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Vat Polymerization

Vat Polymerization also known as Stereolithography (SLA) is the origin of the 3D printing technology. Variations of the process have been developed over the last 40 Years to increase the efficiency and the accuracy of the process. Especially the medical sector is taking advantage of the various polymerization processes.

More recently, Area-wise Vat Polymerization, known also as Digital Light Processing (DLP) has been introduced with the benefit of increased productivity. 

Technology principle

How does Vat Polymerization work?

The origin of this technology is the polymerization process, in which a liquid photopolymer is cured by a UV laser to initiate the solidification. The laser selectively generates the x-y-contour of the part within the liquid resin vat. The initial layer of the component is connected to the baseplate by support structures.

After each cycle the base plate is lowered by the means of the defined layer thickness. The characteristics of the layer is dependent on the transmissibility of the photopolymer as well as the laser power and speed. Each new layer is applied by a coater, which provides an even and homogenous layer of liquid.
After the process is completed the part is removed from the vat. The generated part must be cleaned, and support structures need to be removed. Usually, the parts are also post processed with an UV curing oven to solidify the part completely.

Area-wise Vat Polymerization

Technology variant: Area-wise Vat Polymerization

Area-wise Vat Polymerization, known also as Digital Light Processing or DLP, is a modification of Vat Polymerization. Instead of selective curing of a photopolymer by laser, a UV projector is used that cures an entire layer at once. This technology was first introduced by ENVISIONTEC and its founder Al Siblani.

Nowadays this process is especially popular in the medial sector. Dental aligners, hearing aids and other customized products are produced via this technology in high numbers and take advantage of the accuracy and speed of this technology.

This process is based on the polymerization of a photopolymer. To cure the liquid polymer a UV light source or projector is used. A transparent and permeable window allows transmission of light. A series of cross-sectional images using UV light cures the resin during the up movement of the plate. This process repeats after every layer.

After the process is completed the part is removed from the vat. The generated part must be cleaned and support structures need to be removed. Usually, the parts are also post processed with an UV curing oven to solidify the part completely.

The process of solidification of the part is the actual polymerization. This process initiated by light energy is a chemical reaction, which leads to a change of aggregate state. The resin is solidified and therefore shrinkage of the dimension occurs. This shrinkage is typically in the range of 1 % to 2 % and is compensated by the data preparation software of the system manufacturer.

Read more about this topic in the Polymer Technologies Course.

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Sinter-based AM technologies and process chain

Sinter-based AM - a technology overview

Many different printing technologies - one sintering process

The sinter-based AM (SBAM) technologies have, as the name suggests, the sintering process in common. In this process, the printed green part is consolidated into a dense part and receives its final properties. The green part can be printed in advance using different technologies.They all have in common that metal powder is bound to the desired shape by a binder. The best-known printing technologies include Binder Jetting and Filament Material Extrusion.

In this section, you learn everything about the sinter-based AM  process chain and get an overview of the different printing technologies.

Goal and structure of this course

This course is aimed at engineers, designers and other professionals that are working closely with sinter-based AM technologies. The goal is to cover the most important aspects that will enable engineers and designers to fully grasp the capabilities and technical limitations of the printing technologies and the sintering process to succeed in technology selection and part design. Besides going through the course from the beginning until the end, this course can also act as a constant source of knowledge while working on AM projects. 

The course is structured into the following sections.

This section will start with an overview of the sinter-based AM process chain and its printing technologies, followed by a technology deep dive into the most important aspects of the BJT technology, followed by a closer look at the debinding and sintering step also including sintering simulation .

The second section will provide an overview of the different materials that are available as well as part characteristics that can be achieved with the BJT process and typical methods for quality assurance. Finally, several common defects in the BJT process are presented. 

The last section will act as a guideline for designers. Besides generally describing the process when designing for Additive Manufacturing, actionable restrictions and guidelines for the BJT process are provided. The final section will present several design examples from different industries. 

What you will find in this section

Sinter-based AM process chain

From digital model to finished part

Data preparation

Simulation to compensate the deformation during the sintering step, nesting of parts and definition of printing parameters

Printing

Through various printing processes, different feedstocks such as metal powders, filaments, pellets or dispersions are processed into green parts

Unpacking

Unpacking of fragile green parts needs to be done carefully and is typically a manual process.

Debinding

Debinding describes the process of removing the binder which results in a brown part

Sintering

To reach the structural integrity of a metal part, a sinter process is required. The powder particles fuse together to a coherent, solid structure via a mass transport that occurs at the atomic scale driven via diffusional forces.

The brown part shrinks ~13-21 % in each direction.

The process chain of sinter-based technologies differs from other AM Technologies. Especially the post-printing processes (debinding and sintering) are crucial to achieve the intended mechanical properties.

Technology principle

How does Binder Jetting work?

Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder is selectively deposited onto the powder bed binding the metal particles and forming a green body.

The metal powder is applied to a build platform in a typical layer thickness of 40 µm to 100 µm. Subsequently a modified 2D print head apply a binder selectively onto the powder bed. Depending on machine technology a hardening or curing process of the binder is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite.

Working Principle of Binder Jetting

Afterwards the build platform moves downward by the amount of one layer thickness and a new layer of powder is applied. Again, the liquid binder is deposited and hardened in the required regions of the next layer to form the green body. This process is repeated until the complete part is printed. After the complete printing process is finished the parts have to be removed from the “powder cake” meaning the surrounding loose but densified powder. To improve the removal of the excess powder from the green body often brushes or a blasting gun with air pressure are used.

To create a dense metal part the 3D printed green body has to be post-processed in a debinding and sintering process. Similar to the metal injection molding process BJT parts are placed in a high temperature furnace, where the binder is burnt out and the remaining metal particles are sintered together. The sintering results in densification of the 3D printed green body to a metal part with high densities of 97 % to 99,5%, dependent of the material.

Printing Technologies

Metal Binder Jetting

Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder is selectively deposited onto the powder bed binding the metal particles and forming a green body.

The metal powder is applied to a build platform in a typical layer thickness of 40 µm to 100 µm. Subsequently a modified 2D print head apply a binder selectively onto the powder bed. Depending on machine technology a hardening or curing process of the binder is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite.

Working Principle of Binder Jetting

Material Extrusion

Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder is selectively deposited onto the powder bed binding the metal particles and forming a green body.

The metal powder is applied to a build platform in a typical layer thickness of 40 µm to 100 µm. Subsequently a modified 2D print head apply a binder selectively onto the powder bed. Depending on machine technology a hardening or curing process of the binder is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite.

Working Principle of Binder Jetting

Mold Slurry Deposition

Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder is selectively deposited onto the powder bed binding the metal particles and forming a green body.

The metal powder is applied to a build platform in a typical layer thickness of 40 µm to 100 µm. Subsequently a modified 2D print head apply a binder selectively onto the powder bed. Depending on machine technology a hardening or curing process of the binder is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite.

Working Principle of Binder Jetting

Metal Selective Laser Sintering

Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder is selectively deposited onto the powder bed binding the metal particles and forming a green body.

The metal powder is applied to a build platform in a typical layer thickness of 40 µm to 100 µm. Subsequently a modified 2D print head apply a binder selectively onto the powder bed. Depending on machine technology a hardening or curing process of the binder is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite.

Working Principle of Binder Jetting