Causes and cures for shell-related defects.

Causes and Cures For Shell-Related Defects

Defects are part of nearly every foundry's operations, and their causes are many and varied. The booming investment casting industry shares these problems, of course, but its defects are largely shell-related, and differ somewhat from other metalcasting processes.

What follows is a brief outline defining seven common investment casting defects in relation to the shell system used, and several ways to prevent them. Finally, the four most common causes of these defects (drying conditions, slurry composition, dewax/firing techniques and cluster preparation) are examined.

Defects/Causes:

Inclusions

This defect is generally nonmetallic and usually can be detected on the surface or a few microns below the casting surface. Many subsurface inclusions become evident during welding repair of a pinhole-type defect. Jagged in shape, inclusions sometimes will have the ceramic from the shell, which caused the condition, embedded. An example is shown in Fig. 1.

The major causes of shell-related inclusion defects are:

Mold Cracks - If a burned out shell has cracked, erosion from the edges of the crack may occur, disbursing refractory particles throughout the casting. Often, if a seal dip is added to a cracked shell, the slurry will run into the crack and break off during shell firing or pouring.

Mold Handling - Poor shell handling may cause loose stucco particles to fall into the mold prior to pouring. The two most common problems are cup orientation in the firing cycle, or a jagged cup collar remaining after dewaxing.

Spalling or Scabbing - The ceramic shell that spalls or scabs not only causes a positive metal-type defect, but the ceramic is generally distributed throughout the casting to cause inclusions.

Other common inclusion sources are pattern design, ceramic in melt returns, spalled ladle or furnace linings and improperly reclaimed wax.

Prime Coat Buckle

Prime coat buckling defects usually occur on flat surfaces as an island of surplus metal, and normally have hairline fins associated with them, usually at a casting's edges. Figure 2 illustrates this defect.

This problem is caused by predip and/or second coat slurry entering a crack on the surface of the first coat. The binder and/or slurry spreads and dries beneath the first dip. Although the crack is often sealed by subsequent dips, the casting will have an irregular surface. The most common shell-related causes for the formation of the primary slurry cracks include the following:

Environmental Conditions - Temperature and humidity fluctuations in the dip and drying area, after prime coat application, cause expansion differentials between the wax and the slurry dip. The expanding and contracting wax causes stress on the slurry coat causing small cracks in the coating. Air movement around the drying dip can result in uneven drying which puts the coating under stress.

Uneven Prime Coat Thickness - The drying rates of thick and thin coatings differ. This difference can lead to stress concentrations along the thick-to-thin boundaries, particularly at the outside edges of the casting.

Shell Cracking

This defect is characterized by a line(s) of positive metal on the casting surface. The length and thickness of these fins sometimes can be correlated to the cause of the crack. Shell cracking is most common on cylindrical casting sections as shown in Fig. 3.

The most common causes of shell cracking defects are found in several distinct areas of a foundry's operation. These include the following:

Process Control - This principal defect leading to lower overall shell strengths is caused by inadequate process control of shell composition following, inadequate binder concentration, incompatible shell materials, excessive anti-foam or wetting agents and stucco grain fineness distribution.

Dip Room - Flawed dipping techniques and drying practices cause this defect. Excessively long drying times between the prime and backup dips can cause delamination of the shell and subsequent cracking. Overuse of a binder pre-wet can also cause weaker, easily cracked shells.

Dewaxing Control - Expansion stresses caused by improper dewaxing equipment or techniques can cause shell cracking. Slow heat-up rate and uneven firing furnace temperature distribution is common.

Firing - Improper firing cycles in respect to time, temperature, rate and number of times leads to adverse phase changes within the dewaxed shell.

Dry Before Stuccoing - Occasionally, a particular cluster configuration can dry quite rapidly, especially in low humidity, high temperature environments. If the cluster dries prior to stuccoing, the coating will not build up the required strength.

Destabilized Wax - A mold dipped in slurry prior to wax stabilization will cause prime slurry cracking and/or nonadhesion.

Misrun

This defect, shown in Fig. 4., is sometimes called "non-fill". It is characterized by an incomplete casting. The metal does not fill the entire mold cavity. The two most common appearances of a misrun casting are rounded out at the defect front and cupped inward at the defect front. Generally, misruns which are cupped inward are related to gas back pressure within the mold and can be shell related. There are many other factors which can cause misruns to occur in castings, and most of them are not shell related.

Permeability of the Primary Slurry Coats - If the shell is not permeable at elevated temperatures, the back pressures within the cluster can build and prevent metal fill. Several additional factors, though not shell related, that can affect misrun are metal temperature, gating design, shell temperature, gassy metal and incomplete burnout of organic materials.

Rough Surface

The general appearance of this type of defect can be described as positive metal, ranging in diameter from 0.005-0.015 in. and covering all or part of a casting's surface as illustrated in Fig. 5. Roughly surfaced casting can be attributed to a wide range of shell-related inconsistencies. Several of these are described below.

Destabilized Slurry - Colloidal, silica-based slurries are often used beyond their reasonable life. Bacterial contamination can partially gel the binder to cause agglomeration. These agglomerates may be deposited on the wax and are undetectable after stuccoing. When the shell is fired, this high concentration of silica reacts with molten metal to form a rough surface on the casting, particularly in higher temperature areas.

Stucco Penetration - If a slurry is allowed to drain too much, or is too thin, the primary stucco will penetrate the slurry coat and come in contact with the wax pattern causing rough casting surfaces.

Metal-Mold Reaction - If a shell system is incompatible or contaminated with materials (such as Fe) that react with liquid metal, rough surfaces will form.

Bubbles in the Prime Slurry - Any bubbles in the prime slurry will cause a noncontinuous coating to dry on the surface of the wax. Metal will penetrate these areas and form a rough surface.

Buckling

A shell buckle (see Fig. 6) is an inward movement of the shell. As the metal fills this area, indentations occur in the casting. An outward movement of the shell is a bulge leading to excessive casting dimensions in the bulged areas. Shell buckling or bulging are defects usually seen on long, flat surfaces of a casting. Metal breakthrough or penetration can also be associated with buckling.

The possible causes of the buckle or bulge of the shell can be divided into two distinct areas. The first, buckle/bulge on large flat areas, is commonly caused by the following:

Cleanliness of Patterns - Nonadherence of the primary slurry to the wax because of residual mold release is a common cause of buckling or bulging.

Cluster Design and Orientation - Cluster design plays an important role when considering the buckle or bulge defect. The flat surfaces should be oriented perpendicular to the slurry when dipping to minimize the effects of gravity pulling the coating material from the wax.

Stucco Distribution - Rounded stucco material with poor screen distribution (too many fines) causes a delamination of the prime dips to the backup dips. This factor, combined with high metallostatic head pressures, can cause the deformation.

Expansion Differential - Different coefficients of expansion of materials within a slurry composition of a shell system can cause buckling or bulging.

Wax Stresses - Internal stresses in a pattern may be relieved during dipping and cause the shell to pull away from the wax pattern. Stresses are magnified on large flat or bowed patterns.

The second type of buckle or bulge defect is associated with areas on the pattern which do not allow for a consistent shell to be built. This type of buckle normally has metal penetration associated with it. Its common cause is described here.

Improper Shell Construction - After the primary dips and stucco is applied, the next dip is a more viscous one containing a coarser stucco. If the shell is not constructed adequately, the third dip will bridge slotted areas on the pattern. Subsequent dips will not build adequate shell strength, and buckling or bulging will occur. In some cases, metal will penetrate the cracks in the shell and enter the bridged area.

Scab

A scab is a rough deposit of positive metal on an area on the casting's surface. A scab is a worst-case buckle. Sections of the mold buckles inward and the metal runs in back of the associated crack to form a scab which is illustrated in Fig. 7.

Scabbing - Many of the causes associated with scabbing are similar to those associated with buckling/bulging. As noted, scabbing is an extreme buckle. The primary cause of scabbing is delamination of the prime coats and the backup dips. This delamination is associated with drying conditions and/or shell composition.

Drying Conditions - Over-drying prior to a third dip application will set up a two-component shell. This effect can be exaggerated if improper pre-wetting techniques are used between the second and third dip.

Shell Composition - A third-dip stucco with an adequate distribution may cause a similar two-component shell system to form.

Curing the Defect

Once the cause of a defect is understood, the cure should be a straightforward correction of the materials utilized, the process techniques and/or the shell system composition. Table 1 is the metalcaster's reference guide to the most common causes of investment casting shell-related defects and suggested action for curing them.

The Big Four

There are many causes which may be attributed to shell related defects, but the four most prevalent are drying conditions, slurry composition, dewaxing and firing techniques.

The drying conditions surrounding the cluster during and after its primary coats contribute more to various shell defects than any other condition.

Primary dips provide the refractory coating necessary to resist metal/mold reaction. They do not, however, impart strength to the shell system; so careful control must be maintained to assure the dip's contraction as it drys.

Inappropriate air velocity will evaporate water carried from a colloidal binder at a rate that will cause the thin ceramic shell to contract too rapidly around the wax cluster. Additionally, uneven air velocities around the cluster will cause uneven shell drying and contraction.

Temperature or humidity fluctuations beyond [+ or -] 2F or [+ or -] 5%, respectively, also will place undo stress on the thin primary shell, and must be controlled to avoid shell cracking, scabbing or buckling.

The consistent maintenance of a dynamic slurry compositions is the key to repeatable shell properties. The balance of binder sol particles, total solids content, aggregate balancing and additive quantities are difficult objectives to maintain. Deviations may cause serious shell defects.

Maintaining a fixed percentage of [SiO.sub.2] particles within the slurry can alleviate green and fired shell strength inconsistencies. High percentages of [SiO.sub.2] (from condensed slurry) can cause lower strengths in the shell than slurries which have too low a percentage of [SiO.sub.2].

Three additives that contribute to weakened shells are wetting agents, anti-foams and water. Wetting agents or anti-foamers are commonly overused and cause premature failure of the shell in its fired condition. Small amounts of them can reduce shell strengths significantly. Raw water (non-deionized or undistilled) can cause slurry destabilization and gelling.

In dewaxing and firing techniques, care must be taken when developing a process or technique to dewax, prefire and fire a shell for pouring.

The most common mistake made with autoclaves and flash-fire dewaxers is the length of time an investment caster waits between the last dip and the dewaxing procedure. As shell layers build up, water evaporates and the shell layer dries. Unfortunately, the shell absorbs more water than evaporates to the air. This gives a false dryness indication. If a shell still contains moisture as it enters the dewaxing cycle, the chances of cracking or failure rise.

The second most common mistake is overloading shells in the dewaxer. An efficient autoclave requires reaching 60 lb pressure within 6-8 seconds at approximately 350F. This may be easy with some clusters, but as the surface area of the shells increases, so does the number of BTU's necessary to heat the total load. Therefore, the 60 lb in 6-8 seconds at 350F parameters may be insufficient, particularly during peak production situations. A manufacturer should know the maximum surface area of clusters his autoclave will hold, not just the total number of clusters. Additional defects can result in overloading a flash-fire dewaxing unit and give even more detrimental results.

Finally, most shells must cure at 1600F for a minimum of 30 minutes during their firing cycle to assure that fired shell strengths are maximized and that all residual wax has been removed. In addition to these temperature and time requirements, a manufacturer must consider the excess [O.sub.2] available within a furnace. The [O.sub.2] must be available to combine with the carbonaceous wax by-products to form [CO.sub.2] gas. If this does not occur, then "swiss cheese"-type defects will occur like the one shown in Fig. 8. Again, shell overloading in the firing furnace is the major cause of this problem.

Many defects may occur due to inadequate cluster preparation. Three of the most common preparation mistakes include the following.

Part Spacing - Clusters with multiple parts can be assembled with parts too close together, causing several defects. The shell may build in thickness between the parts, thus absorbing much of the shell firing, causing the molten metal to surface shrink. (See Fig. 9). Further, the shell may not build up correctly and cause bridging. Metal runout or shell buckling could result.

Gating System Design - Much has been written on this subject, but it is worth repeating that unless proper flow and thermal gradients are controlled within the cluster, the desired surface and internal results from shell molded castings will be lost. Fig. 10 shows the results of poor gating system design.

Piece Orientation - The ability of a cluster to evenly drain excess slurry after dipping is an important design criteria. Uneven coatings can lead to uneven shell thicknesses and lower the overall fired shell strengths. An example of this would be a part with a cylindrical core to be formed by the shelling process. An assembly with the core dipped parallel to the slurry's surface will build an uneven shell as compared to an assembly which would have the core dipped and dried perpendicular to the slurry's surface. Results of poor piece orientation can be seen in Fig. 11.

Shell defects that are properly identified can be corrected by understanding how they are formed. This has sought to provided some useful insights into some common shell-related defects, their causes and potential cures.

PHOTO : Fig. 1. Surface or sub-surface entrainment of ceramic particles causes inclusion defects.

PHOTO : Fig. 2. Prime coat buckling usually occurs on flat surfaces as irregular islands of excess metal.

PHOTO : Fig. 3. Shell cracking, as this line of surplus metal indicates, is most common in cylindrical castings.

PHOTO : Fig. 4. Misrun defects occur because poured metal does not fill the mold cavity.

PHOTO : Fig. 5. Rough metal defects leave small islands of positive metal dotting the casting surfaces.

PHOTO : Fig. 6. Outward and inward bulges in the shell cause excess metal or dimpling in castings surfaces.

PHOTO : Fig. 7. A scab is a worst-case buckle in which metal runs behind a shell crack.

PHOTO : Fig. 8. "Swiss cheese" defect is caused by insufficient oxygen to combine with carbonaceous wax by-products to form [CO.sub.2] due to furnace overloading.

PHOTO : Fig. 9. Surface shrinkage is caused by uneven shell coating between cluster parts.

PHOTO : Fig. 10. Flawed surface of this part is the result of poor cluster gating.

PHOTO : Fig. 11. Improper dipping and draining results in uneven shell thickness and strength.