Heat deflection temperature (HDT), glass transition temperature (Tg) and mechanical properties are useful parameters for describing the thermal and structural behaviour of polymeric materials. In 3D printed photopolymers, however, the measured values also depend on the printing, curing and post-processing conditions used to manufacture the test specimens.
Key point: Tg, HDT and mechanical properties should not be interpreted independently from the printing and post-processing specifications used to produce the tested parts.
About Tg
The glass transition temperature, Tg, describes the temperature range in which increased molecular mobility produces significant changes in the thermal and mechanical behaviour of the amorphous portion of a polymer.
Below the glass transition region, the amorphous polymer phase behaves in a relatively rigid and glassy manner. As temperature increases through the glass transition region, molecular mobility increases and the material becomes progressively softer and more compliant.
Tg is therefore an important parameter for characterizing amorphous polymer systems. Its measured value can be influenced by factors such as polymer structure, molecular weight, additives, residual species, moisture, degree of curing and crosslink density.
For photopolymers and 3D printing resins, curing conditions can influence the polymer network produced during printing. Consequently, wavelength, light power, exposure time, effective energy dosage and post-curing conditions can affect the measured Tg and the resulting thermomechanical behaviour.
Amorphous structure of 3D printed photopolymers
Many photopolymer 3D resins form predominantly amorphous polymer networks during printing because of their composition and rapid curing kinetics.
Unlike crystalline thermoplastics, thermoset photopolymers do not normally exhibit conventional melting behaviour. Upon heating, they may soften or lose stiffness depending on their polymer structure, crosslink density, Tg and processing history, but they do not melt in the same way as conventional thermoplastic materials.
Crosslink density and conversion are influenced by curing kinetics. Changes in printing and post-curing conditions can therefore affect Tg, stiffness and other thermomechanical properties of the final printed material.
About HDT
Heat deflection temperature is the temperature at which a standardized polymer test specimen reaches a specified deformation under a defined flexural load while the temperature is progressively increased.
Under ASTM D648, HDT testing is performed using standardized specimens subjected to a defined flexural stress, commonly 0.45 MPa or 1.80 MPa. ISO 75 uses comparable principles, with specimen dimensions and deflection criteria defined according to the applicable test configuration.
The specimen is heated at a controlled rate while the defined mechanical load is applied. The temperature at which the specified deflection is reached is recorded as the heat deflection temperature.
HDT is therefore a macroscopic thermomechanical measurement. It describes the ability of a specimen of defined geometry to retain stiffness under a defined mechanical load as temperature increases.
Difference between Tg and HDT
Tg and HDT are related to thermal behaviour, but they describe different phenomena.
Tg describes a transition associated with molecular mobility in the amorphous polymer phase. HDT describes deformation of a standardized specimen under a defined mechanical load and temperature programme.
For this reason, Tg and HDT should not be treated as interchangeable values.
HDT also depends on specimen geometry, stiffness and loading conditions. Reinforcing fillers or ceramic particles, for example, may substantially increase stiffness and HDT without producing an equivalent change in the Tg of the polymer phase itself.
Effect of printing conditions on Tg, HDT and mechanical properties
Photopolymer properties are generated during printing and post-processing. The liquid resin alone therefore does not completely define the final thermomechanical performance of the printed material.
Relevant processing variables include:
- Light-source wavelength
- Light power or irradiance
- Exposure time
- Effective energy dosage
- Layer thickness
- Print orientation
- Cleaning conditions
- Light post-curing conditions
- Thermal post-processing, where applicable
Different combinations of light power and exposure time can produce different polymerization kinetics even when the nominal energy dosage appears similar. Consequently, specimens printed from the same resin can exhibit differences in conversion, crosslink density, stiffness, elongation, tensile strength, flexural strength, Tg and HDT.
Undercuring can result in lower conversion and softer material behaviour. Increasing the effective curing level can increase rigidity and thermomechanical performance until the processing conditions appropriate for the material and application are reached.
Very different light-power and exposure-time regimes may also generate different polymer-network structures and mechanical responses. For this reason, results obtained with one printer or curing protocol should not automatically be assumed to be identical to those obtained using another printing system.
Effect of fillers and reinforcement
For unfilled photopolymer systems, Tg and HDT may show a relatively close relationship because both are influenced by the behaviour of the polymer network.
In filled or reinforced systems, the relationship can be different. Ceramic fillers, reinforcement additives and other solid phases can increase stiffness and resistance to deformation, thereby increasing HDT, while the Tg of the polymer phase may change to a much smaller extent.
This distinction is particularly important when comparing unfilled photopolymers with highly filled composite, ceramic or reinforced 3D printing systems.
Geometry and specimen thickness also matter
Absolute Tg and HDT values should also be interpreted in the context of the geometry being evaluated.
Standardized HDT measurements use specimens with defined dimensions, whereas real 3D printed parts may contain walls, lattices, thin sections, ribs or other geometries substantially different from standardized test bars.
Apparent rigidity therefore depends not only on the intrinsic material properties but also on geometry and section thickness. A relatively rigid material may appear flexible in a very thin section, while a more flexible material can behave comparatively rigidly when its section thickness increases.
Why standardized processing conditions are important
Meaningful comparison of photopolymer properties requires controlled and reproducible specimen preparation.
When comparing Tg, HDT, Young's modulus, tensile strength, flexural strength or other mechanical properties, the printing and post-processing conditions used for the specimens should therefore be considered together with the reported numerical values.
This is particularly important when results from different printers, wavelengths, exposure strategies or post-curing protocols are compared.
Conclusions
- Tg describes molecular mobility and thermal transition behaviour within the amorphous polymer phase.
- HDT describes the temperature-dependent deformation of a standardized specimen under a defined mechanical load.
- Tg and HDT are related but are not equivalent properties.
- Printing wavelength, light power, exposure time, energy dosage and post-curing conditions can influence the final polymer network and its thermomechanical properties.
- Young's modulus, tensile strength, flexural strength, elongation, Tg and HDT should therefore be interpreted together with the printing and post-processing specifications used to manufacture the specimens.
- Fillers and reinforcement phases can increase stiffness and HDT without producing an equivalent increase in the Tg of the polymer phase.
- Real-part behaviour also depends strongly on geometry and thickness and may differ from the behaviour of standardized test specimens.