Time and Cost-Saving Additive Manufacturing of Casting Molds for Injection Molding

Figure 1: PEEK mold inserts with ABS(black) and Polyurethane(transparent) molded parts
(PresseBox) ( Karlsruhe, )
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 <1500 to 2500 Wm-1K-1>, carbon nano tubes <2000 to 6000 Wm-1K-1>, graphite <100 to 600 Wm-1K-1> or aluminium <205 Wm-1K-1>) 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 lubricants on the insert surface, high speed filling of the mold at higher pressures and rapid cooling of the insert. Therefore, additives which bring along lubricating properties can be included in the polymer material. The stiffness requirements of the mold insert can be realized or compensated by designing the part for thicker sections, in case of fairly ductile/flexural materials, or filling the polymer with materials which have relatively high mechanical stiffness.

The size of the molds to be produced using FFF 3D printing is limited by the fact that objects in the volume range of 200 mm x 200 mm x 200 mm can best be printed; above this size dimensional challenges increase. Although FFF 3D printers which have effective build volumes in the order of 1 m3 now exist, it still remains a challenge to control the tolerances of parts fabricated within such a space. Also, mold features below 0.5 mm in lateral thickness can be difficult to produce using an FFF 3D printer thus there might be a need to conduct post processing steps such as drilling and other material removal processes to meet design specification.

Due to the compositional nature of polymers, the thermal cycling and mechanical loading during injection molding processing allows for only a limited use of the mold material. The thermo-mechanical fatigue resulting from these cyclic processes leads to structural failure of the material. This means that the number of injection shots derivable from the polymer mold is limited (50 to 100 shots) in comparison to metallic mold inserts where shots of up to 10,000 are achievable. Clearly mold inserts fabricated by FFF 3D printing technology is not for extended mass production purposes.

The business strategy for FFF 3D printed mold inserts

Given that the economics which presently accompanies mold manufacturing is poor. This is because most molds take a long time to be designed, machined out from metallic blocks, tested and then deployed to production lines. Some reports suggest that the time taken for the production of molds using traditional manufacturing methods such as machining can be reduced by up to 90% if done using 3D printing technology. Also, the overall manufacturing cost of the mold can be cut down by up to 70% using 3D printing technology. This is a huge form of savings in development and manufacturing cost. Therefore, based on this fact and even notwithstanding some of the constraints alluded to above regarding FFF 3D printed molds, it goes without saying that 3D printed mold inserts bring along key benefits to the economics of an injection molding business.

It is typical that in the development cycle of most consumer products, several models of different designs are required before conclusive decision is made on which would be launched as a final product. Each of the models in review often requires a mold insert so that tangible sample pieces can be fabricated and assessed under real operating conditions. The cost of producing the mold inserts is simply high thus a technology like 3D printing which offers low cost mold insert fabrication and ease of implementing alterations in designs is attractive for industry.

Conclusion

FFF 3D printed mold inserts is currently the most affordable and quickest way of fabricating mold inserts for small batch production of parts. Materials for 3D printing the mold inserts is a critical factor. The development of polymeric based materials which exhibit mechanical stability up to 260 °C as well as thermal conductivity suitable for FFF 3D printing processing is imminent. Research efforts back this imminence where new knowledge on behaviour of advanced materials stable at extreme operating conditions and commercially accessible for industrial applications now makes entry into mold insert making by FFF 3D printing possible. This together with the fact that understanding of the different applications of 3D printing technologies in different industries is improving rapidly giving encouraging impulse that FFF 3D printed mold inserts for long production runs may well be underway.

Figure 1 shows PEEK mold inserts fabricated using an Apium P 155 FFF technology 3D printer. The mold insert surface is untreated. Production time for the shown mold inserts, measuring 30 mm x 30 mm x 10 mm, added up to 6 hours per part. Raw material costs are 13 € for one mold insert. To generate the required pressure, the mold inserts have been cased in metal blocks and fixated with screws.

Authors: Philipp Renner, Julian Scholz, Brando Okolo, Uwe Popp
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