Additive metal turns up the volume

By removing the need for traditional moulds and tools, it was argued that 3D printing would replace many older manufacturing techniques, ushering in a new world of flexible production and mass customisation. That revolution has yet to happen.

Additive manufacturing technologies have evolved dramatically in recent years, improving in speed, precision and in the range of materials they can handle. The approach has become a mainstay of product development, prototyping and small-volume manufacturing. In sectors from aerospace and automotive to medical devices and sporting goods, manufacturers are finding niche applications that benefit from the unique characteristics of additive approaches. When it comes to the large-scale mass production of thousands or millions of parts, however, additive manufacturing is still too slow and too expensive to compete with well-established techniques such as injection moulding.

Growing behind the scenes

In fact, the relationship between additive and traditional manufacturing techniques has turned out to be one of collaboration rather than competition. It might not be obvious in the end products, but many high-volume production processes increasingly rely on additive manufacturing at some point along their value chain.

In plastics injection moulding, for example, metal additive manufacturing systems are transforming the production of mould tools, helping companies to get ideas into production faster, and driving quality and productivity up.

Conformal cooling

To reach the short cycle times and high productivity rates required for low cost, high volume parts, cooling water needs to be evenly distributed around the mould tool; this is typically achieved by machining water channels within both the base plate and tool inserts. As well as boosting injection moulding productivity, rapid, even cooling is also vital for part quality.

Appropriate control of the cooling rate affects the mechanical properties and surface finish of the part, and if areas of the material are insufficiently cooled within the mould, or cool at a different rate to the surrounding areas, they can shrink excessively after ejection, leading to distortion, poor tolerances and unacceptably high reject rates.

Conventionally, these cooling channels are drilled through the mould material during tool manufacture; and while this approach is simple, where the part geometries are more complex, it can be difficult to run straight cooling channels close enough to the mould cavity for efficient heat transfer.

A further complication arises when cooling channels have to compete for space within the tool with features such as ejector pins or moving inserts. This is a particular problem in the production of box shapes, such as electronic enclosures where the best position for the ejectors is usually at the more structurally strong corners. Unfortunately, these points are also the hardest to cool; even minor shrinkage at the corners of an enclosure due to inefficient cooling can lead to significant distortion of the adjacent walls.

Poor cooling performance creates a dilemma for plastic injection moulders. Either they accept high levels of distortion, or they slow down the production process, allowing the part to cool in the mould for longer. Taking the latter route inevitably increases the overall cycle time, which is detrimental to productivity and drives up part costs.

Additive manufacturing provides a neat solution to complex cooling challenges. The hybrid manufacturing process allows complex, shaped channels to be built into the structure of the tool, ensuring adequate cooling of even inaccessible areas. And as well as allowing cooling channels to take any route through the tool, the process also removes the necessity for those channels to be round. Elliptical, rectangular and even teardrop designs can maximise heat transfer for a variety of applications. Or special features can be incorporated within the channels to promote turbulent coolant flow, which increases the heat transfer rate. Moulds created using this technology have demonstrated cycle time reductions of as much as 20%.

Escaping quality traps

Additive manufacturing also helps to handle some common production challenges experienced in many plastic moulding applications. Gas traps are caused by gas pockets forming as the ‘melt fronts’ of the molten plastic. They can lead to scorching, pinholes and poor finished part quality. Best practice in part design, tool design and the use of mould flow simulation is to alleviate the risks of gas traps, but they can still occur.

Traditionally, gas trap issues are addressed by fabricating the tool with inserts made from specialised porous materials in the critical areas, or by retrospectively adding vent pins (fixed ejector pins) to the exact site of the problem. Both these approaches add cost and time to the toolmaking process.

With additive manufacturing technology, OGM, for example, can build inserts that incorporate large numbers of micro-pores, each just a few microns in diameter, through which gas can escape, without adversely affecting the quality of the finished part. The inserts are 3D laser sintered in steel or other metals, to match the exact requirements of each injection mould tool. The dimensions and characteristics of the gas escape channels are optimised to reduce cycle times and boost productivity, while maintaining high levels of part quality, even for extremely complex designs.

Cutting time to market

OGM was the first UK plastic moulding company to invest in a new kind of rapid toolmaking technology that combines the best features of additive manufacturing and conventional CNC machining. The system builds mould features from powder material layer by layer using a laser. After each layer is added, an automated secondary machining process rapidly removes excess material to generate the finished geometry of the tool. The result, extremely high dimensional accuracy and fine surface finish allow core and cavity details to be manufactured automatically in one hit. The material surface finish produced is also very hard, avoiding the subsequent need for heat treatment to produce production tooling. If required, a full range of textured or polished surface finishes can be applied in secondary processes.

The machine provides the most significant time savings for complex mould features such as the deep, narrow slots required to produce thin internal walls. Such features cannot be produced using a conventional milling cutter. Instead, traditional toolmaking approaches require either the manufacture of complex split tools, or the uses of EDM (spark erosion) techniques followed by time-consuming hand polishing.

To make the most efficient use of its hybrid metal additive manufacturing technology, OGM’s Rapid Production Toolmaking technique combines the process with conventional high-speed machining. The basic shape and simpler features of a core or cavity are machined first from a solid block of tool steel. This becomes the base structure onto which the more complex features are added. The completed core and cavity components are then installed in a standard design of bolster, preassembled and ready for production.

A key benefit of this approach for customers is speed. It can be used to produce steel tools suitable for high volume production in as little as four weeks. Moreover, this approach means tool production can begin in parallel with the end of the design process. If the overall dimensions of a part have been defined, but certain features are still being refined, OGM can start producing the tool, with the complete part definition only required at the final production stage.

Additive and conventional production techniques are not rivals, but friends. Innovative manufacturing approaches that build on the strengths of different techniques are helping companies move faster, cut costs and deliver products that work better for their customers.