Sensor concepts for polymer injection molding

Charles Landreth Thomas

doi:10.17918/00009530

In order to increase the economic competitiveness of injection molding, the process is analyzed to identify areas where either the process capability or efficiency can be improved. While several avenues for improvement are identified, it is noted that each possibility requires, or would be augmented by automated techniques to monitor both the state of the polymer during processing and final product quality. This is only possible with new sensor technology. Hence new concepts and techniques in a number of different sensing areas are explored, including ultrasonic sensing, optical sensing, fluorescence based sensing, and pressure sensing. Changes in the reflection factor of a transient ultrasonic pulse at the polymer/mold interface are exploited giving the ability to sense mold filling, flash, and shrinkage in the mold. The sonic velocity in the polymer during solidification demonstrates sensitivity to temperature and pressure. Features of this signal indicate the cooling rate and the moment of gate seal. Two fluorescent dyes are investigated that, when doped at low concentration into the polymer, provide the ability to monitor both the temperature and solidification state during molding. The emission of one dye clearly indicates the glass transition in an amorphous polymer. The second dye responds smoothly to changes in temperature, without directly indicating the glass transition. In a semi-crystalline polymer, these dyes are sensitive to the heat of crystallization generated upon crystallization. An optical sensor is developed that is sensitive to the formation of the crystal morphology in a semi-crystalline polymer of low optical density. Features of this signal indicate the formation of skin, the end of primary crystallization, and the average crystallinity of the finished part. The physical basis for sensor signals are demonstrated using a process model. This model is used to interpret and explain correlations between observed features of the sensor signals and process milestones such as gate seal or final solidification. Finally, the application of these sensors to process control is discussed. An example application is demonstrated using a cavity pressure transducer and ultrasonic transducer to monitor and control the hold time setting.

Sensor concepts for polymer injection molding

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Sensor concepts for polymer injection molding

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