Abstract:
A method of blow-molding comprises forming a parison of a thermoplastics material, closing a mold around the parison, injecting compressed gas into the parison through a blowing hole to form a molded product that is shaped according to the shape of the mold, then deforming a seal forming portion of the molded product with a heated sealing tool to seal the blowing hole. The seal-forming portion preferably comprises a tubular collar that surrounds the blowing hole.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of blow molding and to a blow-molded product.  
         [0003]     2. Description of the Related Art  
         [0004]     Blow-molding processes are commonly used for manufacturing a wide variety of hollow plastics products including, for example, bottles and containers. Typically, a tube (or parison) is extruded into a mold, which is then closed around the parison. A needle is then inserted into the parison and compressed air is injected to blow the parison into the shape of the mold. The needle is then withdrawn, the mold is opened and the finished product is removed.  
         [0005]     Products made by the above process are left with a blowing hole formed by the needle, which typically has a diameter of approximately 5 mm. For certain products such as food containers, this blowing hole may have to be sealed to prevent water and dirt from entering and leaving the interior of the molding, as this could contaminate foodstuffs within the container.  
         [0006]     An example of a prior art blow-molded product made by the process described above is shown in  FIGS. 1 and 2  of the drawings. The product  2  is a wall panel of a bulk container and the drawings show a portion of the wall panel, partially broken away. The panel includes two parallel rectangular side plates  4  that are interconnected by a peripheral wall  6 . A shallow recess  8  is provided in one portion of the peripheral wall and a blowing hole  10  is located in that recess. To seal the blowing hole, a plug  12  is inserted and fixed in position. The recess allows the top of the plug  12  to lie flush with the edge of the panel, as shown in  FIG. 2 .  
         [0007]     Inserting a plug to seal the blowing hole in a blow-molded product is a difficult and time-consuming process, which requires an additional component and adds to the overall production costs. There is also a risk that the blowing hole could be incompletely sealed and/or that the plug could work loose during use. It is an object of the present invention to provide a method of sealing the blowing hole in a blow-molded product that mitigates at some of the aforesaid disadvantages.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the present invention there is provided a method of blow-molding, the method including forming a parison of a thermoplastics material, closing a mold around the parison, injecting compressed gas into the parison through a blowing hole to form a molded product that is shaped according to the shape of the mold, and deforming a seal forming portion of the molded product with a heated sealing tool to seal the blowing hole.  
         [0009]     The invention allows the blowing hole to be sealed quickly and easily, without requiring an additional component. Production costs are thereby reduced. There is also a reduced risk of an incomplete seal and, because the seal is an integral part of the molded product, it cannot work loose.  
         [0010]     Advantageously, the mold is shaped to provide the molded product with a seal-forming portion, which is preferably located adjacent the blowing hole. The seal-forming portion may include a tubular collar that surrounds the blowing hole and extends outwards from the molded product. The tubular collar may for example be molded between an orifice in the mold and a gas injection needle that is inserted into the mold.  
         [0011]     The mold is preferably shaped to form a locking element that restricts movement between the tubular collar and the mold. The locking element may include a flange element that is provided on the tubular collar. This helps to prevent the tubular collar being pushed into the molded product when the gas injection needle is inserted.  
         [0012]     Advantageously, the mold is shaped to provide the molded product with a recess in the vicinity of the blowing hole and the seal forming portion is deformed to form a seal element that is located within the recess. This construction makes it possible to avoid having a seal element that protrudes outwards beyond the edges of the molded product.  
         [0013]     According to a further aspect of the invention there is provided a blow-molded product including a hollow molded body portion and a seal element that seals a blowing hole in the body portion, wherein the body portion and the seal element are made of a thermoplastics material and the seal element comprises a deformed seal forming portion of the body portion.  
         [0014]     The blow-molded product can be sealed quickly and easily, without requiring an additional component, with reduced production costs. The risk of an incomplete seal or a loose plug is significantly reduced.  
         [0015]     Advantageously, the seal-forming portion is located within a recess provided in the molded body portion.  
         [0016]     The blow-molded product may for example comprise a wall panel of a container, or any other suitable product. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings.  
         [0018]      FIG. 1  is a broken away partial front elevation of a prior art molded product, illustrating the insertion of the sealing plug.  
         [0019]      FIG. 2  is a broken away partial front elevation of the prior art molded product, after insertion of the sealing plug.  
         [0020]      FIG. 3  is a front elevation of one half of a blow-molding tool according to an embodiment of the invention.  
         [0021]      FIG. 4  is a side section showing an extruded parison located between two halves of a molding tool according to the invention, with the mold in an open position.  
         [0022]      FIG. 5  is a side section showing the parison within the closed molding tool.  
         [0023]      FIG. 6  is a side section showing the molded product after blowing (for clarity, the molding tool has been omitted).  
         [0024]      FIG. 7  is a side section of the molded product after removal from the mold, during a sealing operation.  
         [0025]      FIG. 8  is side section of the finished molded product after sealing.  
         [0026]      FIG. 9  is a sectional front elevation of the finished molded product after sealing. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     The embodiment of the invention depicted in FIGS.  3  to  9  of the drawings illustrates a method of molding a blow-molded product  14  such as a simple rectangular wall panel, which may for example form part of a bulk container or crate. The wall panel, which is shown in  FIGS. 8 and 9 , includes two parallel rectangular side plates  16  that are interconnected by a peripheral wall  18 . It will be appreciated that many other products may also be made by substantially similar processes and that the present application is intended to cover such products and the processes for making them.  
         [0028]     The blow-molding tool  20  shown in  FIGS. 3 and 4  includes two symmetrically identical mold halves  22 , each of which includes a mating surface  24  that in use mates against a corresponding surface on the other mold half, and a substantially rectangular mold cavity  26  that comprises a side face  28  for molding a side plate  16  of the blow-molded product and a surrounding peripheral face  30  for forming the peripheral wall  18  of the product. The peripheral face  30  includes a portion  32  that protrudes inwards, to form a recess  34  in the peripheral wall of the molded product.  
         [0029]     A channel  36  is provided in the mating surface  24  of each mold half  22 , at the location of the protruding portion. When the mold halves  22  are brought together, these channels  36  form an orifice  38  that extends through the mold tool  20  from the exterior to the interior of the mold cavity  26 . This allows an air injection needle  40  to be inserted through the mold tool  20  into the mold cavity, for blowing compressed air into the parison, to inflate the molded product  14  within the mold.  
         [0030]     The features of the blow-molding tool  20  as described above are all conventional. Where the tool differs from prior art blow-molding tools is in the shape of the orifice  38 . In the prior art, this orifice generally consists simply of a cylindrical bore having a diameter slightly greater than that of the air injection needle  40 , so as to allow the needle to be inserted. In the embodiment of the invention shown in the drawings, the orifice  38  includes an outer portion  38   a  comprising a cylindrical bore with a diameter slightly greater than that of the needle  40  (for example approximately 5 mm), an inner portion  38   b  of slightly larger diameter (for example approximately 8 mm) and an intermediate portion  38   c  of even larger diameter (for example approximately 10 mm). It will be appreciated that the dimensions quoted above are only illustrative and are not intended to be limiting in any way.  
         [0031]     The inner and intermediate portions  38   b ,  38   c  of the orifice are therefore significantly larger than the needle  40 , thus forming an annular gap between the needle  40  and the sides of the orifice  38 . During use, melted thermoplastics material is molded in this gap to form a tubular collar, which is subsequently deformed to seal the blowing hole.  
         [0032]     The blow molding process will now be described in detail with reference to  FIGS. 4-7 .  
         [0033]     First, as shown in  FIG. 4 , a tube of heated thermoplastics material is extruded through an extrusion nozzle  42  into a gap between the two halves  22  of the mold, which at this stage is open. The extruded material forms a parison  44  comprising a hollow tube that is closed at its lower end.  
         [0034]     The mold  20  is then closed as shown in  FIG. 5 . The upper part of the parison  44  is nipped between the two halves of the mold forming a plug of material within the orifice  38  in the upper part of the mold. The air injection needle  40  is then inserted through the orifice  38  into the parison  44  and compressed air is injected to inflate the parison, so forming a molded product  14  matching the internal shape of the mold, as shown in  FIG. 6  (the mold tool has been omitted in this drawing for the sake of clarity. Once the thermoplastics material has set, the needle  40  is withdrawn, leaving a blowing hole  45 . The mold  20  is then opened and the molded product  14  is removed.  
         [0035]     As can be seen in  FIGS. 6 and 7 , the molded product  14  includes a tubular collar  46  that extends from the upper edge of the molded product, this collar having been shaped within the gap between the wall of the orifice  38  and the air injection needle  40 . The lower part of the collar  46  is substantially cylindrical, having been formed by the inner portion  38   b  of the orifice, and at its upper end the collar has an outwardly extending flange  48  formed by the intermediate portion  38   c  of the orifice. The collar  46  and the flange  48  provide a seal forming portion  49  of the molded product, which may be deformed to seal the blowing hole  45 .  
         [0036]     After the blow-molded product has been removed from the mold tool, it is subjected to a second processing step, in which a heated sealing tool  50  is applied to the seal-forming portion  49 . The sealing tool  50  re-heats and melts the thermoplastics material of the seal-forming portion  49  and squashes it downwards, thereby sealing the blowing hole  45 . The remelted material thus forms a dome-like seal element  52  that is accommodated within the recess  34  in the edge of the molded product  14 , the upper part of the seal element  52  lying flush with or slightly below the edge of the product, as shown in  FIGS. 8 and 9 .  
         [0037]     The process described above thus provides a very simple method for sealing the blowing hole  45 . The seal element  52  is formed as an integral part of the blow-molded product  14 , thereby obviating the need for a separate plug. Sealing the blowing hole  52  is a very simple process involving the application of a heated sealing tool  50 . This process can be carried out manually or it can be automated, if desired.  
         [0038]     The shape of the seal-forming portion  49  is important for successful sealing of the product. We have found that the seal-forming portion  49  preferably comprises a tubular collar  46  having a flange  48  at its outer end. The flange  48  supports the collar  46  within the orifice  38  in the mold  20  and prevents it from being deformed as the needle  40  is inserted. We have found that if a flange  48  is not provided, the collar  46  tends to be forced inwards as the needle  40  is inserted, which makes subsequent sealing very difficult.  
         [0039]     It will be appreciated that various modifications of the process described herein may be made, the process described being merely illustrative of one embodiment of the invention.