Residual stresses and cracking in large ceramic injection mouldings subjected to different solidification schedules

In ceramic injection moulding, the moulding dimensions and the residual stresses are related to the hold pressure history during solidification in the cavity. In conventional moulding, the residual stresses are generally compressive at the surface. In this work, an insulated sprue device was made. It allows prolonged pressure transmission to the moulding and this can result in residual tensile stresses at the surface of thick section mouldings. At very low holding pressures (<9 MPa) or under conditions of premature gate solidification, a reversal of the sign of surface stresses occurs with compressive stresses near the surface. The appearance of cracks during binder removal was related to these residual stresses. For 25 mm thick mouldings which were subjected to a low but persistent hold pressure of 5 MPa no defects developed during the binder removal stage.     

PVC注塑件的密度与收缩

分析了PVC注塑件的密度特征和收缩特征,结果表明:PVC注塑件的密度及其分布与PVC注塑件的收缩相互影响。     

Relaxation patterns in powder injection mouldings made with repeated flow Reversals

Injection moulding suspensions based on both high and low molecular weight organic vehicles and incorporating both spherical and anisotropically shaped particles were prepared. A double end-gated rectangular bar cavity was used for injection moulding. Conventional mouldings were made by injection from one gate followed by static hold pressure. Modulated pressure mouldings were made in the same way but reciprocating flow was induced between the gates during solidification. The mouldings were sectioned, polished and heat treated whereupon a pattern of rings emerged in relief preserving the history of flow reversals as the solid-liquid interface advanced from the mould walls.     

The evolution of residual stresses in thermoplastic bonding to metals

The ability to create strong joints between thermoplastics and metals offers many advantages. Differential properties between the polymer and metal generate residual stresses during cooling. In our study, both amorphous thermoplastic thin films and semi-crystalline thermoplastic thin films are joined to metal strips and the curvature is measured during controlled cooling. A series of five designed experiments uses a fast cooling (30°C/s) and slow cooling (4.5–10°C/min) to create different residual stresses. Experimental evidence shows that the residual stresses begin to develop at 190°C for amorphous Poly Ether Imide (PEI, T_g = 210°C), but at 255°C for semi-crystalline Poly Ether Ether Ketone (PEEK, T_g = 143°C). A mechanics based curvature model, combined with the elasticity and viscoelasticity of thermoplastics, successfully predicts the residual stress development. An elastic behavior is exhibited during the fast cooling (30°C/s), whereas a viscoelastic behavior occurs during slow cooling (4.5°C/min).     

Impact tests on filled and unfilled thermotropic liquid crystalline polymer injection mouldings

Summary Charpy and falling weight impact tests have been carried out on filled and unfilled thermotropic liquid crystalline mouldings between room temperature and 140 °C. There is progressive embrittlement on filler addition in both types of test, and a maximum in absorbed energy as a function of temperature is observed, corresponding to the glass transition temperature near 100 °C.     

Temperature rise in a thermoplastic under completely reversed bending stresses

When a thermoplastic specimen is tested in flexural fatigue, the viscoelastic behavior of the material combines with the heat dissipation mechanism to generate an equilibrium temperature distribution. In order to locate the starting point of an eventual fatigue crack in the most stressed section of a particular geometry, it is important to know where the temperature reaches its maximum value. A first mathematical model has been proposed to evaluate the temperature distribution across the thickness of the specimen. Solutions have been obtained analytically and by a finite difference method. A comparison of the solutions enabled us to optimize the mesh size, the relaxation factor and the convergence criterion in the finite difference scheme in order to get results within a chosen accuracy. Preliminary test results are given and compared with the theoretical temperature distribution for two thermoplastics, nylon 66 and acetal homopolymer.     

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.     

Interfacial instabilities in multimaterial co-injection mouldings: Part 2 - Interfacial mixing in transparent mouldings

Abstract

Previous research in co-injection moulding has produced viscosity ratio guidelines for skin and core components, which must be followed if good core distribution is to be achieved. However, by examining two phase systems of PMMA-PC, which fall within the recommended viscosity range, this work shows that viscosity matching of materials is not a sufficient requirement for interfacial stability. The transparency of the materials allows areas of interfacial mixing to show up as streaks in the mouldings, so that factors affecting stability can be determined. One system is found to be more stable than the others. Explanations for such effects are given by consideration of interfacial mixing, viscosity, tooling geometry, injection speeds, interfacial stresses and shear in multilayered mouldings.     

Residual thermal stresses in injection molded products

Nonisothermal flow of a polymer melt in a cold mold cavity introduces stresses that are partly frozen-in during solidification. Flow-induced stresses cause anisotropy of mechanical, thermal, and optical properties, while the residual thermal stresses induce warpage and stress-cracking. In this study, the influence of the holding stage on the residual thermal stress distribution is investigated. Calculations with a linear viscoelastic constitutive law are compared with experimental results obtained with the layer removal method for specimens of polystyrene (PS) and acrylonitrile butadiene-styrene (ABS). In contrast to slabs cooled at ambient pressures, which show the well-known tensile stresses in the core and compressive stresses at the surfaces, during the holding stage in injection molding, when extra molten polymer is added to the mold to compensate for the shrinkage, tensile stresses may develop at the surface, induced by the pressure during solidification.     

Generation of large residual stresses in injection moldings

Large residual stresses have been generated in injection molded bars by ejecting them prematurely and completing the cooling process by quenching into ice water or liquid nitrogen. The stress distribution formed under these conditions was found to be much closer to parabolic than is the case when the moldings cool conventionally. Limited testing on moldings made in this way indicated significant property enhancement and improved resistance to ultraviolet degradation.     

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.     

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     

Residual stresses and aging in injection molded polypropylene

The residual stresses in injection molded bars of polypropylene have been examined using a stress relaxation method and by the layer removal technique. A positive value for the internal stress parameter was obtained with newly molded specimens and was found to be retained by specimens stored at liquid nitrogen temperature. The stress relaxation parameter reduced to zero both for specimens aged at room temperature and also for those aged at ?40°C. Nevertheless the relaxation behavior of specimens aged at all three temperatures was quite distinct. The layer removal technique showed that the stresses near to the surface were compressive and those in the interior tensile, in apparent contradiction to the interpretation of Kubat and Rigdahl for the meaning of a positive internal stress parameter. A marked reduction in stress levels on aging at room temperature was confirmed, however. The relevance of the relaxation spectrum of polypropylene to these observations is discussed.     

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.     

Residual stresses in injection molded polycarbonate rectangular bars

This study deals with the quantification of the residual stress profiles of injection molded rectangular bars with polycarbonate of various molecular weights. Determinations via the layer removal technique indicated that these bars can possess compressive stress of up to 10 MPa at the surface and up to 9 MPa in tension in the interior. The residual stress profile showed a shape similar to that observed in thermal quenching experiments, i.e., parabolic, for locations far away from the gate of the cavity. At locations near the gate of the cavity and for injection molding at high speed, the residual stress profile changed sign, going from compressive to tension at the surface and vice versa in the interior. These residual stress profiles were very dependent on gating geometry and location.     

Structural order in oriented PVC mouldings

The structural changes occurring upon drawing and annealing of compression and injection mouldings of commercial poly(vinyl chloride) were studied by wide-angle X-ray diffraction. Low temperature drawing appears to lead to a reduction in 3-dimensional order and an increase in oriented 2-dimensional order. The degree of order of drawn and annealed PVC depends on draw ratio, annealing temperature and the restraint during annealing. The maximum in 2-dimensional order occurs on annealing at 110°C. Tensile yield stress is significantly increased by the drawing process and it was shown that the anisotropy of this mechanical property decreased upon annealing. This could not be explained by the reduction in amorphous orientation alone. Electron microscopy of the fracture surfaces shows a structure which appears to be related to the drawing and annealing process.     

Determination of stresses in formed plastics parts

To determine the optimum conditions for vacuum forming quality parts, a precise method of determining internal stress required development. The approach investigated was based on the shrinkage of a heated stressed part. Small stress should be proportioned to strain. By exposing loaded tensile specimens simultaneously with the formed part, the material modulus is determined. Results indicated creep which was minimized by using the correct modulus to calculate internal stress. Stresses in a normally formed part and in a part formed cool were compared at two temperatures. Values were higher in most areas of the cool-formed part and consistent at the two temperatures. This method probably can be used on other materials and possibly on injection molded parts.