# What are Polymers? In this section we will walk through the common issues and challenges encountered in 3D printing with a material science approach. The idea behind the page is to provide more scientific knowledge to common issues in order to easily overcome them. To begin, it is important to understand what material is being used in 3D printing: Polymers Polymers are large molecules, or “macromolecules”, formed by large numbers of repeating units known as “monomers” in the polymerization process. The polymerization process bonds the monomer molecules together in a chemical reaction, forming the backbone of the polymer. The type of polymers produced can vary depending on the chemistry and composition of monomer compounds that construct them. The links created between the monomers will be defined as covalent bonds. ![3D Printing Polymer](https://cdn.shopify.com/s/files/1/0548/7299/7945/files/monomers_480x480.png?v=1709320843) Polymers can be divided into 2 families: thermosets and thermoplastics. Thermosets are polymers that are irreversibly cured from a soft solid or viscous liquid pre-polymer into a solid polymer. The curing process is also known as cross-linking, which proceeds via a chemical reaction that connects all the monomers and pre-polymers to form a network structure. A cured thermoset can no longer be melted and usually is not thermally processable. ![Thermoset Thermoplastic](https://cdn.shopify.com/s/files/1/0548/7299/7945/files/thermoset_thermoplastic_600x600.png?v=1709320909) Thermoplastics are materials which become soft when heated and hard when cooled. Thermoplastics can be heated, molded and cooled multiple times with minimal change in their chemistry or mechanical properties. Unlike thermosets where each of the polymer chain is linked to others with a covalent bond, thermoplastics have their polymer chains linked with each other with weaker links which will be defined as non-covalent bonds. Polymers can also be divided into two main categories depending on their micro-structure: ### Amorphous and Semi-Crystalline One of the ways that different thermoplastics can be identified is through their micro- structure, which can define the properties and behavior of the polymer. **Amorphous** Amorphous polymers are identified for not having a long-range ordering. This means that the polymer chains are randomly oriented. Generally speaking, clear plastics are often made with amorphous polymers, such as PMMA, PS and PC. ![Amorphous](https://cdn.shopify.com/s/files/1/0548/7299/7945/files/Amorphous_480x480.png?v=1709320953) **Semi-Crystalline** Semi-crystalline polymers are identified for having an ordered structure with structural domains known as “crystals”. Crystals are an ordered and tightly packed group of polymer chains. Crystalline domains and amorphous domains co-exist in semi-crystalline polymers, thus the “semi”. The proportion of crystallized areas is defined by the degree of crystallinity. A specific characteristic of semi-crystalline polymers is that this degree of crystallinity can highly affect their mechanical and thermal properties. ![semicrystalline](https://cdn.shopify.com/s/files/1/0548/7299/7945/files/Semi-crystalline_480x480.png?v=1709321000) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://wiki.polymaker.com/printing-tips/material-science/what-are-polymers.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.