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. 2020 Feb 22;12(2):488.
doi: 10.3390/polym12020488.

Dynamic Compression Induced Solidification

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Free PMC article

Dynamic Compression Induced Solidification

Benedikt Roth et al. Polymers (Basel). .
Free PMC article

Abstract

This study presents a method for the determination of the dynamic pressure-dependent solidification of polycarbonate (PC) during flow using high pressure capillary rheometer (HPC) measurements. In addition, the pressure-dependent solidification was determined by isothermal pressure-volume-temperature (pvT) measurements under static conditions without shear. Independent of the compression velocity, a linear increase of the solidification pressure with temperature could be determined. Furthermore, the results indicate that the relaxation time at a constant temperature and compression rate can increase to such an extent that the material can no longer follow within the time scale specified by the compression rate. Consequently, the flow through the capillary stops at a specific pressure, with higher compression rates resulting in lower solidification pressures. Consequently, in regard to HPC measurements, it could be shown that the evaluation of the pressure via a pressure hole can lead to measurement errors in the limit range. Since the filling process in injection molding usually takes place under such transient conditions, the results are likely to be relevant for modelling the flow processes of thin-walled and microstructures with high aspect ratios.

Keywords: no-flow pressure; polycarbonate; solidification.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Melt flow during compression in pressure-induced solidification (CIS); 1: No Compression; 2: Beginning compression; 3: End of Compression; black arrows indicates the compression direction.
Figure 2
Figure 2
Schematic drawing of the counter pressure Rheograph 75 with the pressure sensor behind the pressure hole; adapted according to [53]; rp: piston radius; vP: piston velocity; rc: capillary radius; lc: capillary length.
Figure 3
Figure 3
Calculated isothermal compressibility for evaluation of solidification pressure in pvT measurements, adapted according to [54]; pg,E: pressure of ending solidification; pg,B: pressure of beginning solidification.
Figure 4
Figure 4
Determination of the solidification by evaluation of the pressure difference in HPC measurements; p1: Pressure transducer measuring cavity p2: Pressure transducer counter pressure chamber pp: Pressure calculated from piston force; (a) pressure signals as a function of time; (b) pressure difference pp-p1 as a function of pp; (c) first derivation of (b) with respect to pp.
Figure 5
Figure 5
Isothermal pvT-measurements (a) and calculated specific compressibility according to Equation (1) (b).
Figure 6
Figure 6
Second derivative of the specific volume (a) and evaluation of the temperature dependent glass transition (b); pg,B: pressure of beginning solidification; pg,E: pressure of ending solidification; Tg,B: temperature of beginning solidification; Tg;E: temperature of ending solidification.
Figure 7
Figure 7
Pressures p1, p2 and pp for the Temperatures 220 (a), 230 (b), 240 (c) and 250 °C (d); the velocity of the piston was 0.0056 mm/s.
Figure 8
Figure 8
Evaluation of the beginning of solidification at different compression velocities. The ordinates of the graphs show the first derivative of the difference of the pressure signals p1 and pp. The vertical red lines indicate the range for building the linear fit. The crosses mark the intersections with the x-axis for the different temperatures; (a): 0.0028 mm/s; (b): 0.0056 mm/s; (c): 0.028 mm/s.
Figure 9
Figure 9
Pressure of solidification Pg in relation to the temperature for the four different measurements: isothermal pvT-measurement, dynamic solidification with a closed counter pressure chamber and a piston velocity of 0.0028 mm/s, 0.0056 mm/s and 0.028 mm/s.
Figure 10
Figure 10
HPC measurements as a function of a measurement instrument independent compression rate ψ (a); calculation of a master curve by shifting the measurements with a shift factor α (b); logarithmic shift factor α as a function of the temperature difference to the reference temperature of 250 °C (c).

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