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The Vibration-Assisted Injection Molding Concept
Vibration-assisted injection molding (VA I M) is based on the
concept of introducing mechanical oscillatory energy to the polymer melt by oscillating the injection screw axially during the injection and/or packing stages of the molding cycle. Conceptually,the applied vibration energy appears to affect the rheological behavior of the polymer. This, in turn, affects the morphological state throughout final products, thereby enhancing the final product performance characteristics. It is well known that the rheological behavior of a polymer is dependent on the pressure, temperature, and shear rate distributions that are present during processing. In general, shear stresses induced during the injection stage tend to relax during the packing stage. The material’s temporal rheological behavior along with the rate of cooling affect molecular orientation and relaxation processes, which in, essence,determine the final morphology throughout molded articles. The application of vibrational energy to the polymer melt during the
injection and/or packing stages enables some control over pressure and strain histories, which, in turn, result in altered rheological states. Therefore, it naturally follows that final morphological states would also be modified. In essence, the VAIM technique can be considered to be a traditional molding process with added real-time morphology control.
Experimental VAIM Capability Establishment
To study the effects of vibration-assisted injection molding experimentally, a facility was developed that applies mechanical energy to polymer melts during molding by oscillating the injection screw in a compression-decompression manner. This was accomplished by developing an open-loop hardware/software control system to actuate hydraulic valves appropriately and produce a forward-rearward motion of the injection screw. The detailed description for the development of the VAIM capability can be found in.
The actual injection molding machine utilized during the present study was a BOY 15S. In addition, a LVDT transducer was installed onto the machine to monitor the screw position during the cycle. A Reglomat RT20, closed-loop, hot-water thermolator was used to control the temperature of an ASTM standard tensile test mold with two Kistler pressure transducers embedded in the cavity. Type I tensile test specimens were produced in compliance with ASTM D638.
The experimental system was configured so that the VAIM controller would dictate the action of the machine hydraulic control valve during the injection and holding and/or packing stages of molding processes. To supply oscillatory pressure vibrations to the polymer melt, the VAIM controller simulates the manual operation of pushing and releasing the injection and decompression switches on the injection molding machine in an alternating fashion. Choosing to control the process in this manner enables the completion of vibration-assisted molding tests with constant stroke amplitude while varying other parameters, such as vibration frequency. The oscillatory mechanical action can be applied to the polymer melt in a manner specified by the user throughout the injection and packing phases of each molding cycle as explained in the following section.