Mass finishing is an umbrella term for a group of abrasive industrial processes in which huge quantities of parts or components are economically processed to achieve one or more surface improvement effects. These surface upgradations include, but are not limited to, burnishing, deburring, cleaning, drying, polishing, and brightening. These processes may not necessarily be mutually exclusive to each other. In other words, it is very common to have several or all of these functions happen in a single mass finishing process.
Mass finishing processes revolve around two key elements, viz:
- Media: All mass finishing processes use a free (or loose) abrasive substance referred to as media inside a chamber or container of some kind. Energy is imparted to the abrasive media in order to kickstart motion and make it wear away at surfaces of the components.
- Compounds: These are added to mass finishing processes to aid in burnishing, deburring, cleaning, cutting, descaling, and repressing corrosion. They may be either in the form of dry powders or liquids.
Why Employ Mass Finishing?
Almost all manufactured parts or components need some degree of surface refinement before the final assembly (or coating) so as to make the parts satisfactory to the end-user. Usually, manufacturing firms apply mass finishing techniques because they are aware of the economic benefits of the process, particularly when compared with surface finishing and manual deburring techniques.
Mass finishing processes have the potential to stamp out various procedures that demand considerable part handling or are labor intensive.
Moreover, quality control standards are becoming increasingly stringent. Mass finishing processes have the ability to produce surface effects with part-to-part and lot-to-lot uniformity that simply cannot be replicated when parts are individually handled.
What’s more, well-executed mass finishing processes significantly decrease part rejection and rework rates.
Functions of Mass Finishing
As mentioned before, the purpose of mass finishing is to achieve a number of surface improvement effects. Let us take a look at some of them in detail:
- Cleaning: The chief function of mass finishing is cleaning, and is the foundation for all finishing processes. That is, in order to efficiently deburr, surface refine, inhibit, dry, and more, the part ought to be clean. Basically, cleaning is the elimination of undesired residue from the part. Residues may be present themselves as:
- Oxidation or scale
- Cutting or stamping fluid
- Dust or particulates
- Die lubes or corrosion inhibitors
While the cleaning process is essentially a chemical one, it is greatly supplemented by mechanical action, namely — media and equipment.
- Deburring: Deburring employs the use of abrasive media to grind away machine lines, burrs, sharp edges, belting marks, slag, and establish radius.
- Surface refinement: Surface refinement reduces rough surface finishes to satisfactory standards for painting, plating, polishing, and anodizing operations. Ordinarily, these finishes are measured by a profilometer and denoted in terms of RA, which is the average distance between the peaks and valleys of the surface of the parts.
- Inhibition: Once the part has been cleaned, deburred, and surface refined, it must be shielded from oxidation and corrosion. Hence, inhibition.
- Drying: All the above functions usually result in a wet part. To guarantee optimum corrosion inhibition, the part must be dried. This is achieved in mass finishing equipment with ground corn cob and is typically intensified by heat.
Types of Mass Finishing
There are many types of mass finishing processes and machines. However, the two most commonly used processes are tumble finishing (or barrel finishing) and vibratory finishing. Both entail the use of a cyclical action to produce a grinding contact between surfaces.
Let us take a brief look at both:
- Tumble finishing: This is a technique for polishing a rough surface on relatively small parts. The parts are put in a barrel together with water, abrasive media and an appropriate compounding agent. The rotation of the barrel makes the media mass and parts to tumble down upon itself producing friction and thereby eroding the parts in the mass. This creates a cutting action that results in a swift, efficient deburring of the part’s surfaces.
- Vibratory finishing: This class of mass finishing process is used to deburr, descale, clean, radius, burnish, and brighten a large number of small-to-medium sized workpieces. The process involves placing parts, media, and compounds into a vibratory bowl or tub. The vibratory action generates a force on the parts causing media and parts to grind against each other, resulting in polishing and deburring of a mass of parts. Apart from deburring and polishing, Vibratory finishing can also be used for other mass finishing applications such as cleaning, descaling, and surface preparation. The results obtained are usually unobtainable by hand.
So, which one of the two is better?
By and large, vibratory finishing is the more popular choice for most mass finishing applications. The primary reason for this is the fact that the rotary finishing process is a better fit for automation, which can ultimately save time and decrease labor costs. Besides, in most cases, vibratory finishing is the better alternative for finishing large or delicate parts and is also more appropriate for tight-tolerance projects.
Furthermore, vibratory finishing usually yields a smoother finish without inducing a lot of surface wear, which extends the durability and lifespan of the part.
On the flip side, vibratory finishing systems are more convoluted than barrel finishing systems. Consequently, this makes them more costly to purchase, own and operate. That said, barrel systems are slower and take much longer to complete the finishing tasks. Still, they generate significantly less wear on the abrasive media, thus further saving costs.
However, there is a drawback in the barrel tumbling process. As the parts tumble over one another rather than gently rubbing together, it increases the risk of surface indentations.
Types and Classification of Mass Finishing Media
There are various types of media used in the mass finishing process, such as ceramic, aluminum, synthetic, plastic, and so on. However, the two most popular ones are ceramic & plastic:
- Ceramic: Ceramic media are utilized for processes ranging from heavy grinding applications to fine polishing.
- Plastic: Generally, plastic media is used for mild surface finishing. Also, it is regularly used for softer, non-ferrous stuff such as die castings.
Diving deeper into these, mass finishing media can be further classified by:
- Composition: Be it heavy grinding, fine finishing, or anything in between, the composition of the media plays the decisive role on what it’s designed to do. Each composition comprises variables such as bulk density, defined as the weight of media per cubic foot, and the amount of abrasive material, that are crucial in deciding the composition that best fits your finishing application.
- Shape: The majority of mass finishing media can be classified as pre-formed media. Though, random-shaped media is also available. Pre-formed media is created in a definite shape and meant for a particular finishing application. The shape of the media is pivotal in the success of your application. Simply put, different shapes of media with the same size and composition will produce entirely different results. Here are five of the most common shapes employed in mass finishing applications:
- Size: This can be the deciding factor for your mass finishing application. If your media selection is too big, it may not be able to reach 100% of the surface area of the components you are processing. Conversely, if the media size is too small, it may pose a “lodging” problem for your processed parts.
Mass finishing is increasingly becoming the most desirable and profitable way to finish manufactured parts making hand finishing a practice of the past. It is no surprise considering all the advantages the process offers in terms of speed, quality, and reliability.
Thus, having high-quality mass finishing equipment is becoming a must for all manufacturers, big or small, to compete at a global level.
Introduction of Mass finishing (Vibratory Finishing) Courtesy Professor Brigid A Mullany from Vibrational Finishing Research at UNC Charlotte .