BACKGROUND OF THE INVENTION One of the known methods of molding composite parts is to use the expansion casting system. The expansion casting system consists of filling the mold cavity with a polymeric foam material and a blowing agent which, when activated, foams the polymeric material to the shape of the mold cavity. When the expansion casting technique is used in molding a part having a thin outer skin, the outer skin is often rippled due to the uneven expansion of the polymeric foams. As a consequence, the molded part is ruined or it is necessary to add a further finishing step to the molded part prior to painting in order to remove the unwanted surface irregularities from the outer skin surface. This additional/ finishing step adds to both the cost and time of producing the molded part.

SUMMARY OF THE INVENTION

This invention produces a smooth skin laminated structure using the foam casting technique by interposing a flexible open-celled foam wetted with a thermoset resin between the outer thin skin and an expandable thermoset polymeric foam. The polymeric foam, as it expands in the interior of the mold, comprises the inner surface of the flexible foam. The resin in the flexible foam cures after the expanded foam cures. The flexible foam compresses differentially in proportion to the uneven expansion of the

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expandable foam and thereby prevents any distortion of the • outer thin skin.

DESCRIPTION OF THE DRAWING For a better understanding of this invention, reference may be made to the accompanying drawing in which

FIGURE 1 is a cross-sectional view of a closed mold in which a thin skinned laminated structure is cast b means of an expandable polymerxc foam in accordance with t principles of this invention; and

FIGURE 2 is another embodiment in which the prin ples of this invention are used to reinforce a thin skin body portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Figure 1, there is shown a mold 10 having an upper part 12 and lower part 14 and a cavity 16 a desired pattern shape.

Following the principles of this invention, the first step is to lay up along the surface of the cavity 16 thin sheet of material 18. The thin sheet 18 could be mad of any plyable material such as metal, wood or fibrous reinforced material. The thickness of this sheet, of mater al depends on the particular material employed, for exampl .020 inches for aluminum, .1 inch for wood veneer, and .04 inch for fiber glass laminates. In any event, this thin sheet of material would be of a thickness that causes the skin to buckle upon an expandable polymeric foam being introduced into the cavity 16, which when activated expand outwardly and compresses against sheet 18.

The next step is to lay up onto the thin sheet 1 a sheet of flexible open-celled foam 20. Prior to inserti into the cavity 16, the open-celled foam sheet 20 is impre nated with a resinous material. The flexible foam may be any of the commercially available material such as flexible polyurethane, sponge rubber, cellulose foams, etc. Its impregnation is prefer-

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ably performed in a manner that the open cells of the foam • are left coated with the thin film of a settable resin. A suitable technique for accomplishing this objective is described in my U.S. patent No. 3,269,887. Finally, an expandable polymeric foam mixed with a chemical blowing agent 22 is inserted into the cavity 16. The quantity of this mixture must be sufficient to permit the polymeric foam to expand when activated to fill the volume defined by the inner walls of the flexible foam layer 20.

When the chemical blowing agent is activated, it expands the polymeric foam outwardly until it contacts and compresses the flexible foam. The flexible foam's inner surface 24 compresses differentially, in proportion to the uneven pressure applied by the gases of the expanding foam 22.

From the foregoing description it will be appre¬ ciated that the flexible foam layer 20 acts as a buffer zone between the expandable polymeric foam 22 and the thin skin layer 18 by absorbing the uneven expansion of the gaseous blowing agent. Due to this uneven expansion of the blowing agent, the impression in the inner surface of the open cell flexible foam 20 will vary around the perimeter of the mold cavity 16, but the pressure applied to the outer skin layer 18 will be negligible around the mold cavity perimeter.

It will be appreciated that the resin used to wet the flexible foam sheet 20 must be compatible with the polymeric foam mixture 22. For example, an epoxy resin and an a ine curing agent would be suitable for wetting the flexible foam 20 when using a polyurethane rigid foam mix¬ ture as the expandable polymeric foam material. It is important that the settable resin cures after the expandable foam cures.

When cured, the settable resin bonds the thin skin 18 to the rigidized flexible foam 20.

SECOND EMBODIMENT The principles of this invention can be used to add impact resistance to vehicular bodies or other struc-

tural members in those areas most subject to damage. For ^• example, one of the- areas of an automobile body subject to great damage upon a collision with the passenger compartme of the automobile is the hollow portion extending along th 5 .bottom of -the door sill. As will now be explained, this invention permits this hollow portion to be reinforced by use of polymeric foam techniques without the undesirable side effect of crumpling the outer skin of the automotive body in that area. 10. Referring to Figure 2 there is shown a cross- section, of a portion of. an automobile body comprising the floor 50 and bottom of door sill 52. The bottom wall 52 o the door sill is constructed of a thin skin which may be metallic, plastic composite, or the like, and which extend 15 the length of the door to define a generally U-shaped inte ior channel 54. To add structural reinforcement to the U-shaped floor member 52, a flexible open-celled foam 56 wetted with a settable resin is layed up along the interio walls 58 of the U-shaped channel 54. This flexible foam 5 0 would be wetted i a manner described with reference to

Figure 1 to provide the flexible foam's open cells with th thin coating of settable resin. An expandable polymeric foam 60 in sufficient quantities to fill the U-shaped chan nel 54 when expanded is then added to the interior of the 5 channel mixed with a chemical blowing agent. The bottom portion of the U-shaped channel 54 is left open to permit any excess of the expandable foam to pass through it.

The chemical blowing agent is activated to cause the polymeric foam 60 to expand outwardly toward the flex- 0 ible foam 56. Upon contact, the inner surface 62 of the flexible foam compresses and applies a resultant negligibl pressure outwardly against the thin skin wall members 52. The excess 64 of the expandable foam pushes through the op bottom of U-shaped channel 54. 5 As before, the settable resin in the flexible fo is selected to cure after the polymeric foam has cured. T resulting structure is a high impact resistant laminated structure which is reinforced by the expanded polymeric fo