Abstract:
A thermoformer apparatus having a press driving upper and lower platens by respective pairs of servo motors, each pair driving four toggle linkages to stabilize the platen against the forming pressures. Linear bearings are provided on four corner posts for precision guiding of the platen motion.

Description:
BACKGROUND OF THE INVENTION 
     This invention concerns thermoforming apparatus, and more particularly presses used in thermoforming parts from preheated plastic sheet material. Thermoforming is in wide spread use as a manufacturing method, and involves preheating of thin plastic sheet material in an oven and then advancing the same into a thermoforming press having a mold located between the press platens. 
     The plastic sheet is drawn against the mold surfaces during the forming operation carried out by the application of a vacuum or air pressure causing the sheet material to be drawn against the mold surfaces. Relatively large articles are often manufactured this way and considerable press loads are generated by the fluid pressure forces acting over large areas, which must be resisted by the press platens and their supporting structure. 
     The forces required also may be increased by the use of plug assist tools in which complementary tooling is carried by one of the platens and driven into the forming surfaces on the other tooling piece. The weight of the tooling supported by the platens thus can become considerable. A large servo motor has sometimes been used, the size of the motor in turn resulting in a relatively slow response time of the platen drive mechanism. The relatively thin wall thickness of the part makes accuracy in the forming process more critical. 
     Presses used in thermoforming typically involve a pair of columns supporting the platens in their up and down motion and plain bearings are used in the linkages used to drive the platens up and down and to guide the platens on the machine frame. This construction has limitations in that the deflections of the platens under heavy loading is not sufficient to prevent inaccuracies in the thin walled parts formed due to slight tilting and bending of the platens and the structure supporting the platens under the heavy press loads described. In addition, a single large servo motor has considerable inertia increasing the cycle time required. This construction also absorbs a significant portion of the motor power to overcome the frictional losses. Where the apparatus is taxed to its utmost capability, these losses become critical, and the press may not be able to execute the operation required. 
     Accordingly, it is an object of the present invention to provide a thermoforming apparatus having an improved thermoforming press in which the stability and precision movement of the platens is substantially improved over the prior art thermoforming presses. 
     It is a further object of the present invention to provide a press for use in thermoforming operations in which the frictional losses are significantly reduced. 
     It is yet another object of the present invention to provide a servo motor operated platen mechanism in which the response capability of the servo motor drive is improved over the prior art designs for heavier duty presses. 
     SUMMARY OF THE INVENTION 
     These objects and others which will become apparent upon a reading of the following specification and claims are achieved by a thermoforming press in which the platens are guided on precision antifriction bearings located at each of four corners of each of upper and lower press platens, the bearings mounted on each of four posts forming a part of the press frame. Both the upper and lower platens are thereby guided on each of their four corners with minimal friction losses, highly precision guided movement of the platens is obtained, and the platen is very stably supported by the linear antifriction bearings at each corner post. 
     Actuation of each platen is carried out by a set of four separate linkages connected to each plate at a location adjacent the four corners of the plated, such as to evenly distribute the forces imposed on the platen by the press forces. 
     Two separate servo motors are provided for each platen drive, each servo motor driving a transmission having an output shaft having a pair of crank arms affixed thereto, in turn driving two of the four toggle linkages provided for each platen. 
     Both the upper and lower platen are mounted and driven in the same manner, such that a total of four servo motors driving two sets of four toggle linkages is employed for operation of the platens. 
     The pivotal connections of the links making up the toggle linkages are provided with antifriction bearings at the pivotal connections such as to further reduce the frictional losses in the system. 
     The dual servo motor drive for each platen provides a rapid response press operation. A very stable support of the platens, even under heavy loading is achieved, and an only minimal frictional losses are experienced such as to maximize the capacity of the press for conducting thermoforming operations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a perspective view of a thermoforming press according to the present invention. 
     FIG. 2 is a front elevational view of the press thermoformer press shown in FIG. 1 with a block diagram representation of other components of the thermoformer apparatus. 
     FIG. 3 is a side elevational view in partial section along the lines  3 — 3  in FIG. 2 with a block diagram representation of other components of the thermoformer apparatus. 
     FIG. 4 is an enlarged view of the section  4 — 4  taken in FIG.  2 . 
     FIG. 5 is an enlarged fragmentary view of a portion of the thermoformer press shown in FIG. 1 showing details of the shut height adjustment mechanism. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
     Referring to FIG. 1, a press  10  according to the present invention includes a press frame  12  formed by a top  14  and a bottom  16  joined by four corner posts  18  defining a rectangular in section cavity within which an upper platen  20  and a lower platen  22  are mounted to be capable of vertical motion towards and away from each other. 
     A pair of side plates  24 ,  26  overlie the outer sides of each of the respective pairs of corner posts  18 , secured with machine screws to the outer sides of the respective pairs of corner posts  18  to stiffen the frame and provide outriggers for antitip leveling feet  26  located to the front and rear of the press  10 . 
     According to one aspect of the present invention, each corner of the upper and lower platens  20 ,  22  is supported by a precision linear antifriction bearing sets  28  and  30  each affixed to an inwardly facing surface of each frame post  18 , with a suitable bearing slide affixed to each corner of the upper platen  20  and lower platen  22 . This provides precision low friction guidance of each platen in its up and down motion within the frame  12 . The platens  20 ,  22  may be temporarily held in position by means of a bracket  34  affixed thereto and a drilled plate  36  on one of the posts  18 . The bottom platen  22  may also be held in position by means of a drilled plate  36  and bracket  38  such as to allow the platens  20 ,  22  to be held stationary for maintenance purposes. 
     According to the concept of the present invention, each of the upper and lower platens  20 ,  22  is driven by a drive arrangement comprised a pair of electric servo motors  40 A and  40 B for the upper platen and a pair of servo motors  42 A,  42 B for the lower platen  22 . 
     Each servo motor  40 A,  40 B,  42 A,  42 B, drives a respective one of transmission units  44 A,  44 B,  46 A,  46 B. The motor-transmission assemblies thereby formed are each held on one of the side plates  24  by means of an adapter  48 . An output shaft  50 A,  50 B extending from each of the transmissions  44 A,  44 B oscillates a main crank arm  52 A,  52 B, respectively, as well as auxiliary crank arms  54 A,  54 B. 
     A first connecting bar  56 A drivingly connects auxiliary crank arms  54 A,  54 B together to oscillate insuring simultaneous oscillation of both of the output shafts  50 A,  50 B of the transmissions  44 A,  44 B. This stalls both motors  40 A,  40 B if one stalls to prevent one motor from continuing to operate if the other stalls. 
     The main crank arms  52 A,  52 B are each journaled to one end of one of a pair of doubled linkage members  58 A,  58 B. The pivoting thereon is preferably provided by low friction ball or roller bearings  60 A,  60 B, respectively, in order to reduce the frictional losses in the system. 
     The lower end of each double link  58 A,  58 B is journaled to a respective bearing block  62 A,  62 B again by means of low friction rotary bearings  64 A,  64 B. The bearing blocks  62 A,  62 B are secured to the top of the upper platen  20  by a shut height adjustment means to be described hereinafter. The transmission shafts  50 A,  50 B extend across the frontal width of the machine  10  and over a second pair of double links  66 A,  66 B. Main crank arms  68 A,  68 B are journaled to the upper end of the double length links  66 A,  66 B, while the lower end of the double links  66 A,  66 B are journaled in bearing blocks  70 A,  70 B fixed to the top of the upper platen  20 . 
     A second connecting bar (not shown) is operated by a pair of auxiliary crank arms  72 A,  72 B also mounted to output shafts  50 A,  50 B. 
     Lower transmissions  46 A,  46 B likewise have respective output shafts  74 A,  74 B which each drive a main crank arm  76 A,  76 B which are each pivotally connected to the lower end of a double links  80 A,  80 B on the right hand side of the machine as viewed in FIG. 2, with the upper end of each of the double links  80 A,  80 B pivotally mounted in bearing blocks  82 A,  82 B fixed to the undersurface of the lower platen  22 . 
     A second pair of double links  84 A,  84 B are also driven by the shafts  74 A,  74 B by means of main crank arms  86 A,  86 B pivotally connected thereto with the upper ends of the linkages  84 A,  84 B, likewise journaled in bearing blocks  88 A,  88 B affixed to the undersurface of the lower platen  22 . Auxiliary cranks  90 A,  90 B,  91 A,  91 B are provided, driven by the shafts  74 A,  74 B, respectively, to drive connecting bars  92 A,  92 B for the purpose described above. 
     The transmission output shafts  50 A,  50 B,  74 A,  74 B are each oscillated by the connected servo motor, to produce a dead center condition shown in the drawings at one extreme position, which is the bottom dead center position of the toggle linkage formed by the main crank arms and double linkage, a full stroke of the platens  20 ,  22 . Return oscillation of the crank arms,  52 A,  52 B,  76 A,  76 B draws each of the upper platen  20  and lower platen  22  apart from each other. 
     The combination of the crank arm  62 A and the double links  58 A,  58 B,  64 A,  64 B,  66 A,  66 B,  86 A,  86 B and links  84 A,  84 B act as toggle linkages able to exert a powerful mechanical advantage. In the aligned, dead center position shown, the platens  20 ,  22  are able to be held in position against the enormous thermoforming pressures typically exerted. The four point location of the four links mounted to each platen with a double link adjacent each corner of a platen, together with the location of the precision bearings  28 ,  30 , provide a very precise guided platen movement, and stabilized support for the platens  20 ,  22  despite the heavy forces imposed, such that very good accuracy of the thermoformed parts may be achieved. 
     An auxiliary lifting cylinder  96  for the upper platen and auxiliary lifting cylinder  78  for the lower platen is provided to assist in overcoming the weight of the tooling dies, shown diagrammatically in phantom lines in the drawings. Such assist cylinders are well known and typically comprise pneumatically actuated devices. 
     The press shut height may be adjusted by means of slide wedges  100 A,  100 B,  102 A,  102 B interposed beneath the bearing blocks  62 A,  62 B,  70 A,  70 B. 
     An adjustment motor  104  is operatively connected to cause sliding movement of the wedges  100 A,  100 B and via a cross shaft  106 , also the wedges  102 A,  102 B by means of a power screw device of a type well known in the art and hence will not described herein in detail.