Residual stresses determination in injection molded virgin and recycled HDPE blends: mechanical properties and morphology

Abstract

An experimental study of residual thermal stresses has been carried out in injection molded virgin and recycled high density polyethylene (HDPE) blends. Effects of blend concentrations on residual stresses were investigated under different injection conditions such as melt temperature, mold temperature and cooling time. Layer removal technique was used for measuring residual stresses. In order to determine the relation between the residual stresses and material characteristic of HDPE blends, mechanical and morphological properties of the blends were also investigated. Elastic modulus and impact strength were important key factors for determining the blend characteristics. As a result, it was found that HDPE blends gave higher residual stresses but lower impact strength with higher elastic modulus when recycled concentration was increased. Furthermore, it was seen that shape and size of the crystallites were also effective on residual stresses. Small and spherulitic crystallite structured blends such as 30 % recycled HDPE induced reduction in residual stresses due to easier relaxation with lower elastic modulus and higher impact strength while lamellar crystallite structured blends such as 50 % recycled HDPE gave higher elastic modulus but lower impact strength with higher residual stresses.

Processability and mechanical properties of extensively recycled high density polyethylene

“…Similarly, several authors who investigated the effect of HDPE reprocessing on degree of crystallinity [6,24,26,27], did not observe any significant change on crystallinity throughout 10 reprocessing cycles. This is in line with our findings from the first 20 extrusion cycles.…”

“…increasing of MFI with number of reprocessing cycles [6,26], decreasing [20,23,27,28], or they did not observe significant change [21,24,28]. Many authors concluded that intensity of MFI change can be related to the content of stabilizers [6,23,28].…”

Effect of injection molding conditions on the properties of polyamide 6/calcium carbonate nanocomposite

“…Altan et al 23 noted high residual stresses in HDPE flat plates injection molded at low T mold , which can account for the reduced impact strength. Guevara‐Morales and Figueroa‐López 24 explain that such stresses, which influence the properties of injection molded parts, are mainly associated with melt shearing during the filling and packing stages of injection molding, as well as with the different cooling rates between the skin and core of the molded part.…”

Dimensional Stability and Process Capability of an Industrial Component Injected with Recycled Polypropylene

“…When samples are heated again, residual stresses are released, leading to dimensional changes on the sample [50,51]. According to literature, higher residual stresses are stored in a recycled polymer than in raw materials [52]. Thus, higher stresses will be released at higher temperatures for recycled material when recycling PP, changes related to polymer structure are induced.…”