Printing 3D Objects by Selective Sintering

Selective sintering is a technique by which fine particles of a material (typically a metal) are fused together in specific regions to create a solid mass by applying heat/pressure to those regions. This is done layer by layer, as in a typical 3D printing process, to create 3D objects.

The most common selective sintering technique uses lasers to heat the powder, and is called Selective Laser Sintering (SLS). Selective laser sintering is was one of the earliest 3D printing techniques. It was developed by Dr. Carl Deckard and Dr. Joe Beaman in the mid-1980s.

Selective laser sintering works on a variety of materials like metals, glass, ceramics, and various composite materials.

However, as we will see in this article, selective sintering can also be done without using lasers.

How does selective laser sintering work?

Selective laser sintering (SLS) is a popular technique in 3D printing. In this process, a thin layer of powder is spread in a rectangular region. A laser beam is then directed to specific regions of this layer to fuse the particles in those regions to form a solid mass.

Then a subsequent layer of powder is spread on this first layer, followed again by laser treatment. In this step, the particles don’t only fuse with themselves, but also with the solid mass of the lower layer. Repeating these steps layer by layer results in a 3D object.

Maintaining the right temperature can often be a challenge in SLS. One way to control temperature is to use a machine vision system to monitor and control temperatures in various areas of the current top layer. More details about this are available in this patent.

How a powder is made for metal-laser-sintering?

Metal powder used for laser sintering can consist mainly of iron and nickel powder with one or more of nickel-based alloy powder, copper or a copper alloy powder, and a graphite powder.

The mean particle diameter of the powder should be in the range 5 to 50 μm and its annealing temperature should be in the range 600-700°C.

This has been described in more detail in this patent.

How selective sintering can be done without a laser?

3D printing by selective sintering can be done without the use of lasers or other radiation sources.

This is possible by printing a mask on a powder layer, compacting the layer including the mask, and then sintering the compacted layer. The shape of the mask determines the structure of the 3D object. This process is repeated for each layer until the object is formed.

The compacting step of this process improves the efficiency of the manufacturing process and also improves the quality of the end product.

This has been described in more detail in this patent.

Thermal roller based selective sintering

A laserless device for selective sintering of a 3D printed object can use a thermal roller and an anti-peeling mechanism.

The powder applying system spreads a layer of powder on a tray. Then a mask is printed on the layer. The mask defines the negative portion of the layer to be sintered. A thermal roller sinters the layer by applying pressure and heat, promoting adhesion between layers. The anti-peeling mechanism positions a foil between the thermal roller and the layer.

The stage repeatedly adjusts the building tray to build multiple layers that together form the 3D object.

This has been described in more detail in this patent.

In 3D printing irrespective of the method used, getting a high-quality final product is the goal, which includes reducing distortion and getting high mechanical strength in the object.

How can distortion be reduced in sintering-based 3D printing?

Distortion can be reduced by forming a tray, supports, and a separation layer along with the target object that will be debound from the sintered object later.

A powder filler in a matrix is used as a material for forming the sintering tray and the support. The sintering supports extend up to the tray and prevent the target object from distortion during the sintering process, the distortion generally occurs due to gravity. During the sintering process, the sintering tray shrinks, and the supports allow the sintering tray to slide on the build plate via the separation layer to reduce distortion.

The following image shows this visually. More details are available in this patent.

How can sintering be used to make high-strength objects?

High strength objects can be made by sintering-based 3D-printing by using alternating shaping and binding layers.

The shaping layer forms the structure of the 3D object while the binding layer connects the two shaping layers. Inorganic particles are the core component in both layers, but the initiation temperature of the inorganic particles in the binding layer is lower than the shaping layer. The binding layer also contains thermoplastic resin.

This process has been described in more detail in this patent.

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