Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Starting material is nonhollow planar finite length preform...
Reexamination Certificate
1999-12-02
2002-08-27
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Starting material is nonhollow planar finite length preform...
C264S547000, C264S553000, C264S572000, C264S335000, C264S138000, C425S388000
Reexamination Certificate
active
06440353
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to vacuum forming and more particularly to a method for twin-sheet vacuuming forming parts in a vertical orientation.
BACKGROUND OF THE INVENTION
Vacuum thermoforming generally refers to the process of heating a plastic material in sheet form to its particular processing temperature and forming the hot and flexible material against the contours of a mold by pneumatic means. When held to the shape of the mold and allowed to cool, the plastic retains the shape and detail of the mold.
A particular type of vacuum thermoforming is known as twin-sheet forming. In twin/dual-sheet forming, two sheets of plastic sheet are parallel-laid in a horizontal fashion and heated to a predetermined temperature. A blow pin is inserted between the two sheets and the upper and lower halves of a split mold close onto the sheets and pinch off around the entire perimeter. High pressure is then introduced between the two sheets from the blow pin, and a vacuum is applied to each of the two mold halves. However, a number of limitations exist with current twin-sheet forming technology. Current twin-sheet forming methods require precise controlled heating to ensure quality and consistent output from cycle to cycle. Because the sheets are horizontally oriented, the area in the center of the sheet tends to sag downward from natural gravity. Therefore, very precise and expensive heating controls must be employed to ensure that the plastic is evenly heated. As the material begins to soften it begins to sag downward, drawing away from the top heater and moving closer to the bottom heater, particularly in the center. This action requires precision zone heating controls to prevent the bottom sheet from getting too hot in the center as it moves down toward the bottom heater and the top sheets from being too cool in the center as it moves away from the top heater.
Another cause of sagging is that the melt index of the plastic is too high. Should this be the case, a plastic resin with a lower melt index is required thereby restricting the number of polymers available to utilize in the process.
Another aspect of current twin-sheet forming technology is that of trimming. Because a sheet of material must be clamped on its edges to allow stretching of the sheet into a shape, edge trim must be removed. For low volume, large sized molds, trimming is often manually performed in the forming station while the edge trim is still soft. However, manually cutting the molded article from the sheet is difficult as the sheet is in a horizontal position. This requires the operator to lean over the molded article to reach the outside perimeter of the pinch off. This is often difficult and sometimes dangerous should the operator lose his balance and fall onto the mold itself.
Another problem existing in current twin-sheet forming technology is the clamping pressure required to pinch off the perimeter of the molded article. Because the temperature of horizontally oriented sheets cannot become too hot without sagging, high clamping pressures must be utilized to form an effective pinch off. For high-quality parts, clamping pressures of 150 tons are not uncommon. Exerting this level of force on inexpensive molds often leads to cracking thereby destroying the mold. In addition, high clamping pressure requires more expensive tooling and operation of the apparatus.
Consequently, there is a need in the art for a method of twin-sheet forming that can form large parts without the sheets sagging from the force of gravity.
There is a further need in the art for a method of twin-sheet forming that does not require precise and expensive heating controls to heat the sheet evenly from end to end.
There is a further need in the art for a method of twin-sheet forming that presents the molded part to the manual trimming operator for easy trimming and safe removal of the part.
There is a further need in the art for a method of twin-sheet forming that requires less clamping pressure to pinch off the perimeter of the molded part.
There is a further need in the art for a method of twin-sheet forming that is able to utilize a wide variety of polymers while maintaining a high quality output.
There is a further need in the art for a method of twin-sheet forming that permits the use of less expensive composite molds such as fiberglass, ceramic, epoxy and the like over more expensive molds such as those fabricated from aluminum. It should be noted that aluminum molds may also be used if the part volume justifies the cost on certain models of parts. The present invention can accommodate aluminum and other metal type molds as well. Typically, however, these molds cost more, although usually less than blow molds.
However, in view of the prior art in at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.
SUMMARY OF THE INVENTION
The present invention solves significant problems in the art by providing a method of twin-sheet vacuum forming plastic articles comprising the steps of clamping two parallel sheets of thermoplastic synthetic resin in a vertical orientation to a clamping frame, the two parallel sheets in direct contact with each other about the perimeter of the clamping tray, and blowing air between the parallel sheets thereby creating a hollow cavity between the two parallel sheets. The two parallel sheets are then heated evenly in vertical orientation to molding temperature. The two parallel sheets are disposed between a first opposing mold cavity and a second opposing mold cavity of a vertically oriented split mold wherein the mold cavities have a contour conforming the configuration of the desired article. A negative pressure is applied to suction holes provided in the mold cavities, which draw the two parallel sheets against each opposing mold cavity as the split mold is being clamped together. The split mold is then clamped together to form a pinch off around the perimeter of the hollow cavity thereby forming a molded hollow part still disposed within the clamping frame. Simultaneously, a blow pin inserted into the hollow cavity injects compressed air, further pressing the heated sheets against the mold cavities.
In a preferred embodiment, at least four substantially equidistant spacers are positioned along the circumference of a first mold frame holding the first opposing mold cavity. A second mold frame holding the second opposing mold cavity may be fitted with apertures to slideably receive guide pins projecting from the spacers. The spacers provide a positive stop before the peripheries of the first and second mold frame come into contact. By placing the spacers between the two frames, a gap forms around the perimeter of the first and second mold frames while they are pressed together. This gap provides a pathway for plastic to escape. Otherwise, if the plastic were compressed along the entire perimeter of the first and second mold frames, high press force would be required to press the plastic down in order to properly close the mold. Another benefit of using the spacers is that the pinch offs are protected from damage, particularly when composite materials are used to construct the mold such as fiberglass, ceramic and epoxy. The spacers allow for less press force while providing precise clamping of each opposing mold cavity. Escape holes in the frame under the female guide prevents plastic from building upon the pins.
The molded hollow part is allowed to cool to a set point and the vertically oriented split mold is opened. The mold may be cooled externally with a water spray or may be cooled internally using water conduits.
The molded hollow part may be ejected from the split mold by blowing compressed air through the vacuum suction holes. The clamping tray is withdrawn from the split mold and shuttled away from the mold cavities where the molded hollow part is cut away from the clamping tray, still in a vertical position. In a preferred embodiment, a rail means in provided for sliding th
Hopen Anton J.
Lee Edmund H.
Silbaugh Jan H.
Smith & Hopen , P.A.
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