Mechanical properties and applications for plastics.
The main reason why multiple types of filament spools exist is due to the big diversity in mechanical, thermal, chemical, and optic properties that the polymers can have. Having in consideration these properties can help us improve the quality and performance of our models. Although having profound knowledge of all these properties isn’t compulsory to produce decent 3D prints, being familiarized with them will increase the quality level of the parts or products manufactured.
On this occasion, we’ll mix some mechanical properties relevant to additive manufacturing and for plastic injection processes.
Density: The quantity of mass per volume unit for a material. It can be referred to as net density, apparent density, and compact density. The most important of them is the net density, which corresponds to the measurement of the material in a pure solid state. The density of a certain plastic can become of relevance when purchasing filament spools. For example, ABS has a density of 1.03 g/cm3, and PLA of 1.24 g/cm3, making PLA 20% denser than ABS. Meaning that when buying a 1 kg spool of ABS, it will last 20% longer than a 1 kg spool of PLA for 3D printing.
Molding contraction: When plastics contract or shrink and reduce their volume when in the cooling process. It is a factor of high importance in injection molding processes, and it is recommended to run computational simulations for specific plastics. The shrinkage values can range from 0.4% for Polystyrene, to 2% for PBT.
Humidity absorption: The relative quantity of water absorbed by the plastic when being exposed to a humid environment. It is expressed as a percentage and interpreted as the weight gained from water absorption. Usually, it is preferred to keep plastics dry, failing in doing so, their mechanical properties and appearance can be affected. One of the most hygroscopic plastics is PVB or also known as PolySmooth manufactured by PolyMaker. As an example, PLA has a humidity absorption of 0.25% and PVB of up to 8%. Therefore, it is compulsory for some plastics to use a dehumidifier.
Tension stress: The capacity of material of contra resting a tension force. This study is performed with a universal testing machine and is reported in MPa or kg/cm2. The study consists of grappling a specimen with the two grips of the testing machine. The grips will move and separate each other until the specimen breaks. The higher the value, the greater the resistance to failure.
Compression stress: It is the contrary study of the tension stress test. It is the material’s capacity of opposing a compression force until failure or significant deformation. To perform this test the grips are replaced by a pair of discs and the specimen by a solid cylinder of known dimensions.
Flexural modulus: The capacity of a material to prevent bending. The universal testing machine can also be used to calculate this value. It is also expressed in MPa and a higher value equates to a greater force required to bend the material.
Hardness: The ability of a material to avoid being scratched on the surface. This property can be expressed in the Rockwell scale, which drops a steel sphere onto the material surface and the damage is measured. The Shore scale is also commonly used. In most cases, Shore A is used for flexible plastics, and Shore D or Rockwell for rigid plastics. In the case of 3D printable elastomers, TPU usually has a Shore A that ranges from 30 to 90.
Considering these properties can reduce manufacturing and prototyping costs when knowing beforehand the characteristics that should be taken care of for parts to perform as expected. The filament we sell and use in our products has its own datasheets with information about some of these parameters. In future articles, we’ll cover the rest of the properties such as temperature and chemical properties of different polymers.
The Enciclopedia del Plástico Siglo XXI was used as reference bibliography for this article.