If filling a mold with molten metal were as easy as pouring a cup of coffee, automated pouring systems would be as plentiful in foundries and melt shops as coffee pots in break rooms. But, of course, pouring molten metal into a mold is much more difficult than pouring coffee.
To begin with, molten metal flows differently at different temperatures. As it cools, its viscosity increases, and it flows more slowly. Coffee flows at a constant rate regardless of its temperature, and you don't know whether its hot or cool until you taste it, or spill it in your lap. The viscosity of molten metal greatly affects how well it flows into all parts of a mold, filling all cavities. The fluidity of molten metal, dictated by its viscosity, is key to its thorough dispersion across all mold recesses, filling each cavity completely.
Depending on the metal being poured and its specific alloy composition, deposits may build up in the pouring mechanism, restricting metal flow. This is particularly true of reactive nonferrous metals. Coffee also forms deposits inside the coffee pot, and while these may ruin the coffee's taste, they seldom restrict the pouring stream.
Pouring a cup of coffee also is a lot more forgiving than filling a mold with molten metal. If you pour coffee too fast, some may splash out of the cup. If you pour a mold too fast, metal does not flow properly through the gating system and the turbulence created may wash sand into the casting cavities, forming undesirable inclusions in the metalcasting.
If you pour coffee too slowly, you just waste a little time. If you pour molten metal too slowly, it can cool prematurely in the gating system and produce incomplete metalcastings, resulting in costly part reworks and scrapped sand molds. With coffee, if you fail to fill the cup completely, so what? You can add more anytime. If you fail to fill a mold completely, you’ll lose valuable metalcastings. If you overfill a coffee cup, you've wasted some coffee. If you overfill a mold, you've wasted molten metal and all the costs associated with producing that metal – from scrap purchases to man-hours to power charges.
When molds are poured by hand, the foundry worker making the pour uses his experience to make the adjustments necessary to fill the mold properly. But hand-pouring generates high labor costs and, at its best, cannot consistently produce quality metalcastings. With foundries today using high-speed molding machines, hand pouring simply can't keep pace with today’s mold production.
That's why, despite the difficulties associated with pouring molten metal, several foundry equipment manufacturers have tackled the engineering challenge of building automated pouring systems for a variety of ferrous and nonferrous metals. These companies recognize that most major foundries will need to install automated pouring systems to remain competitive.
Pouring technique
The proper way to pour a sand mold is well known. Pouring begins at a relatively high flow rate to "choke" the mold by filling the sprue cup and the runner system to the desired level. Next, the pour rate must be closely controlled to maintain a constant level in the sprue cup. This creates head pressure that feeds the gating and cavities. By maintaining a full sprue throughout the pour, mold/pattern design can be optimized. The process is complete when the mold is full and the metal in the sprue cup is at the desired level.
Like today's specialty coffees, automated molten-metal pouring systems are offered in different flavors but all have the following common components:
- A refractory-lined vessel to hold the molten metal, such as an unheated tundish, or a heated and/or pressurized vessel.
- A molten metal metering device (valve.) In most cases this is a stopper rod and nozzle that regulates the flow out of the pour vessel.
- A valve actuating mechanism that throttles the stopper rod to regulate the flow of molten metal. This mechanism may be pneumatic, hydraulic, electric, or a combination of these drives.
- A controller. This could be a person manipulating a joystick, a semi-automatic profile following program without feedback (often called a "teach" control), or a fully automatic feedback controller.
It is the feedback controller that makes automated pouring truly automated. Its job is to overcome many of the difficulties inherent in pouring molten metal and maintaining the optimal pouring profile.
Feedback controllers are found on closed-loop control systems where the sprue cup is continuously monitored by a sensor. The sensor information is then sent to a controller. Then, the controller adjusts the stopper rod opening to minimize the difference between the desired and actual levels in the sprue cup. The closed-loop system responds to changes in the pouring conditions by automatically adjusting the stopper rod.
Vision systems are among the most widely used feedback controller systems for monitoring the sprue cup. A typical vision system uses a video camera, lens, and image processing electronics to measure the level of molten metal in the sprue cup through analysis of a video image. This information is used by the control program to adjust the stopper rod.
One such system is Inductotherm’s Visipour® P3™ vision control technology. Visipour® P3™ technology uses a pre-loaded database of optimal pouring parameters based on the type of pouring system, the classification of molten metal being poured, pour weight, pour time, bath temperature, and other such variables. The Visipour® vision control technology actively monitors the flow of molten metal into the mold, triggering minute adjustments to optimize the metal flow throughout the duration of the pour.
Each system provides continuous monitoring of the sprue cup metal level during the pour for positive control of the flow. This continuous monitoring allows the system to adjust for the changing properties of the metal being poured.
As noted, when metal temperature changes its viscosity changes. Cooler molten metal flows more slowly than hotter molten metal. In an unheated system, the temperature of the metal normally drops between recharges. Because automated pouring systems continuously monitor the level of the metal in the sprue cup, they can increase the stopper rod opening during the pour to allow for the cooler metal's greater viscosity, thereby maintaining the desired head throughout the pour.
Controlling metal flow
Of course, no system can pour successfully once the metal cools past its pouring temperature range. Beyond that point, flow problems develop in the mold no matter how carefully the sprue level is controlled.
Laser- and vision-based automated pouring control systems are able to adjust the pour for reductions in the nozzle opening due to the buildup of oxides. The buildup can be particularly severe in pouring ferrous and nonferrous metals alike. As the buildup grows, the flow of molten metal through the nozzle is reduced.
Again, by monitoring the level of metal in the sprue cup and opening the stopper rod further, the automated pouring controller can compensate for the smaller nozzle opening; but as with changes in viscosity, there is a point beyond which a clogged nozzle just won't work. The oxide buildup may prevent the stopper rod from opening and closing properly, or it may reduce metal flow to trickle even when the stopper rod is fully open.
To mitigate the effects of oxidation as molten metal fills the mold cavity, an inoculation system may be used to disperse an inoculant throughout the metalcasting. While an inoculant can be added manually to the metal pour stream, much more consistent results can be achieved with the use of an automated inoculation system.
The iNoc™ Inoculating Equipment and Visinoc® Inoculation Monitoring and Verification Systems work together to monitor the flow of molten metal and cast the optimal amount of inoculant into the pour stream in real time.
Beyond the controller hardware, either vision or laser, the ability of the controlling software to do its job is crucial to the performance of the automated pouring system.
- How accurately does the system track the changes in sprue level?
- How quickly and precisely does it respond by adjusting the stopper rod opening?
- Is the system able to use the pouring profile optimized for each mold being poured?
- Can the system adjust for a change in sprue cup location?
Finally, to be successful, all elements of an automated pouring system must be designed for the metal being poured, the processes being used, and the range of molds being filled. The ultimate success of any ferrous or nonferrous metal pouring system hinges on three basic criteria: furnace construction, furnace maintenance, and furnace power supply.
Induction pouring furnace construction, maintenance
Construction of a metal pouring furnace depends mainly on the type of metal to be poured. For example, a heated and pressurized ductile iron pouring furnace should have a larger metal depth-to-diameter ratio than a conventional gray iron holding vessel. This configuration, when coupled with inert gas as a pressurizing medium, retards magnesium fade. Vertical walls will greatly reduce slag adhesion.
Some molten metals oxidize more easily than others; this too should be taken into account when considering pouring furnace construction. Consider whether the molten metal would benefit from large-diameter tubes to provide increased heat transfer to the metal in the tubes; or if a lining material with greater insulating properties should be used in the spouts to help maintain the metal temperature there.
Inductor shape can also influence the behavior of molten metal in a pouring furnace. For example, an automated pouring furnace for ductile iron should have a "U" shaped inductor with thicker refractory surrounding the channel throat as it enters the upper case. This geometry increases the metal velocity, and the refractory material reduces temperature loss to slow slag accretion at a commonly troublesome choke point in conventional pouring furnaces.
A rigorous maintenance program is critical to getting the best possible performance from an automated pouring system. A few furnace maintenance tips that will maximize performance throughout the furnace’s life cycle include:
- The recharge enclosure should be cleaned at least once per shift and the pour enclosure once per day. Fully formed hard slag takes time to develop. Staying one step ahead of this formation simplifies the cleanup job.
- Knowing the current condition of the inductor is invaluable. Daily meter readings and graphs should be kept either via supplied software or manually charted information. The only reasonable method of cleaning the inductor channel is to rod it mechanically.
- In stopper rod systems, the nozzle is an area of relatively high thermal loss and has a higher chance of becoming closed by oxides. This can be mitigated by using a nozzle refractory material with a lower thermal conductivity, such as fused silica or zirconia.
- To minimize the buildup of magnesium, nitrogen lines and filters in a pressurized ductile iron holding or pouring furnace must be cleaned at least every 24 hours using dry nitrogen gas. Never use air to blow out nitrogen lines or filters as this will cause the magnesium to react explosively.
- Wear appropriate Personal Protective Equipment (PPE), such as non-flammable, full body protective clothing, when opening or cleaning nitrogen lines and filters.
- Wear a full-facepiece chemical cartridge respirator approved for exposure to sulfur dioxide gas when opening or cleaning nitrogen lines and filters.
Induction pouring furnace power systems
While furnace construction and meticulous maintenance play pivotal roles in the efficiency of metal pouring systems, the foundation of any furnace operation is its induction power supply. Choosing the right induction power supply is paramount to achieving optimal system performance and overall operational success.
The construction of a furnace, as outlined, dictates the specific needs and characteristics required for handling different metals. To harness the full potential of these intricately designed systems, it is crucial to align the induction power supply with the furnace’s unique specifications.
Inductotherm's family of VIP® and VIP-I® power supply units stand out as advanced induction power supply systems. Their distinctive features give foundries and melt shops the power to produce more molten metal per kWh and kVA, resulting in lower melting costs and higher operational productivity.
At the core of a VIP® or VIP-I® power supply unit is an intelligent digital control board designed for optimal functionality. Equipped with fiber-optic cables for seamless signal processing and a vivid LCD touchscreen, the control board epitomizes metal pouring precision and user-friendly operation. Configuration of all control board functions is conveniently facilitated via the integrated keypad, ensuring adaptability to specific operational requirements.
Beyond standard configurations, Inductotherm provides a spectrum of power supply options to suit diverse industrial needs. The renowned VIP® Power-Trak® units demonstrate this commitment to customization, with flexibility that aligns seamlessly with the nuanced demands of different applications for ferrous and nonferrous metals alike.
For specific application requirements, Inductotherm offers three-phase and single-phase power supply units. This versatility enables industries to fine-tune their melting and pouring processes, addressing unique challenges with precision. The VIP-I® PWC power supply units are most often used for pouring systems and can also be designed for compatibility with new or existing external deionized (DI) water cooling systems.
In essence, the selection of the correct induction power supply completes the trifecta of efficient metal pouring systems, working in harmony with furnace construction and maintenance practices to elevate operational efficiency and output quality.
Systems built for specific metals or different applications will each have their own characteristics. It is essential to find the right automated pouring system for your metal melting operations to get the best performance. Just don't look for one that pours a double espresso—unless it’s for the break room.
Patrick O’Connor is a District Manager with Inductotherm Corp.