With newer 3D printable materials being researched and released by all brands, it is important to understand some aspects relating to material selection. Engineers use the material datasheet to perform a feasibility study whether the given material suits the needs and requirements of the application at hand. There are several material properties, and each material may have additional properties that cannot be covered in a single shot but in this article we will give an idea of some common properties to consider in a general way without going into too much of technical details. Also one need to understand that 3D printed and injection molded materials have significantly different properties than conventional engineering materials like metals mainly due to how they are manufactured. Hence the datasheet for a given material should only be used as a comparison but actual performance of the part may vary.
Tensile strength is one of the most common properties mentioned in every datasheet so let’s understand it in a simpler form. When it comes to strength, it is the ability of the material to withstand load before failure (breaking or deforming permanently). Permanent deformation is called as yielding which is seen in ductile materials and in brittle materials the part does not show any deformation but breaks suddenly at the ultimate tensile strength. In a typical practical application load is not either in pure tension or bending or compression or shearing but a combination of all of these. Nevertheless, tensile strength could be taken as a reference to measure other forms of strength. Tensile Strength is measured by pulling a sample under tension between two hydraulic jaws till it breaks. This test also measures the stiffness (or Young’s Modulus) and deformation the material undergoes before breaking (expressed in percentage elongation). As an example, Steel has a yield tensile strength of 400 MPa (starts to deform) and an ultimate tensile strength of 600 MPa (break) while material like ABS has strength of 40 MPa meaning that Steel is 10 times stronger than ABS plastic.
Flexural Strength is another property commonly specified in datasheets. While some materials have same strength both in tension and bending, some materials have lesser strength in bending while some others show more strength in bending, hence the bending strength (or strength when the material flexes, hence the name flexural strength) measured under a 3 point bending test is used. Flexural strength and flexural modulus are specified in MPa and GPa respectively.
In reality materials are prone to experience loads due to sudden impacts or shock. Impact strength is measure of energy absorbed under shock load rather than the load applied. Pendulum based impact testers use a heavy weight that swings and hits the test specimen. Based on rebounding height of the specimen, the impact strength is measured. If pendulum returns to same height, no energy is absorbed by the specimen thus has no impact strength, and if the specimen absorbs some impact energy, the pendulum rebounding height will be lower than the point where it was released. Impact strength is expressed as energy absorbed per unit cross-sectional area of the specimen.
Material behaviors can change with the temperature. Out of many thermal properties that may appear in the datasheet depending on the material, we focus on heat deflection temperature and glass transition temperatures here. Heat deflection temperature is the temperature beyond which the material starts to lose its strength and starts deforming under its own load. Glass transition temperature is another property only specified for amorphous solids that gives a temperature point beyond which material starts behaving like soft rubber losing its stiffness. To give an example, PLA has a glass transition temperature of 50 C while ULTEM has a Glass transition temperature of 217 C.
Hardness is the resistance to penetration by sharp objects. It is measured by durometers that have a sharp pointed indenter. Based on how far the indenter penetrates into the test specimen, the hardness of the material is given a number. Higher number stands for more hardness. Although there are several hardness measuring scales like Brinell, Rockwell, Vickers scales etc, Shore hardness is the scale used for plastics and polymers. Scale-A for softer materials like rubber and Scale-D for hard plastics.
Depending on the specialty of the material, additional information such as its resistance to chemicals, resistance to heat and flames, electrical properties, post-processing requirements may be given in the datasheet. As not all properties will be well suited for the application under consideration, only the predominant factor is given more importance for selecting the material and other properties may be compromised to some extent.