Effect of Processing Parameters on the Mechanical Properties of Injection Molded Thermoplastic Polyolefin (TPO) Cellular Foams

In this study, the effects of processing parameters on the mechanical properties of injection molded thermoplastic polyolefin (TPO) foams are investigated. Closed cell TPO foams were prepared by injection molding process. The microstructure of these foamed samples was controlled by carefully altering the processing parameters on the injection molding machine. The foam morphologies were characterized in terms of skin thickness, surface roughness, and relative foam density. Tensile properties and impact resistance of various injection molded TPO samples were correlated with various foam morphologies. The findings show that the mechanical properties are significantly affected by foam morphologies. The experimental results obtained from this study can be used to predict the microstructure and mechanical properties of cellular injection molded TPO foams prepared with different processing parameters.

     

Morphology and mechanical investigation of microcellular injection molded carbon fiber reinforced polypropylene composite foams

Employing microcellular injection molding technology, carbon fiber (CF)/polypropylene (PP) composite foams have been prepared. The influences of injection molding conditions and CF amounts relating to the flexural and impact performances have also been studied. X-ray computed tomography scanning has been used for morphological observation. For the flexural specimens, although the solid skin and foamed core layers can be confirmed significantly, the intermediate layer is indistinct. Moreover, the stretched cells can be confirmed dramatically for the Charpy impact specimens. The cell density increases to 12.0 × 10³ cell/cm² when the nitrogen content is 1%. By contrast, the cell densities decrease with the injection speed and CF content increasing accordingly. Further, the maximum specific flexural modulus and Charpy impact strength of the foams can achieve 14 GPa/(g/cm³) and 6.2 kJ/m², respectively, at the CF content of 30 wt%. Finally, the microcellular structure with the highest cell density can be confirmed with the nitrogen content of 1 wt%, the injection speed of 50 mm/s and the CF content of 10 wt%. Obviously, the CF contents have shown strong influences on the mechanical behaviors of the CF/PP composite foams compared with nitrogen contents or injection speeds.     

Effect of sintering on processability vis-a-vis microcellular foaming of ultrahigh molecular weight polyethylene

Processing of ultrahigh molecular weight polyethylene (UHMWPE) involves sintering due to its high melt strength and no flowability above melting temperature. Variations in compression molding pressure during sintering lead to chain rearrangement at the sintered interphase and the boundary, affecting foamability. UHMWPE particles are sintered using compression molding; samples are prepared at two different pressures: UHPE-HP (80 bar) and UHPE-LP (40 bar) at 180°C. The sintering phenomenon of UHMWPE particles is observed through an optical microscope, and their effect on foaming was observed. UHPE-HP foams are systematically studied to obtain the foaming window. Increasing foaming pressure (80–120 bar) made UHPE-HP foams softer (0.350–0.219 g/cm³) with varying average cell size (26.37–46.1 μm) and foam cell density (3.98 × 10⁷–1.06 × 10⁸ cells/cm³), and compression modulus decreased from 9 to 5.4 MPa. DMA results showed a strong dependence of stiffness on crystallinity, and foamed samples exhibit higher stiffness than their unfoamed counterpart. The storage modulus for foamed samples decreases with increase in the gas content. The UHPE-LP foam is relatively softer, with a lower foam density (0.233 g/cm³), a higher expansion ratio, bigger average foam cells (35.13 μm), and lower foam cell density (9.33 × 10⁷ cells/cm³). This is due to constrained crystallinity at the interphase and pre-existing cavities, favoring the foaming.     

Effect of supercritical nitrogen content on the weld lines and mechanical properties of microcellular injection molded double gate PP/Al parts

Injection molding products made of aluminum flakes and polymer blends exhibit a distinctive esthetic effect. However, during the filling process, the melt flows in different directions converge and collide, resulting in the flop effect of the aluminum flake and consequent weld line formation. Herein, microcellular injection molding (MIM) was employed to fabricate polypropylene/aluminum flakes (PP/Al) composite foamed parts with distinct weld lines using supercritical nitrogen (scN₂) as the physical blowing agent. The scN₂ content has a significant effect on cell diameter and cell density. When the scN₂ content was 0.6%, the weld line width of the foamed part was 13.03 μm, while it was 30.41 μm for the solid counterpart due to the expansion and rupture of cells in the flow front during filling. Moreover, the orientation of Al flakes was mostly along the flow direction for the foamed parts, while it was generally aligned perpendicular to the flow direction for solid parts in the weld line region. In addition, the flexural modulus of foamed parts was increased by 29% compared with the solid parts, although the tensile strength was reduced by 18% due to the alignment of Al flakes and the stress concentration on the cell walls. Therefore, this work provides insight into the improvement of flexural property and the mitigation of weld lines for injection molded composite parts using MIM.     

Natural convection mass transfer at spheres

Mass transfer by natural convection at spheres has been studied by an electrochemical technique involving limiting current measurement for the anodic dissolution of copper spheres in phosphoric acid. Acid concentration and sphere diameter were changed to provide values ofSc. Gr ranging from 2.85×10¹⁰to 2.15×10¹¹; under these conditions the mass transfer data was correlated by the equation:Sh=0.15 (Sc. Gr)^0.33 Nomenclature I Limiting current density - K mass transfer coefficient - F Faraday's constant - C Saturation solubility of copper phosphate in phosphoric acid - Z number of electrons involved in the reaction - Sh Sherwood number=Kd/D - Sc Schmidt number=v _ave/D - Gr Grashof number=gd ³ ( _i– _b)/v _ave ² _i - d Sphere diameter - D diffusivity of copper ions - u _b viscosity in the bulk liquid - u _i viscosity at the interface - _b density in the bulk liquid - _i density at the interface - g acceleration due to gravity - v _ave average Kinematic viscosity - Nu Nusselt number - Pr Prandtl number     

一步法滚塑发泡成型新技术

介绍了一种用于加工具有非发泡外壳和发泡内芯的塑料制品的新方法——一步法滚塑发泡成型技术,包括一步法滚塑发泡成型工艺原理、加工流程及影响因素。     

Polystyrene Foam with High Cell Density and Small Cell Size by Compression‐Injection Molding and Core Back Foaming Technique: Evolution of Cells in Cavity

Many efforts have been made to obtain uniform cell structures from foam injection molding techniques. However, cell nucleation mechanism and complex dynamics during the cell formation have rarely been well understood. Here, high‐pressure foam injection molding (HPFIM) is achieved by combining the injection–compression molding with core back foaming (ICMCBF) technique. The influences of compression pressure and time on the cell structure of polystyrene foam during the foaming process are studied. Compared with low pressure for conventional foam injection molding, high compression pressure (200 bar) and fast pressure drop rate of ICMCBF endow the foam with the highest cell density (1.59 × 10⁷ cells cm^?3), and the smallest cell size (15 µm). The tensile strength and impact strength are enhanced by about 60% (from 22.3 to 35.6 MPa) and 80% (from 3.6 to 6.8 MPa), respectively. This study gives a critical understanding of the cell nucleation and growth mechanism of the foam injection molding and supplies a new strategy for the fabrication of foam with uniform cell structure.     

Lightweight and High Impact Polypropylene Foam Fabricated via Ultra-Low Gas Pressure Injection Molding

Foamed materials play an important role in a lightweight design. Foam injection molding (FIM) is an advanced and convenient way to fabricate lightweight structural materials. Recently, a new foam injection molding machine is developed, which only needs ultra-low gas pressure to fabricate microcellular foam. As a universal plastic, polypropylene (PP) is widely used due to its good mechanical properties. But after foaming, the toughness of the PP tends to decrease. Herein, a lightweight and high-impact polypropylene foam is fabricated via the new FIM technology with an ultra-low nitrogen pressure of 6.5 MPa. PP/polyolefin elastomer (POE) foam with a tiny cell size of 4.13 µm and high cell density of 2.7 × 10⁹  cm⁻³ is successfully obtained. Owing to the superior cellular structure, compared with the pure PP foam, after adding the POE component, the maximum impact performance is increased by 465%. In this work, an easy-to-industrialized method for preparing lightweight and high-impact injection-molded PP foams are presented.     

家具用聚氨酯硬质结构泡沫

研究了家具用聚氨酯硬质结构泡沫的配方。讨论了影响泡沫制品性能的因素。制得的结构泡沫性能为：密度４００－６００ｋｇ／ｍ＾３，压缩强度≥５ＭＰａ，弯曲强度≥２０ＭＰａ，表皮邵尔Ｄ型硬度≥５０。     

Fabrication of foamed polypropylene with excellent behaviors by adding a special foam stabilizer

Foamed polypropylene (PP) has attracted more and more attention in recent years due to its unique properties, such as heat resistance and high flexural modulus. In this work, foamed PP with excellent properties was successfully fabricated by adding a special foam stabilizer, which was prepared by a simple one-step strategy using fatty acid and amino silicone oil as reactants. The two-component stabilizer mixed uniformly with PP and reduced the surface tension during foaming. The foam stabilizer significantly reduced density and cell diameter of the foam. When the amount of foam stabilizer was 1.0 wt%, the density dropped to 0.958 g/cm³, about 2.8% lower vs foam generated without stabilizer. The tensile strength increased to 18.4 MPa from 16.1 MPa, and the elongation at break increased to 495% from 328%.     

Natural convection mass transfer at horizontal screens

The free convection mass transfer behaviour of horizontal screens has been investigated experimentally using an electrochemical technique involving the measurement of the limiting currents for the cathodic deposition of copper from acidified copper sulphate solutions. Screen diameter and copper sulphate concentration have been varied to provide a range ofSc.Gr from 22×10⁸ to 26×10¹⁰. Under these conditions, the data for a single screen are correlated by the equation:Sh=0.375(Sc.Gr)^0.305 Results have been compared with previous work on free convection at horizontal solid surfaces where mass transfer coefficients are somewhat lower.Mass transfer coefficients have been measured also for arrays of closely spaced parallel horizontal screens. The mass transfer coefficient was found to decrease with the number of screens forming the array.Symbols and units A area of mass transfer surface, cm² - C _b bulk concentration of ionic species, mol cm^–3 - D diffusivity, cm²s^–1 - F Faraday number, 96494 C g [equiv^–1] - Z number of electrons involved in the reaction - I _L limiting current, A - K mass transfer coefficient, cm s^–1 - Sh Sherwood number, dK/D - Sc Schmidt number,/D or/D - Gr Grashof numbergd ³/ ² _s - solution dynamic viscosity, g cm s^–1 - solution kinematic viscosity, cm² s^–1 - solution density, g cm^–3 - density difference between bulk solution and electrode/solution interface, g cm^–3 - _s solution density at electrode/solution interface, g cm^–3 - d screen diameter, cm - g gravitational acceleration, cm s^–2 On leave of absence, Chemical Engineering Department, Alexandria University, Alexandria, Egypt.     

The role of mass transfer in the kinetics of anodic dissolution of gas-evolving metallic surfaces

The rate of anodic dissolution of copper in phosphoric acid above the potential where oxygen evolution takes place was studied. Variables investigated were oxygen discharge rate, phosphoric acid concentration and electrode position. The mass transfer coefficient of the anodic dissolution of copper in phosphoric acid was related to the oxygen discharge rate and the physical properties of the solution by the equations for a vertical electrode:k=aV ^0.2(/u)^0.93 for a horizontal electrode:k=aV ^0.21(/u)^0.93 List of symbols k mass transfer coefficient (cm s^–1) - V oxygen discharge rate, (cm³cm^–2min^–1) - a constant - I current consumed in copper dissolution(A cm^–2) - Z number of electrons involved in the reaction - F Faraday's constant - C Solubility of copper phosphate in H₃PO₄,(mol cm^–3) - N rate of copper dissolution, (g-ion cm^–2s^–1) - diffusion layer thickness (cm) - r bubble radius (cm) - g acceleration of gravity (cm s^–2) - ¯V rise velocity of O₂ bubble (cm s^–1) - u viscosity (poise) - density (g cm^–3)     

Prediction of the change in density in systematic series of fibre-forming polymers

The density of aliphatic polyamides is determined by the concentration of amido groups and additively increases in systematic order. The packing coefficient of aliphatic polyamides determined by the ratio _a/ _c increases with an increase in the concentration of amido groups. The density of cellulose acetateis determined by the concentration of acetate groups and decreases additively in systematic order. The calculation with the group contribution method can be used to predict the density of crystallites of fibreforming polymers. but this method only gives approximate results for amorphous regions.St. Petersburg University, of Design and Technology. Translated from Khimicheskie Volokna. No. 2, pp. 21–22, March–April, 1996.     

Optimization of the structure of integral skin foams for maximal flexural properties

The effective flexural properties of integral skin foams (ISF), are modeled using Euler-Bernouli beam theory along with a power law empirical equation relating the properties of a homogeneous foam to its density. The optimal density profile that maximizes the effective flexural modulus of an ISF beam of fixed overall density, and with the density constrained to lie in a given range, is continuous when the power law exponent (n) is less than 1. For n > 1, the optimal density profile is discontinuous with a low density core and a high density skin. The effective flexural modulus of such sandwich beams is maximized for a fixed density ratio (ratio of the core density to the skin density) and fixed overall density. The maximal flexural modulus is found to increase monotonically with decreasing density ratios and increasing values of n. The flexural strength of the sandwich beam is also maximized considering failure to occur by tensile fracture or buckling of the skin. In this case an optimal skin thickness and an optimal density ratio are obtained for a fixed overall density. The results are useful for the design and evaluation of flat ISF panels.     

Effect of drag-reducing additives on the rate of mass transfer in a parallel-plate electrochemical flow reactor

The effect of polyox and CMC drag-reducing polymers on the rate of mass transfer in a parallel-plate flow cell was studied by measuring the limiting current for the cathodic reduction of potassium ferricyanide in alkaline medium. Reynolds number and polymer concentration were varied over the range 3500–21 000 and 10–200 ppm respectively. Under these conditions it was found that polyox and CMC reduce the rate of mass transfer by a maximum of 42% and 35% respectively.Nomenclature a a constant - C concentration of ferricyanide ion (g mol cm^–3) - D diffusivity of ferricyanide ion (cm²s^–1) - d _e equivalent diameter of the cell (4 x cross-sectional area/wetted perimeter) - F Faraday's constant (96 487 C mol^–1) - I limiting current density (A cm^–2) - K mass transfer coefficient (cm s^–1) - L electrode height (cm) - (Re) Reynolds number (d _e /u) - (Sc) Schmidt number (u/D) - (Sh) Sherwood number (Kd _e/D) - u solution viscosity (poise) - flow rate of the solution (cm s^–1) - Z number of electrons involved in the reaction - solution density (g cm^–3)     

Calorimetric,X-ray and infra-red investigations on poly(hexamethylene adipamide)

Summary In dependence on crystallization conditions three ranges with different crystal structure and heat of fusion were found by DSC,WAXS,and IR for unoriented PA 6.6 samples of densities between 1.10 and 1.17gcm^–3: Range I:_I triclinic, _c ^I =1.225 gcm^–3,H _M ^I = 235 Jg^–1. Range II:_II triclinic, _c ^II =1.165 gcm^–3, H _M ^II =185 Jg^–1. Range III:Continuous variation from _c ^I ,H _M ^I to _c ^II , H _M ^II . _a=1.095 gcm^–3 is independent of crystallization. conditions. The transition between _I and _II is probably due to changes of the chain conformation.     

Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings

A discussion of the requirements for hardware and software necessary for collection and analysis of electrochemical impedance spectroscopy data for polymer coated metals is presented. Most authors agree that a simple model can describe the frequency dependence of impedance spectra for polymer coated metals exposed to corrosive environments. The water uptake of the coating can be estimated from the time dependence of the coating capacitance C _c, The pore resistance R _po depends both on the resistivity of the coating and the disbonded area A _d. The polarization resistance R _P of the corroding area under the coating and the corresponding capacitance C _dl both depend on A _d. The breakpoint frequency method is discussed in detail and the dependence of the breakpoint frequency f _b on and A _d is derived. In addition to f _b other parameters can be obtained which depend on the ratio A _d/ or only on A _d or . Since these parameters can be obtained at frequencies exceeding 1 Hz without the need for an analysis of the impedance spectra in the entire frequency region, this approach is considered especially useful for corrosion monitoring. The concepts proposed for the analysis and interpretation of EIS data for polymer coated metals are illustrated using data for Al alloys, Mg and steel exposed to NaCl. For an alkyd coating on cold rolled steel the time dependence of A _d and during exposure to 0.5 m NaCl has been determined qualitatively using the modified breakpoint frequency method.     

The application of renewal theory to anodic dissolution processes

The oscillatory behaviour of a certain class of dissolution processes involving anodic dissolution of a metal and the formation of a passivating substance is analysed in terms of renewals via isomorphism with a Type I counter in queuing theory.Nomenclature E(X) statistical expectation of random variableX - E _i expected frequency in theith class of observation (due to a postulated probability distribution) - m renewal number - N _t number of renewals in time period (0,t) - O _i observed frequency in theith class observation - R(t) renewal function (Equation 3) - T _m time lapsed until themth renewal - t time - density parameter of the experimental probability density function - (u) Gaussian probability density function of random variableu - ² chi-square statistic     

The effects of gas counter pressure and mold temperature variation on the surface quality and morphology of the microcellular polystyrene foams

In this study, we developed a foaming control system using the Gas Counter Pressure (GCP) combined with mold temperature control during the microcellular injection molding (MuCell) process and investigated its influence on the parts' surface quality and foams structures. The results revealed that under GCP control alone when GCP is greater than 10 MPa, part surface roughness for transparent polystyrene (PS) improved by 90%. When GCP increased, the skin thickness also increased, the weight reduction decreased and the average cell size reduced to about 30 μm. For black PS parts, when GCP is greater than 10 MPa, the part gloss reaches the same value as that molded by conventional injection molding. By increasing gas holding time, the cell density decreased and the cell size distribution became more uniform. The increase in amount of supercritical fluid foaming agent also increased the cell density. Applying mold temperature control alone with temperature in the range of 90–120°C (near T_g), the surface roughness improved by 65%. Increasing mold temperature decreased the skin thickness; however, the cell size distribution became significantly nonuniform. It was found that thin skin, small and uniform cell size as well as good surface quality can be achieved efficiently by simultaneous combining of GCP and mold temperature control. The proposed innovative approach may lead to a significant improvement and a more broad application for MuCell process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Simulation of foaming in reaction injection molding

Blowing agents are often used in the reaction injection molding process to compensate for the shrinkage that occurs upon polymerization, thereby creating a structural foam part. Controlling the thickness of the solid skin and foamed core is essential to achieve the intended mechanical properties of the molded part. A numerical model was developed to predict the foaming behavior in reaction injection molding. This algorithm employs a novel primitive cell construction to enable it to analyze complex rectangular geometries, including inserts, with a two-dimensional, finite difference solution method. The analysis was applied to foaming of polyurethane in a rectangular cavity. The predicted skin thickness was found to be in good agreement with actual structural foam parts. Foaming as a function of cavity thickness was also treated. The algorithm Is useful for understanding and interpreting nonlinear phenomena of rapid, exothermic polymerizations such as foam formation adjacent to the mold wall or around a metal insert. The results can be used to formulate design guidelines for achieving desired skin/core thicknesses as a function of design, material, and process parameters.