3D Printed Parts

An interesting thread about deflashing and surface finishing 3D printed parts has been started on the ShopTalk Forum. As this 3D manufacturing process becomes more popular and mainstream, surface finishing using both barrel tumbling and vibratory tumbling will continue to be an issue. Please provide any comments and experience you have either on The Finished Part Blog here or on the Forum.

Stainless Steel Finishes

There are four major types of surface finishes that are designated for stainless steel. While many of these finishes are somewhat subjective, below is a guideline for the most common finishes:

#3 Coarse Finish -- Usually a preliminary, coarse surface finish. Also referred to as rough grinding. Used for removing gates from cast parts, heavy burrs, weld splatter. Often accomplished with a disc or belt grinder but parts can be tumbled part-on-part with an abrasive grit to achieve a #3 finish.

#4 Brushed Finish -- Defined as a directional or uniform surface finish. Removes surface defects in the surface but accepts fine polishing lines. A #4 Sanitary Finish is a slightly smoother, polished finish commonly designated in the food, dairy and medical industries. Tumble polishing can be used to accomplish a close approximation to a #4 Finish.

#6 Fine Finish -- A polished, softer version of a #4 Finish. Often less reflective than a #4 Finish. No polishing lines or surface defects visible.

#8 Polished Finish -- All surface defect are removed. Mirror-like finish depending on the quality of the metal. Highly buffed surface.

Glass Bead vs Aluminum Oxide for Deburring Steel

Machining and work-hardening steel creates burrs and scale that needs to be removed. The part in the middle of the picture above shows the 'excess' material in threads as well as the darkening caused by scale formation in the heat treating process. While barrel or vibratory tumbling can be used to remove these burrs and create a smooth surface finish, threaded areas often need to maintain the square edges and dimensionality on the part.

The other two parts were blasted with different types of abrasive blast media to show how each media can accomplish the deburring task but create a totally different surface finish. The part on the left was blasted with a Medium Glass Bead (70-100 mesh) and resulted in a satin-like surface finish with clean edges. The part on the right was blasted with 120 Mesh Aluminum Oxide Grit and has a more matte, gray surface finish.

While Glass Bead can often result in a more aesthetically pleasing finish, the micro-profile achieved with Aluminum Oxide Grit is often required for future processing of the parts (i.e., coatings application).

Plastic vs Ceramic Media

Selecting the proper media can be as much an art as a science. A clear understanding of all the finishing requirements is necessary. Considering the needed edge rounding, surface finishing, cycle time, metal type and many other factors are critical.

Notice the very heavy flashing on this aluminum part (far left in picture) left over from the machining process. This 'raw' part also had some machining marks on the surface that needed to be removed. The easy solution to finishing this part is to throw it in a vibratory tumbler with some Ceramic Media (see part on right) to remove the flashing and surface marks. Alternatively, Plastic Media will take a little longer to deflash the part but will result in a smoother surface finish and less edge rounding (see middle part).

For this project, the objective was to get a smooth surface finish with slightly rounded edges in order prepare the part for a painting process. Ceramic Media was the best selection in this case due to the shorter cycle time, increased edge rounding and 'rougher' surface finish.

Ceramic vs Plastic Tumbling Media

The selection of tumbling media for a particular application depends not only on the type of material/metal being tumbled but also on the finish required for the next step(s) in the processing of the part. While less aggressive medias such as Plastic Media or Synthetic Media are typically used with softer metals like aluminum, the results above show a comparison of a Plastic Media versus a more aggressive Ceramic Media.

The picture shows the machined part on the far left followed by a part tumbled with a general purpose Plastic Triangle (middle) with a different part tumbled with a Ceramic Triangle on the far right. Both tumbled parts were vibratory tumbled for 1 hour with the same Kramco 1030 mild acid compound.

The machined part shows heavy burrs and machining marks on the surface. Both media types removed the machine marks and the burrs on the edges. The differences show up when looking more closely at the 'roughness' of the surface finish and the amount of edge rounding after the 1 hour cycle time.

The Plastic Media tumbled part shows slight edge rounding and a matte but uniform surface finish. This is a good finish for anodizing the aluminum part to a matte, smooth finish. The Ceramic Media tumbled part has noticeably more edge rounding and a more tumbled, 'rough-looking' surface finish. This finish is better for accepting paint or powder coating.

Increasing Tumbling Abrasiveness

There are a number of strategies that can be used to increase the abrasiveness of both barrel and vibratory tumbling operations. These can be used when a boost to more aggressive surface finishing. deburring or edge rounding is needed.

• Increase the rpm (barrel tumbler) or the amplitude (vibratory tumbler).
• Use a denser/heavier media (i.e., switch from Synthetic Media to Ceramic Media).
• Switch to a more aggressive bond/formulation of tumbling media.
• Reduce the amount of water and/or compound in the process.
• Add an abrasive such as Aluminum Oxide Grit or similar (best with a barrel tumbler).
• Adjust the load level for a more aggressive tumbling action.

Identifying the critical issues with your part will determine which strategy(ies) are best to employ. Of course, all these strategies work just as well in reverse to decrease the abrasiveness.

Etching Stainless Steel

Stainless steel can be polished to mirror surface finishes. Sometimes, the surface needs to be etched in order accept a coating or prepare the part for a process that requires a little 'grab'. While tumbling in a vibratory system or a barrel tumbler will create a matte finish, the surface is generally not rough enough for many coatings. Also, when the parts are small, it can be difficult to separate the media from the parts in a tumbler.

Stainless Steel Etching

When an etch is required and the parts are small, a tumble blasting process is often an ideal solution. The stainless steel parts above (before and after) are less than 1/2" in any dimension. These parts were tumble blasted with 60 mesh Aluminum Oxide Grit for only 20 minutes. Depending on the size of the blasting basket, batch sizes as large as 3 cubic feet can be processed at one time.

Deburring Aluminum Slots

Selection of media shape and size is critical when trying to deburr slots and holes. The material type (i.e., aluminum, steel, plastic) is also important in identifying the proper type of media. The part pictured above (machined on left; tumbled on right) required complete deburring of the part, decent edge rounding and a uniform surface finish. The customer's next step in the process was to create a dull, anodized finish.

The part contained a variety of geometric features including approximately 1/4" wide x 1/4" deep slots, both large a small holes, square interior corners and radiused edges. The finished part was vibratory tumbled with general purpose (KX bond) 1/4" x 1/4" Plastic Pyramids. This shape and size of media was selected in order to get into the corners of the slots and result in a uniform surface finish across the part including interior corners.

The general purpose media resulted in a dull surface finish ideal for creating a matte, anodized final surface. If a brighter anodized finish was specified, a secondary polishing step in a vibratory tumbler would be improve the results.

Bead Blasting Aluminum

Before

Machined aluminum parts end up with lines, swirls and etching on the surface of the aluminum. To prevent these surface imperfections from translating through a coating or anodizing, the surface needs to be made uniform. If a polished surface is ultimately required, abrasive blasting the surface with Glass Bead will produce a smooth finish.

After

The machine aluminum part (Before picture) was blasted with Fine (#13) Glass Bead to remove the machine lines and produce a highly uniform, matte surface finish (After picture). This parts was ultimately clear coated to produce a clean, bright finish. Different size of Glass Bead will each produce a unique surface finish on aluminum. As the Bead size increases, the surface finish will become more a peened finish and also brighter.

Removing Machine Marks

Machining aluminum parts to create the specified dimensions and geometry of the part leaves light etching, 'swirls' and general machine marks on the surface. Part of the finishing process to achieve the desired finish requires the removal of these marks. While hand grinding and polishing will work, a mass finishing process is certainly a better process

Swirling machine marks

The choice of media for a mass finishing process will depend on the required surface finish to prepare the part for the final specified finish. If deburring and heavy machine marks need to be removed, an aggressive media (i.e., Ceramic Media) may be required. If a less aggressive media is needed, a Synthetic Media can be used. Either of these cutting steps can be followed with a polishing media to smooth out the surface finish and remove the 'tumbled' look.

'Tumbled' surface finish

The surface finish above was achieved by vibratory tumbling the part with a general purpose Plastic Media (medium aggressiveness) to completely remove the machine marks on the part. The ultimate goal for this part was a matte, anodized finish so no further processing was required. The appropriate compound (Kramco 1060) was used to keep the part and process clean without darkening the aluminum metal.

Surface Finishing Specifications

The Steel Structures Painting Council (SSPC) has designated fifteen different surface preparation standards including simple solvent cleaning (SP-1) to Commercial Blast Cleaning (SP-6) to Industrial Blast Cleaning (SP-14). Some of these standards are simply visual while others designate specific profiles. These standards are applicable to a large variety surfaces. If a specification has not been designated for a surface to be cleaned or blasted, using an SP standard will provide consistent results from part to part and job to job.