From a string of operations in North and South America, Europe, and China, Nemak supplies automakers with high-quality aluminum cylinder blocks and cylinder heads it produces by precision sand casting. Todd Stockwell, NCC engineering manager for Nemak Canada explains that sand casting is used because it allows each mold and core to maintain the complex designs needed to produce an engine block, including integrated water passages and oil passages, and complex cast surface features. Some of these internal passages open at one end to the casting surface while the opposite end terminates on a blind spot. If a permanent mold made from steel or some other material were used, there would be no way to get these interacted passages formed.
Once a sand casting is finished and ready, the entire mold is heated and the resins that hold the sand grains together burn away. Loosened sand is easily extracted, revealing the intricate internal passages that simply could not be made using by other molding processes.
Nemak uses several different precision sand processes, depending on a customer’s exact needs and product design. This includes gravity casting, in which metal is poured into the mold; and low-pressure casting, in which metal is pumped quiescently into the mold and held under pressure until the casting solidifies. For both blocks and heads, the process selected is matched to customers’ needs, driven by part design and material property requirements.
Sand casting carries with it many inherent challenges that require close coordination during production, in order to achieve the targeted quality for the finished parts.
Everything starts with the initial part drawing. What each automaker wants in its particular engine block becomes a reverse design for each sand core – the inverse of what the final product will look like. The cores hold the spaces that become the water passages, oil passages, and exterior features, such as bolt bosses, lugs, and pockets of the final engine block.
At the Nemak Windsor Aluminum Plant (WAP) in Ontario, core machines blow sand premixed with resin into tool cavities to create the individual core sections. Each core section is robotically assembled to form the complete core package, which then is ready for pouring.
The production lines use foundry-specific robots that are sealed from moisture and dust and are able to sustain consistently high temperatures. ABB Robotics (www.abb.com) has been working with Nemak on planning, support and strategies for robotic handling and assembly of sand packages since1998. It has set up the robotic lines for several Nemak facilities worldwide. Gustavo Sepulveda, ABB’s general manager for Robotics at Nemak Mexico notes that Nemak typically uses 6-axis robots with a 200-kg load capacity. “Each facility is different,” he says. “On average, every line has between 12 and 15 robots. Some of them may be handing extra parts, or they have added additional operations beside the sand core assembly. The minimum number is typically 12,” he says.
Fanuc Robotics America Inc. (www.fanucrobotics.com) provided the robotic lines for WAP. John Utley, Fanuc, project manager explains that a series of core machines, each making a piece of what will be assembled into the final core mold, form the front of the lines. Each core machine has a robot installed in front of it. When the machine produces the sand core, the robot moves it to a table where a sub-assembly process is performed on each core.
“We can load them directly onto the main line where the cores would proceed through all subsequent steps. Or, if the line is blocked upstream for any reason, we can have the robots pick the set of cores and load those to the shelving system on the other side of the line. There are six sub-system cells set up along a main line assembly line — start to finish,” Utley details. “The package gets built up until there is a complete core package at the end of the line.”
Utley notes that the line is set up with both pedestal- and rack-mounted robots. The pedestal mounted robots handle the core machine unloading operations while the rack mount give a more vertical envelope for the buffer shelf positions needed on the other side.
Specific grippers are needed on each robot to extract the shaped core from the coremaking machines, and provide all subsequent handling as they are assembled and move from step to step down the line. Since the plant’s start-up, IPR Automation (www.iprautomation.com) has provided the sand core grippers for ABB robots at Nemak Mexico. This knowledge was transferred to the Fanuc robots for the new coremaking lines at WAP.
Nicky Borcea, President of IPR Automation explains that being a company dedicated to end-effector tooling, everything they do is built universally. “Our tooling is made to help the robot to do its job, whether standard or customized. It has to work with all manufacturers’ robots,” he says.
|It’s important that robotic grippers “cradle” the cores (above), rather than grab them. Core sections are assembled by robots (left) to form a complete core package. (Photos by courtesy of ABB Robotics.)|
For this specific application it is extremely important that grippers “cradle” delicate sand cores rather than grab them. While the resin makes a core somewhat sturdy, the wrong tooling strategy could easily crush it or abrade the surface, or loosen bits of pieces of sand, damaging the integrity of the casting.
“The robots do more than simply pick up sand cores and move them from ‘A’ to ‘B’. They must perform some quality inspections and operations to each core and then place each into what is known as a core package,” Stockwell says. “A core package can be likened to a puzzle where all the sections come together, yielding a finished mold. Once assembled, the core assembly receives the molten aluminum that will eventually become the end product.”
The main carrier starts as an empty conveyor collecting core sections one by one from each robotic station until a fully assembled sand core package is complete. At that point the mold moves to the casting area where molten aluminum is poured into its cavity. As the aluminum is solidifying, the mold is transferred into a Thermal Sand Removal (TSR) oven for sand breakdown and reclamation. The oven is set to a temperature just below that of the last stage of complete metal solidification, to deter incipient melting, but hot enough to allow sufficient thermal breakdown of the sand.
After the aluminum casting emerges from the sand mold, it goes through some primary cleaning operations such as deflashing and rigging removal, and then is heat treated again for dimensional stability and targeted mechanical integrity.
“Finally, the block goes through what is known as a cubing operation,” Stockwell says, “where we physically remove a few millimeters of material from the face of each block. The result is a perfect engine block, ready to be shipped to the customer’s plant for auto machining and engine assembly.”
Stockwell says, “We rely on our systems to pick up cores and manipulate them, inspect them, and then assemble them into a package. There is very little tolerance for positioning. If the core is not gripped properly or not inserted properly into the package, it will break. It’s a delicate process. The way the fixtures are set, the precise repeatability in the way grippers hold the core, the way the robots verify that the core is in the correct position – all of those aspects are extremely important.”
|Six sub-system cells in the assembly line complete core packages. (Photo by courtesy of Nemak.) |
From the robotic point of view, holding the part with the right type of gripper is only one aspect of performance. Utley notes that from the perspective of a robotic assembly process, it is also a challenge to fit the cores together precisely without any rubbing.
“If you don’t do that, you are going to abrade sand that will fall down into areas of the core and compromise the pouring process, resulting in defects in the casting,” he says. “The tooling and everything involved with the robot system must be precise and support the cores in ways that show no variation whatsoever.”
“That’s one of the top quality issues,” Ricardo Cruz, ABB’s account manager for Nemak Mexico agrees. Even a few PPM of sand within the mold will embed in the engine block when they pour the aluminum. We have to be sure from the very beginning how we will move the core and its mold assembly, to minimize aforementioned core damage, and to ensure productivity (cycle time) as well. We have to visualize the core handling from the start in order to have the repeatability to be able to assemble a mold yielding a dimensionally tolerant and scrap-free engine block.”
The IPR tooling used to hold and move the sand cores are developed from initial 3D CAD models in order to ensure precise tolerance. There is only 0.1-mm clearance on all sides to cradle the cores, to achieve the very high degree of accuracy required.
Handling is critical. Even sand that avoids falling into the mold can shed onto the line adjacent to the conveyer. Over a few days of cycling this can build up, affecting productivity of the whole line.
Not all cores are perfect, so it is also imperative that all robotic lines have vision systems to inspect for what are known as “lacy cores.” These less-than-perfect cores may occur when the resin content is not correct or when a vent tube in the tool box is partially plugged. These can break even with the most gentle handling. It’s important to find defective cores and remove them early in production.
|Graphic shows robotic gripper core handling in close up (courtesy of Fanuc Robotics.)|
|All robotic foundry lines are coordinated and tracked for quality control of the manufacturing process. (courtesy of Nemak Inc.) |
Cruz says that for any project on the scale of a Nemak aluminum casting line to work effectively, the goal is not only for the robotic supplier and integrator to execute their tasks precisely, but to be thoroughly collaborative with all other companies involved in the design and delivery of each part of the process.
“There are a many companies involved in a project like this,” Sepulveda says. “Some provide the sand blowers for the casting machines, some make the tooling for the actual sand cores, and there are conveyor people, the robot providers and robotic tooling providers. There are more, but I would say those are the primary ones. Everything has to run like clockwork to give Nemak what they need,” he says.
Utley emphasizes that even with a good production concept and good partner involvement, with each one knowing what the customer wants and what it wants to do for the customer, it still may take 18 months or more to get a line up and running. “Having a good software modeling system in place can make the job a lot easier for both the robot line provider and the customer. It helps to be able to see what the line will look like and how it will work, before it ever gets to the installation level,” he says.
Nemak relies on the experience and expertise of key suppliers chosen for their teams. All business models rely on choosing the right team for the right project, Stockwell notes. Though he acknowledges this appears to be a cost-driven approach, he emphasizes that the most important factor is for each project partner to be comfortable with the others’ technical capabilities.
A good supply team must fit together, with a precision that matches sand core and mold assembly. For Nemak this isn’t a challenge, it is the only way to do business.