Friday, April 9, 2010

Surface Finish & Finishing of DMLS - (Direct Metal Laser Sintering) Parts

Parts “as built” off DMLS machines have a “raw” finish comparable to a fine investment cast, with a surface roughness of approximately 350 R a- μ inch or R a-μm 8.75, or a medium turned surface. This surface roughness can be improved all the way up to 1 R a- μ inch or R a-μm 0.025, qualifying as a super mirror finish. There are several processes available that can be used to achieve the desired surface roughness or finish. These processes include, but not limited to:

Abrasive Blast (Grit & Ceramic)
Abrasive blasting is the operation of forcibly propelling a stream of abrasive material (media) against a surface under high pressure to smooth a rough surface. Abrasive blasting services are included standard for all DMLS projects. If a “raw” DMLS part is desired, this should be noted at the time of the RFQ when addressing the desired surface roughness. Abrasive blasting with grit and ceramic media provides a satin, matte finish of approximately 150 R a- μ inch or R a-μm 24. This finish is largely uniform, but does not provide a 100% uniform finish.

Shot Peen
Shot peening is a process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails the use of media to impact a surface with sufficient force to create plastic deformation. It is similar to blasting, except that it operates by the mechanism of plasticity rather than abrasion. Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Depending on the part geometry, part material, shot material, shot quality, shot intensity, and shot coverage, shot peening can increase fatigue life from 0–1000%. Shot peening is used primarily for foundries for deburring or descaling surfaces in preparation for additional post-processing.

Electrochemical Polishing
Electrochemical polishing also referred to as electro polishing, is an electrochemical process that removes material from metal parts through polishing, passivation, and deburring. It is often described as the reverse of electroplating; differing from anodizing in that the purpose of anodizing is to grow a thick, protective oxide layer on the surface of a material rather than polish. The process may be used in lieu of abrasive fine polishing in micro structural preparation, and is an inexpensive option for DMLS projects that are not tolerance dependent, creating a bright uniform finish. The extent to which electro polishing is successful depends upon the degree of preparation of the treated surfaces.

Abrasive Flow Machining (Extrude Hone) Polishing
Abrasive flow machining (AFM), also known as extrude honing is a method of smoothing and polishing internal surfaces and producing controlled radii. A one-way or two-way flow of an abrasive media is extruded through a workpiece, smoothing and finishing rough surfaces. One-way systems flow the media through the workpiece, then it exits from the part. In two-way flow, two vertically opposed cylinders flow the abrasive media back and forth. The process is particularly useful for difficult to reach internal passages, bends, cavities, and edges. This is an inexpensive option for DMLS projects that are not tolerance dependent, and a more uniform surface roughness. The extent to which AFM is successful depends upon the degree of preparation of the treated surfaces.

Electroplating is a process that uses electrical current to reduce ions of a desired material from a solution and coat a conductive object with a thin layer of the metal material. Electroplating is primarily used for depositing a layer of metal to bestow a desired property (e.g., abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.). Another application uses electroplating to build up thickness on undersized parts. Plating is also an inexpensive method of improving surface roughness, with the reduction in roughness once again hinging upon the degree to which surface are treated prior to plating. DMLS parts can also be plated in their raw state, and then finished in combination with another method.

Optical Polish (Hand Finishing)
When projects have geometries in low quantities that are not tolerance dependent, the best finishing option is an optical polish. Optical polishes are extremely cost effective, and the best way to achieve a brilliant finish. Due to surface porosity of DMLS metals, .003” to .010” of surface material is removed depending upon geometry. If this option is desired, it is imperative that designers or engineers consult with GPI prior to building, as specific surfaces may need to be offset with additional material to ensure part integrity after post-processing. Optical polishing is not ideal for large batches as it lends itself to an inconsistent finish from part to part.

Micro Machining Process (MMP)
Micro Machining Process (MMP) is a mechanical-physical-chemical surface treatment applied to items placed inside a treatment tank, providing highly accurate selective surface finishes. The desired surface finish is obtained by using MMP only on those areas where that particular finish is required. MMP begins with a detailed analysis of the surface state of the item to be treated, establishing the processing parameters required to meet the customer’s objectives. MMP can finely distinguish and selectively apply different primary roughness, secondary roughness and waviness profiles to surfaces. MMP is a batch process that is quite expensive, with costs ranging from $500 to $1000 for sample finish testing. After acceptable samples have been provided, costs for batch runs start at approximately $3000. This process has selective application, and is ideal for projects requiring precision tolerance finishing to a large number of parts, as well as parts with internal passages that cannot be reached by an alternate method.

CNC Finishing/Machining
CNC finishing permits high quality contoured milling applications to achieve tight tolerances. Detail-oriented precision can be accomplished with 3-axis, 5-axis and 6-axis CNC lathes. Conventional fixed headstock and Swiss-style CNC lathes can be utilized to support complex operations such as cross drilling and cross tapping, cross milling and slotting, C-axis milling and off-center work. Proper fixturing can yield tolerances as tight as 1 micron or (.00004). Should this post processing option be desired, pre-build planning is required to add sufficient material to machined features and surfaces so that tolerances can be met.

Cobalt Chrome MP1, media tumbled.

Cobalt Chrome MP1, optical polish #2

Cobalt Chrome MP1, optical polish (mirror finish) #3.

The shell behind the finished part is a "raw" part. You can see the contrast with this finish. This finish is the "optical finish". Stainless Steel. PH1.

This is an insert which had supports removed and abrasive blasted. MS1.

Raw with supports. Stainless Steel.

Stainless Steel PH1, shot peened finish.

Stainless Steel PH1, raw with supports removed.

Tim Ruffner
Account Executive
GPI Prototype & Manufacturing Services, Inc.
940 North Shore Drive
Lake Bluff, IL 60044
Phone: 847.615.8900
Fax: 847.615.8920
GPI on Twitter
Tim Ruffner LinkedIn
*DMLS – Direct Metal Laser Sintering Blog

Specializing in:
* DMLS – Direct Metal Laser Sintering
* 3D Printing - Objet
* RTV Casting and Urethane Molds
* Laser 3D Scanning
* Short Run 1-1000 Prototypes
* Manufacturing