Injection moulding is one of the most widely used manufacturing processes. It can be used to produce a large quantity of parts with high accuracy and quality, very quickly. The process involves melting polymer granules to the point where they flow like a liquid, then injecting them into a mould tool cavity. This is carried out at high pressure, which is maintained until the polymer cools, taking the shape of the mould cavity. The variety of parts manufactured in this way is vast, ranging from small medical devices right the way through to tables and wheelie bins. The initial design and manufacture of a mould tool is traditionally a time consuming, expensive and often problematic process. With the recent advancements in additive manufacturing technology and materials, it is now feasible that mould tools can be manufactured by 3D printing creating significant cost and time savings. LPE in Belfast recently carried out am Intertrade Ireland research project with the Athlone Institute of Technology (AIT) to investigate the viability of using 3D-printed tools for short production runs or as an initial design testing iteration prior to a full metal machined tool. Of particular interest was to investigate stereolithography (SLA) resins, which offer excellent strength and thermal properties as well as metal powder bed fusion (MPBF) printed steel tools. The objectives of the research project were to:

  •  Compare surface finish of parts injected from moulds of different materials;
  • Determine maximum injectable parts from a mould using various injection materials;
  • Perform design iterations to improve the cooling and heating affect by designing adding conformal cooling channels to mould tool;
  • Where possible, reduce cost of tool insert manufacture; and
  • Optimise injection-moulding parameters for rapid tooling application.
[caption id="attachment_38870" align="aligncenter" width="300"] CLIC TO ENLARGE Fig 1 (left): Single block cavity with mould deformation. Fig 2 (right): Tool insert with complex geometry[/caption]

Methods

Following initial discussions with AIT, it was felt that instead of printing the entire mould tool, only the cavity relevant geometry should be printed as a tool insert (Figs 1 and 2), as this would help reduce build times and hence costs that could in turn make it a more viable solution. It was anticipated that the SLA materials would perform worse than the printed metal inserts, so it was decided to investigate these first. A simple stepped block cavity tool insert was designed and printed in variety of SLA materials at LPE’s advanced manufacturing centre in Belfast (Fig 1). The tools inserts were then tested at the Applied Polymer Technology Gateway located at AIT. The printed tool inserts were analysed to determine which SLA materials had the best resistance to thermal and pressure strains. Following the results learnt from this step, a new part design was created and, in order to print a more complex demonstrator tool insert, one with thin walls and complex geometry. This was then printed with the most successful SLA material (Fig 2). This new tool insert was then tested at APT’s campus. Tool insert design improvements were investigated using mould flow analysis. An advantage of additive layer manufacturing is that conformal cooling channels can be incorporated into the design of the print that would otherwise be impossible to manufacture. These channels were designed to help create more evenly distributed cooling, which helps reduce the cycle time and increased the longevity of the tool (Fig 3). [caption id="attachment_38872" align="aligncenter" width="300"] Fig 3: Example of conformal tooling[/caption] The final part of the investigation was to compare these SLA printed inserts to metal PBF steel inserts and also to a CNC machined tool insert from tooling steel for benchmarking purposes. These new tool inserts were tested at APT. The investigations concluded with analysis of the longevity of the tool inserts, as well as analysis of the surface finish and the quality of injection-moulded components.

Results

From the SLA materials that were trialled, it was discovered that the SOMOS Perform ceramic-filled resin achieved the best result of all the SLA materials tested. In fact, the Perform surpassed expectations in terms of heat resistance and longevity of tool insert life. The other SLA resins did not perform so well, as the surfaces of tools made in these resins blistered and deteriorated quickly (Fig 1). [caption id="attachment_38873" align="aligncenter" width="300"] CLICK TO ENLARGE Fig 4: Pre-conformal cooling. Fig 5: Post-conformal tooling[/caption] The ceramic-filled Perform resin also performed well with more complex geometry. However, it was evident that the mould needs another design iteration to allow quicker processing and avoid heat related, warp-age issues. Conformal cooling channels significantly reduced the heat build-up in the moulds during the injection process (see Fig 3 for an example). Fig 4 is a thermal image of a mould without conformal cooling and shows surface temperatures of around 140°C. Compare this to Fig 5, which had conformal cooling channels and you can see a reduction of almost 50°C. This means the cycle time can be reduced and more parts can be produced in a shorter time frame. Being able to add conformal cooling channels to the mould is an invaluable addition to the injection-moulding process and serves two main purposes. Firstly, it allows a mould to be cooled down between shots and, secondly, it provides a method of heating a mould for more abrasive polymers so as not to shock the material when it is shot. [caption id="attachment_38875" align="aligncenter" width="300"] CLICK TO ENLARGE Fig 6: Stereolithography. Fig 7: DMLS. Fig 8: CNC[/caption] Figs 6, 7 and 8 above show pictures of the tool inserts cavity under a microscope. It can be seen that the stereolithography and CNC cavities had a smooth surface finish, whilst the DMLS cavity had a slightly rougher finish. This roughness in the finish of the DMLS tool caused the parts to stick to the cavity upon ejection. This issue could be offset with more time spent finishing and polishing the cavity to achieve a smoother surface finish. For all three tool-inserts, the moulds were able to produce +500 runs. However, it should be noted that the gate on the stereolithography insert broke under the high shear stress, so a number of work around had to be developed to combat this issue.

Conclusions

The results from this project show that rapid tooling using stereolithography and MPBF is a viable and advantageous method for either producing short production runs or as an intermediate step in the design of a full metal machined tool. However, both the SLA and MPBF tool inserts have their limitations, being that of gate strength and printed surface quality respectively. This project was a great success and both LPE and APT gained valuable knowledge. LPE hopes to continue research in this field by investigating use of 3D-metal printed inserts. [caption id="attachment_38876" align="aligncenter" width="300"] CLICK TO ENLARGE Fig 6: Stereolithography. Fig 7: DMLS. Fig 8: CNC[/caption] Author: Wilson McKay graduated from Queen's University Belfast in 2012 with a degree in manufacturing engineering. The following year, he completed a Masters in Software Development. Through Intertrade Ireland he commenced a graduate development programme at LPE in 2014 and is now employed as an additive manufacturing application Engineer and is involved in continuing research and development in this field.