What parameters have an impact on the 3D printing of resin moulds?

Digital orthodontics has led to the introduction of 3D printers in the dental sector. These printers can be used for different purposes: printing models with which we can fabricate aligners on them, printing retainers or even, as we have recently seen at some conferences, printing the aligners directly.

To ensure the fit of any device we are going to manufacture, the printed resin model must be as accurate as possible. So let's first define precision. Applied to the world of 3D printing, precision is known as the reliability of a printer to reproduce the same model repeatedly with as little variation between them as possible. Precision is not the same as accuracy, which is more related to the detail of the print. Depending on the purpose of the device we are going to manufacture, it will demand a higher or lower precision/accuracy. For example, the casts needed to produce a retainer, which is used to maintain the teeth, will not require the same accuracy as resin models used to make aligners to move the teeth.

How do 3D printers work? These machines print or "build" an object layer by layer from a printable material, usually resin. As mentioned above, a critical factor for the success of 3D printing in orthodontics is its dimensional accuracy, especially when applications involve precise tooth movement. Among the various printing technologies, stereolithography (SLA) and digital light processing (DLP) are the most common used by orthodontists. Many factors throughout the digital workflow can affect the quality of the impression, including the accuracy of the digital model obtained with the intraoral scanner.

DLP printers are the most widely used in orthodontics for several reasons: their printing speed is faster than SLA printers, high print quality is easier to achieve, and there is a wide variety of biocompatible materials for 3D printing with DLP.

Once we have our 3D printer... what do we do now? It's time to print the models. At this point, we are going to analyse if there is an "ideal" way to prepare them for printing. Some studies have investigated the influence of the angle at which the models are oriented and its relationship to the height (thickness) of the printing layer.

The impact of model angulation on the accuracy of the print changes as the layer height changes and vice versa. In other words, the two variables are closely related to each other. This finding is important and sets this study apart from previous studies, which evaluated both factors independently without taking into account this relationship between the variables.

The study found that the combination of an oblique impression angle of 30° or 60° and a layer thickness of 20 to 50 micrometres produced the most accurate models. However, it is important to note that the difference in accuracy between this combination and the other groups analysed in the study was clinically insignificant and no single angle or layer thickness was identified as the 'best' for all situations.

In other words, if the 3D printer is of good quality, for orthodontic applications we will not see significant differences in the printing angle. Although this inclination does not affect precision very much, it can have an impact on the cost-effectiveness of the print. The results obtained by Williams et al. demonstrate this:

• The models printed at an angle of 45° were the most cost-effective. This impression configuration used 5.20 mL of resin per model, making it the most economical option.
• If we changed the angulation to 30º, a greater amount of resin was consumed per model, making this inclination the most expensive option.

This variation between angulations is related to the support structures that appear when we prepare the models for impression. Depending on the orientation of the arches, more or fewer supports will appear, affecting the impression time and the amount of material used.

Ko J et al. Effect of build angle and layer height on the accuracy of 3-dimensional printed dental models. Am J Orthod Dentofacial Orthop 2021;160:451-8.

Williams A et al. Effect of print angulation on the accuracy and precision of 3D-printed orthodontic retainers. Am J Orthod Dentofacial Orthop 2022;161:133-9