Reducing cycle times in rotational moulding of plastics: A theoretical and experimental analysis

  • Raji Khouri

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    Rotational moulding is one of the fastest growing sectors of the plastics processing industry today. Yet, in spite of the advantages which it has to offer in terms of the economic production of quite complex, stress-free articles, its competitiveness and widespread growth is still hampered by long production cycle times.

    This work aims to achieve substantial savings in cycle times by focusing on the basic process and heat transfer modes involved in this process, the polymer and mould material properties.

    Analyses were carried out utilizing the process simulation package RotoSim to identify the most significant process and material parameters that affect cycle times. Moreover, in an attempt to predict the processing conditions to optimise cycle time and product quality, statistical analyses were performed using a statistical design of experiments methodology.

    The experimental work consisted of carrying out numerous tests to identify the various heat transfer coefficients involved in the process, to determine the effect of varying oven temperature, oven and cooler fans’ capacity and rotation speed ratio on cycle times. Particular attention was given to internal mould cooling by investigating three different methods; air amplifiers, chilled water coil and liquid carbon dioxide (C02).

    The key finding of this project was that a reduction in overall cycle times of approximately 45% was achieved by simultaneously applying all techniques
    investigated. A case study was carried out using a mould from industry, which confirmed the previous results by achieving a reduction of approximately 35% in overall cycle times.

    It was found that reducing cycle times depends on the factors that increase the heat transfer rates into and out of the system, while these conditions, on the other hand, tend to result in a reduction in wall thickness uniformity. From the RotoSim analysis it was also found that increasing the rotation speed of either the machine arm or plate resulted in reductions in cycle times and the values of standard deviation of thickness.

    Another key finding was that the heat transfer coefficients involved in forced air heating and cooling are very small values if compared to the typical convective heat transfer coefficient values range given in heat transfer textbooks. The results analysis also revealed that higher oven temperature has a larger effect on cycle times than higher oven fan capacity, which in turn has a larger effect than varying the machine arm or plate rotation speed.

    It was also found that using air amplifiers is the most efficient way to reducing cooling cycle times among the three methods tested in this project. The initial tests carried out using liquid CO2 came up with promising results that open up a new possible fertile field for more research with liquid CO2 and liquid N2 as coolants.
    Date of AwardJan 2004
    LanguageEnglish
    Awarding Institution
    • Queen's University Belfast
    SupervisorEileen Harkin-Jones (Supervisor) & Wright Ed. (Supervisor)

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