ENCAPSULATION OF PRE-EXPANDED ELASTOMERIC FOAM WITH A THERMOPLASTIC MATERIAL BACKGROUND OF THE INVENTION It is often desirable to fill cavity sections of assemblies with low density materials to obtain acoustic damping, structural reinforcement or seal. Various foam materials that can be expanded are known in the art. For example, it is known to fill an article with foam comprising a polymeric outer shell and a foam core. It is prepared by compressing an expandable material in a degradable container, then forming a liner while degrading the container. The foam may be expandable urethane, the container degradable (for example by application of heat), a polyethylene bag and the liner, a vinyl polymer. When the container is degraded during the formation of the liner, the non-limited foam expands to fill the molded article. The need for the degradable bag, however, requires time and is expensive and the degraded material is undesired, and sometimes presents other negative aspects. It is also known to obtain a thermally insulating material by using a core of polyurethane foam with a gas barrier material. Due to its deformation capacity it is said that it can be used to isolate an irregular surface to obtain excellent thermal insulation. It is also known to obtain an expanded molded article comprising a porous sheet of a heat-resistant polymer attached to a mold, wherein pre-foamed particles are to be heated uniformly. Various fillers for cavities including pillars and other hollow structures of a car that are used in the automotive industry are also known. Typical fillers are one- and two-part epoxies as well as polyurethanes that expand upon heating due to a chemical reaction or chemical breakdown of a blowing agent. However, polyurethanes contain toxic isocyanates and many epoxy curing agents are agents that sensitize to allergies, which require at least minimum handling precautions that include the use of gloves and ventilation. There is therefore a need for a filling material that, efficiently and economically, can fill and join the internal area of a cavity, and that works based on a physical mechanism instead of operating on the basis of a chemical mechanism. BRIEF COMPENDIUM OF THE INVENTION The invention, in one embodiment, refers to a two-part capsule structure comprising an outer shell around a foam core, the core being a pre-expanded foam capable of expansion upon heating and the cover is optionally capable of adhesion on an adjacent surface. The cover preferably adheres on the hollow structural member that is filling and sealing said member. In another embodiment, the invention relates to a method for filling an enclosed space or cavity comprising the insertion of a two-part capsule into a space and heating to melt the outer cover and allow expansion of the foam. The thermal activation causes the expansion of the encapsulated foam, and causes the softening or melting of the coating by the expanded foam thus forming a hermetic seal against the surface of the enclosed space. In a third embodiment, the invention relates to a process for making such capsules, comprising the compression of a pre-expanded foam core; the supply of an external thermoplastic coating around the core; and the seal of the liner or cover around the core. The fourth embodiment of the present invention relates to a hollow structural member having its cavity filled and sealed by the activated capsule. The invention is useful in several applications, including automotive applications and other types of manufactured products (e.g., aircraft, ships, bicycles, etc.) with a capacity to accurately fill body sections with acoustic or structural foam, employing minimum amounts of material, for scaling and cushioning purposes. DRAWINGS Figure 1 is a partially open front elevational view of a capsule made in accordance with the present invention; Figure 2 is a cross-sectional view taken through Figure 1 along line 2-2; Figure 3 is a perspective of the capsule illustrated in Figures 1-2 with a retainer member mounted on the capsule; Figure 4 is a cross-sectional elevation view showing a partially open capsule within the cavity of a hollow structural member prior to activation of the capsule, and Figure 4 is a view similar to Figure 4, after the Activation of the capsule. DETAILED DESCRIPTION The present invention includes, in general, a capsule 10 illustrated in Figures 1-2 incorporating a compressed pre-expanded foam core 12 that is sandwiched or wrapped between the layers of an outer shell 14 made of a material thermoplastic In the embodiment illustrated in the drawings, the capsule 10 is formed by co-extruding the compressed foam core 12 and the outer shell 14 such that this results in a foam core located between two sheets of thermoplastic material. The sheets are sealed along their marginal edges 16 in order to completely wrap the foam core 12. In this way, the foam core 12 is kept in a compressed condition and therefore can not prematurely return to its original shape. The foam core 12 and the outer cover 14 can be of any acceptable geometric size and shape in accordance with the intended end use. For example, if the capsule 10 is to fill an elongated cavity in a hollow structural member, the foam core 12 and the outer shell 14 can have a corresponding geometry. Similarly, if the cavity is of another type of shape including irregularities in shape, the capsule can generally conform to this specific shape, thus facilitating and ensuring that when the foam expands, the cavity is filled. A sufficient outer coating material is employed to provide, as will be described below, a seal cap which adheres to the internal surface of the hollow structural member and which covers said inner surface of said member. Preferably, the other coating material also adheres to the foam core such that when the foam core is expanded, the foam core acts as a vehicle to ensure that the melted outer shell material will move outwardly. during expansion of the foam core and will come into intimate contact with the inner surface of the hollow structural member, thus ensuring the sealing function of the outer covering material. As best seen in Figure 2, there is a more external cover material along the peripheral margin 16 than along the central portion or sides 18 of the outer cover of 14. This extra material also ensures that there will be a enough outer cover material to form an appropriate seal. If desired, a retaining member 20, such as a pin, could be mechanically inserted and thereby become part of the capsule 10. A retaining member 20 is operable to fit into a corresponding hole in the structural member thus ensuring proper positioning of the capsule 10 in the structural member. Figures 4-5 illustrate a use of the capsule 10. As shown here, a windshield post 22 having a cavity 24 must be filled by the capsule 10. Figures 4-5 show the post 22 which includes a metal structure external 26 which have flanges 28. A retaining pin 20 is inserted into the hole 20 of the post 22. As shown in Figure 4, the capsule 10 is in its initial unexpanded condition. For example, a capsule 10 could be inserted into the cavity 24 before the coating immersion e. When the post 22 is subsequently subjected to heat treatment, in the coating immersion e, the outer coating 14 begins to soften and melt, and sufficiently softens in such a way that it no longer limits the foam core 12. The foam core 12 can therefore expand by bringing the outer coating material melted therewith to completely fill the cavity 24 as shown in Figure 5. Figure 5 also illustrates a seal layer 32 adhered on the inner surface of the post 22 and formed completely against said post to seal the cavity 24. The layer 32 is formed of a material 14. As shown, the foam 12 also adheres to the layer 32 thereby completely filling the cavity. Any suitable material can be used to practice the invention. In selecting these materials, the outer coating must have a softening or melting temperature no greater than the temperature at which the structural member will be heated. Conversely, the foam material must not melt or degrade at this temperature, but must have a higher melting point. Thus, for example, when a capsule 10 is used in relation to an automotive part subjected to a coating e, materials must be selected to suit the conditions of time and temperature at which the capsule will be subjected. Conventionally, a coating e is carried out at a temperature of 375 ° F for a period of about 30 minutes. Accordingly, when a capsule 10 is used for a coating e, the outer coating 14 should be sufficiently softened at a temperature no greater than 375 ° to no longer confine the foam 12, and the foam material 12 should not melt at a temperature of 375 degrees An advantage of the present invention is that the foam material can be any suitable foam such as, for example, polyurethane waste having a low compression set, which has a decomposition / melting temperature higher than the outer coating temperature. . Preferably, the foam core is made of a thermosetting material, examples of which are silicon, rubber and polyurethane polyurethane. More preferably, the foam core is a polyurethane foam of variable density, for example, a foam of a density of 1, 2 or 3 pounds, either virgin or recycled or its capacity to accept the heat typically employed in the industry. automotive A core of foam 12 can be compressed in such a way that when expansion is allowed, said expansion will be in a ratio of 2 to 20 between compression and expansion. For example, a foam core compressed to a thickness of inches can be expanded to a thickness of one inch. The particular foam material is not a critical factor, what is important is that the foam can be compressed and then maintained in its compressed state while it is inside the outer shell and can then expand to its original uncompressed state. The foam core should adhere to the outer cover material 14 and carry it in order to secure the placement of the outer cover material against the inner surface of the polystructural member. The foam core material is preferably substantially completely foam-formed prior to the application of the outer coating or sheath 14 in such a way as to minimize further reaction within the formed capsule 10. Varieties of open or closed cells may be employed. High durometer foams can be specified when optimizing structural reinforcement. Foams can also be selected for their acoustic damping or damping properties. An advantage of the present invention is that the thickness before expansion and shape of the foam before expansion can be matched to the thickness and shape of the cavity provided in order to ensure that there is no excess expulsion which minimizes the post-expansion cleaning procedure. The size of the foam piece is smaller in its X-Y dimensions than the outer shell. The outer cover 14 can be made of any suitable material. Preferably, the outer cover is a thermoplastic material that can have a low module with the objective of sealing or acoustic damping. In such a case, a poly (ethylene-co-vinylacetate) material is a suitable material. If structural reinforcement is desired, a high strength, high modulus thermoplastic material such as polycarbonate can be used. The melting point of the outer coating 14 should be less than the melting point of the foam core 12 such that when heated, the coating 14 will soften or melt, but not the core foam 12. The outer coating 14 will preferably adheres to both the foam core 12 and the internal surface of the structural member. In a preferred practice of the invention, the outer cover material to form a complete seal, although the invention can be carried out without taking advantage of this seal feature. The outer covering material does not have to be a material capable of degrading. It is sufficient that it softens or melts to a degree where it no longer limits the expansion of the foam 12 to its original condition. As noted, any suitable material for external coating 14 can be employed. A "rubber-plastic blend, including about 5-30% and generally about 20% of an elastomeric component such as for example ethylene vinyl acetate (EVA) or Polyvinyl chloride (PVC) can be used, the former is preferred due to its lower consistency and favorable environmental properties.One or more thermoplastic materials such as thermoplastic rubber can be mixed with the elastomer.The choice of such a material will be determined by the substrate with which it must adhere Although the invention may be practiced for use in any hollow structural member made of any material, including plastic, a preferred practice of the present invention is with a metallic structural member, particularly metals used in the automotive industry As noted, Figures 4-5 illustrate the course of the invention with a pole A (windshield). Other members such as a B-post (vehicle door) or a superior defense support can also be used as the hollow structural member. It will be understood, however, that the invention is not limited to automotive parts, but may be incorporated in any application where it is desired to essentially fill a cavity with or without providing a seal layer on the internal surface of the hollow structural member. The invention should be practiced to fill a structural member of any element that requires energy for movement, such as aircraft, ships, bicycles. In its wide practice, the structural member can be anything (mobile or not) that benefits from having a foam filling in its cavity. Any suitable process can be used to manufacture the capsule 10. A preferred practice is to coextrude the foam and its coating, followed by sealing the die; the preparation of a bag type structure around the foam, and heat sealing; then the use of an immersion tank where an external, semi-liquid coating is applied in the tank with a series of rollers used to press the liner into place while it is cooling. The preferred process for manufacturing capsules is generally to compress the foam core 12 and subsequently or simultaneously coextrude the outer coating 14. A hot pattern press manufactures a sheet of connected capsules which will subsequently be cut or cut into individual capsule clamping members or retention such as for example a -alfiler 20 can be fixed on the capsule towards the internal part of an assembly cavity. Said retaining members may also be screws, welding pins, rivets and the like, which can be co-molded or co-extruded at this point. The fixing members as per example tape of two adhesive sides can also be offered subsequently. Figures 1-2 illustrate on a whole scale a practice of the present invention wherein a capsule 10 has from about 2.3 to about 2.9 inches with a maximum thickness in its central part of about 0.28 inches. A marginal edge 16 has a thickness of approximately 0.1 inch and extends approximately 0.3 inch outwardly from the foam core 12. An outer coating 14 has a thickness of approximately 0.05 inch in its central portion 18. A foam core 12 has a - size of approximately 1.7 by 2.2 inches with a maximum thickness of approximately 0.2 inch. In general, the invention can be practiced to fill the cavity of a hollow structural member by loosely placing such as, for example through the use of retaining members, at least one capsule within a cavity and then thermally activating the capsule to the temperature of softening or melting of the outer coating 14. Any suitable thermal activation temperature can be used. In general, but not necessarily, said temperature may be within a range of about 250 ° C to about 400 ° C and preferably at least 350 ° C. An advantage of the invention is that the mixture comprising the outer coating 14 will melt, will undergo curing and the foam will expand at these temperatures which are profitably employed at low in the production line in an automotive processing such as for example The coating chosen for the activation of the capsule is a temperature which is well above the softening point, and preferably above the melting point of the paint. This eliminates the need for a separate baking step. external coating material, but below the melting point and preferably also below the thermosetting point of the foam core As noted, the invention can be practiced with capsules of various geometric shapes such as flat shapes or geometric shapes two-dimensional, including, but not limited to, circles and other curvilinear shapes, rectangles, I after rectilinear forms, hexagons and other polygons and the like. The capsule may also have an irregular geometry that generally fits the geometry or shape of the cavity in which it must be placed.