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SLS (Selective Laser Sintering) is one of the additive manufacturing technologies. It employs a laser to sinter plastic powder into a solid object according to the 3D model. For decades, this technology has been the engineers’ favorite choice in product development. Higher productivity and lower cost compared to other manufacturing processes make it perfect for a variety of projects, from creating fast mock-up to small batch production of finished products, like car parts.
Recently, there have been great improvements in machines, printable materials, and applications of SLS printing, which makes it available to a broader range of companies. Previously, its use was limited to some high-tech companies.
In this article, we will specify the process of laser sintering, currently available various printable materials, and when to consider using SLS printing services.
The History of SLS Printing Services
SLS printing is one of the earliest additive manufacturing technologies (>>>> The History of 3D Printing: How Much Do You Know). It was developed in the mid-1980s. Ever since then, SLS printing has been applied to work with a variety of materials, such as glass, ceramics, plastics, and various composite materials. Today, SLS printing together with SLM/DMLS printing is collectively known as powder bed fusion. SLS is plastic-based and SLM/DMLS is metal-based. Even though there have been over three decades since developed, both SLS and SLM/DMLS printing processes are still expensive and complex, which limits their application to a small number of high-value or tailor-made parts, for example, critical aerospace parts or medical equipment.
As innovation in this field continues, SLS printing services will become accessible to more industries.
How is Fast Mock-up Printed Using SLS
1. The powder is dispersed in a thin layer on the build platform.
2. The SLS printer warms up the powder just below its melting point. This preheating process can make it easier and more efficient for the laser to increase the temperature of specific areas of the power layer.
3. The laser scans a cross-section of the 3D model. Then it heats the power to right at or slightly below the melting point. In this process, particles are mechanically fused together and form a solid part. For powder that is not fused functions as supports, thereby eliminating the needs for other specialized support structures.
4. The platform lowers by a layer into the build chamber, usually within 50-200 microns. A new layer of powder is then dispersed on top of the previous layer and the next cross-section of the 3D model is laser scanned.
5. The dispersing, scanning and heating processes are repeated on each layer until the complete 3D model is printed. And then leave the fully printed fast mock-up to cool down gradually in the printer.
6. Once mock-ups have completely cooled, the build chamber then is removed from the printer and goes to a cleaning station where the printed mock-ups are separated and the excess powder is removed.
Characteristics of SLS Printed Fast Mock-up
In SLS printing, there is really excellent bond strength between layers, which means that isotropic mechanical properties can be found in every SLS printed mock-up. And compared to those manufactured using other technologies, SLS printed parts are more brittle because of their porosity structure, meaning less durable. But they have great tensile strength and modulus.
The porosity of common SLS printed parts is about 30%. Porosity gives SLS parts a granular finish. This means that SLS printed parts have good water absorption which makes them easy to be dyed into a variety of colors in hot water. Therefore, if your fast mock-up is to be used in a humid environment, special post-processing, such as polishing, dyeing, lacquering, or spray painting, may be required to prevent discoloration. Applying a watertight coating or a metal plating can also help.
Shrinkage & Warping
SLS printed part shrinks and warps easily. As the new sintered layer cools, its size decreases and internal stress increases, pulling the bottom layer upwards.
A 3 to 3.5% of shrinkage is common. But the manufacturer should take this into consideration and adjust the dimensions of 3D design accordingly.
If the fast mock-up to be printed has large flat surfaces, the printing result is prone to warp. The solution is to minimize the thickness of flat surfaces or introduce cutouts to the design, which can reduce material use as well as overall cost.
When radiant heat fuses unsintered power together is called oversintering. This can cause small features of printed parts such as slots and holes to be lost, which depends on the wall thickness and the size of these features. For example, a hole with 1mm diameter or a slot with 0.5mm wide can still be seen on a 2mm thick wall but will be undiscovered on a wall with thickness over 4mm.
It is easy to print a fast mock-up with a hollow structure using SLS technology as no support structure is required in the process. SLA is an excellent solution to create lighter-in-weight parts. Escape holes are required to remove unsintered powder from the interior of the part.
To ensure high stiffness, usually, the mock-up is printed completely solid. But there is an alternative - make a mock-up of a hollow design and remove the escape hole. And a cellular lattice structure can be added to the hollow interior to further enhance the stiffness of the parts. Hollowing a part may also effectively prevent warping.
Rules of Thumb about SLS printing Services
1. SLS printing services are adopted in more and more industries to produce functional mock-ups from a variety of engineering plastics. The most commonly used material is nylon.
2. The common build volume of an SLS printer is 300 x 300 x 300 mm, excellent for producing a small or medium number of prototypes or parts.
3. SLS printed parts show good mechanical and isotropic properties.
4. SLS process requires no support. So it is easier to produce geometrically complex fast mock-up.
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