Designing Castings With Thin Sections

8/4/2019 Designing Castings With Thin Sections

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Designing castings with thin sections.By Tomovic, Mileta

Publication:Modern CastingDate: Wednesday, October 1 2003

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Casting processes provide design freedom by

allowing engineers to place metal where neededto meet desired criteria. Typically, functionaldemands of the part as well as induced stressdistribution result in a non-uniform massdistribution in a designed structure. This meansthat a casting shape can assume non-uniform

distribution of interconnected thick and thinsections.

An example of such a configuration ofinterconnected unequal sections is shown in Fig.1, in which heavy sections are surrounded withthin sections. This configuration does not allow

the gating system or side riser to feed the heavysections properly and fails to eliminate shrinkagedefects in these isolated masses. As a result,this type of mass distribution requites theapplication of strategically located top risers tofeed the isolated heavy sections.

[FIGURE 1 OMITTED]
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In this configuration, each heavy section mayrequire a separate top riser, resulting in low yield,increased labor for casting cleaning, andincreased final part cost. Sometimes,configurations like this are required to satisfy thestructural and functional demands imposed bythe operating conditions.

However, there ate certain instances that can

lend themselves to redesign to make the shapemore readily castable without additional risers. Inthose circumstances, a designer's knowledge ofthe metalcasting process or close collaborationbetween the designer and foundry engineer canplay a significant role in modifying the shape to

suit the process.

For example, the wheel shown in Fig. 2 has acentral hub that is isolated within a thin section.A side riser probably would not be sufficient tofeed the central section of the wheel so a top

riser on the hub would have to be used.[FIGURE 2 OMITTED]

An alternative solution to this problem would beto add a feeding pad on the cope side, whichwould connect the side riser and central hub

(Fig. 3). A feeding pad is essentially additionalmetal added to the casting that allows directional


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solidification to progress towards the side riser.The addition of a feeding pad would result in asolid casting while maximizing the yield byeliminating the heavier top riser. This type ofmodification can be made in many instanceswithout affecting the part's functionality and onlyminimally increasing its weight. In application,when the part is rotating around a central axis,the additional pad can be added on the oppositeside to maintain dynamic balance.

[FIGURE 3 OMITTED]

Another geometric modification that can beconsidered to improve component castability with

isolated masses is to reallocate the existinggeometric features. For example, ribs are usedto provide additional structural support to isolatedbosses. The size of the ribs is an important factorin casting since it determines how far the moltenmetal can flow and how will solidify.

If the ribs are too thin, this will promote rapidcooling and premature solidification and w resultin either a co shut during pouring inadequatefeeding , the boss, causing shrinkage defects.Alternately, large ribs will add the weight of thecasting. Hence, ribs should be dimensioned

properly to satisfy structural integrity whileallowing castability.


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Figure 4 shows an example of a casting that hasan isolated boss connected with three ribs. Theribs provide sufficient structural rigidity and sizeto feed the boss, but insufficient size to allowprop feeding of the boss. As a result,solidification shrinkage can occur on the boss,resulting in a casting defect.

[FIGURE 4 OMITTED]

A potential solution is to redistribute the centralrib onto two side ribs This would eliminate thecentral rib while increasing the size of the sideribs by one hall of the central rib. Thismodification would not affect structural integrity,

as the part would function as originally intended.However the size of the ribs will now allowproper feeding of the boss and eliminateshrinkage related defects.

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Designing Castings With Thin Sections