Opening, Carding, Drawing and Combing

Blowroom machine sequence

In a modern high-performance blowroom line the machines run in a fixed order. The automatic bale opener breaks compressed bales into tufts, the coarse cleaner removes large impurities at an early stage, the blender mixes tufts from many bales so that properties become uniform, and the fine cleaner removes the remaining small trash just before the card. Coarse cleaning comes before blending so that trash is not spread through the mixture.

The main purpose of opening is to loosen compressed material into small, more voluminous tufts, which improves the performance of later cleaning and mixing. Fibre individualisation and parallelisation belong to carding and drafting, and removal of short fibres belongs to combing, so the blowroom does neither.

Cleaning machines in series remove trash in sequence. If machines with efficiencies E_1, E_2, \dots, E_n act in series, the fraction of trash that survives is \prod_i (1-E_i), so the overall efficiency is E = 1 - \prod_i (1-E_i). For identical beaters of efficiency E_b, E = 1-(1-E_b)^n. Efficiency rises with each added beater but never reaches 100 %.

Opening also gives each tuft an impulse. By the impulse-momentum relation the average force is F = m(v_2 - v_1)/\Delta t. With m in kg, speeds in m/s and time in s, the force comes out in N. For example, a 20 g tuft accelerated from rest to 10 m/s in 0.01 s experiences F = 0.02 \times 10 / 0.01 = 20 N.

Comber machine components

A cotton comber works on a lap through cooperating parts, each with its own motion. The nipper clamps the fringe of the lap and oscillates back and forth. The top comb is a fine comb that reciprocates through the tuft and combs the trailing ends of the fibres, which removes short fibres and neps and straightens the ends. The cylinder comb (circular comb) rotates continuously in one direction and combs the fringe projecting from the nip. The feed roller advances the lap intermittently in one direction, and the drawing rollers in the drawbox rotate forward only to draw the sliver out.

The detaching rollers are the only part in this group that reverses. They turn forward to draw the combed tuft out of the nip and backward to set back the lap sheet so that the next cycle starts correctly.

• Nipper — the clamp that holds the fibre fringe; it oscillates rather than rotates. e.g. The nipper closes on the lap, then opens so the fringe can be combed.
• Top comb — a fine comb working on the trailing fibre ends after detachment. e.g. Combing the trailing ends removes short fibres left behind in the tuft.

Gégauff's noil theory explains how combing settings control waste. Noil rises when the detaching setting is wider, because more short fibres escape the holding nip. Noil falls with a narrower detaching setting in the backward feed, a higher forward feed length per combing cycle (more fibre is presented to the comb), and a lower backward feed length.

Carding machine settings

The number of taker-in teeth acting on each fibre equals the rate at which taker-in teeth pass the feed point divided by the rate at which fibres arrive there. Raising the taker-in speed increases tooth passage, and lowering the feed roller surface speed reduces fibre arrival. Both raise the teeth acting on each fibre. Increasing the linear density of the feed (more fibres per unit time), or using shorter and finer fibres, lowers the teeth per fibre.

The draft in each zone is the ratio of surface speeds of adjacent elements. The feed roller is much slower than the taker-in, so the draft P between them is the largest. The draft Q between taker-in and cylinder is intermediate, and the draft R between cylinder and doffer is the smallest because the doffer runs far slower than the cylinder. The order is therefore R < Q < P.

The cylinder-to-flat setting controls carding intensity. A wider setting weakens the carding action, so fewer entangled clusters are opened and more neps remain in the sliver. A closer setting increases carding action and fibre rupture. Flat strips and trailing hooks arise from other causes, not from this setting.

Neps are small entangled knots of fibre. They are removed by the comber, which combs them out together with short fibres, and by the card, through the cylinder-flat carding action. Blowroom beating does not remove neps and can even create them. The roving frame only draws and twists the sliver, so it does not remove neps.

Carding cylinder and doffer

The doffer is a small roller that strips the carded web from the cylinder. It runs at a much lower rotational speed than the cylinder and carries wire points of lower density.

A periodic defect caused by one damaged zone of cylinder wire recurs once per cylinder revolution. The sliver wavelength is \lambda = V_d / f, where V_d is the sliver delivery speed and f = n N_c is the defect frequency. Here N_c is the cylinder speed in rev/s and n is the number of defect zones per revolution. If the cylinder speed is doubled while the delivery speed is unchanged, the wavelength is halved. For example, one defect zone on a cylinder turning at 2 rev/s with delivery speed 0.5 m/s gives \lambda = 0.5/2 = 0.25 m.

Carding occurs where wire points work fibres against one another. This happens at the cylinder-flat interface, where the flat wires comb against the cylinder wire, and at the cylinder-doffer interface, where the doffer strips the web. The feed roller to licker-in interface is a feeding and coarse opening zone, and the licker-in to cylinder interface is a transfer zone.

Draw frame doubling and sliver orientation

In a drawframe the delivered linear density is \text{Ld}_{out} = n \cdot \text{Ld}_{in} / \text{draft}, where n is the number of slivers fed. When the draft equals the doubling number, the linear density is unchanged. Drafting straightens and parallelises the fibres, so their orientation improves.

Doubling combines slivers by adding their linear densities. For independent variations, variances add, so for n identical slivers with standard deviation \sigma, the doubled sliver has \sigma_d = \sqrt{n}\,\sigma while its mean is n times larger. The coefficient of variation therefore falls by a factor of 1/\sqrt{n}. For example, four slivers each with standard deviation 0.2 ktex give a doubled sliver with standard deviation 0.2\sqrt{4} = 0.4 ktex at four times the mean, so the relative variation halves.