Blow Molding Integrated into Injection Molding
With gas-assisted injection molding, the cross-section of the void or bubble formed is severely restricted. For this reason, a new process that allows the bubble initially formed by gas injection to be enlarged further has been developed as part of a research project. Two versions of the process are available.
In the single-stage process, a preform geometry is formed first. The bubble is formed by injecting nitrogen. After a period of time during which the gas provides the holding pressure, the gas pressure is released. To permit inflation of the bubble, the cavity in the injection mold is equipped with a hydraulically actuated core pull. After or while the part cavity size is being increased, nitrogen is injected into the already existing gas bubble once again. The size of the bubble now increases as the wall of the initially formed bubble is stretched. During this stretching, the material in the wall is still in a thermoelastic for thermoplastic state. Gas pressure is maintained for several seconds until the part can be ejected.
A two-stage process is available as an alternative. This version involves transferring the gas-assist preform within the mold. Upon completion of the initial gas-assisted injection molding step, the mold is opened and the part is transferred to a second, larger cavity, for instance, by means of a rotary platen. While the mold is open, the temperature profile in certain regions of the molded part can be heated using infrared radiation to achieve the optimal temperature for stretching. After the mold closes, the preform is then blow molded to form the final part, while the next preform is already being produced in the other cavity.
Applications for the new process include, for example, fluid conduits and cable conduits for electronic devices, household appliances and automobiles as well as design elements for the furniture industry.
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