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3 Experimental work
3.1 Moulding part
The front panel of a cell phone for deaf users, as shown in Fig.2, was chosen as a case study. It was already been in Fig. 1 Classification of rapid tooling technologies Fig. 2 Front panel of the cell phone Fig. 3 Core and cavity made by epoxy tooling production using a conventional all-steel mould. The shape and geometrical features of the part are typical but difficult to obtain, thus presenting a challenge to the fabrication of the moulding blocks. It is of rectangular box shape, with 1.5-mm nominal thickness. The total volume for the part is of 11.47 cm3.The main features are the numerous holes in the front face and the extensive vertical ribs at the back face. Several snap fits require side movements for ejection. Thickness variation also occurs along the flow path.
4 Moulding block fabrication
The moulding blocks were produced by epoxy tooling and SLSm. The hybrid mould was instrumented with temperature and pressure sensors for online monitoring and data acquisition. The master pattern for the epoxy casting solution was made by stereolithography, using SLA250 equipment (3D Systems, USA). The moulding blocks (Fig. 3) were made using the epoxy composite resin Neukadur VG SP 5 (from Altropol Kunststoffe, Germany). The resin was cured for 4 h at 70ºC. Table 1 shows the main properties of the Neukadur resin. Due to the limitations of the epoxy tooling technique, some of the features were suppressed, namely the larger vertical ribs on the middle of the back face, the snap fits and the big boss on the top of the back face (Fig. 4). Preliminary tests showed that those features were difficult to replicate, due to difficulties in separating the mould insert from the master pattern. The SLSm moulding blocks were produced using the stainless steel-based powder Laser Form ST100 using the SLA250 equipment (3D Systems, USA). The main properties of Laser Form ST100 are shown in Table 2.These blocks where produced in two stages: sintering and bronze infiltration. The metallic sintering was obtained at 700ºC, and the bronze infiltration was made during 3 h at 1,070ºC. During the infiltration cycle, the polymer binder in the “green part” was burned between 450ºC and 650ºC. The parts were then submitted to a cooling stage down to 200ºC. The resulting composite structure contained 40% of bronze and 60% stainless steel.
4.1 Mould instrumentation
The hybrid mould was instrumented for temperature and pressure using a Priamus Mobile DAQ Type 8001B system (Priamus System Technologies, Switzerland) to retrieve temperature and pressure data. Figure 5 shows the location for the sensors in the cavity. Due to geometric limitations related to the cooling and the ejection systems, an additional temperature sensor only, T1, was located at the core, just opposite to T2.
4.2 Injection moulding
The parts were moulded in polypropylene, Hifax BA238G3 (Basell polyolefins, The Netherlands) using an Engel 200/45 injection moulding machine of 450 kN clamping force (Engel Austria, Austria). The polypropylene is a copolymer grade of MFI 13 g/600 s (230ºC, 2.16 kg).
The processing conditions used with the two sets of moulding blocks are shown in Table 3.The processing conditions used for the case of SLSm blocks were the same used on the all-steel mould. Previous studies show the need to reduce the pressure and the temperature in the cavity. Achieving this objective was necessary to increase the melt and mould temperature and cooling time.
5 Results and discussion
5.1 Epoxy tooling
Due to the thermal characteristics andmechanical properties of the epoxy composite, the processing parameters are different from those used with conventional moulds: Cycle times are longer, and pressure settings are lower. The major difficulties in establishing an adequate setup are related to the injection pressure, which should not be too high to prevent damage of the core details. The variation of the temperature at the moulding surface is shown in Fig. 6. It can be observed that the temperature stabilises after 40 s. It also can be observed that as the distance from the gate increases, part temperature decreases: The higher temperature occurs at the position T3, closer to the gate, almost 30ºC higher than at the other positions T1, in the core, and T2, in the cavity. These sensors show a 10ºC temperature difference between the two moulding surfaces. The ejection of the mouldings from the resin core was difficult confirming the observations by Gonçalves et al. [23]. This problem could be overcome by spraying the surface of the core with silicon-based demoulding agent. The degradation of the core started after 14 moulding cycles, when debris of the moulding block were extracted Fig. 7 Degradation areas in the core made by epoxy tooling 444 Int J Adv Manuf Technol (2010) 50:441–448