Introduction to Basic Manufacturing Processes and ... - always yours

238 Introduction to Basic Manufacturing Processes and Workshop Technology
conventional sand molds. The mold walls of a permanent mold have thickness from 15 mm
to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. This
provides the desirable chilling effect. For faster cooling, fins or projections may be provided
on the outside of the permanent mold. Although the metallic mould can be used both for
ferrous and nonferrous castings but this process is more popular for the non-ferrous castings,
for examples aluminum alloys, zinc alloys and magnesium alloys. Usually the metallic molds
are made of grey iron, alloy steels and anodized aluminum alloys. There are some advantages,
dis-advantages and applications of metallic molding process which are discussed as under.
Advantages
(i) Fine and dense grained structure in casting is achieved using such mold.
(ii) No blow holes exist in castings produced by this method.
(iii) The process is economical.
(iv) Because of rapid rate of cooling, the castings possess fine grain structure.
(v) Close dimensional tolerance is possible.
(vi) Good surface finish and surface details are obtained.
(vii) Casting defects observed in sand castings are eliminated.
(viii) Fast rate of production can be attained.
(ix) The process requires less labor.
Disadvantages
(i) The surface of casting becomes hard due to chilling effect.
(ii) High refractoriness is needed for high melting point alloys.
(iii) The process is impractical for large castings.
Applications
1 This method is suitable for small and medium sized casting.
2 It is widely suitable for non-ferrous casting.
12.21 QUESTIONS
1. Explain briefly the main constituents of molding sand.
2. How do the grain size and shape affect the performance of molding sand?
3. How natural molding sands differ from synthetic sands? Name major sources of obtaining
natural molding sands in India?
4. How are binders classified?
5. Describe the process of molding sand preparation and conditioning.
6. Name and describe the different properties of good molding sand.
7. What are the common tests performed on molding sands?
8. Name and describe briefly the different additives commonly added to the molding sand for
improving the properties of the molding sand.
9. What are the major functions of additives in molding sands?
10. Classify and discuss the various types of molding sand. What are the main factors which
influence the selection of particular molding sand for a specific use?