Abstract:
The present disclosure describes a process and an apparatus for delivery of plastic at a viscosity and in the required quantity suitable for compression molding of large, and often odd shaped three-dimensional products, such as large automotive body parts and large appliance parts, etc. These products may consist of plastic only, or may require additional layers of other plastics or even other materials such as woven cloth, inserted into the cavity before molding. The purpose of this invention is to provide a process and apparatus for use with a compression molding machine to supply plastic to a mold cavity, at a number of suitable locations, and in appropriate quantity at each location as required by product shape and thickness. The apparatus and process includes a delivery head which is mounted on rails attached to the machine clamp or the mold cavity, and a connection between the injection unit of the molding machine. With the mold open, the delivery head moves over the mold cavity, and delivers exactly metered globs of plastic directly into the cavity, or on top of an inserted sheet of other plastic, decorative cloth or other film, as specified for the product. The head then retracts and permits the clamp to close and to compress the plastic into the desired shape.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of accurately metering plastic over the cavity of an open compression mold, advantageously for large products made from plastics, such as polyesters, polyolefins, etc., plain or reinforced with glass, etc. It is particularly suitable for depositing such plastics on top of decorative materials, which may be plastic or other materials or combinations of materials. Such a system is shown in copending U.S. patent application Ser. No. 09/130,864, the disclosure of which is incorporated herein by reference. 
     Large components used often in large quantities, such as in the automotive and appliance industry, can be produced both by injection molding and by compression molding. A problem in injection molding is that the molds are often quite complicated, especially due to the need to distribute the injected hot plastic over large areas, and in relatively heavy and often unequal thicknesses. Also, the injection process makes it difficult to decorate the product during the molding process with a special finish, as for added appearance and sales appeal, and injection requires the added cost of thermoforming for deep draw parts and in general is more difficult than the extrusion/compression process. 
     In order to overcome these disadvantages, the almost century old compression molding technique, much ignored during the last 50 years, has resurfaced and is now often successfully used for molding large, usually decorated products. The typical compression molding is done in vertical presses. Thermosetting molding material is placed into the heated cavities either in the cold or preheated state and then cured in the closed, hot mold. In a limited way, decoration and reinforcements, with other layers, are possible with this method. 
     However, thermosetting materials are not as suitable, for better quality, for a wider range of applications, and economically, as thermoplastics could be. The problem now is to supply hot plastic in sufficient quantity, evenly distributed, into the cavity or over an inserted decorative sheet. it is important that little time is wasted between the application of the plastic and the closing of the press. Some methods have used extruded, hot or reheated, very thick film (possibly 5-10 mm), cut into size and shape and then place it over (or into) the cavity. Another attempt is to move the extruder right over the cavity and extrude a heavy sheet right over the cavity while slowly retracting the extruder and extruding die head. The problem here is to coordinate the motion of the extruder so that the proper amount of plastic is dispensed. This is dependent on the speed of motion and the viscosity of the plastic. 
     Another problem with this method is that a conventional extruder (especially for this application requiring often several kilograms per shot) must be very large, weighing several tons, and is difficult to move accurately; by necessity, such motion is very slow. A serious difficulty is to guide the die head over the cavity, properly aligned, while moving the heavy extruder back and forth. While it is not too difficult to maintain alignment in the vertical plane with the center line of the press, the heavy extruder die is not held from vibrating up and down. On a large product of for example 1.5 meters width, the extruder die head could weigh one ton or more, and this would be at the end of an extruder barrel of 3 meters or more in length. Horizontal alignment would be very difficult to achieve. 
     Also, the extruder delivers only intermittently, since the time to deliver takes maybe 10 seconds while the cooling process in the mold may take 2 minutes or more. To run a conventional extruder intermittently to this extent is not desirable because it makes it difficult to control and achieve a uniform viscosity of the plastic, and an evenly thick plastic sheet. 
     The shape (contour) of the product may not be square, and it would be practically impossible to vary the width of the extruded sheet, thus creating a large amount of wasted plastic outside of the actual shape of the cavity. 
     The sheet thickness could possibly be varied over certain areas if required, using a specially designed and very expensive sectioned die head, in combination with a modulated speed of retracting the die head while delivering the plastic into the mold. Such a method would require one to control and correlate all the critical conditions of speed, temperature, extruder output and extruder die opening, in order to achieve different plastic wall thicknesses in the product. While this would be possible, it would not be very practical. 
     Another problem is the necessarily slow horizontal motion of the extruder as it dispenses the plastic while retracting; this may take from 5-10 seconds. Added to this is the time it takes to complete the withdrawal of the die head from the molding area so that the clamp can close to start the molding process, which may, due to the heavy mass of the extruder, take another 5-10 seconds, so that from the time the depositing begins it may be 10-20 seconds before the mold can close and start to shape the plastic. During this time, the temperature (and viscosity) of the extruded sheet may be substantially different from one end to the other. 
     It is, therefore, an object of this invention to limit the moving masses of machinery, and to insure that the plastic is deposited only where required, at an adjustable temperature and viscosity and at an adjustable volume suitable for the desired product. By reducing the moving masses, the time for depositing the plastic will be greatly reduced, and the delivery system out of the way of the closing mold in a much shorter time than before. 
     Hot, plastic globs will weld together to form a homogeneous mass as the molding pressure is applied under the fast closing mold. Any film used as an insert will bond to the plastic while under pressure. 
     SUMMARY OF THE INVENTION 
     The present invention prepares a molded article by the process which comprises: providing a mold cavity and a mold core adjacent said mold cavity, said mold core and mold cavity forming a molding area therebetween; providing a hot plastic delivery means positioned adjacent said molding area and delivering hot plastic from said hot plastic delivery means to at least one shooting pot which maintains said plastic at an elevated temperature for delivery to said molding area; and delivering hot plastic to said molding area from said at least one shooting pot and forming said hot plastic in said mold cavity into a molded article having the shape of said mold cavity at least in part by said mold core which forms the plastic into the shape of the mold cavity. 
     The apparatus of the present invention comprises: a mold cavity; a hot plastic delivery means positioned adjacent said mold cavity; a mold core adjacent said mold cavity, said mold core and mold cavity forming a molding area therebetween; at least one shooting pot connectable to said hot plastic delivery means for receiving hot plastic from said delivery means, wherein said shooting pot maintains said plastic at an elevated temperature for delivery to said mold cavity; means for delivery of hot plastic from said shooting pot to said molding area; and means for forming said hot plastic in said mold cavity into a molded article having the shape of said mold cavity including said mold core which forms the plastic into the shape of the mold cavity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more readily understood from a consideration of the accompanying drawings, in which: 
     FIGS. 1 and 2 are partly schematic views of the apparatus of the present invention; 
     FIG. 3 is a partial view showing an alternate embodiment; 
     FIG. 4 is a top view showing an array of shooting pots; 
     FIGS. 5,  6 A and  6 B are detail views of the shooting pot assembly and plastic delivery; 
     FIGS. 7 and 8 are detail views of alternate embodiments. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to this invention, a plastics delivery system is associated with and desirably mounted on a vertical clamp of an injection molding machine and is linked to the extruder of the molding machine. The delivery head makes direct contact with the extruder to permit charging of the shooting pots. The delivery head preferably consists of a heated manifold plate which desirably carries an array of heated, individual shooting pots, located for example in a rectangular matrix pattern, or at any locations wherever globs of plastic should be deposited. The plastic is extruded into these shooting pots while the delivery head is outside the molding area, and kept at the appropriate temperature for loading into the mold. 
     The shooting pots may be individually adjusted for proper volume, and individually heated in case different viscosity is required. 
     The extruder has the entire time while the mold is closed as during the cooling cycle, available to fill the shooting pots. This makes it possible to use a relatively small extruder, and use either the back pressure alone to fill the pots, or, if necessary use a reciprocating screw which comes standard with injection molding machines. The plasticizing screw can then make as many strokes as required to completely fill the shooting pots in time for the next cycle. 
     The shooting pots may be filled by a variety of means, including the following. 
     (a) If the mass of the extruder is relatively small compared with the delivery head, a fixed, heated connection is satisfactory, and the extruder can move together with the delivery head, at every cycle, to position the pots over the mold. 
     (b) When the extruder is very large, it will be advantageous keep it stationary so as not to move very heavy masses at every cycle, and to connect it to a moving delivery head as with a couple of articulated, heated arms. 
     (c) In another preferred embodiment, in the out-position, a sprue bushing in the hot runner manifold, which is provided with a built-in check valve or other device to trap the transferred plastic, will butt against a standard machine nozzle and permit the transfer of the plastic from the stationary extruder to the delivery head. There should be a provision to press and lock the manifold and the extruder together, if necessary, so that the nozzle and the sprue bushing will not separate and leak plastic during the transfer of plastic to the delivery head. 
     The hot manifold plate is desirably carried by a cold main plate which is supported and aligned on each side by rails, and moves in and out of the molding area. The plastic volume within the shooting pots is easily and very accurately adjustable using individual, mechanical stops; all plungers in these pots may be pushed down simultaneously to deposit the content of each pot, with a common pusher plate driven by hydraulic actuators. 
     Hydraulically actuated switches may control the flow in and out of each shooting pot. In the delivery position, the switches connect each pot with its associated gate from where the plastic is dropped into the molding area. After delivery, the switch returns to the charge position to block the plastic flow to the gate, to prevent drooling and to readmit plastic from the extruder when the head is in the out position. An optional flag or the like can be fastened to the switching lever to block the gate externally, if this should be required for very low viscosity plastics. 
     As soon as the delivery head is outside the molding area, the extruder can start charging the pots for the next cycle. The plungers are driven back up until each one arrives at its own stop. A pusher plate is driven back by the plungers as the plastic enters the pots, or, the same hydraulic actuators can be used to return the pusher plate back, if this should be required. 
     The extruder needs to be sized only to have enough plasticizing capacity to make it possible to make a certain number of shots per minute, times the weight of the plastic per unit produced, divided by a time factor (in percent) which is approximately the cooling time, divided by the cycle time, times 100. In the following example, the time factor is assumed to be 75%. 
     Example: a 1.5 kg shot at a cycle of 25 per hour, needs only 1.5×25÷0.75=50.0 kg per hour, which is a comparably small extruder for such a large product. 
     Referring to the drawings, FIGS. 1 and 2 show a vertical clamp of an injection molding machine  10  including a mold cavity half  12  having a mold cavity  12   a  therein and cooperating mold core half  14  mounted on respective platens  16 ,  18 . Mold cavity  12   a  has a shape of the desired final molded article. At least one of the cavity half and core half is reciprocable in the direction of arrow  20  from an open position as shown in FIGS. 1-2 to a closed position and from a closed position to an open position via suitable motive means (not shown). The mold cavity half and mold core half form a molding area  21  therebetween. 
     If desired, a sheet or film  22 , which could be a clear plastic film, a colored plastic film, a web or decorating cloth, for example, is held in frame  24  above mold cavity  12   a.    
     Hot plastic delivery means  26 , as an extruder/injection unit, having a nozzle  28 , is arranged adjacent molding machine  10  and molding area  21  to couple with delivery head  30  which carries at least one shooting pot  32 . Hot plastic is delivered from the hot plastic delivery means  26  through nozzle  28  to the shooting pots  32  in the position shown in FIG.  2 . The delivery head  30  is then moved into the molding area  21  above sheet  22  between mold cavity  12  and mold core  14  as shown in FIG.  1 . In this delivery position the plastic to be molded is deposited from the shooting pots  32  in a number of precisely metered hot globs to any desired location, as on sheet  22  if used or directly into mold cavity  12   a , at the precisely controlled temperature and viscosity. As soon as the plastic is delivered from the shooting pots, the delivery head  30  is withdrawn into the out position as shown in FIG. 2 so that the mold core  14  can close into the mold cavity and form the desired molded product while the shooting pots are being recharged from the hot plastic delivery means or extruder. 
     The delivery head desirably includes a hot runner manifold  34  and a sprue bushing  36  at the end of the manifold for connection to nozzle  28 . A check valve (not shown) is preferably included in the sprue bushing to prevent any back flow of plastic from the manifold after delivery of hot plastic thereto. The delivery head  30  may be supported by a cold main plate  38  which slides on tracks  40  by rollers or the like between the in and out positions. In the in position, the delivery head position may be accurately controlled by stops, and in the out position the delivery head position is determined by pressing the sprue against machine nozzle. 
     In the embodiment where the extruder is quite small compared with the masses of the delivery head, the extruder can be mounted directly on the delivery head so that the extruder may move with the delivery head. 
     In a further embodiment, shown in FIG. 3, which could be advantageous with very large masses, the hot plastic delivery means or extruder  26  could be stationary and connected to the delivery head  30  by articulated, heated arms  42  and joints  44 , with the delivery head also travelling on tracks or rails as in FIGS. 1-2. Naturally, the delivery head  30  would carry the shooting pots as in FIGS. 1-2. 
     The hot runner manifold  34  would naturally be heated as by heaters  50  shown in FIG. 5 to control the temperature of the plastic within the shooting pots and includes a desired number of shooting pots  32  mounted thereon. Any number of shooting pots may be used depending on the particular application. For example, four (4) such shooting pots  32  are shown in FIG.  4 . Manifold channels  46  connect the plastic coming from the injection unit through sprue bushing  36  to a two-position switch assembly  48  shown in detail in FIGS. 6A and 6B. This assembly includes a housing  52 , a base plate  54  and a rotor  56 . In the position shown in FIG. 6B, plastic from the extruder is directed from manifold  34  to shooting pot  32  via passage  58  in switch assembly  48  with rotor  56  turned by suitable motive means (not shown) to block outlet passage  60  as shown in FIG.  6 B. In the second position shown in FIG. 6A, plastic is delivered from the shooting pots  32  via passage  58  and outlet passage  60  to the mold cavity or on top of sheet  22 . The switching levers  62  at the end of the rotor  56  are connected to suitable links and actuators (not shown) to permit the switching from the position shown in FIG. 6B to that shown in FIG.  6 A and vice versa as required. 
     Plastic enters the shooting pots  32  via manifold channels  46  (see FIG. 4) and branch channels  64  and through passage  58  into the shooting pot via rotor  56 . After the shooting pots are filled and the delivery head positioned in the molding area  21 , the rotor  56  is turned and the plastic can flow from the delivery head via a mold gate if used (not shown). The system does not require a balancing of the flow paths from the extruder to various mold gates as is common with hot runner manifolds. 
     Referring to FIG. 5, hot runner manifold  34  carries the desired number of shooting pots  32 . Inside the shooting pots are pistons  66 , shown in FIG. 5 in the fully discharged position. A number of support pillars  68  connect the manifold with main plate  38 . An adjustable stop or rod  70  is connected to the main plate  38  and extends adjacent to the center of each shooting pot. The tip  72  of these stops limits the upward stroke of the pistons during the charging cycle. The location of the stops can be adjusted so that the volume of the shooting pots is precisely set. The main plate  38 , which is not heated, carries rollers or gliders (not shown) which travel on the rails or tracks  40  to permit easy movement of the delivery system as described above. 
     Below the main plate  38  is movable pusher plate  74 , also unheated, which is guided with pins  76  and bushings  78  for proper alignment with the main plate. At least two (2) hydraulic cylinders  80  are mounted on the pusher plate  74 , and the piston rods  82  are held in the main plate. Loosely surrounding each adjustable rod  70  are pushers  84  which are fastened to the pusher plate  74 . 
     As the shooting pots  32  are gradually filled with hot plastic, in the charging position outside of the molding area  21 , the pistons  66  rise in the heated shooting pots until they come to their respective stops. As the pistons rise they can push the movable pusher plate  74  upwards; however, to insure that the pusher plate moves all the way up without cocking because for example of different stroke lengths, it would be advantageous to energize cylinders  80  and rods  82  in reverse to bring the pusher plate all the way to the desired upwards position. 
     As soon as the pots are full or contain the desired amount of plastic and after the shot from the previous cycle has been removed and if applicable, a new sheet placed over the now empty mold cavity, the system is ready to move the delivery head  30  into the molding area  21  to deliver plastic to the mold for the next cycle. 
     FIG. 7 shows schematically the moving joints  44  shown in FIG.  3 . Each joint section includes two machined matching blocks  86 ,  88  with each block having a block half and the block halves connected together. The two block halves are located with dowels  90  and held together with a sufficient number of screws  92  to contain the plastic pressure. Heaters  94  maintain temperature so that the plastic will be kept at its desired temperature, and also to permit easy, fast start-up from cold after a shutdown of the system. A swivel  96  connects the two joints as shown. The arms  98  include outwardly extending pipes  100  surrounded by heaters  102 . When assembled, the swivel is free to turn in its swivel  96 ; however, the pipes are solidly clamped in their corresponding sockets  104 . Naturally, thermocouples (not shown) may be strategically located in the swivel joints and in the pipes. The joints connecting these units to the manifold may be similar in construction, except that one of the two joints carries a connecting flange to the manifold and not a pipe. 
     FIG. 8 shows sheet or film  22  delivered to molding area  21  from a roll  106 , with plastic material  108  thereon and with mold core half  14  and mold cavity half  12 . The film remaining in frame  24  must be cut around the mold cavity  12   a  before opening the mold so that the molded-in film does not remain attached to the remainder of the film including the scrap film within the frame. 
     This is achieved by a ring of knives  110  surrounding the mold cavity adjacent the edge thereof. The knives or cutters are mounted on a spring-loaded holder  112  and guided and positioned by a frame  114  surrounding the holder. The frame also acts as a stop surrounding the mold cavity to limit the downward stroke of the mold clamp. As the core descends into the mold cavity, the cutters bear down on the film with the cutting force generated by springs  116 . Near the end of the mold clamp stroke when the core is almost completely within the mold cavity, the knives separate the molded in film from the remaining film. Desirably, a soft strip can be placed beneath the film at the cutting location to improve the life of the knives. 
     It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.