The effect of flow on cavity surface temperatures in thermoset and thermoplastic injection molding

In the Injection molding process, nonuniform heat transfer between the polymer and the mold caused by flow during the cavity filling stage can lead to spatial variations in the cavity surface temperature. This can result in an increase in cycle time or poor part quality. An investigation of the flow-induced, nonuniform, cavity surface temperature is reported here. A flow model for a thin, rectangular, end-gated cavity and a model for the steady-state temperature distribution in a simple mold are developed. These are applied to some thermosetting and thermoplastic systems. For both filled and unfilled thermosets, it is found that a simple plug flow model gives a good approximation for the heat transfer during flow. For thermoplastics, however, the full flow solution must be used. For the cases considered in this study, the steady-state temperature variation along the cavity surface is less for the thermoplastics than for the thermosets.     

Rheological behavior of thermoset/thermoplastic blends during isothermal curing: Experiments and modeling

The evolution of the viscoelastic properties of a molten thermoplastic/thermoset system during the course of the isothermal polymerization of the thermoset precursors has been investigated and modeled. Such systems are initially homogenous and phase separate upon polymerization of the monomers. In the present study, atactic polystyrene (85 and 60 wt%) is blended to a stoichiometric mixture diglycidyl ether of bisphenol A with 4,4′-methylenebis(2,6-diethylaniline). During the polymerization, polystyrene becomes the thermoplastic-rich matrix and an epoxy-rich dispersed phase appears. Both phases experience changes in their composition and viscoelastic properties. A rheokinetic model is proposed to take into account four contributions to the viscoelastic behavior: progressive deplastification of the polystyrene matrix involving a modification of the glass transition and thus of free volume, dilution of the network of entanglements of the matrix by the non yet converted low molar weight molecules, emulsion behavior after the separation of the epoxy-rich phase and finally interparticular interactions being assimilated to a mechanical percolation. Provided that the glass transition temperature of the matrix and the dynamic moduli of the neat components are known, the changes in the viscoelastic behavior of the system with time can be predicted with no ad hoc parameter and model calculations are in good agreement with the experimental data.     

Reaction injection pultrusion of thermoplastic and thermoset composites

Coventional pultrusion of thermoset composites is under increasing examination for emissions of harmful volatiles from the resin wetout tank. Even though the pultrusion of thermoplastic matrix composites produces no emissions, it is difficult to wet individual fibers due to their high melt viscosities. This paper addresses both the issues of volatiles and wetting with a process called Reaction Injection Pultrusion (RIP). A prototype RIP machine was used to make both thermoplastic polyurethane and thermoset polyisocyanurate matrix composites. The RIP process produces pultruded parts with low void content, good surface finish, and acceptable mechanical properties. The low viscosity constituents used in RIP help improve fiber impregnation, while the small volume of the impregnation bath reduces emissions. Processing parameters such as line speeds, catalyst levels, and die temperaures were varied to establish processing guidelines for sustained production.     

Water‐assisted injection molding of thermoplastic materials: Effects of processing parameters

The objective of this study was to experimentally investigate the effects of various processing parameters on the water‐assisted injection molding of thermoplastic materials. Experiments were carried out on a lab‐developed water‐assisted injection molding system, which included a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. Two types of water injection pins designs were proposed to mold the parts. After molding, the lengths of water penetration in molded parts were measured. The effects of different processing parameters on the lengths of water penetration were determined. It was found that the shrinkage rate and the viscosity of the polymeric materials, and the void shapes of the hollowed cores mainly determined the water‐penetration lengths in molded products. In addition, a comparison has been made between the parts molded by water assisted injection molding and gas‐assisted injection molding. It was found that water‐assisted injection molded parts exhibit less uniform void sizes along the water channel. The cycle time for water‐assisted injection molded parts was shorter than that of conventional injection molded parts and gas‐assisted injection molded parts.     

Wetting measurements as a tool to predict the thermoplastic/thermoset rubber compatibility in two‐component injection molding

Recently a novel two‐component injection molding process has been developed combining thermoplastics with thermoset rubbers. Since the adhesion strength between the two materials strongly depends on the combination of a specific thermoplastic and a thermoset rubber, there is a need to predict their compatibility, defined as the formation of a strong interface. In this study, the wetting behavior of molten thermoplastics on rubber substrates is used to predict their compatibility since wetting is an essential step in the formation of a strong interface. Contact angle measurements at high temperatures showed that the wetting of polypropylene and polyethylene is the best in combination with ethylene propylene diene monomer rubber while nitrile rubber is best wetted by polycarbonate. The subsequent two‐component injection molding tests confirm that it is possible to combine these materials. Material combinations with a poor wetting behavior on the other hand are not suitable for two‐component injection molding. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46046.     

Melt temperature profile prediction for thermoplastic injection molding

A computer system is developed to quantitatively reveal how the melt temperature is affected by the operating conditions during the plastication, dwell and injection stages of the injection molding process. The variables considered in this study are rotation speed, back pressure, barrel heater temperatures, nozzle heater temperature, dwell time and injection velocity profile. A set of Artificial Neural Networks (ANN) has been developed to predict the effect of the operating conditions on the melt temperature during plastication. The dwell period is treated as a heat conduction problem. A free boundary model for the injection phase is developed to simulate the temperature development and melt flow due to the forward motivation of the screw. The overall prediction of nozzle melt temperature is in good agreement with the experimental measurement, validating the proposed procedure combining ANNs and mathematical modeling. This work enhances the understanding of the process and provides a basis for future work on the optimization and advanced control of the process.     

Microwave processing of syntactic foam from an expandable thermoset/thermoplastic mixture

In this work, a microwave expansion process to produce thermoset‐matrix syntactic foam containing thermoplastic foam beads was designed and developed. Expandable polystyrene (EPS) microspheres and epoxy resin were chosen as a model material system. This process is featured with a capability to effectively expand EPS microspheres in syntactic foam with high EPS loading. The resin viscosity and specific microwave energy are found to be the two primary control parameters determining the process window. Mechanical characterization showed that the specific flexural strength and modulus of the syntactic foam are similar to those of the neat epoxy. By comparison, the flexural moduli over density squared or cubed of the foam are much higher, especially at high EPS loadings, than those of the neat resin. The foamed EPS microspheres can also effectively toughen the syntactic foam, preventing propagation of cracks. Furthermore, the microwave expansion process was found to be capable of molding syntactic foam parts of relatively sophisticated geometry with smooth surfaces. POLYM. ENG. SCI., 55:1818–1828, 2015. © 2014 Society of Plastics Engineers     

Sequential injection molding of thermoplastic polymers. Analysis of processing parameters for optimal bonding conditions

A systematic approach was used to study the effect of the process variables that control bonding of the injected melt to the previously injected parts, in sequential injection molding of thermoplastic polymers. Three polymer pairs were sequentially injected in a mold with the same geometry, in a range of mold and injected melt temperatures, and packing pressure conditions. Standard Peel Tests were conducted on injected samples to measure the resulting bonding strength. The analysis of the experimental results allows the quantification of the relative importance of the processing parameters involved. A direct correlation is found between the calculated interface temperature and the packing pressure needed for bonding. A generalized procedure is proposed to establish the processing conditions, which allow polymerpolymer bonding to be optimized.     

Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application

This study aimed at developing highly conductive, lightweight, and low‐cost bipolar plates for use in proton exchange membrane fuel cells. Injection and compression molding of carbon‐filled polypropylene, PP, and polyphenylene sulfide, PPS, were used to fabricate the bipolar plates. Loadings up to 60 wt% in the form of graphite, conductive carbon black, and carbon fibers were investigated. The developed compositions have a combination of properties and processability suitable for fuel cell bipolar plate manufacturing, such as good chemical resistance, sufficient fluidity, and good electrical and thermal conductivity. Two bipolar plate designs were successfully fabricated by molding the gas flow channels over aluminum plates to form a metallic/polymer composite plate or simply by direct injection molding of the conductive polymer composite. For the first design, overall plate volume resistivities of 0.2 and 0.1 Ohm‐cm were respectively attained using PP and PPS based blends as the conductive overmolded layer. A lower volume resistivity of around 0.06 Ohm‐cm was attained for the second design with injection molded plates made of the PPS‐based blend. Polym. Eng. Sci. 44:1755–1765, 2004. © 2004 Society of Plastics Engineers.     

Toughening of thermoset/thermoplastic composites via reaction‐induced phase separation: Epoxy/phenoxy blends

Phase morphology and phase separation behavior of amine‐cured bisphenol‐A diglycidyl ether epoxy and phenoxy mixtures have been investigated by means of time‐resolved small angle light scattering, optical microscopy, and scanning electron microscopy. The starting reactant mixtures composed of epoxy, phenoxy, and curing agents such as diaminodiphenyl sulfone (DDS) and methylene dianiline (MDA) were found to be completely miscible. Upon curing with DDS at 180°C, phase separation took place in various epoxy/phenoxy blends (compositions ranging from 10–40% phenoxy), whereas the MDA curing showed no indication of phase separation. The mechanical and physical properties of single‐phase and two‐phase networks were examined, in that the DDS‐cured epoxy/phenoxy blends having a two‐phase morphology showed improved ductility and toughness without significantly losing other mechanical and thermal properties such as modulus, tensile strength, glass transition and heat deflection temperatures. The energy absorbed to failure during the drop weight impact event was also found to improve relative to those of the single‐phase MDA‐cured blend as well as of the neat epoxy. Such property enhancement of the DDS‐cured blends has been discussed in relation to the two‐phase morphology obtained via scanning electron microscopy micrographs of fractured surfaces. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1257–1268, 2000     

Reaction-induced phase decomposition of thermoset/thermoplastic blends investigated by energy filtering transmission electron microscopy

Energy-filtering transmission electron microscopy (EFTEM) was employed to investigate the morphology developments of thermoset/thermoplastic blends of poly(2,6-dimethyl-1,4-phenylene ether)/bis(vinylphenyl) ethane, PPE/BVPE. Neither conventional TEM images nor energy-filtered images at any energy loss levels showed any evidences for the phase separation of the blends, while those could be shown and characterized by the oxygen maps representing the differences in the oxygen concentrations between the two phases, which could be characterized as the PPE-rich phase (oxygen-rich phase) and the BVPE-rich phase (oxygen-poor phase). The blends undergo phase decomposition into the two phases through the crosslinking reaction of BVPE. The effect of the composition and the curing time on the phase decomposition behaviors was investigated by quantitative analysis of EELS spectra acquired from the two phases. Reactive functional moiety was introduced onto PPE and the effect of the reaction between the two components on the phase decomposition behaviors was also investigated. We show the possibility of EFTEM for the investigation of the mechanism of reaction-induced phase separations.     

Characterization of wood-filled thermoplastic polyurethanes for the injection molding process

Fiber fillings of wood plastic composites (WPC) are almost exclusively limited to standard plastics such as polyethylene and polypropylene. At the Kunststofftechnik Paderborn of the University of Paderborn the wood fiber filling of engineering plastics is being promoted. WPC with different fiber types and fiber contents based on two thermoplastic polyurethanes (TPU) were compounded and subsequently characterized. We found that the physicochemical properties of the materials differ from standard plastic-based WPC. Wood filling with increasing fiber content did not immediately correlate with an increase in density. A decrease in density and swelling of the compound was detected with reaching a critical fiber content. Our compounds showed an increased water absorption at high-fiber contents over time, which can be described logarithmically. The observed viscosity curves obey the Ostwald and de Waele power law, but an increased viscosity at increased fiber content was not apparent for both TPU matrices.     

Fracture toughness of weldlines in thermoplastic injection molding

Weldlines are fatal defects in many injection moldings of thermosetting resins and thermoplastics. Significant strength reduction by weldlines in thermoplastics is caused by poor adhesion, molecular orientation, and a V-notch effect. These factors have been little investigated in detail, in spite of being well known. In the present article, the V-notch effect on strength is discussed for two types of thermoplastics, polystyrene and polycarbonate. The depth of weldlines was obtained by milling on the weldline surface, and the fracture toughness was measured with a double edge notched specimen. Polystryrene, which was drastically weakened by weldlines, had relatively deeper V-notch regions and the fracture toughness was also reduced by weldlines. Although polycarbonate had the same fracture toughness as polystyrene, it had strong weldlines since the depth of weldlines was negligibly small.     

Reactive processing of polystyrene-co-maleic anhydride/elastomer blends: Processing-morphology-property relationships

The morphology and impact properties of polystyrene-maleic anhydride/bromobutyl rubber blends have been studied as a function of interfacial modification and melt processing conditions. It is found that dimethylaminoethanol (DMAE) serves as a reactive compatibilizing agent for these blends and that the addition of DMAE results in a five-fold reduction In the size of the dispersed phase. Evidence for covalent bond formation between the DMAE and the elastomer and reactive polystyrene phases is presented. The volume average diameter of the minor phase increases significantly as the screw rotation speed and the material throughput increase. In fact, control of various material and processing parameters can be used to effectively control the particle size distribution during compounding. Impact strength measurements are shown to be clearly dependent on the quantity of DMAE in the system as well as the concentration of elastomer. Saturation of the interface with DMAE is shown to be an important consideration.     

Microstrcture development during the injection molding of PET/LCP blends

A liquid crystal polymer (LCP) was blended polethylene terephthalate (PET) in different concetrations to improve the barrier properties of PET in injection stretch blow molded bottles. The improvement depends on the microstructure developed at various stages of the process.In this work, the emphasis is on the injection molding stage of the preform. The characteristics and number of morphological layers were directly related to the amount and type of LCP in the blend and the loction within the perform. It was found that at 10% LCP, three morphological layers were found across the thickness of the part, while at 30% LCP, five morphological layers could be identified. The LCP structure can be classified into four types: droplets, thick rods, thin fibrils and ribbons. Each morphological layer is made up of one or more types of structures. The evolution of on type structure to another depends on the particular flow regime ongoing at various locations in the mold. This microstructure development, during the flow, was examined in detail.     

Numerical simulation of thermoplastic wheat starch injection molding process

A numerical study is carried out on the thermoplastic wheat starch injection molding process. The simulation is performed using currently available molding software to determine optimal molding parameters. The molding of a standardized sample for tensile test is considered. It is shown that the conventional continuum mechanics equations can be used for modeling the injection molding of thermoplastic starch. These equations are solved using the finite element method. Comparisons with some experimental results are presented, indicating good agreement. Data on the processing of thermoplastic starch and several other basic aspects are also provided.     

Alternating multilayer structure of polyethylene/polypropylene blends obtained through injection molding

The formation of multilayer structures in the high‐speed thin wall injection‐molded samples of high‐density polyethylene/isotactic polypropylene blends is reported. Based on the morphology development in injection runner and mold, a possible formation mechanism of multilayer structure was proposed in this study. Injection molding could be used as a simple and an effective method for the fabrication of multifunctional multilayer structure. This work is interesting and important for scientific research as well as several potential applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Compounding and injection molding of polybutene‐1/polypropylene blends

The effect of shear‐controlled orientation injection molding (SCORIM) was investigated for polybutene‐1/polypropylene blends. This article reports on the methods and processing conditions used for blending and injection molding. The properties of SCORIM moldings are compared with those of conventional moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated with mechanical tests, differential scanning calorimetry studies, and polarized light microscopy. The application of SCORIM improved Young's modulus and the ultimate tensile strength. The gain in stiffness was greater for higher polybutene‐1 content blends. A drastic decrease in the strain at break and toughness was observed in SCORIM moldings. The enhanced molecular orientation of SCORIM moldings resulted in a featureless appearance of the morphology. Interfacial features due to segregation were visible in the micrographs of SCORIM moldings. Both conventional and SCORIM moldings exhibited form I′ in polybutene‐1. This article explains the relationship between the mechanical properties and micromorphologies. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 806–813, 2003     

Investigation of adhesion phenomena in thermoplastic polyurethane injection molding process

Adhesion is a serious problem when ejecting products from mold cavities in the injection molding process. This study developed a mechanism for measuring adhesion force in injection molding process. The apparatus measured and quantified adhesion force and engineers could use the measurements to understand the effectiveness of mold release agent and different adhesion release coatings. Three different surface roughness conditions (Ra = 0.01, 0.08, and 1.20 μm) without surface coating were compared and the results showed that the smallest adhesion force would be obtained when Ra = 0.08 μm. With Ra = 0.08 μm and 0.01 μm, it was found that single‐layer CrN (chromium nitride) coating and modified CrN coating were effective in alleviating adhesion force whereas multi‐layer CrN coating was not effective. In addition, the adhesion force was proportional to the ejection speed, melting temperature, and cooling time. Because thermoplastic polyurethane (TPU) behaves like a rubber‐like material and is thus different from other stiff resin after cooling, the release of adhesion of TPU is time‐dependent. Comparing the peak and integrated impulse values of the measured curve in continuous experiment, it was found that these two results were in good agreement with each other. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers