The automotive industry has been at the forefront of manufacturing breakthroughs and significant advances since Henry Ford established the modern assembly line. The industry’s growing interest in metal additive manufacturing (AM) is an ongoing example of this.
Current discussions tend to categorize AM for low-volume use, but for conventional manufacturing processes like stamping or casting for high-volume production—the picture is changing.
There is a third, growing category of production called “AM-enabled conventional manufacturing.” This label refers to the integration and sequencing of AM with post-processing tools like CNC machining, surfacing, and heat-treating approaches, with AM’s supporting use, alongside conventional shape-forming processes. In implementing AM with conventional manufacturing for selected, higher-production programs, automotive manufacturers may again be leading the charge on innovation and productivity.
One example is tooling creation, a mainstay of high-volume conventional manufacturing. Recent advances in Laser Bed Powder Fusion (LBPF) technology are providing unparalleled design freedom, larger capacity print volumes, advances in automated calibration routines, and scalability.
Design freedom plus efficiencies
Anyone working with high-pressure diecasting (HPDC) or injection molding (IM) machines knows the importance of cooling for better part quality, reduced cycle time, and improved final yields. Conformal cooling is a technique used to accelerate and control temperature declines in tooling inserts (and sometimes in new core and cavity designs.)
In aluminum diecasting, for example, the molten metal arrives at the die at a temperature of around 600°C or more, and is injected into the part cavity in about 20 milliseconds. The part then solidifies with the help of liquid flowing through cooling channels that surround it, decreasing in temperature to approximately 60°C within 60 seconds or so, depending on the part solidification strategy.
When the injected component has solidified, the die separates and pins eject the part from the die. In diecasting production, cooling of the diecast insert constitutes around 70% of the cycle time to manufacture a part. By accelerating the cooling rate, tooling designers can speed-up production times. They also increase the service life of the cooling inserts, and even have the ability to improve the material properties of the end-use part.
Conformal cooling has been around for a while. In fact, in principle, it is similar to a solution used in spacecraft design—regenerative cooling.
In regenerative cooling, cold fuel flows through a rocket nozzle to prevent the nozzle from melting. The result is a nozzle that is cryogenically cold on the exterior but experiencing full rocket exhaust combustion on the interior. Engineers have found such solutions preserve their parts’ survival even when exposed to environments that exceed the melting point of the metal. And now, design solutions similar to those used to launch spacecraft are being used in automotive plastic-part production.
Challenges with cooling channels
Engineers have limited design freedom to create cooling channels when using conventional manufacturing methods. Most approaches involve drilling (or sinking EDM) straight, intersecting lines for cooling fluid to flow through. However, this type of cross drilling can cause a number of issues like stress concentrations and leaks.(Image 2.)
In addition, because these cooling channels are formed in straight lines, hotspots often remain beyond the coolant's reach. The thickest parts of the casting that also have the longest solidification times—along with high-aspect ratio features, such as pins—do not see optimized cooling amounts. Since these sections take the longest time to cool, speeding up the process in these areas can help accelerate the entire manufacturing process. It also can mitigate the risk of opening the die too soon and exposing molten metal to the atmosphere.
Conforming makes the difference
In contrast, metal AM can be leveraged to print an insert with conformal cooling channels that follow the contours of a part and provide more optimized cooling. By integrating conformal cooling channels, manufacturers can achieve a number of beneficial results:
● Reduced cycle times (time per shot goes down)
● More uniform temperature gradients to eliminate hot spots
● Better, more complex designs that improve the material properties of diecast parts. (Image 3.)
With conformal cooling, it becomes possible not only to optimize manufacturing but engineers also may be able to thermally balance the tool, resulting in a casting that solidifies near-simultaneously.
In casting, the last sections to solidify also tend to have a higher concentration of air pockets. By creating a cooling strategy that solidifies the more critical sections faster, conformal cooling can help to move these air pockets away from harmful areas. By controlling the cooling balance, engineers can improve the material properties of their parts.
Higher production, lower insert costs
In the past, one of the roadblocks to AM in cast/mold production has been limited printer sizes and corresponding build volume. These issues are being overcome quickly as increasing numbers of large-format, multi-laser printers have entered the market. (Image 4.)
The expanding dimensions of metal 3D printers increases the opportunity for creating larger diecast inserts. The likelihood of additional lasers also has the potential to accelerate the die-insert manufacturing process and lower insert costs.
What’s more, engineers now are examining the possibility of increasing cooling-channel size and cross-section design. And this is where many current AM technologies falter: for channel cross-sections larger than ~6 mm in diameter, inserts have started to experience premature failures.
Can you print it?
Some of these failures have been due to the challenges inherent in conventional metal AM systems.
Metal additive manufacturing, specifically Laser Powder Bed Fusion (LPBF) printers, deposit a planar surface of powdered metal onto a build plate. Next, the lasers inside the printers follow a two-dimensional pattern of one layer of the part, melting the metal powder in specific locations dictated by the design. Then, the metal cools and solidifies quickly.
As the metal cools, it contracts. When printing layer after layer, this process of melting and cooling builds stress. This stress, and the need to remove heat from the melted layers, can result in potential deviations from the design.
There is a higher frequency of deviations on channels of any kind. A circular channel, printed layer by layer, results in two sides—or curved walls—of a cylinder that must eventually meet and close out at the top. During this close-out the stress on these two walls is the greatest, leading to deformation in the structures.
The end result is a channel with a non-uniform, upper close-out section with poor surface finish. A rough finish results in stress concentration, eventual cracking of the part, and premature failure. (Image 5.)
In addition, with more lasers comes the potential for each laser to drift out of alignment with the others. Conventional systems require a complex, manual calibration process involving external equipment and testing routines. Typically, due to the manual and time-consuming nature of the calibration, operators only adjust the lasers a few times a year. This leads to drift over time and within the build if the chamber temperature rises and components shift. As a result the overlay—the area where the two laser zones meet—can become compromised during printing, leading to weak points in the part.
Larger channel dimensions
New-generation AM systems offer a solution on a number of fronts. With larger build plates, engineers can access larger-format printing for their die inserts. Automated calibration routines and runtime layer-by-layer calibration checks ensure virtually invisible overlay zones. (Image 6.)
In addition, engineers can realize a circular channel cross-section at significantly larger diameters. The most highly integrated AM systems are capable of printing channels up to 100 mm in diameter with a quality surface finish, a full order of magnitude beyond what exists in the industry today. This capability helps to mitigate local accumulation of stress that is common on the teardrop shape typically produced in the design-for- additive manufacturing (DfAM) process.
Integration of pre-print design software and printing hardware largely avoids the design compromises and part defeaturing endemic to the DfAM approach. This new level of attainable design sophistication and performance works to eliminate the risk of cracking. And it serves to optimize the flow of the coolant (water/oil) and achieve higher heat- dissipation rates.
The future of metal AM for tooling
Advanced metal AM systems will be the main driver of future tooling innovation due their powerful, integrated, end-to-end solution sets serving both part and tool optimization. These solutions are comprised of sophisticated pre-print design software; high-capacity, highly accurate printing; and quality-assurance software for in-situ monitoring and quality-control reporting—all on a unified platform.
For example, such solutions translate into the time-saving ability to assign geometry- specific scanning instructions automatically. This produces a robust printing process that yields a higher-quality surface finish and consistent mechanical properties without compromising the design. It also gives non-AM experts confidence that their designs will print to the standards they require for both simple and complex parts and tooling. (Image 7.)
By exploring the latest technologies for AM tooling, manufacturers can discover breakthrough efficiencies in HPDC and IM performance, and unprecedented part longevity. With those engineered improvements come lower costs and faster delivery times for satisfied customers.