Troubleshooting Casting Defects in Nobake Molding.
The goal of every foundry is to produce quality cast components at a low production cost. Regardless of process, metal or type of components, every casting operation works toward this objective with every mold made and poured.
For nobake molding operations, this goal often faces extra stumbling blocks that present themselves in the way of mold binders and resins, refractory coatings and sand reclamation. As a result, casting defect detection, recognition and resolution for the nobake foundryman can be as much about detective skills as metalcasting knowledge.
This article examines casting defect analysis for nobake molding operations. Each section defines common defects associated with nobake molding and identifies their possible causes and remedies to ensure casting quality.
Pinholes and Blowholes
These gas defects in nobake molded castings arise from:
* nitrogen (N) porosity;
* hydrogen (H) porosity due to H arising from water vapor (which can occur when aluminum is used as an alloy in iron).
N and H porosity is limited to steel and iron nobake castings and primarily is due to gases arising from the resin and catalyst decomposition at pouring. The gases will be present in the mold from the last resin added to the sand, in the refractory, from a build-up on recycled sand or from the melt's additives or metallurgical processing. These gas defects also can form due to too low a pouring velocity or a melt that has been contaminated with aluminum-containing inoculant.
These gases are first absorbed by the molten metal and try to be released as it solidifies. If the gases aren't released, a defective casting can result (Fig. 1). Concentrations of N in the sand above 0.15% greatly increase the probability that defects will occur. The porosity forms under the skin of the surfaces touching the mold surface. In steel castings, the defect primarily is seen in castings up to 30 mm in thickness, while iron castings don't have a size limit.
Prevention of gas defects can begin with the formulation of the melt. In a cupola melt, scrap content should be limited to less than 35%. In addition, inoculating the melt with 0.02-0.03% titanium (Ti) reduces the propensity for the defect. Also, the melt should not be overheated and pouring temperatures can be reduced to compensate.
Other remedies to reduce this defect are mold-related. A foundry can use a N-free refractory that contains Ti. A resin binder with low N content can be used at the minimum binder addition levels. A foundry can focus on improving the loss on ignition of its recycled sand or use a higher proportion of new sand in each mold. Last, a foundry can add 12% red iron oxide or 2-3% black iron oxide to the nobake sand mixture.
This defect is caused by surface cracks in the mold that fill with metal. These cracks are due to the heat expansion of the silica sand when hot spots in the molten metal cause stress that the sand binder can't contain. The binder failure may be due to its fragility or to a lack of mechanical strength at high temperature. The binder must have excellent heat resistance and a degree of plasticity. These properties diminish as the resin ages.
An excess of resin or catalyst can reduce the work time of a mixture and the mechanical strength of the mold. As a result, the mold surfaces may move and crack when the casting is poured. If the sand has a wide range of screen size distribution, the mold may be able to adsorb the expansion movement.
The choice of resin for furan nobake molding with the correct quantities of furfuryl alcohol and urea-formaldehyde is critical as they reduce the fragility of the resin layer surrounding the sand grains. In addition, iron oxide additions can be used in the phenolic urethane nobake molding process to achieve this. The recommended quantities are 1-2% for red iron oxide and 2-3% for black iron oxide.
Foundries also can look at changing the design of the mold to eliminate hot spots. Also, foundries must avoid storing molds in damp conditions, which may cause them to deteriorate before use.
If a casting exhibits an orange peel effect (Fig. 2) on its surface (hollow blemishes all over the surface of the casting), it has arisen from a metal-mold interaction due to contamination of the sand with acid waste products from the additives (such as furan resins). In a sodium silicate binder system, this is caused by moisture. The two remedies for this defect are to add a large amount of new sand into the system and to improve the efficiency of the sand processing and/or reclamation system.
Rough Surface Finish
Since nobake molding often is selected to achieve a fine surface finish, this defect can be especially troublesome. Some of the causes of this defect can be:
* too coarse a sand grain;
* incorrect refractory coating selection or improper application;
* unsuitable mold release agent;
* pattern imperfections that result in scabs on the mold surface;
* inadequate pattern draft that retainssand on the pattern after withdrawal.
In low carbon steel casting, the metal has an affinity for carbon and readily adsorbs it from the decomposition of some resins. This may affect a casting's surface layer to several millimeters thickness, especially near high metal flow. As a result, the upper surfaces of the casting, which are in contact with the mold for only a short period of time due to volumetric contract during solidification and cooling, are less likely to be carburized. The lower surfaces of the casting, which are in continual contact with the mold during cooling, are more likely to be affected by carbon migration.
In iron casting, the carbon is visibly deposited on the casting surface.
The possible molding problems that cause this defect are an excess of resin in the mold, an inadequate use of vents, too low permeability in the mold, or insufficient time between mold construction and pouring for the complete evaporation of the solvent. Solutions to this defect include:
* using an inorganic binder such as sodium silicate instead of an organic binder to eliminate the carbon;
* using a zircon-based refractory coating;
* reducing the amount of binder used;
* adding 2-3% black iron oxide or 1-2% red iron oxide to the sand mix.
Abnormal Graphite Nodules
This defect is encountered when the sand has been recycled many times and has a high content of sulfur containing compounds such as the catalyst. During casting, the sulfur can migrate from the mold to the surface of the casting. In ductile iron, this can lead to misshapen nodules or pseudo-plates. This defect is accentuated in thick-wall castings when the magnesium content is at the lower limit. A solution is to ensure the melt has a higher residual magnesium content. Also, metallurgical changes can help.
A mold that is too rigid can hinder the contraction of the casting and cause hot tearing (Fig. 3). Nonferrous alloys with low hot slippage have a greater tendency for this defect than ferrous castings due to their lower pouring temperatures, which are too low to promote sand collapse.
The defect can be solved by altering the hot characteristics of the resin by using a sand with a wider grain fineness distribution and by using a core in the problem area of the mold. In addition, a foundry can reduce the amount of binder used or use less heat-stable binders to reduce the defect.
Sand Erosion/Metallic Infiltration
Several factors can cause the sand to erode in a nobake mold and allow the molten metal to infiltrate. Within the mold, possible causes can be:
* an insufficient amount of refractory it is used or it is not applied uniformly;
* the mold sand is too coarse;
* too high of a metallostatic pressure;
* the mold lacks mechanical strength;
* the release agent applied to the pattern isn't properly applied;
* the sand isn't compacted properly over the pattern;
* the binder isn't thoroughly distributed through the mold;
* the sand work time has been exceeded.
Possible causes with the metal can include:
* too high of a pouring temperature;
* improperly distributed metal flow;
* too high of a metallostatic pressure.
In addition to the obvious remedies that relate to the possible causes, there are three other suggestions. These include:
* using binders that give the sand mixture increased flow characteristics;
* altering the work time of the sand mixture to delay the beginning of the hardening process so that the best flow characteristics can be fully exploited;
* reducing the water content of the mixture and taking precautions against air moisture adsorption.
Proper Refractory processing, Application Critical to Defect-Free Nobake castings
There are three generic types of refractory coatings used for nobake molding:
* The first type is a coating based on refractory minerals with a sharp-pointed granular structure. The refractory system may have one mineral or a mixture. The advantage of this system is a minimal amount of oxides or gas generation. The disadvantage is that the system is not effective in preventing defects related to silica sand expansion.
* The second type is a refractory based on a plate structure. During sand expansion, the plates slide on each other and the rendering film covers the cracks that develop in the mold surface. The advantages of this system are that it is effective against defects due to sand expansion and protects against metal penetration. The disadvantage is that the system develops gas and may result in gas defects. If there is any water or crystallization present, oxidation of the molten metal may occur. In addition, application of this refractory is limited due to its low melting point.
* The third type is a refractory with combinations of several elastic minerals. The advantage of this refractory is better protection against sand expansion defects and metal penetration. Also, it provides better casting surface shakeout. The disadvantage is that the elastic phases react actively with the molten metal's oxides and slag. The contents of these phases must be adapted to the thickness of the casting wall.
Refractory Coating Application
Each of these three refractory coatings can be applied by brush, spray, flow or immersion techniques.
Before refractory application, how-ever, the foundry's refractory technician must ensure that the coatings characteristics are in specification (as provided by the refractory supplier). These characteristics include viscosity, density, pH and settling time. In addition, there are several other variables such as:
* the thickness of the coating on the dried surface of the mold;
* the loss on ignition of a sample as-is and one dried at 248F (120C)
* the percentage of solids after drying;
* the tendency of the refractory to crack or peel;
Foundries also should perform practical tests to confirm the refractory performance with the application method used.
When, the refractory is applied, a layer should not be thicker than 3 mm. In the case of large steel castings in which a heavy refractory coating may be required, foundries should apply a second refractory coat after the first is dried. In general, the first coat will be dense while the second will be more dilute. Whether one or two coats is used, the final flame hardening of the refractory must not burn the binder that anchors the paint to the layer beneath nor should the surface color change.
To assess the adherence of paint to the subsurface mold, foundries can mark a network of small squares in the refractory with sides of 0.5-1 cm. When the refractory is scratched, if it comes off in small particles, there is good adherence; if it comes off in flakes, there is not.
In nobake casting, many defects can be attributed to the incorrect manipulation and/or application of refractory to the nobake mold. The following list details four areas of concern for foundries to guard against.
1. Air bubbles caused by prolonged violent stirring of the refractory or by degradation of organic materials in the refractory can cause gas formation during casting.
2. An orange peel effect or veining can occur during refractory coating drying on the mold due to a lack of uniformity in the thickness of the refractory layers or excessive refractory density. If the refractory is too dense, its penetration of the mold surface may be insufficient to guarantee adequate anchorage.
3. Thick refractory coating layers on molds may result in excess layers in mold cavities and pockets. This is due to the accumulation of thick paint deposits that result in a lengthened drip time. Also, whenever a mold takes time to reach its maximum draining position and the paint stays in a thick layer, it can't drip-drain.
4. Thick paint layers are difficult to dry, peel easily and may alter the dimensions of the mold.