Background of the Invention This invention concerns thermoforming methods and apparatus.

Thermoforming is a process for making articles from heated plastic sheet material by drawing the heated plastic sheet material into conformity with mold surfaces by the application of fluid pressure and/or vacuum. So called plug assists are used with female molds to properly control the stretching of the heated plastic sheet material as it is drawn into conformity with the mold surfaces.

Articles are sometimes formed from two thermoformed sheets, each molded separately, and the formed sheets then fused together. Fuel tanks and other hollow articles are commonly formed by this technique. See U. S. patents 3,779, 687 and 3,868, 209 for a description of this technique.

Thermoforming apparatus often involves rotary or linear transfer of precut sheets mounted in clamping frames, the frames successively transferred into one or more heating stations, thereafter into a forming station, and then to a load/unload station where the completed article is removed and fresh sheets are loaded into the clamping frame. Other processing stations may sometimes be included, such as a cold forming station located next to the main forming station.

Roll fed thermoforming machines are also in use particularly for making thinner products such as disposable plastic cups in which a continuous strip is advanced through the forming station where an array of cups are formed into the sheet and the cups then cut from the sheet in a trim press.

Efforts have been made to increase the cycle rate of twin sheet thermoforming processes.

U. S. patents 6,187, 252 and 5,759, 591 describe a twin sheet thermoformer involving a plug assist which is shifted in the forming station to be aligned with either of two molds used to form each respective sheet, the plug assist carrying one formed sheet over the other formed sheet in order to enable fusing of the two sheets together to form an appliance door.

U. S. patents 5,658, 523 and 5,843, 366 show such an apparatus which involves simultaneous transfer of two sheets into a forming station, both sheets being formed at the same time in respective molds in the station.

The sheet parts are thereafter nested together and fused to create a plastic pallet. The pallet is then transferred into a cold forming station and thereafter to an unload/load station.

The simultaneous forming of the two sheets saves time but the transfer of both sheets together sometimes makes proper heating of the individual sheets more difficult. Also, the load and unload location are the same, making unloading of a formed article and loading of new sheets slower and necessitating more complicated apparatus.

In copending U. S. application U. S. serial no. 09/441,767, filed November 17, 1999, there is described a twin sheet thermoformer in which sheets are loaded successively

into a transfer member at the same location thereon but one sheet is moved to a shifted position on the transfer member in the forming station in order to align the sheets with a respective one of two tooling sets, and the article unloading is carried out from this shifted position, thus simplifying the loading/unloading process and equipment since the loading process is at an offset location on the transfer member.

However, this approach does not allow simultaneous forming of the two sheets so that the cycle time is slower than it could be with simultaneous forming.

A further development of the twin sheet thermoforming process involves emplacing an insert to be sealed within the hollow article, such as a level sensor and/or fuel pump assembly for fuel tanks. It has been found to be advantageous to seal such an insert within the tank at the time of forming the hollow article. Such insert installation has heretofore slowed the cycle rate of the machine by necessitating an additional step in the process.

It is an object of the present invention to provide a method and apparatus for twin sheet thermoforming in which a simultaneous forming of each sheet is carried out to speed the cycle rate but which does not involve a complicated loading-unloading process.

It is a further object to provide an insert installation step in such method and apparatus which does not lengthen the process cycle time.

Summary of the Invention These objects and others which will become understood upon a reading of the following specification and claims are achieved by a method and apparatus for making a hollow article which transfers both sheets into a press in the forming station at the same time

on a common clamping frame, each sheet in alignment with one of a pair of respective tooling sets including oppositely facing female molds, and both sheets are released from the clamping frame and then simultaneously formed into respective parts of the article by closing of press platens.

After forming each sheet into an article part, the mold holding one of the formed sheets is shifted on its platen to be aligned with the other mold, the plug assist for the other mold being simultaneously shifted away from the other mold, and the two formed parts are fused together by a second press cycle to complete the article.

After the molds and plug assists have been separated by retraction of the platens, and at the same time as one mold is being shifted to a position in alignment with the other mold, a robot or other pick and place device emplaces an insert within the cavity of the formed sheet in the one mold so that there is no substantial increase in the cycle time.

After the article is formed, and the two molds are separated by retraction of the platens, a second robot or other pick and place device removes the completed article containing the insert from one of the molds in the forming station.

This technique allows simultaneous forming of both sheets in the forming station without complicating and slowing the loading/unloading process.

Description of the Drawing Figures Figure 1 is a diagrammatic representation of the apparatus according to the present invention.

Figure 2 is a diagrammatic representation of the forming station components with the mold and plug assist sets separated but with the respective molds and plug assists in

alignment and the sheets in position to be formed.

Figure 3 is a diagrammatic representation of the forming station components with the mold and plug assist tooling sets in the condition in which the respective sheets have been formed into article parts.

Figure 4 is a diagrammatic view depicting the shifting of the upper mold and the emplacement of the insert in the part in the lower mold by a robot.

Figure 5 is a diagrammatic view of the forming station components showing the upper and lower platen and molds brought together to fuse the two parts into a completed article.

Figure 6 is a diagrammatic view of the forming station components with the platens and molds separated and the completed article removed by a robot.

Figure 7 is a diagrammatic elevational view of an alternate embodiment in which one mold and plug assist mounted to the upper platen is mounted a rotary tooling plate, shown in the position in which each mold is aligned with a respective plug assist.

Figure 8 shows the embodiment of Figure 7 is the rotated position with the two molds aligned.

Figure 9 is an elevational diagrammatic view of another alternate embodiment in which the mold and plug assist on the bottom platen are mounted on a rotary tooling plate.

Figure 10 is a plan diagrammatic view of the embodiment shown in Figure 9.

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 the Drawings, Figure 1 is a simplified diagrammatic representation of a thermoforming apparatus 10 according to the present invention.

The apparatus 10 includes a loading station 12 in which a rectangular clamping frame 14 has a pair of openings each receiving a plastic sheet A, B which are releasably clamped in the well known manner.

A transfer system 16 which also be provided by of any conventional design, is adapted to transfer the clamping frame 14 into a heating station 18 to preheat the sheets A and B in the well known manner prior to being transferred into a forming station 20 by the transfer system 16.

The transfer system 16 can be either a linear or rotary type such as is shown in the patents referenced above.

The details of the forming station 20 are depicted in Figures 2-6, which depict diagrammatically a press 22 including an upper platen 24 and a lower platen 24.

Lower platen 24 mounts a lower or first female mold 28 having an upwardly facing cavity 30, with a male first plug assist 32 mounted normally aligned above on the upper platen 24.

An upper or second female mold 34 is mounted on the upper platen 24 alongside the first plug assist 32 having a downwardly facing cavity 36 such that the respective cavities 30,36 face each other.

A second plug assist 38 is mounted aligned below the upper mold 34 on the

lower platen 26.

The sheets A, B are transferred into the forming station by movement of the clamping frame 14, bringing each sheet A, B into alignment with and between the components of a respective mold-plug assist tooling set, as indicated in Figure 2.

When the platens 24,26 are closed together as shown in Figure 3, as by an actuator 40 lowering the upper platen 24, the sheets A, B are simultaneously formed into a bowl shaped part of the article to be made. It will be understood by those skilled in the art, that vacuum and/or fluid pressure is applied to the sheet during this forming process by a well known means to cause the sheets to be conformed to the female cavity contours. The plug assists 32,38 aid and control the stretching of the preheated sheets A, B into the cavities 30, 36 for the purpose and in the manner also well understood in this field.

The platens 24 and 26 are then separated as by retracting of the upper platen 24 as shown in Figure 4. The bowl shaped oppositely facing article parts A1, B1 are retained in the first and second mold cavities 30,36 having previously been released from the clamping frame 16.

The upper second mold 34 is shifted laterally on the upper platen, as by an actuator 44 on a slideway 42 to be moved into alignment with the lower first mold 28. The upper first plug assist 32 is attached to the mold 34 so that at the same time it is shifted out of the way.

Such shifting could also be carried out using a rotary mounting of a tool plate on which one of the tool sets is mounted to allow switching position of the mold and plug assist on one of the platens, as described below.

While the shifting of the upper second mold is taking place, an insert 46 is

being emplaced within one part, typically within the cavity of the lower part B-1, by a pick and place device such as a robot 48 shown in Figure 4. Such inserts have been developed for automotive fuel tanks desirably sealed within the tank.

As shown in Figure 5, the platens 24,26 are again closed to bring the molds 34,36 and parts A1-B1 together fusing the rims of the parts together and forming the article C.

As shown in Figure 6, after retracting the upper platen 24, the article C is withdrawn from the upper mold 34 to be held exposed in the lower mold 36. A robot 50 grips the article C and removes the same while the upper mold and plug assist 32 are being returned to their original positions.

Thus, a rapid cycle time is provided while the use of pick and place devices eliminates the complexities of involved in transferring the article to a load/unload station as has been the practice in the past.

Figures 7 and 8 show an embodiment in which a rotary tooling plate 54 is rotatably supported an upper platen 24 so that the upper mold 34 and plug assist 32 can be shifted from a position respectively aligned with the lower plug 38 and mold 28 to a position where the molds are 34,28 aligned (Figure 8) by rotation of the tooling plate 52 by a rotary actuator 58.

Figure 9 shows the lower mold 28 and plug assist 38 mounted to a tooling plate 56 rotatably supported on the lower platen 26 by a bearing 58 to allow shifting of the relative position with respect to the upper mold and plug assist (not shown) as indicated in Figure 10.