Size Add-on Falling Below Target After a Drying Fix
A sizing plant runs a 6,000-end warp of 60 Ne combed cotton at 55 m/min for a 190 cm reed width. The target size add-on is 10.0% using a 70/30 PVA–modified starch recipe at 8.5% concentration, size box at 92 °C and drying cylinders at 130 °C. To raise drying speed, the squeeze roller pressure was increased last week from 2.5 kN to 3.5 kN and cylinder temperature to 145 °C; the size box level control also failed and the operator has been topping up manually. A 20 m sample now weighs 1.181 kg unsized and 1.266 kg sized (oven-dry). Loom warp break rate rose from 1.2 to 3.4 per 10⁵ picks, efficiency fell from 92% to 84%, and the sized warp shows hairy, loosely bound fibres and size flakes at the back rest. Size costs Rs 190/kg; the looms run at 700 rpm on a 60 picks/inch quality for 7,000 hours/year.
1. Calculate the actual size add-on and identify the process changes responsible for the fall from target, including the effect on yarn hairiness and size shedding.
Read the worked answer
Add-on calculation
- Oven-dry size mass = 1.266 − 1.181 = 0.085 kg
- Add-on = 0.085/1.181 × 100 = 7.2%, against a 10.0% target (a 28% shortfall)
Causes, in order of importance
- Squeeze pressure raised from 2.5 to 3.5 kN. Pick-up is governed mainly by wet pick-up, which falls as nip pressure rises. A 40% pressure increase can easily remove 2–3 percentage points of size. This is the dominant cause and it was made deliberately, so it is the first thing to reverse.
- Cylinder temperature raised to 145 °C. Fast surface drying drives size migration to the yarn surface. The core stays under-sized and the surface film becomes brittle, which explains both the flakes at the back rest and the hairy fibres: the film cracks, abrasion removes it, and unprotected fibres lift.
- Faulty level control with manual topping up. Concentration and viscosity drift through the shift, so add-on is unstable along the beam even if the average looks acceptable. Rising viscosity also increases nip pick-up unpredictably and can cause size streaks.
- Possible size penetration loss. Higher nip pressure and quick surface drying reduce size penetration into the yarn body; the yarn then relies on a surface film that abrasion removes quickly, raising loom breaks and stopping the loom at the wrong moment.
The rise in breaks from 1.2 to 3.4 per 10⁵ picks and the 8-point efficiency loss are consistent with abrasion failure at the back rest and dropper zone, not with a yarn quality fault.
2. Set out a corrective plan with parameter targets and a verification method, and explain why simply adding more size solids is the wrong first response.
Read the worked answer
Corrective plan
| Parameter | Action | Target |
|---|---|---|
| Squeeze pressure | Return to the validated setting | 2.5–2.8 kN, both ends of the nip equal |
| Cylinder temperature | Reduce and use a drying profile | 120–130 °C, gradual, not 145 °C in the first cans |
| Size box temperature | Hold steady | 92–95 °C with agitation |
| Concentration | Restore automatic level control; calibrate refractometer | 8.5% ± 0.2%, checked every 2 h |
| Viscosity | Check cooking cycle and batch consistency | Within ±1 s of the standard flow cup |
| Add-on | Re-measure with a proper sample | 9.5–10.5% oven-dry |
Verification
- Take a 20 m sample from the sized beam, condition it, weigh unsized and sized portions from the same warp, and calculate add-on by oven-dry mass, not by difference on a moisture meter.
- Check penetration by splitting a sized end and examining the cross-section, or by a tint test; a surface-only film is the failure mode here.
- Measure sized yarn hairiness and abrasion resistance (for example a yarn abrasion tester) before and after the change.
- Run a short loom trial on one beam and record breaks per 10⁵ picks; the target is to return below 1.5.
Why not just add more solids
Adding solids raises viscosity and surface coating without restoring penetration. With a crushed nip and over-fast drying the extra size sits on the surface, flakes off, and the extra cost is wasted. The fault is mechanical and thermal, so it must be corrected at the nip and the cylinders first; only then should concentration be trimmed to hit 10%.
3. Quantify the size saving at 7.2% versus 10% for a 12,000 m beam and compare it with the production consequence of the 8-point efficiency loss.
Read the worked answer
Yarn mass in a 12,000 m beam
6,000 ends × 9.84 tex (60 Ne) × 12 km = 708.5 kg
- At 10.0% add-on: 70.9 kg size → Rs 13,470
- At 7.2% add-on: 51.0 kg size → Rs 9,690
- Apparent saving: Rs 3,780 per beam, about Rs 0.32 per metre of warp
Cost of the efficiency loss
Loom output at 700 rpm and 60 picks/inch = 700/60 = 11.67 in/min = 0.296 m/min = 17.78 m/h at 100% efficiency.
- 8 points of efficiency = 1.42 m/h lost per loom
- Over 7,000 h: 9,940 m per loom per year
- At Rs 12/m contribution: about Rs 1.19 lakh per loom per year
A sizing machine typically feeds several looms. If it supplies four looms, the loss is roughly Rs 4.8 lakh/year, while the size saved is about Rs 3,780 per 12,000 m beam. Even at 35 beams/year the saving is only about Rs 1.3 lakh. The calculation therefore says clearly that the over-squeezed, over-dried setting is a false economy: the mill is buying a small size saving with a much larger loss of loom output, plus the risk of start marks and seconds.
The correct economic target is the lowest add-on that keeps breaks low, not the lowest add-on that can be measured. Verify by a break-rate trial at 9.5%, 10.0% and 10.5% add-on and choose the minimum-cost point, including size cost, energy for drying and loom efficiency.