Additive Manufacturing and 3D

Time and cost-saving additive manufacturing of casting molds for injection molding Introduction Additive Manufacturing is without a doubt an empowering manufacturing tool. Though with inherent disadvantages, its benefits extend to industrial situations of diverse economic ramifications. For instance, in the mass production field of injection molding an attendant set of challenges have often been the production of mold inserts which have complex forms as well as the machining cost and fabrication time. These constraints prompt a search for alternative ways of manufacturing mold inserts. Additive manufacturing methods such as 3D printing technologies now make it possible for the fabrication of mold inserts in a short time as well as the implementation of an iterative mold testing routine proven to be expensive in conventional manufacturing. Due to cost considerations, polymers have in recent times been exploited as choice material for the 3D printing of molds. However only few polymeric materials can structurally withstand temperatures up to 260 °C which is typically applicable in commercial injection molding of polymers. These polymers still do not feature in 3D printing technologies although the high temperature polymer PEEK has only recently been demonstrated as technically processable using selected laser sintering/melting (SLS, SLM) and fused filament fabrication (FFF) 3D printing technologies. The laser based systems are generally cost exorbitant in terms of machine price and system running cost whilst the FFF system is markedly affordable requiring comparatively minimal running cost. The combination of high temperature stability of PEEK and the FFF 3D printing technology especially leverages for applications in cost effective mold insert manufacturing. Mold making flexibility of Fused Filament Fabrication 3D Printing technology The FFF 3D Printing technology is especially designed for thermoplastic materials i.e. those polymers that exhibit a melting range. Such polymers can be applied in various engineering scenarios in pure form or filled with different materials to create properties previously not found in the pure polymer. One of the key challenges in mold material design is the conduction of heat from the liquid material which is filling the mold to regions far off the filling cavity. The reason for this being that if the heat in the mass melt remains latent in the cavity then timely solidification process and form shape formation by the melt material are negatively affected. The thermal mass of the melt needs to quickly reduce once the mold cavity is filled and this process is aided using mold insert material that exhibit appreciable thermal conductivity. Most engineering polymers have thermal conductivity in the range 0.03 to 0.5 Wm-1K-1. This value is markedly insufficient to quickly transfer heat away from the melt to other portions of the machine. Even in the absence of cooling media assisted processing, the use of mold insert materials which ensure adequate thermal conductivity remains a preferred engineering solution. Therefore, mold inserts made from polymeric materials filled with highly thermally conducting materials (such as graphene , carbon nano tubes , graphite or aluminium ) can provide a viable solution. Surface quality is a concern inherent to parts manufactured by 3D printing technologies because the material build-up process leading to part formation/fabrication occurs in a layer by layer fashion. This layering process creates features at the layer – layer interface which add to the surface texture and topography of the 3D printed part. Therefore, for mold inserts where smooth surfaces are desirable a polishing (mechanical or chemical) step or coating step may be applied to the surface of the 3D printed mold part. There are mold design rules which are still not fully developed for 3D printing technologies. These rules consider aspects of injection molding process such as placement of ejector pins, gate designs and the location-specific nature of these features with respect to the direction of the melt flow into the mold cavity. Whilst meeting these design rules is imperative, once met though structurally fit mold inserts can be fabricated from FFF 3D printers. Materials and technology based challenges on FFF 3D printed mold inserts Demolding can be a process challenge in injection molding. This challenge can be mitigated by applying Figure 1: PEEK mold inserts with ABS(black) and Polyurethane(transparent) molded parts