Abstract:
A gas injection moulding device with a mould cavity which communicates via a valve with spill chambers. The injection moulding process is controlled by a control unit so that the valve is first kept partially open during the injection of fluent plastic material for purging the mould cavity of air and for spilling a first small amount of plastic material to the spill chambers, whereas during the subsequent injection of gas, the valve is completely opened.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of manufacturing a plastic article with a cavity enclosed in the same, comprising the following steps: injection of plastic material under pressure into a mould cavity, cooling the plastic material closest to the walls of the mould cavity, injection of gas under pressure into fluent plastic material, which is surrounded by the cooled plastic material, pressing out, by means of the injected gas, fluent plastic material to at least one spill chamber located outside the mould cavity and communicating with the mould cavity injected gas, to form a cavity in the plastic material, and separation of the plastic material in the spill chamber from the plastic material in the mould cavity. 
     The invention also relates to an injection moulding device with a mould cavity defined by movable walls, comprising means for injecting under pressure fluent plastic material into the mould cavity, means for injecting gas under pressure into the plastic material in the mould cavity, at least one spill chamber located outside the mould cavity and communicating with the mould cavity, said spill chamber being arranged to receive plastic material forced out by the injected gas, and means for opening and cutting off the communication with the spill chamber. 
     DESCRIPTION OF THE RELATED ART 
     The method of manufacturing hollow plastic articles by first filling the mould cavity completely as in normal injection moulding and thereafter, with the aid of the gas, pressing the core material out to a spill changer, so that the cavity in the article is formed, is usually called “blow-out gas injection” and is described, for example, in U.S. Pat. No. 5,204,051. In the known method, the communication between the mould cavity and the spill chamber is kept closed during the injection of the plastic material and is opened after the mould cavity has been completely filled, and only after the surface of the plastic material has hardened somewhat. The purging here is done by leakage between the mould halves, and this means that the counter-pressure against the plastic material will be relatively great. 
     In this process, all of the plastic material injected into the mould cavity will remain in the cavity. This means that the plastic material, which, at the beginning of the injection, is in and nearest to the nozzle of the mould injector and which has a lower temperature and/or poorer quality than the rest of the plastic material, especially when the flow channel areas are large, can end up in the mould cavity at a location farthest away from the inlet to the mould cavity. This can occur in particular when injecting thermosetting resins and/or so-called cross-linkable plastics. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to achieve a method and a device, through which the above mentioned disadvantage can be eliminated, so that more even wall thickness and uniform plastic quality can be achieved in the manufactured plastic article. 
     This is achieved by virtue of the fact that a communication with a predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the fluent plastic material, and that a communication with a larger predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the gas. 
     The communication with smaller flowthrough area is maintained for such a long time that all the air is pressed out of the cavity and possibly a small amount of the plastic material is pressed out to the spill chamber. The method according to the invention achieves not only a product with an even wall thickness and uniform plastic quality but also makes it possible to shorten the production cycle by virtue of the fact that purging of the mould cavity of air can be done much more rapidly than when the purging is done by normal leakage between two mould halves. More rapid purging also reduces the risk of small air bubbles forming in the plastic material. 
     An injection moulding device for carrying out the method is characterized in that a communication with a predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the fluent plastic material, and that a communication with a larger predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below with reference to examples shown in the accompanying drawings, where 
     FIG. 1 shows a section through a schematically represented injection moulding device according to the invention, 
     FIG. 2 shows a perspective view of the rear of a plastic panel in the form of a radiator grill for motor vehicles, which can be manufactured by using the method and the device according to the invention, 
     FIG. 3 shows a section along the line III—III in FIG. 2, and 
     FIG. 4 shows a section along the line IV—IV in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Element  1  in FIG. 1 generally designates a mould, which comprises upper and lower mould halves  2  and  3 , respectively, which define together a mould cavity  4  and which can be moved away from each other, by means not shown in more detail here, from the position shown to open the mould cavity  4 . A nozzle  6  of a mould injector opens into a channel  5  through the upper mould half  2 . The mould injector has a cylinder  7  and a piston  8  axially movable in the cylinder, by means of which fluent plastic in the cylinder  7  can be pressed into the mould cavity  4 . 
     The mould halves  2  and  3  define, in addition to the mould cavity  4 , a pair of so-called spill chambers  9 , which communicate with the mould cavity  4  via individual channels  10 . A bore  11  opens into each channel  10 , and an ejector pin  12  is arranged displaceable in each bore  11 . A corresponding ejector pin  13  is arranged in a bore  14 , opening into each spill chamber  9 . A gas needle  15  extends directly opposite the channel  5  and is provided with an opening  15   a,  through which gas can be injected into the plastic material in the mould cavity  4 . The gas needle  15  can be fixed in the position shown or be extractable out of the mould cavity  4 . 
     Element  16  in FIG. 1 designates a control unit,  17  designates a compressed gas source and  18  and  19  are drive means for driving the piston  8  and the ejector pins  12 , respectively. FIG. 1 is symmetrical relative to a center plane A and to the left of the plane A the state is illustrated after the piston  8  has come to the bottom of the cylinder  7  and the mould cavity  4  has been completely filled with fluent plastic “b” while air and a small amount of plastic material “c” have been pressed out to the spill chamber  9 . During the plastic injection phase, the control unit  6  keeps the ejector pins  12  in the position shown to the left in FIG. 1 via the drive means  19 , in which position the upper end surface  12   a  of each ejector pin  12  leaves a narrow passage  10   a  open, through which first air and then plastic material can pass to the spill chamber  9 . 
     When the injection of plastic is finished, all air, and possibly a small amount of plastic, has been evacuated to the spill chambers  9 . The control unit  16  then activates the compressed gas source  17  so that gas under pressure (preferably nitrogen) is introduced via the gas needle  15  and out through its openings  15   a  into the plastic which is not yet hardened, which is then pressed out through the channels  10  and into the spill chambers  9  until they are completely filled, as is illustrated to the right in FIG.  1 . During the gas injection phase, the control unit  16  keeps the ejector pins  12  in the position shown to the right in FIG. 1, in which position the end surface  12   a  is at a lower level to open the entire flowthrough cross-sectional area of the channel  10 . 
     When the plastic material has hardened, the mould cavity  4  is opened and the control unit  16  activates the drive means  19  of the ejector pins  12  to push the pins  12  up to lift the plastic article from the lower mould half  3 . As the pins  12  move through the channel  11 , the communication between the plastic material in the mould cavity  4  is cut off from the plastic material in the spill chamber  9 , so that the latter can then be ejected with the ejector pin  13 . 
     FIGS. 2,  3  and  4  illustrate a plastic article in the form of a radiator grill  20  for a motor vehicle, which can be manufactured with the method and gas injector moulding device described above. 
     The radiator grill shown comprises a rectangular frame, generally designated  21 , which consists of a horizontal upper frame member  22 , a horizontal lower frame member  23  and two vertical side frame members  24 , which connect the upper and lower frame members  22  and  23  to each other. Between the frame members  22  and  23 , a pair of vertical mouldings  25  extend and between these and each respective side frame member  24  a pair of horizontal mouldings  26  and  27  extend. Finally, a horizontal moulding  28  extends via the vertical mouldings  25  from one side frame member  24  to the other  24 . 
     All of the components  22 - 28  shown and described are made in one piece with each other in a gas injection moulding process m the above described manner, so that cavities are formed in the frame members  22  and  23 , respectively, and in the mouldings  26 ,  27  and  28 , respectively. These cavities, which extend over the entire length of the frame members and the mouldings are designated  29 ,  30 ,  31 ,  32  and  33  in FIG.  3 . 
     Element  40  in FIG. 2 designates the holes formed after the gas needles  15 , through which gas under pressure is blown into the still not hardened plastic material during the gas injection moulding process. By using, as can be seen in FIG. 1, a mould cavity which has a gradually decreasing cross-sectional area towards the ends, a more balanced filling of the mould is assured than would be the case if the channels had had the same cross-sectional area along their entire length. Thus, if the plastic material on one side of the needle  15  should tend to flow out more rapidly towards the end of the mould cavity than the plastic material on the other side of the needle, the counter-pressure against the former plastic material would increase when it reaches the tapered portion of the mould cavity, so that the gas pressure increases against the latter plastic material which has still not reached the corresponding opposite tapered portion of the mould cavity, thus balancing the filling.