Casting for the Home Workshop by Henry Tindell Dave Cooper

Author:Henry Tindell,Dave Cooper
Language: eng
Format: epub
ISBN: 9781785003547
Publisher: Lightning Source Inc.

Figure 40 Shrinkage cavities in alloys (typical).

With a uniform wall thickness casting, the objective is to restrict the freezing distance in order to avoid the plane of weakness associated with centreline shrinkage. These solidification modes can result in the defects shown in Figure 40, and for critical work it is preferable to cut a sample casting along the centreline, to check for such problems.

Figure 41 Two heavy sections and connecting member shrinkage effects.

However, the norm for castings is for non-uniform wall thicknesses and sections. Hence, the large sections can often be utilized to feed the smaller ones, but an understanding of solidification is required – often apparent with a careful examination of the drawing, and proven with sample cast sectioning. An example of feeding/chilling/heat application, or control, is shown in Figure 41, using these devices in order to feed late-solidification areas.

GATING DESIGN

Gating is the important means by which the liquid metal is conducted, in a controlled manner; from the mould feeder, to the casting, and onward to exit at the riser(s). It should be appreciated that this is a rapid process, often taking only seconds from beginning to end, and therefore reactions are fast – with all the expense and time-consuming business of heating the furnace charge to a superheated liquid, and all of the mould preparation – and proven, successfully or otherwise, in the blink of an eye as the liquid rushes through the gating system and into the mould cavity.

Mould Filling Parameters

Normally, a fast mould filling is sought because in thin sections the rapid loss of heat from the liquid, through the mould walls, can lead to a premature freezing before the cavity is properly filled. The resulting surface defects, such as scabs and cold laps, require reworking, as described in Chapter 6, ‘Post-Casting Processes’.

One method of reducing this effect is to increase the superheat at pouring, but this risks introducing problems with gas absorption and erosion of the mould walls.

Liquid Flow

The liquid flow through the gating system, essentially a pipe-conduit arrangement connecting feeder to mould cavity, has been touched upon in the section on behaviour of liquid metal flowing through pipes. The behaviour of, for instance, liquid en route to producing cast iron, is similar in fluidity to that of water. Therefore, the geometry and rate of flow determines if lamellar-, boundary-, or turbulent-flow will occur in the gating system, during the casting process. Whilst generally a good plan to design for lamellar flow as an ideal, in practice casting flows inevitably involve turbulent flow in the bulk fluid, where not of great concern. However, of crucial importance is the flow rate on pouring, at the liquid/free surface interface. And for this surface, Re is not an appropriate measure. Instead, we need to invoke the Weber number (We), after Moritz Weber (1871–1951), which relates the liquid inertia to its surface tension, thereby giving a measure described as ’surface turbulence’, see Bibliography, Campbell, 2011. The Weber number can be defined as:

We = ( ρ V2 L )/γ

where

ρ =

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