Chair And Seat And Back Unit Therefor

Abstract

A chair comprises a frame, which may be somewhat flexible, and a unitary blow-molded plastic seat and back unit including a hollow, relatively non-bendable seat portion, a hollow, relatively non-bendable back portion, and a relatively bendable portion intervening between and connecting the seat and back portions. The seat and back unit is blow-molded in substantially, i.e. generally, flat, over-all condition, in which it may be stored and shipped, and is conditioned for mounting on the chair frame by bending about the intervening connecting portion. The latter is formed to provide for controlled bending with avoidance of sharp localized bends, and to provide for such lateral flexing of the seat and back portions as to conform to a sitter's position, such as a slouched position, and to prevent stress-induced edge cracks. The seat and back unit is attached to the chair frame by bolts engaging internally threaded fastening elements molded in situ in the plastic unit, the bolts extending through and beyond the fastening elements and biting into the plastic to lock the bolts against unintentional unscrewing.

Description

This invention relates to chairs and more particularly to chairs of the general class comprising a supporting frame and molded plastic seat and back means mounted on the frame. Chairs of this general class have been in wide use for many years, one important field being that of schoolroom furniture. In some previously known chairs of this general class, the plastic seats and backs have been molded separately and attached separately to the frames. In others, the seats and backs have been molded unitarily in such angular relation to each other as to conform to the relative angularity of the frame seat and back supporting structures. One disadvantage of the known unitary molding of seats and backs at an angle to one another has been the size and cost of the molds required. Another and attendant disadvantage has been the large amount of space required for storing and/or shipping the unitary seat and back structures before being assembled to frames. Still further, prior art unitary molded seat and back structures known to the applicant have lacked the coexistent properties of flexibility and conformability to certain positions, such as slouched positions, of sitters, and resistance to structural failure, such as formulation of stress-induced edge cracks resulting from flexing.

An object of the invention is to provide a unitary blow-molded chair seat and back unit free or substantially free from the above pointed out disadvantages.

Another object of the invention is to provide such unit including a hollow, relatively non-bendable seat portion, a hollow, relatively non-bendable back portion, and a relatively bendable portion intervening between and connecting the seat and back portions.

Another object of the invention is to provide an improved chair construction including a chair frame and a blow-molded seat and back unit of the character outlined above.

Another object is to provide an improved, self-locking attachment of a chair frame to a unitary blow-molded seat and back unit.

A further object of the invention is to provide an improved method of blow-molding a unitary chair seat and back unit and mounting it on a chair frame, wherein the unit is molded in generally flat form, providing for storage and/or shipping with saving in space, and is then bent to angular form for mounting on the chair frame.

Other objects will become apparent from a reading of the following description, the appended claims, and the accompanying drawings, in which:

FIG. 1 is a perspective view of a chair including an integral blow-molded seat and back unit embodying the invention;

FIG. 2 is a side elevational view of the chair shown in FIG. 1;

FIG. 3 is an exploded perspective view showing the chair frame and integral blow-molded seat and back unit in separated relation, the blow-molded seat and back unit in separated relation, the blow-molded unit being shown tipped from its normal position of assembly on the chair better to show its construction;

FIG. 4 is a diagrammatic fragmentary side elevational view of portions of the seat and back unit, indicating the bending of the unit from its substantially flat form as molded to its condition conforming to the chair frame;

FIG. 5 is a section on the line 5--5 of FIG. 2;

FIG. 6 is a section on the line 6--6 of FIG. 2;

FIG. 7 is a section on the line 7--7 of FIG. 2;

FIG. 8 is an enlarged scale plan view of the seat and back unit as molded in substantially generally flat over-all condition, i.e. before being bent to conform to the chair frame, showing the configuration of outer major walls of a seat portion and a back portion, the outer major wall of the seat portion being that wall which is to face downwardly when mounted on the chair, and the outer major wall of the back portion being that portion which is to face rearwardly when mounted on the chair frame;

FIG. 9 is a longitudinal section on the line 9--9 of FIG. 8;

FIG. 10 is a transverse section on the line 10--10 of FIG. 8;

FIG. 11 is a cross section on the line 11--11 of FIG. 8;

FIG. 12 is a cross section on the line 12--12 of FIG. 8;

FIG. 13 is a cross section on the line 13--13 of FIG. 8;

FIG. 14 is a cross section on the line 14--14 of FIG. 8;

FIG. 15 is a cross section on the line 15--15 of FIG. 8;

FIG. 16 is a cross section on the line 16--16 of FIG. 8;

FIG. 17 is a cross section on the line 17--17 of FIG. 8;

FIG. 18 is a schematic vertical sectional view of molding apparatus for blow-molding the unitary seat and back unit, two mold halves being shown separated and a plastic parison being shown in the process of being extruded between the mold halves;

FIG. 19 is a view similar to FIG. 18, but showing the mold halves closed upon one another to encompass the extruded parison which is shown as being partially blown;

FIG. 20 is a view similar to FIG. 19, but showing the parison fully blown to produce the integral seat and back unit;

FIG. 21 is a view similar to FIG. 20, but showing the mold halves separated, and the completed seat and back unit therebetween suspended from the extruding nozzle;

FIG. 22 is an enlarged scale fragmentary detailed sectional view through part of one of the mold halves and part of the molded unit, showing a threaded nut molded in place in the seat and back unit after having been positioned in the mold cavity by a locating pin carried by the mold half;

FIG. 23 is an enlarged scale fragmentary detailed sectional view of part of the chair frame and an adjacent part of the molded seat and back unit, showing an attaching screw extending through a frame part and being engaged in the molded-in nut, the screw having biting engagement with the plastic to lock the screw against unintentional unscrewing. This section may be considered as being taken on the line 23--23 of FIG. 7; and

FIG. 24 is a fragmentary sectional view on the line 9--9 of FIG. 8 showing a molded-in groove in the seat and back unit for attachment of a cushion.

The drawings show a representative embodiment of a chair and blow-molded seat and back unit, and illustrate a preferred method of blow-molding the seat and back unit to obtain the advantages of the present invention. As shown in FIGS. 1, 2 and 3 particularly, the chair comprises a chair frame CF and an integral blow-molded seat and back unit generally designated SBU.

In the illustrative embodiment, the supporting frame CF is of the kind disclosed in U.S. Pat. No. 3,276,818, Sidney A. Durfee et al., issued Oct. 4, 1966. The frame components are formed of tubing of steel or other suitable metal, bent or formed to desired shapes and then assembled. The chair frame components are semi-rigid, preferably having a degree of yieldability or resilience such as to permit adaptation to small unevenness in floor surface and avoidance of shock when being sat upon.

The chair frame CF comprises two unitary combination leg and under-seat frame members LUF, and one back and under-seat frame structure BUF formed and assembled as described below.

The two leg and under-seat frame members LUF are identical, but are reversed relatively to one another when assembled. Each member LUF is formed of a unitary, elongated element so as to have a bight 1 intermediate its ends. Each bight comprises a curved central connecting portion 1a and two side portions 1b which extend from opposite ends of the connecting portion 1a and terminate in downwardly extending leg portions 2a and 2b. The leg portions diverge relatively to one another fore and aft from the center of the chair frame considered as a whole.

In assembling the two leg and under-seat frame members LUF, the curved connecting portions 1a of the bights 1 are brought into contact with one another and are secured together for example by welding indicated at 3. The legs of the two assembled members LUF diverge from each other transversely of the chair frame, as well as fore and aft.

The back and under-seat frame structure BUF is formed integrally from a unitary, elongated element, e.g. of steel tubing, so as to have a rear upstanding back supporting part or post 4 in the shape of an inverted V, and two laterally spaced portions 5, 5 which extend forwardly respectively from the bottom portions of the upstanding inverted V shaped post part 4.

In assembling the back and under-seat frame structure BUF and the sub-assembly of the two leg and under-seat frame members LUF, the back and under-seat frame structure BUF is so positioned that each of its laterally spaced portions 5, 5 crosses over the two side portions 1b of the associated bight 1. Adjacent to their rear ends, the laterally spaced portions 5, 5 are welded at 5a to the rear side portions 1b of the bights 1.

Adjacent their front ends, the laterally spaced portions 5, 5 of the structure BUF are suitably secured to the respective front side portions 1a of the bights 1. Preferably, this securing is effected through spacers 6 interposed between and connected to the foremost bight side portions 1b, 1b and the forward ends of the transversely spaced portions 5, 5 of the BUF structure. Each spacer 6 is of inverted U shape as shown in FIG. 5, and provides a seat for the associated portion 5 of the structure BUF. Welds secure the spacers 6 respectively to the portions 5 and the bight portions 1b.

With its components formed and assembled as described above, the chair frame CF has such an over-all configuration that a number of chair frames may be stacked and supported one upon another. This stackability of the chair frame per se is frequently convenient when a number of chair frames must be held awaiting application of back and seat structures.

Preferably, glides 9, which may be of conventional form, are applied to the lower ends of the legs 2a and 2b.

The integral blow-molded seat and back unit SBU is molded from any of a number of known thermoplastic materials, such as polyethylene and polypropylene, having the property of being extrudable in the form of a parison while hot and molded to predetermined shape. A suitable material is that known commercially by the trademark MARLEX 5202, a high density linear polyethylene obtainable from Phillips Petroleum Company, Bartlesville, Oklahoma.

As shown in FIGS. 8 and 9, the unit SBU comprises a relatively non-bendable hollow seat portion HSP, a relatively non-bendable hollow back portion HBP, and a bendable connecting portion BCP intervening between and connecting the portions HSP and HBP. Preferably, the connecting portion BCP is solid as distinguished from being hollow.

Considering the configuration of the flat molded unit SBU, the hollow back portion HBP comprises an inner major wall 10 and an opposed outer major wall 11 spaced from the wall 10, the walls 10 and 11 being connected by a free end edge wall 12, side edge walls 13, 13 and the bendable connecting portion BCP. The wall 10 is designated the inner major wall because it is to face inwardly toward the sitter in the assembled chair, and the wall 11 is designated the outer major wall because it is to face away from the sitter in the assembled chair. The outer surface of the inner major wall 10 may be textured in molding, if desired.

To enable nesting or partial nesting of the upstanding frame parts 4, 4 in the outer wall 11 of the hollow back part HBP, the latter is molded to have an inverted V shaped recess 14 which receives the upstanding frame parts 4, 4 as shown in FIG. 2, thus contributing to the appearance of the assembled chair.

As shown in FIGS. 9-13, the major walls 10 and 11 of the hollow back portion HBP are spaced from each other more widely in a section 15 extending along the top or free end edge 12, and in hollow sections 16 extending along the side edges 13, 13 than in some parts of the area surrounded by the sections 15, 16 and the bendable connecting portion BCP. This construction provides relatively greater stiffness and strength throughout the periphery of the back portion HBP with relatively more yieldability throughout the central area so as to provide for comfortable resting of the sitter's back against the back portion HBP. As shown in FIGS. 9 and 10, the inner and outer major walls 10 and 11 are united in molding at 17 in a relatively narrow strip area extending transversely and just below the enlarged top edge section 15, opposite the bendable portion BCP, from one side section 16 to the other section 16. The uniting of the inner and outer major walls 10 and 11 in the narrow area 17 is effected by pushing opposite parts of the molding parison against each other and welding them together to form a reinforcing box structure during the molding operation, which is described more particularly hereinafter with reference to FIGS. 18-21.

FIGS. 7, 9, 11 and 12 show fastening nuts 18 molded in situ in the outer major wall 11 for attaching the hollow back portion HBP to the chair frame CF. The method of molding the nuts 18 in situ is described hereinafter with reference to FIG. 22.

The form of the hollow seat portion HSP may best be understood by reference to FIGS, 3, 5, 6, 8, 9, 15, 16 and 17. The seat portion comprises an inner major wall 19 and an outer major wall 20, side edge walls 21, 21, and a front or free edge wall 22 which is curved as shown in FIGS. 2 and 9 to provide a comfortable drop-off contour at the front of the seat portion.

As shown in FIGS. 3, 8, 15, 16 and 17, the outer major wall 20 of the seat portion HSP is molded to provide two fore and aft extending grooves 23, 23 which are spaced suitably respectively to receive the fore and aft extending parts 5, 5 of the frame CF. The wall 20 is of undulating peak-and-valley formation, the peaks 24 and valleys 25 of which extend transversely and die out and terminate close to but short of the edge walls 21, 21. The peak-and-valley formation provides additional stiffness of the outer major wall 20 which, in assembly, rests on the frame CF. The inner major wall 19 is of smoothly dished contour so as to be relatively yieldable and comfortable when sat upon. Additional strength of the hollow seat portion HSP in its entirety is provided by mold-welding the major walls 10 and 20 together in two relatively small intermediate areas 26, 26, as shown in FIG. 16, or so as to touch in those areas.

As shown in FIGS. 5, 6, 8, 15, and 17, fastening nuts 27 are molded in situ in the outer major wall 20 similarly to the molding of the nuts 18 in the outer major wall 11 of the hollow back portion HBP.

The bendable connecting portion BCP is molded unitarily with the hollow seat portion HSP and the hollow back portion HBP in such manner that the connecting portion BCP is solid, i.e. not hollow, as shown in FIGS. 8 and 9, and so as to be somewhat flexible in all directions. The molding steps followed for so forming the connecting portion BCP are described hereinafter with reference to FIGS. 18-21. At this point, it is sufficient to state that opposite sides of the parison being molded are pressed together while hot so as to be heat-united or welded together. The construction of the connecting portion BCP thus produced is constituted by an extension of the inner major walls 10 and 19 respectively of the hollow back portion HBP and the hollow seat portion HSP from one to the other, and an extension of the outer major walls 11 and 20 respectively of the hollow back portion HBP and the hollow seat portion HSP from one to the other. These extensions are merged or united in molding to provide the solid bendable connection BCP between the hollow back portion HBP and the hollow seat portion HSP. As shown in FIG. 9, the bendable portion BCP is twice as thick, or substantially twice as thick, as the inner and outer major walls 10, 19, 11 and 20. As also shown in FIG. 9, the relatively bendable portion BCP is substantially less in cross section than the hollow seat portion HSP and hollow back portion HBP.

When the molded seat and back unit SBU is to be mounted on the chair, it is bent from its substantially flat as molded condition as indicated in FIG. 4 to position the portions HSP and HBP for being connected respectively to the frame under-seat members 5, 5 and the frame back-supporting member 4. Bending about the connecting portion BCP is facilitated by providing molded-in grooves 28 extending transversely with respect to the chair close to but terminating short of the side edges of the bendable portion BCP. Terminating the grooves 28 short of the side edges leaves ungrooved marginal or lip portions 29 which provide for controlled bending of the connecting portion BCP and avoidance of sharp localized bending. The grooves 28 are formed in the part of the bendable portion BCP which is constituted by extensions of the outer major walls 11 and 20.

FIGS. 18-21 schematically illustrate apparatus for molding the seat and back unit SBU, and the step-by-step molding procedure. The molding apparatus is of generally known or conventional construction including two mold halves M1 and M2 mounted for relatively horizontal reciprocation from the open positions shown in FIGS. 18 and 21 to the closed positions shown in FIGS. 19 and 20, and vice versa. The mold halves M1 and M2 are actuated respectively by hydraulic rams R1 and R2.

The mold half M1 is contoured as a reverse image of the outer major walls 11 and 20 of the unit SBU and the extension of these walls into the bendable connecting portion BCP. The mold half M2 is contoured as a reverse image of the inner major walls 10 and 19 of the unit SBU and the extension of these walls into the bendable connecting portion BCP.

An extrusion nozzle N is mounted in fixed position directly above the molding apparatus so that when the mold halves M1 and M2 are open as shown in FIG. 18 the extrusion nozzle N will be directly above the space between the two mold halves. The nozzle N is of known or conventional construction having an annular orifice through which heated plastic material is extruded between the mold halves in the form of a closed bottom end, cylindrical parison P as shown in FIG. 18. The parison P is shown in full lines as partly extruded with the complete extent of extrusion indicated in dotted lines.

The mold half M1 is provided with an upper blowing needle 30 and a lower blowing needle 31 as commonly used in blow-molding operations.

When the mold is open and the parison is being extruded between the mold halves, the needles 30 and 31 are retracted so as not to extend inwardly beyond the cavity forming walls of the mold half M1, as shown in FIG. 18.

When the extrusion of the parison P has been completed, the mold halves are closed to the position shown in FIG. 19 and the needles 30 and 31 are projected so as to extend inwardly beyond the cavity wall of the mold half M1, through the adjacent wall of the tubular parison, but to terminate short of the opposite wall of the parison. Air or other gas is then injected through the needles 30 and 31 and thus into the parison P so as to force the walls of the latter into contact with and to assume the contour of the mold walls. The partially blown parison is shown in FIG. 19, and the completely blown parison forming the seat and back unit SBU is shown in FIG. 20.

As seen in FIGS. 19 and 20, the top needle 30 is positioned to blow into the enlarged section 15 extending along the top of the hollow back portion HBP and above the narrow area 17 in which the inner and outer major walls 10 and 11 are united by being pressed together while hot. However, the hollow section 15 communicates with the hollow sections 16 extending along the sides of the hollow back portion HBP so that air blown into the section 15 by the needle 30 flows into the hollow side sections 16 and thence in between the inner and outer major walls 10 and 11 so as completely to expand the parison in forming the hollow back portion HBP, thus maintaining the inner and outer major walls spaced from one another throughout most of their areas.

Substantially vertically midway of the parison and mold, the mold halves press opposite sides of the parison into contact with one another while hot, thus welding or uniting them to form the solid bendable connecting portion BCP having the grooves 28.

After the parison has been blown completely to form the blow-molded seat and back unit SBU, the mold halves M1 and M2 are opened as shown in FIG. 21. The molded unit SBU is then severed from the following extruded material at 32 and removed from the mold. Thereafter, the scrap or flash at the bottom of the molded unit may be trimmed off at 33. The unit SBU will then be ready to be bent to the configuration shown in FIGS. 1-3 and mounted on the chair frame.

FIG. 22 illustrates how one of the fastening nuts 18 is molded in situ in the outer major wall 11 of the hollow back portion HBP. The mold half M1 is locally bulged to provide an enlargement 34 formed with a recess or bore 35. A positioning pin 36, fitted snugly in the recess 35, has a reduced diameter end generally designated 37 providing a shoulder 38 at its juncture with the larger diameter part of the pin. The pin 36 is inserted into the recess 35 with a moderate friction fit so as normally to remain in the bore 35, but to be removable for replacement if damaged. When the mold is open the nut member 18 is slipped onto the reduced diameter pin portion 37, the crests of the nut threads clearing the pin portion 37 just enough to prevent binding while still maintaining the nut substantially axially aligned with the pin 36. The mold is then closed and the parison P blown into molded form as previously explained. During the blow-molding operation the parison material will surround the part of the pin 36 extending beyond the bulged mold enlargement 34, the nut 18 and the reduced diameter part 37 of the pin 36. Pressure exerted by the plastic against the outer face 39 of the nut 18 due to blowing forces the nut against the shoulder 38. When the mold is opened, the pin 36 will remain in the mold half M1, and will be withdrawn from the nut 18 and the surrounding blow-molded material. This will leave the nut 18 securely embedded in the mold outer major wall 11 of the hollow back portion HBP. Removal of the pin 36 from the nut 18 leaves an unthreaded opening 41 in the molded material beyond the nut 18. The wall of the opening 41 has a diameter less than the diameter of the roots of the nut 18 threads and substantially equal to the diameter of the nut thread crests.

FIG. 23 shows the attachment of the hollow back portion HBP to the chair frame CF. The upstanding post 4 of the frame shown in part in FIG. 23 is apertured to receive a bolt 42 which is screwed into the nut 18 to clamp the back portion HBP to the post 4 securely. The threaded end of the bolt 42 extends beyond the nut 18 and bites into the plastic molding at 43 so as to cut its own threads and thus lock the bolt against unintentional unscrewing. Other nuts 18 are similarly molded in situ in the outer major wall 11 of the back portion HBP and are similarly engaged by other frame-connected bolts 42. The nuts 27 previously referred to for fastening the hollow seat portion HSP to the frame are molded in situ in the outer major wall of the portion HBP similarly to the molding in of the nuts 18 described above. Bolts 44 cooperate with the nuts 27 for securing the hollow seat portion HSP to the parts 5, 5 of the chair frame.

FIG. 24 shows a modified hollow seat portion HSP which is molded to provide a groove 45 adapted to receive a projection 46 on a seat cushion 47 for holding the cushion in place on the seat portion HSP.

A blow-moulded unitary seat and back unit, exemplified by the unit SBU in the present disclosure, has a number of advantages and improvements as compared to previously known chair seat and back equipments. The hollow blow-molded forms of the seat portion HSP and the back portion HBP provide sufficient yieldability and cushioning effect for increased comfort, while still being strong and durable. Strength and durability are provided, inter alia, by the perimetral stiffness or reinforcement afforded the back portion HBP by the enlarged marginal hollow sections 15 and 16, and the bendable connecting portion BCP. The undulating peak-and-valley formation of the outer major wall 20 of the hollow seat portion HSP provides for relative stiffness of the lower or outer wall 20 of the seat and for thus firmly supporting the seat unit on the frame members 5, 5, while the grooves 28 and edge lips 29 in the bendable portion BCP provide for evenly resisted bending from flat, as molded, form to the angular form required for mounting the unit SBU on the chair frame CF. Very importantly, the bendable connecting portion BCP is sufficiently flexible to allow the seat portion HSP to flex laterally to conform to a slouched position of a sitter so that the sitter's buttocks still will contact and be supported by the seat, thus affording comfort through good weight distribution. Similarly, the flexibility of the bendable portion BCP also allows the lower part of the back portion HBP to shift or flex laterally to conform to a slouched position while still contacting the sitter's lower back or pelvic area, thus affording better support and comfort through weight distribution. In short, the formation of the unit SBU permits the flexing required for maximum comfort without, however, creating such stress, particularly in the waist area, as would be likely to cause structural failure, such as edge cracks.

Aside from the above mentioned advantages in the chair construction as such, there is a very important practical advantage in the molding of the entire unit BSU unitarily in a generally flat condition and then bending it to shape for mounting on the chair frame CF. The blow-molding of the unit SBU in generally flat condition permits the use of molding equipment which is less cumbersome, less heavy, and therefore less expensive than if the unit SBU were to be molded with the seat and back portions in such angular relation to one another as to be mountable on the chair frame. Additionally, the generally flat blow-molded unit SBU is compact and may be stored and/or shipped with considerable saving in space. The economic advantage is apparent.

The construction and method disclosed are preferred and are representative of the invention; but the disclosure is intended to be illustrative rather than definitive of the invention.

Claims

I claim:

1. A chair comprising a supporting frame; an integral blow-molded seat and back unit comprising a relatively non-bendable hollow seat portion, a relatively non-bendable hollow back portion, and a relatively bendable portion intervening between and connecting said seat and back portions, said relatively bendable portion being of the same material as and being substantially less in cross section than said relatively non-bendable seat and back portions; and means attaching said unit to said frame.

2. A chair according to claim 1 in which said bendable portion is solid, that is not hollow.

3. A chair according to claim 2 in which said seat portion includes opposed spaced inner and outer major walls, said back portion includes opposed spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner major walls from one to the other and an extension of said outer major walls from one to the other, said extensions being united to each other to provide said solid bendable portion.

4. A chair according to claim 3 in which said solid bendable portion is substantially twice as thick as the inner and outer major walls of said seat and back portions.

5. A chair according to claim 3 in which said solid bendable portion has a plurality of spaced grooves extending transversely with respect to the chair.

6. A chair according to claim 5 in which said grooves are in the part of said solid bendable portion constituted by the extension of said outer major walls of said seat and back portions.

7. A chair according to claim 5 in which said grooves extend close to but terminate short of the edges of said solid bendable portion respectively at the sides of the chair.

8. A chair according to claim 1 in which said seat portion includes opposed spaced inner and outer major walls, said back portion includes spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner walls from one to the other and an extension of said outer major walls from one to the other; and in which the outer major wall of said seat portion is of undulating peak-and-valley formation, the peaks and valleys extending transversely with respect to the chair.

9. A chair according to claim 8 in which said frame comprises two laterally spaced fore-and-aft extending seat portion supporting members; and in which said seat portion outer major wall has similarly laterally spaced fore-and-aft extending grooves respectively receiving said supporting members.

10. A chair according to claim 8 in which said peaks and valleys of said peak-and-valley formation extend close to but terminate short of the edges of said seat portion respectively at the sides of the chair.

11. A chair according to claim 8 in which, in an intermediate part of said seat portion fore-and-aft of said chair, said seat portion inner and outer major walls are united or at least touch one another and in which other parts of said seat portion inner and outer major walls are spaced from one another.

12. A chair according to claim 1 in which said seat portion includes inner and outer major walls, said seat portion inner and outer major walls being united or at least touching one another in a fore-and-aft intermediate part of said seat portion, said inner and outer major walls being spaced from and in non-contacting relation in other parts of said seat portion.

13. A chair according to claim 1 in which each of said seat and back portions comprises opposed mainly spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner major walls from one to the other and an extension of said outer major walls from one to the other; and in which the inner and outer major walls of said back portion are spaced from one another throughout most of the area of said back portion, said back portion inner and outer major walls being united in a relatively small area adjacent to but spaced from the free edge of said back portion opposite said bendable portion, thereby to provide a reinforcing box structure extending along said free edge of said back portion.

14. A chair according to claim 13 in which said extensions of said inner and outer major walls are united to each other to provide said bendable portion.

15. A chair according to claim 1 in which said means attaching said unit to said frame comprises an internally screw threaded member molded in the material of said unit in axial alignment with an opening in said material having a diameter less than the diameter of the screw threads at their roots and substantially equal to the diameter of said screw threads at their crests; and an externally threaded fastening screw extending through said frame, threaded into said internally screw threaded member, and projecting beyond the latter and into said opening, the threads on the projecting part of said screw biting into the wall of said opening and thereby locking said screw against unintentional unscrewing.

16. A chair according to claim 1 in which said frame comprises a rear upstanding back supporting part and two laterally spaced fore-and-aft extending seat portion supporting members; and in which both the seat portion and the back portion of said seat and back unit include opposed spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner walls from one to the other and an extension of said outer walls from one to the other; said seat portion being of undulating peak-and-valley formation with the peaks extending transversely with respect to the chair, the outer major wall of said seat portion having two laterally spaced fore-and-aft extending grooves respectively receiving said seat portion supporting members of said frame; the outer major wall of said back portion being grooved vertically to receive said frame rear upstanding back supporting part; and said bendable portion being flexible substantially in all directions, whereby to enable said seat portion to adjust in position so as comfortably to accommodate a sitter sitting in such position that the sitter's weight would not otherwise be evenly distributed on said seat portion, as when sitting in a slouched or non-erect position.

17. A chair according to claim 16 in which said chair frame is resilient.

18. A chair according to claim 16 in which the inner and outer major walls of said back portion are more widely spaced from each other in sections extending along the side and top edges of said back portion than in the area surrounded by said sections and said bendable portion.

19. A chair according to claim 16 in which said solid bendable portion has a plurality of spaced grooves in the part of said bendable portion constituted by the extension of said outer major walls, said grooves extending transversely with respect to the chair and terminating close to but short of the sides of said bendable portion, thereby leaving ungrooved marginal edge lips providing for evenly resisted bending of said bendable portion without localized sharp bending thereof.

20. An integral blow-molded seat and back unit for a chair, said unit comprising a relatively non-bendable hollow seat portion; a relatively non-bendable hollow back portion; and a relatively bendable portion intervening between and connecting said seat and back portions, said relatively bendable portion being of the same material as and being substantially less in cross section than said relatively non-bendable seat and back portions.

21. Seat and back unit according to claim 20 in which said bendable portion is solid, that is not hollow.

22. Seat and back unit according to claim 21 in which said seat portion includes opposed spaced inner and outer major walls, said back portion includes opposed spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner major walls from one to the other and an extension of said outer major walls from one to the other, said extensions being united to each other to provide said solid bendable portion.

23. Seat and back unit according to claim 22 in which said solid bendable portion is substantially twice as thick as the inner and outer major walls of said seat and back portion.

24. Seat and back unit according to claim 22 in which said solid bendable portion has a plurality of spaced grooves extending transversely with respect to said unit.

25. Seat and back unit according to claim 24 in which said grooves are in the part of said solid bendable portion constituted by the extension of said outer major walls of said seat and back portions.

26. Seat and back unit according to claim 25 in which said grooves extend close to but terminate short of the longitudinal edges of said solid bendable portion.

27. Seat and back unit according to claim 20 in which said seat portion includes opposed spaced inner and outer major walls, said back portion includes spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner walls from one to the other and an extension of said outer major walls from one to the other; and in which the outer major wall of said seat portion is of undulating peak-and-valley formation, the peaks and valleys extending transversely with respect to said unit.

28. Seat and back unit according to claim 27 in which said peaks and valleys of said peak-and-valley formation extend close to but terminate short of the edges of said seat portion respectively at the sides of said unit.

29. Seat and back unit according to claim 27 in which, in an intermediate part of said seat portion fore-and-aft of said chair, said seat portion inner and outer major walls are united or at least touch one another and in which other parts of said seat portion inner and outer major walls are spaced from one another.

30. Seat and back unit according to claim 20 in which said seat portion includes inner and outer major walls, said seat portion inner and outer major walls being united or at least touching one another in a fore-and-aft intermediate part of said seat portion, said inner and outer major walls being spaced from and in non-contacting relation in other parts of said seat portion.

31. Seat and back unit according to claim 20 in which each of said seat and back portions comprises opposed mainly spaced inner and outer major walls, and said bendable portion is constituted by an extension of said inner major walls from one to the other and an extension of said outer major walls from one to the other; and in which the inner and outer major walls of said back portion are spaced from one another throughout most of the area of said back portion, said back portion inner and outer major walls being united in a relatively small area adjacent to but spaced from the free edge of said back portion opposite said bendable portion, thereby to provide a reinforcing box structure extending along said free edge of said back portion.

32. Seat and back unit according to claim 20 in which said seat and back portions both include opposed spaced inner and outer major walls and said bendable portion is constituted by an extension of said inner walls from one to the other and an extension of said outer walls from one to the other, said inner and outer major walls of said back portion being more widely spaced from each other in sections extending along the side and free end edges of said back portion than in the area surrounded by said sections and said bendable portion, whereby to provide more stiffness in the periphery of said back portion than in portions of the central area thereof.

33. Seat and back unit according to claim 31 in which said extensions of said inner and outer major walls are united to each other to provide said bendable portion.