Thermoplastic Extrusion Research Articles

Rapid Prototyping Materials Rheology

A new technology for printing three-dimensional solid objects from computer-generated images is described. The technology is based on extrusion of a thermoplastic polymer melt as small fibers that bond to form the solid object. This technology is referred to as thermoplastic melt extrusion rapid prototyping (RP). Polymer materials for thermoplastic melt extrusion RP must have the ability to form the structural building blocks of the part. These building blocks are walls, shelves, and bridges. The flow and deformation (rheology) of the melt as it is extruded through the circular die and adheres to the layers already in place govern the process. At this exploratory stage of development, there is no model to specify the extrusion conditions and materials properties relationship to RP performance. Ten marginally acceptable RP materials were identified through extensive RP testing. This study presents the preliminary rheological and thermal analytical characterization of these materials. Characterization of the melt properties is fundamental to the development of a constitutive equation for modeling and to an understanding of the process. The most promising RP materials were found to be crystallizing polymers. Isothermal rheometry and calorimetry measurements were performed to study the crystallization kinetics. Solidification due to crystallization during extrusion is expected to play a key role in RP materials performance.

Extrusion of polypropylene. Part II: Process analysis of the metering zone

AbstractThis is a study of the extrusion of thermoplastics with special focus on the extrusion of polypropylene. A multidimensional numerical model is presented for computational simulations of the extrusion in the metering zone. The model contains a non‐isothermal analysis of the extrusion of non‐Newtonian melts in single‐screw extruders. The computer code developed is tested against an analytical solution and against literature data for the case of a high density polyethylene. Computer simulations are carried out for the extrusion of two grades of polypropylene at two different screw rotation speeds. Special attention is paid to the evaluation of variables of practical interest, such as residence times and measures of mixing quality.

Finite element analysis on PCs with an ms—windows graphical user interface

A tailored graphical user interface (GUI) for finite element analysis, fully integrated into Microsoft Windows 3.1, has been developed. The current application is the simulation of flat sheet extrusion of thermoplastics, but many of the features would be common to a wide range of finite element analyses. Microsoft’s C/C++ Professional Development System 7.0, including the Software Development Kit 3.1 (SDK), has been used as the programming tool for the GUI. The interface is based on the Common User Access Advanced Interface Design Guide, which is part of the IBM Systems Application Architecture Library, and The Windows Interface: An Application Design Guide, which is part of the SDK. A memory handling technique is proposed to break the imposed 64 KB data segmentation. Connected finite element calculation routines are written in Fortran and compiled by the Salford FTN77/x86 32‐bit compiler. The protected mode interface of the Fortran compiler allows direct access by the GUI, and allows the computation to run as a 32‐bit background application, without memory limitations, in the multitasking environment. Finite element routines are supported by pre‐ and post‐processors comprising mesh generation, post‐processing for derived results, and graphical displays. A convenient contouring algorithm is proposed to generate contoured plots of nodal quantities in the form of iso‐lines or iso‐fields.

An Investigation for Improving The Quality of Thermo-Plastic Extrusion Products by Using New Statistical Method.(Dept.M)

The traditional quality control procedures are adopted to reduce the variation in the process during the manufacturing stage. Modern quality control techniques are utilized as quality engineering in the design stage. In this paper a statistical quality control technique based on Taguchi method has been developed. The objective is to determine the optimum combination of the process characteristics that reduce performance variation while keeping the process average close to its target value. As well, various aspects of off line quality control procedures are discussed in this paper. Off-line quality control refers to quality and cost control activities at the product and process design stage in the product development cycle. The proposed technique used statistical application methods for improving product design. It was app lied on a practical case in one of the existing factories at the 10th of Ramadan City. It was found that the amount of improvement in shrinkage reduction was as much as 63%, as well, the cost of the product is also reduced by 37%.

Thermoplastic extrusion—the mechanism of the formation of extrudate structure and properties

Systems processed by thermoplastic extrusion can be regarded as heterophase polymer melts of incompatible water‐plasticized biopolymers. In the process of thermoplastic extrusion, proteins and polysaccharides are melted at high pressure and temperature below the temperature region of their thermal decomposition. Dispersed particles of these systems can be deformed in flow. The mixed‐melt anisotropic structure, formed in flow, is fixed by rapid conversion of the melt jet that lets the extruder die from a viscous state to a rubber‐like state and then to a glassy state caused by cooling and drying. Incompatibility of proteins and polysaccharides in their water‐plasticized melt mixtures impacts on structure formation and texturization during thermoplastic extrusion.

Mechanical properties of the products obtained by the thermoplastic extrusion of potato starch-soybean protein mixtures

The mechanical properties of extrudates produced by thermoplastic extrusion of potato starch-soybean protein isolate mixtures have been studied over the full range of starch contents. Extrudates were obtained through a water cooled nozzle in order to avoid explosion-like water evaporation. It was shown that the mechanical properties of the extrudates were affected by their multiphase nature. The authors conclude that phase inversion occurs in the extrudates over the starch content range 60–80% (on dry weight basis). This conclusion is drawn from an analysis of the dependence on starch content of the extrudate expansion ratio at the nozzle outlet, its apparent density, initial elastic modulus, real tensile strength, failure strain and apparent cutting stress. It was shown that the extrudates have maximum anisotropy at starch contents of 10–30%.

Biaxial orientation of polymers by solid state extrusion through convergent-divergent dies

Solid-state extrusion of crystalline thermoplastics is a well known method for the production of monoaxially oriented filaments exhibiting very high modulus and strength. This is achieved primarily by forcing the polymer through a converging conical die at temperatures below its melting point.

Structure of protein texturates obtained by thermoplastic extrusion

AbstractThe structure of texturates of moistened proteins (0.33 g water per 1 g protein) obtained by thermoplastic extrusion was investigated using X‐ray scattering, differential IR‐spectroscopy, optical microscopy and mechanical cut testing. Texturates were cooled at the output of the extruder using additional die preventing explosionlike evaporation of water. It was shown that textures of individual proteins (ovalbumin and BSA) had amorphous isotropic structure, whereas texturates of soybean protein isolates (SPI) exhibited mechanically anisotropic amorphous structure. The fraction of β‐sheet structure in polypeptide chains increased during extrusion process. Besides non‐covalent interactions and S‐S bonds other covalent bindings are suggested to stabilize texturate structure. The obtained data was used to discuss possible mechanisms leading to the creation of fibrous texturates.

Flow of thermoplastics in an annular die under orthogonal oscillations

Extrusion of several thermoplastics has been carried out through an annular die having the internal surface oscillating in the sound frequency range at different amplitudes. Oscillating shear flow has been imposed orthogonal to the main pressure flow. The die characteristics and average melt temperature at the exit from the die have been measured with and without imposition of oscillations. Modelling of orthogonal superposition of oscillations upon pressure flow has been performed by using the Leonov viscoelastic constitutive equation. Three cases are considered: (i) the process is considered to be isothermal and the change of pressure is only due to a nonlinear interaction of oscillatory and pressure flow; (ii) the process is assumed to be nonisothermal and adiabatic and pressure reduction is due to the nonlinear interaction and the dissipation of oscillatory energy leading to a temperature rise in a polymer melt; (iii) the process is considered to be nonisothermal with heat transfer due to transient heat conduction and the dissipation of oscillatory energy. Pressure reduction occurs due to both nonlinear interaction and temperature rise. It is found that case (i) cannot explain the observed die pressure reduction, while case (ii) does only at high flow rates. The predicted results for case (iii) are found to be in fair agreement with experimental data.

Prediction of residual stress distribution in plastic pipe extrusion

AbstractIn thermoplastic pipe extrusion, the extrudate emerging from the die is typically sized and cooled from outside in a quench tank. This process causes quick solidification of the external layer, while the inner mass of molten material cools only gradually. The slow cooling and crystallization, and the associated shrinkage of this material, can lead to build‐up of severe stresses in the final part that can affect the long term service performance. In this paper, a simple theoretical analysis of this process of residual stress build‐up is presented. The pipe undergoing quenching is modeled as an annular cylinder of molten polymer being cooled at a controlled rate from outside. The overall stresses are derived numerically by adding up the stress contributions due to incremental advance of the solidification boundary. The results of the analysis are found to be in qualitative accord with the experimentally measured stress profiles in thermoplastic pipe.

Analysis of the performance of cooling extruders in thermoplastic foam extrusion

AbstractThe performance of cooling extruders widely used in thermoplastic foam extrusion was analyzed, by numerically solving the equations of motion and heat transfer. Analysis of cooling extruders does not require a consideration of the melting behavior of polymers, thus simplifying the system equations considerably. The flow geometry analyzed was an unwound screw channel of a single‐screw extruder, i.e., a rectangular channel of uniform height followed by diverging and converging sections. Due to the cooling of both the extruder barrel and the screw, the heat transfer equation considered includes the terms describing the convective heat transfer in the down‐channel direction and the conductive heat transfer in the cross‐channel direction, in addition to the terms describing the viscous shear heating. For the analysis, a power‐law model was used as the constitutive rheologlcal equation, describing the viscosities of a mixture of a fluorocarbon blowing agent and a low‐density polyethylene melt (or polystyrene melt). The parameters in the Theological model were determined using the data of Han and Ma (13). In obtaining numerical solutions of the equations of motion, an integration method was employed to overcome the problem of numerical instabilities. The present analysis predicts the profiles of developing temperature and velocity in the down‐channel direction, and the profiles of temperature, velocity, shear rate, and viscosity in the cross‐channel direction. In presenting the results of computer simulation, emphasis is placed on the effects of cooling the extruder barrel and screw on the performance of cooling extruders, in terms of the pressure drops along the extruder axis and the mechanical power consumption. This study provides a rational basis for the design of cooling extruders widely used in thermoplastic foam extrusion and for the selection of optimum extrusion conditions in producing thermoplastic foams.

Correlations between extrusion and injection molding process variables for PVC dry blends

AbstractAlthough thermoplastic extrusion and injection molding have been extensively studied by a large number of authors, very little is known about the correlations between the extrusion and injection molding process variables. This paper describes the various comparable process variables between extrusion and injection molding of PVC dry blends. The Brabender extruder of 19 mm diameter and 25:1 length to diameter ratio and the Szekeley reciprocating single screw injection molding machine were used. PVC dry blends of industrial importance were prepared using a high speed mixer. The four mix formulations based on a commercial grade of PVC were used. Process variables studied during the injection molding were the melt temperature near the nozzle, injection pressure, injection speed, and energy consumption. Process variables studied during extrusion were the melt pressure at the die, power consumption, and the melt temperature at the die orifice. The correlations between the extrusion and injection molding process variables for PVC dry blends have been established.

Influence of sunflower flour on the extrusion behaviour of soya grits and the functional properties of the extrudates (Short communication)

AbstractTexturizing is the key to produce meat‐like products from vegetable proteins; by application of texturized protein products it is possible to increase the substitution rate of meat in meat products, because such products have their own structure, and their addition will not cause such a structure loss as it is connected with addition of proteins as a powder. Thermoplastic extrusion is a relatively simple procedure to process flours to structurates [1, 2].Our aim was to study the extrusion of mixtures of soybean and sunflower flours, because the amino acid compositions of the two proteins supplement each other.

Studies on the development of texturized vegetable products by the extrusion process.

Thiamin retention in extruded cowpea, mungbean, defatted soybean and air classified mungbean products was studied by a least squares regression equation using process temperature, screw speed, buffer pH and moisture content as the major independent variables. The increase in process temperature, pH and screw speed variables effected the decrease of thiamin retention values, while the increase in moisture content increased retention. The heat generation due to increase of stress force caused by increased screw speed presumably resulted in the thiamin retention values which were lower than expected. The carbohydrate and protein of the feed material flours pro‐tected the thiamin during the thermoplastic extrusion process.

Recovery and utilisation of by‐product proteins of the meat industry

AbstractThe methods available for the recovery and utilisation of proteins from blood and offals are reviewed: special reference is made to the use of controlled enzymic hydrolysis. The texturisation of these proteins into meat‐like analogues, by fibre spinning or thermoplastic extrusion, is discussed.

Effect of lipid-protein interactions on extrusion of offal protein isolates

Lung protein isolates after defatting with carbon tetrachloride, chloroform, dichloromethane, iso-propanol, ethanol and methanol, were extruded with and without admixed soya grits. Extrusion of these isolates showed a clear dependence of the process on the lipid content. Extruded products with better characteristics were obtained by defatting the samples. Progressive removal of lipid, as the polarity of the solvent used increased, however, did not promote a corresponding progressive improvement in the textural quality of the extrudates. Protein-protein interaction was assessed by the primary mechanical parameters, hardness, springiness and cohesiveness. It was maximal at intermediate lipid contents. These broadly corresponded to the range of lipid contents where maximum hydration and affinity towards water occurred. It was concluded that residual lipid is necessary in order to maximize protein-protein interactions during extrusion. The effects of lipid-protein interactions on the texturization of offal protein caused a wide variation in the degree of cross-linking between protein molecules during extrusion. The results showed that hydrophobic and electrostatic interactions can play an important rôle in texture formation during thermoplastic extrusion.

Response surface methodology in the control of thermoplastic extrusion of starch

Effect of extrusion temperature, screw speed and moisture content on the gelatinisation properties of extruded corn starch using a single-screw extruder were studied using response surface methodology. The degree of gelatinisation can be effectively controlled by controlling the moisture content of the raw material and the extrusion temperature; control by manipulation of screw speed is also possible when the raw material has a high moisture content. The extrudate produced from starch with a low moisture content exhibited a very low retrogradation capacity, which was independent of the screw speed. It was evident that variation of operating conditions permits the production of an extrudate with various technological characteristics to meet varying food and industrial applications, and the use of response surface analysis seems to be an effective means of studying and optimising these conditions for extrusion technology.

Solid‐phase backward extrusion of thermoplastics

AbstractThe backward extrusion process was studied as a means of forming projections and ribs on flat sections of thermoplastic parts. The simplified, open‐die axisymmetric and plane‐strain extrusion cases were mainly considered. The results were applied to the development of a continuous solid‐phase‐forming process. Commercial‐grade polycarbonate (PC), polypropylene (PP), and polystyrene (PS) were used in the investigation. The projection height and extrusion loads in the unconstrained extrusion were measured as a function of the initial temperature of the material, the load hold time, and geometric variables. Cold dies were used in the experiments, and the samples were kept below their melt temperatures. The dimensional recovery of the flange section of the formed samples was also measured as a function of the hold time for given initial temperature and geometric variables. The slab and the upper‐bound methods‐in plasticity analysis were used to theoretically derive the projection heights and forming loads. The degree of correlation between the predictions and the experimental results depended on the estimates used for the frictional factors at the interfaces between the deforming material and the dies.

Physical and Chemical Characteristics of Extruded Rice Flour and Rice Flour fortified with Soybean Protein Isolate

ABSTRACTTexturized products were produced from rice flour and rice flour fortified with Soy Protein Isolate (SPI) by extrusion cooking. Water absorption capacity (WA) of the extrudates was slightly depressed in the presence of SPI. Scanning electron microscopy revealed that the file structure of the textured products differed subtly from each other and that SPI formed fine strings in the starch matrix. Solubility tests indicated that thermoplastic extrusion processing increased noncovalent interaction and disulfide bonding resulting in decreased protein solubility. 7S subunit of the soy protein seemed more affected than the 11s component. Insolubilization was significantly less when rice flour and SPI were processed simultaneously indicating a sparing effect in their mutual presence.

Future uses of skimmmed milk‐texturizing processes

There is a need to texturize skimmed milk to increase the sales of this product within the EEC, not least as a defensive measure against possible changes in the Common Agricultural Policy. In creating new products based on skimmed milk, competition with existing dairy products must be avoided. Human foods must be the prime aim of such work because these give higher returns than animal foodstuffs. Possibilities for texturization could exist in substituting for bread, potatoes, fish. and meat. The most attractive possibility is that of a meat substitute made by one of several methods such as wet or dry spinning, gel formation, or thermoplastic extrusion. In addition, there are other texturization processes which should not be forgotten, including the production of cheese analogues and plastics materials as well as conventional cheese and yoghurts.