The Occurrence of Flow Marks during Injection Molding of Linear Polyethylene

Abstract

Injection molded parts often show several types of surface defect. It has been hypothesized that wall slip is associated with some of these defects. Wall slip of molten plastics has been observed above a certain critical shear stress in rheological measurements. The objectives of the proposed research were to analyze the occurrence of flow marks during the injection molding of linear polyethylene and evaluate its possible relation to wall slip. Various variables were studied in terms of their influence on flow mark formation: mold thickness, mold temperature, melt temperature, gate, injection speed and TeflonTM and silicone oil coatings. It was found that injection speed is the controlling factor for the generation of flow marks during injection molding of the linear polyethylenes studied. Since one of the resin studied had shown no tendency to slip in capillary flow experiments, and since a Teflon TM coating on the mold walls did not affect the occurrence of flow marks, we conclude that there is no relationship between wall slip and the generation of flow marks. Microscopic observation of molded surfaces suggests instead that flow marks result from the filamentaton and stretching of semi-solidified material in the neighborhood of the three-phase contact line.

Flow marks in injection molding of polypropylene and ethylene–propylene elastomer blends: Analysis of morphology and rheology

“…Surface defects known as tiger stripes have been observed on injection molded parts with a variety of thermoplastic materials, including neat polymers,1–3 filled polymers,4, 5 and polymer blends 5–11. Tiger stripes are alternating glossy and dull bands that form perpendicular to the flow direction.…”

Rheological properties of elastomer‐modified polypropylene and their influence on the formation of flow marks

“…In many cases, these flow disturbances are attributed to flow front instabilities, 7,8,10-13 but a slip-stick behavior may also be responsible 5,14 and there may also be other causes. 9 In reality, a slip-stick phenomenon can also result in a snake-like motion of the flow front. The reasons for the appearance of the flow marks are hence quite complex and there might not be one single cause; i.e.…”

Identification des contraintes de rupture et de cisaillement à l'interface polymère injecté/moule métallique à l'aide du test couronne

“…Le profil de vitesse est donc modifié par le glissement ( Fig. 3), ainsi que les températures, en accord avec la littérature [14][15][16][17][18]. La température du polymère diminue sur la quasi-totalité de la paroi quand l'écoulement de celui-ci se fait selon des conditions de contact collant.…”