Apparatus and method for heating pipes made of thermoplastic material

Abstract

Described is an apparatus for heating end portions of pipes made of thermoplastic material, comprising an internal heating element, designed to be inserted at least partially inside an end portion of a pipe made of thermoplastic material for heating an inner cylindrical surface of the portion, the heating element comprising at least one infra-red ray radiation unit which has an operating zone designed to face at least partially the inner cylindrical surface, the operating zone extending longitudinally along a predetermined direction, parallel to a central axis of the pipe to be heated.

Claims

1. A method for uniform heating of an end portion of a pipe made of thermoplastic material, comprising the following steps: providing an external heating element for heating an outer surface of an end portion of the pipe, providing an internal heating element including an infra-red ray radiation unit, for heating an inner surface of the end portion of the pipe, partially screening infra-red rays emitted by the infra-red ray radiation unit in an intermediate position of the infra-red ray radiation unit, along a longitudinal direction of extension parallel to a central axis of the pipe, using a first screening element having an annular extension about the infra-red ray radiation unit, inserting the infra-red ray radiation unit and the first screening element at least partially into an interior of the end portion of the pipe such that the first screening element is positioned in the interior of the pipe, radially between the infra-red ray radiation unit and the pipe; adjusting the intermediate position of the first screening element relative to the infra-red ray radiation unit in the longitudinal direction depending on a configuration of the pipe.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The technical features of the invention, with reference to the above aims, are clearly described in the claims below and its advantages are more apparent from the detailed description which follows, with reference to the accompanying drawings which illustrate a preferred, no-limiting example embodiment of it, and in which:

(2) FIG. 1 is a schematic lateral elevation view, with some parts cut in cross section to better illustrate others, of a heating apparatus made according to this invention;

(3) FIG. 2 is a schematic front elevation view, with some parts cut away and others in cross section, of the apparatus of FIG. 1;

(4) FIG. 3 shows a schematic perspective view of a detail of a variant of the apparatus of the preceding figures;

(5) FIGS. 4 and 5 show two diagrams.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(6) As illustrated in FIG. 1, the numeral 1 denotes in its entirety a preferred embodiment of the heating apparatus according to this invention.

(7) The apparatus 1 is designed to carry out, with the method described below, the heating of end portions 2 of pipes 3 made of thermoplastic material, introduced through an access opening 4.

(8) Mainly, but not necessarily, the apparatus 1 according to this invention is used for heating pipes 3 made of polypropylene (PP) and made of high-density polyethylene (HDPE).

(9) The end portion 2 of the pipe 3 is heated for being subjected to a thermoforming step, such as the forming of an end bell, not illustrated.

(10) As shown in the accompanying drawings, the apparatus 1 comprises a containment casing 5 inside of which is defined an area A for heating an end portion 2 of the pipe 3.

(11) Inside the area A there are a plurality of heating elements 6 designed to operate by radiation on an outer cylindrical surface 2a of the portion 2 of the pipe 3, also indicated as external heating elements.

(12) The casing 5 comprises a front wall which, near the access opening 4 of the apparatus 1, has a cylindrical portion 7c projecting inwards, that is, towards the above-mentioned heating area A.

(13) The cylindrical portion 7c of the wall 7 comprises a protective screen to prevent the heating of the pipe 3—by the external heating elements 6—outside the desired end portion 2.

(14) In other words, the portion of the pipe 3 which protrudes from the cylindrical portion 7c towards the heating area A constitutes, with the relative length P, precisely the end portion 2 to be heated.

(15) The apparatus 1 also comprises an internal heating element 8 on which is fitted, externally, the pipe 3, at least for a stretch equal to the above-mentioned end portion 2 to be heated.

(16) The internal heating element 8 is designed to heat an inner cylindrical surface 2b of the end portion 2 and, for this purpose, comprises one or more infra-red ray radiation units 9.

(17) The embodiment of the internal heating element 8 shown in FIGS. 1 and 2 has a single radiation unit 9.

(18) The infra-red ray radiation unit 9 is advantageously of the filament type and has an operating zone 10 extending longitudinally along a predetermined direction D for a respective length Q and facing the above-mentioned inner cylindrical surface 2b of the end portion 2.

(19) When the end portion 2 is fitted on the internal heating element 8, the above-mentioned predetermined direction D is parallel to a central axis C of the pipe 3 to be heated.

(20) With reference to FIG. 1, the internal heating element 8 comprises a frame 11 for supporting the radiation unit 9, the frame 11 extending longitudinally along the above-mentioned predetermined direction D.

(21) The internal heating element 8 also comprises, also supported by the frame 11, a first screening element 12 having an annular extension around the radiation unit 9.

(22) The first screening element is designed to neutralise at least partially the infra-red rays emitted by a portion of the operating zone 10 of the radiation unit 9.

(23) The first screening element 12, hereinafter also referred to as the annular screening element, is advantageously made in the form of a thin metal plate.

(24) The first screening element 12 is located in an intermediate position relative to the longitudinal extension of the radiation unit 9.

(25) Preferably, the first screening element 12 is located in an intermediate position along the longitudinal extension of the above-mentioned operating zone 10 of the radiation unit 9.

(26) Advantageously, the internal heating element 8 also comprises, supported by the frame 11, means, not illustrated, for adjusting the relative position of the annular screening element 12 with respect to the radiation unit 9 along the above-mentioned direction D.

(27) The above-mentioned and not illustrated adjustment means therefore allow the position of the annular screening element 12 to be varied along the direction D.

(28) The above-mentioned and not illustrated adjustment means are advantageously of the lead nut and screw type, for a fine adjustment of the position. Alternatively, or in addition, to the lead nut and screw type coupling, the above-mentioned and not illustrated adjustment means comprise a slidable coupling, made between the annular screening element 12 and a rectilinear guide extending parallel to the direction D and supported by the frame 11.

(29) As illustrated in FIG. 1, the frame 11 supports a second screening element 13 positioned at a longitudinal end of the frame 11 projecting inside the pipe 3.

(30) The second screening element 13 is designed to shield the pipe 3 from the infra-red rays directed towards parts of the pipe different from the portion 2 to be heated.

(31) The second screening element 13 therefore has a substantially axially symmetrical shape.

(32) According to the embodiment shown in FIG. 1, the second screening element 13 has, towards the inside of the apparatus 1, a cylindrical extension adjacent to the surface of the pipe 3 with a cavity which surrounds the end part of the radiation unit 9 and, towards the outside, a truncated cone shape designed to facilitate the insertion of the pipe 3 on the internal heating element 8.

(33) According to the embodiment shown in FIG. 3, the shape towards the outside of the second screening element 13 is disc-like.

(34) The apparatus 1 also comprises rotation means, not illustrated and of a substantially known type, for rotating the pipe 3 with the respective end portion fitted on the internal heating element 8, relative to the radiation unit 9, about the central axis C of the pipe 3.

(35) The purpose of this rotation is to render uniform in a circumferential direction the heating of both the inner 2b and outer 2a surfaces of the end portion 2 of the pipe 3 operated by the internal 8 and external 6 heating elements, respectively.

(36) The embodiment of the internal heating element 8 shown in FIG. 3 has, unlike that of FIGS. 1 and 2, two radiation units 9, positioned with the respective filaments parallel to each other and to the direction D, preferably, but not necessarily, without longitudinal offsetting along the direction D.

(37) The annular screening element 12 defines, for the heating apparatus 1, means 100 for conditioning the infra-red rays emitted by the radiation unit 9, the conditioning means 100 being designed to limit, in an intermediate position along the longitudinal extension of the operating zone 10 of the unit 9, the action for heating the end portion 2 of the pipe 3.

(38) According to variant embodiments not illustrated, but falling within the scope of this invention, the conditioning means 100 are defined, in a filament radiation unit 9, by discontinuities in the extension of the filament.

(39) In practice, these discontinuities can consist in a reduced winding of the filament, such as to reduce the energy emitted per unit length, or by an actual absence of filament for a predetermined section of the longitudinal extension of the operating zone 10 of the radiation unit 9.

(40) The longitudinal extension of this section of discontinuity is comparable and similar to the length L indicated above with reference to the annular screening element 12.

(41) In other words, in the case, not illustrated, of eliminating or reducing a longitudinal section of radiating filament of the unit 9, the optimum length of this section can be compared with the optimum length L of the annular screening element 12.

(42) In use, as illustrated in FIG. 1, with methods and apparatus known and not illustrated nor described further, the end portion 2 of a pipe 3 made of thermoplastic material to be heated is fitted on the internal heating element 8.

(43) Whilst the outer cylindrical surface 2a of the portion 2 is heated by the action of the external heating elements 6, the inner cylindrical surface 2b is struck by rays emitted by the radiation unit 9 of the internal heating element 8.

(44) At the same time, the above-mentioned and not illustrated rotation means are activated to rotate the pipe 3 about the relative central axis C.

(45) The operating zone 10 of the radiation unit 9 faces the end portion 2 of the pipe 3 to be heated.

(46) It has been experimentally seen that it is advantageous to extend the operating zone 10, even in a limited manner, beyond the end edge 3a of the pipe 3, for a section 14, as shown in FIG. 1.

(47) With this arrangement of the radiation unit 9, the part of the pipe 3 subjected to greater heating would without doubt be that facing the central part of the above-mentioned operating zone 10.

(48) It has been seen experimentally that in order to achieve an almost uniform heating along the direction of the axis C of the pipe 3, the radiating filament can be configured with a non-uniform power density, that is, suitably reducing the power density at the above-mentioned part of the pipe 3 subjected to greater heating.

(49) Similarly, the elimination of a small portion of radiating part (in practice, the radiating filament) at the central zone subjected to greater heating has been found to be particularly effective.

(50) For this purpose, that is, the creation of a discontinuity in a longitudinal direction in the emission of radiating power, the first screening element 12 is positioned around the radiation unit 9.

(51) The first screening element 12 therefore has a substantially continuous annular surface S extending longitudinally along the direction D for a predetermined length L.

(52) This length L is determined experimentally as a function of certain parameters, including the diameter of the pipe 3 to be heated, the heating capacity of the radiation unit 9 (or units, if there is more than one, as in the embodiment shown in FIG. 3).

(53) Advantageously, in the processing of the pipes 3 most commonly used, the length L is between 1 and 15 mm.

(54) Preferably, the length L is between 2.5 and 9 mm.

(55) It has been seen experimentally that values of the length L greater than those indicated can adversely affect the correct operation of the radiation unit 9, until causing the failure by overheating due to the reflective effect of the annular screening element 12.

(56) The diagrams of FIGS. 4 and 5 indicate how it has been experimentally possible to observe the distribution of heating energy which strikes a portion 2 of pipe 3 facing an infra-red ray radiation unit 9.

(57) More specifically, FIG. 4 represents a diagram showing the energy per unit of surface area relative to a pipe exposed to the radiation of a normal radiation unit 9 (for example, of the filament type normally used in the filed of processing pipes made of thermoplastic material) as a function of the longitudinal extension of the radiation unit (in the axis of abscissas).

(58) FIG. 5 shows instead, in a respective diagram, the energy per unit of surface area relative to the same pipe exposed to the radiation of the radiation unit 9 with the latter equipped with an annular screening element 12 made according to this invention. Also in this diagram, the energy value (axis of ordinate) is represented as a function of the longitudinal extension of the radiation unit (in the axis of abscissas).

(59) The experimental tests performed, the results of which are shown below in a qualitative form, have been carried out with pipes having nominal diameters of 32 mm, 40 mm and 50 mm, (and relative zone to be heated which extends axially for approximately 70 mm) using as the radiation unit an infra-red emission lamp having a power density of 60 W/cm.

(60) It has been seen experimentally that the optimum uniform distribution of heating energy is obtained, with the characteristics listed above, with an annular screening element 12 extending axially with length L of just less than 3 mm.

(61) In short, the adoption of the annular screening element 12, even though it has a very limited extension (in the case described L is less than 3 mm), it has nevertheless allowed the heating energy distribution along the end portion 2 of the pipe 3 to be kept uniform in a particularly effective manner, as can be seen in the diagram of FIG. 5.

(62) For pipes 3 which are larger, for example with nominal diameters of 63 mm or 75 mm, the bell of which requires an end portion 2 to be heated of approximately 75 mm, with a radiation unit comprising an infra-red lamp having a power density of 80 W/cm, the optimum length L of the annular screening element 12 is approximately 5 mm.

(63) For pipes 3 which are even larger, such as those with diameters of 90 mm, 110 mm, 125 mm and 160 mm, the bell of which requires an end portion 2 to be heated extending for approximately 85-100 mm, and an internal heating element 8 comprising two radiation units 9 (lamps with power density of 60 W/cm) the optimum length L of the annular screening element 12 is approximately 9 mm.

(64) These experimental results have also shown that the adoption of the annular screening element 12 results in an overall reduction in the radiating power of the radiation units 9 which is negligible.

(65) Therefore, the adoption of the annular screening element 12, as well as being absolutely effective in achieving a uniform longitudinal heating of the inner cylindrical surface 2b of the end portion 2 of the pipe 3, does not penalise the heating times.

(66) The apparatus 1 according to this invention has major advantages.

(67) A first of these advantages is that of allowing, making uniform the heating effect, the use of short wave radiation units, thus fully exploiting the speed of heating.

(68) A further advantage is the possibility of effectively adapting the heating apparatus to different needs linked with the processing of pipes even with different dimensions.

(69) Moreover, the apparatus 1 according to this invention is suitable for being used, due to the fast heating times which can be achieved, to make belling machines even with a single heating station. In these belling machines, the pipes are transferred to the belling and cooling station at the end of the step for heating in the single oven.

(70) Alternative embodiments of this invention, which are partially equivalent even if less efficient, comprise the making of radiation units with non-uniform power densities, that is to say, wherein a central part has a reduced capacity of emission of infra-red rays. This reduced capacity is achieved, for example, as mentioned, by reducing or eliminating radiating filament for a predetermined section.