Abstract:
An apparatus for injection molding or forming of work pieces, including closed profile work pieces, is provided that is less expensive and requires less space than known press apparatus. The apparatus locks a molding tool assembled from two or more tool pieces to define the shape of a molded or formed product using a frame, and one or more presses. The frame defines an interior space extending from a first end of the frame to an opposed second end of the frame for receiving a molding tool. The presses are placed within the frame and act in concert with the frame to apply pressure to a molding tool. The interior space is large enough to receive an assembled molding tool into the interior space of the frame, while not permitting removal of the tool pieces from the product within the interior space.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 08/737,567, filed Nov. 12, 1996, now abandoned, which is the U.S. National Phase of PCT Application No. PCT/SE95/00507 filed May 9, 1995, both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This application relates to an apparatus for formation of work pieces using a molding tool assembled from two or more tool pieces. The apparatus and method can be used for injection molding of plastics and some metals, and for hydrothermal forming. 
     BACKGROUND OF THE INVENTION 
     Injection molding and hydromechanical forming processes frequently employ large presses designed for press forces in the range of several tons for opening, closing, pressing together and locking the molding tool pieces. Presses of this type are expensive and require a lot of space. In addition, the large size requires distribution of forces over substantial distances, which can lead to distortions which must be compensated for at increased cost and complexity. 
     When forming closed profiles, e.g., pipes, the profile is put into a tool which is subsequently locked. A pressure medium is supplied internally to the profile, which expands until it conforms to the inner surface of the molding tool. It is advantageous to work several work pieces simultaneously in one molding tool, but this raises the demands upon the locking mechanisms. 
     SUMMARY OF THE INVENTION 
     The present invention provides an efficient apparatus and method for injection molding or forming of work pieces, including closed profile work pieces, that is less expensive and requires less space than known press apparatus. Thus, in accordance with an embodiment of the invention, an apparatus is provided for locking a molding tool assembled from two or more tool pieces to define the shape of a molded or formed product, comprising: 
     (a) a frame, said frame comprising a wall member that defines an interior space extending from a first end of the frame to an opposed second end of the frame, said interior space opening through the first and second ends of the frame and being large enough to receive an assembled molding tool into the interior space of the frame, while not permitting removal of the tool pieces from the product within the interior space; and 
     (b) one or more hydraulic cylinders disposed within the frame and acting in concert with the frame for applying pressure on a molding tool received within the interior space in the frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 in a perspective view shows an apparatus for locking molding tool halves in accordance with an embodiment of the invention; 
     FIG. 2 is an end view of an alternative design of an apparatus for locking molding tool halves in accordance with a first embodiment of the invention; 
     FIG. 3 is a section along line III—III in FIG. 2; 
     FIG. 4 schematically in a side view shows hydraulic pistons for controlling locking of the took halves; 
     FIG. 5 schematically in a plane view shows the hydraulic pistons of FIG. 4; 
     FIGS. 6A and 6B show an example of a molding tool half in plane and cross-sectional views; and 
     FIG. 7 in a larger scale shows a connection which is used in the molding tool half of FIGS.  6 A and B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus according to the invention is adaptable to many different purposes and fields of use. Hereinafter, two specific embodiments are described, but the person skilled in the art will realize that different modifications and applications lie within the scope of the invention. 
     While the terms used in this application are generally used in a manner conventional in the art, certain terms will be first defined in the interest of clarity. As used in the specification and claims of this application: 
     “molding tool” refers to a structure which defines the shape of a product to be produced by a process such as injection molding or hydrothermal forming; 
     “tool piece” refers to a segment of a mold In general, two or more tool pieces are utilized to create the molding tool; 
     “work piece” refers to the product produced using the apparatus or method of the invention. References to “work piece” refer to single products, or to multiple products formed in a common forming step. 
     FIG. 1 shows a first embodiment of the apparatus of the invention in which a number of quadratic (i.e. square) plates  10  are placed closely together in a stack. The plates  10  can have varying thicknesses. They are held together by means of five pull rods (not shown) which can be passed through five holes  11 . 
     The plates  10  do not have to be quadratic, but they can have any other multi-sided, oblong or round form, and they can be held together by means of more or fewer than five pull rods. The pull rods are placed in such a way that the press force from the pull rods is distributed as evenly as possible through the frame. 
     An opening  12  is cut out in the central portion of each plate  10 centrally in plate  10 , such that the plates together cooperate to form a frame with an interior space extending from a first end of the frame to an opposed second end of the frame, said interior space opening through the first and second ends of the frame. In the embodiment shown in FIG. 1, the opening  12  is substantially quadratic with a rounded indentation at one corner. 
     Within the interior space formed by the openings  12 , presses formed from hydraulic cylinders  13  and press plates  16  are positioned to apply pressure on an assembled molding tool placed within the interior space. In the embodiment of the invention shown in FIG. 1, there are two presses formed from hydraulic cylinders  13  and press plates  16 , disposed at an angle of 90° with respect to each other. The presses work against anvil plates  15  to lock a molding tool placed within the interior space in the frame. The press plates  16  and anvil plates  15  may be exchangeable to allow adaption to the shape of a molding tool. 
     The size of the interior space remaining between the press plates  16  and the anvil plates  15  in the unlocked position is sufficient to allow the introduction of an assembled molding tool. The interior space is not, however, sufficient to permit the insertion of the tool pieces without prior assembly, or the stripping of the tool pieces from a completed work piece without first removing the molding tool and work piece from the interior of the frame. 
     FIGS. 2 and 3 show a second embodiment of the apparatus of the invention in which a cylindrical pipe  19  is used as the frame. Filling inserts  20  are inserted into the pipe to act as anvils for the presses  13 . The pipe  19  can be wound with a high tensile wire  21  which creates an inner pretensioning of the frame. As in the case of the embodiment of FIG. 1, the interior space  12  defined by the filling inserts  20  and the presses  13  is sufficient to allow the introduction of an assembled molding tool, but not sufficient to permit the insertion of the tool pieces without prior assembly, or the stripping of the tool pieces from a completed work piece without first removing the molding tool and work piece from the interior of the frame. 
     The apparatus of the invention is utilized by first assembling two or more tool pieces to form a molding tool of the desired conformation in a space exterior to the apparatus. The assembled molding tool in then inserted into the interior space within the frame, and the presses are activated to lock the molding tool. A work piece is then formed within the molding tool, for example by injection molding or hydromechanical forming. The molding tool and work piece are then removed from the interior space within the frame prior to the removal of the tool pieces from the work pieces. 
     Because the molding tool is assembled and opened outside the apparatus of the invention, the presses used can have a very short stroke, for example a stroke of about 30 mm. With such a short stroke, the apparatus can be very compact and cost efficient, in spite of the high pressure forces which are applied. Indeed, the construction cost of an apparatus in accordance with the invention can be as much as 70 to 90% less than the cost of a long stroke press with comparable productivity. Furthermore, the use of short stroke presses results in more even force distributions and reduced deformation of the frame. 
     In the apparatus of the invention, as exemplified in FIGS. 1-3, the molding tool can be pushed into or be withdrawn from the interior space through either end. This reduces set up time considerably, and allows emptying of fluid to be effectuated outside the press apparatus. In some cases, however, it is desirable to have closures at the ends of the frame during the actual molding process. Such closures cooperate with the frame elements  10 ,  19  to lock the molding tool in a third dimension. One or both of the closures can be provided with an additional press, so that press forces can be applied from at least three different and substantially orthogonal directions. 
     FIGS. 4 and 5 show a type of press which can be used in the apparatus of the invention in greater detail. As shown, press plate  16  is maneuverable in response to three hydraulic pistons, two pistons  17  disposed on either side of the press plate, and a centrally disposed piston  18 . Piston  18  may be considerably more powerful than pistons  17 . In a preferred embodiment, pistons  18  is used for pressing and locking the molding tool, and pistons  17  are used for pulling the press plate back from the molding tool at the end of the process. Alternatively, all three pistons can be used for generating press force. 
     As a specific example, piston  18  of one press can be dimensioned for a press force of 300 tons, while the other press has a piston with a press force of 500 tons. These presses can be disposed around an interior space for receiving a molding tool having the following approximate dimensions: 300×400×1000 mm. 
     FIGS. 6A and B show one of two tool pieces  14 ′ which can be assembled into a molding tool for insertion into the interior space of an apparatus in the accordance with the invention. This tool piece is designed for the simultaneous molding of four work pieces  22  in one work cycle. The work pieces each have an enlarged part  22 ′. The energy for forming the work pieces is supplied hydromechanically by means of water pressure which is supplied to each work piece  22  via connection  23  which is attached to the molding tool. These are in turn connected to pumps (not shown) which generate pressure impulses. As shown in FIGS. 6A and B, the pressure surfaces are integrated close to the work piece in order to improve tightening and form stability. The area closest to the mold at the interface between two tool pieces is somewhat higher (less than 1 mm, and probably less than 0.1 mm, depending on the size of the tool) than the surrounding surface of the tool piece. This means that the force applied to keep the tool parts together will be concentrated in this area, forming a stronger closing bond between the two tool pieces and allowing for a better molding operation. 
     FIG. 7 shows a connection  23  in greater detail. The connection has an inner displaceable part  24  which protrudes for some distance into the work piece  22 , and is sealed to the work piece  22  with a sealing ring  70 . The displaceable part  24  is traversed by a conduit  24 ′ for the hydromechanical medium. Piston flange  25  is displaceable within a cylindrical space  26  in the connection  23 . The piston flange  25  is provided with a sealing ring  27  which abut the cylinder wall. Pressure medium is provided to the work piece via conduit  24 ′ via pressure medium connection  30 . 
     The displaceable part  24  can be maneuvered in one direction or the other within the connector  23  because each side of the piston flange  25  is in communication with a controllable pressure medium via conduits  28  and  29 . In this way, the distance that the displaceable part  24  protrudes can be adjusted in response to changes in the work pieces (such as the inclusion of enlarged part  22 ′) to maintain the sealing engagement of the connector with the work piece. This compensation is made with balanced pressure since both small ends of the displaceable part  24  are of equal size, and are subjected to equal pressure from the pressure medium introduced through connector  30 . 
     Alternatively, the pressure medium connection  30  can be directly connected to the displaceable part  24  via a piston rod which protrudes through the end wall of the connection means. 
     The apparatus of the invention is preferably used in combination with a surveillance system for monitoring the progress of the injection molding or forming process. In this cases, sensors adapted for sensing different parameters of relevance to safety and to production quality can be included in the apparatus, or in the tool pieces used in the apparatus. These parameters include dimensional changes in the work piece with reference to the applied force, nosie, vibrations and temperature. All measurements can be data processed in real time, so that the work cycle can be interrupted or altered when a fault is registered. In particular, it may be desirable to allow for the separate control of the hydraulic cylinders to permit for alteration and balancing of the locking pressure on the tool during formation of the work piece. 
     The invention is not limited to the above-described embodiments, but may be varied in various ways. For example, the molding tool can be made up of more than two part. Similarly, the connection means for pressure medium or for supply of injection molding material can be located on the molding tool, as shown in FIG. 6, or they can be arranged separately as a dockable unit.