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.

Machined Magnesium Parts

Similar to aluminum, lighter weight magnesium metal parts are often machined to create the required shapes and features (see picture above). The machining process leaves burrs, sharp edges and (most noticeably) machining lines and swirls. These imperfections in the surface need to be removed prior to further processing of the part.

As similar to aluminum and other reactive metals, magnesium is subject to oxidation. When vibratory tumbling the part, using the proper chemistry is important. This part was deburred and made smooth using a Ceramic Angle Cut Cylinder tumbling media and a solution of Kramco 1030 Mild Acid Compound (see 2nd picture above).

Ceramic media was used to keep the cycle time to a minimum with maximum surface finishing. The Angle Cut Cylinders removed the machining marks all the way into the interior corners of the machined areas as well as provided an even deburr and edge rounding across the entire part. The slightly acidic pH of the Kramco 1030 prevented the part from oxidizing during the tumbling process and resulted in a clean, white-metal finish.

Polishing Medical Parts

Once cut, machined and heat treated, stainless steel parts to be used for medical applications (see tweezer parts above) need to be deburred and polished to a high shine. The part in the top of the picture above is ready for surface finishing and some light edge rounding.

To remove the heat treat discoloration, the part was first blasted with a Coarse Glass Bead. This created a uniform, satin-like surface finish on the part. In this situation, a larger bead was used on the hard metal in order to keep the finish a bright as possible and make polishing a little easier..

After blasting, the part was barrel tumbled with a Steel Polishing Media to smooth out the finish, provide some light edge rounding and prepare the part for an additional polishing step. A final step such as a burnishing wheel or electro-polishing would create a near-mirror finish.

Edge Rounding Aluminum Parts

Aluminum is a relatively soft metal and care needs to be taken when tumbling to ensure that the finish is uniform. It is easy to 'over work' some areas of a part if the proper media shape, size and type if used. Often light cutting media such as the Plastic or Synthetic Media is used to build a margin of error and prevent too much edge rounding or a rough surface finish.

Sometimes, parts need to have a heavily rounded edge. Ceramic Media is much denser and more aggressive media than Plastic or Synthetic and is used on aluminum in these scenarios. The picture above shows the raw, cut aluminum part (left in picture) with the rough edges and the smooth surface finish.

After vibratory tumbling with an aggressive Ceramic Media, the edges are rounded and the part has a matte, tumbled surface finish. This result was accomplished in less than 1 hour. These results can be achieved with steel parts but the cycle times are typically longer.

Wood Parts Tumbling vs Hand Sanding

Hand sanding wood parts can be a tedious and labor intensive process. The ability to smooth the surface, remove cutting marks and round edges on a large quantity of parts can same time and money.

The part pictured above needed to be sanded down prior to the application of a stain and coating. The part on the right shows the sharp edges and some flashing after the initial cutting of the part. A two step barrel tumbling  process was used to achieve the end result on the left.

Step 1 involved using a Hardwood Peg Media and relatively coarse White Aluminum Oxide Grit. This produced the edge rounding and major 'sanding' of the part.

Step 2 used a similar Peg Media along with a much finer grade of White Aluminum Oxide Grit to result in a smoother, sanded finish. If necessary, subsequent steps with even finer grades of Grit or a quick hand sanding to the final required surface finish can be used. Barrel tumbling the parts in this dry process can eliminate most if not all of the hand sanding often done.

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.

Tumbling with Abrasive Grit

Tumbling parts to achieve a deburred, deflashed or rounded edge is a common process. Both vibratory and barrel tumblers are used for this purpose. Choosing the proper media type is critical to achieving the desired finish.

Most applications require the use a preformed tumbling media. These media contain abrasives that are released into the tumbling process to remove the burr, etc. The (relatively) large mass of the media assists in this process by acting as a sanding block of sorts on the edge and surface of the part. After this abrasive process, parts can be tumbling in other media (including grits) to achieve a smoother or polished surface finish.

Tumbling parts simply with an abrasive grit such as Aluminum Oxide, Silicon Carbide or Walnut Shell does not typically achieve the deburring, deflashing or edge rounding required. While the grit may be very sharp, hard and abrasive, there is very little force acting against the edge of the part. The surface finish will probably be affected (roughening, smoothing or even polishing) but the edges will be left mostly untouched.

There are some exceptions to this 'rule' (see this post) but generally preformed tumbling media is superior to grit media for deburring, deflashing and edge rounding.