Flow Marks in Injection Molding of PP

Flow marks in injection moulding is quite often encountered on different type of polymers. This defects is known as successive and periodic bright and dull bands oriented perpendicular to the flow direction. Their appearances meanly depend on processing and observation conditions. This study was lead on two industrial polypropylene polymers with a simplified polished rectangular mould geometry. We show that this defect appears during the filling stage and comes from a local change of morphological and molecular orientation along the filling direction on the surface. An instability of the flow front due to a limited sliding at the contact combined to the strong changed of molecular orientation before and after the stagnation point are invoked to explain this instability.

Analysis of the No-Flow Criterion Based on Accurate Crystallization Data for the Simulation of Injection Molding of Semi-Crystalline Thermoplastics

It is well known in practice that the shape and dimensions of injected parts are highly dependent on the packing-holding stage. A major problem in semi-crystalline polymers is the prediction of the solidified layer, whose thickness has an important effect on shrinkage and warpage. We propose a pragmatic approach based on the concept of no-flow temperature. This temperature should be related to crystallization temperature, but the choice is not easy because it depends on cooling rate and pressure which are functions of time and position. The objective of the work is to evaluate the sensitivity of an injection molding computation to the no-flow temperature and to evaluate the relevance of its choice. The crystallization temperature of an isotactic polypropylene is determined as a function of cooling rate and pressure in laboratory experiments. The pressure dependence is measured using the original Cristapress cell. As a case study, we simulate the filling and post-filling of a plate mold using Rem3D, a 3D code for injection molding. Three no-flow temperatures and two sets of parameters for temperature dependence of viscosity are tested. Their respective influences on the pressure evolution are shown, and the crystallization temperature calculated a posteriori using the experimental material data is compared to the “arbitrary” no-flow temperature.

Crystallization of Polymers in Processing Conditions: An Overview

In polymer processing, crystallization generally occurs in complex, inhomogeneous and coupled mechanical (flow, pressure), thermal (cooling rate, temperature gradient) and geometrical (surface of processing tools) conditions. A first route to understand crystallization in processing conditions is to design model experiments to isolate the specific influence of a given parameter. The emphasis will be laid here on the influence of: (i) shear flow through rheo-optical measurements using the commercial RheoScope module, (ii) high cooling rates obtained with the modified hot stage Cristaspeed (up to 2 000 °C min−1) and (iii) high pressures in the original Cristapress cell (up to 200 MPa). Numerical simulation is also a useful tool to understand and predict the coupled phenomena involved in crystallization. Based on Avrami's ideas and equations, a general differential formulation of overall crystallization kinetics has been proposed by Haudin and Chenot (2004). It is able to treat both isothermal and non-isothermal cases, and has been extended to crystallization in a limited volume without and with surface nucleation inducing transcrystallinity.

Stretch Blow Molding of Mineral Filled PET

Injection stretch blow molding (ISBM) is a two-step process that was designed and optimized mainly for unfilled PET resins. The present study focuses on stretch blow molding of a PET filled with different amounts of sub-micronic mineral fillers. The influence of fillers is analyzed thanks to DSC, DMA, tensile testing and processing in different configurations on a prototype apparatus and an industrial machine. It is demonstrated that fillers enhance crystallization kinetics, which leads to a reduction of the processing range. Differences in strain hardening induced by fillers make it necessary to adjust blowing temperature. However, the main effect occurs during the heating phase of the preforms. Temperature within a filled preform is much less homogeneous than in neat PET, making the temperature gradient totally different if the heating protocol is kept unchanged. Once the heating of filled preforms is controlled to reach equivalent temperature gradients, blowing is possible and rather equivalent to that of pure PET.

Numerical and Physical Modeling of Polymer Crystallization

In this first paper, we have revisited Avrami's model and cast its basic equations into a differential system. This system is integrated numerically, which avoids unnecessary simplifying assumptions generally used in order to get analytical expressions. This allows us to introduce the variations of nucleation and growth parameters as a function of processing ones (temperature, cooling rate, shear rate, etc.). Our analysis shows that it is necessary to take into account the variation of the initial number of potential nuclei with temperature, which was usually ignored. Finally, an outpout of our calculations is the size distribution of the morphological entities, i. e., a quantitative information on microstructure.

Numerical and Physical Modeling of Polymer Crystallization

Crystallization of thin polypropylene films was performed in isothermal, constant cooling-rate and mixed conditions. The experiments were first analyzed using the classical procedures based on simplified forms (Avrami, Ozawa) of the general Kolmogoroff-Avrami-Evans (KAE) theory. These analyses, which can be applied over an unusually wide transformation range, show that the crystallizations are actually 2 D. Then, a procedure has been established for the determination of the nucleation and growth parameters involved in the theoretical model presented in the first paper of this series. These parameters have been introduced into the model in order to predict the crystallization behavior in isothermal, constant-cooling-rate and mixed-conditions: transformed fraction, number of activated nuclei, final size distribution of semi-crystalline entities. A very good agreement is generally found between predictions and experimental results.

Structure Development in Injection Molding: A 3D Simulation with a Differential Formulation of the Kinetic Equations

The purpose of the present work is to introduce a crystallization law into Rem3D, a 3D code written in C++ and dedicated to the injection molding of polymers. We kept the basic hypotheses of Avrami's model and cast the kinetic equations into a differential system that is solved numerically. The variation of the density of potential nuclei with temperature is taken into account. Furthermore, the distribution of mean spherulite sizes can be deduced from the calculations. The second part of the paper is an experimental study of crystallization in well-controlled conditions (2D, isothermal or constant cooling-rate). It establishes a procedure for the determination of the nucleation and growth parameters used in the theoretical model, and gives a first validation of this model. Finally, the crystallization equations are introduced into Rem3D, in order to assess the feasibility of our new approach. Some typical results concerning the evolution of the transformed volume fraction in injection-molded parts are presented.

External Calibration in PA12 Tube Extrusion

Elongation at break is one of the major end-use properties of polyamide 12 extruded tubes. It is strongly affected by the tube microstructure and the molecular orientation resulting from extrusion conditions. Molecular orientation was characterized by X-ray diffraction and birefringence evaluation in light microscopy. Measurements were carried out on (r, z) sections obtained by polishing and microtoming. On the other hand, polymer drawing was measured on line by tracer techniques. Calibration stage was determined as the key step of the process that generates orientation in tubes: as the tube is drawn through a cylindrical calibrator under vacuum and cooled from its outside surface, calibration leads to a highly oriented zone in the twenty external microns. Calibration conditions and elongation at break have been connected through orientation level in this region. Molecular orientation was found to strongly depend on the draw ratio in the calibration tank. Finally, birefringence of the tube external layers and elongation at break were successfully correlated. Elongation at break can be enhanced by reducing orientation resulting from calibration conditions.

External Calibration in PA12 Tube Extrusion

In PA12 tube extrusion, calibration or sizing is the key step of the process that affects subsequent mechanical properties. The extruded tube is pulled through a cylindrical calibrator under vacuum. A water flow rate is applied at the inner side of the calibrator, creating a lubricant water layer at the polymer outer surface. The scope of this article is to show how this lubrication influences the elongation at break of tubes through drawing kinematics of the polymer. Lubricant water layer thickness measurements and on-line video recording have been employed to monitor the lubrication dependence of the velocity profiles from the extruder die to the end of calibrator. Velocities were measured through three independent innovative methods and thirty-two calibration conditions have been carried out to validate our work. Three main calibration parameters were found to determine the water layer thickness: the level of vacuum applied in the calibration tank, the water flow rate at the calibrator entrance, and the line speed. The influence of each parameter on lubrication level was found out. Simultaneously, the draw ratio in the calibration tank was deduced from velocity profiles. This parameter was found to affect tensile properties and to depend strongly on the level of lubrication during calibration. We showed quantitatively that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank and an increase of the elongation at break. This implies that we are now able to optimize tensile properties by fitting the main calibration parameters to improve lubrication and restrict draw ratio in the calibration tank.

Stretch-Blow Molding of PET Copolymers – Influence of Molecular Architecture

The purpose of the paper is to study the influence of molecular architecture of poly(ethylene terephthalate) (PET) on its ability to be processed by stretch-blow molding, which is not well documented in the literature. To evaluate this process ability, it proposes an original strategy combining laboratory analyses and experiments on a prototype machine. PET copolymers were prepared from three types of comonomers: diethylene glycol (DEG), isophthalic acid (IPA) and trimethylolpropane (TMP). It is first shown, through laboratory experiments, that the nature of the polymer in terms of chain constitution (copolymerization), chain length (intrinsic viscosity) and purity (catalytic residues) greatly affects many properties: melt crystallization, thermal properties, polymer rigidity and drawability. These different properties obviously induce very different behaviours at the different steps of the stretch-blow molding process: injection-molding of the preform (quenchability), heating (IR absorption), stretch-blow (rigidity and drawability). The stretch-blow step has been simulated on a prototype apparatus designed in our laboratory. It has been shown that free blowing can be used to characterize the process ability of the polymer. A statistical analysis has confirmed the great differences between the materials investigated and pointed out the complexity of the material response during blowing.

Numerical Simulation of Extrusion Coating

In packaging industry, many structures are produced by extrusion coating. In this process, a polymer film is extruded through a slit die, then stretched in air, coated on a substrate (paper, aluminium or steel foil) between a chill roll and a flexible pressure roll, and finally cooled on successive chill rolls. Due to their non-polar character, polypropylenes are not suitable for extrusion coating on metallic surfaces. Adhesive properties can be improved by grafting on the polymer chain a polar group like maleic anhydride which may react with the aluminium surface.

Our purpose was to develop a general model in order to predict the temperature field in the thickness of the multilayered structure along the stretching, laminating and cooling steps, and especially near the polymer/metal interface. This model includes crystallization kinetics and accounts carefully for the heat transfer coefficient with the successive rolls and surrounding air. Its predictions have been successfully compared to experimental temperature measurements along the coating line for various processing conditions (velocity, roll temperature, etc). Moreover, thermal history has a real impact on structure and morphology in the film. All these aspects are revealed by microscopic observations of thin microtomed sections of the film and X-ray diffraction experiments. Finally, adhesion properties of the laminate have been tested for the same process conditions. A good correlation has been established between adhesion properties and the thermal history experienced by the grafted polypropylene near the interface with aluminium before crystallization.

A Mechanical Model for Stress Developmentin PA12 Tube Extrusion

In polyamide 12 tube extrusion, calibration is the key step of the process that affects the subsequent mechanical properties. In previous work it has been shown that according to the calibration conditions, a very oriented skin layer may be created, which has been correlated to an important decrease of elongation at break. In the present paper we develop a thermomechanical model which consists of two parts: a viscoelastic mechanical model for the calculation of stresses and a thermal model for the determination of the temperature field. Three types of inputs are necessary: processing parameters, material data (e. g., crystallization kinetics and rheology in the melt, the solid state and the transition zone) and heat transfer coefficients to describe the heat exchanges along the extrusion line. The model allows us to propose a physical interpretation of the oriented layer. During cooling high axial stresses are frozen in the first solidified layers. They induce a plastic deformation of the polymer leading to a high level of orientation in the outer zones.

External Calibration in PA12 Tube Extrusion

In polyamide 12 (PA12) tube extrusion, calibration is crucial for the major final properties such as elongation at break or burst pressure. In external calibration, the extruded tube is pulled through a cylindrical calibrator located in a water tank under vacuum. A water flow rate is applied at the calibrator inner side, creating a lubricating water layer at the polymer outer surface. A combination of a quenching and a mechanical drawing was highlighted during calibration by on-line measurements. A subsequent high molecular orientation in the outer tube layers was also featured by X-Ray diffraction and birefringence evaluation in light microscopy. Besides, influence of lubrication level in the sizing-sleeve was investigated by an estimate of the lubricating water layer and a characterization of the tube final surface state in different calibration conditions. Firstly, we quantitatively showed that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank, DrCAL (ratio between line velocity and velocity at the calibrator entrance). Excellent correlation has been found between tube superficial orientation and DrCAL: a reduced elongation of the polymer in the calibrator leads to a lower level of superficial orientation. Secondly, two kinds of surface defects were detected on the tubes by light microscopy and microtopography. At last, several correlations were featured with mechanical properties. Elongation at break was found to strongly depend on the molecular orientation resulting from calibration. Moreover, origin of rupture was investigated during tensile testing in light microscopy. We emphasized that surface defects alignments perpendicular to the extrusion initiate the rupture by creating a significant crack via coalescence of initial defects. Thus, microstructure, surface state and tensile properties can be controlled by fitting the calibration parameters to improve lubrication and reduce the draw ratio in calibrator.