Include nothing: step-by-step reduction of investment casting inclusion defects.

In the stringent global market metalcasters face today, the need for precision is grossly imperative. Although North American plants still lead global investment casting with 50% of the market share, competition from world sources has never been more acute, as they duel their end-products against manufacturers worldwide. Any faults or complications may cause a firm to lose its business.

One such fault or complication investment casters can improve upon is the detection and reduction of inclusion defects in their components. An inclusion defect occura when loose inaterial, such as that of a chipped mold or wax that wasn't sufficiently burned out, is collected in the casting during the pouring process. these particles may enter a casting due to a number of reasons, such as poor wax or mold processing or the use of burnout ovens that have cracked shells from previous castings.

By establishing a step-by-step approach to inclusion reduction for all departments to follow, investment casters can begin the path to inclusion reduction and elimination.

The result is a subsequent reduction in rework of finished castings, reduced time, labor and money, and efficiency is increased. But, true reduction or elimination only can be achieved through strict adherence to procedure. This procedure is based on three principles:

* eliminate the generation of materials that could cause the defects;

* remove the materials generated;

* block the transport of the generated materials.

This article addresses the sources and control of inclusion defects in wax pattern molding, shell building, melting and pouring for investment casting and assists firms in adopting a procedure to fit their production of flawless cast components.

Circle the Loop

To simplify your examination of inclusion Inside This Story:

* Investment casters always face the dilemma of inclusion defects. By establishing preventative procedures, quality will improve.

* Provided in this article are steps and factors to consider in reducing or preventing recurring inclusion defects. detects, consider a closed loop approach to your casting processes. This approach asks casters to view the entire investment casting process (from wax processing to casting) from afar and locate the procedure where inclusions may occur the most. The investment caster then should make the necessary improvements to reduce the inclusion rates and gradually go through the process again and again, like a loop, to gain optimal reduction in casting defects. A control part with a low inclusion level also might be helpful to ensure effective evaluation.

First, observe each segment of the casting process: wax processing, mold processing, dewaxing, burnout and mold preparation, and pre-heating and casting (further detail in each area is presented later in this article). Focus on the characteristics of patterns, molds and metal at each step in the process. Be diligent in observing any sources of loose material anywhere in the process.

Next, characterize the appearance, location and frequency of inclusion defects, and, where possible, the composition of inclusions. To easily evaluate the findings, create a table to record the data. Analyze the results for trends within molds, molds within a lot and between different lots. Compare the inclusion trends and characteristics with in process observations, process controls and techniques, environment, materials and equipment.

Finally, complete the initial loop by evaluating the more significant factors and implementing the changes that would have an influence. On a regular basis, check that the machine controls, such as those in pressurization and burnout, are at the appropriate settings.

As the looping investigation progresses, the quality of parts being directly monitored will improve, allowing for the observation of other parts. Further inclusion factors and system limitations of the process will be better understood, and casters can develop and evaluate new processes, equipment, techniques or materials.

If the closed loop process is believed to be unnecessary, alternate approaches exist that are more generalized than the steps above. These do not include running trials or following the whole process in cycles, but rather examining previous data and looking at which practice works best.

One approach is analyzing past results to identify trends or correlations between inclusion defects and processes, materials, equipment, production locations and other factors. Test and confirm the significance of the factors in relation to inclusion defects and apply those that will help reduce inclusion rates. This may involve an assessment of whether the standard level of performance is adequate of if the system has been under control.

A second approach is to implement "best practices" that are known from general experience at your foundry of from studies to have an influence on the frequency of inclusion defects. Examine the influence that these practices have on the casting results obtained in your process.

A third possible approach is having operators be diligent in controlling observable loose material being generated anywhere in the process. Some sources are infrequent and random, so constant attention is required. Periodically review each process area for sources of control and variability.

The closed loop approach requires investment casters to closely look at each area of the operation. The more that metalcasters meticulously scrutinize each process for potential inclusions, the fewer inclusion defects that will result. Following is a more detailed approach to each area of the process.

Wax Processing

When first creating the wax pattern, investment casters should make certain that the sprue/wax assembly design is adequate. The patterns, gates, sprues and pour cup must be properly attached because gate and sprue designs control metal flow. This can influence mold erosion and how inclusions and dross are pushed through the parts. If using ceramic pour cups, they ought to be free of loose materials, properly attached and completely dip-sealed inside to prevent further inclusions.

To achieve optimal mold cavities for casting, the wax assembly should not allow wax to be trapped when the mold is dewaxed There should be no sharp edges, undercuts, bubbles, negatives or rough surfaces that could form sharp or weak features in the prime coat. Weak areas are likely to break off during dewax or when the metal enters the mold.

Be sure that openings (vents) in the mold don't act as paths for outside contaminants to enter the casting cavity. Inside contaminants in the pattern wax or other materials used in the assembly also must be sought. If any contaminants are being picked up through the process, ash levels should be low for fewer inclusion particles. Further, if causing problems, core quality and core handling and floating practices should be reevaluated.

Shell Mold Processing

Shell mold quality will have a direct correlation to the ceramic inclusion rate. For some processes, the vast majority of inclusions can be traced to mold ceramics. Obtaining strong, uniform and defect-free molds will always have a beneficial impact on inclusion defects.

When beginning the molding process, adequate slurry conditioning and control is essential. Make sure the tests that give the best control of the system are used. Record and adjust to the viscosity cup control ranges, raw materials used, batch size and weights effect of age of slurry and calibrate tests, such as percent silica. Also, try using pH and density measurements, recording all additions made.

For each dip applied, the slurry application should be uniform, and each prime coat should be examined for each mold and all subsequent coats. Each robot program should be closely examined for the quality of the mold produced, and the pour cup should remain clear of the slurry.

In the drying of each dip there are two approaches. Both are advantageous, even though one might work better in one foundry than another:

* only dry the dip sufficiently to gel the colloidal silica. This leaves some water in the coat, and the coat will not reach its maximum strength. The final dry completely dries the mold, so the finished mold is strong enough to be dewaxed without defects;

* dry every dip completely to adhere the maximum strength. Although this method might take up to 24 hours longer than the other, it's used when the mold's strength is not high enough to survive through dewax without some damage.

After the drying process, each mold should be checked for damaged parts, such as broken gates of joints of buckled of spalled prime coats, and further investigated to what causes damage. They could be buried deep inside the mold and only observable on the final castings. If necessary, open a portion of the mold after dewax or firing to examine it better. Loose material on the mold's outside also must be removed of affixed before the mold goes to dewax. Otherwise, such material could come in contact with future molds and cause inclusions.

Dewaxing

During dewax, high stresses can be generated, causing mold cracking and prime-coat defects. This also is the first stage of the process where the mold is open to materials being transported inside of the mold. The steam injection into the autoclave might blow loose materials into the mold during the process, so it is essential that the autoclave and dewax carts are clean.

To prevent mold cracking, be sure the autoclave process and pressurization and depressurization cycles are under control. Pressurization must be rapid--within 6-10 sec. to 80 PSI--but depressurization must be slow. Allow at least two minutes for the pressure to uniformly drop to zero to prevent water condensed in the mold from expanding violently if the pressure is reduced too fast.

If cracks are found internally or externally, look for face-coat and core defects and begin to qualify the degree of damage. It is best to report the damage against a known rating system than to say "cracks or no cracks."

Burnout & Mold Preparation

Burnout and preheat firing may be one and the same cycle. For many commercial foundries, the mold goes right to pouring from the hot furnace. Any "mold prep" would be done before firing, and examination of the mold after burnout is not possible because it's already hot and ready to cast. Only after casting, in which no runouts occur, and after the castings pass inspection, can the process be considered successful.

If poor castings result, the exact cause is often hard to find.

If the process includes a cooling and examination procedure before casting, then you must observe certain rules for an all silica shell (with or without zircon). Silica molds have a maximum temperature of firing if the mold is to be cooled before casting. This temperature is 1,600F (870C). Above this temperature, cristobalite will form, depending on time temperature and particle size of the flours and grains. When cristobalite cools below 480F (250C), it transforms and goes through a large contraction and the mold disintegrates. This is useful after casting, because the mold readily comes off the metal. Molds that are not all-silica don't reach the high-temperature strength that a silica mold does due to the presence of alumina.

When the mold is placed in the burnout furnace, no loose materials should be in proximity, and it is recommended the mold is covered or pour cup-down to minimize the potential for inclusions. Also, watch for the heat-up rate and oxygen levels. Excess oxygen of at least 10% is recommended to prevent carbon formation. A low oxygen level and a low burnout temperature (less than 1,500F (816C) can create coke from some types of residual pattern materials. Once coke/carbon is formed, it's nearly impossible to remove.

Once burnout is complete, observe for indications of spall, buckle, weak (soft), chalky of dusty areas on internal surfaces as well as cracks externally. All dewaxed molds should be handled around clean areas and surfaces because molds are open and can be exposed to a wide variety of inclusion-causing materials.

Watch for chipping or rubbing on the pour cup during handling as well. If the mold is being washed, develop a rating system based on residues captured from wash water.

Preheat & Casting

The factors to watch for at this step are similar to those seen in previous processing steps, but they cannot be overlooked.

Continue inspecting for any possible loose materials, chipping of cracking that may cause inclusions in your casting, and try to have alloy bar stock cleanliness, preparation, storage and internal shrink properly controlled.

Before casting, make the observations depending on which method you use. For vacuum casting, check for loose material in the lower of upper chambers of the casting furnace because the blower and vacuum pumps could ingest some material. For pouring, be sure the crucibles or ladles aren't spalling, eroding, deteriorating or developing too much buildup. Look at crucible conditioning, accurate temperature monitoring, filtering, leak checks, minimum melt hold time, minimum super heating and smooth, rapid accurate pouring Further, when the alloy is charged, examine the edges of the ingot and see that they aren't chipping, nicking or abrading the crucible or the crucible wall build up.

Follow the Path

Although these steps are not mandatory for investment casters to produce quality castings, they should be considered. Inclusion defects can find their way into the final product from a number of sources. Then, these defects can require many man-hours to repair, or they can cause the casting to be rejected. In any case. the cost to the foundry is significant.

By following the preceding steps in the casting process, casters must practice a meticulous investigation and constant observation to rid the investment castings from inclusion detects. In order to compete in the market all investment casters face today, fewer defects will lead to quality castings and help retain costs for future production, This article was adapted from a paper presented at the 51st Annual Investment Casting Institute Technical Conference and Exposition.

Inside This Story:

* Investment casters always face the dilemma of inclusion defects. By establishing preventative procedures, quality will improve.

* Provided in this article are steps and factors to consider in reducing or preventing recurring inclusion defects.

For more information

"Quiescent Filling Applied to Investment Castings," Mark Jolly, Mike Cox, Ric Harding, Bill Griffiths and John Campbell, MODERN CASTING, December 2002, p. 36-38

"Gating & Feeding of Investment Castings," Cast Metals Institute Course, May 19-20, AFS.

About the Authors

James D. Jackson, Nipendra Singh and Thomas Thornton assist in providing engineering and management consulting at S&A Consulting Group LLP, Cleveland.