Modeling programs that describe mold filling have made tremendous progress in the past 20 years in their ability to calculate metal velocities that can be used to minimize splashing, reoxidation, bubble trails, folds, and similar anomalies that degrade casting properties. Now, the industry is accelerating into new core design and production technologies.
Historically, core box designs were based on experience. Foundrymen have focused on metallurgy for a hundred years, but comparatively little has been published on core making technology such as box design, vent theory, and core quality measures.
Trends in Tooling Design
A review of tool design since the phenolic urethane cold box (PUCB) binders were introduced in the late 1960s will help understand the current trends. The 1970s were the practical beginning of the cold box era. During this time, significant investments were made to convert many shell, hot box, and sodium silicate bonded cores to the PUCB process. The economic benefits of the process derived from high production rates, and reduced core costs per unit produced.
The growth of cold box processes was impressive throughout the 1980s. New processes, including SO2-cured acrylic epoxy and ester-cured alkaline phenolic resins, were introduced, and improvements were made in the PUCB system. Equipment manufacturers introduced the appropriate gas generation, core making, and scrubbing equipment for the cold box processes. As the economic benefits found in the 70s became harder to improve upon, an emphasis was placed on understanding the effects of chemical variations and the physics of blowing and gassing all of the binders. Significant research was done in tooling design, and guidelines were developed to ensure that core makers could use the new technology and save money. These guidelines have appeared in AFS Transactions and have been published by several industry suppliers.
During the 1990s, high production cold box processes became affordable to foundries of all sizes and came to dominate the market. But the intellectual property once present in most captive pattern shops was lost because of downsizing and early retirements. In addition, some foundries began to outsource tooling design and core production to independent pattern and core shops. However, some “industry experts” kept up with the technology and continued their research and publication
In one case, an automotive company, a resin manufacturer, and some experienced computational fluid dynamics (CFD) professionals joined forces to develop ‘math models’ to simulate cold box core making. Arena-flow computer aided engineering (CAE) coupled cold box tooling design with process optimization, and the engineering model became a reality in 2001
This technology was built on over 30 years of experience with cold box core-making and fluid dynamics research. The technology will be as useful as solidification and machining software now used in the foundry industry. The technology offers foundries reduced time to market, tooling design flexibility, and core making process design.
The primary benefit is performance analysis of proposed tooling designs. This CAE technology assists designers with core geometry and layout for multiple cavity core boxes, blow tube selection and location, vent selection and location, and other aspects of tooling design. Complete processes can be modeled and optimized with respect to gassing equipment, core-making equipment, and consumables (resin, catalyst, sand, etc.) selection.
Tool design requires some input values to set up the geometry. A grid pattern for a water jacket core is illustrated.(Image A) Needed information includes:
- CAD files representing 3D geometry of the core blowing machine, blow tubes, and core box geometry.
- Sand characteristics (type, size distribution, density, and binder loading).
- Vent characteristics (type, size, location, open area).
- Process conditions (blow pressure and time, gas pressure and time, purge pressure and time).
The software then solves equations for flow and calculates core blowing and gassing parameters. (Image B) Output information can be in animation and an image formed for detailed analysis. Blowing and gassing simulation can provide optimized tooling and process parameters, and the design can be finalized based on core quality, cycle time, and desired behavior, including cavity filling pattern, tool wear, and cleanliness.
The technology, coupled with precision core boxes, allows more complicated and accurate core and mold assemblies than previously possible. Progress is being made in automation to assemble complicated core packages with minimum labor.
New Binder Technology
Oxides, bubble trails, and subsurface porosity are some of the most aggravating defects encountered in castings. These anomalies are difficult to detect and are often not found until castings are machined. A blow hole and an associated oxide trail in a high performance head casting is illustrated. The gas came from a water jacket core, produced a layer of oxides, and collected in a boss where it produced a hole that was discovered during tapping.
There is a large amount of development work going on worldwide to improve the performance of core binders and to make them more environmentally friendly. There is renewed interest in sodium silicate, modifications of the PUCB and alkaline phenolic resins, and new binders being introduced. PUCB binders are the benchmark in the industry, and an industry goal is to develop products that are as fast as PUCB, provide an equally good casting surface, improve the shakeout behavior, and eliminate the use of noxious gaseous catalysts and scrubbers. LKBinder, GMBOND, and ECOLOTEC, are examples of recent innovations.
LKBinder is intended primarily for aluminum castings. It is reported to be free of odor, dimensionally stable, produce smooth casting surfaces, and be used in a variety of molds including green sand. The binder is in a water carrier and is thermally cured in a warm box. The heat evaporates water from the binder, causing cure at the pattern surface. When the core is partially cured, it can be taken from the core box, thoroughly dried, and placed in a mold.
After solidification, cores can be mechanically removed or placed in water and the binder dissolved. The system eliminates the need for a gas scrubber around the core machines and reduces ventilation requirements around work areas.
GMBOND is based on a unique protein that forms a reversible gel in water and dries to a rigid, partially crystalline polymer when the water is removed. Cores are made by removing moisture with heat. No reactive chemicals are used, so the core making area is safe for the workers. The binder is biodegradable, and there are no hazardous chemicals or wastes to deal with.
Cores made with GMBOND are dimensionally stable, produce a smooth casting surface, and are easily removed from aluminum castings. Emissions from castings made with protein binders are reported about a tenth of the level of Hazardous Air Pollutants (HAPS) found with PUCB binders.
The ECOLOTEC Process is based on an alkaline water-based P-F resin, cured with carbon dioxide (CO2). This material is also reported to provide a clean, safe, high-production, gas-cured core making process for aluminum, copper based alloys, iron, and steel castings.
Emissions during sand mixing, core making and core storage (SARA 313, Form R reportable compounds) are said to be dramatically reduced, as are emissions during pouring, cooling, and shakeout. The process involves no flammable components, and since cores are cured with CO2, no gas scrubber is required. This binder is a virtually odorless.
Core production rates with the ECOLOTEC Process often exceed those of other organic gas-cured processes. The curing gas cycle may be slightly longer than used for PUCB processes, but no purge cycle is required.
Casting surface quality is reported to be exceptionally good. Veining defects are virtually non-existent — without the use of “anti-veining” sand additives.
CORDIS, an inorganic system introduced in the early 1990s, is based on polyphosphate chemistry. The binder provides strength, collapsibility, humidity resistance, and enhanced flow of the coated sand. This system is in commercial use today. It reduces defects, and has excellent dry or wet shakeout characteristics.
Phenolic urethane systems are also being modified and improved for performance and environmental characteristics.
These developments include “Biodiesel,” or methyl ester solvents for lower VOC emissions during core making and lower HAP emissions at pouring, cooling and shake-out. Biodiesel solvent-based PUCB’s offer better release, dip and redry performance, and improved curing speed, which lowers the use of amine catalysts.
A new catalyst called DMIPA (dimethylisopropylamine) has also been introduced that has a higher reactivity than TEA. The high reactivity reduces catalyst consumption, causes faster curing, and improves the work place by reducing odor. This catalyst is not regulated under the new MACT standards for iron foundries.
Work also continues on improving the ester-cured phenolic cold-box system (ECPCB). This technology features a water-based binder with low VOCs, high hot strength, and low odor during mixing and core making and low smoke and HAP’s during pouring, cooling and shake-out. This product technology also exhibits lower expansion defects, especially veining in iron castings and hot tears in steel.
There is also a renewed interest in sodium silicate binders sparked by the environmental advantages, performance breakthroughs, and historically stable pricing. DevCo 285 is a modified, hybrid sodium silicate that produces dimensionally stable cores for all types of metals. It can be cured with liquid hardeners, CO2, heat, and with microwaves. The binder has virtually no odor or smoke during or after pouring.
The future of tooling design has become relatively clear. Computer aided design will play a significant role in complex cold box tooling, and the skills of manufacturing engineers will change to use it. Tool design will become more fully integrated into the design chain internally in Tier I foundries, Tier II jobbing foundries, and contract pattern shops.
Precision cores will open the way for mechanized core assembly to reduce the labor content.
Binders are rapidly evolving to minimize casting defects, minimize emissions, and produce smooth cast surfaces. Core removal from aluminum castings has historically been troublesome, but innovations in water soluble binders are rapidly eliminating this problem. The future for core technologies used in the foundry industry is full of promise.
Acknowledgement is made of many companies that provided information and helped review this article. The contributors in alphabetical order include: Arena Flow LLC, Ashland Specialty Chemical Company, Foseco Metallurgical Inc., HA International LLC, Hormel Corporation, JB DeVenne Inc., and Laempe-Reich Company.
Arena-flow is a trademark of Arena-flow, LLC; ECOLOTEC is a trademark of the Foseco Group of Companies; GMBOND is a trademark of Hormel Corporation; CORDIS is a trademark of HA International LLC; DevCo is a trademark of J. B. DeVenne Inc.; and LKBinder is a trademark of the Laempe Corporation.