Mould Temperature and Cycle Time in PBT: Making the 60–80 °C Window Pay

Polybutylene terephthalate (PBT) is a semi-crystalline polyester with a melting point of 223–225 °C and a glass transition near 45–60 °C when measured by differential scanning calorimetry per ISO 11357-2. The 60–80 °C mould-temperature band is not a nominal setting; it places the cavity wall above the glass transition but sufficiently far below the crystalline melting point to produce a rapidly solidified skin while allowing the core to develop enough crystallinity before ejection. At cavity-wall temperatures below 60 °C, the skin freezes before the core has completed spherulitic growth, leaving amorphous regions that continue to densify after demoulding. Above 80 °C, conductive cooling time rises without a proportional improvement in dimensional stability for unfilled grades. The economic value of the window is therefore tied directly to cooling-time arithmetic, gate-seal behaviour, and post-mould shrinkage control.

How Does the 60–80 °C Tool Face Alter Solidification in PBT?

With a cavity wall at 70 °C, the melt experiences supercooling of approximately 150–160 K relative to the crystalline melting point. Under these conditions, isothermal crystallization studies reported in polymer science literature place the crystallization half-time for neat PBT below 60 s, and nucleated grades are faster. The result is a layered microstructure: a rapidly cooled skin with finer spherulites surrounding a core with coarser crystalline growth. The core must be allowed to crystallize before the gate seals and packing pressure is lost. If the tool is too cold, gate closure occurs before packing is complete, leaving the core under-crystallized. If the tool is too hot, morphology improves but cooling time lengthens because the driving temperature difference between melt and cavity wall is reduced.

Unfilled PBT can reach a bulk crystalline fraction of 30–50% depending on calorimetric method and thermal history. Crystallinity has direct dimensional consequences: amorphous regions show greater densification after ageing or exposure to service temperatures above the glass transition, while crystalline regions stabilise dimensions. The 60–80 °C band therefore operates as a practical compromise between developing enough crystallinity and avoiding excessive cycle-time extension.

Thermal Diffusivity, Wall Thickness, and the Cooling-Time Equation

Cooling time in a thin plaque is dominated by conduction across the cavity wall. The one-dimensional approximation is:

t_cool = (s² / (π² α)) × ln[(8/π²) × ((T_melt − T_tool) / (T_eject − T_tool))]

where s is wall thickness, α is thermal diffusivity, T_melt is melt temperature, T_tool is cavity-surface temperature, and T_eject is ejection temperature. Unfilled PBT has an approximate thermal diffusivity of 1.3 × 10⁻⁷ m²/s at 80 °C, based on a thermal conductivity near 0.25 W/(m·K), a specific heat capacity near 1500 J/(kg·K), and a density near 1.31 g/cm³. For a 2 mm wall, a melt temperature of 250 °C, a tool temperature of 70 °C, and an ejection temperature of 100 °C, the conductive term is 3.1 s and the logarithmic multiplier is 1.58, giving a theoretical cooling time of 4.9 s. Increasing wall thickness to 4 mm increases the conductive term to 19.6 s, showing the square-law effect of part thickness.

The logarithmic term changes from 1.35 at 60 °C to 1.93 at 80 °C under the same boundary conditions, a 43% rise in mathematically predicted cooling time. For thin walls the absolute time penalty remains small, but the percentage change explains why tool-temperature verification matters in high-cavitation connector moulds. Production technical bulletins typically report total cycle times of 15–30 s for small electrical connectors with 0.8–1.5 mm wall stock, including fill, hold, gate freeze, robot extraction, and clamp movement that the conductive approximation excludes.

When Mould Temperature Falls Below 60 °C, Post-Mould Shrinkage Becomes a Quality Liability

Low cavity-wall temperature reduces cooling time, but it also freezes in non-equilibrium amorphous material. Under ISO 294-4, mould shrinkage is measured after conditioning at 23 °C and 50% RH. Unfilled PBT parts moulded at 70 °C typically show mould shrinkage in the 1.5–2.0% range. The same grade moulded at 40 °C may show similar initial shrinkage but can undergo additional post-mould dimensional change of 0.1–0.3% after exposure to service temperatures near 85 °C for 2 h, because amorphous regions densify as they relax above the glass transition. This movement is frequently detected in dimensional audits of connector housings where hole-to-hole pitch shifts after thermal ageing.

The relationship is non-linear: below 60 °C, the crystalline fraction may fall below the threshold needed for stable long-term dimensions, while above 80 °C, additional crystallinity develops slowly and may be limited by thermal degradation during extended residence at elevated temperature.

Before any mould-temperature trial is interpreted, pellet moisture content must be controlled below 0.02% by mass. PBT hydrolyzes at melt temperatures above 240 °C, reducing molecular weight and causing splay on the part surface. Desiccant dryers operating at 120 °C with a dew point below −30 °C for 4–6 h are typical. Moisture content should be verified by ISO 15512 or an equivalent calibrated method. Moisture-related viscosity loss changes gate-seal time, alters filling pressure, and makes the nominal 60–80 °C mould-temperature setting non-representative because the melt no longer solidifies at the expected rate.

On the injection unit, a general-purpose screw with a compression ratio near 2.5:1 and an L/D ratio of 20:1 is normally used for PBT. Screw speed is commonly set between 50–100 rpm, with back pressure between 0.3–0.7 MPa. Excessive shear heating above 270 °C should be avoided because PBT degrades by ester cleavage and may discolour. Melt temperature is typically held between 240–260 °C for unfilled PBT, with nozzle temperature set 5–10 K above the melt setpoint to prevent freeze-off.

Gate Freeze-Off, Holding Pressure, and the Cost of Premature Solidification

Gate geometry interacts directly with the mould-temperature band. A lower cavity-wall temperature shortens the time to gate closure and reduces the available packing window. In production-scale toggle-clamp machines with 80–120 t clamp force and cold-runner tools, dropping the tool from 70 °C to 55 °C can shorten gate-seal time by 30–50% for small connector gates of 0.8–1.2 mm diameter. The resulting cavity pressure cannot be maintained, and shrinkage compensation is lost. Sink marks, voids, and increased orientation near the gate follow.

Holding pressures of 50–70 MPa are commonly specified for PBT. A mould-temperature reduction of 10 K may require holding-time compensation or an increase in gate diameter of 0.2–0.5 mm to restore the same gate-seal time. The pvT path of PBT shows a steep volume contraction during crystallization; if holding pressure is removed before the gate seals, the material cannot be packed. In practice, cavity pressure near the gate should be maintained above 30 MPa until gate seal to avoid short shots and internal porosity.

Reinforced PBT Grades Demand a Different Thermal Contract

Glass-fibre PBT does not treat the 60–80 °C band as its optimum. The recommended mould temperature for many 30% glass-fibre grades is 80–100 °C. The 60–80 °C band remains a lower-bound processing condition for thin-wall glass-fibre parts where cycle time is critical, but it can sacrifice surface quality and dimensional stability.

Glass fibre increases the thermal diffusivity of the compound and reduces mould shrinkage, but the fibre-rich skin layer freezes more quickly than the unfilled matrix. At 60 °C, the skin can become a rigid shell around a still-shrinking core, leading to fibre orientation differences and anisotropic warpage. Raising the tool to 80 °C may increase cycle time by only a few seconds in a 2 mm wall but can reduce post-mould warpage in glass-fibre connectors. The choice between a 60–80 °C and an 80–100 °C setting therefore follows filler content and the shrinkage anisotropy measured across flow and transverse directions per ISO 294-4.

What Influence Do Hot Runners, Valve Gates, and Thermal Nozzle Design Exert?

Hot-runner systems uncouple the gate from the cavity thermal boundary. An externally heated manifold held at 250–260 °C keeps the runner molten while the cavity still operates at 60–80 °C. Valve-gate systems maintain a mechanical seal that prevents drool and can hold the gate open slightly longer to pack the cavity even when the mould is at the lower end of the temperature band. This allows a moulder to run the tool at 60 °C for faster skin solidification while retaining packed cores through delayed gate closure. The pressure drop across a hot runner, typically 20–40 MPa, must be added to the machine pressure calculation. In direct-gated cold-runner designs, the same 60 °C tool may gate-seal too early because the runner itself freezes near the gate, so the hot-runner advantage is configuration-specific rather than material-specific.

A 60–80 °C setpoint is meaningful only if the cavity steel maintains that temperature within ±2 K during cyclic heat load. Cooling circuits should be positioned 1.5–2.0 × diameter from the cavity surface, and water flow should remain turbulent with Reynolds numbers above 10,000 to maintain heat-transfer coefficients high enough to remove heat from the melt. In high-cycle connector moulds, cooling water at 70 °C supplied at 0.4–0.6 MPa through circuits of 8–10 mm diameter is common. Cores with restricted cooling often show local temperatures 15–20 K above the setpoint, producing non-uniform shrinkage and warpage that no amount of holding pressure can correct. Beryllium-copper inserts, conformal cooling channels, and validated thermal camera or thermocouple audits are practical measures for maintaining the window across the entire cavity surface.