Emre Ayaroglu - 4th Week Unanswered

Surface protection

The surface of a component frequently needs to be protected from its immediate environment or enhanced to resist wear under normal operating conditions. Selecting the correct treatment and applying it correctly can make the difference between a well-made product and a shoddy, unreliable, short-lived one.The most familiar surface protection methods involve either applying a coating of material that is resistant to environmental attack or that changes the surface layer chemically to achieve the same effect. These are called direct protection processes. There is, however, another approach that relies upon a completely different principle to resist surface attack, and this is termed sacrificial protection (Waters F., Fundamentals of Manufacturing for Engineers, p. 152).

Vitreous coatings

These protective coatings use ceramics, porcelain and enamel as the surface sealing medium. While they can be applied to aluminium, their main applications are for coating iron and steel components subject to high temperature, and they offer a glass-like surface with excellent chemical and abrasion resistance. They are frequently used as coatings for cooking utensils, but unfortunately the coatings tend to be brittle and are therefore prone to surface chipping.Numerous methods of application are possible, but the most common is by applying the coating in either granular form or as a water-based slurry. Heating to approximately 800°C for a few minutes produces a visually attractive glass-like surface. Typical coating thickness is in the range of 75–100 μm (Waters F., Fundamentals of Manufacturing for Engineers, p. 161).

Ultrasonic cleaning

This cleaning process is very different from those so far described, and an understanding of its operating principle is essential if its many potential uses are to be fully appreciated.The operating principle is as follows. If a low-viscosity liquid such as water is subjected to high-frequency (25–40 kHz) ultrasonic wave energy, or ultrasound as it is called, intense cavitation is generated within thefluid. Cavitation is the generation and growth of vast quantities of gaseous bubbles within the fluid which,upon reaching a certain size, typically 0.15 mm diameter, implode (collapse inwards). As each bubble collapses , a microscopic inward moving jet of fluid is created at an astonishing 250 mph, and it is this jet that blasts away the contamination from the dirty surface.The sustained generation, growth and implosion of huge numbers of bubbles throughout the ultrasonic bath,and on all surfaces of the submerged component to be cleaned, is referred to as the cavitation effect. Commercial ultrasonic cleaning units usually consist of a bath of fluid specially selected to suit both the material to be cleaned and the type of contaminant to be removed, plus an electrical transducer unit for generating the required ultrasound. The transducer(s) may be attached to the bath in a number of ways but is frequently externally bonded to the bottom of the fluid tank. Portable units can also be obtained for fitting into existing tanks, transducer power required being approximately 10W per liter of tank capacity. Cleaning times are typically 2–5 minutes depending upon the level of contamination and component geometric complexity. Cleaning speeds are generally quicker and more thorough than is achievable with most other methods, and both intricate and delicate components can be cleaned without risk of damage (Waters F., Fundamentals of Manufacturing for Engineers, p. 146).

Jet blast cleaning

One method of avoiding the physical impact between components that occurs in tumbling and barrel finishing is to strike the surface to be cleaned with a suitable fluid, the most commonly used being compressed air containing either steel shot or coarse sand. To isolate the jet cleaning operation from the surrounding environment, it is usually carried out in anenclosed cabinet. The component to be cleaned is loaded into the working area of the jet blaster cabinet and the stream of fluid is then directed either manually at the dirty surfaces or, less commonly, the workpiece is moved around under one or more fixed jets. Manual manipulation of either the jet nozzle or component is carried out through a rubber glove box. The spent shot, sand, etc. falls through a hole in the floor of the working area and is then recirculated.The compressed air used in jet blasting is at a pressure of up to 6 bar, which is conveniently the typical pressure found in most factory piped air supplies. Much lower pressures are used for non-ferrous parts. Jet blasting enables the impact fluid to be efficiently directed at internal surfaces, although it is alwaysdifficult to clean complex internally cored holes completely or, more importantly, to know whether or not theyhave been properly cleaned. An advantage of jet blasting is that the surface finish achieved provides a particularly good key for subsequent painting operations.Special portable units that invariably use water combined with chemical cleaning agents are frequently used to remove atmospheric contamination from the outside of buildings (Waters F., Fundamentals of Manufacturing for Engineers, p. 143).