The Effects of Cooling Time on the Dimensional Stability of Thermoforming Polypropylene

Abstract

Introduction Documented by the International Committee of the Red Cross, consolidation time for polypropylene (PP) plastic must be left on the plaster model for 24 hrs. If not followed, this will result in an incorrect shape and volume of PP plastic. To the best of our knowledge, there are no studies that address the comparison of PP plastic expansion at various time domains of plastic cooling after being thermoformed. The purpose of this research was to compare plastic expansion or shrinkage and heat ratio (dimensional stability [DS]) of PP plastic through cooling periods of 6, 12, 18, and 24 hrs, respectively. Materials and Methods Forty-one identical plaster models were made. Vacuum was applied at times of 6, 12, 18, and 24 hrs, respectively. PP was cut and removed. Measurements were taken and recorded at initial times and after 24 hrs. Results DS was not significantly different across the four time groups of 6, 12, 18, and 24 hrs, respectively (P = 0.10). There was a statistically significant linear relationship between DS with room temperature (P = 0.01) and oven temperature (P = 0.04). Conclusions There is no scientific foundation to perform a 24-hr wait before finishing a device. This translates to an effective time frame for removing the plastic from the model, saving time and resources, quicker delivery to the patient, and improvement in whole patient care. Clinical Relevance This article has important implications for the OP clinic. Creating more dimensionally stable PP devices faster translates to better fitting braces fabricated in a shorter time, thus benefiting patients and clinics and increasing the success rate when delivering this device.