The latest research in the journal Applied mechanic by Italian researchers is focused on evaluating the effects of curing on photosensitive resins using stereolithography (SLA) additive manufacturing.
Study: Effects of curing on photosensitive resins in SLA additive manufacturing. Image Source: Acumen / Shutterstock.com
The final qualities of 3D printed materials are determined by how all production parameters are set and / or varied. This also includes those involved in the hardening step. Stereolithography (SLA) curing effects depend on numerous factors including temperature, curing time, and radiation wavelength.
Importance of additive manufacturing
Additive Manufacturing (AM), often also known as 3D printing, is one of the most transformative technologies of our time. 3D printing can be used to create a high-precision virtual model, which is particularly valuable in the aerospace and automotive sectors.
Three-dimensional printed 5A samples (Form 2, Standard Clear Resin). Image source: Riccio, C., Applied Mechanics
It has a wide range of uses including wearable technology with various biological functions and other biotechnological applications such as custom implanted devices.
Limits of additive manufacturing
However, while additive manufacturing is one of the most well-known methods, it has some limitations in terms of its applications. The limited manufacturing throughput and the associated costs are important reasons that limit the use of AM. Additionally, 3D printed items may have different dimensions than CAD models due to differences in manufacturing processes.
Innovative stereolithographic technology
The stereolithography is the procedure of particular importance for this investigation (SLA). The goal of SLA is to print tiny layers of a corrosion-resistant material, add them up on top of each other and combine them into solid, three-dimensional things. This approach enables the creation of designs with innovative geometry that would be difficult or even impossible to achieve using layered manufacturing techniques. These geometric features can in turn increase the engineering performance of a module.
SLA uses photopolymerization technology to produce solid components through selective solidification. It enables excellent component precision, precise surface quality and strong mechanical properties.
Stress-strain curve green and post-curing of BioMed Clear (a), Hard (B), and BioMed Amber (C) Resins. Image source: Riccio, C., Applied Mechanics
The importance of the healing treatment method
To accelerate curing, three-dimensional printed articles can be treated in an ultraviolet (UV) oven. UV radiation causes the development of additional chemical bonds that help the hardening resin achieve increased structural strength and stability.
The temperature dependence is a key feature of the curing process, as heat speeds up the process. As the hardening temperature increases, less time is required to reach a state in which the material has the best conceivable mechanical properties, which leads to faster hardening.
The increased movement of free radicals in the molecular chains can be associated with the influence of UV curing at high temperatures on the mechanical properties. It was found that annealing with a 405 nm light source produced the best elasticity and compressive strength.
A comprehensive assessment of the curing effect of a large number of regularly used SLA products is missing, which was the aim of the latest research.
Commercial SLA resin synthesis
Commercial resins for 3D stereolithography typically consist of oligosaccharides and monomers, coupled with one or more photoinitiators, and additional components. These components are used to stabilize the resin, increase its reaction rate and improve its other properties.
The most recent study used six Type 5A samples that were 3D printed for each resin according to the BEN ISO 527 standard.
Stress-strain curve green and post-hardening of Rigid (a), Dental LT Clear (B), Custom (C), and high temperature (D) Resins. Image source: Riccio, C., Applied Mechanics
The researchers found that curing increased the tensile strength and elasticity values of each resin. The substance became more friction resistant in all situations, with a minimal increase in strength properties of 29% for Tough Resin and a maximum increase of 215% for Custom Tray Resin.
All three resins improved in their breaking load and Young’s modulus values as a result of the curing treatment. The curing process increased the durability of both resins (the breaking load increased by 36% for Tough 1500 Resin and 59% for Tough 2000 Resin) as well as the yield strength and stiffness.
The resilience of the material and its inherent toughness have also been significantly increased.
Although some materials showed notable variance, the materials became more brittle after the process. According to the results of the tests, the curing process may cause some resins to have more fragile properties.
Limitations of the hardening process
In general, the hardening process has the decisive disadvantage of significantly increasing the brittleness of the material – the shift from ductility to brittleness leads to a significant loss of toughness.
In short, the elastic modulus and compressive strength of all resins improve during the curing process. However, the technology reduces the flexibility of all resins and under certain circumstances leads to brittle behavior. Still, the benefits of the hardening process outweigh the limitations and are the only reason it is used in various industries.
Riccio, C., Civera, M., Ruiz, OG, Pedulla, P., Reinoso, MR, Tommasi, G.,. . . Surace, C. (2021). Effects of curing on photosensitive resins in SLA additive manufacturing. MDPI Applied Mechanics. https://www.mdpi.com/2673-3161/2/4/55