In injection moulding, the cooling time of a finished product can constitute up to 70 per cent of the cycle time. One reason that explains this situation is that in conventional mould making, temperature control or cooling channels can only be drilled in a straight line. As a consequence critical hotspots can often remain out of the coolant’s reach – and cannot be avoided…that is until now.
Using Direct Metal Laser Sintering (DMLS) technology on the AgieCharmilles AM S 290 Tooling machine however makes it possible to integrate optimised, conformal cooling channels into the mould during the production process itself. This ensures faster and more even heat dissipation. It reduces thermal stress in the mould and prolongs service life. The plastic product quality and dimensional accuracy of the parts are also increased. In addition, this also allows for a drastic reduction in cycle times.
In 2016 GF Machining Solutions collaborated with GF Piping Systems on a project to improve the quality of a plastic valve part used in the chemical industry.
The project involved the use of GF Machining Solutions’ new Additive Manufacturing (AM) technology to solve a real life production issue - the objective being to use AM to create (build) metal mould inserts with integrated, intricate and optimally positioned conformal cooling channels to ensure improved part quality, quicker cycle times and reduced scrap rates.
The GF Machining Solutions technology used in the project was an AgieCharmilles AM S 290 Tooling solution which incorporates powder bed-based laser sintering (DMLS) technology from the EOS M 290 system with necessary software, automation, secondary processing technologies (i.e. EDM, milling, laser texturing etc.), and measurement and inspection equipment.
The plastic valve parts in question - type PRV DV50 - often present post-injection defects which can often mean they are rejected and scrapped. Typical ‘defects’ relate to sink marks being visible on the surface of the parts and/or insufficient ‘O’ ring flatness. Another problem area involves ‘welding lines’ - a common visible phenomena across many plastic injected parts. With the PRV DV50 valves such weld lines can become the ‘weak’ spots and are where leakages can occur.
The primary cause of these quality issues relates to non-homogenous thermal regulation during injection or, in other words, parts and features of the plastic component cool at different rates throughout the mould.
A major reason why this occurs is due to the non-optimised positioning of cooling channels in the mould/mould insert and manufacturers inability, when using traditional metal cutting technologies like milling, drilling and EDM, to locate them in the optimum position.
With AM conformal cooling channels can be designed-in and built-in to the insert manufacturing process, allowing them to follow the contours and features of injected parts which were previously inaccessible using conventional methods.
GF Machining Solutions’ AM team, in conjunction with Innovation Plastique Composites (IPC), a French Institute specialising in plastics and thermal mould tool management, worked collaboratively on behalf of the customer (GF Piping Systems) to design the channels (core and cavity)and in determining the simulation stages. (Simulation is an important issue as it enables evaluation of the conformal channels’ design, and ensures that the structure and hardness of the insert are not compromised.
GF Machining Solutions and IPC developed a ‘technology strategy’ for the project - i.e. defining what parts of the mould insert would be created using AM and what parts would be produced using more traditional technologies.
Says Martin Spencer, Managing Director at GF Machining Solutions UK:
“We only wanted to ‘print’ those areas where AM adds real discernible value to the manufacturing process. There’s is no point using AM to ‘reinvent the wheel’ - especially if there is no economic advantage in doing so.”
After researching and then realising that conformal channels in the lower base of the core and cavity could not be optimised, it was decided that these channels would be manufactured using GF Machining Solutions’ conventional machining technologies. The additive manufacturing technology instead would be focused on producing the upper core with new and integrated conformal channels.
The building process
GF Piping Systems wanted stainless steel mould inserts to be used - so Corrax powder was selected as the AM build material. The insert building process required 2,500 layers of powder (approx 50 microns per layer) to be used and 60 hours of production time was required.
The mould inserts were printed directly onto the building platform and after separation, using a CUT E350 wire EDM machine were heat treated to ensure the inserts reached a hardness of 52 - 54 HRC. The inserts were then milled using a Mikron 5-axis machining centre and finished on FORM 20 EDM die-sink machine to ensure high insert surface finish.
The new inserts were delivered to GF Piping Systems. The inserts were identical in terms of their size and geometry to the previous AMPCOLOY ones used by GF Piping Systems - the main difference being their internal structure. This meant that they were able to fit into the injection moulds.
During tests the new inserts, with their integrated conformal channels, helped reduced part cycle times by 16% and, thanks to their new design, GF Piping Systems was able to employ Variotherm technology to inject water at different temperatures into the moulds to further optimise the process.
As a result, plastic parts exhibited fewer sink marks, ‘O’ ring flatness was improved and the welding line has been reduced in depth.