The most common 3D printing technologies in dentistry and orthodontics are based on photopolymerization techniques and the photosensitive liquid resin is used as material. The resin is cured only under light irradiation provided by different UV light sources . Without irradiation, the resin keeps liquid. The resin 3D printing is represented by three technologies accordingly with the different light source:
- SLA (Stereolithography): the liquid polymer is exposed to light where the UV laser draws a cross-section layer by layer
- DLP (Digital Light Processing): a digital light projector generates the UV light that selectively cure the resin in the resin tank
- LCD (Liquid Crystal Display): an array of LED’s illuminating on the LCD. The LCD is used as a mask, which creates the curing pattern to cure the resin in the resin tank.
RESIN CALIBRATION GENERAL RULES
DLP and LCD printers are more similar to each others than SLA printers but for all of them there are general rules to calibrate correctly any resin:
- The polymerization must be limited to the areas defined by the slicing process, that is the process converting the 3D model into instructions for the 3D printer through all the printing settings;
- The software must compensate the size variations due to post-curing effects. In fact, after printing, the photopolymerized resin is additionally subjected to a post-curing process in an ultraviolet (UV) oven to improve its mechanical strength by polymerizing unreacted monomers while ensuring that the polymerization is even and complete in all regions.
The illuminated area at the bottom of the tank is a square having a side of 50×50. This square is “drawn” by the projector at the bottom of the tank with a precision given by the calibration of the printer manufacturer and that is part of the technical features of the device. What the dental printer user must do is to control precisely everything above the bottom of the tank, that is to make the resin solidify correctly within the defined area.
OVERVIEW OF THE RESINS ON THE MARKETPLACE
In addition to proprietary resins, on the market there are many resins of different brands (Pro3dure, Nexdent, Detax, Dentona, Applylabwork …). Some manufacturers offer the same type of resin already diversified for the specific technology of the printer (SLA, LDP or LCD). Other manufacturers distribute resins that are compatible with all technologies. On the marketplace there are also resins that react only if affected by a specific light spectrum or resins that are very reactive to the visible spectrum, called daylight resins.
Not all printers can work well with third-party resins, such as those printers that use a light source at a frequency of around 460 nm.
Therefore, this article excludes Daylight Printers on which it is not possible to calibrate 385 nm and 405 nm resins (the standards for most biocompatible and non-biocompatible resins). Instead, we consider the DLP and LCD printers that FeniQX, one of the main distributors of DLP printers in Europe, has used to test its resin calibration procedure
FENIQX CALIBRATION METHOD FOR A SMART CALIBRATION PROCEDURE OF A THIRD-PARTY RESIN ON SLA, DLP AND LCD PRINTER
The goal of resin 3D printer calibration is to balance these three core settings:
- Exposure time of a single layer
- XY resolution (pixel or laser spot size)
- Layer height
To have the security of printing the files correctly both from the point of view of exposure and from the dimensional point of view, FeniQX has identified 2 possible calibration procedures for printers that must be performed correctly one after the other:
- Exposure calibration
- Dimensional calibration
Below is the detail of the proposed calibrations.
The most important parameter is exposure time of a single layer. It defines light exposure duration for a single layer of an object that you are trying to print. This term applies to DLP and LED/LCD based 3D printers, because with such equipment entire layer is exposed whereas SLA printers’ laser “draws” each layer.
A correct exposure allows to achieve a hardening of the layers exactly confined in the area elaborated during the Slicing process. In fact, the software cuts in layers the objects prepared in the printing area and defines the areas that must be projected (in the case of DLP or LCD) or “drawn” (in the case of SLA lasers).
The exposure calibration, which is sometimes illustrated with very complicated and difficult to understand procedure, has been made very simple by FeniQX. A specific STL file is available on FeniQX website: it contains the instructions for making two blocks, one male and one female, which once printed and treated with an adequate post-curing (washing and curing in suitable UV boxes) must be inserted into each other.
Dopo aver incastrato i pezzi l’utente dovrà leggere nel blocchetto dotato di finestra il valore numerico raggiunto che va da 1 a 5.
In pratica se i due blocchetti non si incastrano o se si incastrano appena vuol dire che l’esposizione della resina è stata eccessiva e l’indurimento si è propagato oltre i perimetri delle immagini proiettate causando un ingrossamento dei pezzi stampati.
After inserting the blocks, the user must read in the block the numerical value reached ranging from 1 to 5.
In practice, if the two blocks do not fit or if they hardly fit, it means that the resin has been over-exposed and the hardening has propagated outside the area of the projected images (it increases the size of the printed blocks).
If, on the contrary, the blocks fit fully to the end of the scale, it means that the exposure was not sufficient and the resin hasn’t hardened correctly inside the area of the projected images (it decreases the size of the printed blocks).
The procedure to calibrate the exposure (or editing the resin profiles in the printers that do not allow to change the printing settings) consists in repeating the print test until the two blocks fit up to 3 – 3.5
In general, you can follow this rule :
In the dimensional calibration the purpose is to scale the model in X and Y (Cartesian coordinates) in order to compensate the effect of the shrinkage (expansion or contraction) due to the curing process (both during printing or inside the curing unit). This scaling is usually allowed within the printer software, but it can be done also in an external software such as Meshmixer freely downloadable at http://www.meshmixer.com/download.html)
In order to compensate the natural deformation of each thermoplastic element during the molding phase, FeniQX has released a simple but extremely precise method to make sure of the correct dimensions of the 3D model at the end of the printing process. It consists of matching a printed jig and an aluminum cut-out milled by a precise NC milling machine with tolerances far below our needs (the control template and the STL file of the caliber can be purchased at FeniQX)
Here below the pictures of the printed jig and the relative aluminum template:
The following images instead represent the relative fitting that shows us the goodness of our print:
Additionally, to make everything even easier, a series of notches have been marked onto the jig and through a digital caliber you can check all the most important measures to correctly calibrate in advance the scaling of model along the XY axes.
If on the printed jig (after proper post-curing) you measure 54.2 mm instead of the theoretical 54 mm, then the scaling to apply along that axis 54/54.2 = 0.996
Anyway, the most important reference available is doubtlessly the perfect fitting between the two parts (the aluminum and the printed jig) , taking care that the printed caliber fully reaches the base of the template..
If the two calibration processes described above are successfully completed it means that the printer is ready to properly produce correct models in terms of both exposure and dimensions.
For info about how to buy the aluminum jig please fill the form below :