Point density (Number of points per unit surface area)
The point (or tip) density has a significant influence on the carding operation. However, the number of points and the speed of rotation of the cylinder must be considered together. It is not simply the total number that is significant, but also the number available per unit of time, i.e. the product of the point density and the speed of movement of the surface. Thus, low point populations can be partially compensated by higher cylinder speeds. (This is not always possible, since the overall result may be deterioration in some quality parameters.)
It must also be kept in mind that the populations of the main cylinder and doffer clothing have to be adapted to each other. In general, the higher the point population, the better the carding effect – up to a certain optimum. Above that optimum, the positive influence becomes a negative one. This optimum is very dependent upon the material. Coarse fibers need fewer points, as they need more space in the card clothing; finer fibers must be processed with more points, since more fibers are present if the material throughput is the same. Point density is specified in terms of points per square inch or per square centimeter, and can be calculated as follows:
Base width (a1)
This influences the point density. The narrower the base, the greater the number of turns that can be wound on the cylinder and, correspondingly, the higher the point population.
Height of the clothing (h1)
The height of metallic clothing on the cylinder today varies between 2 mm and 3.8 mm. The height must be very uniform. It can also exert an influence on the population, since shorter teeth – for a given tooth carding angle – leave space for more teeth. Where shorter teeth are used, the fibers are less able to escape into the clothing during carding and better carding over the total surface is obtained. Clothing with smaller teeth is also less inclined to choke with dirt particles.
Tooth pitch (T)
The population is also determined by the tip-to-tip spacing.
Carding Angle ()
This is the most important angle of the tooth:
- the aggressiveness of the clothing; and
- the hold on the fibers
are determined by this parameter. The angle specifies the inclination of the leading face of the tooth to the vertical. It is described as positive (a, Fig. 124), negative (b) or neutral. The angle is neutral if the leading edge of the tooth lies in the vertical (0°). Clothing with negative angles is used only in the licker-in, when processing some man-made fibers. Since the fibers are held less firmly by this form of tooth, they are transferred more easily to the cylinder and the clothing is less inclined to choke. Carding angles normally fall into the following ranges:
|licker-in||+5° to -10°|
|Cylinder||+12° to +27°|
|Doffer||+20° to +40°|
The tooth point
Carding is performed at the tips of the teeth and the formation of the point is therefore important (Fig. 125). For optimum operating conditions the point should have a surface or land (b) at its upper end rather than a needle form. This land should be as small as possible. To provide retaining power, the land should terminate in a sharp edge (a) at the front. Unfortunately, during processing of material this edge becomes steadily more rounded; the tooth point must therefore be re-sharpened from time to time. Formation of a burr at the edge (a) must be avoided during re-sharpening. The tooth must only be ground down to a given depth, otherwise land (b) becomes too large and satisfactory carding is impossible – the clothing has to be replaced.
The base of the tooth
The base is broader than the point in order to give the tooth adequate strength, and also to hold the individual windings apart. Various forms can be distinguished (Fig. 126). In order to mount the wire, the normal profile ((a) for the licker-in, (b) for the cylinder) is either pressed into a groove milled into the surface of the licker-in (a) or is simply wound under high tension onto the plain cylindrical surface of the main cylinder (b). (d) represents a locked wire and (c) a chained wire. Both can be applied to a smooth surface on the licker-in; in this case a milled groove is no longer necessary.
In order to be able to process as much material as possible with one clothing, the tooth point must not wear away rapidly. Accordingly, a very hard point is needed, although it cannot be too hard because otherwise it tends to break off. On the other hand, to enable winding of the wire on a round body, the base must remain flexible. Each tooth therefore has to be hard at the tip and soft at the base. A modern tooth has hardness structures as shown in Fig. 127 (Graf).