Water or air? Examining permanent mold cooling methods.
In an era when casting jobs can be won and lost with quotes that differ by pennies, permanent mold foundries must look for every opportunity to extend the life of their permanent mold tools to increase job profitability. One measure used to increase mold life (and casting quality) is mold cooling systems. Using either air or water, these systems regulate heat flow to ensure the production of sound castings and prolong mold life.
This article will examine both air and water permanent mold cooling systems and discuss the advantages and disadvantages of both. In addition, a common problem for permanent molds due to improper temperature control--cracking and erosion--will be explored to provide solutions for foundries.
Mold Cooling Methods
In terms of casting quality, mold temperature control is important to ensure the desirable thermal gradients in the mold are established, regulating the casting's solidification. In terms of mold life, hot or cold spots in the mold must be controlled. Uniform cooling will eliminate localized areas that could be subjected to thermal shock or liquid metal erosion.
Abrupt changes in the cooling rate within the permanent mold cavity caused by cold or hot spots also are detrimental to casting quality by causing shrinkage-related defects and hot tearing. In addition, uneven solidification produces high residual stresses in the casting that result in distortion and dimensional instability during processing and service.
To ensure regulated temperatures in permanent molds, two primary methods for cooling exist-air cooling and water cooling. Following is a look at both methods.
Air-Cooled Permanent Molds
Air-cooling represents one of the simplest methods of permanent mold cooling. However, this method has limited cooling capabilities because the heat transfer from the mold to the air often isn't fast enough for the production levels required by most foundries.
To enhance air's cooling capabilities, cooling fins and compressed air can be used as well as designing contours into the mold to allow the air more access to the internal segments of the mold.
Figure 1 illustrates the typical design of an air-cooled permanent mold in which the back of the mold shell is contoured to maintain uniform mold thickness around the mold cavity. The contour of the mold back may be thickened or thinned out as needed to increase or decrease the cooling rate in localized areas. Cooling fins are placed across the back of the mold to aid in mold cooling.
The back of the mold also can be designed flat rather than contoured with the addition of a large quantity of cooling fins with either a circular or rectangular cross-section. Rectangular fins have an advantage due to the increased surface area.
The main advantages to air-cooled permanent molds are: a longer service life because the mold cools gradually without high stress in comparison to the immediate cooling effects of water-cooled molds;
* in isolated casting designs and materials, reduced cooling rates may aid casting solidification and casting quality.
Water-Cooled Permanent Maids.
A variety of different designs for water-cooled permanent molds exist each with its own advantages and disadvantages.
Cooling Jacket Welded to Mold-In this design of water-cooled systems (Fig. 2), the water jacket is welded directly to the back of the mold, Effective heat dissipation in this design is achieved by circulating water through the jacket. This method allows the back of the mold to be contoured (instead of remaining flat) in an effort to allow the cooling source (water) to reach throughout the mold and maintain uniform cooling.
This cooling system works well to reduce or eliminate hot and cold spots on the mold, drastically increasing mold life due to the elimination of thermal shock. The only limitation is that the permanent mold must be made of steel to allow the water jacket to be welded onto the back of mold. Low carbon steel water-cooled permanent molds with a wall thickness of 1-1.5-in, exhibit longer life because mold cracks and other deteriorated areas may be periodically repaired by welding.
Cooling Jacket Affixed to Casting Machine-Compared to the water jacket welded directly to the back of the mold, this type of cooling system (Fig. 3) is more moderate because it provides indirect mold cooling. To ensure uniformity and the ability to use this cooling system on a variety of molds, both the face of the cooling system and the back of the permanent mold (where the cooling system interacts with the mold) must be flat. As a result, cooling is slower and less aggressive.
The water jacket can be made from a variety of materials, including steel and iron, and is suited for small and thin-wall casting jobs as well as operations with frequent mold changes. With small and thin-wall casting jobs, the less aggressive cooling approach works well. In addition, the requirement for flat faces on the back of the permanent mold and the face of the water jacket make mold changes easier.
The main drawback of this system is the air-gap that often occurs between the mold and cooling jacket. This gap is due to thermal-related stresses and distortion occurring in the mold during operation. As a result, some areas of the mold will not have contact with the water jacket and will not cool as effectively.
Drilled Cooling Passages-This method (Fig. 4) allows drilled cooling passages to be placed precisely in the mold; often adjacent to difficult-to-cool casting sections within the mold cavity. The major disadvantage of this design is the inability to set up uniform or differential cooling and to avoid hot or cold spots in other regions of the mold.
This cooling technique also presents problems during the drilling of the passages. Since the passages must be straight and perpendicular to each other, elaborate drilling patterns are required to cool the mold. Then, after drilling the paths, the excess openings must be closed with special threaded plugs. However, these plugs may still leak due to differences in thermal expansion between the mold and plug material.
Once plugged, the mold needs to be pressure tested to ensure against leaks. If this testing shows a leak formed by porosity in the mold material, the mold must be scrapped. However, the leak may not be evident during testing as some leaks only are discovered when the mold is heated during use, increasing the size of a small defect in the material to a large (and dangerous) problem. Since the drilling and testing follows machining of the mold, the cost of machining also would be lost with the scrapped mold.
Instead of drilled passages, another option for permanent mold foundries is to have the tool maker cast prefabricated steel pipe into the mold.
Prefabricated Piping in the Permanent Mold-Prefabricated cooling passages (Fig. 5) may be designed with one water circuit or two separate circuits. In this method, steel pipe is prefabricated to the desired cooling pattern and then cast into the iron during mold production. This method allows water to pass directly across any areas that would need cooling without restriction to straight-line distances. Another advantage of this system in comparison to the drilled passages is the virtual elimination of problems related to the mold leakage.
Spray or Vaporization Cooling of Permanent Molds-In this cooling system design (Fig. 6), vertical passages in the mold are sprayed with water from nozzles located in the top manifold. As water contacts the hot surface of the passages, it changes its physical state from liquid to steam. The necessary energy required for this state change is captured from the mold heat. Therefore, this significantly reduces the temperature of the mold. The condensed water then is collected at the bottom by the collector manifold.
The cooling passages can be cast into the mold blank or machined so that an elliptical cross-section (rather than a circular one) is achieved. The elliptical shape provides greater surface area for more effective heat transfer. The commonly used mold materials for this cooling method are iron or copper.
While this process is one of the most effective methods of mold cooling, it is a less uniform method. This lack of uniformity causes more thermal shock to the mold and, therefore, reduced the mold life.
Water Cooling Concerns
All water-cooling systems require proper maintenance to ensure the passages do not become clogged with scale from minerals in the water. To combat this issue, de-mineralized water can be used or the passages can be cleaned regularly with a cleaning solution.
A second concern with all water-cooling systems is constant flow rate because permanent molds require different water flow control throughout the solidification process. Computer-based systems can monitor temperatures at various locations throughout the mold and send a signal to the water flow-control unit to vary the flow of water to the mold. In addition, these systems can supply more coolant when mold temperature is high and less coolant when temperature is low.
Both technical and economic analyses should be performed before choosing the suitable cooling system design for a jobs tool. In selecting the most cost-effective mold cooling system, several technical factors must be taken into account such as the casting weight, casting moduli and section thickness, production cycle times and quality, production run of castings, and expected mold life. Overall, the final decision should be made based on the estimated cost of finished castings.
Solving Mold Cracking, Erosion
Permanent mold cracking and erosion is a common problem. Crack and erosion defects can occur anywhere throughout the mold and its gating system.
In both vertically and horizontally parted permanent molds, the area that suffers the most deterioration is the sprue well. Due to the nature of design, the sprue well receives the highest initial thermal shock (from the metal temperature and the energy of the falling stream) as the molten metal is introduced into the mold. The resultant cracks and erosion in the sprue well (or elsewhere in the mold) require repair. To enhance mold life, solutions can be employed during the repair process.
Solution 1: Grind the mold area and create a new sprue well by enlarging the old one- This practice is only a temporary cure yielding mixed results. Though the mold is repaired, it will eventually succumb to the same fate. It also is important to note that the mold only can be ground to within a reasonable length (usually from 2.75-3 in. of the mold edge) to ensure against leakage. If this distance has been compromised, the mold must be scrapped. In addition, if the choke is located in the sprue, it also may be burned. Grinding and enlarging will eliminate the predetermined ratio between the choke and other elements of the gating system.
Solution 2: Replaceable in serts--One possible solution may be the use of replaceable inserts that can be put in the sprue well area. The inserts can be designed either in a circle or square shape. The circular design has an obvious advantage as its shape can be more easily machined in the mold. The insert also may be shaped on both sides so it can be flipped if needed.
The insert is bolted from the back of the mold, and precautions must be taken to insure that molten metal will not enter the thread holes from the inside of the mold. In addition, the insert material must be the same as the base mold with equal coefficient of thermal expansion. When choosing the insert's method of production, as-cast surfaces should he given preference over machining.
There are some disadvantages to the insert solution. Flash occurring at the contact area and between the mold and the insert may make the ejection of the casting from the mold difficult. This area also may be prone to the same form of deterioration. With this in mind, replaceable inserts must be considered only as a temporary measure that prolongs the life of the mold.
Solution 3: Repair by welding--Permanent molds made of steel exhibit longer life because deteriorated areas may be repaired by conventional welding. Restoration of deteriorated areas as well as other areas of the mold cavity subjected to thermal shock or liquid metal erosion may be done using special surfacing material Ni-base alloy electrodes. With this process, the repaired mold areas do not require subsequent heat treatment as the hardness of the welded area is in the 171185 HB range and easily can he machined to the desirable shape and size.
When iron is used as the mold material, repair by conventional welding is not effective. One method to extend iron mold life is the use of thermal spray welding. A powder Fe-Ni-based alloy can be used to fill fissures and points of wear on the iron mold surface, With this method, it is critical that the thermal expansion of the sprayed metal must be similar to that of the mold.
Solution 4: Cast to the net shape mold blanks--Another effective measure used to extend mold life is the use of cast to net shape tools with their as-cast gating system and cast to net shape mold cavity. The as-cast surface is an advantage with the presence of burned-in sand on the surface. This as-cast "skin" features a good thermal resistant barrier, extending the life of the most deteriorated areas of the mold.
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About the Authors
Yury Lerner is the FEF Key Professor for the Univ. of Northern Iowa and has been teaching in the U.S. for eight years. His career spans 35 years of practical experience in metalcasting, and he has authored or coauthroed 150 technical papers, holds 43 patents, and has 30 former students working in U.S. foundry industry.
For More In formation
"Status and New Developments in Gravity Diecasting of Iron," Y.S. Lerner, Foundry Trade Journal, p. 27-30, May 1999.
"Mold Life Improvement in Permanent Mold Casting," Y.S. Lamer, 5th AFS international Conference on Permanent Mold Casting of Aluminum, AFS, Des Plaines, IL (2000).
RELATED ARTICLE: Enhance Cooling Via Metallic inserts
Metallic inserts (heat sinks) made of a material with higher thermal conductivity than the mold material are another way to control casting solidification and reduce the threat of hot tearing and shrinkage in permanent mold casting. Placing metallic inserts in the mold will alter the heat transfer rate out of thick sections of the casting and accelerate the solidification process in that area. The goal is to establish directional or progressive solidification. Normally, a proper risering system is used to achieve the goal; but, in some cases, casting design makes it impossible.
For example, in the case of a thick hub wheel/pulley, an insert can be placed in the thicker casting section to enhance solidification. Figure 7 illustrates a thick hub wheel/pulley design and four possible locations for inserts to reduce hot tearing.
Figure 7a illustrates uses a flat-ended insert on one side of the casting at the hub location. The insert will draw heat out of the hub so the hub and adjacent rims can solidify simultaneously. The main advantage to using inserts is their wide selection of size, shape and material.
Figure 7b illustrates how a projected-end insert is used to draw heat out of one side of the hub. The advantage of this design compared to the flat-ended insert is that the projected-end can reach farther into the casting to draw the heat out more effectively.
If this setup is not sufficient, two inserts can be used as shown in Fig. 7c. This setup also, can be used with the flat-ended insert design.
If more rapid cooling is needed, a water-cooled insert can be used (Fig. 7d). This setup is the same as a single projected-end insert design. However, this insert has water passages inside that allow cool water to pass and draw heat from the huh at a faster rate. mold.
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|Comment:||Water or air? Examining permanent mold cooling methods.|
|Author:||Lerner, Yury S.|
|Date:||Feb 1, 2002|
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