Thermoplastic Polymer Shrinkage in Emerging Molding Processes

Abstract

In this paper, in situ experiments have been designed using the full-field deformation technique of Digital Image Correlation (DIC) to characterize non-uniform shrinkage in thermoplastic polymers commonly used in traditional and emerging molding processes. These experiments are capable of characterizing the differences in strains that develop due to thermal gradients and stiction as the polymer shrinks from the molten to the solid state during molding processes. The experimental set-up consists of simulated open molds, a heating stage, thermocouples for temperature measurements, and a video imaging system for DIC. From these experiments, it has been shown that there is a large increase in shrinkage strain associated with the transition of the polymer from the molten to the solid state in a mold with reduced side rigidity, and as it is cooled below the Vicat softening point. Changing the cooling rate from air-cooled to quasi-steady state can eliminate the transition at the Vicat softening point. Furthermore, substantial decreases in shrinkage strain are observed when the polymer is melted in an open mold without mold release, while using mold release produces results similar to that observed with reduced side rigidity. A simple one-dimensional model reasonably explains and predicts the observed trends in the shrinkage behavior due to temperature differences through the thickness of the polymer melt when using high conductivity molds as well as constraint in the polymer melt near the mold resulting from stiction.

Replication of micro-sized pillars in polypropylene using the extrusion coating process

“…Gershon et.al. suggests that the difference between the rapid cooling on the surface of the polymer close to the mold and the opposite side of the melt not in contact with the shim surface could give rise to strain 14 .…”

Application of modern measurement techniques for analysis of injection moulding shrinkage

“…The process of injection moulding design is determined by the type of material, the injection method used and its technological conditions, as well as the injection form structure and facilities of the injection moulding machine. These factors influence directly or indirectly final dimensions, the structure and the shape of the moulded piece, in accordance with the specification range established in the construction process and the processing anomalies (material and surface), essential from the point of view of the user [8][9].Processing shrinkage depends on many factors, which can be grouped as follows [2;5;7]:…”

“…Processing shrinkage decreases together with an increase of the mass melt flow index [5;7]. It is known that crystalline materials have greater processing shrinkage than amorphous materials [5][6]8].The shrinkage process should be identified on the basis of measurements of standardized samples in the form of a dog-bone shaped specimen. DOI: 10.22616/ERDev2019.18.N294…”

Use of a digital image correlation method for full-field shrinkage measurement in injection molding

“…DIC technique was also used to study the induced displacement field during molding process with the identification of the effects of stiction and thermal gradient in the molten polymer on shrinkage strain. 19 This technique was also used to measure shrinkage induced by chemical or thermal treatments, typically in medical industry for the study of dental composite shrinkage. 20,21 This technique was used to measure the in-situ cure shrinkage of a thermosetting resin 22,23 especially to relate the chemical shrinkage, the degree of cure and the coefficient of thermal expansion.…”