Plasticity is a material’s ability to permanently change shape under force without fracturing. In 3D printing, this property determines an object’s performance and suitability for applications, ranging from rigid prototypes to functional components that must endure stress.
Understanding plasticity allows creators to select the right material for a specific task. This ensures the final product has the required mechanical characteristics for its intended use, whether it needs to be unyielding or bend without breaking.
The Mechanics of Plastic Deformation
A material’s response to stress is shown on a stress-strain curve, which graphs the force applied against the resulting deformation. This curve illustrates two primary types of deformation: elastic and plastic. In the elastic region, a material returns to its original shape after the force is removed, as its atomic bonds stretch but do not break.
As the applied force increases, the material may reach its yield point. This threshold on the stress-strain curve marks the transition from elastic to plastic deformation. Beyond this point, the material undergoes a permanent, non-recoverable change in shape because the atomic structure has been altered.
A material’s ductility is the extent to which it can plastically deform before it fractures. A highly ductile material can be stretched or bent significantly, absorbing energy without failing. In contrast, a brittle material exhibits little plastic deformation and will fracture suddenly when stress exceeds its elastic limit. The total area under the stress-strain curve represents the material’s toughness, its ability to absorb energy and deform without fracturing.
Plasticity in Common 3D Printing Materials
The wide range of 3D printing materials exhibits different plastic behaviors, making material selection an important step in the design process. The choice of filament influences the final part’s durability and ability to deform under stress.
Polylactic Acid (PLA) is a common 3D printing material known for its rigidity. However, it is also quite brittle and has low plasticity, meaning it is more likely to fracture than bend when subjected to high stress. This makes PLA a good choice for detailed prototypes and objects not intended to bear significant loads.
Acrylonitrile Butadiene Styrene (ABS) and Polyethylene Terephthalate Glycol (PETG) offer a better balance of properties for functional applications. ABS is known for its high impact resistance and ductility, allowing it to absorb shocks and undergo considerable plastic deformation before failing. PETG provides a good combination of strength and impact resistance, making it more durable than PLA and allowing parts to bend under load.
For applications requiring flexibility, Thermoplastic Polyurethane (TPU) is an excellent choice. Unlike materials valued for plasticity, TPU is highly elastic, meaning it can stretch and bend repeatedly and return to its original form. Its rubber-like properties make it ideal for parts like flexible seals, gaskets, and protective cases.
Designing for Plasticity in 3D Printing
Engineers and designers can leverage material properties to create robust 3D-printed parts. By understanding how a material will deform, it is possible to design features that intentionally bend or flex. This approach is used to create components like snap-fit enclosures, which rely on a material’s elasticity to deform temporarily to lock into place.
Living hinges are another design element that relies on plasticity. These are thin, flexible sections of plastic that connect two larger, rigid parts, allowing them to bend. Materials with good ductility, such as PETG and Nylon, are well-suited for living hinges because they can endure repeated bending cycles without fatiguing.
Conversely, some applications demand high rigidity where any plastic deformation is undesirable. In these cases, design strategies must focus on minimizing stress concentrations that could lead to yielding. Adding fillets to sharp internal corners helps distribute stress over a wider area, reducing the likelihood of permanent deformation.
Modifying and Analyzing Plastic Behavior
The plastic properties of a 3D-printed part can be altered through post-processing techniques. One common method is annealing, which involves heating a printed object to a temperature below its melting point and then allowing it to cool slowly. This process can relieve internal stresses and modify the material’s microstructure, often leading to increased ductility and strength.
For example, annealing PLA improves its heat resistance and mechanical durability. The heat allows the polymer chains to rearrange into a more ordered, crystalline structure, which enhances its properties. Similarly, annealing PETG can improve its impact durability, making parts tougher and more resilient to sudden loads.
Engineers can also use simulation tools to predict a part’s behavior under stress. Finite Element Analysis (FEA) is a computational method used to simulate how a 3D model will respond to applied forces, including how it will deform plastically. By breaking the digital model into many small elements, FEA software calculates the stress and strain throughout the object. This allows designers to identify potential failure points and analyze plastic deformation virtually, enabling them to optimize the design before printing.