The additive manufacturing technology, laser sintering, is a faster, lower cost way to install conformal cooling channels into your mold tooling parts.

By Tim Ruffner – VP New Business Development / Marketing Manager, GPI Prototype & Manufacturing Services, Inc.

Conformal cooling in tool design is a fundamentally sound concept. It’s the mechanics that can be problematic. Available machining processes, such as CNC machining, impose design boundaries that often put the merits of conformal cooling out of practical reach. Additive manufacturing technology, however, can make conformal cooling a profitable reality. One additive manufacturing technology is direct metal laser sintering (DMLS), a term coined by EOS GmbH, the developer of it. Recently, the ASTM F42 committee developed company neutral terminology for all of the additive manufacturing, 3D printing, and rapid prototyping technologies. Thus, DMLS can also be known as powder bed fusion or laser sintering.

Conformal cooling is the holy grail of injection mold temperature systems. It’s the most effective means for maximizing tool performance. But implementation has been historically problematic due to straight-line methods or time and cost-consuming secondary operations. With laser sintering, you can create tools and inserts with precisely placed and seamless channels.

Our featured project came to us from Phillips Medisize. The design for the conformal cooling tool insert incorporated channels that quickly and evenly cooled individual cavities within the piece, which would not have been possible with conventional mold technologies. Customer expectations included reduced cycle time and higher quality parts.

Powder bed fusion is an additive metal technology that builds directly from 3D CAD files. Like all additive manufacturing (AM) technologies, it takes a CAD file and slices the object into thin layers, in this case the layers are 20 microns (0.0007 in.) or 40 microns (0.0015 in.) thick. The layers of the part are built using a 200 W fiber optic laser that locally melts each metal powder layer onto the previous layer, eliminating the need for a binder. The result is a fully dense metal part.

It is possible to orient a part diagonally to build parts that are longer than the AM machine’s base dimensions. Another option is to build in segments and weld the parts together.

Typical tolerances of laser sintering technology are 0.005 in. on the first inch and an additional 0.002 in. each inch thereafter, but with some fine-tuning respective to individual projects, machines are capable of tighter tolerances.

Materials for these systems are made from wrought metal that has been water or gas atomized into a fine powder. They are almost identical to current alloys on the market. Most laser sintering materials meet or exceed ASTM standards.

Secondary Processes

The inherent design freedom of this method of additive fabrication often reduces the need for secondary processes. For example, text may be incorporated in a CAD file to build an engraved part. However, when desired or needed, parts are secondary process friendly. The options include machining, tapping, welding, coating, plating and texturing, electrical discharge machining (EDM), and engraving.

Polishing requires some pre-build planning. Laser sintered parts can be polished to a mirror finish, but the size of the part may need to be altered in the CAD file (0.008 in. up to 0.030 in. depending on desired finish) to account for material removed during the polishing process.

Conformal Cooling: It’s in the Channels

An effective temperature control system saves time and costs in the process of injection molding. Conventional cooling relies on things such as the conductivity of the mold material and straight line drilled channels. Its focus is on cooling the mold. Costly inserts made with alloys that have higher conductive properties aid the process.

Conformal cooling uses strategic channels concentrated around the product. Its focus is on cooling the injected melt.

When plastic melt cools evenly, internal stress is minimized. The result is higher quality part with little to no warping or sink marks. An added bonus comes in the form of drastically diminished scrap rates. Productivity gained in upgrading from a conventional tool to a tool with conformal cooling channels can be upwards of 30-60%.

Properly maintained mold temperature also tends to improve tool life and the high costs associated with replacement tools.

The use of CNC machining to create conformal cooling channels is often too expensive for most parts. The intersecting channels, complex coolant routing, and zero velocity areas are difficult to manage and the process can’t achieve a controlled or uniform distance from the melt. Secondary sinker EDM to complement CNC tooling can prove costly too, both in time and money.

 

Powder bed fusion builds conformal cooling channels into the tool. Channel geometries around the cavity handle the fluid, but fluid velocity in the channels from the fluid pressure is important. The molder will control the flow of the water depending on how fast the cycle time is and how quickly the insert needs to be cooled. Sometimes the operator will change the flow depending on conditions inside the mold.

Even combined systems with separated cooling and heating channels are possible, or split between main systems (for the control of the global temperature) and specific systems (for the handling of close to cavity critical temperatures). This opens up the potential for future applications. Turnaround times can be as short as 5-7 days.

Guidelines for conformal cooling

The recommended material for tooling is MS1 Maraging Steel. It is the hardest, most durable material and is used in 95% of all DMLS tooling. It can be post-hardened to 54 Rockwell and withstand temperatures up to 750°F before yielding.

In cases where a tool insert created with directed energy deposition must be polished, it’s best to have it done by a professional mold polisher to ensure tight tolerances.

Powder bed fusion can build channels down to 1 mm in diameter; however, channels this fine can only be put into service with specially treated fluids to avoid clogging. Simulation software helps find the right layout in such critical cases.

According to experience, the optimal diameter should be chosen between 4-12 mm (depending on the design of the product). These values are preferred values to be used in ideal cases; in practice, sometimes tool inserts are too slim to exactly follow this rule (for example a closely placed pair of ejector pins, thin walls, and so on). In cases of complex geometrical conditions, it can be necessary to design much smaller diameters, for example when eliminating a hot spot.

In addition to circular cooling channels, designers can use more complex shapes to reach greater cooling performance. The feasibility criterion supposes a cross section that is self supporting, that is, an angle of overhanging areas should be above 40° to horizontal.

Some overhanging angles require support during the build process. Supports cannot be removed in channels. Your operator should be able to help identify suspect angles prior to building.

Conformal cooling in many applications was considered a lost cause due to the problems faced in execution. Additive manufacturing technology reopens the door for tool designers and mold makers to effectively design high performance, high efficiency conformal cooling tools.

This technology has consistently proven to create tools that categorically improve productivity. The drawbacks are size limitations imposed by the build envelope and gaps in designer to operator communication. MPF

Case Study – Phillips Medisize

Phillips Medisize came to us with a design for an insert that was to be part of a four-cavity mold to replace the one-cavity conventional mold currently in operation.

GPI Prototype used an H13 tool steel plate and built laser sintered tool inserts with conformal cooling channels incorporated onto the plate. We sent the plate with the completed build to Phillips Medisize for them to post machine and wire cut the inserts.

Phillips Medisize confirmed conformal cooling allowed for a shorter cycle time. It also produced a higher quality part (flatness and dimensionally correct).

• Tonnage 80 ton Netstal

• Amount of Material– 0.0774 pounds per shot

• Type of Material Celanex 2401MT

GPI Prototype & Manufacturing Services, Inc.
www.gpiprototype.com