Optimization of 3D models for 3D printing using SLS technology
In the realm of 3D printing, Selective Laser Sintering (SLS) technology has emerged as a powerful tool for transforming digital designs into tangible objects. However, to fully exploit the capabilities of this additive manufacturing technique, it is crucial to optimize 3D models specifically for SLS printing. By meticulously tailoring designs to leverage the unique characteristics and limitations of SLS, manufacturers can enhance print quality, minimize production time, reduce material waste, and unlock a new level of precision in 3D-printed objects. This article delves into the key aspects of optimizing 3D models for SLS technology, exploring the critical considerations and techniques that can revolutionize the world of 3D printing.
Optimization of technological processes is crucial in any manufacturing field, and the mass 3D printing industry is no exception. When assessing 3D-printed parts on a three-sided scale – Quality, Time to Market, and Cost per Cubic Centimeter – SLS printing often emerges as a more favorable option compared to traditional manufacturing techniques, such as die-casting. SLS printing enables the creation of extremely complex and durable functional parts with a high-quality surface finish that does not require assembly (the 3D printer can produce them as a whole). This gives engineers the ability to design and create objects with the most intricate geometries while saving time and money. The best results are achieved by utilizing numerous technological tricks and techniques developed by the industry to optimize the 3D printing process.
The key factors to consider when optimizing models are friction, sufficient strength, and the ability to remove excess powder that remains inside the printed part. The simplest parameter to deal with is the removal of residual powder. To do this, one or two openings are often left in the non-loaded areas of the part, through which the polymer can be blown out with compressed air. It should be remembered that nylon powder is a good mediator material, so it can be left in the cavity of the product in places where there will be (minor) friction with other parts. To counteract more severe friction that generates a noticeable amount of heat, bearings will need to be added to the design.
Other engineering techniques allow optimizing the parameters of the finished parts.
Droplets – If you need to introduce a connection point for components inside a channel (for example, in an air intake) – design an aerodynamic node in the form of ‘droplets’ that will not obstruct the airflow, but will provide sufficient strength for attachment.
Polymer bearings – As already mentioned, nylon is a natural frictional mediator and therefore is well-suited for printing internal bearings for parts that will not be subjected to prolonged loads and friction. In case of prolonged loads that generate a lot of heat, it is better to provide a loading port for ceramic bearings in the design. After loading such a port, it can be sealed.
Sylphons – SLS printing is well suited for creating flexible bellows-type joints. However, technological nuances of materials should be taken into account. For example, nylon is not very good at withstanding multiple bending-unbending cycles, especially with high amplitude. For printing bellows intended for such loads, it is better to use powders based on polyethylene. Another nuance is that round bellows work best when they can evenly distribute the points of application of stretching force over the entire diameter of the cross-section. Any deviation from a round shape will lead to uneven stress accumulation. In this case, it is better to design the structure according to the principle of “Dir-dorff’s bellows”, which consists of a series of alternating rectangles. However, keep in mind that a joint with this profile is more sensitive to stress concentration due to the small radius of the corners.
Blind channels – Blind channels pose a problem for removing residual powder. The solution lies on the surface – make them no longer blind. Add a small hole with a diameter of more than 2 mm at the base, through which excess polymer can be blown out.
Buttons – There are many approaches to designing integrated buttons. However, there are several general nuances that should be taken into account regardless of the chosen design. Firstly, leave a gap of at least 0.3 mm between the button and the hole in which it will move to avoid accidental fusion. Secondly, it is better to plan for the final position of the button to be higher than initially intended during CAD design. This is because the nylon “springs” of integrated buttons tend to deform quickly (although this is entirely acceptable).
Cells – SLS is great for printing a multitude of small and complex parts. To prevent losing such parts during unpacking and post-processing, design a thin protective cage around them with bars around 1mm thick and clearances of around 5mm.
Multi-link chains – The complex geometry of chains and their interweaving offers a great opportunity for engineers and designers to showcase their creativity. Professional advice: when designing a chain (or interweaving, such as a 3D-printed chainmail), set the thickness of the base link >0.75 mm, leave gaps between links of more than 0.5 mm, and make the links polygonal in both longitudinal and transverse sections. This will drastically reduce the size of the STL file and speed up the design process. As for the chainmail fabric itself, fold it in several layers in CAD to reduce the volume it will occupy in the growing chamber.
Springs – The most important aspect of designing 3D-printed springs is reinforcing the points where they attach to other parts. Add as much bonding plastic to these points as your part’s design allows. As with springs (point “Buttons”), the final shape that the finished spring will take will only become clear after several cycles of stretching and compression.
Reinforcement frames and stiffening ribs – SLS printing allows for the creation of highly optimized products with incredible geometry and spatial complexity. To support and strengthen such constructions, reinforcing frames integrated into the structure of the parts should be generously used. In traditional manufacturing methods, creating such frames is a costly operation, but SLS eliminates any additional costs associated with geometric complexity.
Sealing of connections – To ensure a reliable seal of the connection, provide a radial channel and two ports for input and output in the model, as shown in the diagram. After assembling the parts, pour a two-component epoxy resin into the channel and draw it through using a vacuum pump. IMPORTANT! The pump should only operate on suction and never on blowing. Otherwise, the resin is likely to find the path of least resistance and not fill the channel completely. A connection sealed in this way cannot be disassembled without damaging the parts.
Integrated hinges – SLS printing and polymer powder are perfect for creating axial ball joints, consisting of spherical elements that move in internal grooves. This connection is very precise and develops low friction with very high stability. The only caveat is to provide a clearance of 0.2 mm between the spherical part of the joint and the walls of the groove to avoid accidental fusion. It is also recommended to extend the groove channel and bring it out to the end of the part to remove excess powder.
Flexible hinges – SLS technology is not optimal for creating flexible hinge joints. The reason is that such hinges are originally designed and optimized for injection molding. However, the behavior of the nylon used in SLS differs from that of molded thermoplastics – in particular, it reacts worse to cyclic deformation loads. However, SLS is suitable for printing hinges that are planned to be bent only once and then kept in a closed state. In this case, it is recommended to immerse the entire assembly in boiling water for 10 minutes to anneal the nylon before deformation.
Lattices – Lattice structures are highly attractive from an engineering perspective as they provide a high strength-to-weight ratio while reducing mass. They also absorb energy well and can be used to create effective thermal or acoustic insulation. However, designing lattice structures is a highly non-trivial and labor-intensive process, so we recommend writing a macro or using third-party software specifically designed for this purpose.
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