1. Field of the Invention
This invention relates, in general, to wheels and methods for their manufacture thereof, and more particularly, relates to injection molded plastic wheels.
2. Description of Related Art
The use of plastic wheels on a variety of products has dramatically increased in recent years. One area where such plastic wheels have been widely employed is on wheeled refuse or garbage carts or bins of the type commonly employed by homeowners for their trash. Typically, these carts are formed of plastic or other materials and have injection molded, blow molded, rotational molded, or compression molded plastic wheels which are mounted on the ends of a metal axle by a hub member or push-on hat fastener (sometimes referred to as a “pawl nut”). The wheels can also be secured to pre-machined axles with an integrated snap-on lock.
These carts, and the plastic wheels thereon, are typically used in rugged applications and/or in harsh environments. For example, the carts often carry chemical waste and/or heavy loads. The wheels thus may be subject to strong forces and exposed to harsh environments. As such, the wheels for such carts have been manufactured from resilient plastic materials that are able to withstand high forces and maintain a “clean look” after repeated exposure to dirt, grime, trash, chemicals, and other materials that tend to tarnish and degrade the surface of metals. Molded-in plastic dyes also provide a better choice over painted metals in such harsh environments. As such, plastic has become the material of choice over metal and other materials.
Cost-efficiency has also played an increasing role in wheel design. For refuse carts and such other carts that are not carrying fragile materials or passengers, it is preferable to use a single material. The additional step of combining materials (e.g., placing a rubber tread on a plastic wheel) adds complexity, cost, and cycle time to the manufacturing process. Nonetheless, consumers desire a low-cost wheel with ornamental details in addition to functionality.
Performance characteristics also drive wheel design. Manufacturers' desire to reduce costs must be balanced with users' increasing demands for reliable products that maintain a superior look and finish in the face of exacting conditions. As a result, there is a desire to design wheels that achieve the same performance and appearance characteristics but at a lower cost.
Prior methods include blow molding a wheel resembling a tire with tread. The blow molding process allows for manufacturing a product with an intricate pattern and smooth surface. Because blow-molded products generally do not have internal bracing and generally lack superior strength, the blow-molded design may be modified to have curvature or projections instead of planar faces. An example of such a design is shown at U.S. Pat. No. 6,464,305 to Markling. Using both injection and the blow molding process, an ornate wheel may be produced with a separate injection molded tread piece stretched about the outer flange of a preform blow molded rim.
Moreover, blow-molded wheels may lack strength in the hub, outer rim of the wheel, and/or the web portion between the rim to the hub. In general, the outer rim has the important role of stabilizing the wheel when impacting the ground. As such, the outer rim preferably has high-impact strength yet cushions the wheel as it rolls along the ground. Similarly, the intervening structure between the rim and the hub must support the rim. Preferably, the intervening structure is shaped to mimic spokes or create a pattern or design. It also must have good lateral strength.
Prior injection-molded wheels lack the appearance of a wheel having a tire that can be produced with blow-molding. U.S. Pat. No. 3,888,545 to Braun shows an injection molded wheel with projecting fins on an inner face to add additional strength and a tire-like design with little additional material usage. The fins on the outer rim appear as a solid rim with channels cut out, which decreases material usage. The design illustrated by Braun, however, raises several design and processing concerns. Over time, narrow channels on the outer surface of the wheel generally trap dirt and other materials thus detracting from the appearance of the wheel.
The projecting channels may also present manufacturing concerns. At the junction between the nominal wall of the product and the projection there is an increase in thickness. Thus, there is a risk of sinking as the thin ribs cool faster than the surrounding material. Sink marks can be unsightly, especially in comparison to the clean, flat surface of a blow-molded product. Projecting fins also increase the risk of molded-in residual stress, which negatively affects the performance of the wheel.
Sinking may be minimized using packing, gas assist, foaming, and other methods, but the most effective method of reducing the risk of sinking, deformation, and molded-in stress is to maintain proportionality and uniform wall thickness throughout the part. When using ribs, it is thus preferable to avoid long and tall shapes with minimal thickness like the wheel design taught by Braun. Such a design likely leads to heat deflection and other problems detracting from the overall appearance of the product.
U.S. Pat. No. 6,095,543 to McMahon et al. shows a front wheel having an annular rim with an integral wheel web. The wheel web includes a multiplicity of ribs forming recesses on each side of the wheel. The recesses are pattern-shaped and extend transversely on each side of the wheel to form an outer face of the rim. Although these ribs may form a pleasing design, they also lead to the trapping of unsightly dirt on the face of the wheel. Additionally, a user viewing this wheel does not see a pleasing smooth surface that is typically associated with blow molding.
Thus there is a need for a wheel having favorable performance characteristics and a pleasing outer appearance. What is needed is a wheel with ornamental details resembling a conventional tire-mounted-on-a-wheel configuration with the performance of a reinforced injection molded wheel. Further, the wheel preferably has a design minimizing the risk of manufacturing defects. Specifically, what is needed is a high-performance injection molded wheel that has the visual appearance of a blow-molded product from the outside.