Applications for Plastic Media

Q. I have been involved with vibratory finishing for about 12 years, mostly deburring large steel stampings (about 1- × 8-inch to 3- × 12-inch pieces) and machined parts of cast iron. I recently incorporated an aluminum sand cast housing that has two large holes (2 1/2-inch diameter) with some internal passageways. Our equipment consists of a flat-bottom, 20-ft round vibrator with no separation and a 25-ft round vibrator with a screen deck. We have been using 1-inch tri-cyl ceramic media in both machines, but this doesn’t work well with the aluminum parts. The media enter the part and get lodged inside. We tried a larger, 1 3/8-inch tri-cyl ceramic media, and it solved the lodging problem but damaged the ID of the large holes. It also introduced some dents on a ground surface that we don’t like. Next, we tried a 1 1/4-inch cone-shaped plastic media, but it does not efficiently debur the ID of the large holes. Also, the media leaves a difficult-to-remove white residue on the parts that requires washing in a separate operation.

We would like a single media for all of our parts, but we can dedicate the smaller machine to the aluminum parts if we have to. Before we conduct further experiments with different media, what is your opinion? B.B.

A. Your inquiry made me realize that I have not provided much information about plastic media, and now is the time to do so. Before addressing your specific question I will share some basics about these interesting products.

There are two types of plastic media: resin-bonded polyester plastic and urea methanal. The first is generally referred to as “resin bond plastic” and the latter as “synthetic” media. Both are lumped together as “plastic” in general references. It is useful, however, to know which one is intended.

Plastics are made by the slip-casting method, and ceramics (most of them) are extruded. Slip casting can be compared with making ice cubes in a plastic tray. The liquid state is poured, or slipped, into cavities in a flexible mold. After the product is cured, the mold is flexed and the pieces fall out. Ceramics are made by pushing clay through a die that might be shaped as a triangle, circle or star. The extruded clay goes on to a moving conveyor and is cut into pieces of the desired length. Most shapes can be made by either process. There are, however, some shapes unique to each method. For example, plastic can be made in bowtie and five-sided pyramid shapes, while ceramics can be made in angle-cut cylinders. Cones are easily produced by slip casting, and many years ago slip-cast ceramic cones and tetrahedrons were being promoted.

The two types of plastic media each have their advantages. The largest manufacturer of plastic media makes about equal quantities of resin-bonded and synthetic. Resin bond can yield a somewhat smoother surface, resulting in very bright plating and very good anodizing. The synthetics produce a shinier finish on stainless steel and other metals that will not be plated. Also, very thin or delicate parts are treated more gently in small synthetic media with its 50- to 55-lb/cu ft bulk density.

Both types of plastic media can be made with added abrasives using the same abrasive technology. Most commonly used abrasives are silica, zircon and variations of aluminum oxides. The size of the abrasive grains and the quantity used in the media are variables available to meet a wide variety of applications.

Resin bonds produce a foamy residue that can dry quickly on parts and become very difficult to remove. This foam also floats for many hours in the waste-settling tanks, whereas synthetic foam settles within minutes and there is much less foam volume. With either media, it is very helpful to use non-foaming-compounds-containing rinse aids.

I am a proponent of ceramic media whenever possible. Three areas exist in which plastic is really better than ceramic: when very large media pieces are necessary to prevent lodging, but ceramic pieces would damage the parts; when very light and delicate parts would be damaged by the weight of the media; and when very smooth, nick-free surfaces are required, such as for pre-plate or pre-anodyzing finishes, or for a shiny final finish. Two examples of parts requiring a fine finish are medical devices and highly decorative motorcycle parts. An example of needing a large media that does not dent the parts is the case in point. Large ceramic media solved the lodging problem, but it damaged the cast aluminum part.

The selection of a large cone eliminated the lodging and nicking, but deburring of the ID was inadequate. I have written often regarding the poor performance of cylindrical media, and much of the same can be said about cone shapes when it comes to ID finishing. The number of contact points between a cone and the ID are very limited. Consider how much more ID contact will occur with four or five-sided pyramids, or with a large, three-pointed star. The vendor of the media should be able to sample process your parts in some of these shapes and make a valid recommendation. I suggest you stick with the synthetic compositions.

You can then try the final media selection for your other parts. I can’t predict the outcome, but even if deburring is satisfactory, you may find the ceramic you are now using will be more cost effective. Let me know how this comes out.