Optical monitoring of polyesters injection molding

Abstract

An optical fiber sensor similar to the one developed by Thomas and Bur 1 was constructed for the monitoring of the crystallization of three polyesters during the injection molding process. The polyesters studied were: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene terephthalate (PET). With this optical system it was possible to obtain, in real time, some essential parameters of the polyester crystallization kinetics at different processing conditions. Thus, a study of the influence of injection molding variables on the nonisothermal crystallization kinetics of these polyesters was done. The processing variables were: mold wall and injection temperatures, T w and T i , respectively; flow rate, Q; and holding pressure, P h . The experiments were done following a first order central composite design statistical analysis. The morphology of the samples was analyzed by polarized light optical microscopy, PLOM. The signal of the laser beam during the filling and the crystallization stages of the injection molding of these materials was found to be reproducible. The measurements showed that this system was sensitive to variations of the crystallization of different types of polymers under different processing conditions. The system was not able, however, to monitor the crystallization process when the crystallinity degree developed by the sample was very low, as in the PET resin. It was also observed that T w and T i were the most influential variables on the crystallization kinetics of PBT and PTT. Due to its slower crystallization kinetics, PTT was found to be more sensitive to changes in these parameters than the PBT.

Optical monitoring of polypropylene crystallization during injection molding

“…During the filling stage, the polymer at the mold wall begins to crystallize first and the polymer at the center of the mold crystallizes later. In both cases, the growing spherulites will create new interfaces (high interfacial area) which will scatter the light, decreasing the intensity signal up to a minimum value (I min ) [14,15,19,20]. At some molding conditions, however, this decrease occurs after an induction time, t i .…”

“…After attaining a given size, the spherulites, however, begin to impinge one with another, decreasing the interfacial area and therefore decreasing the light scattering and increasing again the light intensity signal. When the crystallization ends, a plateau (I f ) is attained [14,15].…”

“…This monitoring has been done using ''on-line'' ultrasound techniques (measurement of waves' velocity changes during crystallization) [11,12], dielectric spectroscopy (measurement of the dielectric capacitance as a function of time), [13] and optical measurements [14][15][16], among others. The ultrasound techniques require the use of more expensive transducers and the detector signals are more complex to interpret.…”

“…Marinelli et al [15] also developed an optical sensor to monitor the crystallization kinetics of polyesters during injection molding. The sensor was based on the Thomas and Bur principle; however, it was assembled on the cavity wall.…”

Correlation between runner pressure and cavity pressure within injection mold

“…There are many methods of monitoring the physical quantities inside a cavity during the process, including measuring the melt flow front [4][5][6][7][8][9], mold heat flux [10,11], mold temperature [11][12][13][14][15], cavity pressure/ temperature [1,2,[14][15][16][17][18][19][20][21][22][23], and dimensions and properties of the molded part [3,[24][25][26][27]. The sensor types include the frequently used contact type, noncontact type [28][29][30][31], and even a wireless type of sensor [32][33][34][35][36]. The above studies used different types of sensors, mostly flat specimens in the same thickness for research, while seldom discussing the molded part in nonuniform thicknesses.…”