Chapter 1: Introduction
Background
Manufacturing Industry and the Environment
Life cycle of a product begins with the raw material extraction and then goes through various manufacturing activities until the product is delivered to customer. During product life cycle, energy is consumed at various stages along with generation of waste and emission gases that causes pollution. This waste and emissions cause environmental degradation which is one of the ten threats officially cautioned by the high level threat panel of the United Nations (Moy 2004). Environmental degradation means destruction of natural habitats, depletion of natural resources and environmental pollution. The main cause of environmental damage is excessive industrial activities and non-judicious use of natural resources. Environmental policies, aimed at limiting the environmental impact due to industrial activities, are pushing the industries to take preventive actions to minimize the environmental impacts. Therefore, development of sustainable products is one of the key challenges faced by the industry in the 21st century.
Sustainable manufacturing is defined as, manufacturing the products by the use of processes that minimize negative environmental impacts, conserve energy and natural resources (Leahu 2008). To fulfill objectives of sustainable production companies must minimize all kinds of wastes as well as use of natural resources, raw materials and energy throughout the product life cycle. Hence, fundamental re-think in the design of a product at all stages of a product life cycle is required.
Industrial sectors like pulp and paper sector, chemical industries and the metal industries are a step ahead in improving their environmental performance. However, a large number of studies have been conducted on metal industries with focus on primary and secondary metals industry. Only a few studies have been reported for metal casting industry (Neto 2007). Therefore the focus of present study is to analyze the environmental aspects of Sand Casting process. In the next section Sand Casting process has been discussed which is followed by discussion on the present state of Sand Casting and environment.
Sand Casting
Basic Process
There are six steps in this process:
- Place a pattern in sand to create a mould.
- Incorporate the pattern and sand in a gating system.
- Remove the pattern.
- Fill the mould cavity with molten metal.
- Allow the metal to cool.
- Break away the sand mould and remove the casting.
Components
Patterns
From the design, provided by an engineer or designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or a plastic such as expanded polystyrene. Sand can be ground, swept or strickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as contraction allowance. Pattern-makers are able to produce suitable patterns using "according to the percentage of extra length needed). Different scaled rules are used for different metals, because each metal and alloy contracts by an amount distinct from all others. Patterns also have core prints that create registers within the molds into which are placed sand cores. Such cores, sometimes reinforced by wires, are used to create under-cut profiles and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in engine blocks.
Paths for the entrance of metal into the mold cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed', and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via risers, which are added either in the pattern itself, or as separate pieces.
Tools
In addition to patterns, the sand molder could also use tools to create the holes.
Molding box and materials
A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with a molding sand. The sand is packed in through a vibratory process called ramming, and in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes casting possible without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.
Chills
To control the solidification structure of the metal, it is possible to place metal plates, chills, in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings, the effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core. In other metals, chills may be used to promote directional solidification of the casting. In controlling the way a casting freezes, it is possible to prevent internal voids or porosity inside castings.
Cores
To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.
With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze, brass, aluminium, magnesium alloys, or various pot metal alloys, which often include lead, tin, and zinc. After being filled with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed, revealing a rough casting that, in the case of iron or steel, may still be glowing red. In the case of metals that are significantly heavier than the casting sand, such as iron or lead, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the Sand of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.
After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening—that adds resistance to tensile cracking and smooths the rough surface. And when high precision is required, various machining operations (such as milling or boring) are made to finish critical areas of the casting. Examples of this would include the boring of cylinders and milling of the deck on a cast engine block.
Design requirements
The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. A slight taper, known as draft, must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be embedded in the final casting, forming a sand pit, which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed.
Processes
Green sand
These castings are made using sand molds formed from "wet" sand which contains water and organic bonding compounds, typically referred to as clay. The name "Green Sand" comes from the fact that the sand mold is not "set", it is still in the "green" or uncured state even when the metal is poured in the mould. Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). Unlike the name suggests, "green sand" is not a type of sand on its own (that is, not greensand in the geologic sense), but is rather a mixture of:
- silica sand (SiO2), chromite sand (FeCr2O4), or zircon sand (ZrSiO4), 75 to 85%, sometimes with a
Impressum
Verlag: BookRix GmbH & Co. KG
Tag der Veröffentlichung: 10.01.2018
ISBN: 978-3-7438-4959-4Alle Rechte vorbehalten