Chair back assembly

Abstract

A chair back assembly (10, Figure 1) comprises a two-part seat back frame 12. A first part 16 of the seat back frame comprises a hollow first frame member comprising a pair of outer walls 22’,22’’ hav

Description

Field of the Invention The invention relates to chair construction, in particular to the construction of a back assembly for an office chair. Background of the Invention Office chair backs and back frames are typically fabricated from one-piece moulded plastic constructions. These constructions are heavy, bulky, and require complex plastics moulding techniques, meaning such chair backs and back frames are expensive to manufacture. The chair backs are then typically covered by an upholstery covering which, for example, may be fabric, leather or mesh. Various techniques are known for attaching the covering to the chair frame. For a chair back, an upholstery material may be stretched over padding or foam, the fabric being attached to the frame using stitching, staples or adhesive. Chairs with mesh backrests are also popular. These chairs have a backrest made of a breathable mesh fabric that is stretched across a frame. The mesh allows for airflow, which can help keep the user cool and comfortable. The edges J JL JL CO of the mesh may be secured to the chair frame using stitching, clips or extrusions for example. These methods of construction are time consuming and require additional tools or skills. Chairs upholstered using these methods are also more difficult to re-upholster. Removal of metal staples or glue is time consuming and can be difficult. This has environmental repercussions as it can be more cost effective to purchase a new chair rather than renovate an existing one, meaning that chairs often end up in landfill, rather than being re-upholstered and reused. EP2721967 describes a chair back having a one-piece frame to which the upholstery is secured by a clip. US8246113 describes a backrest for an office chair which includes a support frame and a flat covering element, such as fabric or webbing, secured and tensioned across the frame. The support frame comprises two parts which form a groove in which a welt strip is received. The two parts are secured together with screws. It would be desirable to provide an improved chair back frame construction, along with an improved upholstery covering in which the covering is held more securely to achieve an improved end product. Summary of the Invention A first aspect of the invention provides a chair back assembly for use with a chair including a base, and a seat supported on the base, the chair back assembly comprising a two-part seat back frame; wherein a first part of the seat back frame comprises a substantially hollow first frame member comprising a pair of outer walls having a substantially U-shaped cross-section, and an elongate inner wall located substantially centrally between the pair of outer walls; and wherein a second part of the seat back frame comprises a second frame member having a substantially T-shaped cross-section, the second frame member having an outer wall, and an inner wall, the inner wall depending substantially centrally from the outer wall; wherein the first and second frame members each provide one part of a two-part cooperating clip means to allow the first and second frame members to be clipped together to form the seat back frame, such that the first frame member inner wall and the second frame member inner wall are adjacent to each other and located inside the frame, wherein the part of clip means provided by the first part of the frame comprises a plurality of spaced apart protrusions each extending from an upper surface of the elongate inner wall and in the longitudinal direction of the inner wall, and wherein the part of the clip means provided by the second part of the frame comprises a plurality of spaced apart apertures, each aperture aligned with a respective one of said protrusions extending from the upper surface of the elongate inner wall of the first member; and wherein the first and second frame members cooperate to form a channel located between the outer wall of the second frame member and one of the outer walls of the first frame member, the channel extending around the seat back frame, adjacent to a perimeter edge thereof. Preferably, the two-part cooperating clip means comprise a plurality of teeth provided on one of the first and second frame members and a plurality of recesses provided on the other of the first and second frame members, the recesses shaped to receive the teeth. Preferably, the two-part cooperating clip means are provided on the inner walls of the first and second frame members. Preferably, the inner wall of the first frame member is provided with a plurality of protrusions, for example in the form of teeth, and the inner wall of the second frame member is provided with a plurality of recesses shaped to receive the plurality of protrusions. Alternatively, the protrusions may be provided on the inner wall of the second frame member and the recesses on the inner wall or the first frame member. Preferably, the inner walls of the first and second frame members are resilient to allow the plurality of protrusions to each locate in a respective one of the plurality of recesses. Preferably, the chair back assembly further comprises an upholstery covering, the upholstery covering having a perimeter edge and a trim member secured thereto. The channel is preferably shaped to receive the trim member and secure the trim member therein. Preferably, the trim member has a substantially J-shaped profile, comprising substantially parallel first and second side walls extending from a curved base portion, wherein the first side wall is secured to the perimeter edge of the upholstery covering, and the second side wall has a free edge distal from the base portion, and wherein the trim member is fabricated from a resilient material, such that the free edge of the second side wall is flexible. Preferably, the channel includes at least one trim securing element which, in use, engages with the free edge of the second wall of the trim member. The at least one trim securing element may be in the form of a plurality of protrusions. Alternatively, the at least one trim securing element may be in the form of a recessed ledge onto which the free edge of the second wall of the trim member locates. Preferably, the upholstery covering is stretched over, and rests against the outer wall of the second part of the frame. The chair back assembly may further comprise a layer of upholstery padding located between the outer wall of the second frame member and the upholstery covering. The free edge of the first trim member side wall may be secured to the perimeter edge of the upholstery covering by a method selected from the following: sewing, adhesive, and ultrasonic welding. The upholstery covering may be selected from the group comprising: fabric, leather, artificial leather, and mesh. A second aspect of the invention provides a chair comprising a chair back assembly according to the first aspect of the invention. A third aspect of the invention provides a method of forming the substantially hollow first frame member of the first part of the seat back frame of the first aspect of the invention. The chair back assembly of the invention provides an improved back frame construction which is simpler and cheaper to manufacture, and which has an aesthetically pleasing appearance. The back frame may use less material in its manufacture and hence be lighter in weight and present a slimmer profile than at least some of the back frames known in the art. Brief Description of the Drawings In the Drawings, which illustrate preferred embodiments of the seat back assembly of the invention: Figure 1 illustrates a rear view of a chair back assembly; Figure 2 illustrates an exploded view of a chair back frame of the chair back assembly of Figure 1, viewed from the front; Figure 2a is a front view of the chair back assembly illustrated in Figures 1 and 2; Figure 2b is a front view of a part of a component of the chair back assembly illustrated in Figures 1 to 2a; Figure 2c is a schematic representation of another component of the chair back assembly illustrated in Figures 1 to 2a; Figure 3 illustrates an exploded cross-sectional view through a part of the chair back frame of Figure 2; Figure 4 illustrates a cross-sectional perspective view through a part of an assembled chair back frame; Figure 5 illustrates an exploded cross-sectional perspective view of a part of the chair back assembly of Figure 1; Figure 6 illustrates a cross-sectional perspective view through a part of an assembled chair back assembly; Figure 7 illustrates a cross-sectional perspective view through a part of an alternative assembled chair back frame; Figure 7a is an exploded view of the chair back frame illustrated in Figure 7; Figure 7b is a schematic representation of a component of the chair back frame illustrated in Figures 7a and 7a; Figure 7c is a schematic representation of the components of the chair back frame illustrated in Figures 7 to 7b; Figure 8 illustrates a cross-sectional perspective view through a part of an alternative assembled chair back assembly; and Figure 8a is an exploded view of the components of the chair back assembly illustrated in Figures 7 to 8. Detailed Description of the Preferred Embodiments Figure 1 illustrates a back assembly for an office chair, viewed from behind. The seat back 10 comprises a frame 12 and a fabric covering 14 stretched across the frame. The fabric covering 14 may be a mesh fabric. As shown in Figure 2, the frame 12 is in two parts. A first, rear part 16, and a second, front part 18. These two parts clip together to form the frame 12. The frame also includes attachment means 20 for attachment to a seat base (not shown). The frame 12 may be fabricated from a moulded plastics material. Figure 2a shows the front part 18 of the frame 12 attached to the rear part 16. Each of the teeth 30 formed in the rear part 16 of the frame engages with a respective one of the apertures 19 formed in the front part 18 of the frame. Figure 2b is a front view of a part of the frame 12. The teeth 30 can be seen extending through the apertures 19. The arrows ¥ indicate the direction of movement of individual sliding cores in the plastics moulding tool that is used to form the rear part 16 of the frame. Such sliding cores form the male teeth 30 by sliding in and opposite to the direction Y. Figure 2c illustrates the rear part 16 of the frame 12. The rear part 16 is formed in a mould (not itself shown), the arrows Z, Z’ indicating opening of such a mould. Figure 4 illustrates a cross-section through an assembled frame 12. The first part 16 of the frame is substantially hollow and has a substantially U-shaped cross-section. It comprises a pair of angled outer walls 22’, 22” which meet at the base of the U-shape, and an inner wall 24 located substantially centrally between the pair of outer walls and extending away from the base of the U-shape. The second part 18 of the frame has a substantially T-shaped cross-section, comprising an outer wall 26, and an inner wall 28 depending substantially centrally from the outer wall. As shown more clearly in the exploded view of Figures 3 and 5, each part of the frame comprises one part of a cooperating clip means, allowing the two parts to be easily clipped together to form the back frame 12. In this example, the inner wall 22 of the first part 16 of the frame is provided with a plurality of teeth 30 which interlock with a plurality of recesses 32 located on the inner wall 28 of the second part 18 of the frame. The inner wall bearing the teeth 30 is sufficiently resilient to allow the teeth 30 to ride over the inner wall 28 of the second part 18 of the frame and to come to rest in the recesses 32. The second part 18 of the frame includes a plurality of apertures 19 which are aligned with the recesses 32. This allows insertion of a tool (not shown) to prise the teeth 30 away from the recesses 32 so that the two parts of the frame can be separated. Formation of the chair back frame 12 in two parts in this way means that the frame is more easily manufactured and typically uses less raw material than the equivalent one-piece frame. The front and rear frame members cooperate to form a channel 34 which is shaped to receive a trim member 36 which is secured to the edge of the fabric covering. The channel 34 is located between the outer wall 26 of the front part 18 of the frame, and one of the outer walls 22’ of the rear frame part 16. By forming the channel between two frame parts the channel can be made more accurately compared to a one-piece construction. Figure 5 illustrates an example trim member, which is preferably fabricated from a plastics material. Other forms of trim member may be used. As shown in Figure 5, the preferred trim member 36 has a substantially J-shaped cross-section, comprising a pair of side walls 38’, 38” which meet at the base of the J-shape. The first, longer side wall 38’ is secured to the edge 40 of the fabric covering 14, this could be achieved by sewing adhesive, or any other appropriate means. The second, shorter side wall 38” has a free edge. The trim member 36 also includes a protruding ridge 42 which helps with the alignment of the edge 40 of the upholstery covering on the trim member. Figure 6 illustrates the trim member 36 inserted into the channel 34 in the frame. One of the frame members includes a plurality of trim retaining elements, or protrusions 44 which extend into the channel 34. In the example illustrated, the protrusions 44 are located on the first frame member 16 and extend towards the second frame member 18. The trim member 36 is fabricated from a material which is sufficiently resilient to allow insertion of the trim member 36 into the channel 34, but sufficiently rigid to resist removal of the trim member 36 from the channel 34. As the trim member 36 is pushed into the channel 34, the second side wall 38” is deflected elastically towards the first side wall 38’ allowing the trim member to be pushed past the protrusion 44. Once past the protrusion 44, the second wall 38” returns to its original position and comes to rest behind the protrusion 44, securing the trim member 36 within the channel 34, and preventing withdrawal. Once inserted into the channel, the first side wall 38’ of the trim member sits flush with the underside of the outer wall 26 of the front part 18 of the frame. The protrusions 44 are preferably tapered on one side to aid insertion of trim member. The tapered surface adjacent to the channel opening helps the second side wall 38” to ride over the protrusion. The channel width is preferably narrow with respect to the channel depth. The channel needs to be sufficiently deep such that the free edge of the second wall 38” of the trim member 36 can pass the protrusion 44. Figures 7, 7a, 7b, 7c, 8 and 8a illustrate an alternative embodiment of the invention, in which the trim retaining element on the first frame member 16 is in the form of a continuous recessed ledge 46, defined by an upstanding wall 48, rather than a series of protrusions. The channel 34 is formed between the two frame members 16, 18, as with the previous example. As shown in Figure 8, the second wall 38” of the trim member comes to rest on the ledge 46, behind the wall 48 securing the trim member 36 within the channel 34, and preventing withdrawal. Once inserted into the channel, the first side wall 38’ of the trim member sits flush with the underside of the outer wall 26 of the front part 18 of the frame. As can be appreciated from Figure 8, once inserted into the channel 34, the trim member 36 is not visible. The wall 48 adjacent the ledge 46 is preferably slightly tapered as illustrated to aid insertion of the trim member. The tapered surface adjacent to the channel opening helps the second side wall 38” to ride over the protrusion. A chair back frame which comprises a continuous ledge 46 is more aesthetically appealing since there are no interruptions within the channel which may draw the eye to the region where the cover is fixed to the frame. The provision of a continuous ledge 46 also allows a wider variety of materials to be used since the channel can accommodate a variety of thicknesses of fabric cover. For example, the channel may be 5mm wide, but could equally accommodate a fabric cover with a thickness of 1mm or a leather cover of thickness 3mm. Furthermore, the configuration of the rear part 16 of the frame provides for a slim profile using less material than prior art chair back frames. As explained below, such a profile does not suffer from distortion in the same way that chair back frames of the prior art do. This provides for the width of the channel between the front and rear parts of the frame into which the trim member 36 is inserted to be substantially constant around the frame. Such accuracy in manufacture provides for an aesthetically pleasing appearance and a well functioning engagement of the trim member 36 in the frame 12. To remove the fabric covering from the frame, a blade, or similar tool may be inserted into the channel 34 and used to prise the free edge of the second side wall 38” of the trim member past the edge of the protrusion 44 or ledge 46. Figures 7a to 7c illustrate the front and rear parts 16, 18 of the frame 12 in greater detail, in particular, it can be seen that the inner wall 24 has elements 24a which protrude from and extend in the longitudinal direction of the inner wall 24. The tooth 30 extends from the element 24a. The elements 24 are formed by individual sliding cores mounted in the plastics moulding tool in which the rear part 18 of the frame is formed. The arrow X illustrates the direction in which said sliding cores slide. The cores may slide 20mm or more within less than 10mm of the visual width of the rear part 18 of the frame and may form a latch 30 that is 5mm or greater in length. The configuration of the rear part 16, with the elements extending in the longitudinal direction of the wall 24 and from the upper surface thereof, allows the sliding cores to be located within the width of the rear part 16. This allows the profile of the rear part 16 to be much slimmer when compared to profiles of the prior art that would require the sliding components of a moulding tool to move in a direction perpendicular to the longitudinal direction of the wall 24. Not only is a slimmer profile more pleasing aesthetically, but less material is used. The use of less material is an advantage in itself, but also provides a further advantage. When a component is removed from a mould the component is hot. Having less material in the component means that the component can cool more rapidly and more evenly than a similar component of larger dimension containing more material. In the case of frames moulded from plastic for chair backs, it has been found that distortion during cooling can present a significant problem and hence reducing distortion is beneficial. Figures 7a to 7c also illustrate ribs 22c, which extend between the inner wall 24 and the outer wall 22”. The ribs 22c stiffen the rear part 16 of the frame. The outer wall 22” provides an inner ledge formed by surfaces 22a and 22b. As can be seen from Figures 8 and 8a, a surface 26a of outer wall 26 of the front part 18 sits against the surface 22b, with the underside of outer wall 26 sitting on the surface 22a. The upper surface of the outer wall 26 and the end face of outer wall 22” sit in substantially the same plane, thereby ensuring that the surface of the fabric covering 14 is smooth in the region of the frame 12. The chair back assembly of the invention allows an upholstery covering to be stretched tightly over a back frame without any folds. The frame is made of only two components, without any further fastening elements required, meaning it is easily put together, and also easily recyclable. The end product has a very neat finish and the cover is held very securely on the frame.

Claims

1. A chair back assembly for use with a chair including a base, and a seat supported on the base, the chair back assembly comprising a two-part seat back frame;wherein a first part of the seat back frame comprises a substantially hollow first frame member comprising a pair of outer walls having a substantially U-shaped cross-section, and an elongate inner wall located substantially centrally between the pair of outer walls;and wherein a second part of the seat back frame comprises a second frame member having a substantially T-shaped cross-section, the second frame member having an outer wall, and an inner wall, the inner wall depending substantially centrally from the outer wall;wherein the first and second frame members each provide one part of a two-part cooperating clip means to allow the first and second frame members to be clipped together to form the seat back frame, such that the first frame member inner wall and the second frame member inner wall are adjacent to each other and located inside the frame, wherein the part of clip means provided by the first part of the frame comprises a plurality of spaced apart protrusions each extending from an upper surface of the elongate inner wall and in the longitudinal direction of the inner wall, and wherein the part of the clip means provided by the second part of the frame comprises a plurality of spaced apart apertures aligned, each aperture aligned with a respective one of said protrusions extending from the upper surface of the elongate inner wall of the first member;and wherein the first and second frame members cooperate to form a channel located between the outer wall of the second frame member and one of the outer walls of the first frame member, the channel extending around the seat back frame, adjacent to a perimeter edge thereof.

2. A chair back assembly according to Claim 1, wherein one of the protrusion and the aperture includes a recess and the other includes a notch that engages with the recess when the first and second frame members are clipped together.

3. A chair back assembly according to Claim 2, wherein the recess and the notch of two-part cooperating clip means are provided on the inner walls of the first and second frame members.

4. A chair back assembly according to any preceding claim, wherein the channel extends continuously or discontinuously around the whole or a part of the perimeter of the seat back frame.

5. A chair back assembly according to any preceding claim, wherein the inner walls of the first and second frame members are resilient and the notch and recess are snap fit.

6. A chair back assembly according to any preceding claim, further comprising an upholstery covering, the upholstery covering having a perimeter edge and a trim member secured thereto, wherein the channel is shaped to receive the trim member and secure the trim member therein.

7. A chair back assembly according to Claim 6, wherein the trim member has a substantially J-shaped profile, comprising substantially parallel first and second side walls extending from a curved base portion, wherein the first side wall is secured to the perimeter edge of the upholstery covering, and the second side wall has a free edge distal from the base portion, and wherein the trim member is fabricated from a resilient material, such that the free edge of the second side wall is flexible;and wherein the channel includes at least one trim securing element which, in use, engages with the free edge of the second wall of the trim member.

8. A chair back assembly according to Claim 7, wherein the at least one trim securing element is in the form of a plurality of protrusions.

9. A chair back assembly according to Claim 7, wherein the at least one trim securing element is in the form of a recessed ledge onto which the free edge of the second wall of the trim member locates.

10. A chair back assembly according to Claim 9, wherein the trim member is situated between the upholstery cover and the second frame member and behind one of the outer walls of the first frame member, such that the trim member is not visible.

11. A chair back assembly according to any of claims Claim 6 to 10, wherein the upholstery covering is stretched over, and rests against the outer wall of the second part of the frame.

12. A chair back assembly according to any of claims 6 to 11, further comprising a layer of upholstery padding located between the outer wall of the second frame member and the upholstery covering.

13. A chair back assembly according to any of claims 7 to 12, wherein the free edge of the first trim member side wall is secured to the perimeter edge of the upholstery covering by a method selected from the following: sewing, adhesive, and ultrasonic welding.

14. A chair back assembly according to any of claims 6 to 13, wherein the upholstery covering is selected from the group comprising: fabric, leather, artificial leather, and mesh.

15. A chair comprising a chair back assembly as claimed in any of Claims 1 to 14.

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