Protein-Based Binder Update: Performance Put to the Test.

The results of further development and testing of GM'S protein-based sand binder system now validates aluminum casting benefits as well as its environmental friendliness.

In the early 1990s, researchers at General Motors began to evaluate many different materials for their binder-like qualities. Each had to meet two criteria for consideration: environmental friendliness and strengths comparable to other bindersystems. They eventually focused on developing a binder from purified proteins.

In 1994, a new binder system was developed that used a mixture of various molecular weight proteins intimately mixed with a metal oxide catalyst to enhance thermal degradation. At that time, General Motors was issued a patent for the new GMBOND sand binder, which it recently licensed for production.

The binder was introduced to the foundry industry in 1996. Since then, casting trials have been performed to evaluate casting quality and enhance the sand binder's effectiveness. Following up on an October 1996 modern casting article introducing the system, this article updates readers on what has taken place with the system since then. Detailed are specifications for preparation and use based on laboratory work, as well as the results of foundry trials.

How It Works

The protein-based binder system's raw materials are derived from renewable natural sources. They are non-toxic and environmentally benign. These raw materials are purified and processed to give the binder the characteristics that allow it to be used as a sand binder.

Most of the binder system is a combination of various types of polypeptide molecules, or long chains of amino acids. A small amount of iron oxide uses the oxygen trapped within the core's porous matrix to increase thermal degradation of the binder. The addition of iron oxide accelerates thermal degradation at temperatures as low as 842F (450C).

The binder, which is a dry, fine, slightly tan powder, is not flammable or reactive. A small amount of preservative is added to ensure bench life while the mixed core sand is wetted--this preservative and the binder are safe if ingested.

Bonding is accomplished by dehydrating the wet core sand mixture. No chemical reaction ever takes place. The biopolymers form bonds as water is removed from the core to form a crystalline structure.

One of the most important features of the binder is its water solubility. The binder behaves more or less plastically as water is either added or removed. This allows the bonding mechanism to be reversed if water is added back to the protein.

Preparation

Dry binder is added to hot sand during mixing. Next, water is mixed into the binder and sand until dry. Dry coated sand looks like fresh raw sand. The sand must retain enough heat to evaporate the water. Using more binder justifies a higher water addition to ensure even coating. The coating process increases in effectiveness as more mechanical force is applied to the mixture.

The batch method is an excellent way to prepare and use sand on an as-needed basis. Coated sand can be stored indefinitely. In a production sand coating facility, silica sand was coated with the binder. A sand heater preheated the sand before placing it into the muller. Batches of 1000 lb were coated with 1% binder based on sand weight in 5 min or less. Effective coating was verified with LOI and dogbone tensile strength tests.

Core Production

Prior to blowing a core, the cool sand is conditioned with water and mixed to evenly hydrate each coated particle. Sand should be cooled to give the hydrated sand particle better flowability. Excessive water has a detrimental effect on the core production process. In laboratory work, a vibratory mixer was used to condition a 50-lb batch in less than 1 min. Binder additions of 0.75-1.25% based on sand weight have been used to produce cores with adequate strength.

Coremaking operations are completed by the controlled dehydration of the sand mixture within the corebox. After blowing and packing the sand in the corebox, a drying cycle gives the core strength by the dehydration or curing of the core. Excessive heat in the sand mixture changes the sand's flowability, as the sand becomes sticky and begins to gain strength. It then becomes more difficult to blow thin, lacy sections. A lacy core with a higher surface area-to-mass ratio will generally activate and dry faster than a chunky core with a lower surface area-to-mass ratio.

Air pressure also affects airflow and drying time. Gradually increasing the air pressure will remove moisture and decrease cure time. Too much pressure can strip binder from grains located near the intake vents. Maximizing the venting is the most important variable in increasing airflow.

The drying cycle time must ensure adequate handling strength, but the core will continue to dehydrate and cure after it has been stripped from the corebox.

Production Trials

GM and Teksid S.p.A. collaborated in production trials at Teksid's Carmagnola, Italy facility on an aluminum suspension arm casting. This part was converted from a steel weldment. It employed a square tube design with thin walls (3-4 mm), which improved strength and reduced weight. This part had a high surface area-to-weight ratio, which does not provide much heat energy to thermally degrade the binder in the core.

Because it was to be used as an automotive safety part, conventional intensive shakeout would likely damage the casting.

A casting trial was performed using a core made with the new binder in the low-pressure semi-permanent mold process to evaluate shakeout and dimensional accuracy. Dry shakeout was accomplished by blowing compressed air through the casting after it had cooled for 30 mm in ambient air temperatures. Wet shakeout was performed by direct quench of the part after casting. A water jet to flush away sand trapped in narrow cavities followed the quenching. Dimensional accuracy was verified and showed no thermoplastic deformation. The casting surface finish was excellent despite a 45% lower binder content than the originally proposed furan hotbox core. In addition, the 11-hr heat treat and sand removal process could be avoided.

The production trial work was performed on a coldbox core machine that was modified to cool sand before mixing and blowing. The drying air was heated with an immersion type air heater. After the second day of trials, core scrap was reduced to less than 2% with a 60 sec overall cycle time.

While currently no U.S. foundry is using it, a number of aluminum and magnesium operations are performing additional application testing to determine practicality of the binder system in production.

Binder Considerations

The binder's cores are typically stripped with some residual water remaining. The drying cycle will purge most of the moisture from the core. The heat retained during the activation of the binderwill provide a "through-cure" as the remainder of the core dries and gains additional strength. Cores can be handled safety out of the box and used immediately in casting. Most of the strength is obtained within the first few hours of manufacture.

The full strength of the binder is most often not necessary when producing high-production castings. However, in certain applications, strength would be beneficial to the selected casting process. Full strengths are achieved when all water is removed and the binder is 100% crystalline. This can be accomplished via three different methods: conventional, radio frequency or microwave oven.

Removing all moisture from the core will increase the core's strength and maximize dimensional accuracy (Table 2). Some degradation of the binder may occur if exposed to severe dehydration cycles.

The binder will become crosslinked when exposed to aldehyde compounds or gases. Both the binder and coated sand must be kept away from any source of aldehyde type compounds, especially during coremaking. In addition, all equipment used during the process must be free of any formaldehyde containing resins.

Storage

Cores retain most all of the original surface hardness but lose tensile when exposed to high humidity. If additional strength is necessary, the cores can be redried and wilt regain full strength (Table 2). A refractory coating also can be applied to improve the humidity resistance. During developmental work, cores never lost handling strength. Cores that have been kept for years still retain their surface finish without becoming friable.

Refractory Coating

As with other binder systems, refractory coatings can be used to improve the surface finish and reduce humidity degradation. Alcohol- or water-based coatings may be used with this binder.

Also, a process was developed and patented by the GM researchers for coating the binder's cores. It involves waterproofing the core prior to coating with a water-based refractory. An alternative method was developed that involves the powder coating of a refractory material, which would be used instead of the water-based coating. Also, it would make it easier to specify which areas are to be coated. Neither of the two methods affects the core's environmentally friendliness because the materials used to waterproof the core are biodegradable.

Sand Effects

The raw sand moisture content is of little concern as the binder uses water as the solvent. Raw sand moisture content will not affect the tensile strength or core performance. In practice, sand should be preheated prior to coating so most of the moisture would be removed prior to the coating process. However, if a sand heater were not employed, the coating process would only be lengthened by a part of the total coating cycle time. Any coated sand should be kept away from moisture until it is to be conditioned.

The coated sand bench life (which can be several hours, as long as the mixture remains "wet") is not affected by sand pH. There is no chemical reaction that may be influenced by sand pH or the ADV.

Raw sand and coated sand temperature are not a concern until the sand is ready for use. Raw sand is heated before the coating process, but the heat alone will not bond the dry coated sand. Therefore, seasonal temperature variations in stored sand are not of much concern.

Rounded grains work well as they facilitate good migration of the binder to the contact points. Angular and subangular sand also have shown excellent strengths. Sand surface area should be considered in determining the proper binder addition.

Environmental Performance

Any scrap core made with the binder can be recycled without having to add any additional binder. This includes any leftover sand that will not be used before it will dry (sand left in sand hopper at the end of the production shift). Tests have shown that cores can be made with little loss of strength or surface hardness. To reuse scrap cores and leftover sand, simply break the coated sand into individual grains and add back into the system as sand that has just been coated.

The binder does not require the use of toxic chemicals or chemical scrubbers. Improved shakeout and water solubility will eliminate dusts generated during intense core removal processes. There is relatively no odor during coremaking, and employees do not need special personal protective equipment. In addition, water can be used to clean tooling, and core machines do not need to be enclosed.

The ability to easily reclaim and reuse sand reduces the waste generated. Spent sand easily can be reclaimed (thermal, wet or mechanical) without creating additional hazards. Flawed cores can be reused and don't have to be thrown away.

Casting Performance

In trials, castings made with the binder had low core-related scrap problems. In addition, the binder did not lose strength until after aluminum was poured.

The binder forms a crystalline structure when it is dried. Therefore, it does not undergo any thermoplastic deformation.

The binder can be used in iron casting applications where there is a higher surface area-to-weight ratio, but heavier sections have a tendency for penetration defects.

In aluminum and magnesium casting, the cores' hot strength is adequate.

The binder breaks down at lower temperatures with the aid of the catalyst. The gas evolution is predictable and somewhat accelerated. This is how shakeout can be performed with greater ease. The amount of total gas evolved during casting may be decreased since less binder can be used to produce cores of adequate strength.

One of the most important features of this system is the simplified core removal in aluminum and magnesium casting. The heat at those pouring temperatures will degrade the binder sufficiently to allow for dry shakeout of the core, without having to perform thermal core removal. Often, the core sand will flow relatively freely from the casting.

Another important feature is the ability to do a wet core removal. Submersing the casting in water will dissolve binder that has not degraded thermally. Any remaining sand can be flushed away from the casting.

Further Research

Research is ongoing in the effects of contamination into the binder with partially decomposed binder (e.g. a semipermanent mold), profiling the binder to see the full spectrum of usefullness with varuious aggregates at various binder levels, and to optimize the system.

The binder also has been submitted to the Casting Emission Reduction Program foundry, Sacramento, California, for emissions testing, and results will be available by year's end.