METHOD OF FORMING MULTILAYER COMPOSITE PANELS WITH SPECULAR SURFACE

Abstract: Thermoplastic panels are formed and laminated together simultaneously by using a roller to provide verti­ cal pressure to two heated thermoplastic sheets, one a composite thermoplastic sheet, the other a monolithic thermoplastic sheet. The monolithic thermoplastic sheet allows laminating protective layers to structural panels for improving scratch resistance, improving appearance or reducing chemical permeability.

Description: The present invention relates to a method of forming multilayer composite thermoplastic panels with a glossy finish using roller forming. Roller forming is a viable process for solid phase forming of thermoplastic sheet into large deformed panel shapes which could be used for automotive exterior body applications or building panels. Monolithic, that is, unreinforced, thermoplastic sheets which originally had a glossy surface finish can be formed into a panel type part with the same glossy finish as the original sheet. However, when attempting to apply the same forming process to composite sheets, that is, glass fiber reinforced thermoplastic sheets, problems arise. The problems do not involve the actual forming of the composite sheet but rather that the final surface finish on the composite sheet is not glossy, even when the original composite sheet had a relatively glossy surface finish. The reason for this degradation in surface finish when composite sheets are used, can be observed when the sheet is being heated prior to forming. The composite sheet, due to its method of fabrication, contains a considerable amount of trapped air beneath the surface. Upon heating, the trapped air bubbles rise to the surface and pockmark the surface. In addition, as the sheet surface softens during heating, the gloss fibers which are usually constrained by the polymer matrix are free to poke through the surface. Thus, the surface finish is damaged simply by heating the sheet, and if anything, roller forming slightly improves the final surface finish. The composite panel could be finished by coating the panel with a glossy layer or paint. However, this would involve at least one additional finishing step. It is an object of the present invention to provide a method of forming composite thermoplastic panels with a high quality glossy surface finish without adding any additional finishing steps. In one aspect of the present invention a method of forming multilayer composite thermoplastic panels is provided. The method comprises the steps of overlaying a reinforced thermoplastic sheet on a monolithic thermoplastic and heating the two sheets to forming temperature, with the reinforced sheet stacked on top of the monolithic thermoplastic sheet, placing the two sheets on a form having a predetermined curvature, and exerting a normal force on the two sheets through a roller bending the two sheets to the desired shape and simultaneously laminating the two sheets together. The objects and advantages can be more readily ascertained from the following description of a preferred embodiment when read in conjunction with the accompanying drawing in which : Figure 1 is an isometric view of a roller forming apparatus used in carrying out the method of the present invention; and Figure 2 is a partial side view of Figure 1. A glass fiber reinforced thermoplastic sheet is overlayed on a monolithic sheet and the two sheets are heated together in an oven. A radio-frequency oven is preferably used to bring the two sheets to forming temperature within a matter of seconds, beneficially raising the temperature of the inner portions of the material more than the outer portion due to cooling of the exposed outer surfaces. The outer surface of the monolithic sheet is not heated above its resin melting point. This preserves the quality of the finish and avoids introducing any undesired surface variations. It is desirable to heat the composite sheet to a higher temperature than the monolithic sheet. This can be accomplished by heating the composite sheet first before heating the two sheets together, if the radio frequency absorption characteristics of the two materials are similar. If the radio frequency absorption characteristics are different, the length of time each sheet is heated is adjusted to heat the composite sheet to a higher temperature. The inner portions of the two sheets are raised above the glass transition temperature. The composite sheet is placed on top of the monolithic sheet so that the entrapped air is free to escape up from the composite sheet during heating. Once the desired forming temperature is reached, the two sheets are removed from the oven. The desired forming temperature is one at which the contact interface becomes tacky. Referring now to Figures 1 and 2, apparatus for carrying out the solid phase forming and laminating step is shown. The heated monolithic and composite plastic sheets 5 and 7, respectively, are placed on a metal form 11. The metal form has a heated surface of predetermined curvature with the heated surface reducing heat loss from the composite sheet. The sheets, with either the monolithic or composite layer on top, are held in position on one end of the form by a clamping mechanism 13 that can be hydraulically operated. A normal force is applied to the sheets through a roller 15 which rolls in a direction away from the clamp. The roller can be heated but not above the resin melting point of the sheets. The form and roller can be coated with a friction reducing coating such as PTFE to reduce sticking and friction. The roller 15 can be fabricated of metal for geometries with uniform curvature in the direction of roller movement. A pliable roller can alternatively be used, which can comprise a hard rubber roller with a metal core. A pliable roller which is shown in the Figures, can accommodate more varied mold geometries. During the forming operation, the roller bends the plastic sheet to the desired panel shape while at the same time laminating the two sheets to one another by applying adequate vertical pressure through the roller. The vertical pressure causes the tacky interface between the sheets to bond. The surface finish of the bonded monolithic plastic layer is of the same quality as the original sheet stock before lamination and forming allowing composite sheets with glossy surface finishes to be achieved. Monolithic and composite sheets have been roller formed and laminated together using a multi layer composite comprising glass reinforced Lexan® polycarbonate resin providing load bearing strength and a layer of high gloss monolithic Lexan® polycarbonate resin sheet. Laminated composite sheets have been formed using Xenoy® polycarbonate and PBT blend high gloss monolithic sheet and glass reinforced Xenoy® polycarbonate and PBT blend. Xenoy and Lexan are trademarks of the General Electric Company. Sheets of the two materials have also been laminated and formed to one another. The sheets were heated to 176.6-232.2 °C (350-450°F) and a pressure of approximately 0.41-0.689 MPa (60-100 psi) was applied through the roller. The composite sheets contained 40% chopped glass fiber. The monolithic sheets had a thickness of 1.524 mm (0.06 inch) and the composite sheets a 2.54 mm (0.1 inch) thickness. The bonded face between the two plastic sheets was of very high quality and no visible voids or delaminations were present. A 2.54 mm (0.1 inch) thermoplastic composite sheet can also be joined to a monolithic sheet 0.254 mm (0.01) to 0.508 mm (0.02 inches) to provide a glossy finish. If a multilayer composite with more than two layers is desired, additional layers can be heated together and then formed and laminated simultaneously by roller forming. Layers on either side of the composite layer can be used to achieve a symmetric design which could be helpful in minimizing warpage. When monolithic layers are used on both sides of the composite layer the surface on which the glossy surface is desired is placed on the bottom during heating so that entrapped air will not damage the bottom exterior surface. As previously described, the inner composite layer is heated to a higher temperature than the monolithic layer which will provide the glossy finish surface. The monolithic sheet can also serve as a protective layer in structural panels for improving scratch resistance, improving appearance or reducing chemical permeability. The foregoing has described a method of forming composite thermoplastic sheet and at the same time impart a high quality specular surface finish to the sheet.

Claims: A method of forming multilayer composite thermoplastic panels comprising the steps of : overlaying a reinforced thermoplastic sheet (7) on a monolithic thermoplastic sheet (5); heating the two sheets to forming temperature with the reinforced sheet stacked on top of the monolithic thermoplastic sheet; placing the two sheets on a form (11) having a predetermined curvature; and exerting a normal force on the two sheets through a roller (15), bending the two sheets to the desired shape and simultaneously laminating the two sheets together. The method of claim 1 wherein the water surface of said monolithic thermoplastic sheet has a specular finish. The method of claim 2 wherein said reinforced sheet is heated to a higher temperature than the monolithic sheet. The method of claim 3 wherein said specular surface finish of said monolithic thermoplastic sheet is not heated above its melting point. The method of anyone of claims 1 to 4 wherein said step of heating the two sheets is accomplished in a radio frequency oven. The method of anyone of claims 1 to 5 wherein the overlaying step comprises symetrically overlaying the reinforced thermoplastic sheet (7) with a monolithic thermoplastic sheet (5).

Inventors: INZINNA, LOUIS PAUL, NIED, HERMAN FREDERICK
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1987
Main classification: B29D9
IPC codes: C08J5, B32B37, B29C70, B32B27