DESIGN FOR CASTING

Instructional Objectives

The purpose of this lecture was to outline various casting processes, several defects that appear in casting and corresponding remedial measures, and general recommendations to achieve a good quality casting. 

Casting Processes

The casting process involves pouring of liquid metal in to a mold cavity and allowing it to solidify to obtain the final casting. The flow of molten metal into the mold cavity depends on several factors like minimum section thickness of the part, presence of corners, non-uniform cross-section of the cast, and so on. The casting processes can be broadly classified into expendable mold casting and permanent mold casting processes.  

Expendable Mold Casting

 Expendable mold casting was a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) molds. All these methods use temporary, non-reusable molds. After the molten metal in the mold cavity solidifies, the mold was broken to take out the solidified cast. Expendable mold casting processes are suitable for very complex shaped parts and materials with high melting point temperature. However, the rate of production was often limited by the time to make mold rather than the casting itself. Following are a few examples of expendable mold casting processes.

Sand Casting

 Sand casting was widely used for centuries because of the simplicity of the process. The sand casting process involves the following basic steps: (a) place a wooden or metallic pattern in sand to create a mold, (b) fit in the pattern and sand in a gating system, (c) remove the pattern, (d) fill the mold cavity with molten metal, (e) allow the metal to cool, and (f) break the sand mold and remove the casting. The sand casting process was usually economical for small batch size production. The quality of the sand casting depends on the quality and uniformity of green sand material that was used for making the mold. Figure 3.2.1 schematically shows a two-part sand mold, also referred to as a cope-and-drag sand mold. The molten metal was poured through the pouring cup and it fills the mold cavity after passing through downsprue, runner and gate. The core refers to loose pieces which are placed inside the mold cavity to create internal holes or open section. The riser serves as a reservoir of excess molten metal that facilitates additional filling of mold cavity to compensate for volumetric shrinkage during solidification. Sand castings process provides several advantages. It can be employed for all types of metal. The tooling cost was low and can be used to cast very complex shapes. However sand castings offer poor dimensional accuracy and surface finish

Figure 3.2.1 Schematic set-up of sand molding / casting process [1] 

Shell molding

Shell molding was similar to sand casting. Normally a machined pattern of grey iron or aluminum was used in this process. The pattern was heated to 2500C to 2600C and the sand resin mixture was poured over its surface. The heated pattern melts the resin creating bonds between the sand grains. After a dwell period the pattern and sand inverted and extra sand was cleaned off. The mold cavity was now formed by a hardened shell of sand. The mold was then heated in an oven for further curing. The shell thus formed constitutes one half of the mold. Two such halves are placed over one another to make the complete mold. The sands used in shell molding process are usually finer than the same used in sand casting. This process was ideal for complex shaped medium sized parts. Figure 3.2.2 represents the steps of shell mold casting. This method can be employed for making an integrate shapes, thin and sharp corners small projection which are not possible in green sand mold. Subsequent machining operations are also reduced due to more dimensional accuracy.

Figure 3.2.2 Schematic set-up of shell mold casting process [1] 

Investment casting Investment casting was also referred to as lost-wax casting since the pattern was made of wax. The wax patterns are first dipped into a slurry of refractory material and subsequently, heated so that the wax melts away keeping a refractory mold. The mold was then further cured to achieve proper strength. Very high melting temperature material can be cast in investment casting process because of the refractory mold. Figure 3.2.3 schematically shows an investment casting process.  The molten metal was poured into the mold and was taken out after solidification by breaking the mold. Very high dimensional accuracy and surface finish can be achieved in investment casting process. However, the tooling cast was usually high and hence, investment casting process was primarily used for large size batch production or for specific requirements of complex shape or casting of very high melting temperature material. Vacuum Casting In this process, a mixture of fine sand and urethane was molded over metal dies and cured with amino vapor. The molted metal was drawn into the mold cavity through a gating system from the bottom of the mold. The pressure inside the mold was usually one-third of the atmospheric pressure. Because the mold cavity was filled under vacuum, the vacuum casting process was very suitable for thin walled, complex shapes with uniform properties. Figure 3.2.4 schematically shows typical vacuum casting process.  

                      

Figure 3.2.3 Schematic set-up of and sequences involved in investment casting process [2]  

Figure 3.2.4 Schematic set-up of vacuum casting process [3] 

Plaster mold casting

Plaster mold casting, also called rubber plaster molding (RPM), was a method of producing aluminum or zinc castings by pouring liquid metal into typical plaster (gypsum) molds. The plaster molds used as negative molds are created from gypsum and water. After mixing and forming the mold shape, the plaster molds are dried and baked in an oven to remove any water remaining in the mold. Often, the molds are made in two halves – i.e. cope and drag molds – and the halves of the plaster molds are clamped together with any required cores positioned appropriately in the mold. Molten metal was subsequently poured into the negative plaster mold and allowed to dry. The final part was taken out after breaking the mold. The final cast may require machining operation depending upon the requisite dimensional accuracy. This process was often used for producing prototypes of final part or component.  

Ceramic mold casting

The ceramic mold casting was used to produce split molds from a quick-setting ceramic investment. Blended ceramic particles are mixed rapidly with liquid binder to form free flowing slurry that was poured quickly over a pattern. The casting does not require wax patterns and there are no limits to size or alloy. Foundry applications are large and complex impellers, valve bodies, and military hardware. The green strength of ceramic mold casting was high. Ceramic mold casting method uses a ceramic slurry prepared by mixing fine grained refractory powders of Zircon (ZrSiO4), Alumina (Al2O3), Fused Silica (SiO2 ) and a liquid chemical binder (Alcohol based Silicon Ester) for making the mold. Figure 3.2.5 schematically shows a set-up for ceramic mold casting process. 

   

  Figure 3.2.5 Ceramic mold casting process [5] 

Permanent Mold Casting processes

Permanent mold casting processes involve the use of metallic dies that are permanent in nature and can be used repeatedly. The metal molds are also called dies and provide superior surface finish and close tolerance than typical sand molds. The permanent mold casting processes broadly include pressure die casting, squeeze casting, centrifugal casting, and continuous casting.

Pressures die casting

 The pressure die casting process was the most common for Al, Zn and Mg castings (low melting point). The liquid metal was injected into the mold under high pressure and allowed to solidify at the high pressure. The solidified cast was then taken out of the mold or the die which was ready for the next cast. Pressure die casting was suitable for large batch size production. Two types of pressure die casting are generally common in the industry – (a) high pressure die casting and (b) low pressure die casting. Very high production rates can be achieved in pressure die casting process with close dimensional control of the casting.  However, the process was not suitable for casting of high melting temperature materials as the die material had to withstand the melting (or superheated) temperature of the casting. Pressure die castings also contain porosity due to the entrapped air. Furthermore, the dies in the pressure die casting process are usually very costly.

Figure 3.2.6 schematically presents the hot-chamber and the cold-chamber die casting processes. In the hot-chamber die casting process, the furnace to melt material was part of the die itself and hence, this process was suitable primarily for low-melting point temperature materials such as aluminum, magnesium etc.          

Figure 3.2.6 Set-up of (a) hot-chamber and (b) cold-chamber die casting processes [6] 

Squeeze casting

Molten metal was poured into a metallic mold or die cavity with one-half of the die squeezing the molten metal to fill in the intended cavity under pressure as shown in Figure 3.2.7. Fiber reinforced casting with SiC or Al2O3  fibers mixed in metal matrix have been successfully squeeze cast and commercially used to produce automobile pistons. However, squeeze casting was limited only to shallow part or part with smaller dimensions.      

Figure 3.2.7 Schematic set-up of squeeze casting process [7]

Centrifugal casting

In centrifugal casting process, the molten metal poured at the center of a rotating mold or die. Because of the centrifugal force, the lighter impurities are crowded towards the center of the case. For producing a hollow part, the axis of rotation was placed at the center of the desired casting. The speed of rotation was maintained high so as to produce a centripetal acceleration of the order of 60g to 75g.  The centrifuge action segregates the less dense nonmetallic inclusions near to the center of rotation that can be removed by machining a thin layer. No cores are therefore required in casting of hollow parts although solid parts can also be cast by this process.  The centrifugal casting was very suitable for axisymmetric parts. Very high strength of the casting can be obtained. Since the molten metal was fed by the centrifugal action, the need for complex metal feeding system was eliminated. Both horizontal and vertical centrifugal castings are widely used in the industry. Figure 3.2.8 schematically shows a set-up for horizontal centrifugal casting process. Figure 3.2.9 typically shows large pipes that are made using centrifugal casting process. 

     

Figure 3.2.8 Schematic set-up of horizontal centrifugal casting process [7]   

 

Figure 3.2.9 Metallic pipes made using centrifugal casting process [7]   

Continuous casting

Continuous casting process was widely used in the steel industry. In principle, continuous casting was different from the other casting processes in the fact that there was no enclosed mold cavity. Figure 3.2.10 schematically shows a set-up for continuous casting process. Molten steel coming out from the furnace was accumulated in a ladle. After undergoing requisite ladle treatments, such as alloying and degassing, and arriving at the correct temperature, the ladle was transported to the top of the continuous casting set-up. From the ladle, the hot metal was transferred via a refractory shroud (pipe) to a holding bath called a tundish. The tundish allows a reservoir of metal to feed the casting machine. Metal was then allowed to pass through a open base copper mold. The mold was water-cooled to solidify the hot metal directly in contact with it and removed from the other side of the mold. The continuous casting process was used for casting metal directly into billets or other similar shapes that can be used for rolling. The process involves continuously pouring molten metal into a externally chilled copper mold or die walls and hence, can be easily automated for large size production. Since the molten metal solidifies from the die wall and in a soft state as it comes out of the die wall such that the same can be directly guided into the rolling mill or can be sheared into a selected size of billets.

          

Figure 3.2.10 Schematic set-up of continuous casting process [8]

Defects in Casting Processes

Figure 3.2.11 schematically shows various defects that are experienced during casting, in particular, sand casting processes. A brief explanation of some of the significant defects and their possible remedial measures are indicated in the text to follow.

Shrinkage

Shrinkage of molten metal as it solidifies was an important issue in casting. It can reduce the 510% volume of the cast. Gray cast iron expands upon solidification due to phase changes. Need to design part and mold to take this amount into consideration. The thickness of the boss or pad should be less than the thickness of the section of the boss adjoins and the transition should be gradual. The radius for good shrinkage control should be from one half to one third of the section thickness. Shrinkage defect can be reduced by decreasing the number of walls and increasing the draft angle.

 

Figure 3.2.11 Schematic pictorial presentation of various casting defects [2]

Porosity

Porosity was a phenomenon that occurs in materials, especially castings, as they change state from liquid to solid during the manufacturing process. Casting porosity had the form of surface and core imperfections which either effects the surface finish or as a leak path for gases and liquids. The poring temperature should be maintained properly to reduce porosity. Adequate fluxing of metal and controlling the amount of gas-producing materials in the molding and core making sand mixes can help in minimizing this defect.

 Hot tear

Hot tears are internal or external ragged discontinuities or crack on the casting surface, caused by rapid contraction occurring immediately after the metal solidified. They may be produced when the casting was poorly designed and abrupt sectional changes take place; no proper fillets and corner radii are provided, and chills are inappropriately placed. Hot tear may be caused when the mold and core have poor collapsibility or when the mold was too hard causing the casting to undergo severe strain during cooling. Incorrect pouring temperature and improper placement of gates and risers can also create hot tears. Method to prevent hot tears may entail improving the casting design, achieving directional solidification and even rate of cooling all over, selecting proper mold and poured materials to suit the cast metal, and controlling the mold hardness in relation to other ingredients of sand.

Scar 

It was usually found on the flat casting surface. It was a shallow blow.

Blowhole

Blowholes are smooth round holes that are clearly perceptible on the surface of the casting. To prevent blowholes, moisture content in sand must be well adjusted, sand of proper grain size should be used, ramming should not be too hard and venting should be adequate.

Blister

This was a scar covered by the thin layers of the metal.

Dross

The lighter impurities are appearing on the top of the cast surface was called the dross. It can be taken care of at the pouring stage by using items such as a strainer and a skim bob.  

Dirt  Sometimes sand particles dropping out of the cope get embedded on the top surface of a casting. When removed, these leave small angular holes was known as dirts. 

Wash

It was a low projection on the drag surface of a casting commencing near the gate. It was caused by the erosion of sand due to high velocity liquid metal.

Buckle

It refers to a long fairly shallow broad depression at the surface of a casting of a high temperature metal. Due to very high temperature of the molten metal, expansion of the thin layered of the sand at the mold face takes place. As this expansion was obstructed by the flux, the mold tends to bulge out forming a V shape.

Rat tail

It was a long shallow angular depression found in a thin casting. The cause was similar to buckle.

Shift

A shift results in a mismatch of the sections of a casting usually as a parting line. Misalignment was common cause of shift. This defect can be prevented by ensuring proper alignment of the pattern for die parts, molding boxes, and checking of pattern flux locating pins before use.

Warped casting

Warping was an undesirable deformation in a casting which occurs during or after solidification. Large and flat sections are particularly prone to wrap edge. Wrap edge may also be due to insufficient gating system that may not allow rapid pouring of metal or due to low green strength of the sand mold or inadequate / inappropriate draft allowance in the pattern / mold cavity.

Metal Penetration and Rough Surfaces

 This defect appears as an uneven and rough external surface of the casting. It may be caused when the sand had too high permeability, large grain size, and low strength. Soft ramming may also cause metal penetration.

Fin

A thin projection of metal, not intended as a part of casting, was called a fin. Fins occur at the parting of the mold or core sections. Molds and cores in correctly assembled will cause the fin.

High metal pressures due to too large downsprue, insufficient weighing of the molds or improper clamping of flasks may again produce the fin defect.

Cold Shut and Mis-Run

A cold shut was a defect in which a discontinuity was formed due to the imperfect fusion of two streams of metal in the mold cavity. The reasons for cold shut or mis-run may be too thin sections and wall thickness, improper gating system, damaged patterns, slow and intermittent pouring , poor fluidity of metal caused by low pouring temperature, improper alloy composition, etc. 

Inspections of Casting

Visual inspection

Visible defects that can be detected provide a means for discovering errors in the pattern equipment or in the molding and casting process. Visual inspection may prove inadequate only in the detection of sub surface or internal defects.

Dimensional inspection

Dimensional inspection was one of the important inspection for casting. When precision casting was required, we make some samples for inspection the tolerance, shape size and also measure the profile of the cast. This dimensional inspection of casting may be conducted by various methods:

• Standard measuring instruments to check the size of the cast.

• Contour gauges for the checking of profile, curves and shapes

• Coordinate measuring and Marking Machine

• Special fixtures

X-Ray Radiography

In all the foundries the flaw detection test are performed in the casting where the defects are not visible. This flaw detection test was usually performed for internal defects, surface defects etc. These tests are valuable not only in detecting but even in locating the casting defects present in the interior of the casting. Radiography was one of the important flaw detection test for casting. The radiation used in radiography testing was a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light.  The radiation can come from an X-ray generator or a radioactive source.

Magnetic particle inspection 

This test was used to reveal the location of cracks that extend to the surface of iron or steel castings, which are magnetic nature. The casting was first magnetized and then iron particles are sprinkled all over the path of the magnetic field. The particles align themselves in the direction of the lines of force. A discontinuity in the casting causes the lines of the force to bypass the discontinuity and to concentrate around the extremities of the defect.

Fluorescent dye-penetration test

This method was very simple and applied for all cast metals. It entails applying a thin penetration oil-base dye to the surface of the casting and allowing it to stand for some time so that the oil passes into the cracks by means of capillary action. The oil was then thoroughly wiped and cleaned from the surface. To detect the defects, the casting was pained with a coat of whitewash or powdered with tale and then viewed under ultraviolet light. The oil being fluorescent in nature, can be easily detect under this light, and thus the defects are easily revealed.

Ultrasonic Testing

Ultrasonic testing used for detecting internal voids in casting was based on the principle of reflection of high frequency sound waves. If the surface under test contains some defect, the high frequency sound waves when emitted through the section of the casting, will be reflected from the surface of defect and return in a shorter period of time. The advantage this method of testing over other methods was that the defect, even if in the interior, was not only detected and located accurately, but its dimension can also be quickly measured without in any damaging or destroying the casting.

Fracture test

Fracture test was done by examining a fracture surface of the casting. it was possible to observe coarse graphite or chilled portion and also shrinkage cavity, pin hole etc. The apparent soundness of the casting can thus be judged by seeing the fracture.

Macro-etching test (macroscopic examination)

 The macroscopic inspection was widely used as a routine control test in steel production because it affords a convenient and effective means of determining internal defects in the metal. Macroetching may reveal one of the following conditions:

• Crystalline heterogeneity, depending on solidification

• Chemical heterogeneity, depending on the impurities present or localized segregation and

• Mechanical heterogeneity, depending on strain introduced on the metal, if any. 

Sulphur Print test

Sulphur may exist in iron or steel in one of two forms; either as iron sulphide or manganese sulphide. The distribution of sulphur inclusions can easily examined by this test.

Microscopic Examination

 Microscopic examination can enable the study of the microstructure of the metal alloy, elucidating its composition, the type and nature of any treatment given to it, and its mechanical properties. In the case of cast metals, particularly steels, cast iron, malleable iron, and SG iron, microstructure examination was essential for assessing metallurgical structure and composition. Composition analysis can also be done using microscopic inspection. Distribution of phase can be observed by metallographic sample preparation of cast product. Grain size and distribution, grain boundary area can be observed by this procedure. Distribution of nonmetallic inclusion can also be found from this process of inspection.

Chill Test

Chill test offers a convenient means for an approximate evaluation of the graphitizing tendency of the iron produced and forms an important and quick shop floor test for ascertaining whether this iron will be of the class desired. In chill test, accelerated cooling rate was introduced to induce the formation of a chilled specimen of appropriate dimension. It was then broken by striking with a hammer in such a manner that the fracture was straight and midway of its length. The depth of chill obtained on the test piece was affected by the carbon and silicon present and it can therefore be related to the carbon equivalent, whose value in turn determines the grade of iron. 

Design Recommendations for Casting

1. Compensate the shrinkage of the solidified molten metal by making patterns of slightly oversize.

2. In sand casting, it was more economical and accurate if the parting line was on a flat plane [Figure 3.2.12(a)]. Contoured parting lines are not economical. Further, some degree of taper, or draft was recommended to provide to the pattern for its easy removal [Figure 3.2.12(b)]. The recommended draft angles for patters under various conditions are given elsewhere [8].  

3.  In sand casting, it was recommended to attach the raiser near to the heavier section. The thinnest sections are farthest from the raiser and solidify first and then the solidification proceeds toward the direction of raiser i.e. towards the heavier section [Figure 3.2.12(c)].

4. Sharp corners in a casting design cause uneven cooling and lead to formation of hot spots in the final cast structure. Moreover sharp corner in a casting structure acts as a stress raiser. Rounding the corner decreases the severity of the hot spot and lessens the stress concentration [Figure 3.2.12(d)].

5. Abrupt changes in sections should be avoided. Fillets and tapers are preferable to sharp steps [Figure 3.2.12(e)].

6. The interior walls and sections are recommended to be 20% thinner than the outside members to reduce the thermal and residual stresses, and metallurgical changes [Figure 3.2.12(f)].

7. When a hole was placed in a highly stressed section, add extra material around the hole as reinforcement [Figure 3.2.12(g)].

8. To minimize the residual stresses in the gear, pulley or wheel casting, a balance between the section size of the rim, spokes and hub was maintained [Figure 3.2.12 (h)].

9. An odd number of curved wheel spokes reduce cast-in-residual stresses [Figure 3.2.12(i)]. 

10. Similar to sand casting, permanent mold castings also require draft for the easy withdrawal of the casting from the mold. The recommended draft angles are given elsewhere [8].

11. Due to pattern shrinkage, investment shrinkage and metal shrinkage during solidification, there was always a tendency for an investment part to “dish” (develop concave surfaces where flat surfaces are specified). This condition takes place in areas of thick cross section. Dishing was minimized by designing parts with uniformly thin walls [Figure 3.2.12(j)].    

Figure 3.2.12 Recommended designs for casting [6] (continued).

12. When keys and keyways are required, the recommended ratio of width to depth was 1.0 or more. The minimum castable key width was 2.3 mm for ferrous metals and 1.5 mm for nonferrous metals [Figure 3.2.13].

13. Heavy bosses connecting to the surface can cause “sinks” due to the shrinkage of the large mass of the metal in the boss during cooling. This shrinkage problem can be reduced by moving the boss away from the surface and connecting it to the surface with a short rib [Figure 3.2.14].          

Figure 3.2.13 The recommended casting design for key and key ways for (a) ferrous metal,   (b) non ferrous metal [6].        

Figure 3.2.14 The recommended cast design to avoid surface shrink [8].