Abstract:
The invention relates to a mold for the manufacture of holders, comprising at least two mold halves moveable relatively to each other in a first direction of movement and a mold cavity, the mold cavity, the mold being substantially defined by a female part and a matching male part, while at least the female part and/or the male part is arranged so as to be moveable such that within the mold, the male part and/or the female part can be brought into a first position wherein between a forward end, facing, in a first direction of movement, towards the mold cavity, and an opposite bottom part of the female part, there is a relatively large, first distance, and a second position wherein between said forward end and said bottom part there is relatively small second distance.

Description:
This application is the U.S. National Phase of International Application Number PCT/NL2003/000765 filed on 4 Nov. 2003, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a mold for the manufacture of holders. The invention further relates to a method for the manufacture of holders. 
     As a rule, when holders such as tub-shaped containers are injection molded, a mold is used wherein plastic is forced from the bottom surface into a longitudinal wall. With small wall thicknesses and/or long flow paths and/or a large projected surface and/or low melt plastics, particularly high closing pressures and injection pressures need to be used, particularly if a multi-cavity mold is used. As a result thereof, especially bottom surfaces of such holders are of relatively thick design. 
     SUMMARY OF THE INVENTION 
     The invention contemplates a mold for manufacturing holders with which, with relatively low closing pressure and/or low injection pressure, relatively thin-walled holders can be injection molded. To that end, a mold according to the invention is characterized by at least two mold halves moveable relative to each other in a first direction of movement and a mold cavity, the mold cavity being substantially defined by a female part and a matching male part, while at least the female part and/or the male part is arranged so as to be moveable such, that within the mold, the male part and/or the female part can be brought into a first position wherein between a forward end, facing, in a first direction of movement, towards the mold cavity, and an opposite bottom part of the female part, there is a relatively large, first distance, and a second position wherein between said forward end and said bottom part there is a relatively small second distance. 
     By using a moveable male and/or female part, the plastic can be introduced into the mold cavity with relatively little pressure. The fact is that since the space into which the plastic is introduced is initially kept relatively large during use by creating additional space between the male and female part, initially, the plastic needs to be introduced over a smaller distance, while, moreover, the counter pressure as a result of flow resistance is relatively low. Only when the plastic has been introduced at least virtually completely, the two parts are moved relatively to each other, towards each other, so that the plastic is moved further into the mold cavity. It is preferred that the mold is closed before the plastic is introduced, so that the plastic is prevented from flowing away inadvertently. 
     As the above-mentioned relatively large distance is located between the forward end of the male part and the opposite bottom surface of the female part, the advantage is achieved that, in a simple manner, additional space can be created, since the distance is located approximately parallel to the closing direction of the mold, at least the ejecting direction of the ready products. Preferably, the plastic is then introduced in the space between this forward end of the male part and the opposite bottom surface of the female part, so that minimum resistance is experienced and a favorable flow pattern is provided. 
     It is further preferred that the distance between the male and the female part in the first position is selected such that virtually the entire volume of plastic required for the holder can be introduced into this space between the forward end of the male part and the opposite bottom surface. As a result, a particularly advantageous initial flow pattern is obtained. 
     With a mold according to the invention, in a particularly advantageous manner, holders can be manufactured with a truncate shape. Herein, truncate is understood to include a holder with (a) slightly inclining longitudinal wall or walls and a bottom, such as frustoconical or truncate beam-shaped, which latter has a substantially rectangular base shape with a longitudinal wall inclining substantially outwards from a bottom surface. The first-mentioned shape can, for instance, be found in cups and the like, the latter in, for instance, butter tubs, ice-cream containers and the like. However, these examples should not be construed to be limitative in any way. 
     With a mold according to the invention, in particular holders can be formed with a relatively thin bottom and longitudinal wall, for instance thinner than matching the melt-flow index for a similar product with conventional molding. In particular, a bottom can be obtained which is relatively thin, so that material can be economized on. Furthermore, a mold according to the invention can simply be arranged for in-mold labeling, as the pressure of the plastic upon injection is relatively low, especially when the plastic is introduced into a bottom surface which is not to be labeled. 
     When using a mold according to the invention for in-mold labeling, while both the bottom and the longitudinal wall or walls are to be provided with a label, it is preferred that the plastic be introduced through the part against which the label is not provided. With a customary holder, this will mean that the plastic is introduced through the male part into a bottom forming part of the mold cavity, while the label is inserted at the opposite side thereof in or over the female part. As a result, the outside of the holder is provided with a label, while damage to and displacement of the label is prevented. 
     With a mold according to the invention, the design of the male and female part is preferably such that in the first position and the second position, the distance between the female part and the male part substantially only varies in or near the bottom forming part of the mold cavity, between the forward end of the male part and the bottom surface of the female part. As a result, a simple mold is obtained with favorable load of the different parts. 
     Preferably, the male part and/or the female part is biased in the second position. As a result, it will have to be actively brought into the first position, and it will automatically be moved to the second position. A pressure member can, for instance, be used in the opposite mold part for pushing the respective male or female part into the first position. Such a pressure member can, for instance, be a stripping plate or stripping ring or the like, which can also be used for forcing a ready product off the male part or from the female part. The respective part, biased in the second position, is preferably biased in the second position by simple spring means, for instance a gas filled spring means. In a further advantageous embodiment, during use, the respective part, biased in the second position, is moved by the injection pressure of the plastic to the first position and only when the injection pressure falls is it pushed back to the second position by the spring means or the like. 
     In an alternative embodiment, naturally, also the respective biased male and/or female part can, conversely, be biased in the first position and, with the aid of drive means such as a screw spindle, piston-cylinder assembly or the like, be moved to the second position so that, for instance, more pressure can be obtained or a more controlled, for instance, faster or more uniform pressure build-up can be obtained. 
     With multi-cavity molds according to the invention, the advantages of lower injection pressure and, in particular, lower closing pressure are achieved to an even larger extent. 
     The invention further relates to a method for manufacturing holders and such hollow products characterized the steps of bringing a male and/or female part to a position moved backwards, while or whereupon plastic is introduced into the mold cavity, whereupon the male part and/or the female part is brought to a position moved forward, while forcing at least a part of the plastic away such that the entire mold cavity is filled, while in the position when moved backwards, between a forward end of the male part in the direction of movement and a bottom of the female part, there is a larger distance than in the position when moved forward, while the distance between the remaining parts of the male and female part remains virtually the same. 
     With such a method, thin-walled products can be manufactured rapidly and with low pressures. Such a method can be particularly advantageously used for in-mold labeling products, for multiple products and for products with specific properties such as transparency, or from special materials for instance low melt plastics such as PET, PC and the like, or crystalline plastics, in particular when these are to be, become and/or remain transparent. PET is a clear but non-limitative example thereof too. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantageous embodiments of a mold and method, and of holders to be obtained therewith are given in the subclaims. In clarification of the invention, as example, exemplary embodiments of a mold, method and holder according to the invention will be further elucidated with reference to the drawing. In the drawing: 
         FIGS. 1 and 2  show a frustoconical and a truncate beam-shaped holder according to the invention; 
         FIG. 3  shows, in cross-sectional side view, a part of a first embodiment of a mold according to the invention; 
         FIGS. 4A-C  schematically show, in cross sectional side view, a mold according to the invention, in three steps of a method according to the invention; and 
         FIG. 5  schematically shows, in cross-sectional side view, a mold according to the invention, in an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In this description; identical or corresponding parts have identical or corresponding reference numerals. As examples of holders according to the invention a cup and a butter tub are given, as frustoconical holder and as truncated beam-shaped holder. Naturally, many different shapes can also be manufactured according to the invention with a mold according to the invention. 
       FIG. 1  shows a cup  1 ,  FIG. 2  a butter tub  2 . Both are holders  3  with a relatively small wall thickness, for instance some tens of millimeters, and have an internal space  4  which is relatively large. The holders  3  are relatively high in relation to the wall thickness. Preferably, the bottoms  5  of these holders  3  have a thickness which is approximately equal to or smaller than the wall thickness of the walls  6 . However, these thicknesses can be selected to be different and can, for instance, also taper over the wall and/or bottom. The holders can be transparent, opaque or completely non-transparent. 
     In  FIG. 3 , schematically, a mold  10  is shown for forming a holder, for instance according to  FIG. 1  or  2 . This mold  10  comprises a first part  11  and a second part  12 , moveable relative to each other. In the exemplary embodiment shown, the second part can be moved actively, as will be elucidated further with reference to  FIG. 4 , for opening and closing the mold. A supply device  13  is provided in the first part  11 . 
     The first part  11  comprises a male part  14 , which is substantially formed by a frustoconical or truncate block-shaped part  15 , fixedly connected to a first base plate  16 . In the base plate  16 , a stripper ring  17  or such pressure means is provided which extends around the wide end of the part  15  and is moveable in the direction S with the aid of an ejector plate  18 . In the position shown in  FIG. 3 , the ejector plate  18  and hence the stripper ring  17  are shown in a retracted, second position. 
     The second part  12  comprises a female part  19 , which is substantially formed by a block  20  having a cavity  21  therein which corresponds as to shape to that of part  15  and has dimensions such that, as shown in  FIG. 3 , with the mold  10  in the closed position and the stripper ring  17  retracted, between the male part  14  and the female part  19 , a mold cavity  22  is left open, at least defined which has, substantially, the shape of the desired holder S. The block  20  is included in, for instance, a cylindrical space  23  in the second part  12 , in which it can be shifted in the direction S. Between the end of the block  20  facing away from the first part  11  and the closed end  24  of the space  23 , a chamber  25  is formed, filled with, for instance, gas or oil or a similar medium for forming a pressure chamber. Upon movement of the block  20  in the direction of this end  24 , in the chamber  25 , an increased pressure will occur, biasing the block  20  in the direction of the first part  11  and hence in the direction of the second position shown in  FIG. 3 . 
     In this mold  20 , a holder  3  can be formed by introduction of plastic in substantially liquid, at least, moldable form. To that end, steps can be traversed as shown in  FIGS. 4A-C . 
     In  FIG. 4A , the mold  10  is shown in opened condition. The block  20  has been pushed by the medium into the chamber  25  into the second position, maximally forward (towards the right in  FIG. 4 ). The stripper ring  17  in the first part  11  is moved outwards (towards the left in  FIG. 4 ) with the aid of the ejector plate  18 . 
     In  FIG. 4B , the mold  10  is shown in closed condition. The stripper ring  17  is held in the position moved in outwards direction, shown in  FIG. 4A . The stripper ring  17  then abuts against the block  20 , so that the block  20  is moved backwards, in the direction of the end  24 , thereby reducing the volume of the chamber  25  and compressing the medium therein. As a result, pressure is exerted on the stripper ring  17 .  FIG. 4B  shows the second position. As clearly appears from  FIG. 4B , the foremost end  26  of the male end  15  (facing the left in  FIG. 4 ) is at a relatively large distance D from the bottom surface  27  of the cavity  21 . This distance D is, in particular, larger than the distance between the male part  15  and the wall of the cavity  21  in the female part  19 . Between this end  26  and the bottom surface  27 , a bottom forming space  29  is formed into which the plastic is introduced via the supply means  13  (not shown in  FIG. 4 ). In the second position, shown in  FIG. 4B , the volume of the bottom forming space  29  is relatively large, for instance approximately equal to the volume of material required for the entire holder. Consequently, the plastic can be introduced into the mold cavity  22  rapidly and in a simple manner, with little counter pressure. 
     After the plastic has been, at least for the larger part, introduced into the mold cavity  22 , at least in the bottom forming cavity thereof, the ejector plate is moved backwards (to the right in  FIG. 4 ), to a first position as shown in  FIG. 4C . The stripper ring  17  is then no longer retained (or actively pulled back) so that the block  20  can be pushed by the medium into the chamber  25  to the first position (to the right in  FIG. 4C ). The bottom forming space  29  is then considerably reduced. The distance D 1  between the end  26  and the bottom surface  27  is reduced to the desired wall thickness, for instance equal to the wall thickness of the wall  6  of the holder. The plastic is then pressed, for the larger part, from the bottom forming space  29 , between the male part  15  and the wall of the cavity  21 , for completely filling the mold cavity  22  and hence forming the desired product  3 . 
     After the holder  3  has been formed and, optionally, has cooled down, the mold can be opened again to the position shown in  FIG. 4A , while the ejector plate  18  and hence the stripper ring  17  are moved forwards, thereby forcing the ready product from the male part  15 . 
     In  FIG. 5 , schematically, an alternative embodiment of a mold  10  according to the invention is shown, wherein two male parts  15 , having a substantially cylindrical or slightly conical shape, are attached to a moveable base plate  40 , which base plate  40  is moveable in a chamber  41  formed in the first part  11  of the mold  40 . In the second part  12 , fixed cavities  42  are provided with stripper rings  17  around them, biased in the direction of the base plate  40  by springs  43 . Supply means  13  terminate in the cavities  42  through the second part  12 . A supply duct  44  with pressure pump  45  and a discharge duct  46  with a suction pump  47  open into the chamber  41 , so that the pressure in the chamber  41  can be actively regulated. With this, it can be regulated whether the stripper rings  17 , under the influence of the springs  43 , can force the base plate  40  with male parts  15  backwards in the direction away from the second part  12  or not, or the base plate can be actively withdrawn from the second part or, conversely, be moved thereagainst. With this, each time, the distance D between the end  26  of the male parts  15  and the bottom surface  27  of the cavities  21  can be set. Also, the pressure in the chamber  41  can be set to be such that, due to the pressure of the plastic flowing into the mold cavities  22 , the base plate can be pushed away and only be moved back if the injection pressure falls after all plastic has been introduced. Optionally, the springs can then be omitted or be replaced by controlled gas springs with which the stripper rings  17  can be actively moved for ejecting the holders and/or abutting against the base plate  41 . 
     Before the mold  10  is closed, a label can be inserted, for instance over or in the cavity  21  or on the male part  15 , for in-mold labeling. In  FIG. 4A  this is symbolically shown by the chain-dotted line L. 
     Naturally, all combinations of parts of molds according to the invention can be used. 
     The invention is not limited in any manner to the embodiments as shown and described. Many variations thereon are possible within the framework of the invention. Naturally, the pump  45  and  47  can be combined in a double acting pump. 
     For instance, other means can be used for bringing and/or holding a moveable male part and/or a female part in the first and/or second position, such as screw spindles, piston-cylinder assemblies, electric magnets and such means known per se. Other numbers of mold cavities can be provided, single or stack molds. The movements of moving parts can be actively controlled. All sorts of other shapes of holders can be formed with a mold or method according to the invention. 
     These and many comparable variations are understood to fall within the framework of the invention.