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Ogila et al. – eXPRESS Polymer Letters Vol.11, No.10 (2017) 778–798 Table 2. Cycle time reduction for surface enhanced molds from Abdullah and Liu Shot weight [g] Part wall thickness [mm] Rotational speed [rpm] Oven temperature [°C] Cycle time reductions [%] Reference Pyramid Pin – 3.2 – 300 17 32 [88] – 6,0 – 19 28 – 3.2 – 380 16 26 – 6,0 – 20 25 – 9,0 – 16 28 Rectangular Pin Triangular 150 – 200 – 12 270 250 – 11.18 13.93 14.56 [89] 11.37 15.51 14.94 12.10 14.09 14.81 enhanced mold, the pin surfaces located on the side opposite to the direction of air flow were partially obscured and thus not fully utilized. 4.3. Pressurization As discussed earlier, mold pressurization in RM may be applied as a means to hasten bubble dissolution in the polymer melt [54, 60]. A reduction in this cur- ing period leads to a reduction in overall cycle time and lowered PIAT. Mold pressurization is also ap- plied to reduce warpage [37, 90]. Pressure is intro- duced into the mold during cooling to ensure con- stant contact of the polymer with the mold surface. Such conditions improve heat transfer through the mold wall from the polymer, resulting in shorter cooling times. Crawford et al. [30] observed an in- crease of approximately 11 to 18 °C/min in the cool- ing rate, with the introduction of pressurized air at room temperature. 4.4. Internal cooling Internal cooling as a means to reduce the molding cycle time in the period between PIAT and de-mold- ing was first suggested by Ramazzoti [91]. Subse- quent authors attempted, with varying degrees of suc- cess, the application of compressed air [92], liquid nitrogen [93], liquid carbon dioxide (CO2) [92] and water sprays [6, 34, 35, 92] as coolants. Among these coolants, compressed air and water sprays emerged as the most viable mediums for internal cooling. The other two methods, i.e. Liquid nitrogen and CO2, pre- sented with concerns such as increased warpage due to steep in-mold pressure drops [93], complicated mold configurations and component failures due to excessively low temperatures [92]. A comprehensive review of these and other internal cooling techniques is given by Tan et al. in reference [94]. Khouri [92] attached two air movers to inlet and ex- haust ports on the mold inner surface. The inlet port and air mover provided air into the mold while the exhaust port and air mover prevented pressure accu- mulation within the mold (Figure 6). The authors measured the effect of compressed air on the cooling time at inlet and exhaust air velocities of 3.6 and 2.4 m/s respectively and manifold pressures of 0.1 MPa. They achieved a reduction of 19% in cool- ing time compared to a mold with no internal cool- ing. Increased air velocities and manifold pressures achieved improved reductions of up to 25% in cool- ing duration. The convective heat transfer coefficient of evaporat- ing water is approximately 1000 times greater than that of quiescent air. Therefore atomized sprays of water provide a highly efficient means of cooling the mold interiors in RM. McCourt and Kearns [6] de- livered water sprays with an average droplet size of 50 μm into the mold cavity using a pneumatic noz- zle. At the end of heating, an opening on the mold wall enabled manual spray delivery for 40 seconds; after which external water cooling took over for the remainder of the duration until de-molding. The ex- periment achieved a 30% reduction in cooling dura- tion as compared to an air cooled mold. Abdullah et al. [95], used plain and surface enhanced molds, as well as a methodology, similar to their pre- vious work [88] to examine the cycle reduction effects of a combination of physical techniques. They ob- tained optimal results by using a combination of sur- face enhanced molds, higher oven flow rates, internal mold pressure, and water spray cooling. The plain, 789PDF Image | Rotational molding: A review
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