1. The Field of the Invention
This invention relates to joining plastics and, more particularly, to novel systems and methods for joining plastics of dissimilar grade.
2. The Background Art
In spin welding two parts together, also known as rotary friction welding, one part is typically held stationary, while the other part is rotated in relative rotation thereagainst. The heat generated by the friction between the two parts causes portions of both to melt. Accordingly, once relative rotation between the parts ceases, the melted material cool sand solidifies, leaving the parts chemically bonded together.
It has been discovered that the processes for friction welding or spin welding of polymeric resins of similar grade operate within certain parameters. For example, in conventional spin welding, with proper contact pressure, less than one full turn is required to generate the necessary heat and subsequent melting. These parameters however, are ineffective when resins of differing grades (i.e., cross-grade materials) are involved. The material with the lower melt point or higher melt flow index cannot maintain contact pressure or provide heat sufficient to melt the other material.
For example, it has been found that the typical 50 to 200 feet per minute of frictional travel between parts (e.g., 50 to 200 RPM on a 4 inch diameter part) is inadequate to obtain bonding in a cross-grade structural system. Likewise, it has been found that conventional interferences (required to provide friction and the cohesive material) are completely inadequate if the materials in the structure are selected from cross-grade resins.
Any friction-based welding of cross-grade components results in a preferential melting of the component having the higher melt flow index. That is, the component that melts at a lower temperature, or which flows easier, will preferentially melt, and may completely melt, before any substantial melting has occurred in the other component having a lower melt flow index.
Accordingly, a new set of operating parameters is required in order to make a friction-based or thermal weld using friction as the thermal mechanism. That is, for example, two materials that are chemically substantially the same, or closely similar in molecular structure may be melted in an injection molding machine or an extrusion machine, and form adequate components or parts. However, friction-based welding, and in particular, spin welding between a molded component (e.g. high melt index) and an extruded component (e.g. low melt flow index) has not been found to work previously.