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PROCESSING PARAMETERS The heating and cooling stages of the rotational moulding process have significant effects on the quality of the mouldings, as measured by appearance, impact strength, tensile properties, stiffness and environmental stress crack resistance (ESCR). The Heating Cycle In the heating stage, sufficient heat must be transferred to fuse the polyethylene powder within the mould. The rate of heat transfer is controlled by both the oven temperature and the heating time, often termed the Oven Residence Time. The amount of heat required will depend on the desired wall thickness of the moulding, the complexity of the mould shape, and the characteristics of the powder and of the polymer melt. Too little heat (under-fusion) will give a porous low strength moulding; too much heat (over-fusion) will lead to degradation of the polymer and a drastic drop-off in the physical properties. At this point the coating has a very rough sandy (powdery) inner surface and contains a large number of trapped air bubbles and voids, i.e. it has a high degree of porosity. These observations are indicative of under-fusion and the mouldings are characterised by poor tensile properties, low impact strength and ESCR (see Table 3). With further heating, most of the air bubbles are released and the coating completely fuses into a homogeneous densified layer of molten polyethylene. Optimum fusion occurs when a smooth dull inner surface is obtained, and impact and ESCR properties are at or very close to their maximum values. Figure 12: Schematic of the Densification of Polyethylene Powder in Rotational Moulding The few remaining bubbles will generally have little effect on the final properties. However, when the product is one that will be subjected to severe service conditions, every attempt should be made to minimise the porosity. If the powder is heated for too long, this results in thermal degradation of the polyethylene. Alkatuff LLDPE powders for rotational moulding are stabilised with antioxidants to protect against this effect. Once this antioxidant is consumed, the inside surface of the moulding will begin to oxidise and small cracks can be created. This over-fusion is easily recognised by a glossy inner surface, a yellow discoloration, and a strong odour. As the degradation proceeds, physical properties such as impact strength and ESCR will be seriously impaired. ROTATIONAL MOULDING 6 Figure 11: Typical Rotational-moulding Cycle A typical moulding cycle is shown in Figure 11. After the mould enters the oven, its temperature rises, which heats the tumbling polyethylene powder within the mould. When the melting point of the powder is reached, the powder particles falling against the hot wall of the mould begin to melt and adhere to it until the mould surface is completely coated. Eventually all of the free powder melts, or sinters, the molten polymer then builds up to form a porous coating of uniform thickness. Qenos Technical Guides 13PDF Image | ROTATIONAL MOULDING Guide
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