The grinding of polyethylene powders for use in rotational moulding

Abstract

Powders are the only form in which polyethylene is used extensively in rotational moulding. The powder characteristics are recognised as being important for the production of good quality parts, and the grinding process is a crucial step in the preparation of the powders. The primary objectives of this research were to analyse the grinding process as a whole and identify those key factors influencing the process efficiency, and also to relate powder characteristics to the quality of the moulded parts.

Grinding trials identified the processing temperature, cooling temperature and post heating temperature as having the major effects on polyethylene powder quality. In general it was found that quality was improved by annealing the powders at high processing temperatures or over time at a slow cooling rate. The investigation of grinder throughput for a range of gap sizes and screen mesh sizes under closed and open circuit grinding has highlighted the importance of the regrind and processing temperature in defining the rate of powder output. The screening function of the classifier was also found to be essential to minimising the rate of regrind and optimising the throughput rate. It is shown found that the pellets are reduced in size by the removal of shavings from the surface of the pellet as it moves across the narrowing gap in the grinding zone. Features of the grinding zone such as the length, angle of rotating plate, number and angle of rotating teeth were found to have a great influence on the throughput efficiency of the process. Moulding trials established that powder quality is critical for reducing surface porosity in moulded products but has less effect on physical properties. Parts moulded with micro-pellets were found to have mechanical properties comparable with powders but they exhibited larger surface pores. Surface porosity was shown to be improved by addition of fine particles to the micro-pellets or by the use of internal air pressurisation.

Date of Award	Jul 1998
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Robert Roy Crawford (Supervisor)