EFFECTS OF BLOWING AGENT CONTENT AND INJECTION MOULDING CONDITIONS ON SELECTED PROPERTIES AND SURFACE QUALITY OF POLYPROPYLENE MOULDED PARTS

The effects of blowing agent content and injection moulding conditions on the selected properties of polypropylene moulded parts containing 0,125-0.5 % endothermic blowing agent, were investigated. Four variable injection process parameters: the percentage of blowing agent (c), mould temperature (T(f)), injection temperature (T(w)) and injection velocity (v(w)) were used in the experiments. It was found that the mass and density of moulded parts depend mainly on the mould temperature with an increase in temperature, the decrease in mass and density of the parts was observed (Figs. 4-7). The mechanical properties of the parts were mostly influenced by the mould temperature and injection temperature (Figs. 8 and 9). The parts moulded at higher temperatures showed lower elongation at maximum force (epsilon(M)). High mould temperature, which is conducive to the growth of pores, is advantageous because of the longer time of polymer crystallization and, as a result, the increase in tensile strength of the obtained parts. The moulded specimens made of polypropylene with the addition of blowing agent have lower impact strength than the parts obtained from solid polymer (Figs. 10 and 11). The surface state of the moulded parts (sink marks, colour and gloss) was mainly influenced by the blowing agent content and mould temperature (Figs. 12-19). The addition of blowing agent results in the reduction of sink marks, particularly at low mould temperature, but can lead to a loss of gloss and change of colour of the moulded products.