Abstract:
An apparatus for the manufacture of corrugated plastic pipes comprises half shells. Each half shell has a base body which consists of a metal of a higher thermal conductivity and a lower specific gravity than steel and which holds a core. Provided in the core is a mold recess in which to form the corrugated pipes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus for the manufacture of corrugated plastic pipes with cross grooves, in which half shells, which are provided with a mold recess and combine in pairs to form a mold in a molding path which runs in the direction of production, are disposed to circulate on a horizontal machine bed, which is provided with a cover plate of antifriction metal; the half shells resting by their lower bearing surfaces on the cover plate and being pressed together transversely of the direction of production by means of a guide; an extrusion head of an extruder being disposed upstream of the molding path; the half shells being provided with vacuum ducts, which are connected to the respective mold recess and which, in the molding path, open into vacuum connections formed in the machine bed; and the half shells being provided with coolant lines, which, in the molding path, open into coolant connections and coolant outlets formed in the machine bed. 
     2. Background Art 
     In an apparatus of the generic type known from U.S. Pat. No. 4,492,551, the half shells, which combine in pairs to form a mold, are displaced while sliding on a horizontal machine bed. For a sliding motion almost free from friction and wear to be attained, the machine bed has a cover plate of an antifriction metal, for instance of bronze, on which to displace the half shells of steel. Cooling water supply and discharge takes place via corresponding connections in the machine bed, the water line inlets and outlets which are formed in the half shells overlapping these connections on the molding path. The same applies to the vacuum ducts. 
     EP 0 359 089 B1 teaches an apparatus for the manufacture of corrugated plastic pipes, in which the half shells, which combine in pairs to form a mold, are guided on two chains which rest one on top of the other. Support elements are fixed to the chains, on which the half shells are replaceably mounted. The vacuum ducts and the cooling water lines must be guided through these support elements to the half shells. This requires extraordinarily complicated constructions. The guidance of the water lines and the vacuum ducts is complicated, which can lead to losses of cooling water and vacuum after a comparatively short operating time. Wear occurs on the parting planes between the half shells and the support elements; this can lead to a misalignment of the half shells after a short operating time and thus to increased tolerances in the corrugated pipes to be produced. 
     DE 197 02 647 C1 teaches an apparatus for the manufacture of corrugated plastic pipes, in which the half shells are replaceably fixed on guide and slide elements. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody an apparatus of the generic type so that an increase in capacity is feasible without an increase in wear at a low construction cost. 
     According to the invention, this object is attained by each half shell having a core, which consists of a metal of a higher thermal conductivity and a lower specific gravity than steel, in which the mold recess is formed, in which the vacuum ducts are formed at least substantially, and which is sheathed at least in the shape of an L by a base body of steel, on which the bearing surface is formed and on which rests the guide. Based on the measures according to the invention, only unimportant constructional requirements are needed to ensure that the half shells consist at least to a considerable extent of an excellently heat conducting and lightweight material. This offers the possibility of accelerating the production rate, accompanied with sufficiently good heat dissipation. No substantial wear will occur due to the fact that the guides, which press the half shells together, bear against steel and the half shells slide by their lower bearing surfaces on the cover plate of antifriction metal. The coolant and the vacuum are transferred via only one sealing and sliding surface, namely via the machine bed directly into the half shells—as in the apparatus of the generic type. 
     The embodiment according to which the core is at least substantially provided with a rectangular cross section with outer surfaces, on which rest inner surfaces of the base body, ensures that the core rests over its full surface in the base body, which ensures good thermal conductivity. When the base body is formed in one piece and when the base body is approximately C-shaped and holds the core, this reflects a simple design, featuring an especially stable half shell which nevertheless possesses the advantages of the invention. In particular in the case of this design, the core may have varying mold recesses; the base body may be used for cores of varying mold recesses. In this embodiment, the cooling water lines may be formed in the base body so that no problems of sealing will occur. A multi-piece, in particular two-piece design consists in that the base body is comprised of a lower plate and a plate-type vertical web and in that the base body is formed in two pieces. 
     The direct contact between neighboring half shells takes place via the base bodies, i.e. via the metal of greater stability, namely steel, which helps avoid wear of the core. This is especially advantageous in the embodiments according to which half shells which adjoin in the direction of production in the molding path bear against each other by their base bodies and according to which the half shells which combine in pairs to form a mold bear against each other by their base bodies in the molding path. 
     A particularly low-wear guide comprises guide rollers mounted by means of rolling bearings. A preferred material for the core is aluminum or an aluminum alloy. In another modified embodiment the core has a semi-cylindrical outer surface. 
     Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a plan view of an apparatus according to the invention; 
     FIG. 2 is a frontal view of a pair of half shells of the apparatus according to the invention; 
     FIG. 3 is an interior view of the half shell according to FIG. 2; 
     FIG. 4 is a frontal view of a modified embodiment of a half shall in accordance with the arrow IV of FIG. 5 
     FIG. 5 is an interior view of a half shell in accordance with the arrown V of FIG. 4; and 
     FIG. 6 is a frontal view of another modified embodiment of a half shell. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen in FIG. 1, the apparatus for the manufacture of plastic pipes with cross grooves, so-called corrugated pipes, comprises a machine bed  1 , on which half shells  2  and  2 ′ are disposed, which are joined to each other, forming two so-called chains  3  and  3 ′. To this end, a bracket  5  is articulated by means of a pin  6  to each half shell  2  and  2 ′ in the outward front portion thereof which leads in the direction of production  4 ; this bracket  5  is mounted on the succeeding half shell  2  in the corresponding place likewise by means of such a pin  6 . By their rear end seen in the direction of production  4 , the chains  3 ,  3 ′ thus formed are guided along feed rollers  7  which serve as deflection wheels. Upon circulation of the chains  3 ,  3 ′ in the direction of the arrows  8 ,  8 ′, the individual half shells  2 ,  2 ′ are moved into a molding path  9  where two half shells  2 ,  2 ′ are united to form a pair; pairs of shells which are successive in the direction of production  4  lie close together. For rapid closing of the half shells  2 ,  2 ′ into a parallel and adjoining position, so-called closing rollers  10  are provided, which accelerate the joining of the—in the direction production  4 —rear ends of the half shells  2 ,  2 ′. 
     In the molding path  9  itself, the adjoining half shells  2 ,  2 ′ are pressed against each other by means of guide rollers  11 , which are mounted into guide rails  12  for rotation by means of rolling bearings. The feed rollers  7  are mounted on the machine bed  1  for rotation about axle ends  13 . 
     At the front end, seen in the direction of production  4 , of the machine bed  1 , return rollers  14 , which also serve as deflection wheels, are mounted for rotation about axle ends  15 ; the chains  3  and  3 ′ are deflected by these return rollers  14  and guided back to the feed rollers  7 . As seen in FIG. 1, the guide rails  12  with the guide rollers  11  terminate by the length of several half shells  2  and  2 ′ before the return rollers  14  so that the half shells  2  and  2 ′ can be moved apart parallel to each other and crosswise of the direction of production  4  prior to being deflected by the return rollers  14 . 
     An indentation  16  is formed on the upper side of the half shells  2 ,  2 ′, the two indentations  16  of the half shells  2 ,  2 ′ which are allocated to each other in pairs being in alignment so that a common driving pinion  17  can engage with this indentation  16  from above, pushing the half shells  2 ,  2 ′ in the molding path  9  as a closed mold through the molding path  9 . Actuation of this driving pinion  17  takes place in the usual way by a motor (not shown) via a driving gear  18  which is non-rotatably fixed on a shaft  19 , the shaft  19  again carrying the driving pinion  17 . The shaft  19  is run in a bearing  20  which is supported on the machine bed by way of spacers  21  and joined thereto by means of fasteners  22 . 
     The apparatus illustrated serves for the manufacture of corrugated plastic pipes  23  having a cross-groove profile, i.e. with cross grooves  24  which encircle over the periphery thereof, as they are used for instance as protecting tubes for electric cables. An extruder is provided to this end, only the extrusion head  25  of which is roughly outlined, extruding a tube (not shown) which, while still in a thermoplastic condition, arrives in the mold which is formed in the molding path  9  and in which the cross-groove profile is formed. So-called twin-wall pipes can be produced in the same way by this apparatus, which are externally similar to the pipe  23  and which are internally provided with a continuous smooth pipe. 
     The half shells  2 ,  2 ′, which are allocated to each other in pairs, are cooled in the molding path  9 ; moreover, molding the cross grooves  24  takes place by vacuum actuation of the mold cavity  26  formed in the molding path  9 . As far as described hereinbefore, the apparatus is known from U.S. Pat. No. 4,492,551. 
     As seen in FIG. 2, the machine bed  1  comprises a base plate  27  of steel and a cover plate  28  of an antifriction metal, for instance bronze, placed thereon. Each half shell  2 ,  2 ′ comprises an outer base body  29 ,  29 ′ of steel and a core  30 ,  30 ′ of aluminum or an aluminun alloy. The core  30 ,  30 ′ has plane outer surfaces  31   a ,  31   b ,  31   c  of as a rule rectangular arrangement, which rest over their full surface, i.e. tightly, on inner surfaces  32   a ,  32   b ,  32   c  of the base body  29  and  29 ′ which define a recess that holds the core  30 ,  30 ′, so that direct heat conduction may take place from the core  30 ,  30 ′ to the base body  29 ,  29 ′ via all the outer surfaces  31   a ,  31   b ,  31   c  and inner surfaces  32   a ,  32   b ,  32   c . As seen in FIG. 2, each base body  29 ,  29 ′ has the shape of a C. It has a horizontal upper leg  29   a , a vertical web  29   b  and a horizontal lower leg which constitutes a lower plate  29   c . As seen in FIGS. 2 and 3, the core  30 ,  30 ′ is fixed in the direction of production  4  by means of a centering pin  33  which is disposed in the contact area between an outer surface  31   c  and an inner surface  32   c . Fixing a core  30 ,  30 ′ in the base body  29 ,  29 ′ is effected by means of fasteners  34  which—as seen in FIG.  2 —are screwed exclusively into the base body  29  and  29 ′, only their head  35  engaging with the core  30 ,  30 ′. 
     The half shells  2 ,  2 ′ rest on the cover plate  28  by their bearing surfaces  36  which are formed on the respective base body  29 ,  29 ′. Cooling water lines  37  extend from the cover plate  28  through the base body  29 ; they are formed by bores executed in the proximity of the vertical inner surface  32   b . The water lines  37  are formed exclusively in the base body  29  and  29 ′ at a very short distance a from the inner surface  32   b . Cooling water is supplied to the water line  37  at the inlet  38 ; it flows through the water line  37  in the direction of flow  39  and emerges from the base body  29  and  29 ′ at the outlet  40 . 
     Vacuum ducts  41  extend from the bearing surface  36  upwards through the base body  29 ,  29 ′ and the core  30 ,  30 ′ as far as into the direct neighborhood of the mold cavity  26 . From the vacuum ducts  41 , numerous vacuum slits  42  open into the mold recesses  43  which are provided in the half shells  2 ,  2 ′ for the formation of a mold cavity  26  and which have a shape complementary to the contour of the pipe  23 . There is no need of a special seal between the outer surface  31   c  and the inner surface  32   c  in the vicinity of the respective vacuum ducts  41 , since these surfaces rest tightly one upon the other and a small amount of misled air is of no importance in practice. The airflow direction in the vacuum ducts  41  is marked by the arrows  44 . Vacuum connections  45  are provided in the machine bed  1 , which the vacuum ducts  41  overlap in the molding path  9 . These vacuum connections  45  open into a vacuum chamber  46  which is attached to the underside of the base plate  27  and into which opens a connection  47  of a vacuum pump (not shown). 
     In a comparable manner, the inlets  38  and the outlets  40  of the water lines  37  overlap cooling water connections  48  and corresponding cooling water outlets  49  formed in the machine bed  1 . The water connections  48  are connected to a central cooling water inlet line  50  and the cooling water outlets  49  to a central cooling water outlet line  51 . 
     For a change of mold, cores  30 ,  30 ′ of identical outer dimensions can be used, which have varying mold cavities  26  and correspondingly varying mold recesses  43 , the same base bodies  29 ,  29 ′ being used. 
     In the embodiment according to FIGS. 4 and 5, a core  52  of aluminum, an aluminum alloy or any other suitable and excellently thermo-conducting material is available, which is greater as compared to the total cross-sectional surface of the respective half shell  2 ,  2 ′ and which is provided with an indentation  16  on its upper outer surface  53   a . The core  52  is sheathed by a base body  54  which is comprised of a plate  54   b  which forms a vertical web and of a lower plate  54   c  which forms a horizontal lower leg. As seen in FIG. 4, the base body  54  is of a two-piece design. In the same way as with the embodiment according to FIGS. 2 and 3, the base body  54  of the embodiment according to FIGS. 4 and 5 extends over the full length of a half shell  2 ,  2 ′ so that half shells  2 ,  2 ′ which adjoin in the direction of production  4  bear against each other by their base bodies  29 ,  29 ′ and  54 , respectively. The base bodies  29 ,  29 ′ and  54  also extend as far as to the parting plane between two half shells  2 ,  2 ′ of a shell pair. 
     The vertical web  54   b  and the lower plate  54   c  are united with the core  52  by means of dowel screws  55 . Water lines  56  and vacuum ducts  57  run from the bearing surface  36  through the core  52 , in this embodiment also the water lines  56  being formed substantially within the core  52 . Consequently, a seal  58  must be provided between the lower plate  54   c  and the core  52 . 
     With this embodiment, the entire half shells  2 ,  2 ′ are replaced for a change of mold. 
     FIG. 6 illustrates an embodiment which is extraordinarily similar to that according to FIGS. 2 and 3 and to which fully applies the interior view according to FIG.  3 . Therefore, in FIG. 3 the reference numerals newly introduced for FIG. 6 are put in brackets beside the reference numerals used for the embodiment according to FIGS. 2 and 3. As far as parts are identical, identical reference numerals are used. If the parts are functionally identical, but differ constructionally, the reference numerals of FIGS. 2 and 3 are used, however provided with a double prime. 
     The embodiment according to FIG. 6 differs from the embodiment according to FIGS. 2 and 3 by the outer base body  29 ″ of steel having a core  30 ″ of aluminum or an aluminum alloy which has a semi-cylindrical outer surface  31 ″. Correspondingly, the recess in the base body  29 ″ which holds the core  30 ″ is defined by a semi-cylindrical inner surface  32 ″ on which the core  30 ″ rests over its full outer surface  31 ″, i.e. by tight fit. The way in which the core  30 ″ is fixed in the base body  29 ″ and the otherwise design of the base body  29 ″ and the core  30 ″ corresponds to the embodiment according to FIGS. 2 and 3.