New techniques in additive manufacturing are delivering major benefits in injection moulding processes by enabling significantly more effective cooling performance. By Professor Milan Brandt, Dr Maciej Mazur, and Associate Professor Martin Leary of the RMIT Centre for Additive Manufacturing, and Dr Paul Brincat, Dr Chris Friedl and Robert Larson of Autodesk Australia.

The effectiveness of an injection moulding cooling system is directly linked to productivity and part quality. Cooling efficiency is particularly dependent on cooling channel proximity to the mould cavity, as variation in the proximity can result in uneven heat dissipation, leading to: increased cycle time; part warping and sink marks; internal part stresses; and reduced tool life due to thermal stress.

The design of conventional cooling channels in injection moulds is constrained by traditional manufacturing constraints such as the linear nature of the drilling process, limiting the ability to conform the channel to the contour of the mould. Hot spots are traditionally addressed by the inclusion of targeted bubblers or baffles that channel coolant to a specific location; however, these are not always feasible due to interference with other mould components.

In particularly challenging cases, mould halves have to be sectioned, and cooling channel profiles machined into each mating section before assembly into the final mould. This not only significantly increases tooling costs, but often shortens the life of the mould, resulting in productivity and part quality losses.

However, emerging additive manufacturing techniques such as selective laser melting (SLM) can enable the construction of injection moulding tools with significantly improved cooling system performance, by allowing new design freedom in comparison with traditional manufacturing methods. SLM is a process for building solid metal parts additively using a laser beam which selectively melts and fuses accumulating layers of powder. A range of metals can be processed (such as high strength and stainless steels as well as aluminium alloys) to form fully functional parts with highly customised and complex geometry in short lead times.

The design freedom enabled by SLM can allow for the manufacture of injection moulds with integral cooling channels, which conform to the mould cavity shape and improve heat dissipation, addressing the limitations of traditionally drilled cooling channels. Additionally, the geometric flexibility of SLM can enable the manufacture of cooling channel cross sections with varying profiles and surface finish as well as the design of separate heating and cooling channels, for further performance and productivity gains.

Demonstrable benefits

The performance benefits of conformal cooling have been recently demonstrated by researchers at the RMIT University Advanced Manufacturing Precinct and Autodesk Australia, through the design, manufacture and testing of a conformally cooled injection moulding tool. The objective was to:

  • Investigate the SLM manufacturability of injection mould tooling from high-strength H13/DIN 1.2344 tool steel.
  • Experimentally compare performance of conformally and conventionally cooled tooling.
  • Quantify the predictive performance of Autodesk Moldflow Insight injection moulding simulation software.

The test mould developed for investigating conformal cooling improvement is designed around a rectangular box moulding, the geometry of which exhibits dimensional sensitivity to cooling non-uniformity; inward deflection of side walls is expected when cooling uniformity is poor. The associated mould assembly accommodates: exchangeable core inserts; a traditionally machined and conventionally cooled insert with four cooling baffles; and SLM-manufactured inserts with integrated conformal cooling channels. Both inserts were integrated with channels for thermocouple inserts to record temperature.

Following careful experimentation, material testing and analysis to identify optimal laser processing parameters, the conformal mould insert was manufactured at RMIT facilities on an SLM Solutions 250 HL selectively laser melting machine out of H13 tool steel powder. The manufactured insert was stress relieved after SLM manufacture and subsequently machined to achieve the required surface finish.

The manufactured mould inserts were experimentally tested at Autodesk Australia’s laboratory to compare cycle time, temperature uniformity and part quality. The results of the experiments were compared with simulation predictions generated using the Autodesk Moldflow Insight 2016 simulation software. A Nylon6 (BASF Ultramid B3WG6 BK00564) polymer was used to mould the part. Thermocouples channels positioned on the mould cores and cavities were used to record temperatures thought-out the cycle.

Generally, the conformally cooled insert was as much as around 10 degrees Celsius cooler than the baffle-cooled insert depending on location. Furthermore, the temperature uniformity on the conformal insert was noticeably superior to the conventional mould. The lower temperature and improved uniformity resulted in a cycle time reduction of approximately 20% for the SLM manufactured conformal cooling insert.

To test the conformal cooling system temperature prediction capability of Autodesk Moldflow Insight 2016 simulation software, experimental data obtained for mould moving and fixed inlet temperatures of 80 degrees Celsius were compared to simulation estimates. A 3D Cool FEM analysis was applied to simulate transient temperatures within the conformally cooled insert throughout the injection, packing and cooling phases. Generally there is a good agreement in the temperature peaks, where the simulation prediction follows the general pattern based upon the sensor’s location. As such conformal cooling designs can be simulated quite accurately with the Moldflow Insight Cool (FEM) solver to predict the mould temperatures throughout the cycle.

The result highlighted in this project demonstrates the cooling advantages associated with additive manufacture of conformal cooling mould tools with SLM. The extensive research, design and manufacturing capabilities of the RMIT Advanced Manufacturing Precinct can assist the local injection moulding industry to pursue productivity and quality gains.

www.rmit.edu.au

www.autodesk.com.au