Acrylic-epoxy binders for coldbox core and moldmaking.

Today, the metalcasting industry faces a number of challenges, including tough environmental regulations, unfair trade practices, and productivity and quality improvement demands.

Together, these concerns are causing foundries to re-evaluate some sand casting methods. The selection of a sand casting process is the first step in meeting today's challenges - and those of the future.

The selection process depends on production and environmental needs. First, identify bottlenecks in your plant and design the casting process to eliminate them. This helps reduce waste and eliminate nonproductive labor, avoid casting defects and improve environmental compliance.

The free radical cure (FRC) process has been selected for many new installations because it enables the foundry manager to operate a core- and/or moldmaking process without the high costs of downtime associated with mixer and machine cleanup. Furthermore, scrapped cores are reduced because the sand mixture has a shelf life of days - instead of hours.

The process can also eliminate waste sand and costs associated with its disposal. Environmentally, it requires strict adherence to coldbox coremaking procedures. The result is a clean, productive workplace where the process parameters are clearly understood and maintained.

Process Description

The FRC process is based on acrylic-epoxy binder technology, and involves the use of two liquid binders and sulfur dioxide ([SO.sub.2]) gas for curing. Part A and Part B binders are free of nitrogen, water, phenol, formaldehyde and isocyanate. The sequence of introducing the binders isn't important because the two parts don't react at this stage of the process.

An example of this process is the Isoset[R] acrylic-epoxy binders,which are compatible with a variety of sands and aggregates, including silica, lake chromite, olivine, zircon and fused silica refractories. The binders are coated on sand at levels ranging from 0.5-2.0%.

Several binders are available, depending on specific metalcasting needs. For example, there are binders developed for iron and steel casting that exhibit higher hot strength and erosion resistance, zero nitrogen, no water and good veining resistance.

For aluminum and nonferrous foundries, specific binders provide improved shakeout and core storage properties. Acrylic-epoxy binders are low in volatile organic compounds (VOCs), contain no water and exhibit excellent coremaking properties that are needed for the designs required by today's aluminum market.

The binders and sand are mixed and blown into a vented pattern and then gassed with [SO.sub.2]. Equipment manufacturers have recently introduced gassing equipment that provides blends of [SO.sub.2] and nitrogen ([N.sub.2]) for curing acrylic-epoxy binders. The blends of [SO.sub.2] and [N.sub.2] enable metalcasters to reduce the amount of [SO.sub.2] left in the core or mold without sacrificing productivity or quality of the core or mold.

After the gas is introduced, it is followed by a dry-air purge at a dew point of -10 to 40 at 1 atmosphere, which further cures the core and removes the excess [SO.sub.2] from the core and coldbox tooling.

The [SO.sub.2] is then sent to a packed tower-type scrubber, and is scrubbed with water and sodium hydroxide (NaOh) at 5%. The by-product is a sodium sulfate and water solution that is accepted by most municipal sewer systems. The scrubbing process is automatic and the sodium sulfate solution is discharged at a pH of 8.5.

Core & Mold Properties

A key benefit in using these binders is the mixed sand bench life properties (Fig. 1). Once the acrylic-epoxy binders are mixed with sand, no reaction occurs until the sand mix is gassed with [SO.sub.2]. Mixed sand won't harden in the mixer, hopper or blow magazine, and no cleanup is needed in these areas. A reduction of scrap sand of 90% or better from the core or moldmaking operation is possible when converting from a hotbox process or another coldbox process.

In addition to the lack of cleanup and the reduction of waste sand, the sand doesn't change and the foundryman isn't chasing a moving target of sand properties. This results in a significant reduction in core scrap.

One foundry, which produces block cores, reduced core scrap from 4% to 1% when it converted to the acrylic-epoxy process from its phenolic urethane coldbox process. Another foundry reduced core scrap from 7.7% to 2.2% when it converted from the hotbox process.

The bench life properties result in easy startup after long production delays, benefiting on-time manufacturing concepts.

After the cores are produced, they are ready for coating or assembly. Alternatively, they may be used immediately to produce castings.

If the casting process requires coating the core or mold, several coatings can be used, including alcohol, air-dry and water-base. if a water-base coating is used, the core or mold should be coated and dried as quickly as possible by using maximum air flow and temperatures of 200-300F.

During the coating and drying process, the cores lose strength when they are hot and wet. It is important to design an oven that dries the coating quickly and that also has a cooling zone before the cores are handled. This minimizes the threat of core breakage during handling.

The key considerations are coating the core as soon as possible (zero to four hours), drying the core completely and cooling the core before it's assembled or handled.

The storage properties of these cores are desirable and these binders demonstrate good humidity resistance. All binders are affected by high humidity, and like all binders systems, these cores can lose strength in high-humidity conditions.

Ferrous Applications

This process is used in numerous iron and steel applications throughout the world and can produce a variety of cores, ranging from knock-off cores to complex cylinder heads.

The chemical composition of the acrylic-epoxy binders lends itself to iron and steel applications. The chemistry of the binders consists of carbon, oxygen and hydrogen - there is no nitrogen. Because it's free of nitrogen, there's no need to add iron oxide and other nitrogen scavengers to the sand mix.

In addition to carbon, oxygen and hydrogen, these cores contain small amounts of sulfur after they are cured. The binders have only one-third the sulfur content of the systems commonly used to produce ductile iron and steel castings, such as green sand and furan or phenolic nobake binders catalyzed with toluene sulfonic acid (TSA) or xylene sulfonic acid (XSA) catalysts.

Applications for these cores include motor blocks, diesel cylinder heads, pump parts, intake and exhaust manifolds, steering gear components, pistons, disc brake rotors, boiler castings and structural parts.

The features that benefit iron and steel applications include:

* elimination of nitrogen or associated nitrogen fissure defects in the binder;

* elimination of formaldehyde, which allays concerns about employee exposure and facilitates compliance with new OSHA standards;

* elimination of nonproductive labor for cleaning hoppers, magazines, blow tubes and mixers;

* desirable shakeout properties and reduced possibility of hot tears.

* reduction or elimination of veining defects in some iron and steel applications.

* improved productivity due to the elimination for startup or shutdown of the equipment holding mixed sand and fast cure times.

Nonforrous Applications

The process also offers improved nonferrous casting properties because they are free of water and some new systems contain no solvents.

The solvent-free systems reduce the threat of gas-related defects that are associated with water or solvents in the binder system. The use of low binder levels and the lack of solvent in these systems offer lower gas volume. It is important to minimize this gas volume, but the most effective methods of doing so are good veining practices and proper core print design.

Core Deflection - These binders may be formulated to reduce the threat of core distortion in the mold. The acrylic-epoxy blends are designed to minimize deflection at semi-permanent mold temperatures and during the casting process.

Shakeout Performance - In aluminum applications, the process exhibits maximum shakeout when the castings are still warm (350-400F). This enables foundry engineers to design the casting process to shakeout castings in a just-in-time or synchronous manufacturing system. Shakeout time can be reduced up to 80% over room temperature castings. In addition, it reduces retained sand and streamlines the casting process - removing bottlenecks for improved efficiency.

Aluminum applications are as equally diverse as the ferrous, and include complex aerospace components and magnesium parts that require complex pipe cores. The high-production automotive applications include manifolds, cylinder heads, motor blocks, semi-permanent mold castings and structural aluminum parts. The absence of water in these binders and the solvent-free properties of the newest binders help reduce the threat of hydrogen pinholes that are traced back to the core process.

Another feature that promotes its use in nonferrous applications is the lower binder levels that may be used with the process. Binder levels as low as 0.5% based on sand (BOS) are being used. Low levels are critical when the cores are very complex and require good sand flowability, high tensile strength and indefinite mixed sand bench life.

Sand Reclamation - The shakeout sand from semi-permanent mold operations using these binders can be readily reclaimed by mechanical and thermal reclamation methods. Thermal reclamation is preferred because it minimizes or eliminates the organics left on the sand grain, and controls loss on ignition and the gas evolution of the cores. In addition, thermal reclaimers may be engineered to ensure binder decomposition products are totally combusted to carbon dioxide ([CO.sub.2]), carbon monoxide (CO) and [H.sub.2]O, thereby removing potential hydrocarbon emissions.

Environmental

Benefits

As the metalcasting industry moves into more semi-permanent mold and aluminum sand-casting applications, the process may offer a workplace that doesn't contain isocyanates or formaldehyde as decomposition products. The lack of isocyanates or formaldehyde represents a significant benefit over existing coldbox processes.

The process is used in more than 100 foundries worldwide. There is reluctance, however, to implementing the process due to handling and control of [SO.sub.2].

Worker Exposure and Control- [SO.sub.2] gas has a pungent, irritating odor and an odor threshold of less than 0.5 ppm. Employee exposure must be controlled to 2 ppm over an eight-hour time weight average (TWA) and may not exceed 5 ppm during any 15-minute period. While these levels are low, they are similar to those recommended for amine-cured systems and are significantly higher than the control needed for formaldehyde emissions from heat-cured coremaking operations. In addition, the irritating odor of [SO.sub.2] gas makes the user pay attention to good coldbox practices. In other words, the low odor threshold helps maintain a clean atmosphere.

Control of [SO.sub.2] in the process is well documented in several tooling manuals and in an industrial hygiene appraisal presented in AFS Transactions. Basic controls can minimize and contain any leaks during the curing process.

The core storage area is the other area of exposure to [SO.sub.2], and residual [SO.sub.2] may be left in the core and mold after it is ejected from the tooling. Two proven methods have been in use over the last three years to reduce [SO.sub.2] exposure from freshly made cores and molds. Equipment suppliers introduced [SO.sub.2] nitrogen blenders that provide a wide range of [SO.sub.2] and [N.sub.2] blends for curing cores and molds produced with acrylic-epoxy binders.

The second method of reduction comes from new products that have been developed to reduce retained [SO.sub.2]. Several acrylic-epoxy binders have been introduced over the last three years.

More importantly, the combined use of both these methods exhibits the best reduction in residual [SO.sub.2]. The combination of reduced concentrations of [SO.sub.2] and new binder formulations has provided foundries that produce large cores and molds the ability to reduce worker exposure below the 2 ppm limit.

When using blended [SO.sub.2] and [N.sub.2], the foundry must have the capability to measure the concentration being blended. This may be accomplished through the use of infrared testing of the [SO.sub.2] and [N.sub.2] curing gas system.

Sand Disposal - There are no wastestreams from the cleanup of mixers, hoppers or magazines because sand doesn't harden until it is cured in the tooling. Nevertheless, cores cured with the acrylic-epoxy binder and mixed sand have been tested by using the federal toxicity characteristic leaching procedure (TCLP) criteria. Results show the sand and waste cores don't exceed federal proposed guidelines for metals or organics.

Air Emissions - The air emissions from the process during casting have been identified for ferrous and nonferrous casting applications by hood stack studies.

Hood stack studies on the binder systems monitor the type of air emissions and the quantity of decomposition emissions produced during pouring and cooling. The hood stack technique, a recognized method for collecting pointsource emissions, collects airborne decomposition emissions. The molds used for the test were less than 24 hours old, and were prepared by using the irregular gear pattern developed by AFS.

The hood stack was lowered over the mold immediately after pouring Class 30 gray iron at 2600F. Essentially, all emissions were drawn up through the chimney, which was fitted with appropriate sampling devices. A constant flow rate was maintained through the chimney by using a variable-speed exhaust fan. The exhaust fan ensured minimal diffusion of the effluent at the bottom of the hood. Sampling of the airstream was accomplished by using methods developed for stack sampling and industrial hygiene techniques.

Since the concentration of decomposition products measured during the hood stack studies doesn't represent employee exposures, it shouldn't be compared to workplace exposure limits. The data, however, may provide a first-order approximation of potential emission rates found during pouring and cooling so that control technology and ventilation systems may be designed.

A Process for Tomorrow

The metalcasting industry continues to move toward coldbox methods, especially those involving new casting designs and conversion from other heat-cured casting processes. This process is another option for reducing cost and streamlining casting production while meeting or exceeding the environmental regulations required today and into the future.

Choosing a system that meets both the environmental regulations and the production needs of today's metalcaster is a solid investment in the future.