Investment casters examine new techniques, products.

These advances provide new ideas to improve melting, patterns and cores.

New techniques and products need to provide a metalcaster with either cost savings or better quality castings. Although these changes may initially cost a foundry in terms of production time and man-hours, they should not be dismissed immediately. If given a careful look, these technologies may show the promise of increasing production performance and quality.

These three advances in investment casting technology are adapted from papers presented at last fall's 45th Investment Casting Institute Conference. The new technologies utilize an old idea and improve upon it for today's investment caster.

PATTERN FILLER

Investment casting begins with a low temperature, thermoplastic wax material that is used to create patterns. For every pattern wax, there is a different blend of material to produce it, each delivering different results in the casting process. In 1996, M. Argueso & Co., Inc., New York, introduced a new pattern material to investment casting - Hydro-Fill pattern compounds - as an option beyond emulsified or conventional pattern waxes.

The Pattern compound is comprised of pattern materials (waxes, resins, synthetic polymers) and purified water that have been combined with pattern wax in the form of micro-sized spheres. These spheres, which are achieved through chemistry and technology, are distributed uniformly throughout the wax and give it a cellular structure. And, this structure is the base for the fillers' defining characteristic - it is hydrophilic, or in other words, it has an affinity for attracting water.

Conventional pattern waxes are hydrophobic - they resist wetting - and create surfaces that are difficult for bonding. To overcome this property, investment casters wash, scrub, dip, rinse and use aggressive etches to prepare the wax surface for shelling.

Wax surfaces that do not bond or lack adhesion with ceramic coats tend to lift off the pattern, creating a void and air space that is not visible to the naked eye. This void can cause the shell to crack or cause finning on the casting due to the expansion of the entrapped air during the heating of the shell. The addition of the new pattern filler to the wax improves the wetting and adhesion properties of the ceramic to the pattern surface.

Figure 1 demonstrates an adhesion comparison between a conventional wax pattern and a new filler pattern prepared in the same manner. Both patterns were prepared with a primary of zircon flour and a Ludox SK binder, and had no surface preparation. As shown, cracking occurred on the conventional wax pattern along the edge, while the filler pattern showed no signs of lifting or cracking.

Superior Flow

Beyond adhesion, the hydrophilic qualities provide the filler waxes with improved flow and viscosity characteristics. The superior flow is illustrated in the improved fill and detail in the pattern die and the improved pattern surface quality that allows for easy removal of the filler patterns from the die.

The filler's wide range of viscosity aids the pattern during another source of shell cracking - the de-waxing process. As conventional waxes are heated to be removed from the shell, they can also expand and crack the coating. The addition of the filler to the pattern increases the thermal conductivity of the wax and decreases the thermal expansion properties. The filler pattern, due to its superior fluidity and the ability of the cellular structure to collapse during de-waxing, minimizes the tendency to crack shells. Figure 2 demonstrates the expansion difference between conventional pattern waxes and the pattern compound waxes.

After de-waxing, a concern to investment casters is the residue left in the shell before preheat, and ash remaining in the shell after preheat and burnout. Some of the inclusions that force a casting to be scrapped are caused by this ash remaining in the shell after burnout. But the new pattern compound, due to the high amounts of water, is a low ash filler, resulting in an improved cleanliness of shells after de-waxing.

ARGON SWIRL COATIING

Pine Tree Castings, Inc., a 285-employee iron and steel plant in Newport, New Hampshire, has developed a cost-effective gas blanketing system to inert metals melted and treated in an air atmosphere. The new argon swirl inerting system removes more than 95% of ambient air from the molten metal surface utilizing a tangentially injected swirling stream of inerting gases that expands above a hot bath.

Used during induction melting of 300-500 lb metal charges, the swirl blanketing system, developed by Air Products and Chemicals, Inc., Allentown, Pennsylvania, has been shown to improve metal yields, recovery of alloying additions and melt fluidity. In addition, it reduces slag losses, nonmetallic inclusions, porosity/surface defects, reject rates and crucible maintenance. The installation at Pine Tree was the first for an investment foundry.

System Development

The project goal at Pine Tree was to develop a robust, reproducible process to inert the molten metal with gas while minimizing disruption in the plant and involvement after implementation - basically to turn it on and let it go.

Through a series of exploratory steel blanketing tests with a range of gases and gas discharging media - multiple lances and porous diffusing panels - Pine Tree developed criteria for its system:

* the blanketing gas should be discharged as close to the metal bath as possible;

* the density of blanketing gas at the moment of discharging should be higher than the density of ambient air;

* the design of the gas discharging system should minimize turbulence and air entrapment, as well as effects of bath temperature;

* the use of porous metallic or ceramic gas diffusers near the bath surface should be equal for the combined impact of electromagnetic induction field, metal splashes and aggressive furnace charging practices.

The resultant system developed from these criteria is comprised of hollow refractory semi-enclosures shaped like truncated cones and/or rings. The enclosures are positioned directly at the top of the furnace crucible and equipped with simple tangential gas inlets located near the bottom. The basic geometry of these hollow cones and cylinders is shown in Fig. 3.

The tangential injection of a gas denser than air into the blanketing cone or cylinder results in flow patterns above its base plane and/or the metal bath surface. As the injection flow rate and Reynolds number of argon increase, the oxygen concentration inside the cone decreases to reach a plateau below 1% volume of oxygen and then increases again when the swirling velocity of the blanketing gas becomes high enough to aspirate ambient air into the cone. The size and number of argon injectors installed in a given cone is what controls the onset of the back-aspiration effect. If the blanketing cone needs to operate under a strong ventilation draft so that increased flow rates of argon are needed to replenish gas losses, larger and/or more gas injection ports must be installed.

Pine Tree proceeded with trials and foundry testing of the inerting system developed. Its melting system consists of three 350kw induction 3000hz furnaces that are designed to melt 500 lb of a cold charge to a pour temperature of above 3000F (1649C) in 30 min. The metal is tapped into 35 lb teapot ladles for pouring into shell molds that require 10-25 lb of molten steel.

Various features of the system were improved upon during testing, including cover design, materials, assembly and flow rates. Most notable was the need to ensure the system is sealed from outside air. The flow of argon will cause air to be drawn into the atmosphere above the bath unless sealed properly.

These observations helped Pine Tree to complete its effective gas blanketing technology. The results of this technology are shown in randomly chosen test cast bars and the increase in production and cost savings from the inerted molten metal bath. In terms of the castings, the result is a uniform quality that is more consistent in chemistry, contains smaller and fewer inclusions, and yields fewer magnetic particle inspection rejects. In terms of melt benefits, they are:

* a more consistent melting practice derived from the 40% lower gas levels;

* the use of more revert scrap per heat because the pickup of oxygen and nitrogen is minimized in the furnace and lowered overall in the melting and pouting process [ILLUSTRATION FOR FIGURE 4 OMITTED];

* greater control over the nitrogen content in stainless steel because the pickup is a smaller part of the contribution of nitrogen;

* lower slag volumes lessen the erosion rates in the ladle and crucible, offering the opportunity for running more heats or providing less potential contamination during pouring;

* less slag (and in some cases none) must be removed from the melt, and that which is remaining is largely a function of the shell left on the parts prior to melting;

* deoxidization and alloys to make up for losses were reduced;

* lower scrap and rework that probably would not be achievable without good furnace melting control.

HOLLOW CASTINGS

Castalloy, Inc., a 60-employee commercial investment caster in Cleveland, is working to reduce the cost of making its hollow castings. The plant currently forms the hollow cavities in a wax pattern with complex retractable tooling or by leaching out a soluble wax. But, the cost associated with this process is high due to:

* aluminum tooling for injecting the soluble wax;

* materials, including soluble wax and acid for leaching;

* disposal of acid waste;

* production labor time associated with the soluble wax injection and leaching.

In addition to the costs, the injection machine capacity, wax cleanout and scheduling are always a problem for the injection of soluble wax.

Castalloy's dissatisfaction with its current methods sparked its interest in a new process from CADIC International, Inc., Lyndhurst, Ohio. Known as the Convert process, internal casting details are produced through the use of traditional sand cores bonded by hot phenolic or coldbox resins. These cores are impregnated with an inorganic binder. The organic resin binder provides strength to the core at room temperature. The inorganic binder provides strength to the core at preheat and casting temperature.

After being dipped in the binder, the cores are dried and used in a wax injection tool in place of the original soluble wax core. The cores can also be inserted into wax patterns.

This new process offered these advantages to Castalloy:

* lower labor and material costs;

* lower tooling costs and shorter tooling lead time;

* cores can be produced by an outside vendor, eliminating the need for in-house core production equipment;

* the performance of a preformed ceramic core, but with less expensive tooling and core costs;

* cores do not require acid leaching for removal from the wax pattern or high temperature caustic leaching for removal from the final casting.

Some of the limitations of the new process are that it is not suitable for all shapes and sizes of castings and some of the dimensional tolerances will depend on the tool and type of sand used to form the organic resin bonded sand core.

Despite the limitations, the comparison in cost, as shown in Table 1, gave Castalloy enough reason to run a trial.

Test Trial

For its initial testing, Castalloy chose a small, hollow, stainless steel fitting casting about 3 in. long and 1 in. in diameter. The part weighs less than 0.5 lb. The foundry also chose to use vendor-supplied phenolic/ester vapor-cured cores. The cores were made with a 45 AFS GFN silica sand.

The tooling to produce the cores was made in castable urethane by taking direct impressions from the current soluble wax cores. The cost for the tooling was $200.

The cores made with the quick and cheap tool were dipped into the binder for 5 min and air-dried. Once dry, half of the cores were fired at 1850F (1010C) for 1 hr and then cooled, while the other half remained at room temperature. Although the new process doesn't require the firing of the dipped cores, Castalloy wanted to assess this variable.

Both the fired and unfired cores were dipped in hot paraffin wax to strengthen them. After the paraffin dipping, the cores were placed into the standard wax injection tool instead of using soluble wax core. After the injection of pattern wax, some of the fired cores showed cracking around the core print areas. The cores appeared to be slightly over-sized, which was probably due to the silica expansion during firing.

Table 1. Cost Analysis Per Casting of the Soluble Wax Core Method
vs. the Sand Core Method

                                  Soluble        Convert
                                    Wax          Process

Material (costs)                   $0.25            0
Labor (costs)                       0.25            0
Phenolic/Ester Cured Cores           0             0.35
Convert binder                       0             0.20
Wax Injection                       Same           Same
Dipping/Casting                     Same           Same
Caustic Bath                        0.75            0
Shot Blast                          0.75           0.75

Total                               2.50           1.30

Two molds were assembled - one with fired cores and the second with unfired cores. These wax assemblies were then coated with a standard colloidal silica bonded ceramic shell sequence. After one day of final drying, these molds were de-waxed in an autoclave, preheated to 1850F (1010c) and cast in 304 stainless steel at 2950F (1621c).

After casting, both molds were subjected to vibratory knockout and visually inspected. The fired cores showed some pieces with core kiss out. These patterns had virtually no core print left before dipping. Castings with unfired cores performed far better then their counterparts, and maintained their shape and dimensional integrity.

All the casting trees required 4 min tumble shotblasting cycle to remove the residual core sand. No high temperature caustic bath was required. Internal surfaces, normally formed by a soluble wax core, were clean and defect free. However, the surfaces were somewhat rougher than those produced with soluble wax cores.

For Castalloy, the initial results of this trial warranted additional testing of the process. Many of the promises of the new process were proven with this limited trial as the unfired cores performed well. The cored surface finish is expected to be better when finer sand is used.

In terms of the bottom line, the new process costs $1.20 less per casting than the soluble wax core method. This savings, in addition to the decrease in tooling costs, allows Castalloy to believe the new process may prove to be a viable replacement for soluble wax cores.