Patent Publication Number: US-2004047932-A1

Title: Pipe mold apparatus with contact and air cooling of plastic in a mold tunnel

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a pipe molding apparatus and means to provide cooling of the plastic material in the mold tunnel of the apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002] A known pipe molding apparatus includes what is known in the industry as a moving mold tunnel which shapes molten plastic to form the pipe. The molten plastic must be properly cooled in the mold tunnel to hold the shape of the pipe. This is particularly true in the case of a pipe having a profiled outer surface i.e., ribbed pipe or double wall corrugated pipe.  
       [0003] In the known pipe molding apparatus a plastic cooling device known as a cooling plug is provided internally of the mold tunnel. The molten plastic flowing to the mold tunnel is forced onto the external surface of the cooling plug. This therefore provides cooling to the interior surface of the pipe formed in the mold tunnel.  
       [0004] The purpose of the cooling plug is to set the interior dimension of the pipe at the point where the molten plastic enters the mold tunnel i.e., not only does the mold plug cool the plastic material but it additionally provides a molding surface to define the interior shape of the pipe. Once the plastic material has set up sufficiently to hold its own shape which occurs relatively quickly within the mold tunnel the molding surface provided by the cooling plug is not needed. Therefore the length of the cooling plug is relatively short in comparison to the overall length of the mold tunnel.  
       [0005] In the conventional molding apparatus no further interior cooling of the pipe is provided downstream of the cooling plug within the mold tunnel. However, the plastic material of the pipe although as noted above being sufficiently cooled to hold its own shape still maintains substantial heat energy downstream of the cooling plug.  
       [0006] The known methods of cooling the plastic internally of the pipe in the mold tunnel have very little effect on cooling the outer wall of the pipe particularly if the pipe wall is thickened to provide a profiled outer pipe surface. This would be the case with a ribbed pipe having ribs projecting from the outer wall of the pipe and also with a double wall corrugated pipe.  
       [0007] In order to provide cooling of the outer wall of the pipe in a mold tunnel Corma Inc. of Toronto, Ontario, Canada has developed a cooling system which introduces a cooled gas between the inner surface of the mold tunnel and the outer surface of the pipe. This cooling system helps to cool the exterior surface of the pipe. However, even with the Corma innovation it is difficult to provide a cooling of the pipes outer wall sufficient to hold the shape of the pipe over the full length of the mold tunnel or even after the pipe leaves the mold tunnel. This difficultly arises as a result of the outer wall being re-heated by the heat remaining to the interior surface of the pipe as earlier described.  
       SUMMARY OF THE PRESENT INVENTION  
       [0008] The present invention relates to a molding apparatus for molding plastic pipe with a profiled outer pipe wall with additional interior pipe cooling not found in the prior art. This additional material cooling of the pipe helps to hold the shape of the pipe during both travel of the pipe through the mold tunnel and after the pipe leaves the mold tunnel.  
       [0009] More particularly, the molding apparatus of the present invention includes a mold tunnel in which plastic material is shaped to form the pipe with its profiled outer pipe wall. The mold tunnel includes a first plastic cooler against which the plastic material flows for contact cooling of the plastic material. It further includes a second plastic cooler downstream of the first plastic cooler. The second plastic cooler provides an air cooling of the plastic material which surrounds the second plastic cooler in the mold tunnel.  
       [0010] According to a preferred aspect of the present invention the second plastic cooler is of lesser diameter than the first plastic cooler such that it is out of contact with the plastic material. The second plastic cooler then provides cooling air either through a radiant cooling or by positive air pressure from the second plastic cooler onto the interior surface of the plastic pipe. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The above as well as other advantages and features of the present invention will be described in greater detail according to the preferred embodiments of the present invention in which;  
     [0012]FIG. 1 is a schematic view of a pipe molding apparatus according to a preferred embodiment of the present invention;  
     [0013]FIG. 2 is an enlarged sectional view through the upstream region of the mold tunnel including first and second plastic coolers according to a preferred embodiment of the present invention; and  
     [0014]FIGS. 3 through 6 show different arrangements of first and second plastic material mold tunnel coolers according to different preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION IN WHICH  
     [0015]FIG. 1 shows a pipe molding apparatus generally indicated at  1 . This apparatus includes a plastic extruder  3  which delivers molten plastic through a die  5  to a corrugator  7 . The corrugator forms a plastic pipe  9  having a profiled exterior surface.  
     [0016] Provided interiorly of the apparatus between the two endless belts forming the corrugator  7  is an internal pipe cooling system generally indicated at  11 .  
     [0017]FIG. 2 of the drawings shows that the downstream end  13  of die  5  includes first and second plastic delivery channels  15  and  17  respectively. A flow of plastic  16  emerges from channel  15  to form the profiled outer wall of the pipe. This profile is shaped by troughs  25  separated by crests  23  on the inner face of mold blocks  21 . These mold blocks lie side by side with one another within the endless belts of the corrugator  7 .  
     [0018] Also provided in the corrugators  7  are mold blocks  27  having recessed mold faces  29 . These recessed mold faces  29  of the mold blocks  27  are used to form coupling portions of the profiled pipe.  
     [0019] A flow of plastic  18  from the passage  17  is used to form the inner wall of the pipe. This inner pipe wall adheres to the flats between the corrugations of the outer pipe wall.  
     [0020] The description above relates specifically to a double wall pipe having corrugations on the exterior surface of the pipe. It is to be understood that the present invention which is to be described in greater detail below also pertains to the formation of ribbed pipe made from a single flow of plastic used to provide both the ribs to the exterior surface of the pipe and the inner wall of the pipe.  
     [0021]FIG. 2 also shows very generally the interior pipe cooling system generally indicated at  11 . This cooling system comprises a first plastic cooler  31  and a second plastic cooler  35  downstream of cooler  31 .  
     [0022] Plastic cooler  31  referred to as a cooling plug has a cooled exterior surface  33  which is spaced from the interior faces of the mold blocks by a distance which is only sufficient to allow the flow of plastic material such that the plastic rides along the exterior surface of the cooling plug and is forced against the interior faces of the mold blocks. Therefore cooling plug  31  acts as both a cooling device and an interior shaping die which is in direct contact with the interior surface of the majority of the length of the pipe. The only exception is at the coupling regions of the pipe where the faces  29  of mold blocks  27  are spaced outwardly from the exterior surface of the cooling plug.  
     [0023] As will be clearly seen in FIG. 2 of the drawings the second plastic cooler  35  is of reduced diameter relative to cooling plug  31  such that the plastic material on the interior faces of the mold blocks is spaced from and out of contact with the exterior surface of cooler  35 .  
     [0024]FIG. 3 shows further details of the cooling system  11  of FIGS. 1 and 2. Here it will be seen that cooling plug  31  although having a flat external surface  33  to shape the flat inner wall of the pipe includes fluid flow channels  34  directly below surface  33 . These fluid flow channels carry a cooling medium which could either be a chilled liquid or a chilled gas directly below the external cooling surface  33  of the cooling plug. The cooling provided by the medium in channels  34  is transferred through the flat wall  33  of the cooling plug to the interior surface of the pipe by contact between the pipe inner wall and the exterior surface of the cooling plug.  
     [0025] The second plastic cooler  35  as noted above does not make contact with the plastic material in the mold tunnel. Therefore rather than being a contact cooler it provides an air cooling of the plastic material in the mold tunnel.  
     [0026]FIG. 3 shows that according to one embodiment of the invention the exterior surface  36  of cooler  35  is formed by a continuous hollow coil which runs around the exterior surface of the cooler. This coil contains a fluid medium which can either be a liquid or gas which is chilled exteriorly of the mold pipe and then forced to circulate through the coil  36  on the surface of the cooler. Note that coil  36  does not have to be covered by a flat plastic shaping wall since it is not in contact with and does not provide a shaping surface for the inner wall of the pipe. Therefore the cooling effect provided by the medium in the outer surface of cooler  35  is able to radiate without having to go through any type of a barrier onto the inner surface of the plastic material in the mold tunnel which surrounds cooler  35 .  
     [0027] The provision of cooler  35  downstream of the cooling plug provides a number of benefits. Firstly, there is an extended length interior cooling of the pipe. In the past, interior pipe coolers without a secondary cooler of the present invention have not been able to sufficiently cool the inner wall of the pipe so as to prevent it from continuously heating the exterior wall of the pipe as the pipe is moving downstream through the mold tunnel. The secondary air cooler of the present invention downstream of the first contact cooler helps to overcome this problem so that it is much easier to provide effective cooling for both the interior and the exterior surface of the pipe.  
     [0028] In addition, prior cooling systems which have made contact with the inner wall of the pipe have not been able to effectively cool the belled out coupling regions of the pipe. This is because these contact cooling systems do not reach into the recessed faces of the mold blocks for forming the coupling regions. Furthermore they are not designed to radiate cooling to the couplings or other similarly belled parts of the pipe. The secondary cooler of the present invention which uses an outward radiation of cooling rather than a contact cooling is able to move cooling air into the belled regions of the pipe.  
     [0029]FIG. 4 shows a further embodiment of the invention which again uses cooling plug  31  but which uses a modified second plastic cooler  41  downstream of the cooling plug. This second plastic cooler unlike the cooler  35  in FIG. 3 is separated from and located at a spaced distance downstream from the cooling plug. Provided internally of the hollow construction of cooler  41  is a blower  43 .  
     [0030] Cooler  41  not only uses radiant cooling to move cooling air from its exterior surface to the plastic material surrounding the cooler but additionally uses pressurized air for cooling of the plastic material. The positive air pressure provided by blower  43  provides a circulation of the cooling air. This circulation is able to flow around both the upstream and the downstream end of cooler  41  because the downstream end of the cooler is open and the upstream end of the cooler is separated from the cooling plug.  
     [0031] One of the features resulting in the use of a pressurized cooler such as cooler  41  is that any relatively cooled air otherwise trapped within the mold tunnel and normally having a tendency to settle in the lower half of the horizontally extending mold tunnel is circulated by the blower of the second cooler mold. Usually the hotter air will tend to rise to the upper side of the mold tunnel. This results in an uneven cooling of the pipe in the mold tunnel which produces a bending of the pipe as it emerges from the mold tunnel. However, through the provision of blower  43  the relatively cool air which normally drops down into the lower part of the mold tunnel is circulated by the blower producing a more even distribution of the cooling air around the entire inner circumference of the pipe. This in turn produces a straighter pipe emerging from the mold tunnel.  
     [0032]FIG. 5 shows another preferred embodiment cooling system generally indicated at  45  for cooling the interior of a plastic pipe with profiled exterior surface. Cooling system  45  which again locates interiorly of the mold tunnel of the pipe forming apparatus comprises an upstream cooling plug  47  having an exterior surface  48  which is in direct contact with the interior surface of the plastic material and further includes a second downstream plastic material cooler  49 . Consistent with the earlier embodiments cooler  49  is of reduced diameter to cooler  47 .  
     [0033] In the embodiment shown in FIG. 5 cooler  49  is formed by a cooling medium circulation coil  53  with gaps  55  between the helical turns of the coil. Provided internally of the coil is a blower  51 .  
     [0034] In this particular embodiment the blower  51  not only circulates the cooling air to the opposite ends of cooler  49  but additionally produces a positive air pressure directly through the open construction of the main body of the cooler. Therefore, all parts of the cooler are effectively exposed to the plastic material with the air pressure created internally of the cooler sweeping from the inside of the coils through the gaps  55  in the cooler and then outwardly around the coils onto the plastic material surrounding the cooler.  
     [0035] Although FIG. 5 shows a coil style cooler it is to be understood that any other type of open construction cooler will provide the same benefits. For example, a squirrel cage fan housing is presently being used in furnace humidifiers and the like. This same construction modified according to the present invention to additionally include cooling medium channels in the cage construction of the housing could be used in lieu of the coil type construction of the second plastic cooler shown in FIG. 5 of the drawings.  
     [0036]FIG. 6 of the drawings shows yet another preferred embodiment feature of the present invention. This feature is provided in a cooling system generally indicated at  61 . Cooling system  61  includes an upstream contact cooler  65  and a downstream air cooler  69 . The downstream cooler comprises a cooling medium circulation wall  71  and the upstream cooler has a cooling medium circulation wall  67 . In this particular embodiment the cooling wall  71  of the downstream cooler is connected by a fluid passage  73  to the cooling wall  67  of the upstream cooler. This allows both of the coolers to use a common cooling medium which flows between the two coolers. Once again this cooling medium could be a chilled gas or could be a chilled liquid. The upstream cooler provides the benefits of shaping and cooling the portions of the pipe wall in contact with it while the downstream cooler provides the benefit of air cooling to the entire inner wall of the pipe including the belled portions of the pipe wall which will not contact the external surface of the upstream cooler.  
     [0037] A further preferred feature of the present invention is shown in FIG. 2 of the drawings. Here it will be seen that the cooling system further includes a plate  37  downstream of the air cooler. This plate is only slightly smaller in diameter than the inside wall of the pipe. It therefore blocks the cooled air from the second plastic cooler from escaping through the open downstream end of the mold tunnel. Accordingly the cooled air is maintained within the tunnel where it continues to cool the inner wall of the pipe.  
     [0038] Although various preferred embodiments of the present invention have been described in detail, it will be appreciated by those skilled in the art that variations may be made without departing from the spirit of the invention or the scope of the appended claims.