Thermal crystallization in heated injection moulds is controlled by the local temperature history imposed across a cavity wall, and the resulting crystalline morphology determines shrinkage, warpage, modulus, and solvent resistance in semi-crystalline resins such as polyamide 66, polyoxymethylene, polyethylene terephthalate, polyphenylene sulfide, and polyether ether ketone. A heated mould does not eliminate thermal gradients; instead, it raises the average cooling-barrel temperature so that the outer layer solidifies in a regime closer to the polymer's cold crystallization exotherm or, in high-temperature tooling, above the glass transition of the amorphous fraction for sufficient time to allow spherulite growth. On a moulding machine with clamp force in the range of 5000 kN to 12000 kN, variothermal control units using oil at 160 °C to 220 °C are used to hold the mould surface at temperatures above the conventional cooling-water setpoint; this moves the crystallization front away from the filling-phase frozen-skin condition. The morphological response is commonly assessed by differential scanning calorimetry according to ISO 11357-3:2018, while dimensional stability after demoulding is measured by the standard ISO method for injection-moulded test plaques, ISO 294-3:2020. A heated mould surface at 120 °C for polyamide 66 or 180 °C for polyether ether ketone produces a high crystallinity skin that is difficult to replicate with conventional water-cooled steel because the interface temperature falls below the effective cold crystallization threshold within seconds of fill. In industrial practice, the thermal benefit of a heated cavity is largest when the measured residual enthalpy after isothermal holding indicates incomplete crystallization at the process temperature; otherwise, the additional energy input only increases cycle time without changing the terminal crystalline fraction. The practical conflict is that a surface temperature within ±5 K of the cold crystallization onset is often required to eliminate an amorphous surface layer, whereas such a narrow mould-temperature window demands active control across the cavity insert and creates non-uniform shrinkage if cooling channels produce temperature variations greater than this threshold.

What Limits Transferability of Isothermal Crystallization Half-Time Data to a Heated Mould Cavity?

Differential scanning calorimetry performed under isothermal or constant cooling conditions according to ISO 11357-3:2018 or ASTM D3418-21 supplies the crystallization onset, crystallization enthalpy, and half-time data used to rank resins for heated-mould processing, but the transferability of these data to cavity conditions is limited by three differences: the local cooling rate, the presence of shear, and the thermal contact resistance at the polymer-mould interface. Isothermal Avrami analysis is carried out at temperatures selected between the melting point and the cold crystallization onset, yet the polymer in a heated cavity experiences a continuously falling temperature at an instantaneous rate that may reach 50 K/min to 200 K/min at the wall and less than 20 K/min near the core. The Avrami exponent and the crystallization half-time obtained from a quiescent melt at 160 °C for polyether ether ketone may not describe a process in which the material has been stretched by the advancing flow front and compressed during packing. Published data for unfilled polyether ether ketone indicate that the minimum isothermal crystallization half-time occurs near 230 °C to 260 °C, but injection moulding with a mould surface at 180 °C shifts the surface material into a lower-temperature regime where secondary crystallization is more rate-limiting. Furthermore, the thermal resistance of the interface between the polymer and the mould steel is not zero; release agents, surface roughness of 0.2 µm to 0.8 µm Ra, and localized melt pressure fluctuations alter the actual heat flux from the part. For these reasons, a moulding trial based solely on DSC half-time can overestimate crystallinity when the mould surface is below the cold crystallization onset or underestimate internal gradients when the part is ejected after insufficient holding. A complete transferable model uses non-isothermal crystallization kinetics obtained at three or more cooling rates and, where possible, the Nakamura extension of the Avrami equation rather than a single isothermal half-time. The model must also incorporate the actual mould steel thermal conductivity, which for P20 tool steel is approximately 29 W/m·K, and the heat-up time of the surface after the cooling phase. Thermal control units with separate high-temperature oil circuits and chilled-water circuits permit the mould surface to be maintained at 170 °C during fill and then cooled to 80 °C for ejection; the transition between these conditions determines whether the crystalline fraction is fixed before or after the ejection threshold. Published comparative studies on semi-crystalline resins show that when the mould surface is held at a temperature near the cold crystallization exotherm peak for at least 10 s to 30 s, the skin morphology changes from a featureless quenched layer to a spherulitic layer with measurable birefringence; below this holding time, the advantage of the heated mould is lost. Moulding operations that do not account for these constraints often record acceptable total crystallinity by DSC but fail because the skin and core have different crystalline structures, leaving residual stress that manifests as warp during ISO shrinkage plaques.

Conformal cooling channel design and variothermal switching behaviour are more decisive than the nominal mould temperature setpoint because the mould-wall temperature measured by a cavity-surface thermocouple can differ from the cooling-channel fluid temperature by 15 K to 40 K depending on steel conductivity, channel pitch, and cycle sequence. Aluminum alloys or high-conductivity copper-based tooling with thermal conductivity from 90 W/m·K to 120 W/m·K are used when rapid surface heating is required, but these materials have lower hardness and can be incompatible with glass-filled resins because of wear. P20 steel at 29 W/m·K requires longer thermal diffusion times, and the surface reheating rate after opening may be 2 K/s to 8 K/s with standard oil units. A heated mould process for a polyamide 66 connector cage, for example, may use an oil circuit at 110 °C for 6 s, followed by a water circuit at 40 °C for 12 s before ejection; the actual mould surface remains above 100 °C only during the initial packing period, after which the polymer skin has already crystallized to a fraction that is not reversed by subsequent cooling. When the cooling channels are placed from 10 mm to 15 mm behind the cavity surface, temperature differences across a 200 mm × 150 mm plaque may exceed 6 K, sufficient to produce measurable variations in post-mould shrinkage along the flow direction. The use of a heated mould to avoid post-ejection annealing is therefore constrained by the thermal uniformity of the cavity steel, not merely by the pump capacity of the temperature control unit. Direct electric cartridge heating behind the cavity inserts can raise surface temperatures to 200 °C while conformal cooling channels are evacuated, but this introduces thermal stress in the tool and requires seals rated for continuous operation above 200 °C. For polyphenylene sulfide, a mould surface of 140 °C to 150 °C is generally needed to develop sufficient crystallinity for chemical resistance, whereas a simple heated platen at the same setpoint may produce only 120 °C at the cavity wall because of thermal stack resistance. The process window is therefore defined by the minimum surface temperature required for the target crystalline phase, the maximum surface temperature allowed by ejection and demoulding forces, and the tolerance on surface temperature that prevents asymmetric shrinkage. Published process capability data from injection moulding of glass-fibre-reinforced semi-crystalline resins indicate that surface temperature variation of more than ±5 K across the cavity can increase warpage by more than the dimensional allowance in precision gears, although published data for this specific configuration is limited when the cavity geometry deviates from a simple plaque.

When Mould Surface Temperature Exceeds the Heat Deflection Temperature of the Solidified Skin

Heated moulds intended to maximize crystallinity can approach or exceed the heat deflection temperature of the solidified outer layer, particularly for unreinforced polyamide 66 and polypropylene, and this condition changes the ejection mechanics and dimensional stability of the part. For unreinforced polyamide 66 conditioned dry, the heat deflection temperature under a load of 1.8 MPa measured by ISO 75-2:2013 is often reported between 65 °C and 85 °C, while a heated mould surface at 100 °C to 120 °C may still be required to obtain the high crystalline fraction necessary for fatigue and creep performance. The part surface can therefore be ejected while its skin is above the heat deflection temperature, causing ejector pin marks, gateswell, or local deformation under ejection forces that would be insignificant at 50 °C. Draft angles below 0.5° and mould surface roughness below 0.1 µm Ra may increase the demoulding load in such processes, and release coatings with chemical lubricants can alter heat transfer. The moulding of a 2 mm thick tensile bar under ISO 294-3:2020 conditions at elevated mould temperature therefore needs an ejection delay or a cooling phase that lowers the surface temperature to a safe value before the ejector stroke, but a cooling delay also allows additional secondary crystallization to occur while the part is constrained by the cavity. Constrained crystallization during packing and holding can reduce warpage immediately after ejection but may create locked-in residual stress that is released during environmental exposure or heat aging. The dimensional stability of a heated-mould part cannot be predicted solely from the average crystalline fraction; the distribution of crystalline lamellae and the level of rigid amorphous fraction at the surface must also be measured. Differential scanning calorimetry on microtomed samples may show a bulk crystallinity of 35% to 40% in polyamide 66, but the skin can be 5% to 10% higher than the core, and this gradient is more significant for warpage than the absolute value. The following table summarizes representative ranges reported for heated-mould processing of unfilled semi-crystalline resins; these values are indicative ranges from technical literature and are not a substitute for material-specific DSC and shrinkage trials.

Process windows derived from these ranges remain sensitive to nucleating agents, filler content, and polyamide moisture condition. A polyamide 66 resin conditioned to 0.2% moisture prior to moulding, measured by ISO 15512:2019, may show a lower melt viscosity and faster crystallization than a dry resin, but the moisture also hydrolyzes the melt if residence times exceed 10 min at temperatures above 290 °C. Polyoxymethylene cannot be heated above 230 °C in the barrel without formaldehyde evolution, and heated-mould processing for polyoxymethylene therefore emphasizes mould temperatures near 90 °C to 120 °C rather than very high melt thermal input. Published data for highly nucleated grades in heated moulds is limited because most data sets merge nucleated and non-nucleated samples, but the direction of the effect is clear: a high nucleation density reduces the critical mould holding time needed to reach a given crystalline fraction at the surface. The practical control measure is to maintain the mould surface below the measured heat deflection temperature at ejection while ensuring the crystalline skin has passed beyond the primary crystallization peak. If the two conditions cannot be satisfied simultaneously, the part must be ejected with a cooled cavity after a short high-temperature hold or transferred to a post-mould annealing fixture.

Flow-induced crystallization in heated moulds introduces an additional variable that is absent in quiescent DSC measurements. The advancing melt front in a thin-walled cavity generates wall shear rates from 10,000 s⁻¹ to 100,000 s⁻¹ in typical injection speeds, and the extensional flow at the gate creates oriented chain segments that serve as row nuclei for subsequent lateral lamellar growth. In a heated mould, the oriented skin is not immediately quenched into a low-crystallinity frozen layer; instead, the elevated wall temperature permits these nuclei to grow during packing, producing a dense transcrystalline or shish-kebab morphology that is mechanically stiffer in the flow direction but more anisotropic in shrinkage. This condition is exploited in thin-wall packaging and connector components but creates hidden residual stress when the mould surface is held above the cold crystallization onset. The packing pressure, typically 60 MPa to 100 MPa for engineering thermoplastics, must be sufficient to compensate for the faster volumetric shrinkage of the crystallizing skin; otherwise, sink marks and vacuum voids appear at the thick-to-thin transition. The interaction of holding pressure and crystallization at the mould wall is measurable through pressure–specific volume–temperature diagrams, but standard ISO moulding conditions for shrinkage according to ISO 294-3:2020 may not reproduce the shear and cooling history of a production cavity. Consequently, a mould qualification cycle for a heated tool should include a short-shot series followed by polarized light microscopy on microtomed sections, with measurement of spherulite diameter at three depths from the surface. Published micrographs of polypropylene moulded at 20 °C and at 120 °C show that the low-mould-temperature part contains a clearly quenched amorphous skin, while the heated-mould part contains a thicker spherulitic skin with larger average spherulites near the core; this morphological pattern is not captured by mass-average crystallinity alone. Industrial moulders therefore use heat transfer simulations that include a crystallization sub-model calibrated by the cooling-rate dependence of the DSC crystallization exotherm, but such simulations require additional input data for the thermal contact resistance, the mould coating emissivity if infrared heating is used, and the local melt pressure during packing. For filled systems, the flow orientation of glass fibre in a heated cavity can enhance crystallization at the fibre surface but can also create anisotropic thermal conductivity that accelerates cooling in some regions and slows it in others. The degree of crystallinity measured after moulding may then appear uniform even when the local lamellar orientation differs strongly, which is why tensile bars cut parallel and perpendicular to flow are required to expose anisotropy in heated-mould parts. Tensile testing according to ASTM D638-14 is often used to compare flow-direction performance, while flexural creep data under ISO 899-1:2017 may reveal delayed post-mould crystallization in the higher stress regions of the part.

Crystallization Half-Time Gradients and Mould Surface Thermal Boundary Resistance

The thermal boundary resistance between the molten polymer and the cavity wall is a frequently underestimated parameter in heated-mould processing because the polymer skin forms a finite-thickness contact layer that does not attain the mould surface temperature during the first seconds of filling. The local heat transfer coefficient at the interface can vary from 500 W/m²·K to 5,000 W/m²·K depending on melt pressure, surface roughness, and the presence of trapped air or mould release; a clean cavity surface with high packing pressure favours the upper end of the range. When the mould is heated to 160 °C, the actual polymer surface may reach only 140 °C to 150 °C before the cooling phase begins, and this shortfall is sufficient to shift the local crystallization half-time from less than 30 s to more than 90 s for slow-crystallizing resins. The resulting half-time gradient across a 2 mm wall produces a surface layer with a high nucleation density and many small spherulites, an intermediate zone with mixed transcrystalline growth, and a core with fewer, larger spherulites formed during slower cooling. The transition between these zones is not sharp; it is governed by the local cooling rate and by the time the local melt spends between the cold crystallization onset and the glass transition of the rigid amorphous fraction. Polarized light microscopy of microtomed sections can reveal spherulite size distributions from 5 µm to 50 µm, while small-angle X-ray scattering provides the lamellar periodicities that correlate with the measured melting points. For polyether ether ketone moulded with a cavity surface at 180 °C, the first 50 µm to 100 µm from the surface may show a highly nucleated, fine-textured crystalline layer, whereas the core may contain larger spherulites because it cooled at a rate closer to 20 K/min. The thermal boundary resistance is especially relevant when the mould surface is roughened for venting or when mould release agents create an insulating layer; these practices reduce the surface temperature experienced by the polymer without changing the setpoint recorded in the control unit. Process optimisation therefore requires a cavity-surface thermocouple placed in the expected flow path, not a thermocouple buried 5 mm behind the cavity or mounted on the mould exterior. The signal from a surface thermocouple with a junction diameter of 0.1 mm has a response time fast enough to track the thermal cycling of the mould, but the measurement is local and may not represent the entire cavity. Repeated thermal cycling of a heated mould also changes the thermal contact between insert and mould plate because of differential thermal expansion; inserts must be fitted with clearances that allow expansion without losing heat conduction or creating flash. For tools that alternate between oil heating and water cooling, the seal glands and O-rings must be specified for the maximum temperature excursion and for the pressure surges caused by steam formation when water enters a hot circuit. Published failure data from production-scale variothermal tools indicate that thermal fatigue at the junction between heated and cooled regions can produce surface cracking after several thousand cycles, and this mechanical degradation can alter the cavity finish and the release behaviour over time.

Post-mould crystallization in heated-mould parts continues after ejection because the centre of the wall leaves the cavity at a temperature above the glass transition of the amorphous fraction, and the thermal energy contained in the part drives additional secondary crystallization during air cooling, assembly, and service. The crystallinity value measured immediately after demoulding may therefore increase by 5% to 15% over 24 h to 48 h depending on the resin and the initial mould surface temperature. This secondary crystallization is accompanied by a small additional volumetric shrinkage and a shift in the amorphous-phase relaxation behaviour; parts held at room temperature may develop increased modulus and reduced creep compliance without any external annealing. When post-mould shrinkage must be controlled, moulded plaques are measured at intervals according to ISO 294-3:2020 and the results indicate that a heated mould reduces the later shrinkage by allowing primary crystallization to approach completion inside the cavity. However, the reduction in post-mould shrinkage is not uniform across the part; thick sections may retain more latent heat and crystallize further after ejection, while thin sections cooled by the heated wall may reach the same mould surface temperature but with a different cooling history. The combination of a heated mould and a post-mould annealing fixture is used for high-precision polyamide 66 or polybutylene terephthalate components in which dimensional stability must be maintained at a service temperature of 80 °C to 120 °C. Annealing is conducted at a temperature below the melting point but above the service temperature; for polybutylene terephthalate, temperatures around 150 °C to 170 °C are common, while for polyamide 66 the annealing interval may be 130 °C to 160 °C. The risk in such operations is that secondary crystallization is accompanied by lamellar thickening and a small increase in brittleness, particularly in the constrained skin of a heated-mould part. Not all heated-mould applications require maximum crystallinity; controlled mould surface temperatures may be selected to limit the crystalline fraction at the surface while increasing it in the core, producing a more uniform residual stress state than fully quenched or fully annealed moulding. This balances the need for dimensional accuracy at ejection against the need for a high-temperature service rating. Published data for post-mould crystallization of specific heated-mould configurations is limited because most resin suppliers provide isothermal crystallization tables rather than component-specific thermal histories, and therefore the final crystalline fraction must be confirmed by DSC on samples taken from the critical section of the part. The DSC heating scan should be run at a controlled rate, for example 10 K/min to 20 K/min, with the sample mass recorded to 0.1 mg before the first heating run; the first heating value reflects the moulded morphology, while the second heating value provides a comparison after erasing thermal history. The difference between first and second heating crystallization exotherms indicates how much quenchable amorphous fraction remained after the heated-mould cycle. In production-scale heated-mould trials, the first-heat cold crystallization exotherm is often reduced but not eliminated, and a residual exotherm of 1 J/g to 5 J/g may be acceptable for applications in which later dimensional movement is not critical; however, for tight-tolerance bearing cages and pump impellers, maintaining the residual exotherm below 1 J/g may be required to avoid service-related dimensional drift. Mould release agents that contain silicones can affect the subsequent crystallization measurement and should be avoided unless their effect on the first-heat DSC trace is known. The final moulded component should be characterized by the same standard procedures as the raw material, but the specimen location must be documented because heated-mould parts are intentionally non-uniform in their crystalline morphology and thermal history.