A number of applications exist wherein it is desirable to provide panels or boards having desirable acoustical and/or thermal properties. For example, it is desirable to provide molded panels for use as liners in heavy machinery for purposes of sound and/or thermal attenuation, such applications including molded liners for the interior of truck cabs and the like, liners for engine compartments and fire walls, and so forth. In many applications, it has been found that panels formed of fiberglass have yielded desirable results although other textile fabrics and fibers may be employed such as resinated cotton, synthetic fibers, reclaimed fibers and the like.
In applications wherein it is desirable to provide a predetermined level of sound attenuation, the desired attenuation may be obtained through the use of boards produced from fiberglass or other suitable fibrous media. The sound attenuation capability of such a fiberglass board is a function of the density and thickness of the board. One technique for producing boards having desirable thermal and/or acoustic properties is to mold the boards from basic blankets which are comprised of loosely gathered fiberglass filaments and a suitable binder. The fibers are preferably saturated with a resin or a binder such as a phenolic or thermoplastic or other thermosetting binders which are utilized, when cured, to hold the fibers together in order to assure the proper molded shape of the molded end product.
In order to obtain the desired density, it is conventional to utilize the aforementioned blankets of reasonably standardized density and to place the desired number of blankets within a mold in order to form a panel or board having the desired acoustic and thermal properties. For example, blankets of the type described may have a nominal uncompressed density of one pound per cubic foot and a nominal uncompressed thickness of one inch. Assuming it is desired to produce a board having a thickness of 0.5" and a density of 8 lbs. per cubic foot, the desired board may be produced of four (4) of the aforementioned blankets which are inserted within a mold designed to compress the blankets to a total thickness of 0.50". Insertion of four such blankets into the mold produces an end product having a density of 8 lbs. per cubic foot. The mold is normally heated and is maintained at a temperature level sufficient to cause the binder to set, thereby obtaining a board having the desired thickness and density. The blankets are retained in the mold for a period of time sufficient to set the binder and thereby provide a finished product having a substantially uniform density and thickness in order to achieve the desired acoustical and/or thermal attenuation properties.
A number of applications exist wherein it is desired to provide acoustical panels and the like with predetermined shapes and with portions thereof having thicknesses which significantly deviate from the nominal thickness. For example, in applications wherein the molded members are designed for use as liners in the cabs of large industrial vehicles, the liners are typically designed to conform to the interior shape of the cab, such as, for example, the cab ceiling, and interior walls, hence the necessity for shaped molded members. In order to facilitate mounting of the liners to the interior surface of the cab, it is frequently desirable to provide openings for receiving fastening members and to provide the marginal portions of said openings with a thickness which is significantly reduced from the nominal thickness of the liners. In the example where a molded sheet formed of four standard blankets having densities of 1 lb. per cubic foot and uncompressed thicknesses of 1", the portions of reduced thickness of the molded end product may be as small at 1/8" making the density in such regions as high as 32 lbs. per cubic foot. The density and/or packing of the fibers in the regions of significantly reduced thickness is so great as to prevent the molds from compressing the original blankets down to the desired thickness, thereby requiring use of fewer blankets or blankets having lower nominal densities in order to allow the portions of the molded member of significantly reduced thickness to be molded to the desired thickness. This results in a reduced density for the major portion of the liner, which yields an undesirable compromise between the thickness and acoustical and/or thermal properties of the molded end product.
It is also required in some applications to provide certain surface finishes for the liner necessitating that the liner have a smooth high density finish capable of receiving and retaining the surface finish to be applied thereto. These requirements further increase the density requirements of the end product, which contributes to undesirable increases in material costs, process costs, labor costs, weight and brittleness.
The problems of panel brittleness result from the fact that the end product requires an increased density sufficient to provide a surface capable of being finished in the predetermined manner with the result that the end product is overly brittle and thus highly susceptible to breakage.
Surface coatings or facings require a good high density surface to promote application and retention, thus requiring higher density panels which necessitate the use of a greater amount of fibrous material.
The criteria to be considered in forming molded parts of the type described above are, in the order of importance, the selection of the materials for achieving the desired thermal and/or acoustic properties; the structural requirements; and the shape and thickness of the molded part. The desired acoustic and/or thermal attenuation is a function of the thickness and density of the molded part. The ideal thickness of the sound absorbent material is one half the wave length of the sound to be absorbed. Assuming the sound to be absorbed has a frequency of 250 cycles per second, one half the wavelength of this frequency is 2.25 ft. which is clearly an impractical design from the viewpoint of both cost and size. Thus, conventional present-day designs are the result of a compromise which is struck between a group of the primary factors which contribute to the design of acoustic and/or thermal panels and the like.
Structural requirements frequently necessitates changes in density to provide adequate supporting or mounting strength for portions of the panel.
The shape and thickness of the molded part impose limitations on maximum permissible density, necessitating reduction in overall density in order to be assured of achieving the desired molded shapes, the practical limitations of the raw material being such that the fibers of the blanket do not "flow" in the same manner as plastic material flows in order to freely and easily assume desired shapes and configurations.
The interplay of the three above-mentioned criteria result in a molded end product having an overall design in which density and thickness are compromised to achieve a satisfactory balance of the desirable operating and design characteristics.
It therefore becomes desirable to provide a novel method and apparatus for producing molded members and novel molded members having desirable acoustical and/or thermal properties and which more favorably conform to the important design criteria for such panels and which have improved characteristics as compared with panels produced through the use of conventional methods.