| 摘要 |
An apparatus for performing a vacuum forming operation utilizing first and second vacuum producing devices connected to an air space beneath a heated plastic film which is located over a mold in which the film is to be shaped or molded. The first vacuum producing device is capable of removing a relatively large volume of air from the air space and producing a relatively weak vacuum. The second vacuum producing device is capable of removing a relatively small volume of air and producing a relatively high or strong vacuum. This enables the initial portion of the vacuum forming method to rapidly remove a large volume of air with a relatively inexpensive turbine type mechanism and produce a small or weak vacuum and once this has been accomplished, then the second vacuum producing device is activated which rapidly removes the small amount of air remaining and produces a relatively high vacuum. A valve member is provided and is biased to a closed position to prevent fluid flow between the first vacuum device and the air space. Upon initial actuation of the first vacuum device, the valve is caused to move to open position against its biasing and upon production of the relatively weak vacuum, the valve member closes. This disconnects the first vacuum device from the air space and thereafter a second valve device connecting the second vacuum device to the air space is actuated connecting the second vacuum device to the air space which creates a relatively strong vacuum and assists in holding the valve member closed.
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