tooling_English_模具英语第二章.ppt

Lesson 2  Mould for Threaded Components 　　It is frequently necessary to design a mould for a component which incorporates a thread. A thread is a form of undercut which normally increases the intricacy of the design. 　　The extent of complication varies and it is dependent on a number of factors, namely: 　　(i) type of thread: internal, external, continuous or discontinuous; (ii) method: thread form molded or incorporated by the use of a metal insert; (iii) type of productions: manual, semi-automatic or fully automatic. Moulds for internally threaded 　　　　　　　　　　　　　components 　　The internal thread comes within the broad definition of an internal undercut in that the thread forms a restriction which prevents the straight draw removal of the molding from the core①. 　　There are a number of alternative designs of mould to consider, and for reference these are summarized as follows: 　　(i) single interrupted thread design; (ii) stripping thread design; (iii) split core design; (iv) fixed threaded core design; (v) loose threaded core design; (vi) collapsible core design; (vii) unscrewing mould design. Stripping internal threads 　　The internally thread component may be stripped from the core using the basic stripper plate design providing the plastic material has sufficient elasticity during the ejection phase②. 　　The same principle applies here as to stripping any internal undercut, i.e. the molding must be free to expand during ejection to permit the molded undercut to ride over the restriction on the core③. 　　This means that the outside form of the component must be such that it can be formed in a cavity which is fully contained in one half of the mould. 　　A mould of this type is illustrated in Figure 2-1. This diagram shows a single impression mould for an internally threaded cap. 　　The mould opens initially and, because of molding shrinkage and the undercut form, the molding is withdrawn from the cavity. 　　During the latter stages of opening the stripper