2.7.2. Designing of moulded pieces Comparing with other details moulded ones have the following advantages:
-possibility of obtaining a product from raw materials by the shortest way;
-possibility of choosing any shape of detail;
-good damping properties of moulding materials;
-good corrosion resistant properties of steel and pig iron pieces,
comparing with ones of structural steel.
Designing moulded pieces, the demands, specified by peculiarities of separate production operations must be taken into account. First of all, these are the properties of metal itself: its fluidity, which wall thickness depends upon; dependence of strength upon thickness of wall; shrinkage during cooling and internal stresses and shrinkage holes, occurring with it.
Due to it, for simultaneous solidification thickness of internal walls must be done equal to about 0,8 thickness of external ones. Transitions from wall to wall are to be done with fillets. In all cases, when design permits, maximum radii of transition are" expedient to be used. Connection of walls at acute angles should be avoided. If it is inevitable, convergence radius must be not less, than 0,5 wall thickness. Walls with large difference of sections are expedient to be connected by means of V-shaped transition sections, and walls, meeting at obtuse angle, are connected by a radius in the limits up to 50 values of thickness. As a snail change of convergence radii influences quality of casting only slightly, then the radii should be unified.
In constructions of moulded pieces local accumulations of metal, bulges, massives are to be avoided. At the sections, where massives are inevitable, rapid quenching should be provided. It is useful to develop surfaces, contacting moulding sand by ribbing of walls. In order to improve filling of mould the connection of massive elements with the nearest walls by means of fillets, V-transitions, funnels and ribs should be strengthened. Corrugated and cambered walls are expedient to be used. Besides improvement of moulding conditions these mating increase rigidity and strength of casting.
Designing moulded piece peculiarities of models and moulds should be taken into account. They must be maximally simple. For this purpose, details must have plain surfaces, small amount of rods, minimum pattern drafts for extracting piece from a mould and minimum machining allowances.
Moulded pieces must allow simple descaling, simple removal of pouring and air gate, rods and flash.
Specified tolerances for dimensions of moulded pieces and cleanness of surface must correlate with the properties of moulded materials, dimensions of a detail and moulding method (table 2.6.).
Production of details by moulding is related to initial forming. Details may be obtained by plastic forming during plastic metal working too.
Table
Note: In numerator accuracy classes of dimensions and masses are indicated, in denominator - allowance rows, their smaller value relates to simple castings and conditions of their mass automated production, greater value- to complex castings of smaiHot and individual production.
Find Russian equivalents for the following wonts :
Moulded, advantages, obtaining, raw materials, damping properties, pig iron pieces, structural steel, fluidity, thickness, dependence, strength, shrinkage, cooling, internal stress, occurring, due to, simultaneous, solidification, internal walls, equal to about, external ones, transitions, fillets, to be expedient, at acute angles, avoided, convergence radius, obtuse angle, convergence, influences, bulges, massive, avoided, quenching, rub, fillets, funnels and ribs, strengthened, corrugated, cambered, mating, allowances, descaling, removal, pouring and air date, specified tolerances, correlate with.
Give English counterparts for the following words in the text:
Сырое, возможность, демпфирующие свойства, антикоррозийные, требования, жидкотекучесть, толщина, зависимость, усадка, охлаждение, раковины, одновременно, затвердевание, гантели, целесообразно, максимальные радиусы, переход, острый угол, сопряжение, клиновидный, тупой угол, радиус, значение, сношение, ускоренное охлаждение, поверхность, формовочная сеть, оребрение, целесообразно, гофрированные, припуск, очистка, применимый, стержень, слой, соотношение, первичное, формообразование, пластическое, давление.
Make up situations using the following questions :
1.What are the advantages of moulded pieces of work?
2.If you design moulded pieces will you take into account such
things as:properties of metal, its fluidity, its strength, its shrinkages during
cooling and shrinkage holes?
3.Why is the thickness of internal walls and external ones different?
4.When are fillets usually to be used?
5.Are maximum radii of translation ever used? Why?
6.When should radii be unified ? And why?
7.In what case is it useful to develop surfaces by ribbing of walls?
8.Speak about corrugated and cambered walls!
9.What are the peculiarities of models of moulded pieces?
10.Look at fig,N2.6! Study it carefully and describe it!
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