Mould Temperature in Polycarbonate: The Variable That Decides Stress and Shrinkage

Polycarbonate (PC) is an amorphous engineering thermoplastic with a glass-transition temperature commonly reported between 145 °C and 150 °C. In injection moulding, the mould wall temperature determines the quench rate of the melt, the thickness of the frozen layer during filling, the decay of molecular orientation, and the dimensional stability of the moulded part. Unreinforced PC grades are processed with melt temperatures from 280 °C to 320 °C and mould temperatures from 80 °C to 120 °C according to supplier processing guides and ISO 7391-1 grade designations. Within that range, the lower bound reduces cycle time but increases moulded-in stress, while the upper bound reduces stress and optical birefringence but increases cooling time and ejection difficulty. The process relationship described as “Mould Temperature in Polycarbonate: The Variable That Decides Stress and Shrinkage” is therefore the governing control for dimensionally critical PC components.

How Does a Low Mould Wall Temperature Lock In Frozen-Layer Shear Stress?

During filling, the polymer layer in contact with the cavity wall solidifies before the core remains molten. At mould temperatures below 80 °C, the frozen layer in a 2.5 mm wall section can occupy a sufficient fraction of the total thickness to restrict flow passage and increase shear stress in the melt front. Because PC is amorphous, the retained orientation does not relax through crystallisation; it becomes trapped in the glassy state as the part cools below 145 °C. The resulting residual stress is measurable as a reduction in tensile elongation at break under ISO 527-2:2012 and as a loss of notched impact strength under ISO 180:2023. On production-scale hydraulic injection moulding machines with clamp forces from 1,000 kN to 3,000 kN, parts moulded at 70 °C frequently exhibit crazing around gates and snap-fit features, while the same tool run at 100 °C does not show the same pattern.

For a wall thickness of 3.0 mm, the local cooling rate at the surface is high enough in tools held below 80 °C to limit free-volume recovery before ejection temperature is reached. The retained free volume and molecular orientation are not linear functions of mould temperature, but orientation decay is a time-temperature process. A higher mould temperature extends the time available for chain segments to relax before vitrification. This effect is more pronounced in flame-retardant PC and PC/ABS blends where melt viscosity differs from unmodified PC. Melt-flow-rate values measured under ISO 1133-1:2022 at 300 °C with 1.2 kg load typically fall between 6 g/10 min and 22 g/10 min for injection moulding grades. Lower-flow grades require higher mould temperatures to avoid residual stress in thick bosses and gated areas.

The published linear mould shrinkage of unreinforced polycarbonate is commonly cited as 0.5 % to 0.7 % when measured according to ISO 294-4:2018 or ASTM D955-21 on a 60 mm × 60 mm × 2 mm plaque. Because PC does not crystallise, mould shrinkage is dominated by thermal contraction and effective packing rather than by crystallisation shrinkage. Raising the mould temperature from 80 °C to 120 °C does not produce the same magnitude of shrinkage reduction observed in semi-crystalline materials; however, it reduces the differential between near-wall and core shrinkage and lowers post-mould dimensional movement. Parts moulded at the lower bound may show transverse shrinkage values closer to 0.6 % to 0.7 %, while longitudinal shrinkage remains lower because residual orientation resists contraction in the flow direction. Post-mould annealing at 120 °C for 2 h can release additional movement, and dimensional checks should be performed only after 48 h at 23 °C ± 2 °C and 50 % ± 5 % RH in accordance with ISO 291:2008.

Pressure-volume-temperature data used in commercial injection-moulding simulation libraries show that at 300 °C the specific volume of PC decreases from approximately 0.95 cm³/g at atmospheric pressure to 0.88 cm³/g at 60 MPa in the melt state. Mould temperature changes the specific volume at which the part freezes: a higher mould temperature generally freezes the part at a lower specific volume because packing is more effective before gate freeze. The resulting lower specific volume contributes to higher part-weight consistency and reduced visible sink marks. Gate freeze time also increases with mould temperature; for a 1.5 mm diameter tunnel gate, freeze time can extend from 2 s to 4 s at 80 °C and from 4 s to 7 s at 120 °C. This extension improves packing but raises cycle time.

When Mould Wall Temperature Is Held Above 120 °C

Processing at mould temperatures above 120 °C is used for thick optical parts, load-bearing components, and mouldings that require minimal birefringence. The benefit is a longer relaxation window before the part crosses the glass transition. The operational cost is not linear: cooling time increases approximately with the square of the wall thickness, and the temperature difference between the melt and the mould wall narrows. In a 6 mm thick plaque, raising the mould temperature from 90 °C to 130 °C can add 20 s to 40 s of cooling time on a 1,200 kN machine. The part surface may remain above the heat deflection temperature reported between 125 °C and 135 °C at 1.8 MPa under ISO 75-2:2013 long enough to allow ejection marks if demoulded too early. Mould release agents must be selected from grades that do not leave silicone residues. Polycarbonate is sensitive to chemical attack by amines, strong alkalis, and certain halogenated contaminants under stress-bearing conditions.

In polycarbonate optical lens moulding, the allowable birefringence of the final part often requires a mould temperature of 110 °C to 130 °C and a moulding process with lower injection velocity than standard housing parts. The processing window narrows because too low a mould temperature freezes orientation, while too high a mould temperature extends cycle time and increases the risk of sticking on polished inserts. Injection-compression moulding is used on vertical presses with platen parallelism controlled to ±0.02 mm to maintain a uniform melt front and to reduce shear-induced retardation. Under these conditions, cavity-to-cavity mould-temperature variation of ±5 °C or less is specified for multi-cavity optical tools. Published data for the exact birefringence change per degree Celsius is product-specific, but supplier validation protocols commonly compare polariscope images after annealing at 125 °C for 1 h.

Tooling Thermal Design and Conformal Cooling Circuit Specifications

Mould temperature is controlled not only by the temperature control unit setpoint but also by the heat extraction capability of the tool. For PC, the low thermal conductivity of the polymer—approximately 0.20 W/m·K in the solid state—means that cavity wall temperature can rise during high-speed filling unless cooling lines are placed close to the forming surface. Conformal cooling circuits with diameters of 8 mm to 12 mm, centreline distances of 1.5–2.5 times the channel diameter from the cavity surface, and turbulent flow with Reynolds numbers above 10,000 are specified to keep cavity temperature within ±3 °C. Heat transfer coefficients in the range of 2,000 W/m²·K to 5,000 W/m²·K are attainable with water-based circuits under turbulent conditions. A tool that relies on simple drilled channels may produce a cavity wall temperature difference of 10 °C or more between the gate and the end of fill, causing non-uniform shrinkage and warpage in long parts. Thermal imaging of the opened tool is used during process validation to map the cavity surface; variations larger than 5 °C are corrected before dimensional capability studies are run.

Glass-fibre-reinforced polycarbonate grades alter the mould-temperature response because the fibre network restricts thermal expansion and creates orientation-dependent shrinkage. The linear mould shrinkage of 10 % glass-filled PC can fall to 0.2 % to 0.4 % in the flow direction and 0.3 % to 0.5 % transverse to flow. Mould temperature remains important not because of crystallisation but because the matrix must remain mobile long enough to allow fibre-matrix stress transfer during packing. If the tool is too cold, the matrix freezes before packing is complete, and the part exhibits higher warpage due to local fibre orientation gradients. On a twin-screw compounded PC-GF grade with fibre length degradation controlled to a weight-average length near 300 µm, mould temperatures of 100 °C to 120 °C are used for structural carriers, while unreinforced grades can tolerate 80 °C in non-critical applications. Shrinkage measurements on glass-filled grades require a minimum of 10 plaques per cavity and should be reported with standard deviation and confidence interval according to ISO 294-4:2018.

Pre-drying at 120 °C for 2–4 h to a target moisture content below 0.02 % is mandatory for most PC grades. If residual moisture is present, hydrolysis during plastication degrades molecular weight and masks the effect of mould temperature because melt viscosity drops and the part may fill at a lower injection pressure. Moisture analyser data should be recorded before each moulding campaign; in production environments with relative humidity above 60 %, open material hoppers should be avoided or covered with dry-air conveying. The melt temperature and mould temperature are not independent: a moisture-degraded melt at 300 °C may behave like a higher-MFR grade, while the reduced molecular weight increases the risk of brittle failure even if the mould temperature is within the recommended range. This interaction is frequently encountered on machines with 25 mm or 30 mm screw diameters and residence times above 5 min.

Ejection force is also a function of mould temperature. In a cold tool, the part shrinks more onto the core, increasing the force required for ejection and the probability of pin push-through or stress cracking near pin locations. With a mould temperature below 80 °C, ejection force can be 20 % to 50 % higher than at 100 °C for a box-shaped component with a 1° draft angle. A higher mould temperature reduces ejection force but requires the part surface to be below the heat deflection temperature before demoulding. Ejection temperature is typically set between 100 °C and 120 °C for thick parts to balance these requirements.

Residual Stress Gradients Persist After Ejection in Thick Section Parts

Even after ejecting the part, residual stress gradients remain because the core cools more slowly than the outer skin. In a 4 mm to 6 mm wall section, the core may exit the tool at temperatures above 90 °C while the surface has already fallen below the heat deflection temperature. The resulting differential contraction can cause post-ejection warpage that continues for hours. Holding the part in a fixture during cooling or post-mould annealing at 120 °C for 1–2 h is used for flatness-critical components. Dimensional acceptance should include a controlled recovery period of 24–48 h at 23 °C ± 2 °C before final measurement. The mould temperature setting directly controls the magnitude of this post-ejection movement: parts moulded at 80 °C show greater post-ejection curvature than parts moulded at 110 °C when all other parameters are held constant. If the part is measured too early, the reported shrinkage will be artificially low and will not represent the dimension after equilibrium moisture uptake.

Weld-line strength in PC is similarly governed by mould temperature. At a mould temperature below 80 °C, two melt fronts may cool before sufficient interfacial diffusion, reducing tensile strength at the weld line to 40 % to 60 % of the base polymer strength. Raising the mould temperature to 100 °C to 120 °C permits chain interdiffusion across the weld interface. Weld-line performance is measured on a double-gated tensile plaque under ISO 527-2:2012, where the weld line is formed by opposing melt fronts. This measurement is relevant for housing components with holes and snap-fit features that force flow-front division.