Removing raw material when designing for additive manufacturing

As explained in a previous blogpost, there are many ways to use additive manufacturing. Materials can be used in very different configurations (powder, liquid, sheets, wire, ink,…) and one of the main concerns in AM is removing the materials after the job is completed.

With some techniques, such as selective laser sintering (SLS) and electron beam melting (EBM), the powder is sintered around the part due to the hot conditions. Sintering means that powder grains are superficially melted and stick together, as if they were partially welded together in all the contact zones between the grains. This condition of heat-affected powder is called a “cake”, and the manufactured parts are inside that cake. It is possible to take that block of sintered powder out of the manufacturing chamber with your hands. The cohesion between the powder grains is strong enough to allow this. Therefore, it is also pretty hard to remove this cake from the narrow internal cavities.

Here are some photos showing how the powder surrounds the parts after a job is completed with the SLS process, and how hard it can be to clean designs that are too complex.

Raw material removal step, at the end of an SLS process

Sometimes the only purpose of removing the raw material is to make the part as light as possible. However, it could also be for cleaning reasons, for example in the medical or aerospace field where no residues (powder grains, droplet of resin, …) may be released during the lifespan of the part. This is something really important to take care of during the design phase.

Example of a cube

The most obvious example is the design of a very complex (and thus optimised) cooling channel in a mould insert. A designer can spend hours of simulation to find the ideal shape to efficiently remove the heat after the injection step. But if he didn’t take care of the raw material that will fill all the channel during manufacturing, his perfect design won’t be usable and so will his expensive part. Because, if this mould were made from a metal powder, and if the channel were about 1 mm in diameter but over 2 meters long when unrolled, how would it be possible to remove the metal powder from that very long and narrow channel? The basic consideration about raw material that has to be taken into account in the design phase, is how to remove it afterwards.

The most obvious example is the design of a very complex (and thus optimised) cooling channel in a mould insert. A designer can spend hours simulating just to find the ideal shape to efficiently remove the heat after the injection step. But if he doesn’t take care of the raw material that will fill the channel during manufacturing, his perfect design won’t be usable and neither will his expensive part. Because, if this mould is made from a metal powder, and if the channel is about 1 mm in diameter but over 2 meters long when unrolled, how would you be able to remove the metal powder from that very long and narrow channel? The basic consideration to make about raw materials in the design phase is how it can be removed afterwards.

A first example to illustrate this “raw material” concern is the making of a simple cube from powder, with the SLS laser melting technology, where the cube has to be hollow :

A hollowed cube, to be made

For the first layers, the powder will be spread all over the surface and all of the squared area of the cube will be melted (red line in the left picture below). When the middle of the part is reached, only the vertical walls will be treated by the laser but the powder will be spread all over the surface:

Left: manufacturing of the first layer, the whole area is processed by the laser; right: manufacturing of mid-layers, side walls are processed by the laser

In the end, the whole area is again processed by the laser, in the same way as the first layers:

Processing of last layers seals the cube

In the last picture can be seen that a large quantity of the raw material used remains trapped inside the cube because, obviously, no outlet has been made through the skin of the cube to remove that trapped material after manufacturing. In this case, powder was used but it is also applicable for resins, sheets, wires,… So it’s really important to take care of this raw material in the design phase, otherwise this hollowed cube will be much heavier than expected.

The last picture shows that a large quantity of the raw material used remains trapped inside the cube because, obviously, no outlet has been made through the wall of the cube to remove any trapped material after manufacturing. In this case, powder was used, but it also applies to resins, sheets, wires, … So it’s really important to take care of this raw material in the design phase, otherwise this hollowed cube will be much heavier than expected.

Configuration of the outlet

Sometimes an outlet to remove raw material is too narrow, or it is not possible to remove everything because of the internal configuration, or maybe sometimes having an outlet between the inside and outside of the part is not allowed.

Some other examples to illustrate a wrong and a better configuration:

In the following case, a simple, small hole has been made to avoid excessive impact on the design. The technician, however, will need much more time to remove the internal raw material for accessibility reasons, so finally it will cost more. Moreover, the sharp internal edges of the cube allow a better anchorage of the powder in those areas and an iron wire will be needed to try, blindly, to scrap off the powder in those areas. Finally, if the technician wants to suck out the powder with a vacuum cleaner or blow it out with compressed air, both will be very hard to do because no other outlet will allow an air flux to circulate through the part to remove the raw material. That’s why the second option may be better than the first one. A third one could be providing a lot of smaller holes, all around the cube, through which the powder could leave the part without clogging.

Design modification with "easier cleaning" consideration

If the internal shape is not a “simple” volume, but a complex channel, several outlets would be needed, with adapted shape to make removal easier. Like always with AM, the objective of the designer is to forget the traditional design and start from the functions of the part to design. The goal is then to adapt the design to the AM requirements. The following image is an example of “function” approach design compared to traditional design:

Design process starting from part functions. Comparison between traditional and AM design approach

Traditionally, material is removed from a block of material. To make such a block lighter, it would be better to make it hollow. This makes the traditional design above very hard to clean after AM, even if it could be made. In AM the starting point is not a block, but 'nothing', therefore, the goal is to make it as simple as possible by trying to get rid of the cavity and make something functional and easy to clean, something like the design on the right above.

AM also allows designers to reduce the assembly step by integrating moving parts such as hinges, rotating axes or springs inside a single design and to assemble those elements directly out of the machine, in a single process. The secret lies in the gap (about 0.4 mm, depending on the technology chosen) between those moving parts, which should be small to have a smooth and accurate motion. But, of course, some of the raw material will fill this gap. If no outlets are made, it would be impossible to unclog the hinge. Here are some examples:

For a ball joint, to reduce the gap between outer shell and inner shell, the outer shell should be pierced to remove remaining raw material, as can be seen below.

Ball join with integrated powder evacuation channels in external component

When a long rotating axis is needed to pass through a block, a variable gap and accessibility of the axis centre is recommended, as shown below:

Axle design with powder evacuation approach

The worst to clean raw material out of are heat exchangers with large lattice structures. Pre-trials have to be conducted to test the cleaning feasibility on samples beforehand. Examples of complex but cleanable parts are shown below:

Complex parts which have been successfully cleaned inside

Finally, skilled technicians can come up with bright ideas to remove raw material after the AM process, but it will be costly and sometimes impossible to do. The best solution, which will reduce cost and time and improve the overall part quality, will be delivered by designers who are aware of this aspect. And each solution will be case-specific.

Sirris offers you support in design for AM. Together with you we check the technical and/or economical feasibility of your idea. Eager to find out if your products are suitable for (re)design for AM? Contact us or attend one of our future masterclasses!

This blog was written in the framework of the Cornet project AM 4 Industry.