According to the ACAM Aachen Additive Manufacturing Center’s sharing of "Additive Manufacturing Technology ‘Deep Potential’-Frontier Development Trends" at the 2021 formnext Shenzhen exhibition, the development trend of 3D printing-additive manufacturing is heading towards a multi-dimensional deepening level. For mass production applications, 3D printing breaks through the limitations of current applications on economic requirements, and one of the development paths that extends to the application side and moves toward industrialization is to achieve more complex products.

In the field of metal 3D printing, indirect metal 3D printing represented by Binder Jetting technology has attracted great attention in the industry with high speed and low cost. Binder-jet metal 3D printing technology, from the perspective of production efficiency and economy, fully meets the application of surface vector production, so what kind of problems does the binder-jet metal 3D printing technology face in terms of realizing products with more complex structures? Challenge?

This article will conduct an in-depth analysis of the debinding and sintering processes in the post-processing links that affect the quality of parts, in order to provide a better overall idea of controlling the binder injection manufacturing process.

Binder jet metal 3D printing technology

Those in the industry familiar with Binder Jetting's 3D printing technology for binder jetting are not difficult to find that the traditional metal injection molding process MIM requires the post-processing of debinding, sintering and binder jetting metal 3D printing technology. The process is consistent.

The post-processing of Binder Jetting metal 3D printing technology has three goals:

· Extract "green" parts from the powder bed and depowder them without damaging the parts themselves.

· Debinding and sintering parts to reduce them to acceptable density and geometric accuracy.

· Finish machining parts to the required accuracy.

Debinding

The green body is composed of metal powder and a binder. Debinding mainly removes the first-stage binder. Only a minimal amount of the second-stage binder is held together. The binder will be removed at the same time during the sintering process. Therefore, the degreased green parts are very fragile.

The green parts with high binder content can be processed by grinding or milling before debinding to achieve the required surface characteristics. It is also possible to remove a supporting structure that is only used for printing and does not require sintering. Currently, the three types of debinding evolved from the MIM industry are as follows:

• Solvent debinding

• Thermal debinding

• Thermal catalytic debinding

For solvent debinding, place the parts in a tank containing a solvent that can dissolve the binder. According to 3D Science Valley, this variant requires only simple equipment and is relatively cheap. However, the handling of hazardous liquids can cause safety issues, and the debinding process requires 24-48 hours to effectively remove the extractable solvent.

The thermal debinding process is based on the thermal decomposition of the binder system. It can be carried out in the same furnace used for the sintering process or in a low-cost pretreatment furnace. However, this process is time consuming and only suitable for green bodies with low binder saturation. Therefore, most Binder Jetting parts are degreased by thermal debinding. However, as the amount of organic materials brought into the furnace has increased, equipment maintenance has increased.

For thermal catalytic debinding, the parts are placed in a gaseous acid environment and heated to approximately 120 °C. The process is fast, but relatively expensive, and requires infrastructure to handle gaseous acid.

Debonding is a slow process because the adhesive must evaporate through the tiny porous material structure. If too much heat and energy are applied, the metal particle matrix will be disturbed, causing the final part quality to be adversely affected. The adhesive is removed from the outer surface at a rate of about 1 cm/hour, so thicker parts may take several days to unbond.

After removing the binder, the parts are in a very fragile state, containing the least amount of secondary binder, so they are very fragile during handling.

Sintering

In order to achieve the structural integrity of the metal parts, a sintering process is required.

When using support/locator materials that require additional processing, conventional support/locator strategies and technologies can lead to failures in the processing of parts, such as deformation or shrinkage of the support/locator structure will affect the structure of the parts At this time, a very matching strategy is needed so that the support/locator structure can not only play a supporting role in the processing process, but also does not affect the accuracy of the parts in the subsequent post-processing.

According to the market research of 3D Science Valley, Desktop Metal applied for a patent for separable support/locator technology in 2018. The patent mentions an interface layer between parts and the support/locator structure to suppress during sintering. The bonding between the supporting structure and the surface of adjacent parts.

The support/locator is made of ceramic material separately, the production cost of the positioner is higher and more time-consuming, but they can be reused, so it can save the time and material cost of mass production. The support/locator must be carefully designed to support the part during the sintering process to compensate for part shrinkage and thermal stress transfer.

The parts are heated in a furnace close to the melting temperature but below the melting temperature. The process is divided into three stages, one of which is defined as the interval between powder particles and pore geometry changes. In the initial sintering stage, the powder particles are combined only by van der Waals forces. When the sintering temperature is reached, a neck is formed between the bonds of the particles. During this period, the holding time of the thermal debinding process is set to the corresponding decomposition temperature of the secondary binder.

The characteristic of the middle stage of the second stage of sintering is to increase the packing density of the particles through the merging of adjacent particles. At this stage, an isolated pore structure is produced.

In the third stage of sintering, the pore size is further reduced until the pores are almost completely eliminated. A relatively large volume shrinkage occurs during the sintering process. Basically shrink by 16% to 21% in each direction. Due to gravity and material compression, the shrinkage is anisotropic and larger in the z direction than in the xy direction (affected by gravity).

In the furnace, the thinner parts of the parts heat and sinter faster than the thicker parts, and these parts introduce stress into the parts with varying thickness. In addition, the cooling of the parts after sintering further magnifies this effect. These thermal gradients and stresses can warp and damage the parts, and may produce non-uniform grain structures that affect material properties.

Shrinkage

Managing and compensating for the large amount of shrinkage that occurs during the sintering stage is one of the biggest challenges facing Binder Jetting's 3D printing technology. The parts shrink by 30-40% in the furnace and linearly shrink by 15-20%. If the part is small and the wall thickness is uniform, then shrinkage can be predicted. However, the sintering process of large parts with different thicknesses will cause very complicated geometric problems. According to 3D Science Valley market research, sintering shrinkage currently severely limits the applicable geometry and application types of Binder Jetting metal 3D printing technology.

The shrinkage rate may also be affected by other parameters, such as:

• Friction between the base plate and the parts

• Gravity

• Unsupported functions for bending

• Powder contamination

• Particle size

• Warpage during printing

• Wall thickness

During the shrinking process, the bottom surface of the part slides on the sintered bottom plate. Although the rest of the part can shrink freely, friction prevents uniform shrinkage. This can cause deformation of the part.

The friction effect can be reduced by sintering the bottom plate or the movable backing plate, and they shrink in the same way as the parts because they are made of the same material.

Due to the shrinkage of the parts during the sintering process, it is necessary to compensate for the distortion. The software tools under development can better simulate and predict compensation schemes, and then adjust the part geometry accordingly. However, this is not a simple solution, and sometimes only makes sense for specific geometric shapes.

According to market observations from 3D Science Valley, take Desktop Metal as an example. In terms of real-time simulation, Desktop Metal works closely with multiphysics simulation software developer ANSYS. ANSYS' Discovery Live platform allows changes to the CAD model to show how fluid or air flow is affected in real time and can be used by anyone, not just experts. Discovery Live allows engineers to immediately check the impact of their design changes. This platform supports fluid, structural and thermal simulation applications. This allows designers to interactively explore the impact of simple and complex changes, and iteration becomes faster and more convenient.

During the sintering process, the parts are fixed on the bracket by supports/locators and placed in a furnace with an inert atmosphere. First, a debonding cycle is performed to burn off the polymer component of the binder. The temperature is usually in the range of 200-600°C. All the binder must be completely removed from the part, otherwise the residual carbon in the binder will negatively affect the sintering process and impair the final part performance.

However, all this may be changing. According to market observations from 3D Science Valley, Desktop Metal has acquired Meta Additive. Meta Additive’s non-sacrificial binder solution reduces the sintering shrinkage from 20% to 2%, which not only eliminates the debinding step. It also reduces the heat level required in the post-processing stage. After normal printing, heat treatment at 300℃ is enough. The heat treatment is to consolidate and optimize some microstructures.

It is reported that during the use of Meta Additive's binder, it is mainly a chemical reaction, not just a physical reaction. This is the research and development of chemical technology based on the technological tree of atomic layer deposition (ALD) and chemical vapor deposition (CVD) and related industrial achievements that were invented in the 1970s. This binder composed of molecular components, nano-components and micro-components. Nano particles are filled in the powder gap to achieve inter-particle bonding and penetration, which is equivalent to depositing the binder uniformly and non-porously on the metal powder during the 3D printing process. Between the interstices of the bed particles.

| Temperature

The sintering process plays a key role in the quality of the final product. Parts need a specific time to compact and reach the final density and mechanical properties. The setting of the temperature curve must be adjusted according to the material and geometry of the part. Therefore, the sintering process parameters depend on many factors such as materials and cannot be generalized.

| Gas

In the sintering process, a specific gas is required to prevent oxidation of the material, prevent decarburization of carbonaceous materials, or reduce the carbon content of stainless steel. Therefore, the gas must be adjusted according to the material used.

Generally, high vacuum or argon atmosphere is required during sintering, such as titanium alloy, while stainless steel such as 316L requires a high-purity hydrogen environment close to the atmosphere to prevent decarburization.

The hydrogen atmosphere is used to reduce the carbon in the binder system and prevent it from diffusing into the metal. Therefore, when sintering stainless steel, H2 is very important to control and minimize the C content.

Another relevant factor is the gas flow rate used in the debinding and sintering furnace. For best results, it is recommended to use uniform airflow and local pressure around the part to obtain uniform temperature distribution and consistent debinding.

| Powder

Several powder characteristics will affect the debinding and sintering process. This includes particle size, composition and powder density. Each powder characteristic has a different effect on the debinding and sintering process.

Especially the debinding process is affected by the particle size of the powder, and the retention time of debinding will increase sharply as the size of the metal particles decreases. This is due to the influence of the gas cell characteristics of the powder due to its smaller porosity.

Not only the characteristics of the powder are important, but also the debinding performance should be studied to design the process correctly.

The difference in the particle size and distribution of the powder will result in the need to sinter at different lengths of time and temperature, and affect the performance of the parts. Due to the higher specific surface area, the smaller particle size supports sintering at lower temperatures and residence times.

Insufficient sintering may also be caused by powder chemistry. Inclusions, additives, and oxidation can cause metal parts to sinter ineffectively. Contamination can also affect the behavior of the part during the sintering process and may cause unpredictable shrinkage.

| Green characteristics

After debinding, the green parts are very fragile due to the lack of the first-level binder. These parts cannot be processed mechanically now and must be placed on a pallet.

The high fragility of Binder Jetting green parts limits the level of detail that can be achieved. During unpacking or handling, thin walls, pins, or sharp edges may break. Nevertheless, the resolution of BJT parts is high.

Factors affecting cost

In addition to volume and size, the unit cost of adhesive injection is also affected by several different factors such as design.

The cost of each part mainly depends on the volume of each part. As the amount of material increases, the cost of the parts increases linearly. High material costs and constant build rates affect the relatively constant cost per cubic centimeter. Although the cost of the sintering process mainly depends on the overall size of the parts and the subsequent utilization of the furnace, the debinding is mainly affected by the thickness of the parts.

The cost of each part can be reduced by increasing the number of parts in a build job. If the unpacking of green parts is done manually, the design is best optimized for unpacking, which can save expensive labor time and even realize an automated solution.