热加工作业考研题目答案分享——metal casting 2

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Metal casting homework two

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• 版权声明
• 1. Describe the differences between expendable and permanent molds.
• 11. List the advantages and limitations of die casting.
• 14. What are the advantages of the lost-foam casting process?
• 15. What are the reasons for the large variety of casting processes that have been developed over the years? Explain with specific examples.
• 21. What differences, if any, would you expect in the properties of castings made by permanent-mold versus sandcasting processes?
• 22. Would you recommend preheating the molds used in permanent-mold casting? Would you remove the casting soon after it has solidified? Explain your reasons.
• 49. Outline the casting processes that would be most suitable for making small toyss. Explain your choices.
• 55. Porosity that has developed in the boss of a casting is illustrated in Fig. P2.55. Show that the porosity can be eliminated simply by repositioning the parting line of this casting.

1. Describe the differences between expendable and permanent molds.

Firstly, for expectancy, expendable molds are temporary molds that will be destroyed for each casting process. Instead, permanent molds could be used many times.
Secondly, considering materials, expendable molds usually made by plastic such foam, but permanent molds usually made by metal such as alloys. Therefore, the former is more likely to be manufactured than the latter. Thus, permanent molds are more expensive than expendable molds.
Thirdly, they are utilized in different circumstances. The latter is a practical method for mass production and can be automated, while the former is applied for small batch production and is usually handmade.

11. List the advantages and limitations of die casting.

Advantages
1.Labour costs are generally low.
2.It is economical for mass production, because the process is semi- or fully automated.
3.The efficiency of the process is extremely high, which is due to the speed of producing is relatively fast.
4.The result products have less porosity.
Limitations
1.Equipment costs are relatively high.
2.Low-melting points alloys are ideal materials, because the molds would be damaged if the temperature is too high.
3.The result products can not be heat treated, because they contain gas inside, which will escape when the ambient temperature is relatively high, thus, they will develop cracks.

14. What are the advantages of the lost-foam casting process?

Firstly, the accuracy of the dimension is relatively high.
Secondly, machining allowance is comparatively low.
Thirdly, unlike sand casting, lost-foam casting can form a whole piece without parting. Thus, it can be applied to manufacture product with complex shape.

15. What are the reasons for the large variety of casting processes that have been developed over the years? Explain with specific examples.

Judging from the size, shape, productivity, application and cost of the result products, efficient and practical manufacturing method is applied. Thus, even though casting is just one technique of production, there are still numbers of types of casting.
For example, sand casting is applied to produce small amount of and relatively low-quality products. Another example is permanent mold that is utilized in mass production and comparatively high-quality products.

21. What differences, if any, would you expect in the properties of castings made by permanent-mold versus sandcasting processes?

higher surface finish
easier to be automated
save much more money when applied to mass production
closer dimensional tolerances
more uniform and better mechanical properties
higher production rate

22. Would you recommend preheating the molds used in permanent-mold casting? Would you remove the casting soon after it has solidified? Explain your reasons.

Yes, it is better to preheat the permanent-mold before casting because metal flow could be facilitated and thermal damage to the dies due to high temperature gradients would be reduced.
No, the casting should be allowed to cool thoroughly in the mold for safe consideration and also it can reduce the development of defects or distortion during shakeout. The larger the casting is, the much more time should be spent in cooling.

49. Outline the casting processes that would be most suitable for making small toyss. Explain your choices.

Die casting with multiple-cavity die, combination die or unit die can be utilized for producing small toys. Semi-centrifugal casting also has the same function.
One the one hand, it is difficult to create the shape which is small by hand, however, people can use die casting with metal mold which is reusable and can be manufactured by machine.
On the other hand, take multiple-cavity die as an example. It is space-saving and efficient because it can produce numbers of parts of the toys simultaneously. Thus, it is easier to reach large production.

55. Porosity that has developed in the boss of a casting is illustrated in Fig. P2.55. Show that the porosity can be eliminated simply by repositioning the parting line of this casting.

Answer:
The boss should be turn -90^o so that it can be supplied with molten metal and it can solidify as well as shrink more effectively. The result is shown in figure 1.

Figure 1 Repositioning the parting line

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