GM develops a breakthrough 'green' binder for core sand.

A new biopolymer-based core sand binder for aluminum casting is nontoxic, recyclable and shakes out easily. It's also. . .edible.

Editor's Note. Of all the questions I've asked in the course of this job about new products, the strangest by far came in working on this article: "How does it taste?" The fact is that this new binder is so environmentally benign that at least some of the authors have tasted it. Incidentally, the response was "It doesn't have much flavor." Guess we won't see it in the supermarket anytime soon.-- Matthew L. Philbin, Associate Editor

The advent of high-volume core production has made it possible for the modern commercial foundry to produce thousands of large, complex castings each day. The production and use of these castings, however, pose several problems of their own. Coreroom workers may be exposed to various noxious and toxic gases used to catalyze the rapid hardening of the cores, and the handling of these gases imposes significant costs for the foundry. Also. because these cores are literally glued together with a polymer plastic material, any flawed cores cannot be recycled at reasonable cost, and therefore become a bulky waste stream.

Other problems with core binder systems relate to the type of castings produced. Since most commercial core binders were developed for cast iron applications, the temperature required to break down the core within the casting is high, making their removal from aluminum and other low melting point alloys difficult. This means that aluminum castings, which: are increasingly demanded by the automotive industry, may require expensive and problematic additional processing to break down the cores for removal.

However, General Motors: Corp.'s Research and: Development Center and Powertrain Group's Advanced Development Lab have formulated a core sand binder that addresses each of these problems, while showing strength and durability equal to conventional commercial binders. This article details the development of GMBond.

Addressing the Issues

In the early 1990s, researchers at the GM Research and Development Center began work to evaluate alternative materials known to have binder-like qualities, and to evaluate potential processes for applying those properties in binding sand.

Generally, the materials had to be nontoxic and not produce toxic combustion products during casting. They also had to function in a process that used nontoxic or minimally toxic solvents, chemical intermediaries and catalysts using current production equipment These products should also have tensile strengths greater than or equal to the phenolic urethane amine binders in wide commercial use, allowing them to be manipulated in the same ways.

Early studies showed good cores could be produced with starch, vegetable gums, biopolymers and other materials. Subsequent testing eliminated candidates based on low tensile strength, thermoplastic flow at casting temperatures, difficulty of use, high cost, low commercial availability and other factors that might limit their applicability to high-volume production.

From this study, the protein-based polymers emerged as the most promising. Since one of the binder's applications would be for use with molten aluminum or magnesium--metals with melting temperatures as low as 1202F (650C)--it would have to be engineered to break down at lower temperatures for easy core removal. To enhance this: quality, researchers added various metal oxide catalysts in different concentrations to the biopolymer as it was applied to the sand. This had the desired effect of reducing the temperature needed to weaken and destroy the binder through oxidation, while reducing the time required for breakdown at any given decomposition temperature

This mixture of different molecular weight protein biopolymers and mete oxides, in its various proportions, was trademarked by General Motors under the name the name GMBond.

Physical Properties

Researchers performed dogbone tensile testing with various commercially used sands and varying binder concentrations. The tests showed that GMBond samples having 1% binder by weight were as strong or stronger than conventional phenolic urethane binder samples. Compression tests on cylindrical samples showed compressive breaking strengths of 1000 psi for lake sand and 3000-4000 psi for zircon sand.

Electron photomicrographs of the fracture surface of core samples reveal that the binder attachments between sand grains appear very similar to those of other binders (Fig. 1). Also. the new binder makes a good contact film over the sand grain before hardening. One difference is not in the attachments themselves, but the webbing or bridging that occurs between attachment points (Fig. 2). The webbing may help to stabilize the structure and create a network connecting multiple sand grains together, thereby strengthening the entire core. This effect, plus the tensile strength of the binder itself, appears to explain the high breaking strength and good handleability of these cores.

[Figure 1-2 ILLUSTRATION OMITTED]

Recyclability

The ability to recycle cores is highly desirable at several steps in the manufacturing process. The first :is after the cores are blown and flawed cores are identified. GMBond is water soluble, and this aspect was investigated by immersing hardened cores in water. The cores completely dissolved in seconds or minutes, depending on their size. This test proves that the cores can be easily recycled by dissolving the binder, adjusting the moisture content in the sand and reusing it in a new core. Depending on the complexity of the core and the scrap rate, such recycling could give foundries considerable savings in coremaking and substantially reduce solid material disposal problems.

Reclamation of the sand further into the casting process depends on its breakdown at elevated temperatures. The GM researchers: studied this characteristic using the loss on ignition (LOI) test, measuring weight loss with time of heating for a core sample. Core samples were prepared with and without the metal oxide catalyst and tested for LOI at two temperatures representative of the postcasting or heat treating environments. Figure 3 shows that at 748F (399C) and 1000F (538C), the core binder will be considerably destroyed after 60 min. However, the addition of a small amount of metal oxide catalyst can significantly accelerate this process. so that most of the binder is gone in 15 min.

[Figure 3 ILLUSTRATION OMITTED]

A third series of tests was run on sets of small, cylindrical core samples made using the iron oxide catalyst to determine the effect of temperature and catalyst content on core collapsibility for easy shakeout and recycling. The core samples were heated in an oven at 842F (450C) and 932F (500C), and checked at 10 min intervals to assess the degree of core collapse and/or sand cleanup. The stages were arbitrarily defined as follows:

Stage 0: new sand core;

Stage 1: charred large pieces;

Stage 2: charred smaller pieces;

Stage 3: sand and fine pieces:

Stage 4: fine sand colored;

Stage 5: fine sand.

At 842F, the plain sand--called control--only reaches Stage 3 after 60 min of heating. This improves with the addition of iron oxide so that the sand is entirely recycled after 60 min. At 932F, the control sand reaches Stage 3 after 30 min and is entirely clean after 60 min without the use of a catalyst. Adding the catalyst accelerates the process so that Stages 2 and 3 are reached in 10 min, Stage 4 in 20 min and Stage 5 in 30 min. These tests suggest that shakeout should be easy with this core binder, and minimal additional heating should be necessary to clean up the sand for reuse.

Practical Experience

Cores of adequate strength for production are usually blown using [is less than] 1% binder by weight. Test cores made with a variety of sand have a surface quality comparable to those made with the current phenolic urethane methods. The cores have excellent humidity resistance, with no surface deterioration after weeks at high ambient humidity. High humidity does diminish the tensile strength of the cores, but large cores are still handlable after weeks, and can, if necessary, be strengthened by redrying.

Complex shapes such as the brake rotor core and the water jacket that appear on the cover of this magazine exhibit excellent shelf life. In making these cores, odor is greatly reduced, and the finished core has no detectable odor after leaving the corebox. Workers were exposed to no toxic gases during production, and it can be seen that the binder considerably improves working conditions in the coreroom.

GM engineers and technicians poured iron, aluminum and magnesium castings with cores made with the binder. While most iron castings require a coating for best results, their shakeout with GMBond cores is excellent. Most of the testing and experience in practical casting with the new binder has been in aluminum, which shakes out well and displays an excellent finish on cored surfaces. The photo at left in Fig. 4 shows an aluminum casting on which all surfaces were defined by cores. It displays a clean, high-luster surface with no burn-in. The core sand was poured out of the part as soon as it was cool enough to handle. By contrast. the photo at right shows the same part produced identically with the exception being cored with the standard phenolic urethane binder. While its surface quality is also good, the cores have not broken down at aluminum pouring temperatures and must be removed through mechanical means or heat treatment or both.

[Figure 4 ILLUSTRATION OMITTED]

As for the future. GM is planning to use this process technology in its own manufacturing facilities, and is also exploring ways to make the technology available to other metalcasters.

This paper (No. 96-189) was originally presented at the 100th AFS Casting Congress. The complete paper can be obtained from AFS by calling 800/537-4237.

RELATED ARTICLE: What's So Revolutionary About These Cores?

They're made with a binder that:

1. Eliminates noxious and toxic gases from the coreroom.

2. Has no odor in the coreroom.

3. Is made from environmentally benign materials.

4. Causes less emissions than present core binder systems.

5. Shakes out easily from aluminum and magnesium allows without further processing.

6. Is usable with cast iron.

7. Can be easily recycled to eliminate solid waste.

8. Is reclaimable at conventional heat treat temperatures.

9. Exhibits the same strength and imparts the same finish as conventional binders.

10. Has excellent shelf life and humidity resistance.