TABLE OF CONTENTS Polyethylene (PE) dominates the rotational moulding industry. The most obvious reasons for this are the ability of PE to withstand the long time-temperature environment of the process, and its relatively low cost. Polyethylenes are generally resistant to water, vegetable oils, alkalis and most concentrated acids at room temperature. With the proper precautions they can be re-ground, re-pulverised and re-moulded. Polyethylene is compatible with a full range of pigments to produce coloured parts.
Polyethylene can be readily pulverised into a free-flowing powder at room temperature. The physical nature of the powder can have an important effect on the performance during moulding and the properties of the final product. Normally the powder particles vary from less than 150 microns to about 500 microns. The particle size distribution is very important as it can affect the way that the powder particles fuse together, which in turn affects the appearance of the moulded part.
Polyethylene is an easy flow material when it is in the melt form. This is quantified by the Melt Flow Index (MFI) value. The number quoted is the weight of material, in grams, which would flow through a standard orifice at a standard temperature in 10 minutes. An easy flow grade of plastic will have a high MFI, which corresponds to a low viscosity. Rotational moulding grades of polyethylene usually have a MFI in the range 2-8. If intricate detail must be reproduced from the mould then a higher MFI is desirable. However, a higher MFI is associated...
