Post-Molding Shrinkage, Structure and Properties of Cellular Injection-Molded Polypropylene

Abstract

Cellular injection molding is a common method of modifying polymer materials aimed at reducing the sink marks on moldings' surfaces while reducing their weight. However, the dimensions of polypropylene (PP) samples as well as their mechanical properties after the injection molding process change as a result of re-crystallization. Knowledge of dimensional accuracy and awareness of the change in mechanical properties of products during conditioning are very important aspects in the polymer processing industry. The aim of this study was to assess the changes in the value of processing shrinkage and the size of the sink marks of porous PP moldings depending on the degree of porosity and the time since they were removed from the injection mold cavity. Studies of the structure and mechanical properties of moldings were carried out after several conditioning time intervals. The maximum conditioning time of samples was 840 h at 23 °C. Based on the analysis of the test results, it was found that the cellular injection molding process with the holding phase reduces the nucleation of gas pores, which results in a smaller reduction of sink marks than in the case of samples produced without the holding phase. However, PP moldings with a porosity degree equal to 8.9% were characterized by a higher shrinkage value after 1 h of conditioning, compared to moldings with porosity equal to 3.6%. The extension of the conditioning time also resulted in an increase in the value of linear shrinkage and the properties determined during tensile tests of solid and porous samples. Furthermore, in the case of samples with the highest porosity, the impact strength was reduced by about 30% after 840 h of conditioning compared to results obtained after 1 h.

Keywords: cellular injection molding; mechanical properties; polypropylene; post-molding shrinkage; sink marks.

Figure 1

DSC curve of PP and blowing agent (CBA) recorded during heating.

Figure 2

Model of the injection mold cavity (a) and a single test sample (b).

Figure 3

Sample preparation process.

Figure 4

DSC crystallization curves of test samples.

Figure 5

DSC melting curve of PP samples conditioned for 1 h (a) and 840 h (b).

Figure 6

Distribution of pores in the cross-section for each of the series 168 h after them from the injection mold: (a) PP sample (b) PP-C4 sample (c) PP-C0 sample.

Figure 7

Changes of processing shrinkage values as a function of the conditioning time.

Figure 8

Changes of tensile modulus (a) and tensile stress at yield (b) as a function of the conditioning time.

Figure 9

Examples of tensile curves of PP samples; after 1 h of conditioning (a), after 840 h of conditioning (b).

Figure 10

Dependence of the force on the time of fracture for the tested samples during the conditioning time equal to: (a) 1 h (b) 840 h.

Figure 11

Storage modulus and mechanical loss factor of polypropylene samples: PP (a), PP-C4 (b), PPC0 (c).

Figure 11

Storage modulus and mechanical loss factor of polypropylene samples: PP (a), PP-C4 (b), PPC0 (c).