,机电技术专业英语,2,Lesson Fifteen,Text,Mould is a fundamental technological device for industrial production. Industrially produced goods are formed in moulds which are designed and built specially for them. The mould is the core part of manufacturing process because its cavity gives its shape. There are many kinds of mould, such as casting & forging dies, ceramic moulds, diecasting moulds, drawing dies, injection moulds, glass moulds, magnetic moulds, metal extruding moulds, plastic and rubber moulds, plastic extruding moulds, powder metallurgical moulds, compressing moulds, etc. The following is the introduction of some of the moulding process and the corresponding moulds used.,Introduction to Mould,Lesson Fifteen,Compression Moulding Compression moulding is the least complex of the moulding processes and is ideal for large parts or lowquantity production. For lowquantity requirements it is more economical to build a compression mould than an injection mould. Compression moulds are often used for prototyping, where samples are needed for testing fit and forming into assemblies. This allows for further design modification before building an injection mould for highquantity production. Compression moulding is best suited for designs where tight tolerances are not required. A compression mould is simply two plates with cavities cut into either one or both plates. Additional plates between the top and bottom plates can be included to create cavities in the moulded part. Figure B.15 shown below is an example of a basic twoplate single cavity mold. The mould does not require heater elements or temperature controllers. The moulding temperature is fully controlled by the pressure it is running in.,Lesson Fifteen,Figure B.15 A compression mould 1. top plate； 2. opening bar slot； 3. handle； 4. dowel pin & bushing；5. flash & tear trim gate； 6. part cavity； 7. bottom plate,Due to the simplicity of the mould, it is the most economical mould to buy. And the economical mould/tool cost keeps small quantity running affordable.,Lesson Fifteen,Injection Moulding Injection moulding is the most complex of the moulding processes. Due to the more complex design of the injection mould, it is more expensive to purchase than a cast or compression mould. Although tooling costs can be high, cycle time is much faster than other processes and the part cost can be low, particularly when the process is automated. Injection moulding is well suited for moulding delicately shaped parts because high pressure (as much as 29 000psi) is maintained on the material to push it into every corner of the mould cavity. Moulds used in injection moulding consist of two halves：one stationary and one movable. The stationary half is fastened directly to the stationary platen and is in direct contact with the nozzle of the injection unit during operation. The movable half of the mould is secured to the movable platen and usually contains the ejector mechanism. The use of a balanced runner system carries the plastic from the sprue to each individual cavity.,Lesson Fifteen,An injection mould can be a simpler two plate mould with a runner system to allow the rubber compound to be injected into each cavity from the parting line or a more complex mould with a number of plates, an ejector system and additional heating elements within the core. Figure B.16 shown below are examples of basic threeplate and twoplate multicavity injection moulds.,Figure B.16 An injection mould 1. injection runner; 2. nozzle bushing; 3. stripper bolt; 4. top plate; 5. sprue； 6. opening bar slot; 7. bottom plate;8. flash 13. movable plate,Lesson Fifteen,Cast Moulding There are two types of casting, open casting and pressure casting. With open casting, the liquid mixture is poured into the open cavity in the mould and allowed to cure. With pressure casting, the liquid mixture is poured into the open cavity, the cap is put in place and the cavity is pressurized. Pressure casting is used for more complex parts and when moulding foam materials. In principle, pressure casting is identical to injection moulding with a different class of materials. Cast moulding can, in fact, produce parts that have identical geometries to injectionmoulded ones. In many cases, injection moulding has been a substitute for casting moulding due to decreased parts cost. However, for structural parts, particularly those parts with thickwalls, cast moulding can often be the better selection.,Lesson Fifteen,The construction of the mould for cast moulding is almost identical to that of moulds for injection moulding. It consists of two major sectionsthe ejector half and the cover half which meet at the parting line. The cavities and cores are usually machined into inserts that are fitted into each of these halves. The cover mould half is secured to stationary platen, while the ejector mould half is fastened to the movable platen. The cavity and machining core must be designed so that the mould halves can be pul