To improve the qualities of our prints we need to rearrange their molecular chains to make them more arranged, ergo, more similar to crystalline polymers. Because a 3D printed part is mostly amorphous, its heat resistance, firmness, and tensile strength are inferior to crystalline materials with organized molecular chains. The plastic is then rapidly cooled down to room temperature, therefore its structure remains the same. Imagine spaghetti, but with polymer chains instead of pasta dough. If you heat up a plastic polymer inside a nozzle, its structure is likely to be amorphous. Crystalline structures are usually harder and more rigid (similar to a diamond, even though that’s not a polymer), amorphous polymers are more elastic and flexible. There is no exact dividing line, though, molecules could have various levels of arrangement. Polymers have two types of molecular structures: amorphous (chaotic) or semi-crystalline (partially organized). Most of the commonly used 3D printing materials are polymers, i.e.
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If you are not much into theory, feel free to skip it and go right to the practical part, where we present actual results of annealing, using various materials at different temperatures. This will help us understand why exactly annealing helps to improve mechanical and heat resistance, and why it’s so important to find the optimal temperature. In the following chapter, we will explain the basic nature of this process from a physics point of view. At such temperature, a printed object still retains its shape but is softened and there are ongoing changes on the molecular level, improving its structural stability. We want to achieve a so-called “glass transition temperature”. In a nutshell, we are talking about heating plastic to a point when its molecules are able to rearrange, thus making their structure more firm, stable, and with lesser inner tension.
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