Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work – To degas or prevent gas entrapment
Reexamination Certificate
1993-01-14
2001-01-09
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
To degas or prevent gas entrapment
C264S161000, C264S254000, C264S261000, C264S276000
Reexamination Certificate
active
06171534
ABSTRACT:
DESCRIPTION
The invention relates to loudspeakers and more particularly to drive units for loudspeakers.
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 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-A-2228391 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.
It is an object of the invention to provide a method by which an elastomeric surround can be injection moulded onto an injection moulded cone to provide a surround which is of an improved standard.
It is another object of the invention to provide a loudspeaker drive unit which requires a minimum of hand assembly so that variations in manufacture and thus in performance, which have previously been commonplace, can be avoided or at least mitigated.
According to the invention, a method of making a loudspeaker cone and surround assembly comprises clamping at least the outer marginal edge of a cone in a mould having a cavity defining a surround and injecting an elastomeric material into the cavity through an annular orifice extending substantially continuously around the said marginal outer edge to form a surround attached to the outer marginal edge of the cone. Preferably the elastomeric material is fed to the annular orifice through an annular feed chamber extending around the mould cavity.
Preferably the annular orifice is narrow in width in comparison to the thickness of the surround and preferably the method comprises the step of separating the surround from an annular ring of sprue formed by elastomer which has solidified in or outside the annular orifice by tearing 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, and 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.
From another aspect the invention is injection moulding apparatus for carrying out the method referred-to above, comprising mould parts defining a surround and for receiving at least the marginal outer edge of a cone, the mould parts being movable in one direction to close the mould and in another direction to open the mould to clamp the said cone edge and to allow access to the mould cavity, means formed between the mould parts and extending substantially continuously round the cavity defining an annular feed chamber for molten elastomer, and a substantially continuous annular injection port communicating between the annular feed chamber and the cavity. Preferably the substantially continuous annular injection port is formed by a small gap between the mould parts 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 feed chamber is preferably large in cross-section compared to the annular injection port which will be small in width compared to the width or thickness of the cavity. Preferably the annular feed chamber will be fed from a single radially extending feed port via a constricted section of reduced cross-section which forms a ‘gate’ controlling the flow of molten elastomer to the cavity. Preferably this constricted section splays-out laterally in a direction towards the annular feed chamber to assist the molten elastomer to flow round the path.
From a further aspect, the invention is a loudspeaker cone and surround assembly made by the method 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 embrac
Leach David Ian
Leach Patrick Arthur
Jacobson Price Holman & Stern PLLC
Ortiz Angela
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