Optimize your coldbox core process!
"The ideal world..."
If a foundryman awoke one morning to discover his foundry in an "ideal world" situation, his phenolic urethane amine (PUA) coldbox coremaking operation might look something like what's described below. That "blue-sky" picture would consist of:
* raw materials consistent in quality, readily available and reasonably priced over the long term;
* cores fully cured - in the box - (and easily released) to high levels of immediate strength that are sufficiently stable and strong during handling/storage and pour/solidification;
* cores with a bench life measured in days that are insensitive to the shop's temperature and humidity;
* no problems due to gas evolution, sand related expansion defects, or unsatisfactory surface strip from casting;
* a consistent, reproducible process that is fume-free during coremaking, pouring and shakeout, has no disposal worries, has excellent shakeout properties and is compatible with synthetic molding sands.
In real life, however, foundrymen compromise "ideal world" conditions for what works best and most economically for their operation. Fortunately for them, technology has advanced over the process' history to overcome many of its previous drawbacks. Stratecasts, Inc., estimates that the coldbox process is used on 44% of all castings produced today - by far the largest of the coremaking processes.
Developed 30 years ago this year, the PUA coldbox core process works as follows. After the sand is coated with a two-part binder system, it is blown into the tooling cavity. Then, a vaporized amine catalyst is introduced into the tooling cavity that cures the two resin components instantaneously. After the amine is introduced into the cavity, a dry "purge air" is introduced. This volume of air travels through the core and out the exhaust vents, distributing the amine catalyst to all cavities of the corebox and then flushing the amine and its residual odors from the core. The result of this sequence of events is a highly accurate reproducible process that provides instantaneous strength buildups - all without the use of external heat.
The intent of this article is to bring foundrymen a bit closer, where possible, to the optimum conditions for the process.
Type - Silica, lake, chromite or zircon media can be used.
Grain Fineness Number - Best is 50-60 GFN. In use, however, foundries often operate at GFN levels of 40-90. Higher GFN numbers result in higher resin demand, poor blowability and increased gas generation, with reduced permeability.
Grain Shape - Rounded sands are best for strength purposes, while subangular sands are best for controlling most types of expansion-related defects.
Acid Demand Value (ADV) and pH - Optimal ADV is 0-5, while 5-20 is considered usable. ADV and pH are important because coldbox resins react prematurely in the basic environment, and bench life is shorter with higher ADV-pH values.
Contaminants - It is best to have low clays, oxides and organic contaminants. Usable limits are 0-0.3% clay, 0-0.3% oxides and 0-0.3% LOI.
Figure 1 shows the strength vs. time after mixing with several different sand options.
Sand Temperature - It is best to be at the 70-80F (21-27C) range, however 50-105F (10-41C) is considered usable. Sand temperature should not veer from that usable range because the temperature increase heightens the reaction rate in the sand hopper; therefore hot sand shortens usable life. In addition, sand cooler than 50F (10C) can cause poor mixing by thickening the Part I resin.
Moisture Content - Best operating moisture content in the sand is 0-0.1%. Although it is not recommended, normal usable range can be up to 0.25%. Above this moisture level, core quality deteriorates rapidly because water reacts with Part II and negatively impacts the ability of the binder to adhere to the sand grain. Therefore, higher sand moisture lowers core strength, hardness, bench life and rigidity. As shown in Fig. 2, strength falls with the increase of moisture in the sand. Bench life is affected in the same manner.
RESINS & CATALYSTS
The PUA process consists of three liquid components - a Part I and a Part II resin and then a liquid catalyst.
Part I - Phenolic resin dissolved in an organic solvent.
Part II - Polyisocyanate components dissolved in organic solvents. These are available to match ADV of the sand and mixing conditions. Selection of Part II depends on ADV and the sand temperature because high ADV sands shorten bench life,as does high temperature sand.
Bench life is considered the usable working life of a prepared sand mix. PUA resin systems contain modified Part II binders to increase their usable lives. Different Part II resins have different abilities to lengthen bench life and will exhibit different lab tensile results. Actual performance of these various Part II resins depends upon foundry conditions (mixed sand delivery systems, sand types, ambient temperatures, equipment, etc.).
Tertiary Amine Catalysts - Triethylamine (TEA) is intended for high-volume use in which less offensive odor and higher flashpoint are desired. It is supplied in 5-gal drums and 110-gal returnable cylinders.
Dimethylethylamine (DMEA) is more reactive and is intended for high-volume use wherein air or [N.sub.2] is the carrier gas. It is supplied in 55-gal drums. Table 1 shows comparative data between DMEA and TEA.
Many types of additives are used successfully with the coldbox process. Additives are typically used to modify sand properties (clays, engineered expansion additives, red iron oxide, black iron oxide, and aluminum and magnesium inhibitors).
Engineered Expansion Additives - These products are recent developments that are used at addition rates of 1-10%. The are used to reduce or eliminate most types of expansion-related defects such as veining, but do not significantly reduce tensile strength.
Clay Blends - These powdered materials are available from several proprietary sources and are used primarily to suppress veining in ferrous castings. They also add hot strength to the mix and are added at 1-1.5% levels based on sand weight. A 2% addition may be helpful for correcting severe veining conditions.
Red Iron Oxide - Levels of 0.25-3% based on sand are typical values to suppress the formation of subsurface pinholing in gas-prone alloys (such as low carbon steels), but certain proprietary grades also suppress erosion at ingates. An 82% or greater purity grade of iron oxide is recommended.
Black Iron Oxide - At the same 0.253% based on sand levels as used with red iron oxide, it is just as effective in suppressing subsurface pinholing. However, it doesn't require as much extra binder to recover the strength lost due to the addition of fines.
Inhibitors - Required for casting of some aluminum and magnesium alloys, these are tolerated by the coldbox system. Regularly used are 0.1-1% potassium fluoborate and 0.25-0.5% sulfur additions based on sand weight.
SAND MIXING EQUIPMENT
Continuous Mixing - Zero retention continuous batch mixers with a direct feed into the blower hopper provide the best flexibility of bench life and delivery weight regulation. Also in use are low-speed, single-screw continuous mixers.
Batch Mixers - Batch mixers that are equipped with either manual or automatic resin/additive systems are best for ease of varying binder levels, mixing efficiency and dry additions. The low heat buildup type is preferred, with batch sizes of no more than 20 min of blower use.
Calibration of Mixer - Mixers should be checked once per shift by weighing the pump and sand delivery. Foundries should establish a minimal resin-to-sand ratio.
Accurate calibration can by maintained by:
* protecting the tank (or dram) openings from debris, sand and moisture;
* locating resin tanks close to pumps, with large exit lines, and above the level of suction;
* locating pumps and tanks in an area of even temperature or installing temperature controls;
* installing filters in tank-to-pump lines;
* maintaining a moisture-free atmosphere over Part II resin tank by using desiccant canisters or nitrogen blanket.
Sand Delivery System - The direct fall of mixed sand into the blower hopper (regulated in short shots of 20-min usage) is best. If a belt, bucket, tram monorail, skip or chute is used, the design must deliver the least possible aeration and easy access for cleaning. Air transport on mixed sand should not be used.
Mixed Sand Quality Tests - Once per shift, foundries should test for loss on ignition, and blown tensiles (specific blow pressure and gassing times and specific curing time between gassing and break) should be spaced over the use life of the sand. As a backup, raw sand should be checked daily for % moisture, ADV and temperature.
Type - Blowing is the best method for high productivity. The degree of core compaction affects core quality. It should be noted that the mix is very flowable at this point in time and the mix will "sag" if jolted between the blow and the cure.
Blow Tubes - Large, straight ID or "neck-down" tubes are preferred for the production of small, light cores. Now successfully in use are sizes from 5/81.25 in ID, steel (with beveled seat gaskets of Viton) and cast urethane or nylon tubes threaded into the blow plate or flange-bolted.
Blow tube size and location are important because large blow tubes control impingement velocity and resin "wipe-off" onto the box. Blowing into deeper sections of the core also reduces the resin wipe-off onto the box. Total blow tube area is critical because the PUA process requires a high core density, with low blow pressures and low sand velocity. As a minimal rule of thumb, a chunky core requires 0.2 sq in./lb of sand, and a rangy core requires 0.35 sq in./lb of sand. Compaction will be best if blow tubes have no more than 5 in. between the centers.
Vents - The best all-around vents are the woven/wire screen-type (25-30 mesh). Slotted types, however, are usable and may be required for casting soundness in some non-machined areas or contoured areas of the box. Laser-cut slotted vents are becoming popular.
An important consideration in choosing a vent type is the open area, or the ratio of open to outside diameter area. Great differences exist between vent types. For instance, the coarse woven/wire type is 40% open, the sheet-screen type is 25% open, the slotted type is 13% open (on average) and laser-cut vents are about 30-35% open. Another consideration is ease of cleaning - slotted type vents require more cleaning time.
Blow Pressure - Preferred blow pressure is low, at 35-45 psi. Although a variety of pressures can be used, lower blow pressures result in reduced resin wipe-off, sticking and box wear. However, tooling must be designed to permit low blow pressure.
Generating Equipment - Satisfactory types include the vaporizer (bubbler), pump injector, timed injector and boiler/proportioner.
Table 1. Comparative Data Calculated at Saturation with Generator Operation at 70F (21C) and 15 psig DMEA TEA Vapor pressure [liquid @ 68F (20C)] 435 mm 54mm % amine by weight 51.1 11.7 Cu ft N[.sub.2]/lb amine 13.2 103.5 Cu ft N[.sub.2]/10 cc amine 0.2 1.6 Note: Therefore, TEA requires 7.8 times as much inert gas or air to deliver the same amount of amine to the corebox and through the sand to cure.
Curing Pressures - Assuming properly rigged corebox, the maximum curing pressure during amine introduction at which a hole will not be blown in core is about 5-10 psi at corebox input. The maximum purge pressure that will not cause seals to leak is 15-30 psi at corebox input.
The proper selection of curing pressure is important because the lower the initial gassing pressure, the richer the catalyst/carrier mixture will be. An amine concentration of 4-12% is recommended. The higher the following curing pressure, the quicker the catalyst/air mixture is driven through the core due to the higher volume of air used.
Input Curing Pipe Design - Best is a large pipe size (based on sand weight) that has as few elbows as possible and has a short line from the generator or accumulator to the corebox input plenum (2-10 ft is desirable).
Design of the input piping is important because a length too small to too long results in longer cure times due to the lower volume of catalyst/air mixture. Gas volumes affect cure rates more than pressure. Input size guidelines are as follows:
1-15 lb of san/blow - 1 in. diameter
15-50 lb of sand/blow - 1.25 in. diameter
50-100 lb of sand/blow - 1.5 in. diameter
More than 100 lb of sand/blow - 2 in. diameter
Exhaust Piping Design - The best design utilizes at least three times the volume capacity of the input line. Only a minimum number of elbows and tees should be used, and the exhaust side of the corebox should be between +/-1 psi during blowing and curing.
Blowing and Curing Air Dryness - For thin core production, it is best with a dew point of -50F (-45C) at ambient pressure. Dew points up to -10F (-23C) at ambient pressure are usable, for production of heavy section cores, however. Dryness of the air used to transport raw sand also is important.
Dry air is important because moisture can condense out in a blow magazine, thereby reacting with the Part II resin to reduce its bonding strength. Moisture - which can emerge from blowing air, the gassing air mixture and raw sand - weakens the core by reducing compaction.
Amine/Carrier Gas and Curing (Purging) Air Temperature - A temperature of 150-200F (66-93C) at the input plenum is preferred. In practice, some units heat air upstream of injection generators. Most units heat-trace the piping from vaporizer (bubbler) generators. Insulated curing lines are recommended regardless of generator design.
Amine vapor and curing air temperature is important because the catalyst mixture is richer in bubbler generators at warmer carrier gas temperatures. The warmer the delivery pipes, the less chance of catalyst or moisture condensing as liquids. In addition, maintaining constant temperature assures uniform curing rates.
Air Flow Inside the Corebox - Major principles for catalyst/air flow are maximum practical input area, an exhaust area less than the input, balanced length of flow through the core, and no reversal of flow inside the core. Good techniques for rigging include flooding one side of the core with a catalyst/air mixture and distributing it throughout the core by size and location of the exhaust vents.
Flow is important because the greater the catalyst/air volume through a balanced box, the shorter the cure time will be. Also, any imbalance (bypass) wastes catalyst. Plus, good balance of flow results in low catalyst odor in the finished core. Figure 3 shows simple airflow and balanced airflow designs, respectively.
Calculation of the Input Area - The required area is determined by the sand weight in the cavity and the configuration of the core. The area to be considered is the actual open area of the vents plus the area of the blow ports.
As a minimum rule of thumb for a horizontal split core, the input area should be at least 0.25 sq in./lb of sand. For horizontally parted coreboxes, 0.5 in. vents placed on 1.5 in. centers in the cope and 2 in. centers in the drag portion of the corebox will give good cure rates. This is due to the even distribution of the air/ amine through the sand mass.
For a rangy core, it should be at least 0.4 sq in./lb of sand. For a vertically split core, a minimum of 0.25 sq in./lb of sand for a chunky-type core should be used. The exhaust area should be 6080% of input area.
Cure Rate and Amount of Catalyst - These characteristics are interdependent, and an excess amount of catalyst can raise the cure rate. Attainable cure rates depend on core configuration, blow weight and efficiency of the box rigging (venting). The cure rate is calculated by:
Pounds of sand blown into the box
Seconds of catalyst/air application
The amount of catalyst required depends on the weight of the sand and efficiency of the box rigging (venting). Amount of catalyst is calculated by:
Ccs of catalyst used per cycle
Pounds of sand blown into the box
Or, it may be calculated as pounds of catalyst per ton of sand.
Target Cure Rate - For chunky cores, the target cure rate will be above 10 lb/sec. For rangy cores, it will be above 3 lb/sec.
Target Amount of Catalyst - For chunky cores, the catalyst amount should be 1 lb or less per ton of sand (0.3 cc/lb). For rangy cores, it will be 1.6 lb per ton of sand (0.5 cc/lb). Due to differences in rigging (venting and sealing), chunky cores may range from 1-2 lb/ton and 1.7-2.2 lb/ton for rangy cores.
The main system design and box rigging factors that cause low cure rate and high catalyst usage are:
* insufficient air supply line to the box;
* insufficient catalyst/air pipe ID from the generator to the box;
* poor catalyst/air distribution to input manifold;
* insufficient corebox venting areas;
* poor corebox vent placement;
* corebox exhaust and disposal piping that is too small or too long.
Seals - An assortment of compression seal shapes are available. Selection of a low compression seal (2-5 lb/in.) is important because of possible interference with closing of the box and/or movement of the box after compaction. Proper sealing is extremely important because it minimizes cure time, catalyst usage and downtime.
RECYCLING OF EXCESS CATALYST
Type of Units - Most accepted is the amine/acid scrubber operation. The following procedure can be used to produce the minimum volume of concentrated and consistent by product from acid scrubbers to remove the amine from the corebox exhaust gases associated with the process and provide a solution that can be economically recovered.
1. The clean scrubbers should be charged with a solution of sulfuric acid and water. If TEA is the catalyst, the solution should contain 23% by weight of 66 baume (95%) technical grade sulfuric acid. When DMEA is used, the acid concentration should be 29%. Volume in the scrubbers should be set at the normal operating level. If the acid solution is prepared in the scrubber, charge the water first, followed by slow addition of sulfuric acid with good agitation. Considerable heat can be generated when mixing sulfuric acid with water.
2. As the unit operates and the volume decreases by water evaporation, only water should be added to maintain operating volume.
3. The pH is continually monitored and when the pH rises to above 4.5, the acid is spent. The entire scrubber should be drained and recharged as in step 1. A small amount of acid can be charged at this point to temporarily extend operation if draining is inconvenient.
The spent scrubber solution contains some free sulfuric acid and must be handled as a corrosive liquid. Contact with strong alkalis should be avoided. These materials are readily recycled.
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|Date:||Mar 1, 1998|
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