Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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e) Gate seal: On the descending side of the curve, the slope should decrease smoothly. If the curve has a sudden drop, the gate may not be sealed. Note that when the cavity-pressure curve arrives at atmo- spheric pressure, this has a 1:1 correlation with part shrinkage. USES OF CAVITY TEMPERATURE Cavity-temperature sensors provide an instantaneous indication of melt-front location, more quickly than would be possible with cavity- pressure sensors. When placed directly or flush mounted in the last 10-15% of the flow path, material viscosity changes and cavity fill-time deviations are ascertained immediately. For example, in a hot-runner multi-cavity tool, temperature sensors placed towards the end of fill in the same location of each cavity can be used for auto- matic balancing* so that all cavities fill equally at the same time. As the cavity-temperature traces become consistent, the cavity-pressure curves also become consistent (see Figs 2 and 3). This is inherent in the PVT behavior of the plastic. For larger molds with multiple gates, temperature sensors placed near each FIG. 2 Cycle #1 152 142 132 122 112 102 92 82 72 62 52 42 32 22 12 2 -8 Cavity Temperature Curves Cavity Pressure Curves 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 Cavity Pressure Curves Time, sec 572 522 472 422 372 322 272 222 172 122 72 22 -28 FIG. 1 The Cavity Pressure Curve d 8568 8068 7568 7068 6568 6068 5568 5068 4568 4068 3568 3068 2568 2068 1568 1068 568 68 e . . . c b. a. -432 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time, sec 8568 8068 7568 7068 6568 6068 5568 5068 4568 4068 3568 3068 2568 2068 1568 1068 568 68 -432 Fig. 1: Typical cavity-pressure curve for a sensor placed near the gate (rather than at the end of fill) shows the switchover/transfer point (c) and gate-seal point (e), among other events in the cycle. FIG. 3 Cycle #61 152 142 132 122 112 102 92 82 72 62 52 42 32 22 12 2 -8 Cavity Temperature Curves Cavity Pressure Curves 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 Cavity Pressure Curves Time, sec Figs. 2 &3: Curves for a cavity-pressure sensor near the gate and cavity-temperature sensor near the end of fill. As cavity temperature becomes more consistent from cavity to cavity in a multicavity mold, so does cavity pressure. FIG. 4 The Cavity Temperature Curve 168 166 164 162 160 158 156 154 152 150 148 146 144 142 140 138 136 c d. a. b. 1 2 3 4 5 6 7 8 9 10 Time, sec Fig. 4: Typical cavity-temperature curve for a sensor placed near the end of fill. It shows filling (b to c) and cooling (d). 40 MAY 2012 PLASTICS TECHNOLOGY Fig. 5: Cavity-temperature and pressure sensors can be used together to monitor and control melt viscosity. Shear rate is calculated from ∆t and shear stress from the pressure at T1. 11 12 13 14 15 168 166 164 162 160 158 156 154 152 150 148 146 144 142 140 138 136 16 FIG. 5 Determination of Material Viscocity 572 522 472 422 372 322 272 222 172 122 72 22 -28 Pressure, psi Pressure, psi Temperature, F Temperature, F Temperature, F Cavity Pressure, psi

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