What Is the Significance of Thermal Diffusivity in Selecting Heating Plate Materials for Cyclic Applications?

The change in temperature of the platen has a direct impact on the productivity of processes such as thermoforming, composite moulding or any process that involves repetitive heating and cooling cycles. Thermal conductivity describes part of the narrative while thermal diffusivity provides a more complete picture of the transient thermal response. In cyclic applications, the choice of a heating plate material with a high thermal diffusivity can result in substantial reductions in cycle time and energy usage.

Thermal Diffusivity: How Quickly a Material Responds to Temperature Changes
Thermal diffusivity (α) is a material attribute that characterises how quickly a material may change its temperature in response to a change in its environment. It has three basic qualities combined:

α=k/ρ*Cp

Where:

k = thermal conductivity (W/m K)

ρ = density (kg/m3)

Cp = specific heat capacity J/kg.K

The numerator ( k ) is the ability to conduct heat . The denominator ( Cp) is the volumetric heat capacity, which is the thermal energy needed to raise the temperature of a unit volume by one degree. Thermal diffusivity is thus a measure of the rate at which heat spreads through a material in comparison to the amount of energy that must be absorbed to change its temperature.

A high diffusivity signifies that the material will heat and cool quickly, while a low diffusivity implies thermal sluggishness.

Thermal Diffusivity of Typical Heating Plate Materials
The following table compares thermal diffusivity for materials commonly used in heating plates, platens and press surfaces:

Material Conductivité thermique (W/m·K) Densité (kg/m³) Chaleur spécifique (J/kg·K) Conductivité thermique (mm²/s) Vitesse relative Cuivre (pur) ~400 8960 385 ~116 Très rapide
Aluminium (6061-T6) ~167 2700 896 ~69 Fast Aluminium (5083) ~120 2660 900 ~50 Moderate-fast
Steel (low carbon) ~50 7850 460 ~13.8 Slow 
Stainless steel (304) ~16 8000 500 ~4.0 Very sluggish 
Titanium (Grade 2) ~17 4510 520 ~7.2 Slow 
Note: Values are approximate and fluctuate somewhat with alloy and temperature.

Aluminium alloys have thermal diffusivity values in the range of 50-100 mm²/s, whereas stainless steel is about 4 mm²/s. This means that the aluminium platen will respond to changes in heater power or cooling flow about 10-20 times faster than a stainless steel platen of the same thickness.

Why is Thermal Diffusivity Important for Cyclic Applications?
In cyclic heating processes like thermoforming, compression moulding, heat sealing or laminating, the platen or heating plate is cycled from a lower temperature to a setpoint, held for a process interval and then cooled to a lower temperature before the next cycle. The entire cycle time is the total of:

Ramp-up (heating time)

Time to hold or soak

Ramp-down (cooling time)

Material time handling

Any decrease in the heating or cooling time directly increases the throughput and minimises the energy consumption per part.

A material with high thermal diffusivity will allow the platen to reach the desired temperature faster when power is applied, and to shed heat faster when cooling flow is activated. Conversely, a material with poor diffusivity exhibits thermal lag, the center of the platen may still be hot while the surface has cooled, or the surface may respond slowly to the heater changes.

In fact, the choice of the thermal diffusivity of the heating plate material will determine whether the platen can keep up with a quick cycling production line or whether it will be the bottleneck.

Practical Example: Aluminium Plate vs. Stainless Steel
Let's say we have two heating plates, 20 mm thick, one of aluminium (6061) and the other of stainless steel (304). Both are heated by identical cartridge heaters and cooled by identical water conduits.

Heating time 50°C to 200°C - The aluminium platen heats up to setpoint around 1/10th as fast as the stainless steel platen due to quick diffusion of heat from the heaters through the entire plate . The stainless steel plate shows a strong temperature gradient around the heaters, the core lags.

Cooling time 200 °C to 60 °C – The aluminium platen cools down fast via heat transfer to the cooling channels and removal of heat. The stainless steel platen's core keeps heat, demanding a lengthy cool-down or active cooling on both sides.

Energy usage - stainless steel has a larger volumetric heat capacity (ρ x Cp = 4.0 MJ/m3-K for steel vs. ≈ 2.4 MJ/m3-K for aluminium) It requires more total energy to reach the same average temperature because of its lower diffusivity and wastes more energy while cooling (unless energy recovery is used).

By moving from a stainless steel platen to an aluminium platen, you can shave minutes off each cycle for a process running 100 cycles per day, boosting daily output by 20-30%.

Thermal Diffusivity vs. Thermal Conductivity: Why They Are Both Important
Engineers with a background in steady-state heat transport generally concentrate on thermal conductivity (k). For a continuously operating heater at constant temperature, the heat flux (for a given temperature difference) is determined by the conductivity. However, for cyclic applications the more relevant statistic is often diffusivity.

A substance can be very conductive but also possess a high volumetric heat capacity and thus a moderate diffusivity. For example copper has an extremely high conductivity (~400 W/m*K), but also a high density and moderate specific heat so that its diffusivity is ~116 mm2/s-still very fast, but not proportionally quicker than aluminium (69 mm2/s) given copper's much greater conductivity. Aluminium provides an ideal compromise: metal has relatively high conductivity, low density and moderate specific heat, producing strong diffusivity at lower cost and weight.

Stainless steel is a bad conductor and has a moderate volume heat capacity which leads to very low diffusivity. Despite its superior corrosion resistance and mechanical strength, this makes stainless steel an unsuitable choice for any application that involves rapid change of temperature.

Selection Guidance for Materials for Cyclic Applications
The most important performance figure is the thermal response time of the cyclic heating plates which scales as (thickness2 / α). The reaction time is decreased by a factor of four when halving thickness. Material selection α has a major effect also.

Use high diffusivity materials (aluminium, copper or aluminum-copper composites) when:

The method includes repeated heating and cooling cycles (e.g., thermoforming, heat sealing, cyclic presses).

Cycle time reduction is a productivity imperative.

Importance of energy efficiency (less energy saved & squandered in each cycle)

The working temperature is below the limits of the material (aluminium: ~400-500 °C; copper: ~300 °C for prolonged usage owing to oxidation).

Select lower-diffusivity materials (steel, stainless steel, titanium) only when:

The process is virtually continuous (steady state) with few heat cycles.

High temperature ( > 500C ) removes aluminium and copper.

Thermal response is more important than corrosion resistance or mechanical wear resistance.

The heating plate is quite thin to compensate for the poor diffusivity.

Note that for very thick platens (50 mm or more) even aluminium will show significant thermal lag. In these circumstances, the effective diffusivity can be enhanced by using designs with embedded heaters and cooling channels scattered through the thickness, or by using copper heat spreader layers (see a prior article).

Materials with High Diffusivity Limitations
For cyclic applications, high thermal diffusivity is desirable, but the materials that provide it (aluminium and copper) have limitations:

Maximum temperature – aluminium strength drops off above 200-250°C, and it starts to soften appreciably above 350°C. Copper rapidly oxidises in air at 300°C. If the temperature is above 400°C the cycling is slow and you might only have steel or nickel alloys to choose from.

Corrosion resistance - Aluminium and copper are not appropriate for direct contact with many chemicals or acidic conditions. Stainless steel is a poor diffuser, but may be necessary for corrosion resistance.

Surface hardness and wear - Aluminium is soft and easy to scratch. Steel hardened or coated surfaces may be needed in sliding or abrasive contact. In such instances a multi-layer platen (thin steel working surface connected to an aluminium base) can offer both wear resistance and high diffusivity.

Conclusion: Thermal Diffusivity is Key to Rapid Cycling Material Selection
Thermal diffusivity is an important feature in applications involving cyclic heating. It defines the rate at which a heating plate can respond to changes in heating or cooling power. Materials with a high diffusivity, such as aluminium (≈50-100 mm2/s), heat and cool quickly, minimising cycle time and reducing energy usage. Materials with poor diffusivity (e.g. stainless steel (≈4 mm2/s)) exhibit heat lag, resulting in longer cycle durations and energy waste.

Where operating temperatures and corrosion limitations permit, aluminium is favoured over steel for processes where repeated heating and cooling cycles are involved. For even faster responsiveness copper or copper-aluminium composites are also an option. Thermal diffusivity is often the critical metric for cyclic applications. Selection of materials should consider both the steady-state and transient thermal performance.