The invention will be discussed in the context of molding of , parts from sheet materials. In the automotive industry, small parts are often molded from thermoformable glass mat. Panels of such material are typically heated, placed between two mold halves, compressed to achieve a desired shape, and then held under substantial pressure until cool. A predetermined pressure must be achieved and maintained to ensure proper setting and often to ensure proper finishing of product surfaces.

In such applications, the mold halves may typically be fastened lo th αppei and lower platens of a press. The upper platen may be displaced with a large hydraulic ram to bring the mold halves together and apply the compressive force necessary to shape the raw sheet material. To improve productivity, multiple identical molds may be installed between the platens. Automotive parts might have a required thickness, when set, of about 5 millimeters. The sheets of glass mat used in such application might typically deviate from a nominal thickness by ±5%. This potentially gives rise to a total difference in the thickness of the materials contained in the various molds of 10% or roughly .5 millimeters. Although small, that difference can produce very significant variations in the distribution of compressive force among the molds. This has been seen to result in one or more defective parts. Disclosure Of The Invention

In one aspect, in a press comprising a pair of platens and means for producing a relative displacement of the platens toward one another along a predetermined axis thereby to apply a compressive force to a plurality of workpieces between the platens, the invention provides the improvement in which one of the platens is adapted to distribute the compressive force among the workpieces in a manner substantially independent of differences in thickness

among the materials placed between separable portions of the workpieces. The one platen comprises a supporting structure, and a plurality of plates oriented between the supporting structure and the other platen such that each workpiece can be located between a different plate and the other platen. The plates are mounted to the supporting structure for displacement independent of one another along respective axes substantially parallel to the predetermined axis. Means associated with each of the plates and acting between the associated plate and the supporting structure apply a predetermined force urging the plate toward the other platen. The compressive force applied to each workpiece is then dependent on the force applied to the plate where the workpiece is located and is largely independent of any variations in the thickness of the material within the various workpieces.

The force applying means are preferably fluid-operated cylinders (hydraulic or pneumatic cylinders). In applications involving identical workpieces, the cylinders may be operated to apply the same force to each plate. The total magnitude of the forces applied to the plates will be set to match the compressive force which would otherwise result from simply driving the platens towards one another. That compressive force is effectively distributed evenly among the workpieces. Although the cylinder might be operated only when the platens have closed the workpieces, the cylinders are preferably operated continually to press the plates against stops which limit travel of the plates. Each plate can then deflect immediately in response to any uneven distribution of force among the workpieces.

Other aspects of the invention will be apparent from a description below of a preferred embodiment and will be more specifically defined in the appended claims. Brief Description Of The Drawings

The invention will be better understood with reference to drawings in which: fig. 1 is a diagrammatic elevational view of a press incorporating the invention; fig. 2 is a diagrammatic isometric of four displaceable plates associated with a lower press platen; and, fig. 3 is a diagrammatic cross-section of one of the plates detailing its mounting to the lower platen and detailing fluid-cylinders formed within the plate.

Best Mode of Carrying Out The Invention

Reference is made to fig. 1 which illustrates a hydraulic press 10 adapted to mold thermoformable glass mat of the type described above. The press 10 has a conventional frame 12 which includes a pair of side slabs 14, 16 and a cross-head 18. The press 10 has a movable upper platen 20 whose movement is guided along a central vertical axis 24 of the press 10 by the side slabs 14, 16. A stationary lower platen 22 is fixed to the frame 12 directly below the upper platen 20. A central hydraulic ram 26, suspended from the cross-head 18, can be operated to drive the upper platen 20 toward the lower platen 22 to compress workpieces installed between the platens 20, 22. Except for the construction of the lower platen 22, the press 10 is conventional. The upper platen 20 carries a bolster plate 28 formed with T-slots (not numbered) to which molds can be bolted in a conventional manner. The lower platen 22 has an operative surface defined by four plates 30, 32, 34, 36 which are also formed with conventional T-slots (not numbered). The four plates 30, 32, 34, 36 are oriented substantially coplanar between the platens 20, 22. Four molds are installed between the platens 20, 22, each located between a different one of the four plates 30, 32, 34, 36 and the upper platen 20. Only two such molds 38, 40 are shown in diagrammatic ciυ&s-scction in fig. 1. The mold 38 which is typical comprises an upper mold half 42 bolted to the upper platen 20 and a lower mold half 44 bolted to the plate 30 of the lower platen 22.

The construction and mounting of the plate 30 is typical of all four plates 30, 32, 34, 36. As apparent in the cross-sectional view of fig. 3, the plate 30 is mounted to a supporting structure 46 in the lower platen 22 with shoulder screws. The shoulder screw 48, which is typical, extends through a vertical passage 50 formed in the plate 30. The screw 48 has a shaft 49 with an external screw thread 52 located at its lower end and threaded into the supporting structure 46. The screw 48 has a head 54 at its upper end which is received in an enlarged upper section of the passage 50. The shaft 49 guides movement of the plate 30 along an axis 56 substantially parallel to the central axis 24 of the press 10. The head 54 of the screw 48 serves as a stop to limit axial displacement of the plate 30 toward the upper platen 20. In fig. 3, the plate 30 is shown in its uppermost axial position, the head 54 of the screw 48 then being engaged with the plate 30. The magnitude of the permitted travel of the plate 30 is indicated with the character β in fig. 3. In the arrangement shown in fig. 3, β corresponds to the maximum separation of the plate 30 from

the supporting structure 46, which has been exaggerated for purposes of illustration. The value β need only correspond to the maximum expected difference in thickness among sheet materials placed in the four molds. For molding of automotive body parts from sheet materials with the characteristics described above, β need only be .5 millimeters, but several millimeters might be allowed.

The plate 30 is machined to form a set of four fluid-operated cylinders which can be simultaneously actuated. Only two such cylinders 58, 60 are apparent in fig. 3. The four cylinders are actually positioned at the corners of a hypothetical rectangle which is inset equally from all sides of the plate 30. Each acts between the plate 30 and the supporting structure 46.

The fluid-operated cylinder 58 is typical. A cylindrical passage 62 is formed in the plate 30 with a blind end 64 within the plate 30 and an open end 66 facing toward the supporting structure 46. A piston 68 is located within the cylindrical passage 62 and defines a compartment 70 between itself and the blind end 64. The piston 68 comprises a seal 72 that permits conventional sliding movement of the piston 68. The seal 72 has been shown as an O-ring to simplify illustration. The seal 72 should be appropriately selected to prevent escape of pressurized fluids in response to the fluid pressures required. A fluid delivery passage 74 is formed in the plate 30 and communicates with the compartment 70. All four fluid-operated cylinders are coupled to the fluid delivery passage 74 for simultaneous actuation with fluid under pressure. When actuated, the four fluid-operated cylinders apply an upward force to plate 30. This urges the plate 30 to displace toward the upper platen 20 until its movement is stopped at the uppermost position illustrated in fig. 3 by the heads of the various shoulder screws.

Each plate 30, 32, 34, or 36 is associated with its own set of four fluid-operated cylinders. These sets are identically formed. As indicated in fig. 1, the four sets are coupled to an accumulator 76 with four fluid lines 78, 80, 82, 84. The accumulator 76 is charged with hydraulic fluid from an appropriate pump (not illustrated). The term "accumulator" as used in this specification should be understood in hydraulic applications as a conventional hydraulic accumulator or in pneumatic applications a gas reservoir. Since the cylinder sets are identical and receive hydrauUc fluid from the accumulator 76 at a common pressure, each set applies an upward force of the same magnitude to its associated plate. A significant advantage is obtained by using an accumulator. When any plate 30, 32, 34, or 36 deflects in response to closing

of the platens 20, 22 about the mold connected to the plate, fluid can be discharged from the associated cylinder set to the accumulator 76. No significant variation in the forces applied by the various cylinder sets to the associated plates 30, 32, 34, 36 occurs. This is significant because the upward force applied by the cylinder sets in turn determines what compressive force is actually applied to the mold secured to the associated plate.

In this particular embodiment, the cylinders of each set are designed to have, together, a pressure-effective area which is one-quarter of the pressure-effective area of the central ram 26. The accumulator 76 will typically be charged to the operating pressure used to drive the central ram 26 during compression phases of operation. Each cylinder set consequently applies an upward force which is substantially one-quarter of the downward force applied by the central ram 26 to the molds. This effectively reacts the force produce by the central ram 26. If the mold 38 attached to the plate 30, for example, contained thicker sheet material than the other molds, resulting in excessive downward force being applied initially by the ram 26 to the mold 38, the plate 30 would displace downwardly,. This in turn causes the upper platen 20 to bear more forcefully on the other molds containing thinner materials. After a brief transition period, the downward force applied by the central ram 26 distributes evenly among the four molds.

Once the accumulator 76 is charged as described above, and the cylinder sets are actuated to urge their associated plates 30, 32, 34, 36 upwardly, molding operations proceed as in a conventional press. The cylinders can also be disabled to allow the plates 30, 32, 34, 36 simply to rest atop the supporting structure 46 of the lower platen 22. The press 10 then operates precisely like a more conventional press, and can be used to handle a large single mold or used in other conventional applications. Separate and distinct hydraulic or pneumatic cylinders can be used for the general purposes of the invention. However, the assembly of plates 30, 32, 34, 36 with built-in fluid-operated cylinders has a significant advantage. The assembly can be used to immediately retrofit conventional presses by replacement of the bolster plate 28 commonly associated with the platens 20, 22.

A variety of alternatives are contemplated as part of the invention. The displaceable plates and fluid-operated cylinders could be installed on the moving platen. Although hydraulic operation of cylinders to apply forces to plates has been described, operation with pressurized gas is an immediate alternative. Shoulder screws are used to simultaneously guide and

limit movement of the plates, but stops and guiding structures can be formed in any appropriate manner. The press illustrated in fig. 1 is a common design, but the invention lends itself to any press design in which platens are displaced relative to one another to compress workpieces.

It will be appreciated that a particular embodiment of the invention has been described and that modifications may be made without necessarily departing from the scope of the appended claims.