Post-Processing Time Dependence of Shrinkage and Mechanical Properties of Injection-Molded Polypropylene.

Artur Kościuszko,Dawid Marciniak,Dariusz Sykutera

doi:10.3390/ma14010022

Abstract

Dimensions of the injection-molded semi-crystalline materials (polymeric products) decrease with the time that elapses from their formation. The post-molding shrinkage is an effect of secondary crystallization; the increase in the degree of polymer crystallinity leads to an increase in stiffness and decrease in impact strength of the polymer material. The aim of this study was to assess the changes in the values of post-molding shrinkage of polypropylene produced by injection molding at two different temperatures of the mold (20 °C and 80 °C), and conditioned for 504 h at 23 °C. Subsequently, the samples were annealed for 24 h at 140 °C in order to conduct their accelerated aging. The results of shrinkage tests were related to the changes of mechanical properties that accompany the secondary crystallization. The degree of crystallinity of the conditioned samples was determined by means of density measurements and differential scanning calorimetry. It was found that the changes in the length of the moldings that took place after removal from the injection mold were accompanied by an increase of 20% in the modulus of elasticity, regardless of the conditions under which the samples were made. The differences in the shrinkage and mechanical properties of the samples resulting from mold temperature, as determined by tensile test, were removed by annealing. However, the samples made at two different injection mold temperature values still significantly differed in impact strength, the values of which were clearly higher for the annealed samples compared to the results determined for the samples immediately after the injection molding.

Highlights

The injection process is commonly used to produce polymer products, even with complex shapes
In the case of the samples produced by mold with TF equal to 20 ◦ C, the shrinkage value increased from 1.51% (168 h) to 1.56% (504 h), while for the samples produced in the mold temperature of 80 ◦ C during the same time, an increase in this value from 1.59% to 1.63%
It has been confirmed that the post-molding time significantly affects the shrinkage and mechanical properties of the isotactic polypropylene molded parts obtained at a temperature of 20 ◦ C and 80 ◦ C of the injection mold

Summary

Introduction

The injection process is commonly used to produce polymer products, even with complex shapes. The test of the shrinkage of the moldings is usually performed 16–24 h after the end of processing cycle (representing the primary shrinkage) After this time, the dimensions of the injection moldings of several polymeric materials are not fully stabilized. Depending on the storage and/or conditions of use, the dimensional changes of moldings over time may continue up to 1000 h from their production, as the secondary shrinkage [5]. This applies in particular to semi-crystalline materials whose glass transition temperature is lower than the service temperature

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