This article is a DMLS 3D printing design guide which includes technical design specifications, materials, limitations and an introduction into the post-processing options available.
Direct Metal Laser Sintering (DMLS) is an Additive Manufacturing method that builds prototype and production metal parts using a laser to selectively fuse a fine metal powder. Traditional manufacturing techniques remove material from a piece of stock to create the desired geometry. Additive Manufacturing is capable of producing highly complex features and all-in-one assemblies that would be difficult to achieve with subtractive manufacturing techniques.
DMLS creates fully functional parts out of metals such as Cobalt Chrome, Aluminum, Stainless Steel, Tool Steel, Titanium, Inconel, and many others. The typical users of DMLS fall under these needs:
A key advantage of DMLS is the ability to produce parts that cannot be made using traditional manufacturing techniques. Manufacturing with DMLS can be advantageous if engineers design parts with complex geometries, such as integrated fastening features, long and narrow channels, custom contours, and metal mesh structures. DMLS allows for production of assemblies in single part form reducing number of parts, assembly time, and opportunity for failures.
In specialized applications, the weight of the part is an important criterion of the design. Using subtractive processes for manufacturing of metal mesh or weight reduced parts will dramatically increase the manufacturing time and cost due to the amount of material removed. DMLS is an optimal process for these parts as both manufacturing time and cost are reduced as volume decreases.
Speed is an important aspect of the design and manufacturing process. Both the quality of the product and the overall time to market are driven by the ability to produce physical models in a timely manner for fit and function tests, peer review, and market feedback.
Here, additive technologies allow for faster and more efficient concept review and prototyping. Thus, DMLS parts are commonly used during pre-launch activities for product testing, whereas the final product is made with a tool (i.e. die casting, metal injection molding, sand casting). DMLS parts are commonly used to validate designs as part of final product quality assurance as well as stand in for parts early in product life.
DMLS parts do not require tooling (e.g. molds, jigs, fixtures, gauges etc.), which reduces initial part manufacturing lead time from months to days. Thus, additive technologies such as DMLS present a tremendous value for product customization and change by offering ways to create short run, customized products without incurring expensive tooling changes.
General tolerances for DMLS parts are ±0.005 inch for the first inch and ±0.002 inch per inch thereafter (±0.2 percent), but +/-0.002 inch is achievable in some materials. If CNC tolerances are required, parts will require post-machining.
DMLS machines come in various build platform sizes, we offer the following build envelops:
DMLS, being a 3D printing process, is falsely associated with the simplicity implied from other 3D printing processes. Preparation of the design before being sent to the DMLS machine and the post processing afterwards can be time consuming. All modern manufacturing processes have before and after steps. CNC, for example, requires the programming of tool paths, machine setup, cutting and grinding, then polishing and de-burring afterwards. Prior to being sent to the DMLS machine, part support structures are designed and built. This step may take up to an hour and may determine success or failure the job.
DMLS post processing consists of:
For additional information on the post processing offered by F3DP, please visit this page.
DMLS parts need support structures for:
Unlike other laser and powder based additive technologies, DMLS parts move around in the build envelope if not properly secured to the build platform. Movement of the part occurs from the act of spreading a new layer of powder over the previously sintered layer or larger cross sections of the metal part warping during the sintering process. Movement of the part during the build will cause failures in part accuracy and could potentially lead to machine crashes.
A further reason support structures are required is to support overhanging geometry because the spreading would move unsupported overhands. Examples of these types of geometry are horizontal surfaces, large holes in the horizontal access, angled surfaces, arches, and overhangs.
During the build process, parts are subjected to forces from spreading and compacting of new layers. Tall, thin parts are susceptible to these lateral forces, causing inaccuracy in the parts’ features due to improper design or lack of support structures.
Load bearing part features require further guidelines for height to cross sectional ratios to ensure feature integrity. The figures at left describe feature height to wall thickness ratios of load bearing walls and pins.
Distance Between Features
During the DMLS process the laser creates a melt pool that is slightly wider than the laser diameter from heat dissipating into the surrounding powder. This will cause features that are close to each other to bond together or create a section of sintered powder that cannot be removed from between sintered areas in the part. Distance between features should be at least 0.4-0.5mm to adequately remove powder and allow for part movement.
Wall thickness – The minimum wall thickness to ensure a successful 3D print with most materials is 0.4mm. Finer structures are possible, but are dependent on material, orientation, and printer parameters.
Pin diameter – The minimum reliable pin diameter is 1mm. Smaller diameters are possible, but will have reduced contour sharpness
Hole size – Holes diameters between 0.5mm and 6mm can be printed reliably without supports. Support free building of hole diameters between 6mm and 10mm is orientation dependent. Horizontal holes with a diameter greater than 10mm require support structures.
Escape holes – Holes are required on hollowed metal parts to remove unmelted powder. A bore hole diameter of 2-5 mm is recommended. Using multiple escape holes will greatly improve the ease of powder removal.
Overhanging Surfaces – The minimum angle where support material is not required on an overhanging surface is 45º relative to the horizontal in most cases. It is possible to reduce this angle further by optimizing the laser parameters.
Unsupported Edges – The maximum length of a cantilever-style overhanging surface is 0.5 mm. An overhanging horizontal surface supported on both ends can be 1 mm long. These rules will apply to embossed and engraved features with unsupported surfaces as well.
Aspect Ratio – The maximum ratio between the vertical print height and the part section is 8:1 to ensure stability of the printed part on the build plate.
The quoted price of parts is heavily influenced by factors such as support structure design and removal of support. Therefore, minimizing the amount of support structure required will decrease design time, build time and post processing required.
The best way to accomplish this is to make the geometry as self-supporting as possible:
EXAMPLE #1: In this example the flanges towards the top of the part will cause a problem. The bottom facing surface of the flange will require some form of support. Adding a chamfer or a fillet to the overhanging geometry makes it self-supporting.
EXAMPLE #2: In this example the sloping angle of geometry is changed, making it self-supporting. Note that angles from 30°- 45° will self-support with some surface roughness and angles >45° will have a smoother surface finish.
EXAMPLE #3: Reduce mass and volume by using self-supporting features in the vertical axis.
EXAMPLE #4: The price of DMLS parts is heavily influenced by build time and the amount of material being used. The surface area to volume ratio of a part plays a large role in determining the quoted price of a given part. A part with reduced mass allows for a lower price because it takes less time to build, uses less material, and has a higher success rate of being produced correctly the first time.
The volume of a part is decreased, either by redesign or by using another manufacturing process to create the geometry, the overall part price will go down significantly. In this example the important features of a mold are built using DMLS and the surrounding material is milled to save on overall assembly cost.
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