GB2322589A

GB2322589A - Moulding loudspeaker cone surround assembly

Info

Publication number: GB2322589A
Application number: GB9704142A
Authority: GB
Inventors: Patrick Arthur Leach; David Ian Leach
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-02-28
Filing date: 1997-02-28
Publication date: 1998-09-02
Also published as: GB9704142D0; TW317068B

Abstract

Injection moulding apparatus for making a loudspeaker cone and surround assembly characterised by at least two mould parts (3,7) defining a surround-forming cavity (34) and for receiving the marginal edge (23) of a cone (21), the mould parts being movable in one direction to close the mould to define the surround-forming cavity and to clamp the said cone edge (23) whereby a surround can be moulded onto the cone and in another direction to open the mould to allow access to the mould cavity, at least one depression formed in at least one of the mould parts and extending substantially continuously round the cavity to define between the mould parts an annular feed chamber (4) for molten elastomer, and a substantially continuous annular injection port (5) communicating between the annular feed chamber (4) and the cavity, which injection port (5) is formed by at least one depression in the mould parts and forming a small gap between the mould parts at the junction between the annular feed chamber and the cavity so that the injection port is in the form of a narrow slit through which the molten elastomer enters the mould cavity radially from the feed chamber, the mould parts being shaped at the injection port such that a knife edge is formed by edges of depressions in at least one of the respective mould parts.

Description

TITLE: LOUDSPEAKERS DESCRIPTION TECHNICAL FIELD The invention relates to loudspeakers and more particularly to drive units for loudspeakers.

BACKGROUND ART It is known to make the cones of loudspeakers from materials such as paper, plastics and metal sheet, although increasingly the chosen material is a plastics material such as polypropylene since this material has good characteristics for this particular use.

Speaker cones are normally provided with a peripheral suspension in the form of a resilient corrugated member of flexible rubber or other elastomeric material and which is sometimes known as a roll surround or simply as a surround.

Conventionally such elastomeric suspensions are compression moulded from thermosetting elastomeric materials and are secured to the material of the cone with the aid of an adhesive. This requires skilled hand assembly to ensure concentricity and a reliable bond and is thus slow and expensive. This form of assembly is however usually justified by the improved acoustic performance which results from the provision of a compliant suspension for the cone.

It is a general object of the invention to provide a loudspeaker cone and suspension or surround assembly which can be made without the need for hand assembly so that variations in manufacture and thus in performance, which have previously been commonplace, can be avoided or at least mitigated.

Many attempts have been made to produce a loudspeaker cone and surround assembly directly by injection moulding one onto the other to avoid the need for costly hand assembly of the two components, see for example GB-A2228391 of Pioneer. Although in theory the manufacturing technique involved appears to be simple, in practice this is not the case. This is probably due to the fact that it is particularly difficult to injection mould an elastomer to form a surround because of the poor flow characteristics of molten elastomers when moulding components of thin cross-section such as are involved in a loudspeaker suspension. Ordinary injection moulding techniques produce a surround which is lacking in concentricity and uniformity of cross-section and which is not flat at its outer margin. This is unacceptable. Our early experiments attempted to remedy these deficiencies by simultaneously injecting the elastomer at multiple positions around the periphery of the cone but still failed to provide the whole answer.

Our U.K. patent GB 2273678 describes a method of making a speaker cone and surround assembly comprising positioning and retaining the outer marginal edge of a speaker cone in a mould having a cavity defining a surround, feeding an elastomeric material through a substantially annular feed chamber extending around the mould cavity, injecting the elastomeric material into the cavity through a substantially annular orifice communicating with the feed chamber and which is narrow in width in comparison to the thickness of the surround and which extends substantially continuously around the entire outer marginal edge of the cone to mould a surround attached to the outer marginal edge of the cone, and after completion of the surround, separating the surround from an annular ring of solidified sprue formed of elastomeric material in the annular feed chamber by tearing said sprue from the surround at the junction between the sprue and the surround formed by the annular orifice, the tearing step being performed while the elastomeric material is still hot from the injection step.

This method and suitable apparatus for carrying out the method has been found to result in loudspeaker cone/surround assemblies which meet the need for concentricity and uniformity of moulding.

There is, however, a residual problem associated with mould wear occasioned by the need for a sharp knife edge and its tendency in use to become blunted over time. It is therefore an object of the invention to mitigate this problem.

DISCLOSURE OF INVENTION According to the invention, there is provided an injection moulding apparatus for making a loudspeaker cone and surround assembly characterised by at least two mould parts (3,7) defining a surround-forming cavity (34) and for receiving the marginal edge (23) of a cone (21), the mould parts being movable in one direction to close the mould to define the surround-forming cavity (34) and to clamp the said cone edge (23) whereby a surround (22) can be moulded onto the cone (21) and in another direction to open the mould to allow access to the mould cavity, at least one depression formed in at least one of the mould parts and extending substantially continuously round the cavity to define between the mould parts an annular feed chamber (4) for molten elastomer, and a substantially continuous annular injection port (5) communicating between the annular feed chamber (4) and the cavity (34), which injection port (5) is formed by at least one depression in the mould parts and forming a small gap between the mould parts at the junction between the annular feed chamber and the cavity so that the injection port is in the form of a narrow slit through which the molten elastomer enters the mould cavity radially from the feed chamber, the mould parts being shaped at the injection port such that a knife edge is formed by edges of depressions in at least one of the respective mould parts, the included angles of the said edges of the depressions being obtuse.

The feed chamber is preferably large in cross-section compared to the width or thickness of the cavity.

The annular feed chamber (4) may be fed from a single radially extending feed port (1) via a constricted section (2) of reduced cross-section which forms a gate controlling the flow of molten elastomer to the cavity. The constricted section (2) preferably splays-out laterally in a direction towards the annular feed chamber (4) to assist the molten elastomer to flow round the cavity.

Preferably the feed chamber (4) is formed in one of the mould parts (3,7) and at least the immediately adjacent portion of the cavity (34) is formed in the other of the mould parts (3,7).

From a further aspect, the invention is a loudspeaker cone and surround assembly made by means of the apparatus described above.

From another aspect the invention is a loudspeaker drive unit comprising a cone suspension e.g. of an elastomeric material, moulded integrally with a rigid annular chassis front member. Preferably the annular chassis front member, which may be moulded from a material such as polypropylene, is arranged to surround the cone suspension. Preferably the cone suspension and the cone are moulded integrally. The material of the suspension is preferably arranged to overlie an axial face of the annular chassis front member and is formed with a raised annular bead which can form a compressible water and/or air seal when the drive unit is installed. The suspension material may be moulded to embrace both opposite axial faces of the annular chassis front member, and may be formed with annular beads on both opposite faces.

The annular chassis front member may be moulded with lugs by which the annular chassis front member can be attached to the rear portion of the chassis, or alternatively the annular member may be formed with recesses for receiving fixing members, e.g. screws, by which the rear portion of the chassis frame is attached to the annular chassis front member.

Preferably the rear portion of the chassis comprises an annular rear member formed integrally with a series of legs by which the rear portion is connected to the annular chassis front member. The chassis may be made from a plastics material such a polypropylene. Advantageously the rear cone suspension is moulded integrally with chassis e.g. from an elastomeric material.

Preferably the cone suspension members and chassis are injection moulded. Advantageously the suspension members comprise a styrene-ethylene butylene-styrene polymer whereby the materials of the surround and chassis and suspension become chemically bonded together during the injection moulding steps. This chemical bonding largely obviates the problem of de-lamination which frequently occurs with hand assembled loudspeaker cone and surround assemblies when using adhesives. The chassis components, i.e. the front and rear annular support members, and the chassis legs are moulded from polypropylene filled with mica or the like.

In assembling a loudspeaker drive unit, it is conventional to attach the cone and surround assembly to the chassis or frame with the aid of an adhesive applied to the outer marginal edge of the surround. As an alternative an annular paper ring or gasket can be interposed between the surround and the drive unit chassis. This procedure involves extra work. This is obviated in the present invention since the chassis or a part thereof is formed integrally with the cone suspension.

The polypropylene used for the chassis components may be a homo-polymer or co-polymer and may comprise a proportion e.g. 40% by volume of a filler such as mica.

A method of making a loudspeaker cone and surround assembly employing the apparatus described above comprises clamping the outer marginal edge of a cone in a mould having a cavity defining a surround such that the outer marginal edge is in the cavity and injecting an elastomeric material into the cavity to form a surround attached to the outer marginal edge of the cone and is characterised by injecting elastomeric material through an annular orifice extending substantially continuously around the said marginal outer edge, the annular orifice being narrow in width in comparison to the thickness of the surround at the junction between the surround and the orifice, feeding the elastomeric material to the annular orifice through an annular feed chamber extending around the mould cavity, and separating the surround from an annular ring of sprue formed by elastomer which has solidified in or outside the annular orifice preferably by tearing at the said junction while the elastomer is still hot from the injection moulding step.

If desired the surround may be formed in two or more separate steps each as described above, to create a surround consisting of two or more annular bands or zones of elastomer which may, for example, be of different physical characteristics or colour.

Preferably the method comprises the step of evacuating air from the cavity prior to the injection of elastomer.

The method may comprise the step of placing a paper or the like gasket into the cavity prior to the injection of elastomer whereby the gasket is directly attached to the surround. In this case a vacuum can be applied to the cavity through the gasket to prevent the elastomer from being drawn into the vacuum ports.

Preferably the cone is an injection moulding.

Preferably the material of the cone and the material of the surround are chosen such that when the surround is injected onto the cone, the two are united or bonded chemically, that is to say they become attached at a molecular level due to cross-linking of the respective materials.

The cone may be of polypropylene and may be a homo polymer or co-polymer and may comprise a proportion of a filler such as mica. The plastics material will preferably have a high melt flow index to allow the material to be moulded in a cavity having a thin section.

The elastomer of the suspension is preferably a styrene-ethylene butylene-styrene polymer such as that sold by Evode Plastics Limited under their registered trademark Evoprene G. Preferably the elastomer has a shore A hardness below 50.

The method may comprise the step of placing a rigid chassis component into the cavity prior to the injection of elastomer whereby the chassis component becomes directly attached to the surround. Preferably the chassis component is formed by injection moulding. The chassis component may be annular in shape. Preferably the method comprises the step of choosing the material of the rigid chassis component such that when the material of the surround is injected onto the chassis component, the two are united or bonded chemically, that is to say, they become attached at a molecular level due to cross-linking of the respective materials.

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated by way of example in the accompanying drawings in which: Figure 1 is a cross sectional side view of injection mould apparatus for forming a surround or suspension on a loudspeaker cone; Figure 2 is a view of a detail indicated by arrow X of Figure 1 to an enlarged scale; Figure 3 is a plan view of a female mould part in the apparatus of Figure 1; Figure 4 is a perspective view of a loudspeaker cone and surround assembly made in accordance with the present invention; Figure 5 is a scrap cross section of part of the loudspeaker cone and suspension assembly of Figure 4, and taken on the line B-B of Figure 4; Figure 6 is a scrap cross-sectional view corresponding to Figure 5 of a modified arrangement; Figure 7 is a partly sectioned exploded perspective view of a loudspeaker drive unit, and Figure 8 is a scrap detail view corresponding to Figure 2 of a prior art arrangement.

BEST MODE FOR CARRYING OUT THE INVENTION In Figures 1 to 3 of the drawings there is shown injection moulding apparatus for making loudspeaker cone and surround assemblies. The apparatus is adapted to injection mould an elastomeric suspension or surround on to the outer periphery of an injection moulded loudspeaker cone, and comprises an opposed pair of relatively movable complementary parts (3,7) the female part (3) of which is shown in Figure 3 of the drawings, and both of which are shown in Figure 1. The mould parts (3,7) define a cavity (34) shaped to form a surround or suspension (22) to be injection moulded onto the outer peripheral edge (23) of an injection moulded cone (21) which is clamped between the mould parts and which edge (23) extends into the cavity.

Rather than attempt to inject the elastomer directly into the cavity (34) the molten elastomer is instead injected via a port (1) through a control gate (2), which is a constriction which in plan view shown in Figure 3 fans out sideways as it approaches an annular flow path or feeder gate (4) which extends continuously around the surround-forming cavity (34) at a position closely adjacent to the outer edge of the cavity. The molten elastomer then enters the cavity radially through a narrow "knife edge" gate (5) extending continuously round the outer edge of the cavity. In this way the cavity is filled evenly and simultaneously at all positions around its periphery. Thus the cavity (34) fills with elastomer to form a surround (22) having an outer flange (6) which is thick in comparison to the thickness or width of the gate (5), and an inner flange (17) attached to the outer margin (23) of the cone (21).

It will be seen from Figure 2 that when the elastomer in the filled mould has solidified to form a surround (22) molecularly bonded by cross-linking to the material of the cone (21), the outer flange (6) of the surround will have an attached annular sprue (33) of rounded cross-section consisting of the solidified elastomer formerly occupying the annular feeder ring (4). Preferably this is detached immediately the moulded component is removed from the mould i.e. while it is still hot, and at that stage can be done manually by tearing since the elastomer will not have cured fully. We have found that the waste elastomer will separate cleanly from the outer edge of the surround since the thin cross-section of elastomer in the knife edge injection port (5) forms a line of weakness.

Referring to the prior art arrangement shown in Figure 8, it will be noted that the area indicated by arrow Y of the mould part (3) forming the knife edge port (5) will be subject to wear in use so that the knife edge becomes enlarged and effectively blunted whereby the separation of the waste elastomer becomes increasingly difficult. To overcome this difficulty the arrangement of Figure 2 is proposed in which the ring gate (4) is formed in the mould part (7) and the portion of the cavity (34) defining the outer flange (6) of the surround is formed in the mould part (3). In this way the portion of the knife edge (5) formed by the mould part (3) is less subject to wear.

In Figures 4 to 6 of the drawings there is shown a loudspeaker drive cone and surround assembly of generally conventional appearance and comprising a frusto-conical drive cone (21) which, in use, is attached at its inner margin (11) in any convenient manner to a loudspeaker drive motor, e.g. a moving coil, not shown, the outer margin (23) of the cone (21) being attached in the manner described above with reference to Figures 1 to 3 to a compliant surround or suspension (22) which in turn is secured in any convenient fashion to a loudspeaker drive unit chassis (not shown). In the present case the loudspeaker drive cone (21) has been injection moulded from polypropylene and the flexible elastomeric surround or suspension (22), which is of generally conventional configuration, has been injection moulded thereon. The surround is annular in shape and comprises an outer flange (6) by which it is attached to the loudspeaker drive unit chassis (not shown), a part circular roll or bellows (16) which is the active part of the cone suspension and an inner flange (17), which is moulded onto the rear face (9) of the cone (21) so as to overlap the outer margin (23) of the cone to some extent.

The overlapping joint is thus invisible in use. The outer edge (8) of the cone (21) is preferably positioned closely adjacent to the roll or bellows (16) of the surround (22) so that the surround does not adversely affect the acoustic properties of the cone.

In the arrangement of Figure 5, the surround (22) is injection moulded in two successive steps, in the first of which the inner annular part (31) is injected directly onto the cone (21), after which in a second step the outer annular part (32) of the surround is injected onto the inner part (31). Each of these steps will be as described above with reference to Figure 1 to 3. In this way the characteristics of the surround can be tailored in any desired fashion so that for example the acoustical properties of the surround can be improved. If desired the inner and outer parts (31,32) of the surround (22) can be of different colours for aesthetic reasons. Also the surround may be moulded in more than two steps, e.g. in three or more steps to achieve a multi-zone surround.

The material of the surround (22) may be a styreneethylene butylene-styrene which bonds chemically under the heat and pressure of the injection moulding process to the polypropylene cone to form an integral unit therewith, thereby obviating the need to glue the cone and surround together as has been necessary heretofore. In this way the alignment between the cone and surround can be controlled more predictably than is the case with hand assembly, and the connection between the cone and surround can be achieved consistently.

In the arrangement of Figure 6, a paper gasket (10) is arranged in the mould prior to the injection moulding of the surround (22), whereby the gasket (10) is bonded to the flange (6). In this way the cone and surround unit can be secured to the chassis (not shown) of a drive unit using conventional, that is, non-harmful, adhesives.

In Figure 7 of the drawings there is shown in exploded form a loudspeaker drive unit comprising a cone (21), a cone surround or suspension (22) around the outer periphery of the cone and an annular chassis front plate or ring (14) supporting the outer margin (6) of the suspension (22).

The chassis front plate (14) is supported on a series of chassis legs (9) attached to, and extending from, an annular rear chassis plate (20). The rear plate (20) carries a rear or inner cone suspension (18) and a magnet plate (24).

The cone (21) and surround (22) are made by injection moulding a thermoplastic injection moulded elastomeric material directly onto a thermoplastic cone to form an integral unit as described above with reference to Figures 1 to 3, and in the same moulding step, the elastomeric suspension (22) is moulded directly onto the chassis ring (14) which is itself an injection moulded member of micafilled polypropylene or the like. The elastomeric material may be extended to form an integral gasket (12) which forms a bead-like flange projecting from both sides of the ring (14) and extending continuously around both sides of the ring. To this end the ring (14) is formed with a series of apertures extending through its surface from side to side to permit the elastomeric material to flow through the ring during the moulding process to form bead-like gaskets on both sides thereof.

The inner periphery (11) of the cone is extended to form a tubular member (25) to facilitate alignment of a voice coil (27). A card or paper ring (not shown) may be moulded into the tubular member (25) to provide a thermal barrier and/or to facilitate attachment of the voice coil (27) using an adhesive. The inner periphery of the cone may also be formed with an annular knife edge or raised bead (26) to facilitate the joining of the cone to the rear suspension (18), e.g by ultrasonic welding or the like.

The rear face of the ring (14) is formed integrally with frusto-conical spigots (13) which are received in correspondingly shaped sockets (28) in bosses (15) formed on the ends of chassis legs (19) and held in position by mechanical fixings such as screws or by welding, adhesives or the like. The legs (19) can be injection moulded integrally on the annular rear chassis ring (24) from a thermoplastic material such as mica-filled polypropylene or alternatively the legs can be made separately and the assembly fixed together e.g. by bolting. It will be appreciated that the chassis could be moulded in other fashions. Thus for example the legs (9) could be moulded on the front ring (14). Alternatively again the chassis could be formed as a cup-like member formed with sufficient louvres to allow air to move freely in response to cone excursions. The rear cone suspension (18) is preferably directly injection moulded onto the near chassis ring (20) from a thermoplastic elastomeric material, such as that used for the surround (22). The moulding method will be as described above with reference to Figures 1 to 3.

INDUSTRIAL APPLICABILITY The invention thus provides a method and apparatus for injection moulding loudspeaker cone and surround assemblies consisting of two different plastics materials, e.g. an elastomer on a rigid plastics. The invention also provides a method by which loudspeaker drive units can be made and assembled using a minimum of hand labour.

Claims

1. Injection moulding apparatus for making a loudspeaker cone and surround assembly characterised by at least two mould parts (3,7) defining a surround-forming cavity (34) and for receiving the marginal edge (23) of a cone (21), the mould parts being movable in one direction to close the mould to define the surround-forming cavity and to clamp the said cone edge (23) whereby a surround can be moulded onto the cone and in another direction to open the mould to allow access to the mould cavity, at least one depression formed in at least one of the mould parts and extending substantially continuously round the cavity to define between the mould parts an annular feed chamber (4) for molten elastomer, and a substantially continuous annular injection port (5) communicating between the annular feed chamber (4) and the cavity, which injection port (5) is formed by at least one depression in the mould parts and forming a small gap between the mould parts at the junction between the annular feed chamber and the cavity so that the injection port is in the form of a narrow slit through which the molten elastomer enters the mould cavity radially from the feed chamber, the mould parts being shaped at the injection port such that a knife edge is formed by edges of depressions in at least one of the respective mould parts.

2. Apparatus according to claim 1, characterised in that the feed chamber is large in cross-section compared to the width or thickness of the cavity.

3. Apparatus according to claim 1 or claim 2, characterised in that the annular feed chamber (4) is fed from a single radially extending feed port (1) via a constricted section (2) of reduced cross-section which forms a gate controlling the flow of molten elastomer to the cavity.

4. Apparatus according to claim 3, characterised in that the constricted section (2) splays-out laterally in a direction towards the annular feed chamber (4) to assist the molten elastomer to flow round the cavity.

5. Apparatus according to any preceding claim, characterised in that the feed chamber (4) is formed by a depression in one of the mould parts (3,7) and in that at least the portion of the cavity (34) immediately adjacent to the feed chamber (4) is formed by a depression in the other of the mould parts (3,7).

6. Injection moulding apparatus for making a loudspeaker cone and surround assembly, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.