FR2469994A1 - Vacuum-forming textured mouldings - where vacuum drawn sheet on male tool moves into evacuated female whilst male is pressurised - Google Patents

Abstract

Thermoplastic sheet is vacuum formed into a hollow article with textured finish. The gripped sheet is heated and applied to the top of a male tool, a vacuum is applied under the sheet, and a female tool is applied so that the edges of the two tools meet leaving a cavity for the sheet. Vacuum is applied above and pressure beneath the sheet which lifts off the male, is drawn against the female, and takes up the desired female shape. Process enables prodn. of articles with steep sharp projections without significant variation in thickness, textured finish is accurately reproduced.

Description

Method and apparatus for vacuum thermoforming a thermoplastic sheet and producing a grained surface thereon.

 The present invention relates generally to a method and apparatus for shaping and producing an imprint forming a "grain" on the surface of a thermoplastic material which is in the form of a sheet and it relates more particularly to a method and an apparatus for vacuum thermoforming a plastic sheet in order to transform it into an object having deep recesses, while reproducing the desired grain pattern faithfully on its surface.

 The expression "vacuum thermoreading" denotes, in general, a process in which a thermoplastic sheet stretched in a frame is heated in order to soften it properly before placing it on a male or female form, a vacuum then being created in the space between the sheet and the surface of the form, in some cases with the help of compressed air, in order to ensure narrow application. To avoid local thickness inequalities in the object produced, the surface of the form must be made smooth by an appropriate process. When it comes to producing objects with deep valleys, this measure turns out, however, insufficient to eliminate local inequalities in wall thickness.

In a variant of the above process, the heated plastic sheet is made convex by preforming it with an upward air flow, the male form being mounted under it and then rising towards said sheet of my plastic material. , then a vacuum is created in the space between them. This preforming using compressed air can more or less eliminate inequalities when it comes to producing objects having relatively deep hollows. According to a variant of this latter process, a trunk shape is lowered for the first time plastic sheet heated for preforming purposes, then lowered further to place the preformed sheet on the female form, leaving a space between them in which a vacuum is created to apply it closely.

 However, not all of these various methods are entirely satisfactory for producing objects having deep valleys with very steep slopes and a pattern of grain on the surface. Indeed, it is inevitable that the wall will be thin at the place of the strongly projecting parts or of the hollows and the grain of the surface will often disappear in places because of the local creep or the extension of the sheet.

 Consequently, the object of the present invention is to provide improved vacuum thermoforming methods for producing objects having deep recesses and the walls of which do not exhibit significant thickness inequalities, while ensuring correct reproduction of the drawing of the grain desired on the surface of these.

 The subject of the invention is a method for forming a thermoplastic sheet and etching an imprint thereon in order to produce an object having deep hollows and the surface of which has a grain pattern, which consists in: heating the stretched plastic sheet; placing this sheet on the surface of a convex male shape for preforming purposes; create a gap between the sheet and the surface of the male form; moving either said shape or a concave female shape for the purpose of preforming and imprinting so as to bring it into contact with the other shape and so that it is applied tightly against the peripheral edges of the latter, while leaving some space between them in the work area; and creating a vacuum between the female form and the surface of the sheet, while blowing air between the male form and the latter in order to transfer said sheet from the male form to the female form in order to continue the implementation grain shape and formation.

 The invention also relates to an apparatus for forming a thermoplastic sheet and etching an imprint thereon in order to produce an object having deep hollows, the outer surface of which has a grain pattern which comprises: a male shape the body of which is limited by a external working surface corresponding approximately to the external surface of the object to be formed and which is adapted to create a vacuum in order to suck said sheet against it and to breathe air in order to detach this sheet g a female form having a body whose working surface corresponds exactly to the external surface of said object and which presents in negative a drawing of the grain to be imprinted and which is adapted to create a vacuum in order to suck the preformed sheet against it and a frame for holding the heated plastic sheet stretched horizontally by tightening its edges, the arrangement being such that said frame and said male shape can approach one of the e the other in order to drape the sheet over the body of the male form and then to suck it against the surface of the latter for the purpose of preforming, said male form and said female form then moving relative to each other to the other to come into contact, while leaving a certain space between them in order to manage the preformed sheet from the first of the second and to complete the shaping and produce the design of the grain thanks to the cooperation of the vacuum created in the female form and air blown in the male form in place of the vacuum, now suppressed.

Other characteristics and advantages of the invention will emerge from the description which follows, given solely by way of nonlimiting example, with reference to the appended drawing, in which
Figure 1 is a perspective view of a dashboard of motor vehicles which can be produced by the method of the invention
Figure 2 is a sectional view of a device for implementing the method of the invention comprising a female form occupying a fixed position and whose cavity is oriented downwards, a movable male form installed in the female form , and a frame to stretch the plastic sheet between the two forms
Figure 3 is a view similar to the previous one but where the male form has risen from its normal position or waiting in Figure 2 so as to drape over it the plastic sheet, sketched in phantom, for the purposes preforming; and
FIG. 4 is a view similar to the preceding ones, but on a larger scale, where the male form has been raised still further so as to come into contact with the female form while leaving between them a certain space for shaping and footprint.

Referring now to Figure 1, we see the dashboard of a motor vehicle. Note in this figure that it is an object having deep hollow parts, as well as strongly projecting parts P1 and P2 and that it also has a grained surface G. All technicians will agree that the manufacture of 'such an object by conventional vacuum methods ensuring a faithful reproduction of the grain design is very difficult if not impossible.

 In the upper part of FIG. 2, a female form is seen designated in its entirety by 10. The form 10 comprises side walls 11, a body 12 having a cavity whose contour corresponds exactly to that of the external surface of the product, but in "negative, the lower edges of the body 12 connecting to the lower edges of said side walls 11 so as to form shoulders 11 ', a cover 13 being mounted on the upper edges of the side walls 11 so as to delimit a chamber hermetically close.

 The cover 13 has --- an opening 14 intended to be connected to a pumping installation (not shown) to create a vacuum in said chamber, as well as heating and cooling means (not shown).

The body 12 of the female form has deep recesses Ri and R2 corresponding respectively to the projecting parts PI and P2 of the product of FIG. 1. The body 12 of the female form is pierced with a number of perforations 15 for the passage of the air and the surface of its cavity have a hollow imprint 16 to reproduce the pattern of the grain
G of the product in FIG. 1. The perforations # 15 have been oversized and their number has been reduced in the drawing, at the same time as the imprint of the grain 16 has been exaggerated. In this embodiment, the female form 10 occupies a fixed position on the frame of the machine (not shown).

 A male form, designated in its entirety by 20, is mounted in the female form 10 so as to be able to rise to come into contact therewith. The form 20 comprises a bottom wall 21 and a body 22 defining a hermetic chamber.

The edges of the body of the male form form shoulders 22 'which abut against the shoulders 11P of the body 12, of the female form. The bottom 21 has an opening 23 called to be connected to another pumping installation (not shown) to evacuate and to inject air into said chamber, which also contains a heating device 24 of any type.

 The contour of the body 22 of the male form does not correspond exactly to that of the product, which amounts to saying that its external surface does not apply closely against the surface of the cavity of the body 12 of the female form, but has been studied to provide a certain space between them, as clearly shown in Figure 40 Depending on the dimensions of the product, the thickness of the plastic sheet and other variable factors, the distance between the surface of the body cavity 12 of the female form and the external surface of the body 22 of the male form can be between 4 and 5 mm in the closest region and between 7 and 9 mm in the most widely spaced region, when it comes to producing dashboards for motor vehicles such as that shown in FIG. 1. It should be noted that this male shape 20 is thus different from those used in conventional methods, which are moreover practically not used because of the difficulties encountered in preparing correctly adapted female forms The body 22 of the male form has a contour similar to that of the cavity of the female form and, consequently, it has projecting parts P3 and P4 but which do not correspond exactly to the protruding parts Pi and P2 of the product of FIG. 1 but only approximately. The body 22 of the Malt form has a certain number of perforations 25 for the passage of air. It should be noted that this shape 20 is thus different from the shapes used in the class methods. The upper surface of the body 22 of the male shape is preferably covered with a coating 26 of a breathable material and conductive of heat, such as fibrous tissue to ensure uniform heating of the body of the form.

 Between the female form 10 and the male form 20 is interposed a sheet 30 of a suitable thermoplastic material, for example an acrylonitrile-butadienestyrene resin which is held stretched horizontally in a frame 31.

The sheet 30 is heated by suitable means, for example between 130 and 1400C, to soften it. This temperature can, of course, vary depending on the material, the thickness and other factors. After the heating device 24 has been activated in order to heat the body 22 of the male form 20, including its covering 26, for example between 90 and 1000C, it is raised to the preforming position shown in FIG. 3. By switching on the pumping installation, not shown, in order to create a vacuum in the chamber limited by the body 22 and the bottom 21 through the opening 23, the plastic sheet 30 sketched in dashed lines in FIG. 3 and which is draped over the outer surface of the body 22 is sucked in by the negative pressure prevailing in said chamber and which acts through the numerous perforations 25 and through the permeable coating 26 and comes to bear against the surface of the body of the form- male.

 After that, the male form 20 rises again to the position for forming engravings of the imprint represented in FIG. 4. The heating device (not shown) of the female form 10 has been activated in order to heat the body 12 thereof, for example between 50 and 700C. When the first pumping device (not shown) associated with the female form 10 is actuated to create a vacuum in the hermetic chamber of this form 10 through said opening 14 and when the second pumping installation (not shown) of the male form 20 has been actuated to inject air into the hermetic chamber of form 20 through the opening 23, the preformed sheet 30 which has been pressed against the surface of the body of the male form is detached therefrom and comes apply against the surface of the body 12 of the female form due to the back pressure acting through the perforations 15 and under the action of the pressure acting through the perforations 25 in the space delimited by the surface of the cavity of the first and the projecting surface of the second, the sealing being ensured by the close application of the shoulders 11 'and 22'.

The object then takes the final exact shape and the grained surface which are printed on it by the surface of the body 12 of the female form.
It goes without saying that numerous modifications can be made to the embodiment shown and described, without however departing from the scope of the invention.

Claims (6)

 1. A method of forming a thermoplastic sheet and etching an imprint thereon in order to produce an object having deep recesses and the surface of which has a grain pattern, which consists in: heating the sheet of stretched plastic material; placing this sheet on the surface of a convex male shape for preforming purposes; creating a vacuum between the sheet and the surface of the male form; moving either said shape or a concave female shape for the purpose of preforming and imprinting so as to bring it into contact with the other shape and so that it is applied tightly against the peripheral edges of the latter, while leaving some space between them in the work area; and creating a vacuum between the female form and the surface of the sheet, while blowing air between the male form and the latter in order to transfer said sheet from the male form to the female form in order to continue the implementation grain shape and formation.  2. Method according to claim 1, characterized in that one raises the male preforming form, which advances upwards, so that the heated plastic sheet, whose edges are clamped in a frame, can be draped over said shape to produce a space in which a vacuum is created for preforming, and then elevate said male shape further to bring it into contact with the female shape, the cavity of which is directed toward bottom, in order to produce a certain space in its effective forming area, in which a vacuum is created and air is blown in order to transfer the sheet from the surface of the preforming male form onto the surface of the female form formatting and imprint.  3. Apparatus for forming a thermoplastic sheet and etching an imprint thereon in order to produce an object having deep hollows, the outer surface of which has a grain pattern which comprises: a male shape the body of which is limited by an approximate corresponding external working surface to the outer surface of the object to be formed and which is adapted to create a vacuum in order to suck said sheet against it and to breathe air in order to detach this sheet; a female form having a body whose working surface corresponds exactly to the external surface of said object and which presents in negative a drawing of the grain to be imprinted and which is adapted to create a vacuum in order to suck the preformed sheet against it; and a frame for holding the heated plastic sheet stretched horizontally by tightening its edges, the arrangement being such that said frame and said male shape can approach one another in order to drape the sheet over the body of the male form and then sucking it against the surface of the latter for the purpose of preforming, said male form and said female form then moving relative to each other to come into contact, while providing a certain space between them in order to transfer the preformed sheet from the first to the second, and to complete the shaping and produce the grain design thanks to the cooperation of the vacuum created in the female form and the air blown into the male form instead of vacuum; now deleted.  4. Apparatus according to claim 3, characterized in that the male form is adapted to move vertically so that by raising this form from its normal waiting position to the preforming position, the plastic sheet, held horizontally stretched by said frame, comes to drape itself over the body of the male form and then faithfully follows its contour under the action of said vacuum, then by further elevating said form to the position of shaping and imprint, the body of the male form comes into contact with that of the female form which occupies a fixed position, leaving a certain space between them in which said preformed sheet is transferred onto the body of said female form for shaping purposes and of formation of the grain as a result of said cooperation of the vacuum and the blowing of air, said male form then descending to its initial position to allow the formed and imprinted object to be ejected out of the body of the for me female and in preparation for the next cycle.  5. Apparatus according to claim 4, characterized in that the male form comprises a bottom and side walls defining a hermetically sealed chamber together with its inner surface which has a number of perforations for the passage of air through which said vacuum and the blown air act on the thermoplastic sheet placed on the outer surface of the body thereof, and in that said female shape has a top and side walls defining a hermetically sealed chamber jointly with the inner surface of its body which is pierced with a number of perforations for the passage of air through which said vacuum acts on the plastic sheet resting on the external surface of the body of the male form in contact with it, then on the outer surface of the body of said female form in order to transfer said object, as well as to shape and imprint it.  6. Apparatus according to claim 5, characterized in that the outer surface of the body of the male form is covered with a coating of air permeable and heat conducting material.