Author: Ben Arnold, Tritone Technologies (Rosh H’ayin, Israel)

ABSTRACT

As sinter-based metal additive manufacturing technologies gain traction in manufacturing applications, binder jet platforms garner the majority of headlines and press as the most common additive green part forming technology. However several new processes for green part manufacturing are emerging. This talk will highlight how one of these technologies, MoldJet, addresses several key challenges facing binder jet adoption and opens additional application space for sinter-based AM.

We will share an overview of the MoldJet technology and some test results demonstrating the potential of this technology.

INTRODUCTION

Metal additive manufacturing (AM) is an important technical advancement as it offers several advantages over traditional manufacturing methods for producing metal parts. These advantages include:

1. Design flexibility: With metal AM, it is possible to create complex geometries and internal structures that are not possible using traditional manufacturing techniques. This can result in lighter, stronger, and more efficient parts.

2. Cost efficiency: Metal AM can be more cost-effective for small production runs or for producing parts with complex geometries that would be difficult or expensive to machine.

3. Time efficiency: Metal AM can reduce lead times for producing parts, as the technology allows for rapid prototyping and fast production of parts.

4. Material efficiency: Metal AM can use less raw material than traditional manufacturing methods, as it only adds material where it is needed.

5. Sustainability: Metal AM can reduce waste and environmental impact by using less raw material, producing less scrap, and reducing energy consumption compared to traditional manufacturing processes.

Over the past few decades metal AM applications and markets have grown dramatically, mostly driven by the Laser Bed Powder Fusion (LBPF) process. With this technique a high-powered laser is used to selectively melt and fuse metal powder particles together, layer-by-layer, to build up a three-dimensional (3D) metal part. This “direct” metal fabrication process is capable of producing complex geometries and parts with high accuracy and detail. It is widely used in industries such as aerospace, medical, and energy where the production of complex, high-value parts is required.

While the capabilities of this process have been clearly proven with many commercial successes, the technology is inherently costly which has limited its influence to high value applications that can justify the expense. These processes are also limited to applications of materials that can be welded.

Sinter-based approaches to metal AM have long been explored as a commercial opportunity to widen the appeal of metal AM by lowering the economic barrier. In recent years this sector, driven primarily by the metal binder jet process has received significant investment and corresponding market attention. Several large industrial players have committed significant resources to develop and industrialize the process. The process is improving each year, however there are some fundamental challenges that will likely remain very difficult to fully solve through incremental advances in a decades old core technology.

An alternate approach is to develop an entirely new method to achieve the promise of sinter-based metal AM. That is the path chosen by MoldJet’s inventors. The new process will be described in detail, but first, a brief overview of the binder jet process and some key challenges will be presented.

BINDER JET PROCESS AND CHALLENGES

In the metal binder jetting process, a layer of metal powder is spread over a build platform, and a liquid binder is deposited (jetted) onto the powder in a pattern that corresponds to the cross-section of the part being printed. The binder selectively binds the powder particles together, forming a solid layer. The build platform is then lowered, and the process is repeated for the next layer until the 3D part is complete.

After printing, the green parts are recovered from the powder bed for further processing. The metal binder jet AM process is an “indirect” process for part fabrication as the green parts must be sintered in a furnace to remove the binder and fuse the metal particles together to form a solid metal part.

Metal binder jet 3D printing is maturing rapidly to address the concerns of customers for cost effective and reliable production. While this process has the desired benefits of high productivity and suitable economics, there remain several key challenges to overcome on the route to mass adoption. Some of these challenges include:

1. Part quality and consistency: The quality and consistency of the printed parts can be affected by factors such as powder quality and binder deposition and saturation. Achieving highly consistent powder spreading and binder saturation in the powder bed is a challenging task. Green parts often have different shrink rates along X,Y and Z axes. Any of these inconsistencies or gradients in the printing step can lead to inconsistent tolerances after sintering.

2. Powder management: Metal binder jet 3D printing requires a significant amount of metal powder, which must be carefully handled, stored, and recycled. If the powder is not properly managed, it can lead to contamination and affect the final part quality. There are also safety concerns with maintaining large amounts of fine metal powder in manufacturing facilities.

3. Part recovery after printing: After printing, parts must be carefully removed from the bed of powder for further processing. The parts are fragile at this stage which can lead to yield loss. Additionally, the process is time consuming and labor intensive which work against the attractive economics of the print process.

While metal binder jet 3D printing has great potential, there are many thorny physics challenges that must be addressed to further improve capabilities.

A Note on Sintering

This document focuses only on green part forming and preparation for sintering. A detailed explanation of the thermal debind and sintering phases is excluded as the intended audience is quite familiar with these processes. It is widely accepted that well-formed and highly consistent green parts that are sintered in a well-controlled and maintained furnace will densify consistently.

A NEW APPROACH TO GREEN PART ADDITIVE MANUFACTURING – MOLDJET®

MoldJet was developed specifically to generate high volumes of very consistent green parts and overcome many of the fundamental challenges of existing processes such as those described with binder jet.

MoldJet is an AM process consisting of two primary process steps; creating a mold as a negative of the desired part geometry and filling of this mold with a metal paste. These two process steps alternate layer by layer to complete a build job.

To understand the thinking behind this new process it is useful to make a comparison to traditional MIM and PM processes. These processes use a proven green part forming technique of compacting a pre-mixed charge of metal powder, carrier and binder into a mold cavity. This technique has been used to form millions of parts annually for decades, with very high levels of consistency and repeatability.

These techniques were the inspiration for the new technology presented here. The challenge was to replicate the proven mold-based processes as closely as possible while eliminating the central (but costly) fixed mold. The goal was achieved by combining several proven manufacturing technologies:

• 3D inkjet printing of a wax-like mold material

• Paste or slurry based feedstock (pre-mixed powder, carrier, and binder as noted above)

• Screen print inspired Application of metal paste

• Rotary index manufacturing workstations

• High resolution 2D camera based inspection

We will explore how these technologies are employed interactively to enable the MoldJet process.

Inkjet Printing of mold material enables the following:

1. Production of mold cavities with very high accuracy, resolution, and repeatability to ensure consistent green part forming.

2. A very solid and stable platform to accept deposits of paste. Mold material solidifies immediately after printing, ensuring that cavities printed anywhere in the build box will have the same dimensions and fill characteristics.

3. The mold material melts and dissolves in post processing, eliminating the need for powder removal. This eliminates labor from post-processing and allows very complex interior geometries.

4. Easy programming of layer heights within the build and the ability to have multiple layer heights withing a build.

5. Long printhead life. Jet heads running low melt temp materials typically last for years in service (versus hours or weeks when jetting complex binders).

Paste (or Slurry) Based Feedstock

Using a paste-based feedstock offers these important operating advantages:

1. A cleaner, safer, factory work environment for employees and machines.

2. Metal particles in the paste maintain complete wetting and homogeneous distribution.

3. Lubrication from the carrier agent ensures homogeneous and very tight packing of particles as green parts are formed allowing very low and isotropic shrink rates (under 14%).

4. Integrated process drying steps also play a role in green part density.

5. Paste based feedstocks keep powder from contaminating the machinery and thereby allow for very easy changeover of materials.

6. Paste based feedstocks create an inherent inert environment for reactive powders, protecting them from oxidation and from generating safety hazards.

7. Paste based feedstocks allow users to run jobs with only a small amount of material, enabling efficient development of new materials.

Fig. 1 – Loose powder is mixed with binder and carrier to form a paste.

Screen Print Style Paste Applicator Mechanism

The novel deposition technique and mechanism designed for this process brings these benefits:

1. Paste feedstock flows easily into the mold cavities ensuring consistent results.

2. An array of hundreds of cavities can be filled with one pass of the applicator, regardless of the complexity of part designs.

3. The mechanism can be easily cleaned enabling fast material changes and eliminating the risk of machine damage when experimenting with new paste formulations.

Rotary Index Workstation Design

While common in traditional manufacturing, rotary workstation designs are rarely seen in AM equipment. This approach is the very hub of this new technology allowing:

1. Increased throughput by parallel processing of multiple, simultaneous print jobs.

2. Ease of design and maintenance by requiring each independent station of the machine to perform just one step in the process.

3. Ease of system upgrades due to the modularity of the design.

Fig. 2 – Rotary Index System Architecture

High Resolution Camera Based Inspection

Another key design feature of the new technology is the ability to inspect each layer for potential defects. If defects are detected the system can automatically remove and re-apply a layer that is out of specification. Additionally, a digital image is recorded for every layer on every job for future reference.

MOLDJET PROCESS OVERVIEW

The first step is creating the first layer of the component mold from a wax-like polymer using an inkjet printing process. In this process, the mold material is heated in a reservoir and jetted onto the substrate by the print heads to form a uniformly mold layer.

Fig. 3 – A printed mold layer prior to filling with paste

The layer of mold cavities is then filled with the metal paste. The paste consists of metal powder, an aqueous carrier, and an organic binder system. An applicator mechanism is used to evenly feed paste from a feed cartridge. The proprietary applicator design ensures a homogeneous transfer of paste into the mold cavities and eliminates density gradients.

Fig. 4 – a: Paste being applied into mold cavities, b: the build layer after cavities are filled and dried.

After the mold cavities are filled on each layer, a series of drying and hardening steps remove the water-based carrier from the paste. This step prepares the build substrate for the next layer of mold material to be applied.

As a final step for each completed layer, a high resolution camera is used to inspect for any deviation from the expected output. Image data of each layer is stored for review and if an error is detected, the error can be removed and re-printed. This ability for in-process inspection and repair is quite unique and advantageous in Additive Manufacturing equipment. In most approaches, any print error means the part (or the entire build) must be scrapped.

Fig. 5 – Camera based inspection provides for real time defect detection and repair

After all layers are printed the complete build tray is removed from the machine for secondary processing. At this stage, the print is a solid block of mold with parts embedded inside. The tray can be handled without PPE.

Fig. 6 – A completed build tray, ready for demolding operations

Demolding

The next process is removal of the mold material to reveal the green parts. This is a 2-step, hands free operation. First the bulk mold material is melted away in a low temp oven, then any remaining material is dissolved with solvents. A range of standard industrial equipment is available “off the shelf” for use in these steps depending on the scale of production required. There is no “de-powdering” step required as there is no loose powder to remove.

Fig.7 – Mold is removed to reveal green parts ready for thermal debind and sintering

Post-Processing of Green Parts

Green parts produced with the MoldJet process are quite dense and rugged, allowing a significant amount of handling (and even shipping) without damage. Parts can be polished individually or in bulk to improve surface finish if required, holes can be tapped etc.

Fig. 8– Tough green parts can be handled and post-processed

RESULTS

The MoldJet process offers several relevant operating advantages in the production of consistent, high-quality green parts. These parts must then be sintered and tested to confirm that performance meets expectations across multiple fronts. Data points are presented below to demonstrate that mechanical properties align with relevant standards and dimensional accuracy has been superior to other additive technologies in head-to-head tests. These validation efforts are ongoing as MoldJet technology matures. Systems and resources are available for independent validation work in multiple global locations.

Mechanical Properties – Test method and results

A build box was designed as shown below to validate consistency of part samples printed in all regions of a build box. ASTM samples were produced in horizontal and vertical orientations in all regions of the build envelope. Samples were then tested for consistency and isotropicity of results.

Fig. 9 – Build box layout for sample parts (Tritone internal testing)

Fig. 10 – 15-5 SS Properties Analysis

Data shows good alignment between measured properties and reference properties. This data set indicates a tensile strength that exceeds specifications and a small deficiency in elongation. A current development project is evaluating modified sintering and heat treat parameters to trade some tensile property for increased elongation.

Additional Material Test Data

In addition to the internal development work ongoing with MoldJet, material development and testing is also ongoing at the Fraunhofer IFAM in Dresden Germany. Engineers at this facility have operated MoldJet equipment at their facility since 2021 and are currently developing new materials for a number of industrial and research based clients. The Fraunhofer team holds regular public forums to discuss the technology and are available to take on projects for customers worldwide. The Institute shares the following data to set expectations across a range of materials they have worked with.

Fig. 11 – MoldJet test data shared by Fraunhofer IFAM across a range of materials [1]

Part Measurement Data

Parts produced by the MoldJet process have been tested extensively to validate claims of geometric accuracy and repeatability. This work is done by organizations as part of their evaluation and daily use of the technology. Below are data sets authorized for publication. These represent measurements of a significant number of the same part geometry produced by MIM, by 2 different metal binder jet technology platforms, and by MoldJet in 2 separate shipments of parts.

While the specific geometries tested are confidential, the resulting measurement data presented here indicates superior accuracy and repeatability to the binder-jetted samples.

Fig. 12 – Measurement data comparing Moldjet data to MIM and two binder jet processes.

CONCLUSIONS

This paper has presented MoldJet, a new technology for green part additive manufacturing. The inventors intent to provide a cleaner, easier to use process has been described and several data points demonstrating success have been presented. Moldjet systems are currently in use at customer production facilities globally and the rate of adoption is increasing year over year. There is significant desire in the market for sinter based additive manufacturing modalities to reach their potential for cost effective operation. Each process must prove itself to be highly repeatable and economically viable to earn a long-term place in the market. In addition to the specific nuances of each technology, global standards for sinter-based AM modalities are needed to establish an environment where more manufacturers will participate. As these standards evolve, the MoldJet process will be challenged to keep pace and maintain a leadership position. Industry challenges and feedback will be crucial in the pursuit of this goal.

References

[1] Dipl.-Ing. Robert Teuber, Dr.-Ing. Thomas Studnitzky, “The New Innovative MoldJet® Technology” Fraunhofer IFAM White Paper, March 2021