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Selected Vibration Related Processing Variables
To study the effects of vibration-assisted injection molding, six parameters related to the imposed vibration were selected as follows: Delay time to begin vibration, vibration duration, vibration frequency, intracycle compression duration, intracycle decompression duration, and duty cycle. The first three of these parameters are self-explanatory. ‘‘Delay time to begin vibration’’ is the amount of time between the start of injection and the start of applied vibration. The vibration duration is the amount of time that the applied vibration continues once it is initiated, and lastly,the vibration frequency defines how many compression and/or
decompression cycles are applied per second while the vibration
is ongoing. The intracycle compression and decompression durations correspond to the amounts of time that the injection screw forward valve and injection screw backward ~decompression! Are activated in each mode during each vibration cycle. The duty cycle is determined basically by the ratio of the screw forward to the screw backward duration per cycle. The latter three parameters combine to indirectly control the amplitude of the screw oscillation.
Experimental Design
Materials and Product Selection. The material type utilized throughout the study was polystyrene (PS). The specific new or virgin material used was STYRON 666DW produced by Dow Corporation. This material has an average molecular weight of approximately 230,000 g/mol and a claimed tensile strength of 44.8 MPa. For a recycled material, PC-9000, which is a very low-grade recycled PS available from Polystyrene Recycling Company of America (PRCA), was selected. This recycled material has an average molecular weight of approximately 100,000g/mol. During the study, the two materials were processed inpidually and in the form of two blends. The two blends selected possessed virgin to recycled weight based mixtures of 75/25 and 50/50, respectively.
The injection molded parts produced and tested during study were ASTM D638 Type 1 specimens. Two pressure transducers were installed, roughly centered in each grip section of the ASTM tensile specimen cavity, to monitor the pressure changes during the cycle due to the vibration energy applied by the injection screw.
The focus product quality characteristic on which the study was based was the ultimate axial tensile strength of the specimens produced. This parameter was determined by performing standardized tensile tests on a Measurement Technology Inc. (MTI)Phoenix mechanical testing machine. During all mechanical testing, the specimen elongation rate imposed was 5 mm/min and the ambient temperature was kept at 23°C.