Thermal Growth Compensation And Mounting For Plastic Dishwasher Tub

Abstract

The lower sidewalls of a unipartite, plastic dishwasher tub are formed with integral ledges or horizontal walls which perform a dual load support function, viz. that of supporting the tub on a base assembly and providing a lower dishwasher rack support track. Compensation for thermal growth of the tub walls inwardly of the wash chamber is achieved by incorporating a deliberate, outward bow into the tub walls. This is accomplished, in the case of the lower, top and back walls by molding the same with an unstressed outward bow while the unstressed, planar sidewalls are deformed mechanically.

Description

BACKGROUND OF THE INVENTION

The invention is directed to large capacity front loading dishwashers of the type normally comprising a permanent installation and employing upper and lower dish supporting racks as opposed to the smaller, low capacity, portable and/or top loading dishwashers. The reason for the distinction is that the ultimate objective of the invention is the employment of a unipartite plastic tub in large capacity, front loading dishwashers and while some of the problems presented by a proposed use of such a tub would be common to dishwashers of either type; the majority are distinct. Typical distinctions relate to increasing wall flexibility with increasing area, the absence of encompassing metal housings, greater tub shape complexity and additional interior tub mountings such as dual rack support structures. Accordingly, the ensuing background discussion relates primarily to front loading dishwashers of the type normally adapted for a permanent, under cabinet, installation.

The high temperature, corrosive environment to which automatic dishwasher chambers are subjected dictates that the surfaces of those internal components in contact with such environment be either highly resistant, or immune, to corrosion. Corrosion resistant metals such as stainless steel represent a substantial cost factor in this highly competitive appliance field and the long term industry trend has been toward the use of inert materials to resist the corrosive effects of the chamber environment which are magnified adjacent the usual metal tub seam welds. Insofar as the tub itself, representing the far greater internal surface area exposure, is concerned; the general trend in the United States has been toward the application of an adherent plastic coating to the tub interior while European manufacturers have gone more to the use of plastic tub liners. In either event, the basic structural and design parameters that have prevailed since the advent of the modern front loading automatic dishwasher remained valid. This for the reason that overall dishwasher design parameters, both as regards utility and structural integrity, are virtually unaffected by whether the conventional metal tub is coated, lined or exposed.

Conceptually, the use of a unipartite plastic dishwasher tub, i.e., one not requiring a more rigid backing or outer support structure, represents a substantial advance in the art. Exemplary are the greatly decreased cost factors of a molded plastic tub vis-a-vis that of a metal tub requiring a further coating or lining operation; reduction in finishing labor requirements and particularly tub welding operations; longer tub life due to corrosion resistance which is independent of faulty coating procedures or chipping; decreased thermal and acoustical transmission thus minimizing the need for separate insulation; and a lesser inventory requirement for separable components susceptible of integral molding with the tub.

The reduction to practice of such a concept, as applied to front loading dishwashers on an assembly line basis, however, present a myriad of problems which arise as a requirement for different approaches to those engineering considerations which had previously been predicated upon the use of a metal tub. The distinctions are primarily those associated with material strength, the unusually high temperature environment to which the tub is subjected and considerations relating to plastic molding procedures. Thus, while the structural integrity of a metallic box beam shape defined by the conventional front loading tub is more than sufficient to resist excessive wall deformation as a function of normal loading forces as during installation, adjustment or use and temperature variations from room ambient to approximately 255.degree. F.; such normally applied loading forces would, in the absence of appropriate compensating structure, produce unacceptable wall deformation in a like configured plastic tub. The use of strand-like reinforcing fillers such as fiber glass would, seemingly, ameliorate the overall problem of wall deformation. In actuality, however, the presence of such reinforcing fibers not only effect tub discoloration as discussed below, but their random distribution precludes an accurate prediction of thermal growth characteristics which is an essential parameter in the design of the necessary compensating structure herein discussed. Similarly, impact strength is normally of little moment in the design and installation of metallic tubs but is a matter of major concern in plastic tub design. Furthermore, various structural features appearing in a formed plastic tub as an incident of the forming operation create problems not previously associated with metal tub fabrication. Exemplary of the latter distinction are the presence, on the finished product, of mold lines and/or mold retention ribs creating localized areas of decreased impact resistance as well as the usual wall draft to permit removal of the product, or tub, from the forming mold. One primary disadvantage arising from this necessary wall draft concerns the mounting of the upper dishwasher rack supporting tracks in parallel.

In addition to the foregoing considerations which relate primarily to mounting and/or assembly procedures for large capacity front loading machines, the desirability of precluding tub discoloration and the desirability for firewall separation of the tub and those electrical components constituting a fire hazard are common to all automatic dishwashing machines. As regards plastic tub discoloration, it is preferred to avoid the use of certain conventional strand type reinforcing fillers, such as chopped strand fiber glass, for the reason that their intersection with the interior plastic surface may provide a "bleed" path for food stains to permeate the tub structure.

The prior art suggestions relating to the use of small capacity plastic tubs of the top loading type, as in U.S. Pat. No. 2,691,986 and the use of separate plastic tub components, as in U.S. Pat. Nos. 2,877,778 and 3,385,306 offer little in the way of guidelines as regards a practical reduction to practice of a large capacity front loading tub for the reason that most of the problems requiring solution are not present in smaller and/or top loading units and particularly those which employ an enveloping metal housing.

The present invention is concerned with constructional and mounting details to compensate for thermal growth of the relatively large area tub walls as well as an integrally molded tub wall configuration providing common wall means for the support of the tub and lower dishwasher rack while other of the aforementioned matters are more fully described and claimed in the following, commonly owned, copending applications:

Ser. No. 319,556, filed Dec. 29, 1972, for Double, Reversed Mounting Boss for Plastic Tub;

Ser. No. 319,201, filed Dec. 29, 1972, for Fill Funnel Construction for Plastic Tubs;

Ser. No. 319,203, filed Dec. 29, 1972, for Front Loading Dishwasher Employing Plastic Tub Construction;

Ser. No. 319,348, filed Dec. 29, 1972, for Unit-Handled Roller Assembly for Plastic Tubs;

Ser. No. 319,202, filed Dec. 29, 1972, for Radiused Mold Walls for Plastic Tubs;

Ser. No. 31,368, filed Dec. 22, 1972, for Angled Levelling Foot for Domestic Appliance;

Ser. No. 319,200, filed Dec. 29, 1972, for Torsionally Reinforced, Skeletal Support Frame for Plastic Tubs;

Ser. No. 319,557, filed Dec. 29, 1972, for Impact Protection for Plastic Tub Employing Mold Retention Ribs;

Ser. No. 319,199, filed Dec. 29, 1972, for Plastic Tub Wall Alignment for Dishwasher Rack Mounting.

Elimination of the conventional front loading metal tub and the substitution therefor of a plastic tub necessitates the presence of mounting and/or support structure to compensate for those inherent characteristics previously available in the use of a metal tub which include a modulus of rigidity sufficient to preclude wall deformation and provide secure anchor points for component mountings and whose "box beam" tub shape insures adequate resistance to external loading forces. It is apparent that if the advantages of the invention, as regards decreased cost and weight, are to be retained the necessary mounting and support structure must not, in effect, comprise a confining tub support housing. Rather, a purpose of the invention is to provide an operative, front loading dishwasher unit having a plastic tub whose outer walls comprise the great majority of the outer dishwasher unit construction and would actually be exposed to view prior to installation of the dishwasher unit within an outer decorative housing, which decorative housing may comprise a free standing, portable unit or a permanent undercabinet installation.

SUMMARY OF THE INVENTION

It is apparent that the relatively large surface area tub walls having an exemplary wall thickness of less than 0.2 inch will have a tendency to undergo substantial thermal growth as a function of temperature variations to which the wash chamber is subjected during a normal wash cycle. The tendency, of course, is for the walls to deform inwardly as a function of increasing temperature gradient across the tub walls in response to increased wash chamber temperatures. The effect of such temperature gradient, i.e., the temperature differential across the tub wall, is heightened by the fact of the plastic composition which is a poor thermal conductor.

Substantial, inward tub wall deformation cannot be tolerated for reason of obvious interference with moving dishwasher racks and the rotating spray bar and the compensation for this thermal growth factor constitutes one of two primary objects of the invention. The second objective relates to the use of a common integrally molded tub wall portion, on each tub sidewall, to support the tub on its base and provide the lower dishwasher support tracks. Secondarily, these integrally molded portions interfit with, and are backed by, the metal base assembly to preclude parts warpage and insure parallelism of the tracks.

It has been found that the requisite structural integrity can be imparted to the overall dishwasher and tub construction through the use of a metal base assembly and a relatively narrow (as compared to the tub depth) metal collar upstanding from the base assembly and extending across the top and sidewalls of the tub immediately adjacent the open end thereof. The front, open end of the tub is then secured to the surrounding, composite "box beam" collar and base assembly thus anchoring the open end of the tub to a relatively rigid metal shape. The savings in fabrication, cost and weight involved in bending a narrow metal strap into an approximate U-shape to form a collar and then joining the same to a base assembly as compared with the utilization of a complex metal shape to either form the tub or envelope a tub liner are obvious.

The tub itself is a unipartite, injection molded tub which is homogeneous throughout and thus exhibits no surface exposed, reinforcing fibers along which food stains could permeate the tub structure. more specifically, the tub is comprised of a homogeneous, unipartite, polypropylene based plastic.

Inward wall deformation may be precluded by forming the tub walls with an initial, unstressed outward bow so that thermal expansion of the plastic results only in a minimal increase in the outwardly bowed walls. This approach is entirely feasible insofar as the lower, top and back walls of the tub are concerned; however, the formation of preformed bows in the sidewalls is not readily accomplished by reason of the particular molding procedures employed in the one piece tub molding herein disclosed. This, broadly, for the reason that the multipart, contractible mold core used to form the tub is contracted vertically to clear the lower mold core part from the complex bottom configuration while the transverse dimensions of the tub forming mold core remain constant throughout formation and removal of the tub. Thus the mold core sides used to form the tub sidewalls do not contract and product removal takes place through surface to surface sliding contact between the tub sidewalls and mold core rendering impractical the integral formation of outwardly bowed sidewall portions. As to the remaining tub walls, i.e., the lower, top and back walls, product removal does not involve sliding surface to surface contact inconsistent with the formation of slightly bowed walls.

Although it is thought that for the purposes of the present invention; the foregoing brief description of the reasons why the tub sidewalls are planar while the remainder of the tub walls include an integral, unstressed outward bow will suffice; reference may be had to the aforementioned copending application Ser. No. 319,202 for a more complete description of the molding procedures.

The unstressed planar sidewalls are mechanically deformed, outwardly, to achieve substantially the same result as with the preformed wall bow by applying a deforming stress through the intermediary of separable fasteners anchored in the mounting collar.

The tub sidewalls are stepped inwardly adjacent the lower edges thereof to define horizontal ledges whose lower surfaces rest upon a metal base assembly while the upper surfaces thereof provide the lower dishwasher rack supporting tracks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective of a front loading dishwasher employing a plastic tub;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a section taken along line 3--3 of FIG. 1 as the part would appear prior to the mechanical introduction of a wall deforming stress; and

FIG. 4 is a similar view illustrating the outward bow following application of the applied stress.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 is illustrated a front loading dishwasher 10 including a metal support frame 12 comprised of a base assembly 14 and a tub mounting collar 16 on which is supported a homogeneous, unipartite, plastic tub 18 as well as the remainder of the usual dishwasher components.

The opposite tub sidewalls 20, adjacent the lower edges thereof, are stepped inwardly to form ledges or horizontal walls 22 continuous with further downward extensions 24 of the tub sidewalls which merge with a lower tub wall 26. The role of each wall 22 in performing, in effect, a dual load support function on opposite surfaces thereof with the attendant saving in cost and assembly time as well as the avoidance of an inventory requirement for separable components, will be obvious from an inspection of FIG. 2. Thus the lower and upper surfaces 28, 30 of each horizontal wall 22 provide opposed load bearing surfaces respectively engaged by upstanding side panels 32 and wheels 34 on which the lower dishwasher rack 36 is mounted. The base assembly side panels 32 are interconnected by a bottom plate 38 underlying the lower tub wall and positioning the side panels in closely spaced relation to tub sidewall extensions 24.

Each of the upper, lower and rear tub walls are preformed with an unstressed outward bow as best illustrated by the outwardly bowed upper wall 40 in FIG. 1. A similar outward bow is mechanically induced in the sidewalls 20, which are preformed as unstressed planar walls, after the fashion illustrated in FIGS. 3 and 4.

It will be recalled from the foregoing discussion that it is not feasible to preform the sidewalls in the desired, bowed configuration; however, such sidewalls are preformed with integral mounting bosses 42 outstanding from the exterior sidewall surfaces to accept a threaded fastener 44 anchored in an offset portion 46 of the mounting collar 16. The spacing of offset portion 46 of mounting collar 16 relative to the initial unstressed position of tub wall 20 and mounting boss 42 (FIG. 3) is calculated in light of known thermal growth characteristics to induce, upon engagement of fastener 44 as in FIG. 4, an outward deformation or bow of sidewall 20, from the unstressed phantom line position, which is sufficient to preclude that inward movement of the same which would result from the expected thermal gradient across the unstressed planar sidewall 20.

While the background discussion of this specification is related primarily to front-loading dishwashers of the type normally adapted for a permanent undercounter installation as opposed to the type of dishwasher having a hinged top closure for top-loading, it should be emphasized that many such machines are commercially produced and purchased for indefinite use as a free-standing model adaptable at a later time by the owner to a built-in unit beneath a kitchen countertop to blend with the matching cabinet structure of the kitchen. Generally, the difference between the unit designed as a "convertible" and the unit to be immediately permanently installed is the inclusion of side, rear and top cabinet appearance panels to the basic unit structure as designed for permanent installation. Therefore, it should be understood that the present invention is specifically directed to the front-loading type of dishwasher structure whether of the generally lower-cost unit having no appearance cabinet enclosure or the more elaborate, temporarily portable unit designed for later modification for use in a fixed location.

Claims

I claim:

1. In a front loading dishwasher including a plastic tub having a generally rectangular cross-section, a tub support frame including a base assembly underlying the lower tub wall and a tub mounting collar upstanding from said tub support frame engaging the top and sidewalls of said tub adjacent the open front end thereof in backing relation thereto, the improvement comprising; horizontal wall means integral with a lower portion of each said sidewall defining opposed load bearing surfaces on opposite sides of each said horizontal wall means for respective support of a lower dishwasher rack on the upper of said load bearing surfaces and support of the tub on said base assembly on the lower of said load bearing surfaces; and said base assembly including spaced support panels having upper tub supporting edges in supporting engagement with said lower load bearing surfaces.

2. The dishwasher of claim 1 wherein the lower portion of each of said sidewalls extend inwardly to define said horizontal wall means and downwardly therefrom to merge with a lower bottom tub wall; and the upper and lower load bearing surfaces on said horizontal wall means being continuous with the inner and outer surfaces of said sidewalls, respectively.

3. In a front loading dishwasher including a unipartite plastic tub of homogeneous construction defining a wash chamber of generally rectangular cross-section, a tub support frame including a base assembly underlying the lower tub wall and a tub mounting collar upstanding from the front end of said tub support frame engaging the top and sidewalls of said tub adjacent the open front end thereof and remote from the closed tub back wall, the improvement comprising; at least the sidewalls of said tub being bowed outwardly, through stressed securing means anchored in said mounting collar a preselected distance which is sufficient to compensate for inward movement thereof resulting from thermal expansion during the operation of the dishwasher, whereby interference of said sidewalls with movable dishwasher racks and a rotating spray bar is avoided.

4. The dishwasher of claim 3 wherein said sidewalls include integral mounting bosses outstanding therefrom; and said securing means extending through said mounting collar and into said bosses to terminate short of the wash chamber.

5. the dishwasher of claim 4 wherein said lower, top and back walls include an unstressed outward bow.

6. In a front loading dishwasher including a unipartite plastic tub of homogeneous construction defining a wash chamber of generally rectangular cross-section, a tub support frame including a base assembly underlying the lower tub wall and a tub mounting collar upstanding from the front end of said tub support frame engaging the top and sidewalls of said tub adjacent the open front end thereof in backing relation thereto, the improvement comprising: horizontal wall means integral with a lower portion of each of said sidewalls defining opposed load bearing surfaces on opposite sides of each said horizontal wall means for respective support of a lower dishwasher rack on the upper of said load bearing surfaces and support of the tub on said base assembly on the lower of said load bearing surfaces and said base assembly including spaced support panels having upper tub supporting edges in supporting engagement with said lower load bearing surfaces, and at least said sidewalls of said tub being mechanically bowed outwardly a preselected distance which is sufficient to compensate for inward movement thereof resulting from thermal gradients across the unstressed, planar sidewalls during the operation of the dishwasher.