Processing, thermoplastics molding parameters

Tianjin P(3HB-4HB) and Tianan PHBV can be molded via injection molding. The temperature profile in the injection molding barrel is different than conventional thermoplastics with the hottest temperature at the first zone and the coolest temperature at the nozzle. PHAs can be exposed to a temperature range between 160°C and 190°C. PHAs viscosity is too large for injection molding at temperatures below 160°C. PHAs can thermally decompose at temperatures above 190 C. PHAs also require a heated mold to enhance crystallinity. The injection molding parameters for P(3HB-4HB) are displayed in Table 4.4. Injection molding process conditions are similar for PHBV. 

One of the key parameters in reactive processing is the distribution of residence times and temperatures for all particles in the liquid, because their reactivity depends on temperature, and the degree of conversion is determined by the dwell time inside the mold. Since the fountain effect changes residence time distribution from that in the hypothetical case of steady unidimensional flow, this factor becomes especially important in chemical (reactive) processing, more so than in standard injection molding of thermoplastic materials.284... 

Liu, S.-J. Chen, Y.-S. Water-assisted injection molding of thermoplastic materials effects of processing parameters. Polym. Eng. Sci. 2003, 43, 1806-1817. 

Many of the relationships described in detail below, in most cases for injection molding of thermoplastics, between the process parameters and the structural component properties are applicable to other processes when modified accordingly. 

Local variations in process parameters result in local shrinkage differentials. The different types of internal stress may certainly occur at the same time, for example due to cooling, dwell pressure, and crystallization during injection molding of semicrystalline thermoplastics. The different types of internal stress are also not sharply differentiated. 

With regard to methods of fabrication, all processes in Table 1.1 that are applicable to unfilled, unmodified thermoplastics can also be used for discontinuous systems (with the exception of expandable bead molding). In addition to thermoforming, hot stamping of reinforced thermoplastic sheets mostly containing randomly oriented continuous or discontinuous fibers is used for the production of large semistructural parts. Fillers can also be used in the thermoset processes in Table 1.1, often in combination with the primary continuous fiber reinforcement. The content and inherent properties of the additive, as well as its physical/chemical interactions with the matrix, are important parameters controlling the processability of the composite. 

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