Plastics in 3D printing

To understand how plastics are processed in 3D printing, one must first get to know the different types of plastics.

Plastics are classified into thermoplastics, thermosetting plastics, and elastomers. The latter currently play a subordinate role in additive manufacturing. Thermoplastics and thermosetting plastics account for about 75 percent and approximately 25 percent (Wohlers Report 2022), respectively, of the additive manufacturing market.

Thermoplastics become soft when heated and harden again when cooled. These processes can be repeated. Thermoplastics have unconnected molecular chains that mostly lie parallel to each other. Instead of being held together by chemical bonds, these chains are connected by intermolecular forces, also known as Van der Waals forces. When a large amount of energy, such as heat, is supplied, these chains can easily slide against each other. This makes the plastic malleable, and with further heating, it begins to decompose. Thus, thermoplastics can be recycled by melting and reshaping them.
Thermoplastics used in 3D printing include Polyamide with a market share of almost 50 percent of all 3D-printed plastics (Wohlers 2022), followed by well-known polymers like ABS (Acrylonitrile Butadiene Styrene), PLA (Polylactide, also simplified as “Polylactic Acid”), or PP (Polypropylene).

Thermosetting plastics, on the other hand, can be molded only once and cannot be liquefied or reformed afterward. Due to their chemical structure, they are not easily recyclable. When heated, they tend to decompose and burn rather than go through a recycling process. This is because thermosetting plastics have a dense network of chemical bonds called “cross-linking,” which makes their structure permanent and heat-resistant. Thermosetting plastics are sometimes (not entirely accurately) referred to as “synthetic resins.” Examples of thermosetting plastics include epoxy resins and many polyurethanes.

Thermoplastics – Differentiation based on Molecular Structure

Thermoplastics are typically divided into two main classes: amorphous and semi-crystalline. This classification refers to the arrangement of polymers at the microscopic level, and certain 3D printing processes deliberately exploit these different properties.

Amorphous thermoplastics have a random, disordered structure, which makes them relatively transparent and gives them lower chemical resistance and lower heat deformation resistance compared to semi-crystalline thermoplastics. They do not have a defined melting point. At certain temperatures, they transition from a rigid to a flexible state, as their molecular chains start to move (this is called the “glass transition temperature”). However, the plastic does not immediately turn into a liquid; this happens only as the temperature increases further.

Semi-crystalline thermoplastics have both ordered (crystalline) and disordered (amorphous) regions in their structure. They are usually opaque and have higher chemical resistance and higher heat deformation resistance than amorphous thermoplastics. These plastics also change shape at certain temperatures, but this applies only to the disordered (amorphous) parts. Often, this temperature is lower than the melting point. Therefore, these plastics can be both rigid and flexible between these two temperatures. They are known for bending without breaking, which is referred to as “ductile.” Semi-crystalline thermoplastics usually have a defined melting point.

There are also elastomers, a subclass of thermoplastics, also known as thermoplastic elastomers (TPEs). These materials combine the elastic properties of rubber with the processability of thermoplastics, which means they can stretch and deform like rubber but can also be molded and recycled like other thermoplastics.

Thermoplastics – Differentiation based on Applications

Comparison of thermoplastics

COMPARISON OF HIGH-PERFORMANCE PLASTICS

Source: https://de.wikipedia.org/wiki/Hochleistungskunststoffe

Thermoplastics can also be divided based on performance, as shown in the figure known as the polymer pyramid.

Standard thermoplastics like ABS or polypropylene are versatile and easy to process. They are suitable for products that do not require high-performance characteristics, such as packaging and toys.

Technical thermoplastics like PA (polyamide) and PC (polycarbonate) offer more strength and stiffness than standard thermoplastics but are less resistant than high-performance thermoplastics. They are ideal for applications that require increased resistance but not necessarily extreme hardness or heat resistance.

High-performance thermoplastics like PEEK are characterized by high heat and mechanical resistance. They are used in demanding fields such as the automotive, electronics, and aerospace industries.

The most important technology for 3D printing amorphous thermoplastics is the Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF) process, also simply referred to as Fused Layer Modeling. In this process, a filament is heated in a nozzle and deposited layer by layer. Increasingly, it is also possible to use granulates instead of filaments.

Examples of polymers used in the FDM process:

Semi-crystalline thermoplastics can be processed as powders. Specifically, this is done in 3D printing using the Selective Laser Sintering (SLS) or Multi Jet Fusion (MJF) process, which 3Faktur specializes in.
In SLS and MJF, the so-called powder bed processes are used. In this process, a closed layer of powder is applied, and it is melted at the desired locations. In laser sintering, this is done using a laser, while the Multi Jet Fusion process uses infrared light. However, this process only works at high process temperatures, just a few °C (correct Kelvin) below the melting point. Therefore, only semi-crystalline polymers are suitable for this process class, as amorphous polymers reach their glass transition temperature at such temperatures. This would result in the material fusing together, making it impossible to separate the remaining material from the desired component.

Semi-crystalline polymers can also be processed using the Fused Deposition Modeling (FDM) process. However, the performance of powder bed processes is generally higher, which is why Polyamide, especially PA 12, is predominantly used as a powder.

Typical polymers for powder bed processes:

Thermosetting plastics are made from liquid starting materials. In 3D printing, primarily photopolymers are used. These are light-sensitive plastics that change their properties under the influence of light, typically ultraviolet light. Key processes in this class are Stereolithography (SLA), Digital Light Processing (DLP), and Material Jetting (e.g., “Polyjet” by Stratasys), where UV light is used to transform the liquid resin form of the photopolymer into a solid state. There are several 3D printing methods for processing photopolymers.

Epoxides are also used in the DLP process. However, DLP also uses other material classes, such as

Overall, the range of materials for thermosetting plastics is extensive and characterized by brand names. For more information about the materials, it is best to directly consult the material manufacturers, such as
Henkel, Evonik, BASF