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cosity given by Equation (23): 0, 7652td nSBR = R~1.5 (23) Ogila et al. – eXPRESS Polymer Letters Vol.11, No.10 (2017) 778–798 Figure 7. flow regimes typical for LPSs that SBR occurred at a certain critical value for vis- because irregularities in wall thickness that arise from flow disturbances (corners) are more likely to freeze into the moldings during gelation. Similarly bubbles in moldings may arise when the corners pick up quantities of material which they cannot support. The material subsequently drips down from the over- head positions back into the pool (re-pooling) where it traps air bubbles [115]. To counteract the effect of these disturbances, slow rotational speeds are rec- ommended for low viscosity materials [108, 115]. On the other hand, too high an initial viscosity can lead to insufficient mold coverage. Harkin-Jones and Crawford [115] applied increased mold temperatures of up to 70 °C in order to lower the initial viscosity of PU and obtain a molding of suitable quality. This increased temperature, however, necessitated the use of higher rotational speeds to balance out the in- creased reaction viscosity. Initial mold temperatures must, therefore, also be regulated to achieve optimum molding conditions. The rate of change in viscosity profile for LPSs in- creases with increasing mold temperature [108, 117]. Mold temperatures do not directly affect the viscos- ity increase, They do, on the other hand, act to reduce the induction time and or increase the polymerization rates of LPSs [117]. Initial mold temperature also af- fects the crystallization mechanism of thermoplastic LPSs [108, 116, 117]. At high processing tempera- tures (˃150°C), Harkin-Jones and Crawford [116] observed a sharp drop in the level of crystallinity for Nyrim; which they attributed to increased branching and chain entanglement resulting from increased molecular weight at these temperatures. Similar re- sults were obtained at low mold temperatures as a result of concurrent polymerization and crystalliza- tion events; due to which the reactive chain ends be- came trapped in the solid crystals before polymer- ization was complete. Barhoumi et al. [108] identified two distinct isotherms for polymerization and crystallization events for Caprolactam at higher temperatures (170–180 °C). At lower temperatures (150–160 °C) only a single iso- therm existed, showing that the two events occurred simultaneously. Both works [108, 116] observed that for LPSs, the number of crystal nucleation sites in- creased, and spherulite sizes were generally smaller; which was due to polymerization and crystallization events that occurred below Tm and Tc of the final polymer PA6. Barhoumi and coworkers [108, 117], further identified a relationship between activator/ where μSBR viscosity at which solid-body rotation occurs, ρ fluid density, d wall part thickness, ω an- gular rotation speed, and R cylinder radius. To ensure fault free moldings SBR should occur just before the steep rise in the viscosity profile which signifies the start of gelation. Otherwise, the hydro- cysts formed will freeze onto the part wall, resulting in uneven part thickness. Harkin-Jones and Craw- ford [115] confirmed this in a cylindrical mold using the viscosity profile of a fault free Nyrim molding; for which they compared the theoretical SBR value and the observed point of gelation. Further, in order to extend the fault free operating region they, in agree- ment with Equation (23), suggested the following: increasing rotation speed, decreasing wall thickness (or producing the molding in double shots) and al- tering the viscosity profile. The last suggestion re- sulted from investigations into the effect of hollow glass microspheres; which when, added to Nyrim (up to 7% by weight), extended its fault free operat- ing region. The glass microspheres reduced the rate at which the temperature profile increased; which in turn led to a more gradual viscosity profile. Indeed initial viscosity and rate of viscosity change are the variables that most significantly affect final part quality. When square molds are used, LPSs hav- ing lower initial viscosity alongside rapidly increas- ing viscosity profiles tend to produce parts with the most number of defects [115, 117, 119]. This is 792PDF Image | Rotational molding: A review
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