In search of a cure: optimizing coldbox core systems.
This article focuses on the many factors that can influence the coldbox coremaking process and provides foundries with ideas to keep their coremaking operations running as smooth as possible.
As the name indicates, coldbox coremaking does not rely on intense heat but instead modem chemistry to produce a solid sand core. This chemistry can be described in six major sub-groups:
* PUCB--phenolic urethane coldbox;
* AECB--acrylic epoxy coldbox;
* ECFCB-ester cured phenolic coldbox;
* [SO.sub.2]CB--sulfur dioxide cured coldbox;
* SSCB--sodium silicate coldbox;
* PRCB--[CO.sub.2] cured phenolic resole coldbox;
Of these six groups, 98% of all coldbox cores manufactured in North America are either PUCB or AECB.
All resin or binder systems function on the same basic principle. First, the resins or binders are evenly distributed over the surface of the sand grains through mixing or mulling. After the mixed sand is blown into the tooling, a catalyst or co-reactant gas is forced into the sand mass to cure the resins or binders. The resultant cured resin forms "bridges" at the points where each of the sand grains come in contact with one another. The finished core can be visualized as a porous "composite" of sand and resin.
When close to 99% of the composite is composed of a specific sand type, the sand characteristics will have a significant influence over the final core's characteristics. However, most foundry binder systems are amazingly robust. Even though only 1-2% of the composite or final core is composed of the cured resin or binder, these resins can have a significant influence on primary core and casting characteristics such as strength and humidity, erosion and veining resistance.
Most foundrymen are aware that sand shape, AFS grain fineness number (GFN) and distribution have a significant impact on core strength properties. However, most are unaware of how these sand variables influence mixing. Three examples include:
* round grain shapes are easily coated while angular ones are harder to coat;
* finer AFS/GFN sands have higher surface areas requiring longer mixing;
* high-density specialty sands such as zircon require longer mixing;
At first glance, the process of mixing coldbox sand is:
1. Measuring a specific volume of sand, part 1 resin, part 2 resin and (if needed) iron oxide.
2. Placing these ingredients in a vessel.
3. Moving the sand grains until they are all equally coated with both resins.
4. Discharging the finished product for core production.
Looking deeper into the process, precise additions of the sand and resin are critical to assure proper cured-sand properties such as finished core tensile strength and combustion gasses generated during pouring [loss on ignition (L.O.I.)]. Equally important is the amount of mixing action applied to the sand. Enough action must be performed to maximize chemical distribution across each grain and minimize factors such as temperature buildup, sand degradation and length of mixing time.
Sand mixers accomplish this task in two basic formats--batch and continuous (Fig. 1). Batch Mixers produce sand in pre-established quantities with the ingredients being added in sequential order. At the end of the mixing cycle, the complete mixed volume (batch) of blended sand is discharged from the mixer at one time. Continuous mixers produce mixed sand with each ingredient flowing into the mixer at a constant flow. The mixing process continues as long as the mixer is activated.
With either type of mixer, consistency of material additions is critical for successful mixer operation and the quality of the prepared sand. Having consistent sand delivery is as important as accurate chemical additions. In addition to the sand and resin controls provided with each mixer, care must be taken to follow the manufacturer's instructions regarding the manner in which materials are supplied to the mixer.
It is said that fire, water and earth are the primary elements in all things. Therefore, it should come as no surprise that temperature and moisture can play a significant role in the quality of the mixed core sand, the blowing of the cores and the storage of the finished cores prior to use.
The effects of moisture in the air used to both blow and purge coldbox cores can vary based on the chemistry in use. Some water-based binder systems can be more resistant to the negative effects of moisture. However, these negative effects always are present and more pronounced when common coldbox systems (PUCB and AECB) are used.
Every coldbox process can benefit significantly from the use of dry air. The use of blow or purge air containing even moderate amounts of moisture can result in the condensation of moisture on the surface of the individual sand grains. If this occurs, the binder system cannot bond properly to the sand surface.
When a stress is placed on the core during storage, handling or casting, the resin bond can pull away from the sand surface causing an adhesive" failure. This adhesive failure usually occurs at stress levels 50-80% of that required to break the bonds created by the resin bridges. This adhesive failure phenomenon can result in a significant increase in core or casting scrap and is difficult to identify during normal root cause investigations.
To establish processes that are robust and resistant to adhesive failure, desiccant regenerative air dryers are required to dry the blow and purge air to dew point levels of -10 F to -40 F at one atmosphere.
Sand temperature, as well as ambient temperature, also can have a major influence on productivity and scrap rates. The impact of sand temperature follows the "10G Rule." Every 10G (18F) increase in temperature will result in a two-fold increase in the speed of resin reaction. Also, every l0C (18F) decrease in temperature will result in a two-fold decrease in the speed of a resin reaction.
The "10C Rule" is a phenomenon that most operations tend to overlook and struggle with during both the summer and winter. Establishing core "production" sheets that contain the values for binder percentage, sand temperature, blow, gas and purge time, and blow pressure will provide the operator benchmark values to compare current results. Deviation from the benchmark values would necessitate investigation and aid in narrowing the possible causes for a defective core or casting defect.
While the plethora of different tooling designs, orientation, materials of construction and blow configuration would seem to present an exponential list of variables to control during coremaking, observing the three basic tenets of contain, control and maintain can help keep the actual core production operating smoothly with minimal waste and an improved production environment.
Contain is the intent to keep the sand and catalyst inside the tooling. This can include providing seals between the tooling halves, between the tooling and the blow head, or between the blow tubes and the tooling invest holes.
Control refers to the corebox rigging, which can affect clamp pressure, blow pressure, cycle times and gas evolution from the finished core.
Maintain is simply the idea that maintaining corebox seals, blow tubes and regular cleaning of the core vents can ultimately increase production and the quality of the finished cores.
Historical techniques for establishing core venting patterns always have been considered an art. Proper venting is one of the most important keys to coldhox coremaking. The type, size, placement and number of vents will have a big impact on the ability to blow and cure a core with the minimum amount of catalyst or co-reactant, and purge any remaining gas from the core.
The first consideration with venting is the type of vent. Selection of either a slotted, sheet-screen or woven wire vent will depend on the location of the vent in the corebox, the surface characteristics required on the finished casting and the open area of the vent needed to input the gas or allow it to escape.
For location, slotted vents are practical for curved shapes because they can be contoured to fit into almost any area but have the least open area (13%). Woven wire vents are preferred due to the increased open area (40%), but cannot be used in many circumstances because they can leave a "track" on the core that can imprint the finished casting. Sheet screen vents have moderate open areas (25%) and can be a compromise.
In the past 15 years, several mathematical formulations for vent utilization were developed based on the total input area of the cope half of the tooling and the total exhaust area of the drag half of the tooling. These mathematical formulations were somewhat predictive, but varied widely based on core size and shape as well as the type of binder system. Additionally, several optimization trials were required to adjust the vent locations and achieve optimum catalyst utilization and minimum cure rates and cycle times.
Recently, the Horton/Lewis Venting Matrix Method ["The Venting Matrix Method for Coldbox Tooling Design," 2001 APS Transactions (01-085)] was developed and found to be simpler and faster to optimize. This method uses simple "grid pattern" venting techniques and is based on the theory that the permeability of the core's sand mass will provide sufficient back pressure to distribute the catalyst or coreactant evenly (Fig. 2).
A simple pattern can be made to locate 0.5-in, diameter input vents on the cope half of a tooling cavity at 1,5-in, centers. A similar pattern can be made to locate 0.5-in, diameter exhaust vents on the drag half of a tooling cavity at 2-in, centers. These patterns can then be overlaid on the tooling to determine vent location.
Care Machines--Pulling It Together
To achieve any sort of production speed and core uniformity, the core machine can make the most of the modern coldbox binder systems. A core machine combines the three cycles of core production--blowing, gassing and purging--into a repeatable, consistent process. While some differences exist between models, core machines function by filling a chamber with mixed sand, sealing the chamber off (except for the passages between the blow head and tooling) and then pressurizing the chamber to "blow" the sand into the tooling.
The "gassing" cycle involves the controlled input of the catalyst or co-reactant through the sand mass inside the tooling to initiate the cure. The third cycle is the air purge that exhausts any excess catalyst or co-reactant from the cured core. With the PUCB and AECB process, the gassing cycle will introduce the minimum amount of catalyst necessary and allow the purge air to push the catalyst through and out of the sand mass.
The effect of moisture in the blowing air on the mixed sand can have an detrimental effect on the operation of the core machine. Excess moisture can accumulate on the screens that allow the blow air to exhaust from the chamber after the cycle. This moisture reacts with the sand and binder to reduce the flow through the vent, requiring more frequent maintenance. For this same reason, the addition of lubrication to the blow air should be avoided.
The pressure of the air used in the blowing of a core can have significant effect on core production. An increase in the air pressure equals an increase in the velocity of the sand grains passing through the chamber and into the corebox. Increasing velocity can cause the resin to "wipe-off" the sand grains and deposit onto the wall and vents of the tooling. These deposits can build up quickly and result in sticking cores and plugged vents.
The manufacturer's recommendations regarding blow pressure should be followed. For silica sand, typical pressure ranges between 30 and 40 psig. This provides consistent core density without unnecessary velocity inside the corebox. Pressure requirements higher that these levels may indicate the need for additional venting on the corebox. A general "rule-of-thumb" is to blow with the lowest pressure possible to achieve a dense core. This will reduce "wipe-off" and cleaning and prolong tooling life.
Catalyst Gassing System
Regardless of the coldbox process, the equipment designed to provide the catalyst or co-reactant as a vapor is a key system component. A good design will provide efficient, repeatable gassing results while maintaining control over the chemicals to address safety concerns and improve the coreroom environment. Maintaining the equipment to the manufacturer's design and recommendations will reduce the potential for inconsistent operation.
Optimizing Coldbox Coremaking
Following are some general suggestions for improving the day-to-day operations in the coreroom.
Garbage In, Garbage Out--One of the simplest ways to avoid problems is to have quality control procedures in place when dealing with the resins and catalysts used in the coldbox process. Binder and catalyst storage requirements vary based on the classification of each material.
In general, minimum storage requirements suggest that binders and catalysts be stored in a clean and dry environment, which will prevent contamination and maintain the temperature between 50-90F.
Specific product storage requirements can normally be found on the material MSDS or in technical literature supplied by product specialists. Most binders and catalysts have a shelf life that exceeds 6-12 months. However, several coldbox binders exist that can self-polymerize or "advance" under elevated temperatures.
Sand Equipment--The fact that each mixer type has a different level of efficiency and that binder level percentages can have a large impact on the finished core suggests that "mixer efficiency studies" are important. Mixer efficiency studies are an easy way to determine the appropriate mixing times for each mixed sand formulation and to obtain the highest performing mixed sand.
Tensile tests evaluating strength and bench life as a function of mix time can be run to determine optimum mix times. By watching the strength and bench life values associated with a single sand mix as mix time is varied, the optimum mix time can be determined for any specific style of mixer.
These efficiency studies can have a impact on productivity and core scrap by identifying the proper mix time for maximum bench life and core strength. Regular sampling and testing of "test cores" can be a valuable tool to help identify a core bond problem before it reaches the mold line.
Calibration--Depending on mixer type, the recommended frequency and method of calibration can vary. However, the methodology of the process doesn't.
1. Follow the manufacturer's recommended frequency and procedure. The purpose of the calibration procedure is to ensure that all the parameters of sand mixing are within specification. It is structured to catch any process deviations before they are harmful to the end product.
2. Establish a procedure and enforce it. The designs of most mixing systems provide consistent delivery of materials. It is common for many systems to run months after their initial setup without any required adjustment in the sand or chemical supplies. When this occurs, it is common for the frequency of calibration to diminish.
Question--Why check it when it never changes?
Answer--Because some day it will. If there is not a regular, consistent calibration policy established and enforced, then the foundry is waiting for the process (or the customer) to tell them when something is wrong.
3. Make calibration simple to accomplish. If mixer calibration is simple and easy to do then the likelihood of its completion is much higher. Make sure that the proper containers and scale(s) are in good condition and readily available. Most newer mixers have calibration timer systems in place to remove the need for manually operated controls. If a mixer is not equipped with one, check with the manufacturer regarding the possibility of retrofitting one to a system.
Daily Mixer Maintenance--Whether batch or continuous, daily inspection and cleaning of the mixer is an important and necessary part of its operation. Buildup of sand and chemicals on the interior of the mixer and the mixing blades is a continual process that, if left unhindered, will continue to grow until it affects the quality of the mixed sand or the mixer's ability to process it.
The cleaning procedure varies depending on the design of the mixer but it generally includes removal of the sand! resin buildup on the mixing blades, drive shaft and discharge door (seal). On most continuous mixers it is advisable to clean only the mixing blades and drive shaft. (The build up on the inside of the mixing chamber is an intentional part of the mixer design and is not intended to be cleaned daily.) it is more cost-effective to spend a small amount of time each day cleaning the mixer as compared to waiting until production is compromised or even interrupted.
Inspection of the chemical system should include checking the points where the chemicals enter the mixer to ensure there is no buildup or blockage. Inspect the chemical delivery hoses to ensure that there are no entrapped air bubbles. Inspect the pumps or metering devises for any leaks.
Chemical supply tanks or drums must be vented to atmosphere for proper chemical supply. It is important that the tank vents include a desiccant filter to remove all moisture (humidity) from the vent air entering the tanks. This is exceptionally important on the Part 2 (PUCB) resin, which crystallizes when exposed to moisture.
Heater/Coolers--While all sand heaters and coolers require periodic service inspections, the majority of maintenance issues come from two areas.
Fluid bed heaters that use compressed air for fluidization have a low tolerance for moisture and oil from the plant air system. In most designs, the water/oil carried in the compressed air collects on the heater's fluidizing membrane, reducing its permeability and ability to fluidize the sand. Each heater should include a water separator/filter device as part of its air system. Periodic inspection, draining and cleaning of this filter helps to ensure that contamination of the fluidization membrane is prevented.
Fluid bed sand heaters and heater-coolers are excellent collection points for non-sand grain objects. objects such as agglomerates, metallics and shot require more airflow for fluidization than a grain of sand. Therefore they collect in the bottom of the fluid bed. At some point, this layer of foreign material will grow until it forces the fluidizing air to another part of the heater (path of least resistance).
Core Machine-Different manufacturers have varying recommendations regarding the maintenance of coremaking systems. Care must be taken to properly clean the machine at the end of production. Daily cleaning of the blow chamber vents can reduce unscheduled maintenance and loss of production.
Maintaining the mechanical motion components of the machine is no different than with any other machine. Providing proper lubrication, verification of free motion and moisture drainage from blow pressure and accumulator tanks and filters takes little effort but can go a long way toward keeping the machine operating properly.
Gassing System--Most gassing systems will convert a liquid catalyst, or co-reactant, into a vapor for curing the core. Whether it is a "dosing" system that accumulates a measured charge or an "on-demand" injection-type system that meters the catalyst as it is needed, both will benefit from a clean, contaminant-free supply of fresh catalyst.
Manufacturer's recommendations must be followed regarding the routine inspection of valves, pumps and other system components where leaks can form. Also, inspect the purge air delivery system to insure that it is free from pipe scale, rust and debris that can reduce or block the flow of air for curing or purging the core. An important diagnostic tool is a standard pressure gauge installed in the gassing plenum of the core machine.
The backpressure generated during the curing and purging phase can indicate proper operation of the gassing system or improper venting of the corebox. Typical values to expect are 2 to 8 psig during the gassing cycle.
Controlled Heat--The proper use of elevated temperatures can improve the overall productivity of a coldbox operation. During core gassing, the various carrier gasses and catalyst can be heated to elevated levels resulting in significant productivity and employee exposure benefits. In the PUCB process, depending on the catalyst, both the amine and carrier gas should be heated to a recommended temperature level of approximately 150F. Elevation of the amine and carrier gas to as high as 250F will have several benefits including:
* the use of less amine;
* faster cycle times;
* less retained amine in the cores;
* higher out-of-box strength.
The same phenomena holds true for the AECB system whereby heating the [SO.sub.2] or [SO.sub.2]/nitrogen blend will have the following benefits:
* the use of less [SO.sub.2];
* faster cycle times;
* less retained [SO.sub.2] in the cores;
* higher out-of-box strength.
The elevated heating of the catalyst gasses and carrier gasses can be coupled with increasing the temperature of the purge air. Elevated heating of the purge air will increase the level of benefits outlined above. In combination, this use of elevated temperature can improve productivity by 40% without negatively impacting the other variables in the process that are temperature dependent (Fig. 3).
Release Agent--Release agents have two general functions--prevent the resin from bonding to the tooling during the curing phase of the process and protect the tooling surface.
Of all the materials used during the coremaking process, release agents are the least understood and most often applied incorrectly. Most modern resin systems contain internal release packages that provide sufficient release characteristics. However, there are cases when an external release agent is needed. During those periods, release agents should be applied sparingly with a fogger (Fig. 4).
All other application methods apply excessive release, which will "puddle" in the depressions of the core cavity and restrict vents. Eventually, the vents will become highly restricted or clogged and will not function properly. This phenomena leads to uncured "soft spots" in the core, which promote resin build up on the box and further sticking.
Core Storage--Figure 5 is a prime example of what can happen when improper core storage techniques are used. At least 20% of the cores originally placed on the rack were broken by the time they reached the molding line. Needless to say, proper core storage can have a big impact on productivity and profitability.
Cores should be stored in a clean dry, well lit and ventilated environment. The cause of more than one type of casting defect has been traced to wet or dirty cores, or the use of the wrong cores (mistakenly used due to similarities in size, shape and configuration to other cores in the shop environment).
For a free copy of this article circle No. 342 on the Reader Action Card.
For More Information
"The Venting Matrix Method for Cold Box Tooling Design," T. Lewis and K.B. Horton, 2001 AFS Transaction (01-185), AFS, Des Plaines, IL.
"Evaluating Phenolic Urethane Cold Box Binders: A Practical Approach," S. G. Baker, 2001 AFS Transactions (01-003), AFS, Des Plaines, IL.
"The Effects of Humidity on a Phenolic Urethane Cold Box System," S.G. Baker and J.M. Werling, 2001 AFS Transactions (01-033), AFS, Des Flames, IL.
About the Authors
Wil Tinker is president of Tinker Omega Manufacturing, L.L.C. and has more than 20 years of chemically bonded sand experience. While Ben Thomas has been involved with coldbox coremaking for the last five years, Gaylord Foundry Equipment has been designing, manufacturing and supporting coldbox coremaking, gassing and scrubbing systems for close to 30 years. Mark Adamovits is Technical Manager for Ashland Specialty Chemical Co.'s Foundry Products Div. and has serviced the coldbox coremaking market for the last 15 years.
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|Comment:||In search of a cure: optimizing coldbox core systems.|
|Date:||May 1, 2002|
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