GSM} \]
For the same yarn count, stitch density can be increased by using a suitable higher gauge machine or by decreasing loop length. However, the quality of the fabric should not be sacrificed merely to achieve GSM. Excessively tight fabric may become harsh, narrow, unstable or difficult to process.
Role of Machine Gauge
Machine gauge means the number of needles per inch in the knitting machine. A higher gauge machine has more needles per inch and can produce finer and denser fabrics. A lower gauge machine has fewer needles per inch and is used for coarser fabrics.
For a given yarn and fabric structure, increasing the stitch density through a suitable machine gauge can increase GSM. However, yarn count must be compatible with machine gauge. A very coarse yarn cannot be knitted properly on a very fine gauge machine, while a very fine yarn may not give sufficient cover on a coarse gauge machine.
Therefore, machine gauge is not only a GSM-control factor. It is also a fabric design decision. The correct machine gauge must balance yarn count, required GSM, fabric cover, hand feel, productivity and quality.
Role of IRO or Positive Yarn Feeder
In modern weft knitting machines, a positive feeder such as IRO is used to regulate the speed of the yarn being fed to the needles. The feeder helps maintain uniform yarn supply and controls the amount of yarn delivered during knitting.
If the speed of the IRO increases, the quantity of yarn passing into the machine increases. As more yarn is supplied, the loop size increases and the GSM decreases. If the IRO speed decreases, less yarn is supplied, the loop size decreases and the GSM increases.
| IRO / Feeder Setting | Loop Size | Effect on GSM |
|---|---|---|
| Feeder speed increases | Loop size increases | GSM decreases |
| Feeder speed decreases | Loop size decreases | GSM increases |
This is one of the most important practical controls available to the knitter on the shop floor. But the adjustment should be done carefully, because extreme feeder settings can affect fabric appearance, yarn tension, fabric width and knitting performance.
Role of Cylinder and Dial Setting
The loop size can also be influenced by machine settings, including the distance between the cylinder and dial needles in machines where such arrangements are relevant. If the distance is more, the loop size increases and GSM decreases. If the distance is reduced, the loop becomes smaller and GSM increases.
However, such settings should be adjusted carefully. Incorrect adjustment may lead to knitting defects, needle damage, uneven loop formation, holes, fabric lines or excessive yarn tension. In production, a GSM correction should never be made blindly. The knitter should first identify whether the problem is coming from yarn, feeder setting, loop length, machine condition or processing.
GSM Before and After Processing
The GSM measured in grey fabric is not always the same as the GSM measured after dyeing, washing, compacting or finishing. Knitted fabrics are dimensionally unstable, and their final GSM depends greatly on relaxation and shrinkage during wet processing and finishing.
For example, after dyeing and washing, the fabric may shrink in width or length. When the same mass of fabric occupies a smaller area, the GSM increases. Compacting can also increase GSM by reducing lengthwise shrinkage and making the fabric more stable.
Therefore, a knitter should not target the finished GSM directly in grey fabric without considering processing loss and shrinkage. A practical factory approach is to maintain a grey GSM range based on previous experience, then allow for dyeing, washing, finishing and compaction effects.
Practical point: GSM should ideally be checked after proper relaxation. If fabric is tested immediately after knitting, dyeing or finishing, the reading may not represent the final stable condition of the fabric.
Practical Example
Suppose a single jersey fabric is being knitted from 30s Ne cotton yarn, and the buyer requires a finished GSM of 160. If the finished GSM is coming lower than required, the factory should not immediately blame only one department. The knitter, processor and merchandiser should examine the complete chain.
If the GSM is lower than required, the factory may check whether the loop length is too high, whether the feeder is supplying too much yarn, whether the yarn count is finer than specified, whether the machine gauge is suitable, whether the fabric is being over-stretched during finishing, and whether the fabric has relaxed properly before GSM testing.
If the GSM is higher than required, the factory may check whether the loop length is too short, whether the feeder is supplying less yarn, whether the yarn count is coarser than specified, whether the fabric has shrunk too much during processing, or whether compacting has been done excessively.
This shows that GSM correction is not merely a knitting issue. It is a combined knitting, processing and finishing issue.
Shop-Floor Checks for GSM Control
| Check Point | Why It Matters |
|---|---|
| Yarn count | Coarser yarn increases GSM; finer yarn reduces GSM. |
| Yarn lot | Different yarn lots may behave differently in knitting and finishing. |
| Loop length | Directly affects fabric compactness, width and GSM. |
| Feeder setting | Controls yarn delivery and loop size. |
| Machine gauge | Influences stitch density and suitability of yarn count. |
| Needle condition | Damaged needles may disturb loop formation and fabric appearance. |
| Take-down tension | Excessive tension can distort fabric dimensions. |
| Grey GSM | Helps predict finished GSM after processing. |
| Finished GSM | This is the final buyer-relevant value. |
| Relaxation time | Fabric should be tested after proper relaxation for reliable GSM. |
Common Mistakes in GSM Control
One common mistake is to adjust GSM only by changing machine settings without checking yarn count. If the yarn itself is different from the approved specification, the GSM problem may continue even after several machine adjustments.
Another mistake is testing GSM immediately after knitting or finishing without allowing the fabric to relax. Knitted fabric changes dimension after relaxation, so immediate GSM readings can be misleading.
A third mistake is ignoring finishing. The knitting department may produce correct grey GSM, but dyeing, washing, squeezing, drying or compacting may change the final GSM significantly. Therefore, communication between knitting, dyeing, finishing and merchandising is essential.
Merchandiser’s Note
For a merchandiser, GSM should not be treated as an isolated number. It is linked with fabric handle, fall, opacity, shrinkage, spirality, costing and garment performance. A small GSM variation may be acceptable if the buyer’s tolerance allows it, but if GSM is outside tolerance, the root cause must be identified.
The merchandiser should ask whether the issue is due to yarn count, loop length, feeder setting, machine gauge, shrinkage or finishing. This approach helps avoid unnecessary arguments between departments and supports faster corrective action.
Simple Summary
GSM in single jersey fabric can be controlled by using a coarser or finer yarn count, increasing or decreasing stitch density, adjusting loop length, controlling positive feeder speed, selecting the right machine gauge, and managing fabric relaxation, shrinkage and finishing.
The most practical day-to-day control is loop length. If loop length increases, GSM decreases. If loop length decreases, GSM increases. In single jersey knitting, GSM is therefore not just a weight value. It is the final result of yarn, loop, machine, processing and finishing working together.
General Disclaimer
This article is intended for educational and practical understanding of knitted fabric GSM control. Actual production settings may vary depending on yarn quality, machine condition, gauge, fabric structure, processing route, finishing method and buyer tolerance. For commercial production, GSM settings should be finalized through trial, testing and approval by the concerned technical team.
