NOISE ATTENUATION PART AND PROCESS FOR ITS PRODUCTION

Abstract

 It is described a noise attenuation part which comprises two outer dense layers (11,12) having noise insulating properties, and an interposed porous layer (13) with noise absorbing properties, in which at least one of the insulating layers has through-holes (14) communicating the absorber layer with the environment.

Description

FIELD OF THE INVENTION

[0001] The present invention refers to a noise attenuation part, endowed with both noise absorption and insulation properties, for use in particular in the automotive field. The invention also refers to the production process of such part.

STATE OF THE ART

[0002] Noise reduction is important in many civil applications, including residential and industrial constructions, and transport means, in particular in the automotive field. In the rest of the description reference is made to the latter field, but the invention has general applicability wherever noise reduction is required.

[0003] In the automotive field there is the need of reducing overall noise levels in the interior and exterior of a vehicle by insulating and absorbing noise of such sources as, but not limited to, combustions engines, electric motors, compressors, pumps, etc. Typical noise reduction parts are found in the engine bay (engine or motor encapsulation, outer-dash insulation, hood-liner, HVAC compressor encapsulation), the interior/cabin (floor insulation and floor carpet, head-liner, inner-dash insulation), the exterior (wheel arch liners, underbody shields), and the trunk (side trims, load floor, parcel shelf).

[0004] Concepts and parts relating to acoustics specific to the automotive industry are often referred to with the abbreviation NVH (standing for "noise/vibration/harshness"), which will be adopted in the present description.

[0005] Sound (or noise) insulation means the avoidance of transmission of noise by its reflection, essentially preventing the noise from entering a reference volume in which a lower noise level is to be maintained, while sound (or noise) absorption means the dampening of sound by energy dissipation into a material.

[0006] Generally, acoustic insulation requires a mass, called a barrier, to be placed between the noise source and the receiver; the more mass is used as a barrier, the better the insulation is. As a result, high density materials are used predominantly for these parts. In terms of automotive NVH parts this translates into highly mineral filled thermoplastic elastomers being used as so called heavy-layers in thermo-molding or in injection molding processes. In addition, larger wall thicknesses allow for better insulation but come at the expense of a larger design volume required. Any holes in the barrier render it ineffective as already a small fraction of opened area leads to a disproportionally large decrease in insulation properties.

[0007] At the state of the art, insulation layers do not possess any or have only very little absorptive properties by themselves, and these properties need to be obtained by addition of other layers of materials (often of a different chemical composition negatively impacting recyclability).

[0008] Sound absorption can be added to insulation layers by addition of layers of absorber materials; a layer of this kind is called the absorber. The absorber can be made from any material with high internal surface area, porosity and tortuosity. Typically, open-cell polyurethane (PUR) foams or fiber-based (textile) materials are used for that. State-of-the-art absorber materials do not possess significant insulation properties.

[0009] An NVH part that needs to possess both properties will have to be designed as a multi-layer or even a multi-material construction using multi-stage manufacturing processes, resulting in increased overall process complexity and part cost.

[0010] Acoustic parts that use this type of insulation can be classified into different subgroups according to their functional principle. One possibility is that the insulation layer is coupled to one absorber layer. This combination of materials is then called an absorber-barrier (A-B) structure. More complex structures may be built such as absorber-barrier-absorber (A-B-A) combinations. In all cases the insulation itself does not contribute significantly to the absorptive properties.

[0011] For instance, in insulation of an inner dash, an A-B-A-construction is placed upon a metal sheet that is part of the body-in-white (BIW). The first A-(absorber)-layer, facing the body-in-white, does not contribute much to absorptive properties, and its function is rather to form a spring while the B-(barrier)-layer forms a mass to yield a spring-mass-system (or A-B-system). Within that spring-mass-system the mass is oriented towards the passenger cabin. Any noise from within the passenger cabin will be reflected off the mass which shows very poor absorption properties. Therefore, the absorption performance of such a part is low with respect to interior noise. The absorber layer that forms the spring will not help in absorbing these noises as it is acoustically hidden between the body-in-white and the mass layer. To give some absorbing properties to such a part another absorber layer is added on top of the spring-mass-system thus creating the aforementioned A-B-A-construction. This second absorber layer is facing the passenger cabin and will absorb the interior noises. This construction is very complex: it requires several process steps, a large design space and a multi-material design which are known disadvantages respectively for cost, cubing (namely, the volume occupied by the part) and recyclability.

[0012] Other state-of-the-art NVH insulation parts are used as encapsulations by wrapping the insulation material around the noise source thus fully enclosing it and capturing the noise at its origin. In a more elaborate part design absorber materials are placed between the insulation material and the noise source. Parts of this kind often are restricted in their design space as they need to fit in tight compartments along many other devices for instance in the engine bay.

[0013] Furthermore, the state-of-the-art combinations of absorbers and insulation materials have optimum performance in a specific, material dependent frequency range. For certain noise sources with high levels of noise in higher frequencies, the peak performance of this kind of NVH material does not match the directional characteristics/noise peak of the noise sources (e.g., motors/compressors,). Finally, while heavy-layer/PUR-foam designs provide superior performance to designs adopting textile-based absorbers, they also pose the more costly alternative of the two. Textile-based systems on the other hand, especially when designed to be a dual-impedance system, have acoustic advantages in the lower/mid frequency range, but lack performance in higher frequencies.

[0014] It is an object of the present invention to provide a noise attenuation part endowed with both noise insulation and absorption properties, which allows avoiding the multi-layer and multi-material features of known noise attenuation parts.

[0015] Another object of the invention is to provide processes for the production of said noise attenuation part.

SUMMARY OF THE INVENTION

[0016] These objects are obtained in the present invention, that in a first aspect refers to a noise attenuation part made of polymeric materials possibly loaded with a filler, comprised of two outer insulating layers and an interposed absorber layer, wherein the absorber layer has an open-cell structure and the insulating layers are dense layers, and at least one of the insulating layers has through-holes communicating the absorber layer with the environment, and wherein:

• the overall areal weight of the noise attenuation part is between 0.5 and 15 kg/m²;
• the thickness of the noise attenuation part is between 1.5 and 10 mm;
• each one of said through-holes has an area, as measured on the outer surface of the relevant insulating layer, between 0.15 and 13.0 mm²;
• the average number of through-holes in the relevant insulating layer is between 0.3 and 12 per square centimeter; and
• the overall area of said through-holes, as measured on the outer surface of the relevant insulating layer, is between 0.06 and 30% of said insulating layer surface.

[0017] In its second aspect, the invention refers to a process for the production of the noise attenuation part described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 

Figs. 1 and 2 shows schematic representations in section of possible noise attenuation parts of the invention;

Fig. 3 shows a micrograph of a section of a noise attenuation part of the invention;

Figs. 4 and 5 shows schematic representations of noise attenuation assemblies comprising a part of the invention and further noise attenuation elements (absorbers);

Figs. 6 to 9 show graphs of the acoustic properties of attenuation parts of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] As used in the present invention, the following terms have the meanings reported below:

• hole density: the average number of through-holes in an insulating layer per square centimeter of outer surface of that layer or a portion of the same;
• percent open area: the percent ratio of the sum of areas of through-holes in a surface of an insulating layer with respect to the geometrical area of that surface;
• the terms "insulating layer" and "barrier layer" are used as synonyms;
• the term "dense", referred to the polymeric material (possibly loaded with fillers) means a part having a density essentially correspondent to the theoretical density of the polymer or of the polymer/filler blend.

[0020] The present invention is about a noise attenuation part of novel structure: while in the prior art are common noise attenuation parts having a stacking of layers according to A-B or A-B-A schemes, the parts of the invention have a B-A-B structure, that is, an absorber layer sandwiched between two barrier layers. Moreover, contrary to the common understanding in the field that barrier layers must be continuous, lest a dramatic loss of their functionality, the present inventors have found that a B-A-B part according to the invention, in which at least one of the two barrier layers is perforated, presents a useful compromise of noise absorption and insulation properties; for the purpose of this invention these properties can be modulated controlling its geometrical features and the size and distribution of through-holes.

[0021] A noise attenuation part of the invention has generally lateral dimensions (i.e., width and length) much higher than the thickness, e.g., normally at least 10 times the thickness; in these parts, "thickness" means the dimension of the part along a line perpendicular to a surface of the part and crossing it from the first B layer to the second B layer, passing across the A layer.

[0022] The parts of the invention are not necessarily flat, and rather, when designed for use in the automotive field, they will generally have a 3D shape adapted to be laid against a section of the BIW. The panels of the invention are designed and mounted in the application site in a configuration such to reduce noise transmittance across their thickness.

[0023] Depending on the production method, the parts of the invention may expose the A layer at their peripheral edge or, when produced according to the preferred process described below, the A layer can be completely surrounded by a casing of dense polymeric material. Despite these two alternative possibilities, the edge of a panel of the invention plays no role in its acoustic properties, which are the same both in case of a laterally "open" panel (exposing the A layer at the edges) and of a "closed" panel (in which the A layer is completely surrounded by the dense polymeric material); since the acoustic properties of a panel of the invention are relevant across its thickness, in the description that follows reference will be made to two barrier layers (corresponding to the main surfaces of the panel), even though these may be joined at the edges of the panel forming one single continuous dense layer encasing the A layer.

[0024] In the description of the figures, to same number corresponds same element.

[0025] Figures 1 and 2 show a schematic representation of the section of two possible embodiments of noise attenuation parts of the invention.

[0026] The embodiment in Fig. 1, part 10, is made of two insulating layers 11 and 12, between which is present absorbent layer 13; insulating layer 12 is continuous, while layer 11 presents a series of through-holes, indicated cumulatively as elements 14, through which absorbent layer 13 is in direct contact with the external environment.

[0027] A second embodiment is shown in Fig. 2: this part, 20, has the same elements 11, 13 and 14 as part 10, but in this case the second insulating layer, 11', is in its turn perforated, presenting a second series of through-holes 14'.

[0028] In both embodiments of Figs. 1 and 2, the holes are not all of the same size and are not equidistant, which corresponds to the more general possibility of holes distribution discussed below; however, according to the invention the holes can be all of same size and/or all of same shape and/or equidistant over the surface of layers 11 and 11'.

[0029] Fig. 3 reproduces a photograph of a part of the invention. The photograph shows elements 11, 13 and 14, while layer 11' or 12 is not shown; this photograph is thus representative of both kinds of parts of the invention described above as parts 10 and 20.

[0030] The attenuation part is preferably made of a single material. Materials suitable for producing a noise attenuation part of the invention are essentially all polymeric materials, that is, thermoplastic materials, elastomeric materials or thermoplastic elastomers; examples of polymeric materials useful for the objects of the invention are polyolefins like polyethylene (PE) and polypropylene (PP), polyamides (PA) such as those known in the filed as PA6, PA6.6, PA5.10 and PA6.10, ethylene-propylene-diene rubbers (EPDM), poly(ethylene-vinyl acetate) (EVA), polyesters like polyethylene terephthalate (PET), polyurethanes (PU), and the like, or blends of these polymers. These polymers may be loaded with fillers, typically inorganic ones, to increase the average density of the material; typical fillers are minerals, such as, just to name a few, talc, barium sulphate, calcium carbonate, iron oxides and mica, in the form of powders; typically, as it is well-known to those skilled in the field, these powders have a grain size lower than about 200 µm, a fraction that can be easily collected by sieving powders with a suitable wire mesh.

[0031] A panel of the invention could be formed by producing separately the insulating layers 11 and 12 (or 11 and 11') and absorbent layer 13 and adhering these thereafter, for instance by gluing. In this case, the materials the various layers are made of may be different. However, for convenience of production, the panels of the invention are preferably produced according to a modified injection molding method described below.

[0032] The preferred production process of the noise attenuation part of the invention, described afterwards, forms the material as a dense part (apart from the holes) in the B layers and as a porous part in the A layer. The porosity of the A layer is not regular, in the sense that the voids in the layer have not a regular shape or volume; the only fixed feature is that these voids are interconnected, namely, a continuous pore volume. Despite the random distribution of shape and size of the voids, these give rise to average mechanical and acoustic properties that are repeatable by controlling the parameters of the production process.

[0033] A noise attenuation part of the invention may have a thickness between 1.5 and 10 mm, preferably between 3 and 8 mm, more preferably between 4 and 6 mm. Each of the two barrier layers B may have a thickness between 0.3 and 1.5 mm; within this range, the thickness of the B layers is selected so that the thickness of the absorption layer A ranges between 0.5 and 7 mm.

[0034] The through-holes are present on one or both insulating layers; preferably, these are present on one insulating layer only, with the other layer remaining continuous. In the discussion that follows, when a perforated B layer is meant, this will be indicated as Bp.

[0035] The through-holes in the Bp layer(s) have an area, as measured on the outer surface of the relevant layer(s), between 0.15 and 13.0 mm², preferably between 1.8 and 10.0 mm², and more preferably between 3.0 and 7.0 mm².

[0036] The through-holes may have essentially any shape, for instance circular, square, rectangular, triangular, trilobal, hexagonal or even irregular shapes. The through-holes need not have all the same shape, holes with two or three (or even more) different shapes may be present in the same insulating layer. Similarly, the through-holes need not have all the same area, any hole in a single insulating layer may have any area in the range reported above. The through-holes need not to be distributed over the surface of the insulating layer according to an ordered pattern or an array and may be present on said surface in a random distribution. Finally, the through-holes need not be present over the whole surface of an insulating layer: said surface may be subdivided in regions, one or some of which may be continuous and another one or some others of which will present a perforation with through-holes as described above.

[0037] In a preferred configuration, the through-holes are circular; in this case, the ranges of areas of holes indicated above correspond to a diameter range of between 0.5 and 4.0 mm, preferably between 1.5 and 3.5 mm, and more preferably of between 2.0 and 3.0 mm.

[0038] In case both insulating layers are perforated, the percent open area and the size, density, distribution, and shape of the outline of the holes on each layer may be the same or different in the two insulating layers.

[0039] The average density of through-holes per square centimeter ranges between 0.3 and 12, preferably between 0.8 and 10, and more preferably between 2 and 7.2 holes per square centimeter. This parameter is given as an average because, as said above, the holes have not all the same area and are not necessarily distributed according to a regular pattern over the insulating layer, nor over the whole surface of said layer. Only in case the holes are present according to a regular pattern over the whole surface of the insulating layer the average density and the local densities measured in different points of said surface coincide.

[0040] The percent open area in each Bp layer ranges between 0.06 and 30%, preferably between 2 and 20%, and more preferably between 5 and 15%. The lower limit of the broad range, percent open area of 0.06%, corresponds to the simultaneous adoption of the lower area (0.2 mm²) and the lower density (0.3/cm²) of through-holes. The upper limit of 30%, to the contrary, is not obtained with the maximum values of hole area and density, which would give rise to open area values > 100%; the maximum open area of 30% is obtained by suitable combinations of hole areas and densities, for instance, relatively high values of hole areas at relatively low average density, or vice versa.

[0041] The combination of the parameters defined above, i.e., the kind of material (polymer or filled polymer), the thickness of A and B layers, the shape, size, and distribution of holes in the Bp layer(s), give rise to an overall areal weight of the noise attenuation part between 0.5 and 15 kg/m², preferably between 1 and 10 kg/m², and more preferably between 1.5 and 5 kg/m².

[0042] The parameters discussed above control the acoustic performance of a noise attenuation part of the invention. Remaining within the limits given above for each parameter, in general the insulating properties of a noise attenuation part of the invention increase by increasing its density and weight (for instance by adding more filler in the polymer), its overall thickness, the thickness of the B layers and decreasing the percent open area by decreasing the size and count of the holes in the Bp layer(s); on the other hand, the absorption properties increase by increasing the surface area of the pores (more smaller pores), the percent open area by increasing the size and count of holes in the Bp layer(s), and the thickness of the A layer.

[0043] Examples of how the variation of constructive parameters of the noise attenuation parts of the invention allow to control their acoustic properties are provided in the experimental section below.

[0044] In the automotive field, the noise attenuation part of the invention may be applied in various positions of a vehicle, for instance as encapsulation (for combustion engines, e-motors, compressors, pumps, etc.) or as inner-dash or rear-seat insulation.

[0045] Any B-A-B noise attenuation part of the invention may be used alone, or else it may be coupled with one or more other noise reduction elements producing a noise attenuation assembly.

[0046] In case the noise attenuation part of the invention is used alone, it may need to be adequately supported, for instance through a frame, spacers, or any other suitable means fixed to the structure of the car bodywork; alternatively, and more usually, the correct placing of the part can be achieved by geometric confinement, that is, exploiting geometric features and matching surfaces of the part and the BIW that interlock and thus restrain the movement of the part.

[0047] In the preferred case, in which only one of the two B layers is perforated, the noise attenuation part may be oriented such that the Bp layer faces the noise source in the instance of encapsulation of the source of noise (for instance the car engine); when the part of the invention is employed as the inner-dash or rear-seat noise insulation element, it is preferably oriented such that the Bp layer faces the passenger's cabin.

[0048] In case a B-A-Bp part of the invention is coupled to other noise reduction elements, these may be made of the same material as the part of the invention, or of a different material. The further noise reduction element(s) may be for instance a textile absorber or an absorber foam; these may be adhered to only one B or Bp layer, or both; besides, different further layers may be used, for instance, the complete noise reduction element can be made of a part of the invention coupled with a textile absorber layer on the Bp layer and a foam on the B layer or on the second Bp layer, or vice versa. Typically, a part of the invention may be coupled to a further absorption layer, to form a spring-mass structure in which the B-A-B part of the invention acts as the mass. Figs. 4 and 5 schematically represent two typical applications of stacking of layers of this kind. The drawings represent the case that the noise attenuation part of the invention is of kind 10 (one Bp layer 11 and one continuous insulating layer 12), but the same stacking could be produced with parts of kind 20 (two Bp layers 11 and 11').

[0049] In the first example, Fig. 4, the noise attenuation assembly comprises a further absorption element, layer 40, that is adhered to (or laid against) the BIW of a vehicle, 41; the part of the invention, 10, rests on the surface of layer 40 away from BIW 41. Fig. 4 represents the case that part 10 is oriented with Bp layer 11 oriented towards the cabin and away from the noise source (schematically represented by the loudspeaker). This construction is particularly useful to insulate the cabin from external noise sources, for instance an inner dash insulation that is placed on the bulkhead to shield the vehicle's interior from the engine noise. In this construction, the part of invention provides added absorption for interior noises as the Bp layer (11) allows sound to enter the absorptive core material (13) where it is dissipated. At the same time the entire part 10 provides insulation properties and mass, the latter one being used in a spring-mass system with absorption layer 40.

[0050] In the second example, Fig. 5, the noise attenuation assembly comprises a part of the invention, 10, and a further absorption element, layer 50; the Bp layer (11) of part 10 faces layer 50, and the other side of layer 50 faces the noise source, so that Bp layer 11 is oriented towards the noise source. This situation represents a typical set-up for encapsulation, for instance of a motor or a compressor.

[0051] A noise attenuation part of the invention can also be mounted in noise reduction systems of more complex construction. For instance, a further absorption layer could be added in the staking of layers shown in Fig. 4; this extra A-layer would be facing the interior, i.e. the cabin, of the car, enhancing absorption of noise generated on the inside (which layer 40 of Fig. 4 cannot); in terms of the spring-mass system constituted by layer 40 and part 10, this extra absorption layer is only a further contribution to the mass.

[0052] In its second aspect, the invention refers to the process for producing the noise attenuation parts of the invention.

[0053] As said before, a panel of the invention could be formed by producing separately the insulating layers 11 and 12 (or 11 and 11') and absorbent layer 13 and adhering these, but this is not the preferred method in view of an industrial production.

[0054] The preferred process is a variant of the well-known injection molding method. In this variant, the material intended for the production of the noise attenuation part of the invention (the chosen polymer plus possible additives and fillers) is injected in the molten state in the cavity of a mold along with a supercritical fluid, typically CO₂ or N₂; the technique is known and described for instance in patent application WO 2004/009320 A2 and leads to a form of material referred to in the filed as microcellular. The supercritical fluid is dosed into the melt and is homogenized/dispersed via the rotation of a screw to form a single-phase solution of supercritical fluid and molten polymer. This single-phase solution is injected into the mold cavity where a rapid pressure drop occurs leading to a spontaneous nucleation of the supercritical fluid. Since the supercritical fluid is intimately dispersed in the polymer, the nucleation happens throughout the entire melt volume in a very even way. The cavity filling, leading to the sudden pressure drop, can be done in two different ways. In the first option, the mold volume is underfilled, i.e., the injected volume of single-phase solution is smaller than the cavity volume; the volume difference is made up for by the expanding foam, and the cell growth replaces the pack and hold phase of the standard injection molding process. In the second option, the mold volume is filled entirely with the single-phase solution; once the cavity is filled, the tool is slowly opened in a controlled fashion (e.g., by moving away one of the two halves making up the mold); again, the cell growth leads to the expansion of the material into the newly formed void. In both options, the resulting part shows a foamed core encased by a continuous outer dense polymeric layer; the main (more extended) surfaces of this encasing layer will constitute the B layers in the final molded part; this encasing layer is also referred to in the following as "skin(s)". The reason for this is the rapid cooling of the polymer melt at the cold tool wall preventing significant foaming to take place before solidification begins. This phenomenon, depending on the starting temperature and thermal conductivity of the polymer in the melt, determines the thickness of the two B layers. The temperature in the center of the part, on the other hand, remains above the melt temperature long enough (due to slower heat dissipation due to the outer polymer layers acting as insulation) for the supercritical fluid to expand to a gas and initiate the cell growth of the foam. The thickness of layer A is controlled by the distance between the two halves of the mold and the sum of thicknesses of the two B layers as indicated above; in case of a static mold (initially underfilled) the thickness of layer A will be essentially fixed and preset by the mold geometry, while in the second option described above the final thickness of layer A will be determined by the distance by which one of the halves of the mold is moved apart from the fix mold half. When the part is fully cooled below melting temperature the remaining gas can only vent without creating additional foamed material.

[0055] Up to this point, the process of the invention is same as common injection molding processes in the field of production of polymer parts. Polymer injection parameters, gas injection parameters, foaming parameters, and part geometry need to be fine-tuned for best results, which is however a task within the reach of the skilled person in this technique.

[0056] The process of the invention adds to the standard injection molding processes the step of forming holes in one or both the outer skins (B layers).

[0057] Perforating the skin(s) of the molded part can be done in several ways that will be apparent to the skilled person. One possible way is mechanically drilling the holes, possibly with a tool consisting of a head with multiple drill bits; this method is suitable for obtaining holes with a circular shape. One method that the present inventors have found particularly suitable for use in an industrial production is by means of heated pins connected to a head or plate (in the following simply "plate") whose movement is controlled by a gantry style robot (or any other robot capable of precise movements). In detail, a heated plate with pins attached to it is mounted to the tip of the robot. The pins themselves are heated to a temperature above the melt temperature of the injected material. When the part produced by injection molding has returned to a temperature below the melting temperature of the polymer, the mold is opened. Instead of removing the part from the mold, it rests in the tool. The mold opening is halted, and the robot enters the gap between the two mold halves. The heated plate with heated pins is aligned with the molded part such that the projection of the pins onto the molded part along the axis of the mold opening movement gives the desired hole locations on the molded part. The robot is programmed to move the heated plate with the pins attached to it towards the molded part which rests on the opened tool half. Upon touching the part surface, the pins are then pushed into the part melting their way through the skin. The pin movement is controlled such that the first skin is penetrated all the way through the skin and is stopped after reaching the foamed core (A layer) but before touching the opposite skin (second B layer). The pins are retracted from the molded part and subsequently the robot is retracted from the injection molding machine space. Then the opening movement of the mold is completed, and the part is ejected from the injection molding machine. If it is desired to produce a part with both B layers perforated (Bp layers), the part with perforations on one B layer can be extracted from the mold, turned upside down, placed in a suitable support and the holing operation described above repeated on the second B layer. Alternatively, the part could be extracted from the mold soon after the end of the injection molding phase, still with both skins unperforated, placed in a tool with wire mesh holding walls, and the punching of the molded part can take place from both sides at the same time or at least without manually removing and flipping it. Also in case of a part with both B layers perforated, no through-holes will be present in the completed part; in parts of the invention of this kind, there will be holes from both sides, but always with a tortuous pathway through the A-core connecting those holes.

[0058] The preferred embodiment is the one on which the B-A-B part of the invention has holes from one side only, reaching slightly into the A-core.

[0059] The direction of holes could also define an angle, i.e., be not perpendicular to the surface of the part.

[0060] The invention will be further illustrated by the examples that follow.

INSTRUMENTS, MATERIALS AND METHODS

[0061] Noise attenuation parts were produced with the following tools, materials, and operating parameters during the injection molding phase:

• Material: polyolefin based TPE with mineral filler and a bulk density of 2.2 g/cm³;
• Injection molding machine: Engel DUO - 1350H - 1350M - 450 Combi M with 60 mm 3-zone screw and attached microcell injection unit;
• Mold: 400 mm by 200 mm plate with hot runner and film gate and water cooling;
• Injection molding parameters: melt temperature 220 °C, injection pressure 1000 bar, back pressure 200 bar for 0.1 s, injection speed 350 mm/s, cooling time 45 s, cooling temperature 30 °C, gas content 1%, gas type CO₂, initial tool gap 3 mm (complete filling) then opening tool to final tool gap of 5.5 mm.

[0062] For the acoustic characterization of noise attenuation parts of the invention and of comparative parts were used an Impedance Tube Kit (50 Hz - 6.4 kHz) Type 4206 of Brüel & Kjær Sound & Vibration Measurement A/S, DK-2850 Nærum, Denmark, and an Alpha Cabin of Autoneum Management AG, Winterthur, Switzerland.

EXAMPLE 1

[0063] This example refers to the evaluation of the absorption properties of a noise attenuation part of the invention, compared to the same properties of a standalone absorption part and of a three-layers part of the prior art. All samples were cut in pieces of circular shape and diameter of 29 mm, which is one of the two diameters suitable for fitting specimens into the impedance tube in which the measures were carried out.

[0064] Noise attenuation parts according to the invention were produced using, for injection molding, the tools, materials, and operating parameters reported above. The resulting properties of the injection molded part (intermediate product), before perforation, were: part weight 582 g, part thickness 5.5 mm, part density 1.17 g/cm³, skin thickness 0.7 mm, areal weight 6.7 kg/m².

[0065] Starting from this intermediate product, two noise attenuation parts of the invention were produced by mechanically perforating one B layer (continuous outer skin), forming holes of diameter 2.5 mm in said layer with a drill; the holes were positioned on the nodes of a square lattice. The first noise attenuation part thus obtained, 1, had a hole density of 144 holes/100 cm², while the second noise attenuation part, 2, had a hole density of 324 holes/100 cm².

[0066] The acoustic absorption properties of these two parts were measured at normal incidence (a₀) in an Impedance Tube B&K Type 4206 according to the standards ISO 10534-2 and ASTM E1050-98; in the testing tool, the noise attenuation part of the invention was oriented with the Bp layer facing the noise source.

[0067] The results of the tests are reported in graphical form in Fig. 6: curves 1 and 2 show the absorption properties of the two noise attenuation parts of the invention of same number.

[0068] For comparison, have been tested and reported in Fig. 6 the noise absorption properties of a state-of-the-art polyurethane foam of density 54.35 g/l and thickness 19.07 mm (generally used to produce layers like the A layer of the parts of the invention), of a non-perforated dense layer alone, and of the intermediate product described above, that is, the molded B-A-B part without perforations in a B layer. The results are shown in Fig. 6 as curves 3, 4 and 5, respectively for the foam, said intermediate product and the dense layer.

[0069] The graphs in Fig. 6 show that, despite the perforation of one B layer, the noise attenuation parts of the invention (curves 1 and 2) have absorption performances much better than a simple dense layer (curve 5) or of the B-A-B part (curve 4), and that the height of the absorption peak of the parts of the invention for certain frequencies can reach absolute values close to those of the reference material (foam, curve 3). Besides, the absorption peak for the parts of the invention can be controlled and tuned depending on the perforation parameters.

EXAMPLE 2

[0070] This example refers to the evaluation of the absorption properties of a noise attenuation assembly made up of a noise attenuation part of the invention combined with a further foam layer, compared to similar structures in which the foam layer is stacked to parts not according to the invention.

[0071] A foam layer of the same kind and thickness as that used for comparison in Example 1 has been coupled with noise attenuation part 2 of Example 1 and with the intermediate product defined above, namely a B-A-B part without perforations in a B layer.

[0072] In the assembly obtained with a B-A-Bp part of the invention, this was adhered to a standard foam with the perforated layer Bp facing the foam. In the experimental set-up of the tests, the piece under measure was oriented with the standard foam layer facing the noise source (i.e., the foam was disposed between the Bp-A-B part of the invention, or the B-A-B part, and the noise source).

[0073] The results are reported in graphical form in Fig. 7.

[0074] Curve 6 represents the absorption properties of the assembly made up of foam and the Bp-A-B part of the invention, while curves 7 and 8 represent the absorption properties of the foam alone and of an assembly obtained by coupling the foam and a B-A-B part with no Bp layer(s).

[0075] The first observation from the graphs in the figure is that curves 7 and 8 are essentially superimposed across the whole spectrum of frequencies explored; this means that the assembly obtained adhering a B-A-B part with no perforations to a foam yields no improvement to the properties of the foam alone.

[0076] To the contrary, curve 6 (obtained with an assembly employing the Bp-A-B part of the invention) leads to better properties than the foam alone.

[0077] Comparing these results, it can be concluded that:

• the foam alone is not 100% efficient in absorbing noise; if it was, an added absorber on its backside would not improve the overall performance anymore as it would not receive any noise to be absorbed; so, the experimental set-up is suitable for highlighting differences in performance due to the B-A-B or Bp-A-B parts;
• the addition of a B-A-B part to a foam yields no improvement to the absorptive properties when compared to those of the foam alone;
• to the contrary, the inventors found an overall elevated level of performance, and an improvement compared to the foam alone, when the Bp-A-B part of the invention was combined with the foam.

[0078] From inspection of Fig. 7, it is also noted that:

• for low to mid frequencies (approximately between 400 and 2000 Hz) the combination of absorber foam with a part of the invention performs better (additive effect). This is a very good result in itself as low frequency absorption is generally difficult to achieve;
• at high frequencies (coinciding with the peak absorption performance of the invention alone) a second "peak area" (3-4 kHz) is found, where the combination of both layers performs significantly better than the absorber foam by itself. This is especially difficult to realize, as a widening of the peak area usually comes at the cost of an overall lower level of absorption and vice versa. Here both effects are achieved: a widening of the frequency range of peak performance and a higher level of absorption.

EXAMPLE 3

[0079] This example refers to the evaluation of the transmission loss (TL) properties of two noise attenuation parts of the invention, compared to the properties of a B-A-B part.

[0080] Samples 1 and 2 and the intermediate product of Example 1 have been tested for transmission loss in an Impedance Tube B&K Type 4206.

[0081] The results of the tests are reported in graphical form in Fig. 8.

[0082] Curves 9, 10 and 11 in the Figure correspond to noise attenuation part 1, noise attenuation part 2 and the B-A-B part, respectively.

[0083] As shown by the curves in the figures, as expected attenuation part 2 (curve 10), having a higher percent of open area, has lower transmission loss properties than attenuation part 1 (curve 9); moreover, despite the perforation, the transmission loss maintains a significant level of performance depending on the perforation parameters.

EXAMPLE 4

[0084] This example refers to the evaluation of the transmission loss properties of a noise attenuation part of the invention alone and combined in two different arrangements with a further foam layer.

[0085] Specimens of noise attenuation part 2 were coupled to the same foam of Example 1; in one case the coupling took place by adhering the B layer to the foam, and in the other case it took place by adhering the Bp layer to the foam. These two assemblies obtained by coupling with the foam, and a simple Bp-A-B structure of the invention, were tested for transmission loss properties in an Impedance Tube B&K Type 4206, oriented in all cases with the Bp layer facing the noise source.

[0086] The results of the tests are reported in graphical form in Fig. 9.

[0087] The three curves 12, 13 and 14 in the Figure correspond, respectively, to the assembly in which noise attenuation part 2 is adhered to the foam via the B layer, to the assembly in which noise attenuation part 2 is adhered to the foam via the Bp layer, and to the Bp-A-B part alone.

Claims

1. A noise attenuation part (10; 20) made of polymeric materials possibly loaded with a filler, comprised of two outer insulating layers (11, 12; 11') and an interposed absorber layer (13), wherein the absorber layer has an open-cell structure and the insulating layers are dense layers, and at least one (11; 11') of the insulating layers has through-holes (14; 14') communicating the absorber layer with the environment, and wherein:

- the overall areal weight of the noise attenuation part is between 0.5 and 15 kg/m²;

- the thickness of the noise attenuation part is between 1.5 and 10 mm;

- each one of said through-holes has an area, as measured on the outer surface of the relevant insulating layer, between 0.15 and 13.0 mm²;

- the average number of through-holes in the relevant insulating layer is between 0.3 and 12 per square centimeter; and

- the overall area of said through-holes, as measured on the outer surface of the relevant insulating layer, is between 0.06 and 30% of said insulating layer surface.

2. A noise attenuation part according to claim 1, in which the through-holes are present on one insulating layer only (11).

3. A noise attenuation part according to any one of claims 1 or 2, wherein said outer insulating layers have a thickness between 0.3 and 1.5 mm, and within this range the thickness of said outer insulating layers is selected so that the thickness of the absorber layer is between 0.5 and 7 mm.

4. A noise attenuation part according to any one of the preceding claims, wherein the through-holes are all of same size and/or the through-holes are all of same shape and/or the through-holes are equidistant over the surface of layers said layers having through-holes.

5. A noise attenuation part according to any one of the preceding claims, wherein the through-holes are circular and have a diameter in the range of 0.5 to 4.0 mm

6. A noise attenuation part according to any one of the preceding claims, wherein said polymeric materials are selected among thermoplastic materials, elastomeric materials and thermoplastic elastomers.

7. A noise attenuation part according to claim 6, wherein said polymeric materials are selected among polyethylene (PE) and polypropylene (PP), PA6, PA6.6, PA5.10 and PA6.10 polyamides, ethylene-propylene-diene rubbers (EPDM), poly(ethylene-vinyl acetate) (EVA), polyethylene terephthalate (PET), polyurethanes (PU), or mixtures thereof.

8. A noise attenuation part according to any one of the preceding claims, wherein the insulating layers and an interposed absorber layer are made of a single polymeric material.

9. A noise attenuation part according to any one of the preceding claims, wherein the filler consists of mineral powders.

10. A noise attenuation part according to claim 9, wherein said mineral powders are selected among powders of talc, barium sulphate, calcium carbonate, iron oxides, mica and mixtures thereof.

11. A noise attenuation part according to any one of claims 9 and 10, wherein said mineral powders have a grain size lower than 200 µm.

12. A noise attenuation assembly made up of a noise attenuation part of any one of claims 1 to 11 and one or two further absorption elements (40; 50) that may be made of a textile absorber material or of a foam, which may be adhered to one or both outer insulating layers of the noise attenuation part, in configurations such that one or two textile absorber elements are adhered to one or both outer insulating layers of the noise attenuation part, or one or two absorber foam elements are adhered to one or both outer insulating layers of the noise attenuation part, or any combination thereof.

13. Process for the production of a noise attenuation part of any one of claims 1 to 11, comprising the steps of:

- providing a mold constituted by at least two halves maintained at a temperature lower than the melting temperature of a polymeric material to be used in the process;

- injecting a polymeric material in molten state, possibly loaded with a filler, into the cavity of the mold, along with a supercritical fluid;

- causing a pressure drop to occur in the mold so that the inner part of the molten polymer mass expands giving rise to a foam zone, while the outer zone of the polymer mass, in contact with the mold walls, solidifies producing a continuous outer dense layer;

- perforating at least an area of the continuous outer dense layer.

14. Process according to claim 13, wherein:

- said step of causing a pressure drop in the mold is carried out either by underfilling with the molten polymer mass the mold volume and allowing a central part of the molten polymer to expand and completely fill the mold, or by completely filling the mold and slowly move away one of the two halves of the mold from the other one, thus increasing the inner volume of the mold; and

- said step of perforating at least an area of the continuous outer dense layer is carried out either by mechanical drilling or by forcing against said area of the continuous outer dense layer a plate provided with pins, wherein the pins are heated at a temperature above the melting temperature of the polymeric material the part is made of, so as to cause the pins to penetrate into said continuous outer dense layer and reach the inner foam zone.

Drawing