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.     

Studies on the flexural modulus of structural foams

An investigation based on I-Beam models was undertaken in this paper for extending knowledge regarding the flexural modulus of structural foam. The applicability of five I-Beams models (I-Beam A, B, C, D and E) including a newly developed one (I-Beam E) were investigated in this work. The square law model was used to predict Young's modulus of uniform density foam, which was subsequently utilized for the calculation of the I-Beam models. I-Beam A, B and E were observed from the configuration analysls of each I-Beam to be the more suitable models for predicting the flexural modulus of the structural foams having either an integral skin or a skin with limited residual bubbles, among which I-Beam E is considered to be better than I-Beam B and A. The comparison of the experimental and theoretical values of the flexural modulus of the structural foams molded with gas counter pressure structural foam (CPSF) and low pressure structural foam (LPSF) molding methods also confirmed that the newly developed I-Beam E is the most adequate model for predicting the flexural modulus of structural foams having either an integral skin or a skin with few residual bubbles. I-Beam B and A were also demonstrated to be in good agreement with the experimental data.Nomenclature B width of I-Beam A, B, C and E - Bc core width of I-Beam A - Bc₁ center core width of I-Beam - Bc₂ half of center core width of I-Beam - C adjustable parameter for density distri-iion of structural foam - CPSF gas counter pressure structural foam injection molding - D thickness of I-Beam in foamed core sec tion or thickness of structural foam in foamed core section - D_s thickness of unfoamed beam - e ratio of skin thickness to half of the thickness of a specimen (reduced skin thickness) - E¹ flexural modulus of structural foam - E_c average Young's modulus for foamed core of structural foam - E_s Young's modulus of unfoamed solid - FLBF flexural load bearing factor - GASF gas assisted structural foam injection molding - HPSF high pressure structural foam injection molding - I_c equivalent moment of inertia of I-Beam - I_s moment of inertia of unfoamed beam - LPSF low pressure structural foam injection molding - R reduced density for center core of structural foam - SCSF sandwich coinjection structural foam molding - T thickness of I-Beam in skin section or skin thickness of structural foam - Y half of the thickness of a specimen - Z dimensionless distance from neutral axis of a specimen subjected to pure bending Greek symbols local density of structural foam - ¹ average density of structural foam - _c average density of foamed core of structural foam - ^f density of uniform density foam - _s density of unfoamed solid     

The effect of density profile on the flexural properties of structural foams

In this study, a comparison is made on the models available to predict the flexural modulus of structural polymer foams. Skin thickness and density profile in the core zone are taken into account to better relate foam morphology with mechanical response. It was found that including skin thickness is not sufficient to predict flexural moduli with high levels of precision and thus a transition zone between the skin and the core must be included. To this end, a single continuous equation is proposed to represent the complete density profile, which enabled us to predict our experimental data within 3%. POLYM. ENG. SCI., 47:1459–1468, 2007. © 2007 Society of Plastics Engineers     

Mechanical properties of rigid thermoplastic foams—Part II: Stiffness and strength data for modified polyphenylene oxide foams

The through-thickness variation in the porosity of structural foam material is known to result in different “material properties” when mechanics based on homogeneous materials is used to interpret data from standard tensile and bend tests. Procedures for determining the mechanical properties of rigid thermoplastic structural foams and for the application of these properties to the design of load-bearing components were developed in a companion paper. This paper reports the mechanical properties of modified polyphenylene oxide foams, such as elastic moduli, ultimate stress and strain, as determined by tests on specimens cut from large, edge-gated foam plates. Tests were conducted to study plate-to-plate variations in properties and to evaluate the effect of specimen thickness. Correlations of tensile and flexural data with the average specimen density are also discussed.     

横向腹板增强复合材料夹层梁受弯性能试验研究

通过四点弯试验研究横向腹板增强复合材料夹层梁受弯性能,得到不同腹板间距、厚度对夹层梁弯曲破坏模式、刚度、极限承载力及延性性能的影响规律。结果表明:横向腹板能改变夹层梁的破坏模式,无腹板增强夹层梁破坏模式为芯材剪切破坏,横向腹板增强夹层梁破坏模式为多区格渐进破坏模式;相对于无腹板增强夹层梁,横向腹板能显著增强复合材料夹层梁的延性特性,最高达229%,腹板间距越小,夹层梁延性性能越好。     

Application of rigid polyurethane integral-skin foams to thermal insulating unit cases for car air conditioners

Integral-skin foams of rigid polyurethane are sandwich structures consisting of a core layer of closed cells enclosed in rigid surface layers on both sides. We examined the layer composition of integral-skin foam with the objective of maximum flexural strength, and then studied possibilities of reconciling the strength and thermal insulating properties in housings for evaporators in car air conditioners; i.e., unit cases. This examination showed that the most practical density range (250 ≦ ρ_pall ≦ 500 kg/m³) provides vibratile resistance and thermal insulating properties. In actual car-running tests, a maximum 0.1 MPa stress was generated on unit cases with overall densities of 350 kg/m³, We found this to be 0.4% of the flexural strength of an integral-skin foam and 2% of the fatigue strength. In the forcible vibratile test, a stress of 0.5 to 1.0 MPa was generated at the resonance point of a unit case with 250 to 500 kg/m³ overall density. We found that these values are 2 to 5% of integral-skin foam's flexural strength and 10 to 25% of its fatigue strength. These values are of the same level as the conventional unit case made of polypropylene blended with talc. An integral-skin foam with an overall density of 250 kg/m³, nearly equal to half the weight of polypropylene, has the same level of resistance to vibration.     

Effective elastic moduli for thin-walled structural foam beams

Because of the nonhomogeneous morphology of rigid structural foams, the elastic moduli determined from tension and bend tests are different, the latter being larger. These moduli also depend on the geometry of the specimen. In general, the elastic bending stiffness of foams is determined by the rigidity tensor, which combines geometry and material information. Although the bending problem for nonhomogeneous materials is more complex than the equivalent homogeneous problem, the analysis simplifies considerably for thin-walled beams. The effective flexural modulus for a thin-walled foam beam is shown to be the tension modulus that would be measured on a flat foam specimen of the same thickness. The flexural modulus measured by bend tests on flat bars is shown to have very little effect on the stiffness of most thin-walled sections. This conclusion is independent of how the “true” material modulus varies across the thickness of the foam part.     

Mechanical properties of commingled plastic from recycled polyethylene and polystyrene

The mechanical properties of commingled plastic in the form of thick beams prepared by the ET-1 process have been examined in flexure and compression. The mechanical properties were evaluated in relationship to the hierarchical morphology described in a previous study. It was found that the flexural modulus was dominated by the properties of the skin and was satisfactorily modeled by approaches based on the observed micro-morphology, such as the Nielsen and Davis models. It was not necessary to consider the skin–core macromorphology because the flexural modulus was dominated by the void-free skin. The compressive modulus was lower than the flexural modulus and was strongly affected by the skin–core macro-morphology. From the differences between the flexural and compressive moduli, it was determined that the core was essentially nonload-bearing in compression. Flexural fracture initiated on the tension side of the beam and propagated rapidly through the thickness, whereas compressive failure occurred by longitudinal splitting of the skin. © 1994 John Wiley & Sons, Inc.     

Effect of processing parameters on flexural properties of 3D-printed polyetherketoneketone using fused deposition modeling

Polyetherketoneketone (PEKK) is an engineering plastic with ultrahigh mechanical performance and has attracted considerable attention in the medical and technical fields. Printing parameters during fused deposition modeling (FDM) for PEKK have a significant impact on final part quality. In this study, a relationship between the process parameters and flexural properties of PEKK was investigated by conducting three-point bending tests, and scanning electron microscopy was employed to analyze the microstructure of fracture surfaces. Nozzle temperature, layer thickness, and infill density affected flexural properties by changing the porosity and interlayer bonding strength. Interlayer separation is the main failure mode of the upright orientation samples, while intralayer failure is likely to occur in the on-edge orientation samples. The flexural properties of FDM-printed PEKK under optimum parameters are comparable to those of mandibular bones, indicating that PEKK is a potential candidate for repairing mandibular defects. The results highlighted in this study are fundamental to the optimal design of complex ultralight, highly efficient structures.     

Flexural Behaviour of Fibre‐Reinforced Syntactic Foams

Summary: Syntactic foams containing 0.9, 1.76, 2.54, 3.54 and 4.5 vol.‐% of E‐glass fibres in the form of chopped strands were processed and subjected to three‐point bending tests. The results showed that introduction of chopped strand fibres into the syntactic foam system increased the flexural strength. The values increased with the amount of fibres in the foam system except for the one containing 3.5 vol.‐% of fibres, which showed a lower value than other fibre‐reinforced systems, thereby deviating from the trend. This exception was attributed to the difference in processing route adopted for this particular fibre‐bearing foam. However, in general, the incorporation of chopped strand fibres improved the flexural behaviour of the syntactic foam system without much variation in density, thus making reinforced syntactic foams to act as better and improved core materials for sandwich applications.

Fibre‐debonding and protuberance, and river pattern in a failed sample.     

Morphology of gas‐assisted and conventional injection molded polycarbonate/polyethylene blend

The skin‐core structure of the gas‐assisted and conventional injection molded polycarbonate (PC)/polyethylene (PE) blend was investigated. The results indicated that both the size and the shape of the dispersed PC phase depended not only on the nature of PC/PE blend and molding parameters, but also on its location in the parts. Although the gas‐assisted injection molding (GAIM) parts and conventional injection molding (CIM) part have the similar skin‐core structure, the morphology evolution of PC phase in the GAIM moldings and the CIM moldings showed completely different characteristics. In the section perpendicular to the melt flow direction, the morphology of the GAIM moldings included five layers, skin intermediate layer, subskin, core layer, core intermediate layer as well as gas channel intermediate layer, according to the degree of deformation. PC phase changed severely in the core layer of GAIM moldings, as well as in the subskin of CIM moldings. In GAIM parts, PC phase in the core layer of the nongate end changed far more intensely and aligned much orderly than that in the gate end. The morphology of PC phase in the GAIM part molded with higher gas pressure changed more severe than that in the GAIM part molded with lower gas pressure. In a word, PC phase showed more obvious fibrillation in the GAIM moldings than that in the CIM moldings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3069–3077, 2006     

The performance of adhesive-bonded thin-gauge sheet-metal structures with particular reference to box-section beams

In an effort to reduce vehicle weight, the automotive industry is developing car body structures made from light-weight materials such as composites, plastics and aluminium alloys. Fabrication of these materials using traditional welding techniques is not feasible and adhesive bonding is now being investigated as a potential assembly method. To assess performance characteristics of bonded vehicles, thin-gauge sheet-metal box-section beams have been used to simulate structural details in automotive applications such as car bodies and commercial vehicles. Beams were fabricated from flanged strips by different joining methods to form box-section structures approximately $\text{1}\text{m long}\text{× 60}\text{mm square}$. Tests were carried out to determine torsinal and flexural rigidity and ultimate torsional and flexural strengths, and in the majority of tests, bonded structures gave better characteristics than the equivalent riveted or spot-welded beams. The failures of beams under 3-point bending have been related to buckling of the side webs and further experimental tests have shown that collapse is critically dependent on flange-bends radius. Finite element techniques have veen used to analyse stress distribution in the beam section and this confirms the experimental observations of beam collapse.     

Phenolic foams modified by cardanol through bisphenol modification

In this study, cardanol, a natural phenol, has been applied to toughen phenolic foam by bisphenol modification. In order to verify the occurrence of Friedel–Craft alkylation between cardanol and phenol on the side chain, FTIR, and NMR had been used to characterize the bisphenol successfully. With the introduction of cardanol, the viscosity of prepolymers increased. The SEM results demonstrated that the some cells with increasingly large size existed, when the dosage of cardanol increased. With respect to the mechanical properties, phenolic foams modified by 10 wt % cardanol increased by 22% in flexural strength and 28% in bending modulus compared to pure phenolic foams, which indicates that the incorporation of cardanol does improve the toughness of phenolic foams. In addition, the effects of different dosage of cardanol on the apparent density and thermal stability of phenolic foams were investigated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39942.     

Compressive and flexural properties of functionally graded fly ash cenosphere–epoxy resin syntactic foams

The present study focuses on developing functionally graded syntactic foams (FGSFs) based on a layered co‐curing technique. The FGSFs were characterized for compressive and flexural properties and compared with plain syntactic foams. The results showed that the specific compressive modulus was 3–67% higher in FGSFs compared to plain syntactic foams. FGSF exhibited 5–34% and 34–87% higher specific modulus and strength, respectively in flexural mode. The microscopic examinations of comparative responses of the filler and matrix to deformation suggest that the failure is dominated by the matrix. The gradient in the composition of syntactic foams helps in effectively distributing the stress throughout the microstructure and results in improved mechanical performance of syntactic foams. From the microscopy studies, it is evident that, the failure mechanism in the FGSF under flexural loading is governed by a crack that initiated on the tensile side of the specimen and propagated through the thickness to cause complete fracture. The microscopic observations further clearly demonstrate the existence of seamless interfaces between the layers and a clear difference in the cenosphere concentration across the interface, affirming the gradation in the prepared samples. The results show that appropriate compositions of FGSFs can be selected to develop materials with improved mechanical performance. POLYM. COMPOS., 36:685–693, 2015. © 2014 Society of Plastics Engineers     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

Tensile stresses in the edges of injection moldings: Roles of packing pressure,machine compliance,and resin compression

The slow spontaneous development of cracks in the edges of injection moldings under “field” conditions has been observed for 30 years or more. While environmental stress cracking agents have long been implicated, the magnitude and distribution of the stresses associated with cracking have been obscure. The current study of these stresses involved polycarbonate as a model test material that was molded under systematically varied molding conditions. Surface tensile stresses, though rarely great enough alone to cause “dry” crazing or cracking were revealed through exposure to environmental stress crazing and cracking (ESC) agents. Using an old technique involving a set of calibrated ESC liquids, edge tensile stresses as great as 18 MPa were found in the edges of the moldings. Other, independent methods of stress assessment gave results in semiquantitative agreement with those of the ESC tests. Packing force, machine compliance, injection hold time, and mold flashing emerged as major variables either raising or mitigating stress levels. The root cause of the edge tensions is the mismatch in the times and pressures at which the skins and cores of moldings solidify. In short, skins quench at low pressure first, while cores solidify later during the packing stage. Upon release from the mold, elastic recovery of the core stretches the skin. More importantly, machine and mold compliances allow expansion of the part in the packing stage, during which certain areas of the skin are stretched. Solidifying the core during the packing preserves part of the skin extension as elastic strain. These effects are capable of outweighing the classical tendency of quenching to generate skin compression and core tension. A number of other effects, including release from the mold before the core has solidified, and flashing of the mold, have been found to limit the rise of skin tension.     

The effect of a polypropylene skin on the structure and properties of PE/PP dual layer pressure pipe

A new and growing family of polyethylene (PE)‐based pressure pipes have a polypropylene (PP) skin. The effect of the PP skin on the structure and properties of the core PE pipe was investigated by comparing the skinned pipe with an uncoated pipe made from the same PE material and with the same dimensions. The annealing effect introduced by the skin changed the PE core pipe density profile across the wall thickness, increasing density in the PE core pipe near to its outer surface. The density at the bore of the coated and the uncoated pipe was similar. The melting temperature and enthalpy of melting data from DSC agreed with the density profile results. The melting temperature of PE core pipe material close to the PP skin increased with increasing skin thickness. Residual stress assessment indicated that, as the PP skin thickness increased, the PE core pipe had a lower level of overall residual stress in the hoop direction. Long‐term hydrostatic strength (LTHS) tests were carried out and showed a higher strength for the coated pipe than the uncoated one. The observed structural changes have been used to explain the relative strength of these two PE pipes. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Chemical and physical blowing agents in structural polyurethane foams: Simulation and characterization

Physical blowing agents such as n‐pentane and methyl formate and, for comparison, chemical blowing agents such as water were used to prepare structural polyurethane rigid foams of different densities by reaction injection molding. Experimental runs were carried out with formulations based on oligomeric isocyanate and a mixture of polyether polyols. The constitutive equations for the vaporization rate of the two blowing agents and the polymerization kinetics data are reported. Experimental results were compared with the prediction of a simplified theoretical model, and they showed a satisfactory agreement in terms of temperatures and density profile. All the specimens were characterized by physical‐mechanical properties such as hardness, impact strength, flexural strength and elastic modulus and the results were reported in function of the densities. The best mechanical performance were obtained with the physical blowing agents, due to a better density distribution profile and a thicker skin layer.     

Mechanical properties of rigid thermoplastic foams—Part I: Experimental considerations

The through-thickness variation in the porosity of structural foam material is known to result in different “material properties” when mechanics based on homogeneous materials is used to interpret data from standard tensile and bend tests. Unresolved issues relating to structural design include the specification of the most useful test specimen, the identification of useful material properties, and the application of these properties to part design and analysis. This paper develops procedures for determining the mechanical properties of rigid thermoplastic structural foams, and for the application of these properties to the design of load-bearing components. Rather than molded specimens, it is suggested that specimens cut from large, edge-gated plates be used for determining mechanical properties of structural foams. By modeling foams as continuous but nonhomogeneous materials, it is shown that data from simple tensile and flexural tests can be used in structural analysis to systematically account for the through-thickness variation of material properties.     

Structure and properties of injection‐molded polypropylenes with different molecular weight distribution and tacticity characteristics

Two series of polypropylenes with different molecular weight distribution and tacticity characteristics were injection molded into flexural test specimens by varying cylinder temperature and the effects of the molecular weight distribution and tacticity on the structure and properties of the moldings were studied. Measured propertied were flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, and mold shrinkage and structures studied were crystallinity, the thickness of skin layer, a*‐axis‐oriented component fraction and crystalline orientation functions. The relations between the structures and properties were also studied. It was found that the molecular weight distribution and tacticity characteristics affect the properties mainly through the molecular orientation and crystallinity, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2142–2156, 2002