Speed is of the essence in the production of injection moulded components. Manufacturers need to get their designs from concept to production as quickly as possible so that time-to-market targets can be met. Moulders also need to achieve the shortest possible part production cycle times to maximise productivity and keep unit costs down.
The manufacture of mould tools suitable for the high-volume production of complex parts is a time-consuming process; lead-times of up to four months between design sign-off and first part production are not uncommon. Moreover, the performance of a tool – especially its ability to cool parts effectively prior to ejection – determines the cycle time, quality and overall productivity of the moulding process.
Water plays a critical role in the plastic injection moulding process, ensuring that the part is cooled and solidified in the mould cavity and gains sufficient structural rigidity prior to it being ejected. To achieve the short cycle times and high productivity rates required for low cost, high volume parts, cooling water is passed through channels created within the mould tool to accelerate this solidification process.
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, 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 with features such as ejector pins, or moving inserts, for space within the tool. Illustrative of this is 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 and even minor shrinkage at the corners of a box due to inadequate cooling can lead to significant distortion of adjacent walls.
Sometimes, as in the case of the slender cores used to create the internal surfaces of thin hollow parts, it is impossible to provide a straight cooling path through the tool and often requires elaborate workarounds during tool manufacture. For example, a toolmaker might drill two parallel channels, connect them with a cross channel and then add material to seal its ends, or they may insert a baffle into a larger blind hole to create inlet and outlet pathways for coolant. These all add cost and complexity to the mould making process, while some mould features may be too small to accommodate them altogether.
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, damaging productivity and driving up part costs.
Changing the shape of the fluid channels within the mould from straight lines to curves allows them to follow the part surfaces more closely, negotiate obstacles like ejector pins, and squeeze into inaccessible areas. This ‘conformal cooling’ approach has been around for a long time, but it is rarely used in production applications, as there is significant manufacturing complexity involved in building tools with such conformal cooling channels.
Using conventional subtractive machining, conformally cooled tools require moulds to be created in laminations. The cooling channels are machined into the surface of these laminations, which are then stacked on top of each other to create the finished tool. The technique adds significant time and cost to the toolmaking process; it can also result in less durable tools and does not provide a solution for all part geometries.
More recently, additive manufacturing technologies have provided an alternative method of incorporating conformal cooling channels in plastic injection moulds. Direct metal laser sintering, for example, allows the formation of complex shapes from powder metallic materials, enabling channels of almost any shape to be incorporated into a design; the process does have its drawbacks, however. It is costly and time consuming, for one, and the surfaces created are not smooth enough for the purposes of injection moulding, requiring extensive secondary machining operations and adding further to costs and tool production lead times.
A new, fast hybrid approach
There is now a new technique which promises to overcome some of the barriers that have prevented the wider uptake of conformal cooling by the industry. Combining additive manufacturing and conventional CNC machining technologies, a special machine now installed at plastic injection moulding specialist, OGM’s site near Oxford – and believed to be the first of its kind in the UK – builds steel mould tools complete with conformal cooling channels layer by layer using a laser.
As each layer is deposited, an automated secondary CNC machining process removes excess material to provide a dimensionally accurate, fine surface finish. The material produced by this process is hard enough (HRC 35) to meet the needs of many production applications without subsequent heat treatment; if required, a full range of textured or polished surface finishes can be applied using industry standard secondary processes.
Cooling channel designs are able to make optimal use of the capabilities offered by this process, to create parts dubbed ‘ConformL Cool Inserts’ by OGM. For example, 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. Moreover, special ‘trip’ features can be incorporated within the channels to promote turbulent coolant flow which increases the heat transfer rate.
OGM says that its new approach allows customers to obtain steel tools suitable for high volume production in as little as four weeks, less than a third of the time required for conventional toolmaking. Furthermore, the technology developed to manufacture complex conformal cooling channels – which can significantly improve the in-mould cooling of complex parts – not only boosts part quality but can also cut moulding cycle times by up to 20%.
OGM is currently taking this development forward with a variety of offerings, including custom-built inserts that can be incorporated into conventionally manufactured tools to address hard-to-cool areas, as well as a range of standard inserts, including ejector units with built-in cooling channels.
The ability to build complete mould tools – complete with complex cooling channel geometries and designs – in a one-hit automated process can lead to significant design-to-part lead-time reductions. Thanks to hybrid additive manufacturing, compelling commercial benefits are offered to companies operating in fast-moving, time-sensitive markets.