RESEARCH OF POLYMERIC EXTRUDATE WITH SEGMENT-WALL CONSTRUCTION

This paper presents the characteristics of the production of extrudate with axial-symmetrical construction, having a segmented construction wall. Examples of machine tools and devices for manufacturing products with segments and the construction characteristics of a single segment are presented. Methods of shaping the segment wall in forming sockets, characteristics and type of movement made by shaping elements in the process of extrusion in the process line are described. Exemplary results of experimental investigations of the displacement of geometric elements of a single segment and the influence of wall thickness on segment deformation at a given force and speed of deformation under laboratory conditions are presented.


INTRODUCTION
Axial-symmetrical products produced in the extrusion process line in a form of ducts, tubes and profiles are successfully used in various technical solutions. They are made of many different materials (PE, PP and PVC or PB), which are selected for the required functions and specific functional characteristics [4,9,10,13]. There are known methods of extruding plastics covering various materials and technologies leading to obtaining products with various characteristics and properties [3,4,14,15]. Due to the well-known process of their production in the process line, it is possible to easily modify the shape of the product [1,7,12]. This is done most often by changing the tool or by performing additional technological measures, such as shaping the slide ribs, shaping the segmented wall or by spraying an additional layer of sliding agent on the desired surface. This leads to axial-symmetrical structures with uniform or geometrically homogeneous inner or outer surfaces with an additional elevated tribological properties [11]. The geometrically inhomogeneous surface usually has characteristic macrofolds or macroscopies, which may be continuous or discontinuous, run longitudinally, transversally, helically or alternately torsionally. Geometric elements of longitudinal macropervation (grooves, ribs) of appropriate shapes and dimensions can be performed in the extrusion process using special cores or forming sleeves. However, the desired transverse macro transitions can be made in devices with circulating forming sockets. An additional technological process of shaping in the sockets is most often carried out with the use of vacuum extrusion of the extrudate wall, which obtains the final shape during cooling [12]. This technique produces embossing, single-layer or double-layer, and synchronizing the boring process with the movement of forming slots, allows for accurate mapping of the required macro conversions [8].

SHAPING PRODUCTS WITH SEGMENTED WALL CONSTRUCTION
To produce extruded structural walls with characteristic segments, tools are used that form the plasticized extrudate in special slots coupled together in a suitable kinematic system (Fig. 1a). Underpressure or also compressed air, supplied by appropriate conduits, extends the extrudate and then deformation of the wall in accordance with the shape of the cavity [20].
The extrudates with a two-layer wall with an inner cylindrical layer, and the outer sectional ones are produced in turn using special heads. A characteristic feature of this process is the use of two plasticizing systems and only one tool. The desired product is obtained by connecting together co-extruded two extrudates having a smaller diameter on the inner wall and a larger one on the outer wall, when two outer forming shaping nests close on its circumference. They give the extrusion the desired shape and move with it for a specific period of time. After cooling the product section, the mold cavities move to the initial position (Fig. 1b).
In case of a two-layer extrudate, the plasticized material of the first layer flows over the compressed air supply duct (Fig. 2), which enables the mold cavities to be precisely filled. The stream of the second material is directed right towards the conical core and forms the inner layer of the product. The appropriately shaped conical part of the core and the intensity of the compressed air stream (regulated by the valve) allow for precise control of the wall thickness of the extruded product. The shape of the conical core surface is sinusoidal with gentle ridges in which the material contacts ducts [16] the core. The compressed air flows in the appropriate channels and is directed perpendicular to the surface of the flowing material. This results in the "lifting" of the flowing stream ( Fig. 3) of the plastic and forming between it and the core of the air cushion, which prevents premature adhesion of the material to the core surface [16].
Another solution for a segmented wall construction product is extruded with closed air chambers. (Fig. 4). In this case, the cavities form the wall in two stages. In the first stage, high ribs are formed perpendicular to the axis of the extrudate. However, in the second stage, the entire extrudate along with the ribs is pulled through a special sleeve (Fig. 5), which, depending on the size of the ribs and the state of their plasticity, bends them to the surface of the product [17]. By bending and pressing, the individual tops of the ribs adhere to the wall creating, after cooling, closed air chambers on the outer surface of the extrudate.

MACHINES, TOOLS AND DEVICES
Axial-symmetrical products are manufactured on specially designed technological lines (Fig. 6), where the basic machine is a single screw extruder. Its task is to provide the tool with a homogenized material at a given flow rate, pressure and temperature. The custom of polyethylene and polypropyl-  6. The appearance of two plasticizing systems and a head for producing a two-layer extrudate ene extruders is customary, single-screw extruders, whereas poly (vinyl chloride) extrusions are made on twin screw extruders [7,12].
The shape of the outer surface of the extrudate can be transmitted in several mold cavities or, which is more often used, in a larger number of them. In this case, the sockets are connected to each other in a single system with a circulatory crawler exhaust (Figure 7) moving along with the extruded. This requires precise synchronization of the time of turning on the forming vacuum with the movement of the sockets and the feed of the extrudate [20], as well as the cooling time in the sockets after the wall has been formed ( Fig. 8 and Fig. 9).

THE CONSTRUCTION OF THE SEGMENT WALL
The products made of polymer material may differ not only in the appearance, purpose and method of manufacture but also in different prop-erties influenced mainly by the wall construction.
In the case of axially symmetrical products, depending on the dimensions of the segments, i.e. their height, width, spacing between them, and the wall thickness, the extrudate may exhibit increased circumferential stiffness, i.e. resistance to compression, bending and stretching or increased flexibility, with slightly reduced mechanical properties. By analyzing the structure of individual segments, it can be shown that their thickness in individual areas can vary. The elements which have the greatest thickness are those which are first in contact with the forming seat, which are the side and top walls of the segment. The smaller thickness, have the elements of the wall that are thinning due to stretching. In addition, in the case of two-layer pomace, excessive clusters of material may be formed at the bonding points of the layers. This increases the cooling time in this area, which affects the flexibility of the extrudate. As a result of external forces, uneven stress dis-  tribution may also occur in these areas. Thus, an even distribution of wall thickness in the entire segment plays a significant role in obtaining satisfactory mechanical properties of the product. There is a large number of standards and requirements describing testing of entire product structures [1] with segment wall, such as tests for resistance to internal pressure, peripheral stiffness, impact resistance at reduced temperature, and others. In recent years, due to the development of FEM modeling methods (Fig. 10), some of the research can be carried out in virtual space [2,5,19]. However, the input data for the calculations is made by conducting tests on selected extrudate elements, in particular in the area of a single segment. Such tests allow for accurate identification of the correctness of the constructed structure, determining the values of indicators describing the resistance of the segment to concentrated forces acting on the outer wall of the extrudate in a specific area, taking into account the conditions similar to its operation.
For the numerical analysis, the results of experimental research on products in a form of axial-symmetric segment slices with a segmen-   tal wall are needed. Therefore, as an example, the results were presented for the shape of geometrical elements (Fig. 11) of segments and their resistance to external forces (Fig. 12). Segments of extrudate were taken from ducts made of high-density polyethylene (PE-HD), having an outer diameter of 110 mm. The values of the obtained results of metrological measurements are included in the Table 1.
Measurements of segment deformation at a given force acting on individual samples were carried out on the AS-102 type press, in the Labo-ratory of the Polymer Processes Department. An exemplary course of deformation of the sample is shown in Fig. 13. The tests of the mechanical strength of the extrudate were made at different lengths of the punch contact surface with the extrudate section being 4 to 30 mm (Fig. 14), with a constant loading speed of V = 39 mm/s. During the tests, the maximum value of the force at which the largest deformation of the segment occurred was read. Wall displacement value was determined according to the diagram shown in (Fig. 11).
Mechanical strength tests of marcing segments were also performed at different loading speeds: V 1 = 0.104 mm/s, V 2 , = 0.39 mm/s, V 3 = 1 mm/s, V 4 = 2 mm/s. In this case, the length of the marcing samples was 25mm, and the measurement results are shown in Figure 15.

CONCLUSIONS
Research on products with segmented wall construction allows to indicate areas affecting the The above conclusions are also confirmed by tests carried out on samples of constant length at varying loading rates. Based on the results of these tests, it can be noticed that the deformation of segments having structural defects has much lower mechanical strength. It results from uneven distribution of stresses depending on the wall thickness of the segment in its particular areas. Deviations from the assumed dimensions of particular geometric elements of this type of extrudate depend to a great extent on the course and conditions of the extrusion process in the extrusion process line as well as the appropriate synchronization of the extrusion rates with the circulating forming elements.