The processing of deformable materials generally involves the transformation of a starting material (i.e., in a solid state or a liquid state), which is in a random form (e.g., powder, beads, granules, pellets, etc.), into a final or intermediate product having a specific shape, dimensions and properties. Processes useful in the transformation of deformable materials from their initial random form to the form of the final or intermediate product are well known to those skilled in the materials processing industry. For instance, if the deformable material is a plastic, examples of such plastic transformation processes include, but are not limited to: extrusion, transfer molding, calendaring, laminating, thermoforming, injection molding, compression molding, blow molding and the like. As used herein, such transformation processes and/or operations are collectively referred to as "molding" processes. Similarly, the resulting final or intermediate product is referred to as "molded", regardless of the specific transformation process employed in its manufacture. The materials processing industry is abundant with teachings in this field of technology.
Most of the conventional molding processes include at least the following steps: (a) transporting an unmolded, deformable material to a molding device, (b) heating the unmolded, deformable material until it can be deformed to take the geometric configuration of the mold or die, (e) shaping the heated material to the geometric configuration of the mold or die to form a molded product, and (d) cooling the molded product. These steps can be done either in sequence or simultaneously or a combination of both.
In order to produce molded products having a specific geometric configuration, it is generally necessary to employ a mold or die. The primary objective of a mold or die is to shape the deformable material introduced therein. Sometimes, molds and dies have a secondary objective, this being to cool the deformed material therein until it is able to maintain its shape when the molded product is withdrawn therefrom.
The physical properties of a molded product depend, in part, upon the specific molding process conditions and steps employed. It has been observed that different molding processes will often result with the final or intermediate products having different physical properties. For example, the amount of shear stress in a molded product determines, in part, the degree of molecular orientation and crystallization within the molded product. This, in turn, has an affect on the molded product's physical properties.
Since there is a need to be able to produce molded products which have physical properties within particular ranges, if a means can be devised for controlling at least some of these physical properties (e.g., by controlling the degree of shear stress) both the process and the product resulting therefrom will be greatly welcomed in the molding industry.
One method of controlling the amount of shear stress in a molded product (and thereby controlling some of the product's physical properties), is commonly referred to as "flow technology". The concept of "flow technology", as it relates to plastic molding processes, is concerned with the behavior of a deformable plastic material before, or while, it is being introduced into a mold and/or being passed through a die.
It has been discovered that the properties of a final or intermediate molded product depend largely upon how the deformable material flows prior to, and/or while, being subjected to a molding process. For example, two products, having identical dimensions and made from the same basic starting material, but which are molded under different conditions (e.g., different hydrostatic pressures and/or shear stresses) and subjected to different flow patterns, will probably have different physical properties.
This phenomena is due, in part, to the fact that, as a deformable material flows prior to, or while, entering a mold or passing through a die, it is subjected to a shear stress which is commonly referred to herein as "flow shear stress".
Flow shear stress induces molecular orientation in the plastic material (i.e., it results in the macromolecules aligning themselves in the direction of flow). The flow shear stress varies from a maximum level at the outside surface of the flowing deformable material to a minimum level at the center where the material is last to cool.
From the above it can be seen that the manner in which the deformable material flows into a mold or through a die, or prior to being subjected to a specific molding process, is of extreme importance in determining the physical properties of the final molded product due, in part, to the degree of flow shear stress which will be imparted thereto.
If a means can be devised for controlling the degree of molecular orientation resulting from flow shear stress, it would be greatly welcomed in the molding industry since it will enable the manufacturer to have a greater degree of control over the product's final properties.
As is well known in the molding industry, during the compensating phase of a typical injection-molding process, the flow of a deformable material into the mold is generally unstable due to the flow occurring in "rivers" which spread out in a delta-like manner. The first material to freeze off shrinks early in the cycle. By the time the material freezes in these "rivers", the bulk of the material is frozen up and the shrinkage has already occurred. Therefore, the rivers shrink relative to the bulk of the molded article.
Since the rivers are highly oriented, shrinkage can be very high. This, in turn, can result in high degrees of stress inside the molded part which can, for example, be a source of warpage. Accordingly, if a means can be devised which reduces the degree of shrinkage from the "rivers" and, thus, reduces the degree of warpage in the final product, it would also be greatly welcomed in the molding industry.
It has also been discovered that the micro structures and the morphology of a molded product (e.g., molecular orientation, degree of crystallinity, etc.) are greatly influenced by the thermo-mechanical history experienced by a deformable material during its molding process steps. And, as can be expected, the ultimate properties of the molded product are closely related to the deformable material's morphology and micro structure.
Specifically, according to U.S. Pat. No. 4,469,649, which is incorporated herein by reference, the control of a material's transformation process, from its random form to its final molded form, can be made at least partially dependent upon the rheological properties of the plastic material as it is subjected to specific molding techniques.
If a means can be devised to control the micro structures and the morphology of a molded deformable product, it would be greatly welcomed in molding industry.
As can be seen from the above, while molded products (e.g., plastics) play a significant role in our daily lives, and are expected to play an even more important role in our future, there are many problems in the manufacturing of such products which still remain unsolved.