Moulding Thin-Wall Polycarbonate: Where the Flow Stops and Why is a processing question that arises from the thermal and rheological boundary of polycarbonate (PC). PC enters thin-wall injection moulding as an amorphous melt with a glass transition temperature near 145–150 °C, a density of 1.20 g/cm³, and a melt volume-flow rate (MVR) reported under ISO 1133-1:2022 at 300 °C and 1.2 kg. When nominal wall thickness falls below 1.0 mm, filling is governed less by bulk viscosity than by the rate at which a frozen skin grows inward from the mould wall. The phrase Moulding Thin-Wall Polycarbonate: Where the Flow Stops and Why describes this boundary: a melt channel that remains open long enough to transmit cavity pressure, or a frozen layer that consumes the cross-section and arrests the front. In production, the stop is observed as a short shot at the last rib, a hesitation line behind a thin hinge, or gate freeze-off before pack pressure compensates shrinkage.

Unfilled injection-moulding PC is normally processed at melt temperatures of 280–320 °C and mould temperatures of 80–120 °C. At the upper end, viscosity falls but the risk of hydrolysis and thermal chain scission rises. At the lower end, flow length falls steeply. A medium-viscosity grade with an MVR near 10 cm³/10 min may fill a 2.5 mm wall with a flow-length-to-thickness ratio above 150:1, but below 1.0 mm the same ratio can drop below 60:1 because the frozen layer consumes a larger fraction of the available cross-section. That shift is not a material failure; it is a consequence of the thermal diffusivity of PC, the mould temperature, and the gate pressure available on a given machine.

What Stops the Melt Front Before the Cavity Pressure Reaches the Packing Setpoint?

The dominant stop mechanism is freeze-off at the melt front. PC has a thermal conductivity of approximately 0.20 W/(m·K), a heat capacity near 1.3 kJ/(kg·K), and a density of 1.20 g/cm³; these give a thermal diffusivity of roughly 0.13 mm²/s. In a 0.8 mm wall, the cavity surface at 90 °C rapidly drops the melt below its no-flow temperature near 145 °C. A frozen skin of only 0.15 mm on each side consumes 37.5% of the wall thickness, leaving a central melt channel of 0.5 mm. As the channel narrows, the apparent shear rate increases but the pressure drop rises because PC exhibits a power-law index of approximately 0.65–0.75 in the injection shear-rate range; the viscosity reduction from shear is insufficient to offset the loss of flow area. At 300 °C and 1000 s⁻¹, high-flow PC grades show apparent viscosities in the range of 400–700 Pa·s, while lower-MVR grades exceed 900 Pa·s. The front therefore stops when the available gate pressure cannot maintain the pressure gradient required to move the melt through the remaining channel.

On a production-scale injection moulding machine producing a 0.8 mm thin-wall frame, a 35 mm screw with 20:1 L/D may deliver a filling pressure of 150 MPa at the screw tip, but the cavity pressure at the last fill point can be below 20 MPa at switchover. The short shot occurs not at the gate but at the end of the longest flow path, often after the melt has passed through a 0.6 mm hinge or a series of ribs. If injection speed is increased from 150 mm/s to 350 mm/s, shear heating at the gate raises local melt temperature and can extend flow by 10–20%, but the same high shear can create gate blush and surface splay. Flow length in thin-wall PC is therefore a pressure-limited, thermally limited balance rather than a simple viscosity setting.

Representative spiral-flow envelope for a high-flow PC with an MVR of 19 cm³/10 min measured under ISO 1133-1:2022, using an ISO 294-1 spiral mould at a melt temperature of 300 °C, a mould temperature of 90 °C, and an injection pressure of 100 MPa:

The values are an envelope, not a specification limit. Published data for specific PC configurations is limited because spiral-flow length shifts with screw decompression, nozzle seat geometry, venting, and mould surface finish. A polished cavity with a surface roughness Ra below 0.05 µm can increase flow length in thin walls by reducing friction and delaying frozen-layer adhesion; a textured surface can reduce flow length and shift the stop position toward the gate.

When the Melt Temperature Is Raised Above 330 °C to Extend Flow, Residence Time Becomes the Stop

Raising melt temperature from 290 °C to 320 °C lowers viscosity and moves the flow stop farther from the gate, but the gain is consumed by thermal and hydrolytic degradation if barrel residence time exceeds 5 min. PC processed at 320 °C for extended periods undergoes chain scission; MVR increases, and the part may show silver streaking, brown discoloration, and reduced impact strength under ISO 179-1. When moisture is present above 0.02%, the reaction shifts to hydrolysis with carbon dioxide evolution and surface splay. Drying in a desiccant dryer to a dew point of -30 °C or lower, typically at 120 °C for 4 h, is mandatory before thin-wall moulding; moisture testing under ISO 15512 is the control method.

The practical processing envelope for sub-1.0 mm PC is therefore narrow. Melt temperatures between 295 °C and 310 °C are typical for high-flow grades, with mould temperatures of 100–120 °C, injection speeds of 200–500 mm/s, and hold pressures of 80–120 MPa. At temperatures above 340 °C, degradation can generate black specks and gate blockage; at temperatures below 280 °C, the melt front freezes before the cavity is full. The process window is not simply a recommended setpoint but a boundary between freeze-off and degradation.

On a production floor, the stop can also originate at the screw recovery stage. Thin-wall PC parts often have a short cooling time because the wall is thin. If the screw is set with a high back pressure of 0.7–1.0 MPa or a low screw speed below 60 rpm, plastication time may exceed cooling time. The material in the barrel then remains at melt temperature for longer than the 5 min residence limit; the next shot contains resin with increased MVR and lower viscosity, which can cause flash and dimensional variation even before the short-shot condition is corrected. For a 0.8 mm wall, cooling times of 6–10 s are possible; recovery times above 12 s force the process out of steady state.

Thermal Diffusivity, Frozen-Layer Growth, and Mould Temperature Limits

Because PC solidifies at a temperature well above typical mould temperatures, the frozen layer grows from both cavity surfaces throughout filling. At a wall thickness of 0.75 mm, the melt channel can be reduced to less than 0.30 mm before the flow front reaches the final 10 mm of the cavity. Raising mould temperature from 80 °C to 120 °C slows this growth and extends the flow path, but it also increases cooling time and can produce distortion in areas with thickness variation. For thin-wall applications, fixed mould temperatures below 90 °C are rarely acceptable for PC unless the flow length is very short or the part is gas-assisted. Conversely, mould temperatures above 125 °C may cause ejection marks and dimensional drift because the part remains soft near the glass transition.

Dynamic mould heating, such as variotherm or induction heating of the cavity surface to 140–160 °C during filling followed by rapid cooling below 100 °C, has been used for sub-0.6 mm PC optical components. Published data for this specific configuration is limited, but the observed effect is an extension of flow length because the no-flow condition is displaced from the mould surface. The trade-off is cycle time and cost; the mould must be heated and cooled every cycle, and thermal fatigue of tool steel must be controlled.

Material selection shifts the stop point before moulding begins. High-flow PC grades with MVR of 19–25 cm³/10 min are used for walls below 1.0 mm; medium-viscosity grades near 10 cm³/10 min may be viable only above 1.2 mm. Low-MVR grades below 6 cm³/10 min retain higher impact strength and chemical resistance but stop earlier in thin sections. The selection of a high-flow grade is not free: notched impact strength measured under ISO 179-1/1eA can fall relative to a medium-viscosity grade by more than 20%, while tensile modulus under ISO 527-2 remains near 2400 MPa but yield strain changes with processing history. Flow improvement must therefore be balanced against mechanical performance and the end-use environment.

Gate and vent design often determine where the flow stops. In a 0.8 mm side gate, a land length above 1.0 mm can freeze before pack pressure transfers to the cavity; a land length of 0.6–1.0 mm with a gate thickness of 0.6–0.8 mm is typically used. Valve-gated hot runners with orifice diameters of 0.8–1.2 mm reduce cold-runner pressure loss and allow a shorter filling time, but the gate vestige may be larger than acceptable. Vent depths for PC are usually kept at 0.015–0.03 mm on the parting line to prevent flash; a short shot caused by trapped air at the end of the cavity is often misdiagnosed as freeze-off. If the vent spacing is inadequate, compressed gas raises the pressure at the front and stops flow even when the melt channel is still open.

At 0.5 mm wall thickness, the viable filling envelope narrows further. The cavity must be heated to at least 120 °C, the valve-gate orifice maintained above 0.8 mm, and the injection speed configured to keep the melt front moving above 300 mm/s. If the parting-line vent depth is increased beyond 0.03 mm, flash appears; if it is reduced below 0.015 mm, venting stops fill. Published data for this specific configuration is limited, but process audits on thin-wall connectors and optical fibre housings show that the short-shot location moves from the last flow path to the gate as gate freeze-up dominates at these dimensions.