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1、Lesson 14 Extrusion,Extrusion is the process in which material is forced by compression to flow through a suitably shaped aperture in a die, usually to give a product of a smaller but uniform cross-sectional area.,The process can be carried out with the starting material at ambient temperature, i.e.
2、,cold extrusion, or heated to a temperature which gives optimum results for the conditions applying and the product requirements, i.e. hot or warm extrusion.,In most cases, the metal is in the form of a solid or hollow cylinder and is moved in a containing tool in the direction in which the product
3、emerges from the die aperture.,However, there are some variations on this such as backward-extrusion, tube extrusion, compound extrusion, and sideways (lateral, transverse) extrusion.,Most metals can be extruded in one way or another, but obviously some more easily than others.,The common metals tha
4、t can be extruded at ambient temperatures are lead, aluminium, copper, low carbon steels and a few alloys of these metals. The softer semi-precious metals can also be extruded cold if required.,Hot extrusion,Traditionally, in hot extrusion a billet of metal is forced through a shaped hole at one end
5、 of a container by the action of a ram pressing from the other end.,The container bore is of uniform cross-section and the ratio of its area to that of the product is known as the extrusion ratio.,In the most common form of the process, direct extrusion, the ram moves through the bore of the contain
6、er, the die being clamped rigidly against the end of the container. The extrudate exits in the same direction as the ram moves.,In indirect extrusion, the die is fitted on to the ram which is forced against the billet held in a container closed at the far end.,In this case the extrudate leaves the d
7、ie through the hollow ram. Although the container bore is normally round it may also be rectangular to permit extrusion of very wide products.,In direct extrusion, a dummy block is generally interposed between the hot billet and the ram end to minimize damage to the ramthe dummy block being a cheap,
8、 easily replaceable item.,For extrusion of hollow sections or tubes the ram carries a mandrel, which serves to form the inner surface of a hollow extrusion and which on some presses may be driven independently of the main ram to permit piercing of solid billets.,The main tooling components for the h
9、ot extrusion process at therefore the container, the die assembly, the dummy block and the mandrel.,Cold extrusion,There are four basic extrusion operations (shown in Figure 14-1), namely forward tube-extrusion, backward can-extrusion, compound extrusion and forward tube-extrusion.,Figure 14-1 Basic
10、 extrusion operations,The punch (see Figure 14-2) is the portion of the tool that forms the internal surface of the workpiece in a can-extrusion, or that pushes the workpiece through a die in rod, tube or open die extrusion.,In can extrusion the punch is highly stressed by compressive and bending lo
11、ading, and the same time subject to heavy wear and increases in temperature at the punch nose.,Compressive stresses of over 2200Nmm2 can occur and on the return stroke, tensile stresses are encountered due to the stripping action.,In forward rod and tube extrusion the punch does not suffer much from
12、 wear but the compressive stresses are similar to can extrusion punches.,Figure 14-2 Punch and punch /mandrel configurations,The die is the item of tooling that contains the work piece and forms the external shape of the product.,Normally dies have to have at least one support ring as the internal p
13、ressures are high enough to cause transverse cracking due to tangential or triaxial stresses respectively.,If the insert is carbide then another stress ring is probably required depending on the ratio of inside diameter to overall outside diameter.,Notes, Extrusion is the process in which material i
14、s forced by compression to flow through a suitably shaped aperture in a die, usually to give a product of a smaller but uniform cross-sectional area.,挤压是在压力作用下将金属坯料从凹模上相应形状的孔口中挤压，（使金属产生塑性变形）获得制品的工艺过程。挤压件的尺寸较小，具有相同的横截面。,句中的in which引导的是后置定语从句修饰the process，usually to give a product of a smaller but uni
15、form cross-sectional area不定式表示目的，作状语。, In can extrusion the punch is highly stressed by compressive and bending loading, and the same time subjects to heavy wear and increases in temperature at the punch nose.,在空心件挤压中，凸模在压力和弯曲载荷的作用下产生很高的应力，且磨损严重，凸模前面温度也会（迅速）升高。,句中的is，subjects，increases是三个并列的动词，by co
16、mpressive and bending loading介宾短语作状语修饰the punch is highly stressed。, Normally dies have to have at least one support ring as the internal pressures are high enough to cause transverse cracking due to tangential or triaxial stresses respectively.,由于切向或轴向压力分别作用使内部压力升高，引发横向破裂时，凹模（外侧）必须至少有一个预紧套圈。,句中的as引出的是时间状语从句，due to tangential or triaxial stresses respectively介宾短语作状语表示原因。,
