Extrusion Line Settings For Polycarbonate Temperature Profile And Die Gap
Polycarbonate (PC) extrusion line settings are governed by the interaction between the resin’s high melt viscosity, sensitivity to residual moisture, and abrupt shear-thinning behaviour. The temperature profile and die gap used on a sheet or profile line determine whether the melt remains within the narrow processing window that preserves molecular weight and surface clarity. A production-scale single-screw extruder with an L/D ratio of 30:1 to 36:1 and a screw compression ratio of 2.5:1 to 3.0:1 is typically specified for PC sheet and profile lines. The resin must be dehumidified to a moisture content below 0.02% by weight, which generally requires a desiccant dryer delivering a dew point of −40 °C to −30 °C and an inlet air temperature of 120 °C to 130 °C for 4 h to 6 h. Residual moisture above this threshold hydrolyzes the carbonate linkage during plastication, reducing melt viscosity and producing gaseous degradation by-products that appear as splay or silver streaks at the die exit.
What Barrel Zone Sequence Prevents Hydrolytic Degradation in Single-Screw Plastication?
The barrel set-point sequence for PC is typically flat or slightly reverse rather than strongly ascending, because the resin’s melting behaviour is enthalpy-driven above 220 °C and excessive energy input in the feed section can compact unmelted granules into a solid plug. The following table lists representative starting set points for a 90 mm single-screw extruder with a 32:1 L/D ratio. These values are references; die-head pressure and melt-temperature probes must be used to confirm the actual melt condition.
| Line section | Set range | Control function |
|---|---|---|
| Feed throat cooling jacket | 40–70 °C | Prevents pellet bridging and protects the gearbox from axial heat transfer |
| Barrel zone 1 | 270–290 °C | Initiates melting without overpacking the solids bed |
| Barrel zone 2 | 280–300 °C | Completes plastication under compression |
| Barrel zone 3 | 290–310 °C | Balances metering and viscous heat generation |
| Barrel zone 4 | 290–310 °C | Homogenizes melt temperature before the breaker plate |
| Adapter and melt pump inlet | 285–300 °C | Maintains seal and constant discharge without adding shear heat |
| Die body | 295–310 °C | Provides uniform flow distribution across the manifold |
Extrusion-grade PC with a nominal melt flow rate of 6 g/10 min to 18 g/10 min at 300 °C under a 1.2 kg load, as determined by ISO 1133-1:2022, should show a deviation of less than 10% between dried pellet and extruded pellet when the temperature profile is stable. A larger shift indicates thermal or hydrolytic chain scission. The actual melt temperature measured by an immersion probe at the adapter should be maintained between 295 °C and 315 °C for sheet extrusion. When die-body thermocouple variation exceeds ±5 °C across the width, transverse flow irregularities produce gauge bands that may not be corrected by flexible lip adjustments alone.
Melt shear viscosity of PC at typical extrusion temperatures follows a power-law index of 0.6 to 0.8 over shear rates from 10 s⁻¹ to 1000 s⁻¹. This moderate pseudoplasticity means pressure-driven flow in a coat-hanger manifold is more sensitive to temperature differences than to shear-thinning correction alone. A manifold temperature drop of 5 °C can produce a viscosity increase of 10% to 15% and a measurable reduction in flow at the die edges. Consequently, edge heaters on the die body are often set 5 °C to 10 °C higher than the centre zone.
Extruder barrel temperature settings alone do not define the thermal history of the melt. At screw speeds from 60 rpm to 90 rpm on a 90 mm extruder, shear heating can raise the measured melt temperature by 8 °C to 15 °C above the highest barrel set point. This offset must be subtracted when the die-body set point is 310 °C; otherwise the die-lip melt may exceed 325 °C and initiate surface defects. The resulting relationship between screw speed, back pressure, and melt temperature is recorded on the line’s SCADA system and compared with the virgin-pellet melt flow rate from ISO 1133-1:2022.
For coat-hanger and multi-manifold flat dies producing PC sheet, the initial lip gap is commonly set between 0.8 mm and 1.5 mm for final sheet thicknesses from 0.25 mm to 4.0 mm. A lip gap below 0.6 mm raises die-head pressure above 150 bar on many 90 mm to 120 mm extruders, increasing shear heating and destabilizing the melt bank. A gap above 2.0 mm may reduce orientation and complicate thickness control when the sheet is drawn down more than 2.5:1.
| Parameter | Typical range | Measurement or reference standard |
|---|---|---|
| Residual moisture after drying | <0.02% | Karl Fischer titration, ISO 15512:2019 |
| Melt flow rate | 6–18 g/10 min at 300 °C, 1.2 kg | ISO 1133-1:2022 |
| Adapter melt temperature | 295–315 °C | Immersion thermocouple |
| Die-head pressure | 100–250 bar | Melt pressure transducer |
| Die lip gap | 0.8–1.5 mm | Feeler gauge or non-contact gap sensor |
| Draw ratio | 1.5:1–2.5:1 | Calculated from die gap and take-off speed |
Die-head pressure is a more responsive indicator of gap change than visual sheet thickness. A sudden pressure drop of 10% at constant screw speed and die gap is generally associated with either moisture release, feed blockage, or screen-pack blinding. A pressure rise of more than 25% over a stable baseline indicates that the lip gap has closed due to thermal expansion or that degraded material has accumulated in the screen pack.
When Melt Temperature Exceeds 315 °C at the Die Lip
When the die-exit melt temperature exceeds 315 °C, the residence-time-dependent reduction in molecular weight becomes measurable as an increase in melt flow rate and a loss of notched impact strength in the extruded sheet. The degradation pathway is accelerated by oxygen ingress at the hopper or by inadequate purging after a previously processed resin. Sheet produced under these conditions may exhibit yellowing, microgel inclusions, and intermittent smoke at the die. On twin-screw compounding lines, the same threshold applies to the last mixing zone; however, thermal damage may be masked by the stabilizer package.
Die-gap changes should not be used to compensate for thermal degradation. Widening the lip gap increases residence time in the die land and can worsen gel formation. Instead, the die-body set point should be reduced in 3 °C to 5 °C increments while observing the melt pressure, and the screw speed should be adjusted to keep the die-head pressure within 100 bar to 250 bar.
On production lines processing 30% to 50% post-industrial regrind, the bulk density reduction in the feed section often requires a 5 °C to 10 °C increase in barrel zone 1. This adjustment is not equivalent to raising the entire profile; only the feed-zone set point is changed to restore the solids-bed pressure before the compression section. If a vacuum vent is present, its pressure should not be lower than −0.08 MPa, because excessive vacuum can pull low-molecular-weight fractions from the melt and produce die-lip deposits.
Gear-pump-assisted lines decouple screw speed from die-head pressure. A gear pump inlet pressure of 20 bar to 50 bar and discharge pressure of 100 bar to 250 bar is typical for PC sheet. The pump’s shear heating is small but measurable; the melt temperature rise across a gear pump operating in this window is generally less than 5 °C.
A screen pack with 40/60/100 mesh layers is often used for PC sheet to remove char particles and protect the gear pump; the pressure drop across a clean pack should be between 10 bar and 30 bar. When the pressure drop exceeds 50 bar, screen changeover is usually required to prevent excessive shear heating.
Die-lip gap should be measured after the die has been heated for at least 2 h, because thermal expansion of the die body reduces the cold gap. A cold gap of 0.8 mm may become 0.75 mm to 0.78 mm at operating temperature depending on die width and steel grade. Operators who set the gap before the die reaches thermal equilibrium often observe an initial sheet thickness that drifts during the first hour of production, a condition that is corrected by re-zeroing the gap measurement after the die-body temperature has stabilized at 300 °C.
