First, it is essential to understand the primary materials used in 3D printing: polymers.

A polymer is a large molecule, or macromolecule, composed of many repeating units called monomers, formed through a process known as polymerization. During polymerization, monomers are chemically bonded together to form the main backbone of the polymer chain.

Different types of polymers are produced depending on the chemical composition and structure of their monomers. The bonds formed between monomers are defined as covalent bonds.

Polymers are divided into two main categories: thermosets and thermoplastics.

Thermosets are polymers that irreversibly cure from a soft solid or viscous liquid prepolymer into a rigid solid. This curing process, also called cross-linking, chemically connects all monomers and prepolymers into a three-dimensional network structure.

Once cured, thermosets cannot be melted or reshaped by heat and generally do not allow thermal reprocessing.

Thermoplastics are materials that soften when heated and harden when cooled. They can be repeatedly heated, shaped, and cooled with minimal changes to their chemical or mechanical properties.

Unlike thermosets, where polymer chains are covalently cross-linked together, thermoplastic chains are held together by weaker interactions known as non-covalent bonds.

Another way to classify thermoplastics is by their microstructure, which determines key performance properties and behavior.

Amorphous polymers are characterized by no long-range order — their polymer chains are randomly arranged.

Generally, transparent plastics such as PMMA, PS, and PC are typically amorphous.

Semicrystalline polymers feature an ordered structure containing organized regions called crystals.

Crystals consist of tightly packed, aligned polymer chains. Both crystalline and amorphous regions coexist, hence the term “semicrystalline”.

The proportion of crystalline regions is defined as crystallinity, which significantly influences the mechanical and thermal properties of the material.