Plastics are some of the world’s most diverse and useful manufacturing materials. While plastics encompass a large number of materials, polypropylene is one of the major plastic types found in many consumer goods, from security seals, car parts, shopping trolleys, water pipes and many more
Polypropylene is a relatively easy material to injection mould in spite of its semicrystalline nature. The absence of any real need for high molecular weight, from the mechanical properties view point, leads to low melt viscosity (easy flow). The pseudoplastic nature of polypropylene enhances this effect at high shear rates (fast filling rates).
Typically melting temperatures for injection moulding are between 200°C and 250°C. Though they can be as high as 280°C, or even 300°C, for short periods of time. With Flame Retardant grades it recommended that 220°C is not exceeded. Mould filling rates are generally on the high side, to ensure good surface finish and strong mouldings, free of weld lines and flow fronts. Adequate venting of the mould is essential to prevent burn marks.
Production of polypropylene takes place by slurry, solution or gas phase process, in which the propylene monomer is subjected to heat and pressure in the presence of a catalyst system. Polymerisation is achieved at relatively low temperature and pressure and the product yielded is translucent, but readily coloured.
The melt flow index gives a rough guide to melt flow behaviour, but as a result of the pseudoplastic nature of polypropylene (which is strongly dependant on the molecular weight distribution) this should not be taken too literally. Spiral flow mould data is sometimes provided by manufacturers as a practical means of assessing the flow behaviour, but this is no substitute for the fundamental melt rheological and thermal data which is now becoming more widely available.
Moulding shrinkage of polypropylene is typically around 1%, but prediction of the actual value is difficult due to the strong influence of moulding conditions. Important factors which affect the shrinkage of polypropylene mouldings.