A Brief Overview of the Thermoplastics Pyramid

Sometimes, the use of simple diagrams is the best way to get into a concept. What better way to start than by introducing thermoplastics through the following pyramid:

Aniwaa AM Thermoplastic Pyramid
This non-exhaustive pyramid lists a few of the most commonly used thermoplastics in AM.

The diagram above is a widespread tool used to sort out the thermoplastics available on the market and improve critical decision-making processes in the industry. 

There are numerous iterations of this pyramid on the web, each one with varying complexity and detail. Most of these pyramids, however, group polymers in the same ways:

  • Horizontally, by crystallinity: Amorphous, Semi-crystalline, and sometimes Elastomers
  • Vertically, by performance: Standard or Commodity, Engineering, High-performance, and sometimes Ultra-high performance

The crystalline dimension enables us to truly understand the nature of thermoplastics and how to make the best out of them in practical applications.

Performance-wise, the higher up the materials are placed, the better their overall performance in terms of mechanical properties, thermal stability, chemical resistance, and more. However, they generally are also the most expensive and require the highest processing temperatures.

It’s important to note that this pyramid is not a standardized classification but a practical tool, offering a quick and easy way to narrow down the material selection for any specific application.

Each level of the pyramid is subject to different interpretations; materials that, from some perspectives, are considered as “commodity” might fall into the engineering category from other perspectives (e.g. ABS, PET, ASA, PETG). Before taking a closer look at each level of the pyramid, let’s explain what performance covers.

What exactly does “performance” refer to?

The properties used to determine the performance of thermoplastic materials are mainly mechanical and thermal. Here’s a quick overview of some of the basics.

Mechanical Properties

To better understand these properties, we must look at a tensile test chart like the one below:

Tensile test chart example
Tensile test chart example for mild steel. Adapted from Wikipedia.

The Y-axis refers to stress, a set of forces within a sectional plane of an object. As stress is applied, the object deforms. 

The X-axis is for strain, the ratio between the deformed and original lengths. Materials first go through an elastic phase where deformation is reversible; just like a spring or a rubber band, they return to their original state once the stress is relieved. Since the slope is linear here, you can define elasticity with a constant: the Young modulus, also known as elastic or tensile modulus. The higher this constant is, the higher the material stiffness is.

Takeaway: Young’s modulus (GPa), a.k.a. Tensile modulus or modulus of Elasticity – The higher, the stiffer.

As more stress is applied, the material manifests what is called a plastic behavior or plasticity, a stage at which deformation becomes irreversible. The point where this transition happens defines the material’s yield strength, and the maximum stress it can withstand before breaking is defined by the ultimate tensile strength (UTS). 

Breaking behaves differently for ceramics, metals, and polymers:

Typical stress-strain curves for ceramic, metal, and polymer materials
Typical stress-strain curves for ceramics, metals, and polymers. Adapted from GlobalSpec.

While ceramics break without reaching plastic behavior (brittleness), metals tend to break after reaching UTS due to necking. As the material is stretched, a local sectional area decreases (imagine stretching a chewing gum) until it reaches a fracture point or elongation at break. The more a material withstands necking, the higher its ductility. In the case of thermoplastics, they tend to perform much better under this condition.

Takeaway: Ultimate Tensile Strength (UTS), a.k.a. Tensile strength – The higher, the more ductile. Ductile materials can be more or less stretched before breaking, like a rubber band. Brittle materials fracture easily, like porcelain.

Brittle vs ductile stress-strain curves
An illustration of brittleness versus ductility via stress-strain curves. Adapted from Wikipedia.

Along with numerous other properties, mechanical performance also covers toughness and hardness.

Toughness defines a balance between strength and deformation; the tougher the material, the larger the area below the curve. In other words, it represents a material’s ability to withstand impacts and deform without breaking. Tough materials are therefore ideal for impact applications.

Hardness (as opposed to softness) is a localized resistance to plastic deformation. Hard materials are resistant to scratches, abrasives, and indentation. Thermoplastics aren’t particularly hard.

Takeaway: Tough materials can resist large impacts (e.g. getting hit by a hammer), while hard materials can resist small, localized friction (e.g. scratching an object’s surface with a knife). It is possible for a material to be both tough and hard.

These are a few examples of typical stress-strain curves for different types of polymers:

Stress-strain curves for different types of polymers
Stress-strain curves for different types of polymers. Adapted from MSE Student.

Thermal Properties

These properties are related to the material’s reaction to heat and changes in temperature.

  • Transition temperatures: Thresholds where molecular bonds change.
  • Heat deflection temperature (HDT): The temperature at which materials deform under a given load. 
  • Continuous use temperature (CUT): The range at which the material should be used for extended periods.
  • Conductivity: The ability to transfer heat in a set time span.
  • Thermal expansion: Dimensional variations due to changes in temperature.

Other Properties

  • Chemical resistance: The capacity of thermoplastics to remain stable under chemical exposure.
  • Radiation resistance: Like UV, gamma, and x-rays.
  • Flame retardancy: The ability to stop or slow ignition. This property depends on additives, in most cases.

Standard Thermoplastics (Commodity)

Standard thermoplastics, also known as commodity plastics, are the most widespread and, by far, the most produced. We can see them everywhere in our daily life, most applications involving packaging and consumer goods.

These materials are easy and cheap to process and mass-produce. However, their low performance makes them disposable, leading to massive sustainability and waste management issues. Consequently, packaging made of standard plastics like PET, HDPE, or PP is frequently labeled with a Resin Identification Code (RIC).

Commodity plastics are relatively easy to 3D print, and most standard FFF 3D printers– including entry-level options for hobbyists– can process them. In 3D printing, the most common materials of this category are PLA, PETG, and ABS, which are mainly used for rapid prototyping in many industries. Prices for these materials in filament form generally start at around $15 to $20 per kilogram.

Engineering Thermoplastics

These polymers are selected for more practical uses in engineering applications, with properties superior to those of commodity plastics. 

Engineering plastics are used for durable consumer products and in high-end industries like automotive and aerospace. Being lightweight, corrosion-free, and easy to process, parts made with these materials are sometimes even used to substitute parts traditionally made of metal.

The most familiar examples in this category are polycarbonate (PC) and the polyamide (PA) family. They can be 3D printed on most professional FFF 3D printers (and a select few low-cost options), though they do generally require a closed build chamber and higher processing temperatures than standard plastic filaments. Nylons are also standardly used in SLS (Selective Laser Sintering) 3D printing.

Engineering plastics have a significantly lower production volume than commodity plastics. Therefore, there are more reasons to leverage 3D printing as an alternative to injection molding for engineering plastics, and especially costly high-performance thermoplastics.

PC and Nylon can generally be found at prices between $50 and $70 per kilogram.

High-Performance Thermoplastics

With their high-end material properties, thermoplastics like PEI (ULTEM), PEEK, and PEKK deserve their own category. But what makes them superior to engineering materials?

These polymers were mainly synthesized to respond to the highest demands in engineering. Considering their particular monomer structures, aromatic rings are the backbone of their composition. 

For example, PEI contains imides, while PAEKs (like PEEK and PEKK) contain aryls. These stable and bulky structures are directly accountable for their outstanding mechanical and thermal properties, chemical resistance, and inherent flame retardancy.

Their continuous use temperature (CUT) range goes beyond 150°C, making them ideal for automotive, electronics, and energy industries. High-performance plastics are also suitable for medical implants due to their chemical stability and resistance to autoclave sterilization.

However, they are much more expensive than lower-grade engineering materials, with PEEK filament prices starting at $450/kg at the very least. These materials are challenging to 3D print, too, requiring dedicated high-temperature 3D printers (a.k.a. PEEK 3D printers). 

PEEK 3D printers boast high-temperature nozzles and build plates, and generally feature active interior heating to ensure proper crystallization and layer adhesion, among other technical reasons. For optimal results, parts 3D printed with high-performance (or “ultra-performance”) plastics can even undergo further heat treatment like annealing or hot isostatic pressing.

An increasing amount of such high-temperature 3D printers are being released on the market, and material manufacturers are constantly improving the printability and properties of their polymers. Consequently, as high-performance polymer 3D printing becomes increasingly feasible, promising opportunities for the low-volume production of aerospace parts and high-temp tooling arise.

Side-By-Side Comparison

We selected materials from each category to compare their properties and costs side-by-side:

Tensile Strength @Break (MPa)4255100
Tensile Modulus (MPa)195019803720
Tensile Elongation @Break (%)1010028
Glass Transition Temperature (°C)10576143
Melting Temperature (°C)200200343
Extrusion Temperature (°C)220 – 240255 – 275380 – 400

Still, it’s important to notice that material performances change from formulation to formulation (especially for composite blends, like PA-CF), and every manufacturer has different approaches to their processes.