Use-Cases of different Recoater Configurations for Direct Metal Laser Sintering (DMLS)

September 30, 2021 | Reading time: 7 min

When it comes to different options of recoating blades, there has always been a debate in the Additive Manufacturing (AM) community whether to use a soft or a hard recoating solution. Here, I want to give an overview of different use cases and the advantages as well as disadvantages of soft and hard recoating. 

Hard Recoater

The standard option for EOS systems is the hard recoater, because of its advantages in terms of part quality and repeatability. The rigid blade assures consistent layer thickness and can remove spatter from the part surface that was ejected from the melt pool during exposure and would be a high risk factor for lack of fusion in subsequent layers. Due to its shape and stiffness, the hard blade is compacting the powder during recoating, which provides a higher density powder bed. All EOS standard process parameters are developed with hard recoater blades, because they are very sensitive against unsuitable process parameters. Thereby, the process window can easily be narrowed down to robust process parameters, which provide a stable process.

A hard recoater shows superior wear resistance compared to soft recoaters, which assures constant recoating properties even for long build jobs. Thereby properties throughout the part are more consistent. Even though hard recoaters are typically more expensive than soft recoaters, the longer life time and reduced amount of required maintenance has to be considered as well for a cost analysis.

Highly regulated industries have expressed reservation against polymer soft recoaters, because wear could potentially cause contamination of the powder and lead to inclusions. For example, the aerospace industry is particularly looking to avoid silicon inclusions in their engine applications. However, this is not a general criterion for exclusion.

Typical applications that benefit from the advantages of hard recoaters are parts with highest demands for repeatable quality in terms of mechanical properties or dimensional accuracy as well as bulky parts, for which a soft recoater would wear off too quickly for long build jobs.

A main challenge of the hard recoater is its rigidity. If the part comes into contact with the recoater, the risk is high to have a job interruption where a soft recoater would adapt to the deformation of the part. Contact between part and recoater occurs for example because of support failure due to residual stress or due to overheating caused by unsuitable process parameters.

However, for high quality parts or serial production this disadvantage can also be seen as an advantage. Even if the build job would finish with a soft recoater, the damage or imperfection would still be present in the end and make scrapping of the part necessary. There is a certain risk that the damage is only detected downstream after heat treatment or other post-processing steps and thereby causes even higher costs. In the worst case, if the defect is not detected at all it could lead to malfunction during operation. Although the job would crash with a hard recoater, the AM engineer gets direct feedback that something with the design, supports or process parameters is wrong and that the part has to go through the next iteration.

A further disadvantage is that all friction forces during recoating are transmitted into the part, because of the stiffness of the blade. This fact limits buildability when it comes to high aspect ratios. During recoating, the frictions forces may bend over the part or cause vibration that disturbs the powder bed. This challenge can be partially mitigated by adapted process parameters, which take into account the thermal situation of the part but it still can be a limiting factor for tall and thin designs.

Fine objects like lattice structures for medical applications however, show that fragile parts can fairly easily be built with a hard recoater when adapting process parameters. Hip cups are a good example for parts with fine structures but high quality demands where a hard recoater can prove its’ their benefit.

There are two different types of hard recoaters available for EOS machines: HSS (high speed steel) and ceramic. It depends on the material used, which recoater configuration has to be selected. For most materials, the HSS blade is used, because even though it is hardened, the blade still shows some ductility. Therefore, the risk of breaking out notches after contact with the part is lower than with the ceramic recoater. However, in case of notches after a rough job, e.g. due to support failure, the blade does not have to be replaced but can be grinded carefully.

If the material is magnetizable, a ceramic type recoater blade has to be chosen. Otherwise, powder would stick to the blade and cause streaks during recoating. Such materials in the EOS portfolio are steels like CX, PH1, 17-4PH or MS1. In contrast to the HSS blade, potential notches cannot be grinded due to the brittle characteristics of ceramic. However, the material is extremely wear resistant.

Soft Recoaters

The major benefit of the soft recoater options is the reduction of recoater forces on the part during recoating. Since the recoater is more flexible, it can give way if friction forces become too high or it can adapt to some extent to the profile of a part if it is deformed and sticking through the powder bed. This results in two main use cases that come with the advantages of soft recoaters:

1. the ability to build high aspect ratio parts and fragile features more easily 
2. the lower probability of job interruptions due to recoater jams. Especially in prototyping, short-term results are often more important than ensured part quality. This is why many service providers typically make use of soft recoaters in order to meet tight delivery schedules. Additionally, every crashed job would increase manufacturing costs.

The main disadvantages of soft recoaters have already been briefly mentioned above in the comparison to hard recoaters. Increased wear of the recoater can affect part properties during long build jobs, because consistent recoating behavior throughout the full build height cannot be assured. Furthermore, in case of deformation of the part, for example due to insufficient attachment to the build plate, the job continues but the part has a high probability to be out of spec regarding dimensional accuracy. Therefore, a successful job completion does not necessarily mean a part, which meets the specifications.

Nevertheless, the soft recoater is a helpful tool and its’ need is application driven. It can become the most economical solution for certain applications that have characteristics like mentioned above: High aspect ratio, fragile features or need for short-term results.

EOS offers three different options of soft recoaters. The carbon fiber brush recoater that has already been available for many years and two types of the above mentioned polymer recoaters for the EOS M 290: Silicone and nitrile butadiene rubber (NBR).

The brush recoater is equipped with a collection of short carbon fibers, which are mounted into a holder. It might have small advantages compared to the polymer blades but it is more expensive. In case of contact with the part, the probability that the recoater is permanently damaged is slightly lower than in the polymer option, because the fibers can bend to the side to a certain extent. Furthermore, the effect of friction forces is more localized because the part interacts only with a certain amount of fibers, which makes it superior for building fragile parts. 

The two polymer recoater types for the EOS M 290, are also available for the large frame systems EOS M 400 and EOS M 400-4. The easiest way to distinguish them is the color: The silicone blade is transparent and the NBR one is black but there are also differences in terms of properties and use cases. The NBR recoater material is specified for use up to 80 °C and the silicone material is suitable for higher temperature build jobs.

In order to show the capabilities of the new polymer soft recoater options, we have built a few high aspect ratio parts with EOS M 290 letters on top of it to further increase the challenge. Materialise Magics has been used to create the tree supports, which have a maximum aspect ratio of 66 (2.5 mm diameter with a height of 165 mm). The EOS M 290 writing on top has been designed with the help of Materialise 3-matic software and consists of a mesh-based lattice with volume graphs inside. Furthermore, the fine support for the EOS logo has been created with Materialise e-Stage, which enables fully automated support generation and can help to save material required for the support as well as significantly reduce time for data preparation. Everything was then printed on an EOS M 290 with EOS Aluminum AlSi10Mg and the new AlSi10Mg 60 µm Core process.

Conclusions

There is the appropriate use case for each recoater type. The hard recoater is considered by EOS as the standard option for parts with highest demands with regard to repeatable quality whereas a soft recoater enables parts with high aspect ratios. Furthermore, a soft recoater is preferred when lead time is more crucial than risk of imperfections. EOS offers both hard and soft recoaters so you can easily choose the one that works best for your application.