Photopolymerization Processes in Additive Manufacturing

Photopolymerization is a very important additive manufacturing technique the UV light source is the main energy deposition on the materials. Here we explain the general characteristics of photopolymerization techniques in general.

What is Photopolymerization?

Photopolymerization is the additive manufacturing technique that the solidification of the liquid material takes place with the application of UV light on the objects. The chemical reaction behind the photopolymerization is very different.

In general, they use thermosetting polymers in this technique. Thermosetting materials have an important characteristic that they become solid when exposed to UV light. This UV light creates extra cross-linking structures inside the molecules of the polymer. So these cross-linking provides a solid.

And another important characteristic of thermosets, they can not be melted back once they become solids. Because the extra cross-linked structures are not turning back upon heating to melt.

Because of this phenomenon, we call these additive manufacturing techniques photopolymerization techniques.

Photo-Curable Materials

We call these classes of materials photo-curable materials. Because we are using light to cure them. In general, we use these materials in the coating applications of parts and dentistry. They can be shaped and solidified with the application of UV light.

But in the 1980s, some research showed that we can use photo-curable materials to obtain 3D shapes with the application of UV lasers. So, this pave the way for the photopolymerizatioın techniques today.

Today, photopolymerization machines work as the layer-by-layer approach. Also, the curing of the resins takes place layer by layer to obtain the 3D parts from scratch.

The most common type of photopolymerization technique is vat photopolymerization.

How Do Vat Photopolymerization Machines Work?

In today’s technology, vat polymerization machines become very dominant in the sector. High technology and automated systems are common in the 3D printing market. But, if we take a look at the general working principle of these machines, we can understand why they are so common in the sector.

Furthermore, in a typical vat polymerization machine, there is a vat that is full of a curable resin inside it. And there is a platform that can make up and down motion inside this vat.

A laser device produces a laser light to cure the material. Also, the laser devices send the UV light to the reflector, and the reflector sends the laser beam to the platform. And also, the first layer of the material is obtained with this laser. The orientation of the scanners or reflectors is adjusted by the software to obtain the required shape.

After a layer takes place on the platform by solidification of the resin, the platform descends only one layer height. And the second layer is cured by the machine. So, with the lots of this cycle, we obtain the complete 3D from the machine.

As you see above, the working principle of the vat polymerization machines is very simple. In more advanced versions of them, there are multiple laser sources to obtain the part rapidly. So, if we take a closer look at the general working principles of general parts;

Recoating System

Recoating system has two major parts. One of them is a blade and the other one is resin delivery. Once the platform descends one layer to form the other layer, the resin is delivered to the blade, and the blade passes on the surface to deliver and line up the resin on the surface. In each cycle, the recoating system works like this.

Platform

We stated above that, there is a platform that descends in each cycle of the photoğpolymerization. There is an elevator system that is operated by an electromechanical system that descends and ascends the platform. And there is a little servo motor that drives the elevator system according to the software installed into the machine.

Vat

Vat is a container that contains liquid inside it. Also, the platform dips inside this vat. And there is an electronic system that adjusts the resin level inside the vat according to the software.

Laser System

This is another important system to direct the laser to the intended sections of the surface of the platform to obtain the required layer shapes. So, there is a laser beam generation system that generates a laser beam in a specific waveşnegth and intensity.

And there are lenses between the scanning system and the beam generation system that direct the light properly to the scanning system. The scanning system is a servo motor operated system that directs the beam on the platform surface to cure the resin.

Also, the software operates the servo motor according to the geometry and STL file.

Electromechanical Control System

The electromechanical control system is the control system that controls all the motor-operated systems simultaneously according to the software. There is an electronic circuit that has the software. And relays on the cardboard operate the different mechanisms according to the software.

In general, there are three types of control.

• Environment Control: This is generally the temperature control of the system. Because the curing of the resin is an exothermic process, the temperature must be under strict control. the application of the beam is adjusted according to this control.
• Beam Control: Beam control is also very important. The direction of the beam is very important to obtain the correct geometry. Beam sensors are taking the information where is the beam on the platform. And also, the scanning is controlled by this feedback to reflect the beam into the intended positions.
• Process Control: Process control is the general control of the building of the part from resin. The platform descending is about process control. Because, once the machine creates the part, the lowering action of the platform stops.

This is the general working principle of the vat photopolymerization machines.

Lights in Photopolymerization

The general light source of the photopolymerization machines is the UV lşights in general. But there are applications that the use of X-rays, Gamma rays, and electron beams. Also, the visible light is around the applications.

With the decreasing wavelength the total efficiency of the photopolymerization additive manufacturing increases. This is because the energy content increases with the decreasing wavelength. But producing very low wavelengths such as Gamma and X-rays is harder and increases the energy consumption of the machines.

By thinking of all of these factors, the use of UV light and electron beams are the most common applications in photopolymerization techniques.

For example, in the SLA-250 machine, the laser is the helium-cadmium electron beam with a wavelength of 325 nm. Also in the mas projection techniques, the use the UV lights and visible lights are more common.

Photopolymerization Materials

Around the materials of the photopolymerization techniques, photocurable resins are the most common ones. But there are lots of kinds of types of these thermosetting resins.

One of the most important considerations is the level of cross-linking in the structures. Because, with the increasing cross-linking, the strength of the material increases. But with the increasing cross-linking, the shrinkage also increases.

For example, we can cure acrylate-based resins up to 46% of completion. So, they are common in applications. Also they important amount of shrinkage and curling.

In enhancing technologies, the use of epoxy resins become common. The general shrinkages of the epoxy resins are around 1-2%. The major disadvantage of epoxy resins is their brittleness after the curing applications. But acrylates have better ductility and brittle behavior if we compare them with the epoxies.

In most photopolymerization techniques, the use of acrylates and epoxy mixtures is very common. According to the intended structures and mechanical requirements, they mix these two resins to obtain the optimum for themselves.

Mechanism of Photopolymerization

As we stated above we use epoxies and acrylates in these processes. In a typical machine and vat, we use monomers of these materials which are liquid in structure. And also, there are additions of photoinitiators which are very important in the formation of the first polymers to obtain the general structures.

Polymer materials have lots of monomers that are connected to form polymers. So, the molecular chains are very long in the polymer materials. The process is called polymerization. Monomers come together and form covalent bonds with each other to form polymers.

In photopolymerization processes, we use the effect of light sources to obtain polymers from monomers of the resins. To initiate the reaction, we use photoinitiators. Once the photoinitiators take the laser beam on them, they start to combine the monomers of the resin liquid to form polymers. After forming f the polymer structures, there are cross-linking structures take place with the application of light.

In general, there are two types of photopolymerization mechanisms in vat photopolymerization additive manufacturing applications.

Free-Radical Photopolymerization

This is the main mechanism of photopolymerization of the acrylates. With the photoinitiators, the monomers come together to form polymers. Once the polymers grow enough and come closer between themselves, they form cross-linked structures. The cross-links will be increasing with the exposal of the light.

The general downside of the free-radical photopolymerization techniques of acrylates, they have problematic shrinkage problems with the formation of cross-links.

Cationic Photopolymerization

This is the second type of photopolymerization mechanism. We see this type is generally in epoxies. In this type of polymerization, the monomers of epoxy have ring shapes. With the application of light, the rings open, and the opened rings are attached.

Because of this mechanism, the general shrinkage rates are lower than free-radical photopolymerization. Because there is no net change in the molecular bonds. Just rings open and bond with other rings.

Photoinitiators

They are very important additions to the photopolymerization techniques. The general working principles of the photoinitiators, they take the energy of the light and convert this energy to chemical energy. And with this energy, the monomers start to bond together to form bigger polymers.

Because of this, we must have photoinitiators in these systems. In general, manufacturers are supplying photoinitiators with acrylates and epoxies.

Creating Geometry for Photopolymerization

Also, we call these machines stereolithography machines or SL machines. Creating CAD geometries for general additive manufacturing techniques is also valid for SL machines.

CAD Modeling

The first step to obtaining the general part is the CAD modeling phase. There are various kinds of CAD software available. You can use one of them to obtain 3D structures.

The important point in the CAD design for photopolymerization, you need to design the support structures also. In the photopolymerization or stereolithography processes, you may require some support structures to hold some sections in the balance up to full solidification.

So, creating support structures is a very important phase in CAD modeling.

CAD to STL Conversion

STL is the general type of file that additive manufacturing machines use in the creation of 3D parts. So, you need to convert the CAD file into STL files to use them in the machines.

In most CAD software, you can make this conversion easily. You can adjust the accuracy and silicon precision in the conversions.

Uploading STL to Photopolymerization Machine

In this step, you need to directly upload the STL file into the SL machines. There can be interfaces where you can adjust the general orientation of the model. So, you can set up the machine with this interface. After setting up the machine, the building of the 3D geometry starts.

Production Phase

After producing the CAD geometry and setting up the vat photopolymerization machine, the production phase starts. The machine sends the light to the platform to obtain the geometry layer by layer. All the geometry is obtained in the vat.

The production of the 3D model is an autonoımous phase there is no requirement for operator attention. But for any kinds of production errors, it is important to check the production.

After the Production

Once the production or building of the part finishes in the vat photopolymerization machine, the operator must remove the part from the machine.

The part has lots of redundant support structures. The operator must remove these support structures. Also, they need to apply some finishing operations such as grinding or sanding to obtain the required surface characteristics.

Post-curing is also very important. You may have a solid structure from the machine. But the cross-linking structures are not complete in the machine. So, you can apply post-curing according to the required mechanical properties.

Important Parameters in Photopolymerization

There are very complex mathematical calculations about photopolymerization applications. Without delving into these calculations, we will try to understand the general important terms about this topic.

Cure Depth

This is a very important parameter in the vat photopolymerization techniques. This is the total depth of the laser that can cure the resin in the vat. So, this gives the maximum layer thickness that we can cure in one pass.

So, with the increasing cure depth, the power requirement of the laser increases. We can understand that this is directly related to the power of the laser.

Depth of Penetration

Also, this is another parameter that we define the total depth of each layer in the various photopolymerization operations. In the calculations of the total time of the process, the depth of penetration is a vert important parameter.

The difference between the depth pf penetration and the cure depth is, that the cure depth is the maximum depth of penetration.

Energy Exposure

This is the parameter of total energy per unit area. So, with the increasing exposure, the total energy that the photopolymer takes increases. The unit is mJ/mm2. This parameter is about the to

Critical Energy Exposure

This is the total critical energy to cure the resin in the vat. The resin starts to form cross-links with this critical amount of energy. So, this is a parameter that depends on the type of energy.

The main difference from the energy exposure is, that this is the limit where the cross-linking starts.

Peak Energy Exposure

This is about the resin’s capability to maximum energy exposure. So with the increasing peak energy exposure, the resin will form cross-linked structures rapidly.

Irradiance

The laser applies a radiant power to the resin. Irradiance is the power per unit area of the resin. The unit of irradiance is W/mm2.

Output Power of Laser

The total power of the electron beam that the laser produces. It is a very important parameter for the cure depth and depth of penetration.

Scan Speed

Total speed of the laser application on the resin. The laser speed depends on different factors. The total curing rate of the resin and the power of the laser are the main factors of the scan speed.

With the increasing curing rate, the scan speed of the laser increases. Also with the increasing power of the laser. So if the power increases, the total curing of the material increases.

Laser Radius

The radius of the laser scan is also very important. It affects the total time and the energy per unit area. If the radius is too big, the energy deposition will be minimal. But with the increasing radius, the total time to cure the resin will decrease.

With the decreasing radius of the laser, the energy depositioın will increase. And also with the decreasing radius, the total time increases.

They are the general important parameters that affect the vat photopolymerization processes.

Conclusion on Photopolymerization Processes

As you see above, the photopolymerizatioın techniques become common and common. So, they are very important manufacturing processes of 3D parts from thermosetting resins. You can obtain nearly all kinds of 3D shapes with this additive manufacturing method.

The general working principles of vat photopolymerization machines are very simple. Also, there is a laser generator that generates a laser beam. And these laser beams are directed by the scanner to the platform. And pğlatform descends in each layer of curing. So, the total shape is obtained.

There are generally two types of photopolymerizatioın mechanisms and two common materials. These materials are acrylates and epoxies. In the market different proportions of them are sold in general. Acrylates form cross-linking with the classical polymerization process of monomers. But the monomers of phenolics are in the shape of rings. The laser must open the rings to attach to form long polymers.

We use the general steps of additive manufacturin in vat photopolymerization processes. We start with the CAD geometry and convert it to an STL file. Upload this STL file and start the machine. Wait until to operation completes. After the process finishes, the finishing operations are applied.

Also, it is very important to consider the general parameters that affect the vat photopolymerization processes.

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FAQs About Photopolymerization Processes