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| Figure
3- Viscosity versus Shear Weight at 575¢µ(302¡É), CALIBRE*300 and 300 EP Series Resins
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Polycarbonate resins have higher melt viscosities
than many other thermoplastics. Figure 3 shows typical viscosity
versus shear rate curves for CALIBRE 300 and
300 EP Series resins of various melt flow rates. At shear
below about 1000 sec-1 (during extrusion), the melt viscosities
of these polymers are essentially Newtonian, so changes
in shear rate have little or no effect on melt viscosity.
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| Figure
4- Viscosity versus Temperature, CALIBRE 300 and 300 EP Series Resins
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However, above 1000 sec-1(during injection molding),
melt viscosity actually decreases as shear rates increase
- making the resins even more processable. Figure 4 shows
a sharp decrease in melt viscosity upon heating, which
indicates that CALIBRE resins are easier to
process at higher temperatures.
The relationship between viscosity and
processability can also be demonstrated by plotting
flow distance versus temperature or pressure. As melt
flow rate increases, viscosity decreases and flow distance
increases. Spiral flow data are useful when doing a
direct comparison of two or more resins. Because the
information depends on the particular mold, machine
and conditions, the flow lengths generated cannot be
applied to actual parts.
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| Figure
5- Flow Length versus Cavity Pressure, CALIBRE Resins, 15 MFR versus 22 MFR
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As shown in Figure 5, spiral flow can
be used to understand the improved processability (i.e.,
longer or easier flow of the higher MFR resins). For
example, the 22 MFR resins flow further down an open-ended
spiral than the 15 MFR resins at a given temperature
and injection pressure. |