Cutting
The cutting of fully cut knitted goods covers the full spectrum of techniques available to producers of woven fabric garments, from hand shears to fully automatic systems.
Cutting systems can be categorised according to the cutting device used. The usual classification is:
(1) hand shears;
(2) hand held power cutters;
(3) fixed location band knives;
(4) die cutter systems;
(5) fully automatic cutter.
Hand shears
These are a widely used tool, extensively used even in fully automated cutting rooms. They are used for the following purposes:
(1) cutting exclusive garments in small quantities;
(2) cutting sample and prototype garments;
(3) cutting shortfalls in orders through miscalculation and rejection during production.
Hand shears vary in weight and blade length and can be designed for right hand or left hand operations. A typical pair of knitted fabric shears will have an overall length of between 20 cm and 30 cm.
In use the lower blade remains in contact with the cutting table surface underneath the fabric, while the upper blade operates above the fabric. The blades, while shearing along a line or curve, are never depressed hilly until the end of a cut, otherwise jagged cutting results. In cutting more than one layer of knitted fabric, great care must be taken to avoid distortion of the lower layer or layers. The more layers, the greater the risk of distortion. Most cutters treat two layers as the maximum that can be cut.
Hand held power cutters
These essentially take two forms: the straight knife and the circular knife.
The straight knife is completely self contained except for its connecting electrical cable. An electric motor, and its associated gearing, drive a vertical blade in a vibrating manner rather like a woodworking jig-saw.
The motor is mounted at the top, with a support column below on a flat plate base. The blade is mounted in front of the support column, the leading edge of which provides a slide track to guide the blade. The column is extremely thin to enable it to - pass through the fabric layers after the cut. The support base rests on the cutting table surface under the fabric and its underneath surface must be made of a low friction material. Roller balls can also be mounted on the base to aid ease of movement, and at least one manufacturer (Bullmerwerk) offers a knife with blown air lift, like a hovercraft.
For safety purposes a vertical guard is mounted in front of the length of the blade exposed above the lay. Also for safety purposes some cutters wear chain mail gloves on their free hand. Sharpening is carried out automatically on the machine when it is clear of the fabric.
Lengths of exposed blade, and thus cutting height, can vary from 12 to 25 cm. The operative manipulates and guides the machine through the fabric layers, along cutting lines, by grasping a centre mounted handle. Sometimes with heavier, taller machines a further handle is mounted on top of the motor.
In many cutting rooms in the underwear and leisurewear industry the straight knife is the tool used to cut multiple layers of fabric into garment portions. It is highly productive; the speed has proved difficult to verify, with different sources giving different figures, but an average consensus suggests
5 m/min. The limitation on speed is the heating of the blade through friction. With some synthetic and with cotton this can be a problem.
With some wide widths of knitted fabric, the ability to get into the centre of a lay may present problems. Often two cutters work as a team one either side of the table. Having cut and removed the edge sections the centre can be repositioned nearer to the edge.
Circular knives
The circular knife machines vary more widely in size than the straight knives. Their construction is similar, with the motor mounted on a support column and base plate. This time, however, the support column contains the axle and drive of a circular rotating blade. It is guarded at the rear and exposed on the front of the machine. Again, a centrally mounted handle is gripped by the operative to support and guide the machine through the work. On the smaller machines the motor itself forms the handle.
Circular cutters are available to cut through layer heights from under 2 mm up to 12 cm. The smaller cutters are, in reality, mechanical shears and are used for that purpose, either cutting single or double plys or cutting across fabric at the end of each layer (ply) to align them.
The larger cutters are used for cutting lays of from 6 cm to 12 cm. Because the blade is essentially cutting on the lower quadrant of the leading edge, when cutting on the maximum depth the upper layers are being cut in advance of the lowest layer by a length equal to the radius of the blade. This is immaterial when the cut is straight, but when a curve is attempted the lower edge is following a different curve from the top. For this reason circular cutters are generally used for splitting a lay up into portions, by cutting straight lines across and, where possible, down the fabric. The portions containing several parts of the garment are then dealt with by straight knife, band knife or die press.
The circular knife cutters can operate at higher cutting speeds than the straight knives, having greater blade mass to absorb friction heat. Straight knives are heavy to push around and cutters soon tire and are prone to repetitive strain injury. Systems are available to support the weight of the cutter while retaining 360° rotation in the plane of cutting. In one example a two piece hinged arm is mounted on a vertical column at the side of the cutting table. The arm has freedom of movement above the cutting table. The arm supports the cutter and carries the majority of its weight while the cutting operative can move the straight knife freely within the reach of the arm without strain. The column can be moved along the edge of the table.
Yet another system uses a pivoted counter-balanced arm mounted on a gantry that spans the cutting table. The weight of the cutter is reduced by the counter-balanced weight, without reducing the freedom of movement.
Band knife
Portions of a lay can be moved from the cutting table to a secondary cutting area, where they are dealt with either by a band knife or a template cutter. This is generally only worth doing for relatively small items and items with intricate detail.
The band knife employs a thin continuous blade that is driven and guided by pulleys in a 'C' frame. A portion of the blade is exposed as it passes through a flat working table on which the work is manipulated. The work itself is moved while the cutting blade remains stationary, so there is a limit to the weight/dimensions of the fabric to be sub-divided.
The table surrounding the blade should have enough space around it to accommodate the sub-divided portions without impeding the work being manipulated. As the work is moved and presented to the blade by hand, band knife cutting is a particularly hazardous task and chain mail gloves should always be used.
Fig. 3.3 shows a manual cutting system with automatic fabric laying up machine (spreader) cutting by straight knife marking with a fabric drill, and finally a band knife for the smaller and intricate pieces.
Die cutting
Template or die cutters are popular with knitted fabric garment producers, particularly those making underwear. The template knives are embedded in compressible foam plastic. The foam/knife composite covers the whole lay and eliminates the need for a marker. The fabric lay is
ed over the composite at the end of the cutting table away from position the spreading zone. The 'sandwich' passes through a hydraulic press that compresses the layers so that the knives cut through the fabric as the foam is deformed under pressure. Typical presses of this type are those made by Samco-Strong of Leicester.
Die cutting is quick, very accurate and excellent for small components. Because the preparation of the die cutters is slow and elaborate, this is not a Quick Response process and is used for the mass production of slow change items such as underwear.
Fig. 3.7 shows a fully automatic system with cloth roll magazine for changing fabric, automatic spreading, lay storage on pallets, air cushion table, and a die cutting machine.
Automatic cutting
Automatic cutters are being introduced into firms making fully cut knitted garments. They are a natural consequence of the computerised pattern making, grading and marker making. Marker information can pass directly to the local computer controlling the cutting machine, or can be conveyed via stored' instructions on disk.
Despite experiments with water jets, laser beams and plasma beams, most auto cutters depend on vibrating straight knives. The knife is carried in a cutting head mounted on a gantry that straddles the cutting table. The knife can rotate through 360° below the head; the head can move freely across the gantry; and the gantry can move on rails along the length of the cutting table. Cutting can therefore take place in any direction over the whole surface of the table (Fig. 3.4).
The tables themselves are specially constructed so that the surface will support the lay and yet allow penetration by the knife through all the layers. Usually such tables have a surface of closely packed nylon or similar plastic bristles (Fig. 3.4).
Spreading is invariably carried out on another table or tables, but sometimes the spreading table is an extension of the cutting table so that a flow pattern is established.
Fig. 3.4 shows an automatic cutting line with cloth roll magazine changer and automatic spreading on to an air cushion table to aid the movement of the lay on to the bristle vacuum cutting table. Cutting is with a CNC-system 2000 auto cutter. The cut portions are assembled into bundles from an end work table.
On one of the Bullmerwerk System 2000 versions, the bristle mat is itself a conveyor.
Essential to the operation of automatic cutters is that the lay remains compact and undisturbed by the progress of the knife at high speed, and by the current of air associated with motor cooling etc. To this end an important feature of automatic cutting tables is that the lay is controlled by an under-table vacuum, maintained by very high powered air pumps.
For example, on Gerbers S-95 table for low and medium ply cutting (up to 10 mm), maintaining the vacuum on a working surface of up to 2 m by 3.6 m requires 30 KVA (kilo volt amperes). On the Gerber 5-91 for plys up to 76 mm on a similar area 70 KVA is required to maintain the vacuum.
The surface of the lay is covered with a plastic film or special paper to contain the vacuum. The lay itself compresses under the vacuum and any movement between plys is eliminated completely.
Cutting speeds are high. Different makers of machines quote up to 20-30 m/min. Such speeds refer to low ply heights or even single layer cutting. On maximum heights with difficult fabrics cutting speeds are reduced and on knitted fabrics are more realistically between 8 and 16 m/min.
Blade heating is the limitation on cutting speed and must be avoided to reduce the risk of fusing or scorching. The cutting heads incorporate automatic blade sharpening.
It can be argued that cutting speeds are irrelevant, and that the gains of automatic cutting lie in the ability to eliminate marker making and to change cutting, fabric type, height of lay, length and width of fabric, etc., to enable a manufacturer to engage in Quick Response.
Some manufacturers producing 'high return' articles in jersey fabrics find it economical to cut single or double layers of fabric automatically and claim that the cost is lower than hand cutting.
Table dimensions of automatic systems are relatively small: 2 m, 2.4 m,
3 m and 3.6 m are common lengths, and 1 or 2 m widths are usual.
Knitted fabrics, like wide woven fabrics, need subdividing longitudinally. Systems are available to carry this out during the spreading process.
Summary
All the cutting methods discussed are used in the fully cut knitted garment industry. All have a valid role to play in particular situations and are tools by which cutting room managers can respond to the ever varying demands of the market.
The balance between low capital cost/high labour cost and high capital cost/low labour cost is complicated by the demands of Quick Response. In this section of production in particular, high output potential associated with high capital cost, is not linked inextricably with mass production.