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.

Tumbling Frustration

A question about deburring, cleaning and polishing brass, copper and aluminum parts.  It's worth reading through the entire question as it provides an excellent description of what may small shops are doing and the frustrations they have.

Question: I primarily machine small parts out of 6061 aluminum, a small amount out of C360 brass and a handful out of C145 copper. I currently use a small vibratory tumbler to finish my parts. I use a small (about .25" square base) green plastic triangle to do the deburr and initial cleaning cycle. The problem with this, is it takes two to three days to run to get satisfactory results. I have tried running dry, but it takes even longer to achieve the results I want. I usually add a small cup of water to add some lubrication. But I have to take the tumbler bowl off and rinse it out and wash off the media every 6-8 hours because I get a thick black foam that fills the bowl and turns the media into a solid mass that just moves around the bowl. After this step, I clean out the tumbler and add a relatively fine red corn cob media to do a polish. But that also takes two to three days of tumbling to achieve a decent finish. I never add rouge or other polishing compound because I have no idea what to use. I just use the pre-treated corn cob media and replace it every 3 or 4 months. I am hoping to add a new machine to my shop and expect to produce several hundred parts a week that need polishing (I do not need to polish to a mirror shine, I merely need to deburr after the machining operations and provide a nice level of shine). Obviously, I would like to find a better way to accomplish this over what I have been doing. My parts are typically small - 1.2" diameter x 0.12" thick discs, 0.75" diameter x 0.25" thick slugs, 0.8" x 0.375" x 2" bars. Many of the parts have threaded holes and small slots (1/16" wide). I spend a lot of time hand cleaning the corncob out of the slots with a small pick. I have tried to do research to find a better solution, but it is one of those ask a dozen people, get 2 dozen answers types of questions. Some people tell me I need flow through on my first step. Some recommend ceramic some plastic and some steel shot. Corn cob and walnut shell seem to be recommended for finishing - some say pre-treated and others say to add rouge. I am at a loss now as to which direction to proceed so I figured it was time to seek more professional advise. Would you be able to help me find the best solution to my problem? I do not have a large space to set up multiple large tumblers, I need a compact solution with at most, two tumblers.

Response: I will try to address your questions and offer a simple and reasonable recommendation.

1. Plastic Media, Ceramic Media and other types of deburring media like this require a wet process using an appropriate tumbling compound (i.e., soap/lubricant). Assuming the proper shape and size of media is being used, the cycle times on the types of metal you noted (aluminum, brass and copper) should not be more than a couple of hours. The cycle times could be longer if you have very heavy burrs or need to get a highly rounded edge. The foam is probably a result of not using enough water and no compound. This is common with Plastic Media with no flow-through system for the solution.
2. Given the surface finish requirement of a 'nice shine' and parts descriptions, I would recommend using a 3/8" x 3/8" KSV Synthetic Cone Media and a solution of Kramco 1030 Mild Acid Cleaner. This media is a light, deburring media that will result in a smooth, pre-plate finish. The Kramco 1030 will leave the metals you are using with a bright finish. This media choice may require a slightly longer cycle time than a more aggressive media (i.e., Ceramic Media) but the surface finish will be much better. In addition, a flow-system is recommended for the best results.
3. The polishing media you are using - treated corn cob grit - may be treated with a rouge or something similar. Cycle times with dry tumbling like this are often long, 24 hours or more, but can produce excellent, near-mirror finishes. Of course, if the parts have holes or slots that are similar in size to the grit, it is common for the media to get stuck. Using the recommendation in #2 above may eliminate the need for a second polishing step (depending on the specific finish desired).
4. An alternative polishing step could be with a polishing Ceramic Media or Precision Ceramic Media instead of the dry process. While this may not result in the near-mirror finish achievable with a Corn Cob Grit, the finish can be a smooth, polishing, bright surface. A solution of Kramco 2030 Neutral Burnishing Compound would be used in this step.

Polishing Machined Aluminum

Taking machined aluminum parts and polishing to a smooth, uniform surface finish is often a multi-step process. Each step is designed to accomplish a different task. The part above has burrs, sharp edges and clear machine marks. Vibratory tumbling is the preferred process.

Step 1 - The edges are softened and the machine marks are removed using a Plastic Cone tumbling media. This media is selected due to it's lower density (vs. Ceramic Media) and lesser aggressiveness. The resulting surface finish is smooth and uniform. This part could be anodized to a matte finish, powder coated or painted with excellent results.

Step 2 - In order to brighten the part and prepare for anodizing to a higher luster, the part was tumbled with Polishing Precision Cearmic Spheres. While the finish is bright and smooth, the proper shape must be chosen to ensure that a uniform surface finish is achieved based on the geometry of the part.

Pre-Anodized Machined Aluminum

Machined Aluminum

Anodizing is an electrochemical process where a very thin portion of the surface of the metal is converted into a protecting coating such as aluminum oxide in the case of aluminum metal. Since this is quite different from a painted or powder coated surface any 'features' in the surface will be translated to the anodized finish. Anodizing will not fill in the gaps and micro-profile on the surface of the metal. Machining marks (seen above) will be still visible after anodizing.

Pre-anodized, Tumbled Finish

To remove the machine marks and create a smooth surface for anodizing, vibratory tumbling with a Plastic Tumbling Media was used on this part. The features of this part required a media that will leave a smooth finish across the large, flat surface area as well as all the way into the corners. A Plastic Cone proved to be the ideal solution.

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.

Smooth Aluminum Finish

Aluminum is a relatively soft metal that is easily nicked, scratched and damaged by handling - both during the part manufacturing process and after. Restoring aluminum to a uniform and smooth surface finish can be accomplished with mass finishing.



The aluminum part on the left clearly shows the scratches and damage due to heavy handling. The part on the right is the result of a simple mass finishing process to create a uniform, matte surface finish. The part (aluminum cylinders) were vibratory tumbled in a wet process with a general purpose (KM) Ceramic Media and cleaning/lubricating compound for 2 hours.

Ceramic Media is typically used with harder metals such as steel; however, in this case, there was no need to further polish the part. If a higher polish is required, a lighter cutting Plastic or Synthetic Media can be used in a (potentially) longer cycle time. This step would be followed with a polishing step.

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.

Aluminum Edge Rounding

Deburring and edge rounding often go together. Aluminum is generally tumbled with less abrasive and lighter Plastic and Synthetic media to minimize wear on the edges. Sometimes a heavy deburr or edge round is specified. The aluminum part in this picture was vibratory tumbled for only 1 hour with a Cylindrical Wedge Ceramic media. I think the finished part on the right speaks for itself especially when placed next to a raw machined part.

Fiberglass Deflashing - Alternative

Removing the flash off various types of molded parts (see last post) can be done in a wet or try process. Vibratory tumbling with a Plastic or Ceramic media in a wet process can be a quick and easy method to achieve the desired results. After the deflashing step, the parts will generally need to be rinsed and dried.

A traditional dry tumbling process involves the use of Hardwood media that have been treated with an appropriate cream compound. The selection of this compound depends on the needed aggressiveness and required surface finish. The challenge with these compounds isthat sometimes the cream can transfer from the media and become deposited in holes, slots or corners of the part. This then requires cleaning prior to further processing of the part.

An alternative to using creams is a similar previously described process for dry tumbling of wood parts. Depending on the type of part and the amount of flashing that needs to be removed, simply tumbling the parts with Hardwood media and an abrasive grit such as White Aluminum Oxide will provide excellent results with only a light dust on the parts. Because the abrasive material is not 'attached' to the media, a barrel tumbler is typically a much better process than vibratory tumbling.

Choosing Tumbling Media (2)

In addition to choosing the proper type of tumbling media, the shape and size of media is also critical to achieving the proper finish. Given the multitude of tumbling media options, the size and geometry of the parts being tumbled will narrow down the choices considerably. Optimizing a process requires using a good quality media that is appropriate for the job at hand.

The most popular Ceramic Media shapes are Angle Cut Cylinders and Angle Cut Triangles. The angled edges help the media to produce a uniform finish across the entire part including inside corners, holes and slots. Cone shapes are popular choices when using Plastic and Synthetic Media. Generally, larger media will work better with larger parts. Smaller media will have a tendency to impact surface finish more than edges.

Aggressive media (whether large or small) will shorter the cycle time to deburr or round edges - and even for polishing (aggressive = dense) operations. Aggressive, abrasive media will break down more quickly than general purpose type media so replacement cost and the waste stream are an additional consideration.

The high quality tumbling media manufactured by Vibra Finish provides the balance between options (size, shape and type), cost (economical) and quality (uniform bonds, even break-down).

Choosing Tumbling Media for Deburring, Deflashing or Polishing

Choosing the proper tumbling media (vibratory tumbling or barrel tumbling) for any deburring, deflashing or polishing application requires balancing the aggressiveness of the operation with the surface finish specifications. As long as a rough, tumbled appearance is acceptable a more aggressive media can be used to remove burrs and 'smooth' a surface resulting in a shorter cycle time . If the surface finish is important (i.e., for anodizing or thin-coat plating), a less aggressive media and longer cycle time is necessary. Often a multi-step process is used with progressively less aggressive media to achieve a deburred and smooth part or even polished part.

Generally, Ceramic Media are used on harder metals like steel or titanium. Plastic or Synthetic Media are better for softer metals like aluminum, brass and copper. Within each type of media, a variety of formulations (or bonds) are available from ultra aggressive to general purpose to polishing.

What is Tumbling Media?

Preformed tumbling media used in both vibratory and barrel tumblers are simply the means to achieving a desired surface finish. The type of media formulation or BOND used can vary widely for each process or application.

Tumbling media is a blend of abrasives and binders. Ceramic Media is generally a clay or porcelain binder while Plastic Media and Synthetic Media are typically made from urea formaldehyde or polyester polymers. These binders are used to hold (bond) and shape the abrasive grit blend needed for a particular media type. Aluminum oxide and silicon carbide are the most popular abrasives used.

The specific type of binder and abrasive used along with the ratio of binder to abrasive will determine the impact of the media on the part. A high abrasive content will lead to quicker break down of the media and a more aggressive operation. Low or no abrasive content will result in a media that is very soft/gentle on the part and used for polishing and fine finishing.

All tumbling media are designed to break down or erode during use. This is necessary to perform the 'work' on the part. Generally, the quicker a media breaks down, the more aggressive the bond. The ideal media for any process will provide the minimum erosion (longest lifetime) but must be balanced with a reasonable cycle time to achieve the desired surface finish.

Plastic Abrasive Blasting Media

Plastic Abrasive Blasting Media is a synthetic media that is available primarily as three different types - Urea (Type II), Melamine (Type III) and Acrylic (Type V). Plastic Media is a dry, thermoset plastics that is specifically designed for specialty applications such as automotive and aviation for stripping (without warping the metal), boat stripping, die and mold cleaning and other sensitive surfaces where etching could be a problem.

Plastic abrasive blasting is a very low dust application. While having a high re-usability rate, plastic media will break down during use into particles that maintain a sharp angular shape adding to the long-life and value of this media. The higher purchase cost of plastic media makes it difficult to call this media an abrasive blasting work-horse but in the applications where surface and part sensitivity are critical, plastic media is definitely the way to go.

Properly produced plastic abrasive media is manufactured to meet the MIL-P-85891-A specification which identifies the various Types.

Polishing Aluminum

Bringing machined aluminum into a highly polished finish is a multi-step and sometimes challenging process.
The first step is to tumble (vibratory usually works best) the parts in an abrasive media such as a Plastic or Synthetic Media. Using as round a media shape as possible - cones vs triangles - will result in a smoother finish producing a better end result.
The second step is a polishing step using a Ceramic Media. Either an Angle Cut Cylinder or a Precision Ceramic Sphere will produce a smooth surface finish - again rounder is better.
In order to burnish the surface (if necessary), a third step can be to dry tumble the parts with a Walnut Shell Grit possibly treated with a fine abrasive such as jeweler's rouge.
It is important to make sure that the chemical compounds you use will not attack the aluminum. Depending on the final finish required, a small amount of hand polishing may be needed.

Mass Finishing Basics (Part 4)

The art of mass finishing really shows itself when trying to select a tumbling media. Media is the material ('stones') that parts are tumbled with in either a vibe or barrel tumbler. The types of media available are as varied as the parts that can be tumbled in a mass finishing process.

Ceramic Media is available in bonds or formulations that range from super aggressive, cutting to non-abrasive polishing. The most popular preformed shapes are triangles, cylinders and variations of these shapes. Ceramic media is typically used with hard metals such as steel. Softer metals such as aluminum can be tumbled with ceramic media but care should be taken so the process is not too aggressive. Precision Media is a high density version for specialty applications.

Plastic Media and Synthetic Media have a lower density and are thus less aggressive than ceramic media. These media are typically used with aluminum, copper, brass and other soft metals. Popular preformed shapes include cones, pyramids and wedges.

Steel Media is used primarily for polishing and burnishing operations in barrel tumblers. Some vibes can be set-up to handle the weight of the steel media which is 3-5 times as heavy as the other preformed shapes.

Other types of tumbling media include Hardwood Media (dry tumbling of wood and some plastic parts), Aluminum Oxide (surface finishing and etching), Walnut Shell (burnishing) and Corn Cob (burnishing) as well as many other specialty media.

Choosing the proper type, size, shape and bond is critical to making sure that the required finishing is achieved. Common problems include excessive media wear, poor and/or inconsistent surface finish, too much (or too little) edge rounding and media 'jamming'.