Plastic article or earthenware shaping or treating: apparatus – Preform assembly means and means for bonding of plural... – Reshaping means utilizing fluid pressure directly contacting...
Reexamination Certificate
1998-10-26
2001-06-26
Nguyen, Nam (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Preform assembly means and means for bonding of plural...
Reshaping means utilizing fluid pressure directly contacting...
C425S216000, C425S388000, C156S382000, C156S468000, C156S475000, C156S494000
Reexamination Certificate
active
06250910
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a thermoforming tool employed for wrap-around lamination of a sheet to at least one edge of a molding loosely resting on the tool's lower half. The tool comprises a sheet-stretching frame that is positioned over the molding and lowers a thermally plasticized sheet tensioned therein at its edges into a holding position below the edge of the molding, means of sealing the sheet off from the lower half of the tool, and means of suction employed in the subsequent lamination process. The means of suction consist of an evacuatable depression in the lower half of the tool and below the edge of the molding.
Thermoforming tools of this type are known. Once the motionless sheet has been statically formed to the surface of the molding, the upper half of the tool is lowered against its lower half, swaging the sheet in the vicinity of the depression. Vacuum generated in the depression wraps the sheet around the edge of the molding. The sheet is formed onto the molding and bonded to it in a single operation. In practice, however, the sheet often does not adequately conform to the contour of the molding, and undesirable wrinkles occur at critical and visible areas. Such wrinkles are detrimental to the product's appearance and distort its surface pattern. The adhesion between the molding and the lamination is also unsatisfactory at these points. The lower half of such a known thermoforming tool also includes many moving parts, which makes it expensive and complicated.
SUMMARY OF THE INVENTION
The principal object of the present invention is to provide an improved thermoforming tool of the type described above that will not have the aforesaid drawbacks and that produces a smooth and attractive lamination of even moldings that have elevated or depressed edges.
This object, as well as other objects which will become apparent in the discussion that follows, is achieved, in accordance with the present invention, by providing a thermoforming tool with a lower half that features a sealing lip at the level of a conceptual plane below the edge of the molding. The lip parallels the edge of the molding and demarcates the depression. The sheet can be applied against and stretched over the lip by a plunger that descends between the lip and the sheet-stretching frame into a lowermost position. The face of the plunger parallels the top of the edge of the molding. Once it is in its lowermost position, the plunger is subject to a return stroke to a position approximately at the level of the conceptual plane of the edge of the molding. The sheet, stretched to a prescribed extent, is accordingly displaced in relation to the lower half of the tool to create the wrapped-around area, rolled over against the surfaces of the molding to be laminated, and pressed down. The stresses and strains that occur in the sheet in the vicinity of the wraparound are accordingly equalized, allowing the thermally plasticized sheet to be heated to optimal forming temperature without being damaged by excessive tension at critical points. In addition to unwrinkled forming of the sheet to the molding at all points of the wraparound, unpleasant distortions of any patterns in the sheet will also be eliminated. Such patterns include for example leather graining and imprinting. Even edges that slope backward or swing in and out can be satisfactorily laminated in accordance with the present invention.
A sealing lip in the sense of the present invention is a wedge-shaped elevation entirely paralleling the edge of the molding and tightly engaging the gas-impermeable sheet as the plunger descends. The lip is not very thick and can even taper in to a narrow edge to facilitate displacing it in relation to the sheet it engages even subject to vacuum. The sheet will accordingly be continuously and progressively formed against the edge and surface of the molding in accordance with the present invention and pressed against and bonded to it without wrinkling even when there are downward projections on the back of the molding. To facilitate displacement of the sheet, the sealing lip should be as round as possible and curve into the surfaces adjacent to it at each side. A lip coated with a low-friction material or consisting partly of PTFE or silicone or entirely of such a low-friction material as PTFE or polyurethane for example has also been proven practical. Coating the lip with a material based on soap or oil is also possible and can further help to prevent tension peaks in specific areas of the sheet during forming.
Satisfactory displacement of the sheet in relation to the sealing lip and to the face of the plunger while the plunger is descending and ascending will introduce the remainder of the sheet uniformly into the depression in the lower half of the tool subject to differential atmospheric pressure and smooth it out against the bottom of the molding while preventing relative displacement at the edge. This constitutes a significant advantage with respect to eliminating wrinkling and to ensuring satisfactory adhesion in the critical areas. Equally effective adhesion can be attained with less adhesive than at the state of the art.
It is practical to coat the areas of the molding that are to be laminated with a thermally plasticizing adhesive, a hot-bonding adhesive. This adhesive is briefly applied hot while the sheet is being applied and pressed down in order to attach the sheet. The sheet must, however, be applied to the molding without wrinkling or slipping, easy in accordance with the present invention. Once the adhesive has cooled, the bond will be particularly permanent even when warmed by the rays of the sun entering a vehicle.
The mold should if possible be of a material that conducts heat poorly to ensure that the polymeric sheet will be inherently hot enough to melt the adhesive applied to the mold. The adhesive must be liquid enough to completely wet the mold while the sheet is being pressed against it. To rapidly stabilize the bond mechanically subsequent to lamination, it has been proven practical to blow air over the mold or to otherwise accelerate cooling. Since most of the sheets employed for this purpose are very thin, enough heat can escape through the sheet itself to ensure rapid and stable adhesion. Such accelerated heat dissipation will also help to secure the sheet, and the pressure of application can be removed sooner with no risk of detachment due to the resilience of the sheet.
The return stroke subsequent to stretching the sheet provides a prescribed reserve of material below the edge of the molding in accordance with the present invention. This reserve allows the sheet to be drawn with slight differential forces against the edge of and into the depression below the molding, forming the sheet against the exposed bottom margin of the mold, and bonding it thereto. The forces exploited in conventional suction presses and derived from the atmospheric differential pressure are entirely adequate for this operation.
To carry out this operation it is necessary to apply the sheet tight to the sealing lip below the edge of the molding, and this is accomplished in accordance with the present invention by means of the plunger. The lower half of the tool is mostly metal. It has been demonstrated practical to increase the temperature to improve the plasticity of the sheet.
The sealing lip can be constituted by one side of a groove that engages the lower half of the tool. The groove can be annular and continuous and accommodate all the edges of the molding.
The area of the plunger, its face, that engages the sheet should also be rounded to facilitate displacement of the sheet in relation to the face during the deformation and return stroke. This feature will distribute the expansion of the sheet over a considerable area and prevent damage to parts of the sheet while it is being laminated. It has been proven practical in this event for the face to be low-friction or provided with means of decreasing friction.
For similar reasons it has also been proven practical for
Lause Jürgen
Nimmergut Edgar
Fritsche Moellmann, GmbH & Co. KG
Heckenberg Donald
Milde Hoffberg & Macklin, LLP
Nguyen Nam
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