Post-Curing is a step that many in the dental sector do not take into consideration or even do not know.
This is one of the biggest mistakes that 3D printers make.
The Post-Curing step is the step that influences in a very important way the final properties of the objects we print.
Let's analyze together and understand together what this step is and why it is so important.
What is Post Curing?
During printing, parts of the resin are exposed to UV light which causes the material to cure. Once this process is complete, the entire part will be cured, so it may not be clear why we need to post-cure our prints. To answer this question we need to understand the properties of photosensitive resins and how they transform from the liquid state to the solid state.
While in its liquid state, the resin is composed of many unbound monomers mixed with photoinitiators. Exposure of this mixture to strong UV light causes the photoinitiators to react and bond between the monomers and form a polymer. This is the main stage of care. The cured resin is a crosslinked macromolecule, which means that every part of it is directly connected to every other part of it. However, after printing there will still be many sections that are not crosslinked to the extent that they could be, which affects tensile strength and other properties. UV light can be used to complete the molecular bonding and ensure that any crosslinks that might be made are applied.
For post-curing purposes, the sun isn't actually a bad source of UV light. Many hobbyists who use consumer SLA printers simply place the printed parts outside in the sun for some time and that gets the job done. However, in more professional contexts it is advisable to use a UV oven, which is much more controllable and reproducible.
Post-curing not only makes parts stronger, it also makes them more stable. Having unreacted photoinitiators and unbound polymers in the printed object will mean that its state may continue to change long after printing, particularly when exposed to sunlight. For parts that need to be precise, such as dental inserts, this is totally unacceptable. Unpainted parts may also change color when exposed to sunlight if not properly post-cured.
The main reason for post-curing is usually to increase tensile strength and remove any surface tackiness. Some printed materials may be slightly soft before post-curing, and weaker sections of the structure may deform, compromising accuracy.
There is also a safety issue as uncured resin is toxic to humans. If parts that are not fully cured are handled with bare skin, there is a risk of irritation or other problems, particularly if the conductor's fingers come into contact with the mouth, so just imagine if this product then needs to be placed in the mouth. The post-curing of a part has the effect of making the resin completely stable and safe both to handle and for its application in the oral cavity.
The Chemistry of Curing
Curing is affected by heat and light. Heat increases the energy in the crosslinked polymer network and causes increased movement at the molecular level. This increase in energy and mobility makes it much easier for reactive groups to contact each other and to form further bonds.
However, UV light is the main ingredient needed for post-curing. Although the equipment is referred to as an oven, in reality the internal temperatures are not all that high. The resin used in 3D printing is photosensitive and its exposure to light causes a reaction. As discussed above, not all bonds that could be activated are made during the original curing step of the print, and therefore further reactions must be stimulated. When exposed to UV light during post-curing, free radicals are formed which bind to neighboring groups and create polymerization.
What is the correct light to use?
The most important factor is the wavelength of light used in relation to the resin from which the part is made. Photosensitive resins are designed to react to certain wavelengths of light and using a different wavelength for this post-cure will give sub-optimal results. That's why one of the reasons professional 3D printing teams use specialist equipment for post-processing. In the case of Free Wild Techology, our resins cure best using light with a wavelength between 270nm and 405nm (may vary depending on the resin). If resins from another supplier are used, this value will differ depending on the supplier. This is a great reason to use the Free Wild Techology UV Oven if you already use resins supplied by Free Wild Techology. Using another type of UV oven or UV nail hardener will promote some additional curing but will not unlock the maximum mechanical properties of the material.
Another problem with using the sun or non-specialist equipment is that the cure will be uneven. The sun will shine more on one side than the other unless you rotate it continuously, and the same goes for nail polish curers that only cast light from above. Specialty curing ovens typically have a turntable or UV lamps that completely surround the part to ensure an even cure.
Why choose our Free Cure Pro over other curing systems?
For those who use our Free Wild resins from the Print Fluid line, the great advantage of choosing our curing system is certainly the fact that we have programs that we have already tested.
But our Curing is designed to be used with more types of resins and more systems.
One of the advantages of our oven is to have 3 different beams of LEDs with 3 different wavelengths 270 nm 380 nm 400 nm, and to be able to manage these LEDs separately.... What does this mean? It means that depending on the requirement of the material we are treating, we can activate only one type of wavelength or all 3 together and we can also manage the power of those lights separately for each wavelength, this allows us to create tailor-made programs for each print material.
The Free Cure Pro is equipped with 85 LEDs that illuminate each face of the internal chamber and has a transparent rotating platform in order to distribute the light on the objects in the most uniform way possible.
Another strong point of our Cure is to have a vacuum pump that allows the removal of oxygen from the chamber and the possibility of injecting argon inside it so that the biocompatible resins can be polymerized in the best possible way. without creating white patinas on their surface and completely closing the bonds between the various molecules that make up the material.
This Cure has been designed and conceived to be able to polymerize the 3D printing materials dedicated to the Dental sector in the safest and fastest way possible, providing a result, after the cure of the prints, stable and respecting the characteristics of the material itself.