Effect of Cooling Time on Demoulding Temperature, Shrinkage and Warpage of Injection-Moulded Polypropylene Parts

In injection moulding, the cooling phase occupies 60-80% of the cycle time, making it the dominant factor for cost-effectiveness and a key driver of dimensional accuracy. Most previous work identified process windows that minimise warpage by simulation, indirectly inferring cooling time. This study experimentally quantifies the effect of cooling time on demoulding temperature and the resulting dimensional deviations. Housing parts were moulded from virgin and post-consumer recycled (PCR) polypropylene (PP). For PCR, a 3³ full-factorial design varied cooling time, cylinder temperature and mould temperature. Data were analysed by analysis of variance (ANOVA). For virgin PP, a nine-level design examined cooling time in finer detail. ANOVA revealed R²_CT of 0.54 for the effect of cooling time on demoulding temperature, identifying cooling time as the principal factor. For dimensional accuracy, the mould temperature showed the strongest effect (R²_MT = 0.44), while cooling-time changes from 0 s to 4 s showed a comparable effect (R²_CT = 0.36). For virgin PP, both the demoulding temperature and dimensional deviations decreased at a diminishing rate with increasing cooling time. Thus, a targeted adjustment of the cooling time in short time intervals leads to a significant improvement in dimensional accuracy without substantially compromising cost-effectiveness.