Furniture

Plastic office furniture, principally just for chairs, panels, and laminates for desks, represents about a $10 billion market or just 3l/2 wt% of all plastics. Unreinforced and glass reinforced PPs are principally used. Lawn and garden furniture consumes about 2xh wt% plastics also principally using PPs. All these products as well as others have a good growth rate.

An exercise follows in designing and fabricated furniture racks. Their function is to serve as a support that can hold several objects in a desired location for storage or for display. Unless a more specific function is defined, the rack can take on a wide range of possible shapes, structures, and materials subjected to very little damaging loads; racks can fall, be twisted during relocations, and so on. One type of rack is a bookshelf compatible with the environment found in a library, suitable for holding five or more books per foot of shelf in a certain location, and for constant reference use rather than for storage.

In setting requirements for this type rack factors to consider includes defining the environment, set load levels and type of loading situation, and with the library environment provide some idea of the possible aesthetics of the rack. A wide range of design choices remain such as to the material to be used, structure, and shape but they would be limited to those normally used in a library environment. The more accurately and completely the function is defined, the more restricted are the design possibilities and the more detailed the specification requirements become.

This product has to fit its function within the confines of the space in which it is used that could involve how many books it will hold or by stating the size of the supporting rack that will be used. The size can then be decided either by burden or by space restrictions. There will be those where one or the other of these consideradons will apply. Also both may apply.

The width of the shelf may be determined by the width of a book. In this example it ranges from 6 to 11 in. (15 to 28 cm). The typical bookshelf is supported at 3 ft (0.9 m) intervals so that the shelf would be sufficiently wide to fit a typical book rack in the library. The shelf will hold about 5 books per foot with each having an average weight of 2 lb (0.91 kg). Maximum load on the shelf becomes 30 lb (13.6 kg). If the shelf were completely filled, it could be considered a distributed load, or it can be considered as a set of discreet loads.

The type of shelf design is another consideration. The shelf can be a solid plate of plastic material, an inverted pan-like structure with reinforcing ribs, a sandwich-type structure with two skins and an expanded core, or even a lattice type sheet that has a series of openings. The choice is usually dictated by a number of factors. An important one is appearance or aesthetics.

The design includes developing its appearance that is to match its performance. There are lattice-type shelf, hanging solid structures from the ceiling, etc. They may all be functionally but may not look appropriate for a bookshelf in a library. A simple plastic beam that will function adequately in terms of strength and stiffness may be rather thin. A shelf of this type can look weak even if it is more than functional. However the solid plate approach results in an uneconomical use of material.

One of the requirements included that it look like a wood shelf to match the atmosphere of the library. To produce the desired thickness appearance using plastics either a lipped edge with internal reinforcement can be used or a sandwich-type structure (Fig. 4.10). In either case the displacement of the material from the plane of bending will improve the stiffness efficiency of the product.

In the materials selection properties to be considered or analyzed are static tensile and compressive strength, stiffness (modulus), impact properties, temperature resistance, expansion during temperature changes, and even dynamic properties. In this study the only major concern is the long-term stiffness since this is a statically loaded product with minimum heat and environmental exposure. While some degree of impact strength is desirable to take occasional abuse, it is not really subjected to any significant impact. Using several materials such as PP, glass-filled PS, and PS molded structural foam sandwich type panel material, the design procedure follows to determine the deflection and stress limitations of the material in the several designs.

From a load carrying requirement the rack is to meet a maximum deflection and relate it to the cost of fabrication (material, labor, amortization costs of equipment including tools, and the usual plant costs of overhead, etc.). If the designer does not have this type experience it is best done with the assistance of a reliable fabricator who potentially will make the product.

The various designs and costs can be tabulated and the ones that are the most economical can be determined. It may become evident that the design life can be long and the cost of increasing the design life small, or, alternatively, it may be that the cost of small increments in the design life are quite cosdy. In the latter case, the design life should be limited to the acceptable minimum.

A value judgment can be made as to the product quality requirements and the final design made to meet requirements. It may included reinforced areas, coating or plating, inserts, and so on. With the product dimension a performance evaluation can be conducted. The colored plastic and fabricating process is selected.

In tliis evaluation what is being presented appears unwanted complications that are not needed for such a simple design. However when going into a complicated design one can apply this basic approach to the more complicated designs. Thus the next move is to produce prototypes to ensure that the product will meet performance requirements (Chapters 2

and 3). Prototypes made by machining or other simplified model making techniques do not have the same properties as the product made by molding or extrusion or whatever process is to be used. Simplified prototype procedures may reduce trial mold cost and produce adequate test data in some cases. Main value in this study is appearance and feel to determine whether the aesthetics are correct. Any testing has to be done with considerable reservation and caution.

Following testing of the prototypes for appearance and structural integrity field is made to ensure all is well. Locations and individuals selected require closely monitored or controlled environment. Perhaps a requirement was overlooked such as learning that the library shelves are often cleaned with a solution that disintegrates the plastic being used requiring a new analysis. Unless there has been a serious misjudgment by the product development designers, the field tests do not lead to redesign. The results of the field-testing can help define the basis for labeling and the instructions to be used with the product so that it can properly operate in service.

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