- Education and Science
Cleaning and Inspection of Castings
A cylinder made through casting
A non-inspected casting will not stand the test of time. It is better that that casting was not made because it may cause more danger in future. Also, packaging is very important in any product being sent into market places. Because of how important it is, that is why cleaning comes into play. A well cleaned commodity will attract buyers in the market.
What is cleaning and inspection of castings? Cleaning and inspection of castings are the processes through which products called castings are made neat and then subjected to examination before they are applied to particular use. Without cleaning and inspection of castings, the castings can fail when used for certain purposes. To avoid the failures, castings must be cleaned and inspected. Many buildings and constructions have failed today because proper inspections were not conducted on the metallic materials before their use. Through cleaning inspections, dimensions and other important factors are taken into considerations.
It is not every worker in engineering casting company that carryout cleaning and inspection. This position is reserved for people who are specialists in the area so as to supply products of high reliability to buyers in the market.
Cleaning is carried out after solidification of castings to the desired temperature as nobody will like to use dirty material. Without cleaning of castings, there will be no marking of the already made castings because they will not tidy to attract customers. This involves the breaking of the castings from the mould. When the shakeout operation is conducted (breaking off of the castings from the flask), it is then subjected to fettling. The runners and risers formed during pouring of the molten metal are cut off from the casting to obtain the original casting. The cutting off of the unwanted parts can be done with metallurgical cutting machine.
The use of moulding sand is conditioned for further use. Conditioning of moulding sand is done in sand conditioning shop. The conditioning is done so that the sand can be reused for further moulding process. The dust generated during this process is collected by cyclone dust collector. Because the dust-laden is harmful to casting engineers, the automation and mechanization of shakeout and fettling operation is applied. Mechanical, eccentric, and inertia grids are most economical mechanization to be used and produce minimal noise.
This book throws more light on this topic
Cleaning or Fettling of Castings
The series of operations performed in cleaning shop may be grouped as follow: decoring, removing of gates and risers, flow-offs, stripping off adhered sand from the castings, trimming burrs, ribs, and scale after heat treatment, straightening and painting of the castings if necessary, and inspecting for acceptance after cleaning and cutting.
From the name, decoring, this simply implies removing of cores or knocking out of cores from castings. It involves important skills. The labour input that is applied before any core is removed is dependent on the core cavity, the retained strength of the core sand, and the stressed state of the core in the casting. If the core sand is of very high strength, more energy input is needed for removal of the cores.
Cores which are made of core sands with the mixture of organic binders (oils, oil substitutes, and synthetic resins) have low retained strength and good collapsibility. In many cases, cores (especially simple shapes) are knocked out from castings in the process of stripping the castings from moulding sand on the knockout grid and in the course of cleaning castings in the shot-blast installations. More complicated cores can be knockout using shakeout cores. Cores can be removed using decoring machines like core vibrators, water-jet core knockout units, and so many others.
An electric-discharge hydro-blasting method can be used for removal of cores. The advantages of this method are: it involves high productivity and effectiveness, excludes hard manual work, and decreases the dust content of air in foundry workshops.
Removal of Gates and Risers
This is another fettling process after solidification of castings. Removal of gates and risers make the casting to take the original shape of the pattern which it was made from. When the casting is made from aluminium metal, it can easily be cut off using metal hand saw cutting machine. This is done using the type of saw because the metal, aluminium, is not too hard. But, when metal alloy of high hardness and hardenability is used for the casting, more advanced cutting saw is used for cutting off of the unwanted part(s). The casting is held in a vice before the cutting off process. Again, metal cutting machine can also be used to carry out this same task. The hardness of any metal is dependent on its grain sizes. Note that the grain sizes of metals are determined through a process called metallography and surface preparation technique.
When the casting is made from steel, it is cut off (removal of risers and gates) using more powerful saw. The kind of saws that can be used in this cutting process are power hacksaws, and on disk-type cutting benches. Hacksaws have high capacities and are used on metals with high strength. During cutting with hacksaws, as the cutting proceeds, the saw blade and the castings are cooled from the heat generated due to frictional force by coolants or water. In other words, coolants or water is used in cooling during cutting using hacksaw. For cutting of gates, hydraulic, crank, friction, and other types of presses can be employed.
Removal of gates and risers can also be conducted using electro-machining, abrasive cut-off wheels, flame cutting and arc cutting. Abrasive cut-off wheels functions using abrasive as a medium. The wheel is coated with abrasive and as the wheel rotates, the gates are removed. Flame cutting involve the use of flame of high intensity for the cutting operation. In arc cutting the “main player” is the arc produced by the help of electricity.
Removal of Burnt-On Sand and Scale
Tumbler can be used for cleaning small castings like shell, lid, trunion, casting picks, casting in-flanges, and drives. Tumbling barrels of periodic action serves for cleaning outer surfaces of small castings produced piecemeal and in mass quantity.
The cleaning process is achieved by rubbing of castings against white iron picks. Tumblers can handle 800 to 1,300kg of castings per hour. Tumbling peels off the burnt sand, adhesives sand, and breaks away flashes (fins) from castings. The time taken for the cleaning process is dependent on the shape of the castings, type of metal, and method of loading the castings into drum. The rotation speed of drum is between 20-30 revolutions per minute. The speed of the drum is dependent on the kind of drum used during the tumbling, and the number of castings contained in it.
Chipping and Grinding of Castings
Chipping of flash from the already produced casing can be done using chipping harmers and chisels. While chipping flash from inaccessible parts of any casting, it is advisable to make use of pneumatic chipping harmers with short stroke, up to 100mm. when trimming castings, pneumatic chipping harmers with long stroke is usually applied.
Chisels made from tool steels are the most durable to be used. The chisel tip should have a hardness of 58-61 HRC. Electric power-driven hammers have recently come into play for chipping and trimming steel castings.
Grinding of casting is a smoothening process. It is also known as snagging. Grinding is conducted to remove excessive metals, flash, and to correct shifts. The machines that are used for are flexible shaft grinders, swing frame grinders, floor stands, and automatic and semi-automatic machines. The part of grinder that mainly performs grinding function is the abrasive wheels. These wheels can be made of ceramics, Bakelite, and vulcanite binders.
The later stage of cleaning is referred to as finishing. Some castings receive their finishing after the grinding process while additional finishing is applied on others. Whether the finishing stage stops after grinding or not is dependent on the material desired to obtain at the end of the process. For example, in casting of aluminium spoon for practical purposes, the finishing stage may stop after grinding. But, in casting of special spoons to be used by kings, the finishing stage stops after coating with alloy to make the casting appear attractive.
Additional surface finishing can be carried out using machining, chemical treatment, polishing, buffing, blasting and painting. Some steels and malleable iron come under heat-treatment after cleaning and this can affect the surface of the casting. For this reason, these metal types still undergo special finishing process.
Non-ferrous castings can undergo finishing like chemical, electrolytic, and through mechanical process. The part of finishing that can also be done on castings is removal of impurities, expels are dust, dirt, scales, sand, and grit using salt-bath cleaning process.
Inspection in this context involves the operations that check the quality of castings, and results to rejection or acceptance of the castings. Fundamental types of inspection are metallurgical, visual, and dimensional inspection.
Visual Inspection: The word, visual, can be replaced with sight. Visual inspection can then be said to be viewing finished castings with eye and microscope to identify the correctness of the casting. It is concerned with surface inspection for discovering casting flaws. It is capable of identifying dirt, blowholes, scabs and others. Visual inspection is carried out before cleaning, and annealing, and after fettling operation.
Dimensional Inspection: This inspection type involves checking the dimensions of the finished casting. Dimensional inspection involves the principle of gauging as it is applied to any machine element. Surface plates, high and depth gauges, layout tables, dividing heads and other types of gauges are used for dimensional inspection of castings.
Metallurgical Inspection: When metallurgical inspection is mentioned, the concern is centred on chemical composition, mechanical property tests, evaluation of casting soundness, and other casting testing. Hardenability-test and machinability is included in this type of inspection. Metallurgical inspection determines the strength of metals.
In fact, metallurgical inspection is so wide that it can occupy many pages of metallurgical textbook. It also involves tensile strength of castings, thermal and electrical conductivity, resistance, heat-treatment, corrosion resistance, coating and electroplating.
What is salvaging operation? Salvaging operation is the means of rectifying the defects in castings after they fail to meet the necessary inspection standards. All castings that have defects on them are not just turned into scraps, but some errors can be corrected. The time taken for the defects to be corrected is dependent on how serious the defects are. Salvaging or methods employed for correcting defects in castings include electric welding, metal spraying, gas welding and impregnating of the castings.
The factors that determine whether salvaging is possible or not are the weight of the casting defects, the size of the castings, difficulty in the repair, the casting alloy, cost of salvage, and availability of the equipment for the salvage. If the cost of the cost for the salvage is equivalent or close to that which can be used for producing new one, salvaging may be ignored. If the defect in any casting is due to leakage under pressure, such can be corrected by use of sealing processes.
Discussed are the cleaning and inspection of castings. Castings are not ready to be used for any purpose until they pass through these processes. Under inspection, the types or the three major inspection processes that can be adopted are metallurgical, visual and Metallurgical inspection. It is important to know that there is a type of inspection called ultrasonic inspection. These inspection types were all explained.
Foundry Technology by O. Ekpe Okafor