Rigid centers made of injection-molded plastic, with perforations to make the tubular walls permeable, are widely used. During use, these centers come under specific stresses, in particular centrifugal radial stresses, due to the shrinkage of the yarn-wound in turns to form the spool-during dyeing treatment. This not only causes the cross section of the center to reduce but also results in a considerable longitudinal elongation that causes complications during the handling of the center.

The percentage of the surface area of rigid centers of the abovementioned sort that is perforated is comparatively limited compared with the total area of the center wall. This creates two difficulties: on the one hand the distribution of the liquid, which travels from the inside of the center out through the mass of yarn, is not uniform and in particular can result in irregularities in the coloration of the yarn; while on the other hand the structure of the center is somewhat heavy, which represents a cost as the tube is used only once.

The present innovation greatly reduces the difficulties referred to above and offers further advantages which will become clear in the text that follows.

Basically, the center according to the innovation comprises, on the cylindrical wall, adjacent longitudinal rows of rectangular or square holes with the holes of one row staggered with respect to those of the adjoining longitudinal rows, in such a way that the longitudinal edges of each hole can move toward each other in the intermediate zone under the stress of radial compression, allowing a substantially uniform reduction of the cross section of the center and limited longitudinal elongation.

The holes of adjoining rows are preferably staggered by a distance equal to about one half of their pitch.

The drawing shows a practical embodiment of a center according

to the innovation in an external view and partial section taken on a diametrical plane.

As the drawing shows, the center 1 has conventional shaping on its upper 1A and lower 1 B ends, with a final reduction of diameter 1 C at the top so as to fit into the bottom edge 1 E while the lower end and also the upper end are shaped to allow winding of a reserve of yarn of a few turns that are independent of the spool of yarn, which latter can be accommodated on the main central section of the center.

The area on which the spool of yarn is to be wound is characteristically made permeable by a series of longitudinal rows of holes 5 that are all rectangular or square with rounded corners. Also characteristically, adjoining longitudinal rows are staggered by a distance of one half of their pitch so that the transverse segments 7 that separate the holes 5 of any longitudinal row from each other correspond to the middle portion of the longitudinal side of the holes of the adjoining rows. To obtain this one-half pitch displacement of the holes of one row with respect to those of the adjoining rows, alternate rows include at one end a hole 9A and 9B of much smaller longitudinal dimensions: in practice alternate rows of holes have these reduced holes 9A at the upper end 1A while the other alternate rows have the reduced holes 9B at the lower end 1 B of the center.

This arrangement gives a structure which in the first place is very permeable in the central cylindrical wall of the center, greatly to the advantage of the uniformity achieved in practice in dyeing with liquids that pass out from inside the center through the holes 5 and through the mass of yarn wound in turns to form the spool, resulting in substantially uniform dyeing. In the second place the structure described is sufficiently stiff and light, the advantage of which is that a limited quantity of material is required to produce the center while providing the same strength and stiffness as in the centers with different configurations of the holes. Moreover, the center according to the innovation exhibits, during the treatment to which it is subjected during handling of the wound yarn and in particular during dyeing handling, a slightly shorter elongation than occurs in rigid centers with a

different hole configuration, while at the same time it accepts a suitable reduction in the diametrical dimensions of the center during the treatments applied to the yarn wound upon it.

It is particularly worth pointing out that the structure has a plurality of continuous longitudinal elements 10 roughly parallel to the axis of the center that are formed between one row of holes 5 and the next. This stiffens the center in the axial direction but allows a slight undulating deformation of these longitudinal elements and hence an axial shortening of the center under the centrifugal radial pressures which the center may be subjected to by the shrinkage of the spool of yarn wound on the center in the case of certain treatments such as dyeing. Furthermore, this same tendency to slight undulation also makes it possible for the cross section of the center to be constricted in a radial direction during the said treatments.