Published 12/1/2017

Vacuum Brazing Meets Additive Manufacturing for Optimized Conformal Cooling

Renishaw and Liechtenstein-based Listemann Technology have joined forces to service moldmakers with the optimum conformal cooling solution. Depending on the application and desired reduction in cycle time, the companies now jointly design and create conformal cooling channels either through vacuum brazing, laser sintering or a combination of both to find the most suitable and cost-efficient design for their customers.

Barbara Schulz

European Correspondent, Gardner Business Media

In this mold insert, the insert is vacuum-brazed, and the core is laser-sintered. IQTemp analysis revealed that to reduce the length of the cooling phase, the mold insert could use less expensive, vacuum-brazed cooling channels for the insert, but required more expensive, more complex channels for the core, for which laser sintering was the only option. Images courtesy of Schulz/wortundform.

The mold insert is divided into several parts to machine the cooling channels, and then the mold insert is fused back together using vacuum brazing. The braze material, in the form of a foil, is inserted between the parts to be fused by heat treating. A high-temperature furnace at temperatures up to approximately 1,000°C performs the heat treating, simultaneously hardening the part. Pins fix the parts in their positions. Images courtesy of Schulz/wortundform.

In the top part of these pieces, the cooling channels are created by SLM. The bottom part is conventionally machined to save costs. Images courtesy of Schulz/wortundform.

Breaking Down Brazing

While most manufacturers are familiar with laser sintering technology these days, vacuum brazing is not as common and is often confused with soldering. Vacuum brazing enables the creation of complex components by joining simpler parts using a braze metal with an adapted melt point that flows by capillary action (or without the assistance of external forces like gravity) into the space between the parts. To create conformal cooling channels, the mold insert is separated into several slices, and the channels are milled into these parts, which are subsequently brazed in a high-temperature vacuum furnace.
The filler metal or alloy is heated to a temperature above 750°C and up to 1200°C. The filler metal or alloy is distributed between the parts homogenously to form an exceptionally strong, sealed joint. “After a period of 8–24 hours, the part comes out of the furnace brazed, heat-treated and ready for final machining. In contrast to welding, the base metal does not melt, which results in low distortion. The joint is resistant to shock, withstands high pressures and is free of voids or inclusions,” Manfred Boretius, CEO of Listemann Technology, says. “The process permits joining dissimilar materials or even non-metals and hardens the part at the same time.”

According to IQTemp, the customer usually delivers the mold pieces that are then heat-treated for stress relief. All parts must be thoroughly cleaned and degreased. A degree of surface roughness, imparted by mechanical machining, will assist capillary flow of the braze material to produce stronger bonds. Parts are assembled to ensure that the desired gap with the selected braze material is inserted in the space between the parts. Assemblies are then placed in a vacuum furnace where, following a carefully-prescribed time and temperature cycle, the components are heated to the braze-material melt point, held at this temperature for a specified time and then subjected to controlled cooling.
The melted braze material will soften and wet out to flow and fill in all of the voids and bond with the parts metallurgically. “Conducting this process under vacuum without any flux ensures that no oxidation occurs on the surfaces, and the resulting bonds are free of voids and are exceptionally strong,” Boretius says. “Using suitable materials for the components of the assembly, the heating and cooling cycle can be designed to solution-anneal, age-harden or harden and temper the component as the brazing takes place, saving significant time and cost.”

Braze materials vary according to technical requirements, but these are typically alloys of nickel, gold, silver or copper depending on the base materials that will be joined. Various types of braze materials are used to meet different needs, including foil, wire, powder or paste. According to Boretius, foils can be as thin as 50 microns, which is enough to create a good joint if the parts are properly prepared, as the brazing gap needs to be 0.03–0.05 millimeters wide to ensure a good capillary flow and good bonds.

“Once you have created the bond, it is nearly indestructible, and the quality is repeatable because you always run the same program once you have established a suitable vacuum-brazing process. This requires substantial metallurgical knowledge and experience,” Boretius says. “There are several factors that have a significant impact on the resulting component, including surface preparation and finish of the parts, correct joint design, correct selection of the best braze material and format (foil, wire, powder or paste) and the design of the optimum heat, hold and cooling cycle. Finally, every mold insert is tested for helium leaks to detect even the smallest defect before delivery to the customer.

Fusing Multi-Component Mold Cores, Cavity Assemblies

“There are several methods of creating conformal cooling systems in molds, including laser sintering and vacuum brazing,” Boretius says. “We have specialized in vacuum
brazing technology to fuse multi-component mold cores and cavity assemblies for conformal cooling systems, because it is a fully developed technology that can join different materials. For example, it can join copper and steel or steel and ceramics.”

Boretius admits that Listemann used to think of additive manufacturing technologies, like laser sintering, as rival technologies. Recently, however, the company realized that overlaps are small. “While vacuum brazing is ideal for large mold inserts and cores, multi-cavity applications and joining materials, laser sintering is more suitable for small cores and inserts, highly customized products and complex geometries. Both technologies can only realize about 5 percent of applications. A clear division exists.”

As a result, the company teamed up with Renishaw Inc., which took over Listemann’s partner for laser sintering applications (LBC Engineering) in 2013 and established a new brand named IQTemp (intelligence and quality for molds and dies). IQTemp formed to offer conformal cooling solutions to its customers using vacuum brazing and laser sintering.

“It’s about finding the most efficient and cost-effective conformal cooling design for our customers,” Renishaw’s Design Manager Carlo Hüsken says. “For example, if the cooling channels are too complex for milling and vacuum brazing, we will suggest laser-sintering them. Since the selective laser melting (SLM) process is expensive, we also might suggest producing parts of the core or insert conventionally and only manufacturing the part with the conformal cooling channels additively.”

According to Hüsken, they use computational fluid dynamics (CFD) and thermal heat transfer analysis to determine the optimal conformal cooling solution. “We design different types of cooling channels in an injection mold virtually and compare the performance in terms of cooling time, temperature profile and part warpage. The injection molding condition during operation, for instance, dictates a minimum distance from the mold surface to the cooling channel. Considerations of pressure and drops in temperature along the flow channel are constraints for a successful design.”

Complex cooling channels can only be created using SLM. If the simulation shows that complex cooling channels offer the most potential for substantial improvement in production rate and part quality, the IQTemp team will suggest this design. The team will make that suggestion even if it is more expensive than using less complex channels, which are created from milling and vacuum brazing. However, if the simulation shows that both designs will produce the same results, they will suggest the less expensive solution of using less complex cooling channels.

While Hüsken and Boretius admit that they cannot yet create hybrid parts that feature laser-sintered and vacuum-brazed areas on a single part because of material constraints, the companies are currently conducting research. In some cases, conformal cooling channels for one mold are created using both technologies—one for the core and the other for the insert.
For example, IQTemp designed and manufactured an injection mold that had cycle times of 32.8 seconds with a cooling time of 18 seconds without conformal cooling. Cooling channels with an enhanced-flow design to create a highly turbulent flow inside the cooling channels could make the mold more efficient. Specifically, CFD analysis showed that this type of conformal cooling channel could reduce the cooling phase to 9.5 seconds, a time savings of about 47 percent. The mold insert was vacuum-brazed, while the mold core was laser-sintered.

IQTemp’s analysis showed that the creation of less complex, less expensive cooling channels using milling and vacuum brazing were sufficient for the mold insert to achieve the required reduction in cooling. However, the mold core needed complex channels to achieve the desired result, and laser sintering was the only option.