Benchmarking the nobake binder systems.
Over the last 40 years, the foundry industry has demanded new developments in nobake binder systems. Since silicate binder technology was developed, metalcasters have recognized the productivity, dimensional accuracy and efficiency of making molds and large complex cores at room temperature.
One important trend in metalcasting has been the change from heat-cured or baked processes to cold-cured and nobake processes. This change has enabled many foundries to produce castings more efficiently, with more defined controls and with reduced manufacturing floor space. Improvements include:
* fewer controls;
* fewer variables introduced;
* reduced core/mold inventories;
* less equipment and associated maintenance;
* more tonnage from fewer foundries;
* reduced emission collection areas and total external emissions.
Along with coldbox and heat-cured systems, the wide variety of nobake processes and limited experience with recently introduced systems have left metalcasters with a difficult selection process. This article benchmarks the general characteristics of eight nobake processes.
A nobake process is based on the ambient-temperature cure of two or more binder components after they are combined with sand. Curing begins immediately after all the components are combined. For a period of time after initial mixing, the sand mix is workable and flowable to allow the filling of the core/mold pattern.
After an additional time period, the sand mix cures and can be removed from the pattern or corebox. This time difference ranges from a few minutes to several hours, depending on the binder system, curing agent, sand type and sand temperature. Eight nobake systems are outlined here.
Phenolic urethane--This system consists of a two-part binder system and a liquid catalyst. Part I binder is a phenol-formaldehyde polymer blended with solvents and additives. Part II consists of polymeric MDI (methylene bisphenyl isocyanate) with solvents and additives. Part III is a liquid catalyst. Nearly all metals are cast successfully with phenolic urethane cores/molds.
New urethane--This modified system minimizes binder levels and reduces-smoke and opacity from pouring floors with all the productivity benefits of its precursor, the phenolic urethane system. A Part I to Part II ratio of 70/30 is required.
Ester-cured phenolic--This two-part binder system uses an alkaline phenolic resin. Alkaline means that it contains sodium or potassium ions that have replaced hydrogen on the phenolic hydroxyl group and has a 7.0 pH or higher. The ester coreactant controls the cure rate. The alkaline phenolic resin has limited storage stability.
After mold- or coremaking, a secondary cure further cross-links the polymer. Higher binder levels are required when compared to phenolic urethanes, but the system is nearly free of solvents as viscosity reducers, offering health and safety benefits. Ester-cured phenolics produce excellent ferrous castings and exhibit less smoke and odor than organic solvent-based binder systems.
Furans--These two-part systems cure by a condensation reaction in which water is produced as a by-product of the reaction. Acid catalysts are TABULAR DATA OMITTED used to promote the cure. The strength and amount of the acid control the cure rate. Furan nobakes are known for their high-temperature strength, erosion resistance and good shakeout properties.
Phenolics--These binders require strong acid catalysts. During the furfuryl alcohol shortages, phenolic nobake binder use grew and became widely accepted in the U.S. Phenolic resins are known for their high hot strength and erosion resistance, but only have limited resin storage stability.
Oil urethanes--These are used as two-or three-part binder systems. Part A is an alkyd blend formulated to react with Part C, an isocyanate. Part B is a catalyst that promotes both urethane and air cure.
These nobake oils cure in three stages. Part A and C cure first, producing a urethane polymer. At this stage, the core/mold is flexible but cross-linked enough to promote stripping. The second stage cure is the cross-linking of the unsaturated double bonds in the alkyd. This is accomplished through exposure to oxygen after stripping. Although the third stage involves an oven-baking option, it is required for maximum erosion resistance.
Oil urethanes are known for good productivity and excellent release properties from the pattern.
Silicate ester-cured--These consist of a liquid sodium silicate binder and a liquid organic ester hardening agent. Odor and gaseous emissions are low during mixing, pouring, cooling and shakeout, but they depend on the amount and type of ester and other organics in the mix. Productivity, humidity resistance, burn-in, long shakeout and difficult reclamation are disadvantages with the system.
Inorganic phosphates--This binder system consists of an acidic, water-soluble, liquid phosphate binder and a powdered metal oxide hardener. Designed to comply with air quality and waste disposal regulations, it reduces or nearly eliminates inorganics, fumes, smoke and odor at pouring and shakeout. Its shakeout properties are the best of any nobake system.
This binder system produces high-quality, defect-free castings for molds and cores with a variety of metals, including various steels, and gray and ductile irons. Erosion resistance of both washed and unwashed molds is good. Veining resistance is good on unwashed surfaces and can be controlled with the proper coating selection. The binder is 100% reclaimable by mechanical methods.
Its biggest disadvantages include mold storage deterioration in high humidity and low handling strengths compared to other nobake processes.
Productivity concerns for nobake binders include work time/strip time, humidity resistance, sand temperatures and through-cure properties. Tests were conducted at typical operating binder levels using a 55 GFN washed and dried silica sand. To illustrate handling strengths, the binder levels and ratios are listed in Table 1.
Flowability of mixed sand: Systems that obtain the lowest binder levels from low-viscosity binders also exhibit the best flowability. The phenolic urethane and new urethane systems remain flowable until just before the desired strip time. The other systems show immediate strength buildup from the time of mixing.
New urethane, phenolic urethane, furan alkyd and phenolic systems show the best one-hour strength development. The ester-cured phenolics and silicate-esters exhibit medium performance, and the totally inorganic aluminum phosphate system exhibits the lowest strength development. Flaskless molding is best accomplished using the highest-strength nobake systems.
Productivity work time/strip time: Productivity directly depends on cure rate and work time/strip time ratios. Work time is the time from sand mixing with the binder until hardening. Strip time is the time from mixing to stripping of the solid core/mold. Strip time is determined by the speed of the curing reaction, which also depends on sand temperature.
The ratio of work time to strip time is a measure of potential productivity and can range from zero to one. The closer to one, the greater the potential productivity of the system. The work time/strip time ratios of the new urethane and the phenolic urethane systems exhibit the best productivity.
Figure 1 shows the compressive strength for all eight nobake systems. The rapid compressive strength development of the new urethane, phenolic urethane and ester-cured phenolic systems after their work time is complete gives a strong indication as to their reliability.
Humidity resistance: Each binder process shows loss of tensile strength at 100% relative humidity. Both the silicate ester-cured and the inorganic phosphate systems lose 80-90% of their strengths at 100% relative humidity. The ester-cured phenolic and the phenolic binder systems exhibit the best resistance to humidity. With the exception of silicate and inorganic phosphate, none of the systems exhibits problems with core/mold storage under high-humidity conditions.
With water-based core wash degradation, all systems show loss of strength while they are hot. In operation, drying ovens with adequate air volume and cooling zones prior to handling and assembly promote the least amount of scrap during core/mold processing. All the systems regain strength after drying and cooling, except for the silicate and inorganic phosphate systems, which demonstrate a significant loss in strength when water-based core coatings are used. Using solvent-based coatings with the silicate and inorganic phosphate systems minimizes core/mold degradation.
Sand temperatures: High sand temperatures accelerate the cure, thereby reducing work time and strip time. Generally, for each temperature increase of 18F, work time/strip time is cut by 50%. Sand temperatures in the 80-90F (26.6-32.2C) range are typical for most nobake operations. Temperature extremes should be avoided. At temperatures less than 50F (10C), the core/mold may not completely cure. At temperatures above 100F (37.7C), the curing reactions are so fast that over-cure is common. Furans are the most sensitive to undercure/overcure while oil urethanes are the least sensitive.
Through-cure properties: Ease of stripping and through-cure properties are critical to the core- or moldmaker. Core/mold breakage and corebox or pattern life are significantly affected by this property. Alkyd oils have the best stripping properties and the most plasticity at strip. The new urethanes, ester-cured phenolics and phenolics are the next best. Furans and silicates are the most difficult to strip and are more brittle when cured. All the systems have good release when a high-quality pattern release agent is used on well-rigged patterns.
Casting quality is never better than the quality of the core/mold used to make the casting. The eight binders considered have proven records of making high-quality castings in applications throughout the world.
Several test castings were used to compare the eight nobake systems. These tests include the step cone, erosion test, 2 x 2-in. penetration, soot plate and 7-in. disk shakeout casting. All test castings were produced using Class 30 gray iron. No coatings were used on any of the test molds and cores, and the same binder levels were used to compare sand performance.
The step cone test casting evaluates a binder's propensity to produce gas defects. It also tests for veining defects and surface finish evaluations. Using the total rating system, the new urethane showed the best results, while the silicate ester-cured reported the worst results.
Erosion tests measure erosion due to premature binder decomposition. All the systems show good or excellent erosion resistance with the exceptions of oil urethane and inorganic phosphate binders. However, the erosion resistance of oil urethanes can be improved dramatically by baking the cores/molds or by adding iron oxide or silica flour to the mix.
The penetration 2 x 2-in. test casting determines the influence of nobake binder processing variables on metal penetration and veining in gray iron and steel castings. This test is more severe for evaluating metal penetration tendencies than the step cone test. Results demonstrate that the density of the core/mold helps prevent metal penetration. The new urethane exhibited the best resistance to metal penetration, offered the lowest binder level and provided the best flowability of the systems tested.
If a refractory coating is applied, all systems excellent resistance to metal penetration and good surface finish.
The lustrous carbon soot plate studies lustrous carbon formation. Lustrous carbon surface imperfections (resin kish and soot defects) are usually encountered with binders that evolve large quantities of carbonaceous decomposition products when metal fills the mold.
Lustrous carbon may be beneficial in controlled amounts because it provides a reducing atmosphere that minimizes oxidation at the mold/metal interface and results in improved casting surface finish. In heavier quantities, however, lustrous carbon films can hinder solidification.
While urethane binders exhibit the greatest amount of lustrous carbon, the new urethane shows a significant reduction in lustrous carbon amount. Lustrous carbon formation may be reduced by increased pouring temperature, reduced resin content, faster pour time, iron oxide additions and additional venting.
The 7-in. disk shakeout casting evaluates the shakeout characteristics of binder systems. The new urethane and furan binders exhibited the best shakeout performance, while the phenolic and silicate binders offered the longest shakeout time.
Gas defects are related to both the amount and composition of gases from cores/molds. Gas evolution data is used to estimate the volume of gas generated from a core/mold. The amount of gas depends on casting geometry, sand-to-metal ratios, metal type, pouring time and temperature, and binder level.
The binders that may be used at the lowest organic binder levels produce the least amount of gas. Venting of all the nobake binders is the least expensive and most effective way to minimize gas-related defects.
The characteristics of decomposition gases depend on the chemical structure of the binder at time of casting. Specific components like nitrogen can lead to pinholes, especially in steel castings. Ester-cured phenolics, urea-free furans, silicates and inorganic phosphate binders contain little or no nitrogen. Nitrogen from furans containing urea evolves more rapidly, or in a different form, than nitrogen from urethanes. This tends to cause more pinhole defects.
Iron oxide and other dry sand additives are effective in eliminating nitrogen-related pinholes during furan and urethane binder use. Gas-related defects may be eliminated by maintaining low loss on ignition in reclaimed sand, low binder levels, proper core wash drying practices, iron oxide additions and source venting practices.
Table 2. Decomposition Emissions of Industrial Hygiene Interest Binder System Airborne Emission Suggested Exposure Guidelines in PPM Long Term Short Term Phenolic Urethane carbon monoxide 35 200 New Urethane total aromatic 0.005 0.02 isocyanates 1.0 2.0 formaldehyde phenol 5.0 not established Ester-Cured carbon monoxide 35 200 Phenolics formaldehyde 1.0 2.0 phenol 5.0 not established Furan Phenolics carbon monoxide 35 200 formaldehyde 1.0 2.0 phenol 5.0 not established Oil Urethane carbon monoxide 35 200 total aromatic 0.005 0.02 isocyanates 1.0 2.0 total aldehydes Silicate Ester-Cured carbon monoxide 35 200 total aldehydes not established not established Inorganic Phosphates carbon monoxide 35 200
Another element affecting metal structure is the sulfur content of the binder system. Sulfuric acid catalysts are often used with phenolic and furan nobake binders. The sulfur can evolve and cause metallurgical changes in ductile and compacted graphite irons. The sulfur level of TSA-catalyzed nobake cores/molds range from 0.05-0.07% at binder levels of 1.25-1.50%.
Each nobake binder can present potential health hazards from prolonged or repeated inhalation overexposures. Harmful exposures may also occur through skin contact and possible absorptions of components.
During pouring, cooling and shakeout, thermal decomposition of the binder system occurs. This can result in the potential liberation of hundreds of compounds. The majority of these decomposition products appears only in trace amounts. Decomposition emissions are listed in Table 2.
To eliminate or reduce costly sand wastestreams, most large nobake foundries are reclaiming them. The phenolic urethane, new urethane, furan, phenolic, alkyd oil and inorganic phosphate binders at one pass have been successfully reclaimed at high levels (90%). Mechanical reclamation of silicate ester-cured sand and ester-cure phenolic sand is limited due to rebonding strength concerns, although work is in progress to improve the reclaiming efficiency of these systems.
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|Author:||Archibald, James J.|
|Date:||Mar 1, 1994|
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