Introduction
In many spinning mills, defects noticed in blow room products often show up later at the carding stage. Basically, many of the following defects are related to the lap-forming process associated with conventional blow room. In a modern blow room line, the material from the last machine in the blow room sequence is directly fed through chute to the card. In India, the rate of change-over from conventional blow room line to a fully modern blow room line with chute feeding has been very slow, and some mills are still working with conventional set-up. Even then, the concepts are applicable in a different sense, for example uneven feeding to a card is applicable both in conventional as well as modern set-up, and rich droppings or lower blow room cleaning efficiency are also applicable in modern set-up.
Uneven Laps
The lap formed in blow room is expected to be uniform along its length and width. Normally, uniform feeding from feed regulating motion is expected to ensure that feeding to the beaters during the equal time interval remains fairly constant. Similarly, the degree of opening has a profound influence in maintaining a uniform distribution of cotton across the cages. It also controls and maintains the uniform density of cotton issued from the reserve boxes. The variation in the lap may be of short term or long term nature.
Possible causes
- Uneven feeding of cotton.
- Faulty working of feed regulating mechanism.
- Shifting of belt over the cone drums not smoothly taking place, either the belt has become too stiff (not pliable) or it has not been pieced-up properly. A belt strap wider in width is not sensitive enough to bring about the changes sensed and conveyed by the regulating mechanism.
- Uneven loading on the feed roller and improper alignment of the feed roller with the beater allow uneven snatching of fibres. The slippage of pedal roller causes uneven delivery of material.
- Feed lattice slipping over its driving shell.
- Too low fan speed, easy and smooth withdrawal of the material through the beater zone is not allowed. The air turbulence due to dirty flue pipes and cages affects uniform distribution of cotton on cages.
Conical Laps or Barrel Shape Laps
If the dust box is too small, the air entering the cage perforations and subsequently being led into dust box experiences backpressure. This leads to uneven distribution of material over cage surface. Similarly, if the cage perforations are partially or fully choked up on any particular portion of the cage, it will draw less air through those sections and hence will result in thinner deposition of the fleece.
Possible causes
- Some of the pedals in feed regulating motion are jammed, thus not effectively conveying the variation in the thickness of the cotton sheet.
- Uncontrolled airflow through air in-let valve placed on either side of the trash box allows uneven drawing of air through the cages.
- Blunt beater blades resulting in uneven striking action and thus withdrawing uneven cotton material from the feed system.
Dirty Laps
The laps contain substantial amount of visible impurities and thus appear dirty. This indicates poor cleaning in blow room.
Possible causes
- Grid bars are too close to each other. They are required to be partially open for the liberated trash to fall down through them.
- Excessive fan speed prevents cotton from being adequately cleaned in the beater zone.
- Beater speed is comparatively slower, causing ineffective cleaning action.
- Setting between the beater blades and the pedal roller is too wide, thus causing lumps of cotton being snatched.
- The mild air currents returning back into beater chamber through the grid bar cause the trash to return back into beater zone, recombination of lint and trash within the beater.
Stringy Laps
Overbeating action of some of the beaters in blow room curls the fibres. A tuft of fibres that is repeatedly beaten up by the beater blades tends to form strings. With heavier beating, the string size also grows. Once formed, the strings are very difficult to open out in subsequent processes and may cause difficulties in drafting process.
Possible reasons
- Too high beater speed.
- Too low fan speed.
- Excessive moisture in the cotton processed.
Rich Droppings
Rich droppings mean that there is considerable proportion of lint or fibrous matter in the droppings under the beater.
Possible causes
- Excessive amount of soft waste in mixing.
- Grid bars set either too deeply and/or are more open.
- Air suction of the fan is too weak, thus allowing cotton to linger in the beater chamber.
- Excessive beater speed leading to more forceful action on the material being processed.
Defective Lap Edges
Defective lap edges usually occur due to accumulation of waste in the holes at the sides of the cages. This weakens the air suction on the sides of the cages, leading to irregular deposition. It leads to torn or ragged edges. Also, it is quite likely that some roughening on the inside surface of the passage that leads to cages causes intermittent deposition.
What causes it
- Waste accumulation in the side holes of the cages.
- Roughening on the inside surface of the passage that leads to cages.
Practical note
An inspection can help in locating the roughenings. A periodic cleaning of the cages helps in preventing these blockages of the perforations on the cage.
Lap Licking
This trouble is experienced only when the lap produced in blow room is put on the lap roller of the card. During unwinding of lap at card, the adjacent layers stick to each other. However, due care needs to be taken during lap building itself, in the blow room.
Possible causes
- The mixture of comparatively long and short fibres is very difficult to be consolidated during the lap formation and hence there is tendency for lap licking.
- Incorrect proportion of soft waste makes the laps more fluffy.
- Atmospheric conditions are too humid or damp-cotton is added in mixing. This increases the tendency of fibres to adhere to each other, thus leading to cross-layer fibre adhesion which is licking.
- Unequal amount of cotton, when made to deposit on the top and bottom cage, reduces this tendency. During calendering, the use of felting fingers or scroll roller becomes more effective, the latter making indentation marks. Forming lighter laps, uneven diameter of top and bottom cage, use of roving to separate the layers, more effective use of dampers to divert unequal material to deposit on the cages, also reduce the lap licking tendency. Some of these are required to be followed, especially while processing man-made fibres.
- If the cage runs too fast, it leads to improper blending of fibres during their deposition on the cage. This may also cause licking.
- Inadequate calendering pressure affects the compactness of the lap. Whereas too high rack pressure on the lap spindle increases the pressure on the neighbouring layers to come closer and stick to each other.
Conclusion
These defects are closely related to the lap-forming process associated with conventional blow room, but the same ideas still matter in modern set-ups as well. Uneven feeding, poor cleaning, rich droppings, and lap licking can all affect the next process if the blow room is not kept under control. The main points are proper feed regulation, correct fan action, clean cages, proper grid bar setting, and sound beater condition. If these basic checks are watched carefully, the blow room can give a more uniform feed to the card and reduce later processing trouble. As mills continue to modernize, these practical controls will remain relevant.
