Practical permanent mold heating & cooling.
* In permanent mold casting, part production rate and quality are governed by the rate at which heat energy is transferred from the solidifying casting through the mold material and into the atmosphere.
* To gain a better understanding of the thermal management practices used for heating and cooling in permanent mold, a survey was conducted revealing the tried and true methods metalcasters use in their own shops.
One of the significant problems with any permanent mold preheat process, which is required for mold coating spray application during set-up and before the start of production, is that commonly used techniques cannot mimic what occurs during the casting process. Open flames or electric heating elements deliver a generally even heat flux that occurs over all exposed surfaces of the mold in a steady, continuous manner.
In the actual casting process, a very intense heat flux is delivered on a cyclic basis to those surfaces with which the molten aluminum comes in contact. Just because a control thermocouple on a mold reads 700F during the preheat and cycle of the permanent mold, doesn't mean the total thermal distribution across the mold surface will be constant (Figs. 1-2).
Without preheating the mold, proper mold spray adhesion will not occur, and the start-up will produce parts with improper dimensions and unwanted shrinkage. The first casting also could stick hard enough to the mold to deform upon ejection. If the mold became too hot, the casting might tear away in parts upon mold opening, which would result in a host of other problems.
Mold preheating is just part of total thermal management of permanent molds, which begins with the heating of the mold to prepare it for production and continues with the proper control of the pouring, cooling and extraction cycle. The process of delivering and extracting heat to and from a ferrous substrate, such as a permanent mold, can in theory be done via several methodologies, including radiation, convection, conduction and induction. However, constraints imposed by economics and custom application requirements, such as simplicity and ruggedness, govern the choice of methodologies.
Because several alternatives are available, the AFS Aluminum Division Permanent Mold Committee (2-E) initiated a survey to shed some light on which methodologies have proven most viable in permanent mold metalcasting facilities. Fifty-five responses representing a range of facilities, from those with fewer than 25 employees to greater than 250, were received.
Following are the results of the survey.
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What kind of heating processes do you use in permanent molding?
Approximately 82% of the respondents stated they use a simple combustible gas/air torch with manual controls. This process is simple, rugged and easily customized for various sizes of molds. The vast majority of the respondents do not use a thermocouple to automatically monitor the mold temperature to measure the effectiveness in heating the mold with the torch. Using a gas/air torch without controls can result in overheated molds (making part of the mold red hot), which can lead to shortened mold life.
One metalcaster reported that its primary method of judging when the mold is hot enough is by the hue of the mold; the personnel simply turn off the torch when the mold begins turning red. Eleven percent of the respondents reported using a thermostatically controlled torch to preheat molds. A small number of respondents also reported using "warm up shots" as their most common method for preheating a mold.
About one-third of the metalcasters reported heating molds in ovens outside the molding machine for at least some of the mold's preheat. This fact is significant, since, generally, changing over a molding machine and having the mold ready for casting production is not going to happen in mere minutes, especially if the mold is still at room temperature during installation.
Why is mold heating used for your molds?
Although most metalcasting facilities reported using mold heating primarily to prepare for the application of the mold spray and to make the mold hot enough to begin production, nearly one-third of the respondents use auxiliary heat to add heat to a mold for the purpose of reducing misruns and/or adding heat to pour cups in the tilt-pour process or to low-pressure-feed nozzles in the low pressure process.
What are your methods for heating control?
Despite the need for proper heating to ensure a smooth production start-up and to keep mold damage to a minimum, use of sophisticated controls on mold heat-up is not widespread. The most common technique employed by a majority of the metalcasting facilities is to simply monitor the heat-up until the desired temperature is reached. About a quarter of the respondents use a torch with a timer control as their primary method. The next most common technique is to simply heat the mold up until some part of the mold turns red, a technique reported by roughly 15% of the permanent mold metalcasters.
What are your mold heating practices?
Often in large permanent molds, certain areas cannot stay hot enough to prevent misruns primarily due to too much mold and not enough heat available. To address this problem, slightly less than half of the respondents insulate the back of the mold either during the initial mold heat-up or during production. The second most common practice is to contour cut the back of the molds to reduce the local cross section. The next most common practice is to install a mold insert, which is insulated from the rest of the mold to conserve heat in a specific area.
MOLD COOLING AND CONTROL
Permanent mold cooling is more complex than heating because it is strictly used as part of the production process rather than predominantly in mold preparation It is more likely to be mold- and location-specific for the technique employed. The practices used in cooling will affect the mold life, part quality and dimensional repeatability, as well as production rates. Mold cooling can be accomplished through mold geometry, convection cooling with air or water, or conduction with various alternative mold materials and components.
What kind of cooling systems do you use?
Approximately 50% of the metalcasters surveyed reported they employ air-cooling using compressed air through drilled lines and/or fountain-style components as their primary cooling method. Using water as a cooling medium passing through drilled lines is nearly as common a technique.
Air-cooling is still the most popular technique for a number of reasons:
1. It is easy to plumb and control with valves.
2. It can be used in cast iron molds with little fear of cracking.
3. It does not require any special treatment, as circulated water may.
4. It will not cause any build-up or corrosion in the lines.
5. Air leaks are far less hazardous than water leaks around molten aluminum.
6. If the metalcasting facility already has an air compressor with some open capacity, no additional capital outlay is required for a new job.
However, the use of air has two significant drawbacks: the effectiveness of the drilled passages and the operating costs. Drilled passages do not offer a great deal of surface area for a medium that does not exhibit high interfacial heat transfer coefficient, as is the case with air when compared to water. Due to its high specific heat, water can absorb a tremendous amount of heat energy at a relatively low flow. Compressed air, with a lower specific heat, can absorb much less heat energy at low or high flow rates.
Although most metalcasters do not track the cost of operating mold cooling devices, the cost of the numerous devices can be considerable. It is not unusual for one fountain cooler to use 10 to 16 standard cu. ft. per minute of compressed air at 90 psi while operating, which translates to about 2.5 to 4 hp (1.9 to 3kw) of energy. If your energy costs are $0.08/kwh, a fountain cooler can cost you $0.25 per hour to operate. This cost may not seem like much until you consider operations that may consist of five molds with four air coolers each.
A closed or open water system with a I hp circulation fan and pump will use far less energy; likely less than 20% of the compressed air system. The drawback with water will be the space required for the system and water system maintenance.
Why is mold cooling used in your molds?
Seventy percent of the participants reported that mold cooling is used most often to control the part solidification time so as to reduce shrinkage defects. The second most common reason (55%) that mold cooling is used is to speed up production rates. The occasional use of mold cooling to increase the solidification rate for better mechanical properties is reported by roughly 40% of respondents. Mold cooling as a means of reducing soldering defects, which can occur on very hot sections of the mold, is reported by less than 25% of metalcasting facilities.
Do you incorporate mold features to enhance cooling?
Seventy percent of the respondents contour their molds for a uniform wall. Finning is used to increase the convective cooling surface. Less heat is needed at the start up, and the distance that thermal energy has to travel from the casting interface to the exterior of a mold is reduced. About a quarter of the facilites have tried finning molds to increase the surface area of the exterior of the mold to enhance mold cooling. This can be done by milling grooves in wrought material or by making fins part of the mold block casting.
If you use water-cooling, what happens to the water after passing through the mold?
For those metalcasters using water-cooling, the water's ultimate destination depends on equipment costs, utility costs and often, regulatory costs. The most common answer provided (60% of those using water) is re-circulating the water through an evaporative system (cooling tower).
Cooling towers are efficient and work even in environments where it freezes outside the building. Some minor water treatment costs are often involved with cooling towers to prevent scaling deposits.
The second most common solution is to send the water through a closed loop system (chiller or radiator) so as to re-circulate the fluid. The capital expense and operating costs are generally higher with a closed loop system, but fluid treatment concerns are lower.
Having the water simply pass through the mold and go down the drain is reported by six of the respondents using water-cooling. If the mold is not run often, this is an easy way of keeping equipment costs down, but water and sewer utility costs will climb as the mold runs more often. This practice is not allowed in some municipal sewer districts that are trying to keep the total volume of wastewater treated to a minimum. It is advisable to check with your local wasterwater regulatory agency on the rules regarding "non-contact cooling water" use before implementing it as a practice.
For More Information
"Water or Air? Examining Permanent Mold Cooling Methods," Y. Lerner, MODERN CASTING, Feb. 2002, p. 23.
Tips & Tricks for Mold Cooling
The following tips were offered by the metalcasters responding to the survey.
* Add a control valve to each air cooling line rather than just turning the air supply on or off. If you are using thermocouples to control air cooling, make sure someone checks that the thermocouple is securely in the drilled hole and reading correctly, or your cooling will cycle out of sync with your cooling demand.
* Control cooling by temperature, rather than time, using a thermocouple.
* Offer soft and hard air, so each can be used during process development. Soft air is air from a blower at low pressure and permits more controllable air flow. Hard air is compressed air at high pressure and does not produce as continuously variable control.
* Use a closed loop system; it will save your company money in the long term.
* Standardize flow rates between all dies.
* Precipitants from the water will clog the cooling lines. If you can get away with using air cooling (based on mechanical properties or solidification) you're generally better off.
* Try and stay with closed systems and use distilled or treated water to prevent calcium deposits from closing off water passages.
* Do not cool closer than 0.625 in. to cast surface.
* Make sure all cooling lines are free from calcium build up.
* Don't use artificial cooling, if possible. Work with the customer to eliminate isolated heavy sections.
* Use thin/thick mold walls to control solidification rates.
Randy Oehrlein is the vice president of engineering for Carley Foundry Inc., Baline, Minn. B. Lee Tuttle is a professor of manufacturing engineering for Kettering Univ., Flint, Mich. Brian Began is an account executive for Foseco Metallurgical Inc., Cleveland.