What’s method about polyurethane injected mold for in-line skate wheels?
Since the invention of the first pair of in-line skates in the early 1980's, in-line skating is rapidly increasing in popularity and is successfully competing and co-existing with traditional roller skating. In fact, in-line skating has become so popular that nearly a worldwide market now purchases in-line skates. To this end, the in-line skate industry is continually developing new skates to meet the consumer's demands for lighter, faster skates.
Wheels of in-line skates are a critical component which manufacturers constantly strive to improve. Early wheels were manufactured of high friction material which resulted in prohibitively slow wheels. The industry quickly started producing the wheels from materials which had a lower coefficient of friction against the pavement and thereby created faster wheels.
In addition to demanding faster wheels, in-line skaters have another demand. In order to be suited for in-line skating, the wheels must contain a certain amount of rebound. One subjective test for rebound is to bounce a wheel on a hard surface such as a table top. If the wheel bounces, it may have desirable and suitable rebound properties for use in an in-line skate. Rebound is necessary to absorb the unevenness of pavement, loose pebbles and gravel which a skater may encounter when skating. The rebound of the wheels in part acts as a "shock absorber" for the person wearing the skates.
Currently, in-line skate wheels are produced only by the method of casting or pour molding from thermo set polyurethane. Thermo set polyurethanes are characterized by rebound suitable for use in in-line skates, however, thermo set polyurethanes are not injection moldable. Two other problems are associated with thermo set polyurethanes for in-line skate wheels. First, the material requires a 3 minute cure time which slows the manufacturing time of in-line skate wheels and raises manufacturing costs. Second, thermo set plastics are not recyclable once they have cured. That is, once a thermo set plastic has set it cannot be melted and recast and it is therefore not environmentally sound to use thermo set plastics.
Several other problems are associated with cast molding of in-line skate wheels. First, the process of cast molding is very labor-intensive. Second, cast molded wheels require machining after the wheels are molded to remove excess material. Third, besides temporal constraints, quality control problems also exist with cast wheels because air bubbles are often trapped in the cast-molded wheels. Air entrapped wheels obviously must be discarded or scrapped thus increasing production costs and creating non-recyclable waste.
In addition to manufacturing problems, the currently available wheels are also inferior because adhesives are routinely used to bond the tread to the hub. With time and use, the adhesive breaks down causing the tread portion of the wheel to pull away from the hub. If this occurred while a skater was skating on the wheels, such an event could injure a skater.
A desirable method of manufacturing in-line skate wheels would be characterized by a method faster than the currently used cast-molding method. Such a method would also eliminate quality control problems such as air bubbles in the wheels, and further would create a safer wheel by eliminating the hub portion separating away from the tread portion. Such a method would additionally result in polyurethane wheels having necessary rebound.
Until now, it has been impossible to injection mold acceptable in-line skate wheels made of polyurethane. Many wheel manufacturers have unsuccessfully attempted to injection mold in-line skate polyurethane wheels . The thermo set polyurethane material which is currently used to produce the wheels is not suitable for injection molding. Conversely, material which is injection moldable lacks the rebound necessary for in-line skates. Thus, all attempts to injection mold in-line skate wheels have failed.
The present invention is aimed at solving the foregoing problems and it is an object of the invention to provide a method to injection mold in-line skate wheels of polyurethane. The inventive method is quickly completed and produces in-line skate wheels in about 50 to about 60 seconds which is a significant advance over 3 minutes required for cast molding in-line skate wheels.
In one embodiment, the present invention is a method of producing injection molded in-line skate wheels by the steps of heating an injection molding device suitable for injecting molten polyurethane. The injecting nozzle is heated to about 380° to about 460° F. A polyether type polyurethane is heated to between about 380° to about 460° F. A polyurethane hub is preferably placed into a mold which is suitable for molding in-line skate wheels. The hub-containing mold is heated to between about 50° to 110° F. before about 46 to about 50 g of polyurethane is injected into the mold at about 75 to about 100 psi. The polyurethane is then cooled to between about 120° and 130° F., and the molded polyurethane, which is in the shape of an in-line skate wheel, is removed from the mold.
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