Selective Laser Sintering Production Guide
TABLE OF CONTENTS Introduction... 2 Typical Materials and Finishes for SLS... 3 Key Components of a SLS Machine... 4 The SLS Production Process... 5 Common Uses and Advantages of SLS... 6 Post-Processing Options... 7 1
Introduction What is Additive Manufacturing? Additive Manufacturing, also known as 3D Printing, is a manufacturing process that adds material, usually on a layer by layer basis, to make a 3D object from an interpretation of 3D data. Traditional manufacturing techniques, such as Subtractive Manufacturing, remove material from a piece of stock to create a desired geometry. Subtractive manufacturing usually requires some form of cutting tool such as machining (CNC, mills, lathes), electro discharge machining (EDM), grinders, and cutters. Additive Manufacturing is capable of making features which could not be produced with traditional subtractive method, such as meshes, freeform design, and complex cavities. What is Selective Laser Sintering? Selective Laser Sintering is a form of Additive Manufacturing under the ASTM category Powder Bed Fusion. Selective Laser Sintering, also called SLS, forms parts on a layer by layer basis by a controlled fusion of a polymer powder using a laser. The laser fuses the polymer powder to its neighbors both on the XY axis as well as at least one layer Z to create a 3D part. By iterating the layering and laser fusion (laser sintering) process, parts can be created with highly customized geometries and details. Figure 1. Powder being fused to become parts using SLS 3D Printing. 2
Typical Materials and Finishes for SLS SLS parts are generally printed in unfilled nylon. The most common material being a white nylon 12 or nylon 11. Parts in nylon are very durable and can flex before breaking. The durability, along with nylon s high chemical resistance, makes the SLS material one of the best 3D printing polymers for end-use parts. SLS materials can also have in-fills such as: Glass Fill (GF) By adding up to 50% fine glass bead to the material parts become much more rigid at the cost of making fine features more brittle. Glass Fill parts tend to look off-white to beige in appearance. Aluminum Fill (AF) Similar to Glass Fill materials, AF materials are stiffer at the cost of making fine features more brittle. AF parts have a metallic-grain gray color. AF parts are not conductive as the metallic pieces are suspended between nylon and do not connect. Carbon Fill (CF) CF parts have small carbon fibers which add strength and reduces weight of the parts relative to unfilled materials. CF parts tend to be very rigid and favor continuous contoured design features making the material great for automotive ducting and UAV housings. CF parts are generally dark grey in color. Because of the recoating/layering process, CF parts are stronger on the XY direction of their build orientation versus the Z direction. CF parts may also be slightly conductive in the XY direction on a layer. Mineral Fill (MF) Similar to GF and AF materials, fine minerals can be used as an additive to stiffen parts. MF parts tend to have a natural beige appearance with slight specks. Other raw materials are also available but may be less common or undergoing R&D such as PEEK, PEKK, nylon 6/6, polyethylene, polypropylene, and polystyrene. Although colored SLS raw material is not commercially available, black powder in both nylon 11 and 12 has become a more common offering. SLS parts have a slightly grainy matte finish straight from the machine but retain high dimensional accuracy and detail. Like many additive manufacturing (3D Printing) applications, build orientation may affect details on surface due to grow lines. Grow lines, also known as stair stepping, are more prominent on gradually sloped surfaces less than ~20 degrees parallel to the build base. Overall the resolution of SLS parts is higher than FDM but slightly less than SLA or PolyJet processes. 3
Key Components of a SLS Machine SLS machines are built to reliably create accurate parts over years of service. Top-end productions machines are highly modular and are built to be both mechanically and thermally stable during the build process. Key components of the machines are: Material Feeds Whether it is a gravity-fed or piston-driven system, the material feed, or powder supply bin, provides new material to be deposited for sintering. Depending on the SLS machine, material feeds may have their own thermal controls to warm up powder before it enters the build chamber. Recoater The recoater may be a roller or a blade which takes material from the material feeds and spreads it evenly across the part piston. The recoater is a critical item during SLS build because any irregularity it may have on depositing powder (e.g., a fine line being drawn across the powder due to dust or monomer buildup) will show up in the final parts. Part Piston The part piston is the effective build area for the SLS machine. It consists of a rectangular container with a flat, movable piston base. When a build starts, the piston is at its uppermost level. As each layer is deposited the piston moves downwards slightly (usually in 0.1-0.12 mm increments) so the recoater can deposit new material for sintering. The part piston also is heated during the build process to mitigate any part distortion due to cooling and slowly reduces temperature once the build is complete. A part piston can be part of the machine or an exchangeable frame. Exchangeable frames, such as those at Xometry, have an advantage for production since they can be removed to cool and a new frame put in place to run the next build. Figure 2. Inside a SLS machine. Note that everything seals airtight to create the perfect partmaking environment. Laser (& Galvos) Positioned above the part piston is a laser window where the laser is directed via galvos and mirrors to hit new powder in the respective layer s pattern. A high wattage is not needed for sintering but reduces the sinter time per layer. Higher wattage SLS machines (70-100W) can often sinter 2-3+ times faster than lower wattage machines (15-25W). The Build Chamber The build chamber, a.k.a. the process chamber, houses the laser window, part piston, and recoater. The build chamber houses several thermal sensors and heaters to ensure the powder remains at a very steady temperature. Overflow Bins Any material left after recoating gets collected in the overflow bins. Overflow material, along with unsintered material in the build chamber, can be refreshed with new powder and reused. Peripheral Components SLS machines also require an inert environment so nitrogen is used to remove oxygen from the entire machine. Nitrogen tanks or generators go hand-in-hand with SLS machines. Also of great importance is the post-processing equipment which includes a vibratory sifting station to reclaim powder from the build while separating parts and a bead blasting cabinet to fully depowder sintered parts. 4
The SLS Production Process The SLS machine starts with full feeders and an empty part piston and overflow. During a warm-up phase the machine environment is inerted with nitrogen and 4-6mm of powder is deposited on the part piston while being heated to temperature. The figure below describes the SLS process once the machine is warmed up and ready to make parts. Once the build is complete, parts are gradually cooled to prevent warping. Material in the part piston area is called cake, and is a block of parts and powder. To separate cake material from parts an unpacking station is used where powder is pushed aside and sieved while parts are collected. Used material can be refreshed with new powder to be reused and parts get blasted with fine mesh glass beads to remove any excess powder bulk. Part Piston Moves down One Layer Recoater adds New Powder Layer Laser Etches Layer Pattern Machine Heats Layer Temperature Figure 3. The SLS build cycle. A very controlled and highly repeatable process. Xometry provides high quality SLS parts for its customers using top-of-the-line EOS SLS machinery. Material used is handled in a controlled environment and refreshed using precise measurement to create a high level of consistency and repeatability between orders. Parts are positioned in each build for optimum print quality and run at 0.12mm layers in the Z direction. Xometry offers standard unfilled nylon 12 as the go-to material but is able to print in many materials or different parameters given special projects or production runs. 5
Common Uses and Advantages of SLS Typical Uses Rapid Prototypes SLS is a cost-effective way to make fully-functional prototypes, complete with moving parts, as well as all-in-one assemblies. It is commonly used for concept demonstration, models and first article production pieces for end-use or design verification. Because of the speed and versatility of the system, product developers often make several iterations of a design as it evolves to create physical snapshots of their design review process. Iterative Design SLS produces prototypes quickly and affordably; thereby, reducing the amount of resources required for the prototyping phase of the design process. This leaves more resources available to improve the quality and functionality of the design through testing and refinement. Fit Checks Xometry sees its fair share of designers printing models of their near final pieces to ensure everything mechanically fits before moving to higher level production tooling. SLS has an accuracy of +/- 0.005 and can mimic several plastics depending on the nylon and fill used. Tooling and Fixtures Tooling and fixtures are not necessary for SLS production because objects are built in a powder bed, dramatically reducing the cost of part production. Spare and Discontinued Part Production Aftermarket and spare parts are often better suited for 3D printing than traditional manufacturing, because it offers shorter lead-times and cost-effective low volume builds without fixture and tooling costs. Artistic Creations SLS offers an economical and detailed venue to show off sculpture and jewelry designs as well as expand the possibilities of freeform creation. The low cost of printing individual parts and the medium s limitless complexity make it a great artistic tool. End-Use Applications Appearance details, build tolerances, and material properties are some of the more stringent requirements placed on parts designed for end-use versus prototypes. SLS printing allows for the accurate production of robust end-use parts. Objects printed using Durable White Nylon, for example, can be produced in high detail with +/- 0.005 accuracy typical in all directions. Additionally, parts printed in nylon have a high impact strength, medium flexibility, and high resistance to harsh environments such as high and low temperatures, water-tight applications, exposure to harsh chemicals, and long-term outdoor use all of which make SLS a good fit for a number of end-use applications. No Support Structures Required Infinite Part Complexity Nesting and Build Packing Speed and Throughput Sparser Material Selection Rough Surface Finish Post Processing Requirement Lower Feature Resolution 6
Post-Processing Options Dyeing Xometry s SLS machine prints white nylon parts. Dye is a great way to add color to any SLS project. The process is similar to dying fabrics. For this process our team agitates the parts in hot dye for around 30 minutes until their outer layers have a uniformly colored surface. Note that only the outer layers of the part will absorb the dye, so any deep scratches, scrapes, chips, threads, or holes that penetrate the outer membrane of the part after the dyeing process will reveal the original white color in the layers below. Media Tumbling Nylon parts fresh out of the printer typically require cleaning to remove any un-sintered powder from the surface. This is part of Xometry s standard cleaning process. After cleaning, SLS nylon parts will continue to exhibit a slightly bumpy surface because of the nature of the nylon material that has been fused together on a powder bed. Media tumbling is a great way to generate a smoother surface finish for nylon parts. Note that this process will remove peaks, but will not create a completely smooth, glossy part. Additionally, fragile and small features may be removed and squares might be rounded out. Sanding Sanding is a further way to generate a smoother surface finish on SLS nylon parts. Sanding is a good fit for parts with larger flat areas, but it can be difficult to reach tight contours and to sand fragile and small features. Sanding SLS parts requires more effort and does not yield the same smoothness as PolyJet or SLA. Plating Electroplating is a great way to build parts that are rigid, lightweight, and have a metallic look. Building SLS parts and plating is still often significantly quicker and more economical than milling parts from aluminum. The plating finish is a bright nickel metal and can vary in thickness from 0.002" to 0.006" depending on its purpose. Common functions for plated parts is for EMI shielding or to create a durable, functional part akin to 6061 Aluminum. For tight tolerances, the 3D part file may need be updated to offset the additional material being added to the surface. 7
Design Tips Please see Xometry s Preparing a File for 3D Printing guide for tips on optimizing designs to ensure that that best results are achieved for any 3D printed part. Covered topics include: Preferred file settings and formats, Overlapping geometry, Stl file resolution, Minimum thickness, Clearance between moving parts, Designs with confined hollows, Fillets, and Light-weighting. Xometry offers you access to the widest range of production grade plastic and metal materials for the most demanding applications. We use the latest subtractive and additive manufacturing systems, in-house, for fast, precise production of your custom designed parts. Visit www.xometry.com to learn more. Need help selecting the right technology for your next custom manufacturing project? Call us at 240-252-1138, or email quote@xometry.com to get started. Xometry 7951 Cessna Avenue Gaithersburg, MD 20879 USA 240-252-1138 8