Polylactic Acid (PLA) is a popular thermoplastic material, particularly in the field of 3D printing, prized for its ease of use and biodegradability. Like all polymers, PLA’s physical properties are highly dependent on temperature, moving through distinct states as it heats up. Understanding these thermal thresholds is necessary for successful manufacturing and product stability. The temperature at which PLA begins to soften is distinct from where it fully melts, and knowing the difference is essential for users.
Understanding the Softening Point (Glass Transition)
The temperature at which PLA starts to soften is defined by its Glass Transition Temperature (Tg). This transition marks the point where the polymer chains gain enough thermal energy to begin moving past one another, causing the material to change from a rigid, glassy state to a soft, rubbery state. The softening point for standard PLA typically falls within a narrow range, usually between 55°C and 65°C. This temperature is not a melting point, but rather a structural shift where the material loses its stiffness and structural integrity. If a PLA object is exposed to temperatures at or above this Tg, it will begin to deform and lose its shape.
The Upper Thermal Limits (Melting and Degradation)
Temperatures significantly above the glass transition point lead to the material’s true Melting Temperature (Tm). At this range, PLA transitions from a softened, amorphous solid into a fully viscous liquid. The melting point for standard PLA generally ranges from 150°C to 180°C. This higher temperature range is relevant to the manufacturing process, such as the extrusion temperature used in 3D printing nozzles. The material must be heated well into this range, typically between 180°C and 220°C, to ensure smooth flow and proper layer adhesion during printing. A third, much higher thermal limit is the degradation temperature, where the polymer chains begin to break down chemically. For PLA, this thermal decomposition generally begins around 290°C to 300°C, which must be avoided during processing.
Practical Implications for PLA Use
The low glass transition temperature of PLA has direct consequences for the longevity and application of printed parts. Exposure to temperatures only slightly above 60°C, such as leaving an object inside a hot car on a summer day, can cause the material to quickly warp or droop under its own weight. This deformation occurs because the part has entered its rubbery state and can no longer resist external forces.
In 3D printing, the low Tg of PLA contributes to a common issue called “heat creep.” Heat creep happens when thermal energy from the hot end travels too far up the filament path toward the cold end. If the heat reaches the section where the PLA is supposed to remain solid, the filament softens prematurely, swells, and causes a jam in the extruder.
To address the low softening point, a post-processing technique called annealing can be used to improve the material’s heat resistance. Annealing involves intentionally exposing the printed part to a temperature just below or at the Tg, often between 60°C and 100°C, for a set period. This heat treatment encourages the polymer chains to reorganize into a more ordered, crystalline structure. The resulting increase in crystallinity raises the material’s effective softening point, making it suitable for applications requiring greater thermal stability.