Injection molding of LFBC
Injection molding is the most extensively used industrial process for fabrication of plastic products. Injection molding process offers rapid processability and repeatability in fabricating products having convoluted geometries (Chaitanya and Singh, 2016a, 2016b). It is an ideal process for mass manufacturing of small to medium sized products. With the growing demand of LFBC, processes like injection molding, which have rapid processability, are desired for commercial applications. Fig. 9.6 depicts the extrusion—injection molding process. The biocomposite pellets are fed through the hopper into a heated barrel equipped with a compression screw which can rotate as well as reciprocate. The biocomposite pellets are melt blended in a heated barrel at a high shearing rate. As the fiber—matrix melt mixture progresses towards the nozzle, the reciprocating screw is pushed back (against back pressure) by the mixture accumulating in front of the screw tip. When the screw is pushed back up to the desired shot volume, the fiber—matrix melt mixture is then injected into the mold of the desired product at predefined processing parameters, i.e., injection pressure, injection speed, injection time, holding pressure, holding time, cooling time, and mold temperature. During the cooling phase, the charging
Figure 9.6 Schematic of (A) extrusion and (B) injection molding machine.
of the next shot of the fiber—matrix mixture starts at predefined processing parameters, i.e., screw speed and pressure, back pressure, suck back distance, etc. The injection molding process produces near net-shaped products and has the ability to uniformly disperse lignocellulosic fibers within the biocomposite.
Okubo et al. (2009) used a microscale injection molder to fabricate tensile test specimens of PLA/MFC biocomposites. The PLA/MFC melt mixture was injected at an injection and holding pressure of 0.7 and 1.5 MPa, respectively, into a preheated mold (40°C). Wang et al. (2013) fabricated jute fiber-reinforced PLA biocomposites using an injection molding machine having a barrel temperature range of 140—190°C and mold temperature and cooling time of 30°C and 15 s, respectively. The biocomposites were injected by varying holding pressure. Bledzki and Jaszkiewicz (2010) developed abaca, jute, and man-made cellulose fiber-reinforced PLA and PHBV biocomposites using an extrusion—injection molding process. The biocomposite pellets obtained from extrusion process were injection molded using an injection molding machine having a nominal clamping force of 850 kN, screw L/D ratio of 21, screw diameter of 40 mm, and a screw speed of 120 rpm. The test specimens were injection molded at a barrel temperature of 180°C, injection pressure of 50 MPa, and injection speed of 0.2 m/s (Bledzki et al. 2009). Kim et al. (2010) injection molded PLA- and PBS-based precompounded biocomposite pellets at a melt temperature of 190 and 145°C, respectively. The injection pressure of
8.2 MPa was kept the same for all the biocomposites developed. Moigne et al. (2014) prepared flax fiber-reinforced PLA biocomposite test specimens using barrel and nozzle temperatures of 180 and 210°C, respectively. Extruded biocomposite pellets were injected into a mold (25°C) at an injection pressure of 50 MPa for 20 s and holding pressure of 80 MPa for 15 s. Tokoro et al. (2008) developed bamboo fiber-reinforced PLA biocomposite test specimens by injection molding at a pressure range of 50—60 MPa and melt temperature of 180°C. The mold temperature and cooling time was kept at 20°C and 30 s, respectively. Huda et al. (2006a,b) reported the use of a mini injection molder to prepare wood flour-reinforced PLA biocomposites at a melt and mold temperature of 183 and 40°C, respectively. Borchani et al. (2015) also employed a micro injection molder to fabricate MaterBi®-based biocomposite test specimens. The precompounded pellets of alfa fiber-reinforced MaterBi® biocomposites were injection molded at a temperature and pressure of 150°C and 10 MPa, respectively. Mofokeng et al. (2011) developed sisal fiber (1—3 wt%)-reinforced PLA biocomposites using injection molding process. The biocomposite pellets of PLA—sisal fiber were injected at an injection pressure of 60, 70, and 74 MPa for 1, 2, and 3 wt% of sisal fiber, at 190°C. The holding pressure, cooling time, back pressure, and mold temperature were fixed at 60 MPa, 30 s, 3 MPa, and 20°C, respectively. Asaithambi et al. (2014) developed treated and untreated banana—sisal fiber reinforced hybrid biocomposites. The precompounded extruded pellets were injection molded into test specimens using an injection molder having a maximum clamping force of 60 ton, screw L/D ratio of 20, and screw diameter of 30 mm. The processing parameters like barrel temperature profile, injection pressure, injection time and mold temperature were selected as 80—160—165—170—175°C (feed to nozzle), 19 MPa, 0.95 s, and 30°C, respectively.
Sykacek et al. (2009) fabricated wood flour reinforced biocomposites based on five commercially available biopolymer matrices of Ecoflex®, PLA 7000D, Ecovio®, Bioflex 467-F, and Tenite Propionate 371 A 4000012. The extruded pellets with varying fiber content (up to 65%) were injection molded into test specimens at a temperature of 185—190°C and a back pressure of 10 MPa. The injection molding machine had a screw L/D ratio of 22 and screw diameter of 30 mm.