Updated: Aug 6, 2020
Image source: Diversified plastics
Introduction to plastics processing for additive manufacturing
Thermoplastic polymers have been around for a while with various technologies and processes due to their high potential to cost-effectively manufacture products in high volume. Recently, thermoplastics are gaining popularity in low volume production as well, thanks to the entrance of additive manufacturing (AM) technologies such as Filament Fusion Fabrication (FFF) also known as Fusion Deposition Modeling (FDM), Selective Laser Sintering (SLS) and AFP (Automated Fiber Placement) for composites. So, thermoplastics are here to stay..
The new digital manufacturing era made many designers and engineers with multidisciplinary background to deal with polymers and plastics, therefore, it is crucial to be on the same page once talking on a specialized discipline, polymers or other.
In this article, we’ll review the basic terminology in plastics processing, when the objectives are to define and clarify the practical meaning of the terms in the world of polymers in general and thermoplastics’ processing, in particular. We’ll review terms like melting point (Tm), glass transition temperature (Tg), molecular interdiffusion and more as they are a part of every day’s life of designers & engineers in the AM industry. By using the FFF process of thermoplastics, we’ll explain the meaning of each term and its relevance and importance.
What are thermoplastics? Thermo (heat)-plastic (shape or form)
Thermoplastic polymers are macromolecules (long molecular chains) that are bonded together with intermolecular electrostatic forces such as Wan der Waals, Hydrogen bonds & dipole-dipole. These intermolecular forces are weaker than intramolecular bonds (covalent bonds). Moreover, the intermolecular interactions are reversible by heating. The heat weakens the intermolecular interactions and the polymer softens or melts to a point it can be molded or formed. Cooling solidifies the thermoplastic polymer and maintains its new shape. Thermoplastic polymers are divided to two main groups: Amorphous & Semi-Crystalline. This is well presented in the well-known, Thermoplastics Pyramid.
Figure 1: The thermoplastics pyramid (Image source: Diversified plastics)
The left side of the pyramid represents the amorphous polymers, whereas, the right side represents the semi-crystalline polymers. Going up the pyramid means increased performance, price, Service temperature and processing temperature. Polymers are long chains, so it is hard for them to rearrange in an ordered manner, hence, amorphous domains are formed. All polymers have amorphous domains.
Figure 2: Schematic illustration of amorphous & semi-crystalline domains.
The efficient packing in the crystalline domains exhibits stronger intermolecular interactions via Wan der Waals, Hydrogen bonds or dipole-dipole.
Amorphous polymer - a polymer that its molecules have a random spatial arrangement and no crystalline structure.
Polymer crystal - alignment or folding of molecules to form a lamella. Lamellas may grow radially or spherically to form Spherulites. Amorphous domains connect between lamellas and spherulites. Some polymers can form repeated ordered domains due to their molecular
structure, which is the most crucial factor in the ability of a polymer to crystallize.
These polymers are called semi-crystalline since they are not fully crystallized.
Semi-Crystalline polymer - a polymer that can rearrange in an ordered and repeated manner to form crystalline domains. Crystals have stronger intermolecular interactions than amorphous domains due to closer packing. Crystallinity level can vary up to 80% (wt.) of the material’s structure. Crystalline domains have stronger interactions than amorphous domains due to a more efficient packing of the polymeric chains, thus more energy (heat) is required to break ththem and allow the molecules to “flow”. Degree of crystallinity may affect thermal, mechanical, optical and chemical properties.
But what that has to do with AM?
Well, after all, AM methods like FFF are plastics processing technologies. To print a plastic filament, heat must be applied. As previously stated, heat weakens the intermolecular bonds up to the required level for the material to flow through the extruder and nozzle. The obvious questions rise, how do we know how much heat should we apply? what should be the operating temperature?
Each thermoplastic polymer has its own transition temperatures: Amorphous polymers only have a glass transition temperature or Tg.
Tg- the temperature where the molecular chain mobility increases dramatically (during heating) in the amorphous domains. Below that temperature the material is stiffer and considered to be in a “glassy state” and at a “rubbery state” when higher.
Semi-crystalline polymers have two additional transition temperatures which are melting temperature (Tm) and Crystallization temperature (Tc).
Tm- the temperature where the intermolecular bonds weaken dramatically in the crystalline domains and crystals dissociate.
Tc- the temperature where crystals are formed
Transition temperatures hierarchy - Tm>Tc>Tg
By exceeding the transition temperatures (Tm for Semi-crystalline & Tg for amorphous), it will be easier for the material to flow and achieve the required shape. Many questions remain open but now that we have covered the basic terms of thermoplastic polymers, Amorphous, Crystalline, Tg, Tm, Tc, it will be easier to understand their relevance to the AM process.
My next article will discuss the practical and applicative meaning of these terms, new terms (for some of you) and other phenomena like crystallization and molecular interdiffusion, that have great impact on the printed part quality.