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9.1 Introduction

Proper design of the venting system is one of the requirements for reducing air/gas porosity. Porosity due to entrainment of gases constitutes a large portion of the total porosity, especially when the cast walls are very thin (see Figure [*]). The main causes of air/gas porosity are insufficient vent area, lubricant evaporation (reaction processes), incorrect placement of the vents, and the mixing processes. The present chapter considers the influence of the vent area (in atmospheric and vacuum venting) on the residual gas (in the die) at the end of the filling process.

Figure: The relative shrinkage porosity as a function of the casting thickness

Atmospheric venting, the most widely used casting method, is one in which the vent is opened to the atmosphere and is referred herein as air venting. Only in extreme cases are other solutions required, such as vacuum venting, Pore Free Technique (in zinc and aluminum casting) and squeeze casting. Vacuum is applied to extract air/gas from the mold before it has the opportunity to mix with the liquid metal and it is call vacuum venting. The Pore Free technique is a variation of the vacuum venting in which the oxygen is introduced into the cavity to replace the air and to react with the liquid metal, and therefore creates a vacuum [#!poro:genickreac!#]. Squeeze casting is a different approach in which the surface tension is increased to reduce the possible mixing processes (smaller Re number as well). The gases in the shot sleeve and cavity are made mostly of air and therefore the term ``air'' is used hereafter. These three ``solutions'' are cumbersome and create a far more expensive process. In this chapter, a qualitative discussion on when these solutions should be used and when they are not needed is presented.

Obviously, the best ventilation is achieved when a relatively large vent area is designed. However, to minimize the secondary machining (such as trimming), to ensure freezing within the venting system, and to ensure breakage outside the cast mold, vents have to be very narrow. A typical size of vent thicknesses range from 1-2[]. These conflicting requirements on the vent area suggest an optimum area. As usual the ``common'' approach is described the errors are presented and the reformed model is described.


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