Method for molding an article with integral heat shield

Abstract: A molded article (8) comprising a molded body (10) and one or more layers of heat shielding material (12) attached to the molded body (10) and a method of making same. The heat shielding material (12) is placed on localised areas (16) of the molded article which are exposed to elevated temperatures. The molded article may be a fibre reinforced plastic (FRP) which comprises a fibrous material containing a resin or alternatively, the molded article may be a thermoplastic material with or without reinforcement.

Description: This invention relates to a method of making a molded article having an integral heat shield. Molded articles are useful in a variety of applications. Molded articles include both thermoset composite and thermoplastic material. Thermoset composite materials are made by combining a fibre reinforcement with a liquid resin. The resin wets the fibre reinforcement material and then cures. Thermoplastic material is thermoformable and may be injection molded or blow-molded to form articles having very complex shapes. Blow-molding is especially useful for manufacturing articles for holding fluids such as containers. Both types of molded articles are generally sensitive to exposure at elevated temperatures. Thermoplastic and thermoset composite articles tend to soften when exposed to temperatures above their melting point. Thermoset composite articles tend to oxidise the resin portion of the composite material. In both cases, the molded article becomes structurally weakened by the exposure to elevated temperatures. When localised areas are exposed to elevated temperature, these localised areas tend to weaken. A variety of methods have been used to protect a molded article from exposure to high temperatures. Various types of heat shields have been affixed either to the elevated temperature source or the molded article. These heat shielding devices include various layers of perforated metal sheets containing spaces or baffles to insulate the molded article from high temperatures. Problems with add-on heat shields of this type include the added bulk and cost of incorporating add-on heat shields to molded articles. In some applications, the weight saved between a molded article and a steel article has been more than made up by the requirement for add-on heat shields. Another method of protecting molded articles from elevated temperatures is the use of high temperature resistant resins and thermoplastic materials. These new materials may permit molded articles to withstand elevated temperatures without need of add-on heat shields. The disadvantage of these new high temperature resistant materials is they are not capable of withstanding very high temperatures in excess of 538°C (1000°F). Their cost and the difficulty in processing these materials do not make them commercially feasible in large production situations. In many applications, the exposure of the molded article to elevated temperature is localised. Manufacturing an entire molded article with these high temperature resistant materials is unnecessary. It is one object of the present invention to provide an integral heat shield for protecting localised areas of molded articles from elevated temperatures. It is a further object of the present invention to provide a low cost method of fabricating a molded article with an integral heat shield. It is a further object of the present invention to provide a lightweight molded article having an integral heat shield to protect localised areas from elevated temperatures. It has been proposed in EP-A 0 305 649 (cf. the preamble of claim 1) to make a heat resistant plastic part which can be used as a shield for hot components by uniting a thermoplastic substrate, over the part of its surface to be exposed to heat, with a laminated foil composed of a fibrous tissue laminated to a metal foil by hot pressing the substrate and laminated foil together in a mould with the fibrous layer adjacent the substrate. According, to the invention a method of making a molded article having attached to a localised area thereof a heat shield comprising one or more layers of heat shielding material and including a resin-impregnatable surface layer comprises placing the heat shield with the resin-impregnatable layer thereof adjacent a resin-impregnatable area of a preform of the body of the article over the localised area, impregnating the resin-impregnatable layer and area with resin in a mold and curing the resin to integrate the heat shield with the preform. The method can be used to effect in one impregnation step the production of a fibre reinforced plastic (FRP) article and its integration with a local heat shield. Provided the heat-shielding material includes a resin-impregnatable surface layer, it may comprise one or more layers of heat insulating material and/or one or more layers of heat reflecting material. Various types of heat insulating material made from ceramic fibre and silicone foam material can be used in the present invention. Of the ceramic materials useful in the present invention, preferred are those made of silica. The ceramic fibre material may be made in a woven, braided, wound, chopped, looped, or structurally engineered fashion. The foam material may be made with either open or closed cells as will be described in more detail. The fibre and open cell foam material is porous and permits resin to wet the material. The closed cell or very tightly woven material is non-porous and does not permit resin to wet significantly the insulation material. Wetting as used herein is the impregnation of resin within a material. Resin wets the porous insulation material as well as the fibrous material. The insulating material may be used alone or in combination with a heat reflecting material. The heat reflecting material is generally an infrared reflecting material such as a metal foil. The heat reflecting material may be partially foraminous to assist in adhering to the molded article. The heat reflecting material may be used with or without insulating material. The heat reflecting material acts to reflect heat away from the molded article whereas the heat insulating material acts to reduce the amount of heat which reaches the molded article. Both the insulating and reflecting material act to shield the molded article from the damaging effects of elevated temperatures. The invention integrates the heat shield within the molded article to insure the heat shield is correctly positioned and to protect the heat shield from being damaged or removed. Integrating the heat shield within the molded article also reduces manufacturing and assembly costs as well as the bulk and weight of article and heat shield. It is an advantage of the present invention to provide a light weight integrally molded heat shield made with relatively low cost material to protect localised areas from elevated temperatures. It is another advantage of the present invention to provide a method of manufacturing a molded article having an integrally molded heat shield. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 shows an exploded view of a preform and heat shield in a mold. Figure 2 shows a sectional view along the line II-II in Figure I with the mold in the closed position. Figure 3 is a detail cross-sectional view along the line III-III in Figure 2 showing the final molded article removed from the,mold. Figure 4 is an edge view of a finished molded article placed adjacent a heat source. Figures 1-3 show the manufacture of an integrally molded heat shield on a thermoset composite cross member 8 for an automobile. A preform 10 generally conforming to the desired shape of the cross member is placed in a mold. Preform 10 may be made by a variety of conventional methods. It may contain a foam core, blow-molded core, metal reinforced core, or maybe a hollow. Shown in Figure 1 is preform 10 having a foam core 13 surrounded by fibrous mat 14. Foam core 13 and fibrous mat 14 may be made from a wide variety of materials which are well known in the FRP art. Specifically preferred for foam core 13 are closed cell urethane materials. Fibrous mat 14 maybe fibrous material having a continuous strand or directional fibre placement. Preform 10 is made generally in the shape of the final molded article and has a localised area 16 for receiving a heat shield. Localised area 16 may be dish-shaped to receive the heat shield. Providing a dish-shape to localised area 16, permits the finished cross member 8 to have a smooth exterior finish. if a smooth exterior finish is not required, localised area 16 may be flush with the surface of preform 10 so that the heat shield extends above the surface of the cross member 8. To attach a heat shield 12 to preform 10 by the molding process of the invention, as described in more detail below, heat shield 12 is placed in a mold 30 and preform 10 is placed adjacent heat shield 12 so that localised area 16 registers with heat shield 12, with the resin-impregnatable layer 22 of the heat shield adjacent the fibrous mat 14. Heat shield 12 may comprise a heat insulating material and/or a heat reflecting material. Shown in Figure 3 is heat shield 12 comprising two layers of heat insulating material 22, 24 and a layer of heat reflecting material 26. Heat insulating material 22 is of porous construction such as a fibrous or open-cell insulating material. Shown is insulating material 22 made of a silica woven insulating material of the type manufactured under the name Thermotec Exobloc (Thermotec and Exobloc are trade marks) by Bisco Products. The Thermotec product combines the porous and non-porous insulating materials into one integral product. Both materials are also available alone and in combination with other materials. Other types of suitable porous insulating materials are commercially available such as silicone foam materials from General Electric, and COHlastic (trade mark) silicone foam from CHR Industries. A wetting agent may be added to insulating material 22, to aid in wetting insulating material 22 with resin. A layer of non-porous insulating material 24 is attached to insulating material 22. Non-porous insulating material 24 is designed to prevent a liquid resin from passing therethrough. A variety of commercially available products exist which are useful for non-porous material 22. Of the commercially available products, those manufactured using a silicone foam are preferred. Shown in Figure 3 is a silicone closed-cell foam insulating material 24, manufactured under the name Thermotec Exobloc by Bisco Company. A variety of commercially available heat insulating materials combining the woven and closed-cell construction are commercially available. As stated, the Thermotec product combines both the porous material 22 and non-porous material 24 into one product. A wet-out agent may be added to insulating material 24 to prevent the wetting of insulating material 24 by resin. Woven ceramic fibres offer a very high temperature protection between 538°C and 1093°C (1000°F-2000°F). Silicon foam products can be used for temperatures between 200°C and 370°C (400°F-700°F). The woven and foam products can be combined in one or more layers as is necessary to protect the molded article from temperature damage. Heat reflecting material 26 is attached to insulating material 24. Reflecting material 26 may be a metal or metalized foil which is capable of reflecting heat. Suitable infrared reflecting materials include aluminium or copper foil having a thickness of between 25 and 50 µm (1-2mils). Reflecting material 26 may be affixed to insulating material 22 prior to molding. Alternatively, reflecting material 26 may be placed atop insulating material 22 and adhered directly thereto by the molding process. Various layers of insulating and reflecting materials may be combined to provide the heat shielding required for a particular application. It is preferable that the outer layer of cross member 8 be made of a heat reflecting material to most efficiently reflect infrared heat radiating on the molded article and to help protect the insulating materials beneath from damage. Cross member 8 is manufactured by a liquid molding process as is well known in the FRP art. Preform 10 and heat shield 12 are placed in a liquid resin injection molding tool 30. Tool 30 comprises first and second die piece 32, 34. Die pieces 32,34 have resin channels 36 which communicate a source of pressurised resin (not shown) with the surface of preform 10. Die pieces 32, 34 are closed and closely follow the surface of preform 10. Liquid resin 38 is injected through resin channels 36. Resin 38 wets fibrous mat 14. Resin 38 also wets porous insulating material 22. Non-porous insulating material 24 and heat reflecting material 26 are impermeable to liquid resin. Resin 38 does not wet heat shield 12 beyond insulating material 22. Resin 38 is allowed to cure. Once cured, tool 30 is opened and finished cross member 8 is removed. Figure 4 shows cross member 8 installed adjacent an exhaust pipe 40. Heat shield 12 is located in the immediate vicinity of exhaust pipe 40 and protects cross member 8 from the damaging affects of exposure to high temperatures. As exhaust pipe 40 warms, heat shield 12 is heated by radiant and convection heating. Reflecting material 26 acts to reflect heat back toward exhaust pipe 40. Heat which is not reflected by reflecting material 26 passes through to non-porous insulating material 24. Insulating material 24 reduces the amount of heat which is transferred to porous insulating material 22. Insulating material 22 acts both as an insulating material and as an attaching mechanism between heat shield 12 and preform 10. Heat shield 12 is designed such that the temperature within insulating material 22 remains well below the glass transition temperature (T 9 ) of the resin. Slight oxidation of the resin within insulating material 22 is permissible without degrading the mechanical properties of preform 10. The combined reflecting and insulating properties of insulating materials 22,24 and reflecting material 26 prevent the transmission of heat to preform 10 sufficient to oxidise or degrade the resin therein. The invention as described teaches three layers of heat shielding material: a heat reflecting film nearest the heat source, a non-porous insulating material, and a porous material farthest from the heat source. The invention may be practised with one or any combination of these heat shielding materials and in any arrangement, providing always, as mentioned, that the heat shield includes a resin-impregnatable layer adjacent the body of the article. While not wishing to be bound by the following theory, it is believed that placing the heat reflecting material nearest the heat source produces the largest amount of heat reflection. The non-porous insulating material 24, produces the greatest resistance to thermal damage. Placing non-porous material 24 between reflecting material 26 and porous material 22 is an efficient means of producing a relatively large resistance to thermal transfer in a small space. Because insulating material 22 includes some resin 38, it is desirable to maintain the temperature in this layer well below the T,, of the resin. Placing insulating material 22 farthest from the heat source protects resin 38 from damage and maintains the integrity of the heat shield/preform bond. The invention has been described using a combination of woven and closed-cell insulating material. Other configurations of insulating material are possible and are within the scope of the present invention. An open-cell insulating material (one having an open-cell structure which permits internal passages within the insulating material to communicate resin throughout the material) may be used in place of a woven insulating material. It is believed, however, that completely impregnating the insulating material with resin reduces the thermal insulating properties of an insulating material. The invention has been described in terms of a cross member. However, the method and article of the present invention may be adapted to form other molded articles for both automotive and non-automotive applications. It is understood that various modifications of the preferred embodiments can be used. For example, the teachings of this inventions are applicable for the manufacture of fluid containing vessels and structural components.

Claims: A method of making a molded article having attached to a localised area (16) thereof a heat shield (12) comprising one or more layers (22,24,26) of heat shielding material characterised by placing the heat shield with the resin impregnatable layer (22) thereof adjacent a resin-impregnatable area (14) of a preform (10) of the body of the article over the localised area (16), impregnating the resin-impregnatable layer (22) and area (14) with resin in a mold and curing the resin to integrate the heat shield with the preform. A method according to claim 1 wherein the impregnation is effected by liquid injection molding. A method according to claim 1 or claim 2, wherein the heat shielding material includes a resin-impregnatable layer (22) of heat insulating material and a layer (26) of heat reflecting material. A method according to claim 3, wherein the resin-impregnatable layer (22) of heat insulating material comprises ceramic fibre or open cell silicon foam. A method according to claim 4, wherein the ceramic fibre is silica fibre. A method according to any one of claims 3 to 5, wherein a layer (24) of non-porous heat insulating material is interposed between the resin-impregnatable layer (22) of heat insulating material and the layer (26) of heat reflecting material. A method according to any one of claims 3 to 6, wherein the heat reflecting material is metal foil. A method according to claim 7 wherein the metal foil is aluminium or copper foil. A method according to any one of claims 3 to 8, wherein the heat reflecting material is partly foraminous. A method according to any preceding claim wherein the preform comprises a core or hollow (13) surrounded by a resin-impregnatable fibrous mat (14). A method according to claim 10, wherein the article is a cross member for an automobile.

Inventors: HOUSTON DANIEL QUINN, STILES ERNEST DALE, WLOSINSKI RONALD KENNETH, HOUSTON, DANIEL QUINN, STILES, ERNEST DALE, WLOSINSKI, RONALD KENNETH
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1992
Main classification: B29C51
IPC codes: B29C49, B32B5, B29C70, B62D29, B60K15