UV Curable 3D Printing – The Significant Role of Photopolymers

UV 3D printing methods make use of liquid photopolymers to create physical objects. What are these methods and why are photopolymers a game changer in modern manufacturing, especially in the UV additive manufacturing? We are going to take a look at the most commonly used UV cure 3D printing methods and their most crucial components – photopolymers.

What is UV 3D printing?

UV 3D printing relies on photopolymerization to build the desired objects layer by layer. Common printing methods include Stereolithography (SLA), Digital Light Processing (DLP) as well as Continuous Liquid Interface Production (CLIP) and MultiJet Modeling (MJM).

Even though the processes differ, a common feature is the use of a liquid photopolymer resin, which is sensitive to ultraviolet (UV) light - such as UV curable resin for 3D printing.

When the printing process starts, the liquid resin is selectively exposed to UV light, which makes it solidify.

UV 3D printing with photopolymers is able to produce high-resolution and detailed objects, while speeding up production time and reducing material waste. Therefore, the technology finds applications in various industries, including 3D printing in healthcare, 3D printed jewellery, UV additive manufacturing in aerospace and 3D printing in the automotive industry.

Common methods of UV cure 3D printing

UV curable 3D printing uses different methods to create physical objects. The most common methods include SLA, DLP, CLIP and MJM.

Stereolithography (SLA)

Stereolithography (SLA) is one of the most widely used methods for creating 3D objects using a layer-by-layer approach, which is particularly known for its ability to produce high-resolution and highly detailed 3D prints.

The SLA 3D printing process starts with a 3D digital model of the object to be printed. This digital model is created using computer-aided design (CAD) software or obtained through 3D scanning techniques. The printer consists of a vat or tank filled with liquid photopolymer resin.

Wherever the UV light strikes the resin, it causes the photopolymerization reaction to occur. During this process, the liquid UV curable resin for 3D printing solidifies and transforms into a hardened, solid material. The build platform is then incrementally raised, allowing the next layer of the object to be printed on top of the previous one. This layer-by-layer building process continues until the entire 3D object is complete.

Post-processing steps are typically required to clean and finalise the printed object. These steps often involve rinsing the object in a solvent or isopropyl alcohol to remove any remaining liquid resin. The object may also undergo additional UV light exposure to ensure complete curing and hardening of the material.

Digital Light Processing (DLP)

Digital Light Processing (DLP) is similar to stereolithography (SLA) in the sense that it uses photopolymer resin layer by layer to create 3D objects. The key difference lies in the way the UV light is used to solidify the resin during the printing process.

In DLP 3D printing, the printing process starts with a 3D digital model of the object to be printed. While the digital model design is similar to SLA, the difference lies in the photopolymerization process. Instead of using a laser, DLP printers use a digital micromirror device (DMD) or an array of micromirrors.

These mirrors project UV light onto the entire layer of the object. By doing so, the process solidifies the entire layer in one exposure, unlike point-by-point scanning in SLA. This exposure of a complete layer allows DLP printers to achieve faster printing speeds. Digital Light Processing has become popular due to its speed and efficiency in producing 3D prints with high resolution and fine details.

Continuous Liquid Interface Production (CLIP)

Continuous Liquid Interface Production (CLIP) is an innovative 3D printing technology developed by Carbon3D (also known as Carbon Inc.). CLIP represents a significant advancement in the speed and quality of 3D printing compared to traditional approaches like stereolithography (SLA) and digital light processing (DLP), as it utilises a combination of UV light and oxygen to create objects from a liquid photopolymer resin.

Unlike layer-by-layer approaches in most 3D printing technologies, CLIP works continuously, without the need for a build platform to move incrementally after each layer is cured. After the CAD model is finished, the CLIP 3D printer projects a sequence of UV images onto a transparent, oxygen permeable window. Between the window and the object being printed lies the "dead zone", a very thin layer of resin that never solidifies, as oxygen diffuses through the window.

Inside this dead zone, the photopolymer resin is exposed to UV light, causing it to photopolymerize and solidify at the interface between the liquid resin and the window. At the same time, oxygen is selectively introduced into the dead zone, creating a thin, liquid interface between the solidified part and the rest of the resin.

The key innovation of CLIP lies in the continuous movement of the build platform, which lifts the object upward out of the liquid resin pool. As the build platform rises, the object is continuously pulled from the liquid resin bath while the liquid interface solidifies, forming the desired 3D shape. This continuous process allows for faster printing speeds compared to traditional layer-by-layer techniques.

MultiJet Modeling (MJM)

MultiJet Modeling (MJM) is a 3D printing technology developed by 3D Systems and known for its ability to produce highly detailed and accurate models with a smooth surface finish. The MJM process uses multiple printheads to precisely deposit droplets of liquid photopolymers layer by layer onto a build platform.

Once a layer of the model is printed, UV light is used to instantly solidify the material. Then, the next layer is deposited, and the process repeats until the entire model is completed.

The ability to use different materials and colours in a single printing process allows for the creation of realistic prototypes and models useful in various applications, such as design, product development or the medical field and dental industry.

More information about Additive Manufacturing: What is Material Jetting?

Photopolymers: The main ingredient of UV 3D printing

Photopolymers make curable 3D printing possible. Vat photopolymer uv curable resins for 3D printing are available in a wide range of formulations, each with unique material properties. The main ingredients are monomers/oligomers, photoinitiators as well as additives.

Monomers/Oligomers

Monomers can be compared to building blocks of photopolymer resins. They are small, molecular units capable of forming chemical bonds with other monomers. In their liquid state, photopolymer uv curable resins for 3D printing consist primarily of monomers. These monomers are designed to have specific chemical properties, such as low viscosity and high reactivity to light.

Oligomers, on the other hand, are larger molecules formed by the combination of several monomers through chemical bonding.

They contribute to the overall mechanical properties, strength, and durability of the 3D printed object.

By controlling the different types and ratios of various oligomers, manufacturers can fine-tune the characteristics of the final printed part. The selection of monomers and oligomers in photopolymers is therefore crucial in achieving specific material properties, such as flexibility, rigidity, toughness, or transparency

Photoinitiators

Photoinitiators are typically organic compounds that are sensitive to specific wavelengths of UV light, a critical aspect of UV additive manufacturing. When the photoinitiators absorb UV light of the appropriate wavelength, they undergo a photochemical reaction that generates highly reactive species, depending on the type of photoinitiator used.

These reactive species initiate the polymerization process by breaking the chemical bonds in the monomers and oligomers present in the UV-curable 3D printing resin. As a result, the monomers and oligomers start to link together, forming long chains called polymers.

This cross-linking of molecules leads to the solidification of the liquid resin, resulting in the desired 3D printed object.

Photoinitiators are chosen carefully to match the specific UV light source used in the 3D printer. Different photoinitiators have different absorption spectra, meaning they are sensitive to different ranges of UV light wavelengths. Therefore, the selection of an appropriate photoinitiator ensures efficient and controlled photopolymerization under specific UV light exposure conditions.

Additives

Additives are various substances that are incorporated into the polymer formulation to modify its properties. Each additive serves a specific purpose, and the overall formulation must be optimised to achieve the desired properties in the final product.

Stabilisers are additives that are used to prevent degradation of the polymer during storage or under prolonged exposure to light, while plasticizers are used to increase the flexibility and reduce the brittleness of photopolymers.

Other additives such as pigments and dyes are used to impart colour or opacity to photopolymers, making them more aesthetically appealing or functional for specific uses. In some applications where fire resistance is essential, it is even possible to add flame retardants to photopolymers in order to reduce their flammability and enhance safety.

The final product of UV 3D printing depends on the right choice of photopolymers

Next to the right production process, the choice of photopolymers is crucial to create high-quality 3D prints that fulfil all requirements. RAHN is your provider of raw materials for 3D printing, including materials suitable for UV additive manufacturing, and develops custom photopolymers for every purpose.

We are experts in the field of photopolymers and our labs are equipped with industry-level SLA, DLP and LCD 3D printers, which enables us to develop custom solutions that perfectly fit your needs.

Get in touch and book a call with our experts to find the best solution for your project.