3D PRINTING RESINS – POST CURING

3D PRINTING RESINS – POST CURING

Having performed 3 D printing, another key step is post curing, which is the complete polymerization of the print to be performed with special equipment that provides UV light at a certain frequency to completely fix the polymer molecules.

Models produced with resin printers are not fully cured through exposure to blue/UV light in the printing process. Providing sufficient exposure to fully polymerize each layer would result in a loss of resolution because light scattered outside the designated exposure area would polymerize a larger area than desired. A greater degree of polymerization during printing also gives rise to mechanical problems. The resin shrinks as it cures, subjecting the bond between layers to tension and distorting the shape of the cured resin. It can also make it difficult to detach each freshly cured layer from the production surface since resin is an exceptional glue. Therefore, in the printing process, the resin is cured only so that it has sufficient strength to hold its shape and can be detached from the production surface.

For this reason, the printer exposure parameters for each resin are generally adjusted to minimize unwanted volume curing.
This results in the need to perform POST CURING.

Post-pattern polymerization

Post-polymerization of models serves to complete the curing of uncured or insufficiently cured resin in the model. Resin manufacturers generally specify a post-print exposure of the cleaned model in a chamber strongly illuminated by LED or UV-emitting lights. Post-print UV exposure, however, presents a couple of problems. The first is oxygen: oxygen inhibits the polymerization process, blocks the normal action of free radicals formed as a result of UV exposure of the photoinitiator and prevents the chaining of monomer molecules. This inhibition extends from the surface of the model to a depth of several microns. Insufficiently cured resin due to this inhibition gives rise to the slightly viscous surface of these prints.
To eliminate this annoying inconvenience we can act in a variety of ways, of course depending on the composition of the resin used we may find difference in polymer behavior.

The methods that have given the most comforting results are 3:

– Exposure to direct UV light. ( You need a lot of power to get a good result)
– Exposure to more or less intense heat. ( The result is’ good if temperatures close to the 93°C are reached, which for some resins can be harmful)
– Exposure to moderate heat in vacuum. (Good result but with the complication of the vacuum chamber.)

Obviously for simplicity Binnion made a comparison between the 3 different methodologies referring always to the same resin base.

Post-production polymerization test with UV

Post-production curing was performed in a chamber with eight 365 nm 9-watt fluorescent tubes arranged around the models . The models were placed on a rotating pact that keeps the parts continuously moving during curing so as to provide uniform illumination to the surfaces of the models. The models were cured for time intervals ranging from 30 minutes to 2 hours. All models showed a slightly viscous surface after exposure. The models were weighed before and after curing and showed little weight loss following the curing process. This, perhaps, was only due to the evaporation of the water and/or isopropyl alcohol used in the cleaning process absorbed by the models, since the resins generally do not contain solvents prone to evaporation.

Post-production curing tests by heat

Curing tests by heat were conducted in a ventilated benchtop oven equipped with a digital PID temperature controller and K-type thermocouple to measure and control the oven temperature. Models were placed on a metal foil resting on the oven grate. Initial tests used a three-stage temperature ramp: 50 °C for 15 minutes, 100 °C for 15 minutes, and 150 °C for 15 minutes. At the end of the treatment all models had cracks and imperfections. During testing, it appears that models left too long in the isopropyl rinse after printing were more prone to crack formation. It is suggested not to use isopropyl alcohol and simply use water with a little detergent followed by a clean water rinse or simply rinse the isopropyl prints briefly, then soak them in water with detergent followed by a clean water rinse for final cleaning.
It also showed a greater shrinkage problem on the X Y axes than on the Z, which obviously produces internal deformations and stresses in the models.

Post-polymerization tests by vacuum/moderate heat

Polymerization tests by vacuum/moderate heat were carried out in a vacuum oven. Models were placed in the center of the platform with the oven at room temperature. The chamber is evacuated and the heat is then turned on. A processing time of 1.5 hours seems to be adequate to produce a sufficiently low level of uncured resin in the tested models. Although extended time in the vacuum oven will result in additional removal of volatile components, longer times may give rise to the same cracks shown by post-curing by heat.

Weight loss after treatment

Post-polymerization models by UV show less than 0.5 percent weight loss. Both heat-only and vacuum oven-based processes show higher weight losses than the models (Figure 1). In the tests, depending on exposure time and temperature, weight loss of 2% to 9% was recorded. In both heat-only and vacuum-oven-based processes, the time and temperatures that resulted in a weight loss greater than 3% also led to cracks and imperfections, so it is clear that excessive weight loss is a problem

Conclusion

To achieve proper polymerization of photopolymer resin models, the post-printing polymerization of the resin must be as complete as possible, while avoiding cracks and imperfections caused by heterogeneous stress resulting from different shrinkage rates between the XY plane and the Z axis. Uncured resin in the model results in surface defects.

It would seem that the use of a vacuum oven for removal of uncured resin by evaporation and final polymerization of partially cured resin is the fastest and most effective way to make a correct model.
After careful analysis of these results, New ASAV chose an intermediate way by creating a device that uses mixed sources of UV light.
In order not to burden production costs too much at Nuova ASAV, we chose to simultaneously use UV light produced by high-intensity LEDs ( 10 W. Cad.) with a wavelength between 385 and 425 nM. and a halogen source that in addition to providing high power ( 100W) also had the peculiarity of providing heat to fully cure the print.
The device also proved to be very good for curing dental composites, thus a positive double value.
The curing times are between 3 and 15 minutes, depending on the type of resin used.

The solution designed by NUOVA A.S.A.V. :