Stress Transmission During the Compaction of a Spray-Dried Alumina Powder in a Steel Die

Stress transmission through the compact, die wall friction, and the radial/axial stress ratio were investigated using a spray-dried alumina powder containing 2.3% polyvinyl alcohol binder, stored at 15, 55, or 92% rh, and an unlubricated/lubricated steel die for pressing. Stress transmission was enhanced by lubricating the die and decreasing the thickness/ diameter of the compact. Increasing the punch velocity in the range 0.006 to 0.6 cm/min improved the stress transmission for powder stored at the highest humidity, but only in the intermediate range of pressures. Radial stress at the highest pressures (>10⁷ Pa) was essentially independent of the properties of the binder phase and the radial/axial stress ratio was ∼0.4. At high pressures, the product of (die wall friction) (radial/axial stress ratio) decreased with lubrication of the die and storage of the granulated powder at a higher percent relative humidity (softer granules).     

Extrusion of highly unsaturated wet powders: Stress fields in extruder barrels

Wet glass bead powders with solid-to-liquid volume ratios between 2 and 3 were extruded through cylindrical dies by means of a ram extruder. The vertical stress exerted by the ram and the wall normal stress above the die were recorded. A continuum mechanical model was developed to calculate stress fields in the extruder barrel as a function of the load superimposed by the ram. The model predicts that the vertical stress, the wall normal stress and the wall shear stress decrease exponentially above the die entry region. A sudden increase in wall normal and wall shear stresses is observed where the powder in the die vicinity starts to be vertically accelerated. The stresses then decrease to zero from this point to the die which is in agreement with experimental observations. Calculated wall normal stresses are consistent with measured values.     

The importance of the amount/thickness of die wall lubricant for UO2 pellets pressing

External lubrication is often used to complete compaction process of powder materials. The main goal of this method is generally to reduce the amount of admixed internal lubricant (Zinc stearate powder) within the raw material. The application of external lubricants enhances the density uniformity and the mechanical strength of the resulting compacts. This study investigates the effects of the external lubricant amount for UO₂ powder compaction and the properties of the corresponding green pellets (corresponding to the compacts before sintering) without any admixed lubricant in the raw powder in order to evaluate the feasibility of this route in the case of nuclear powder. Results show that there is a quantity or number of layers from which the external lubricant applied on the die wall becomes detrimental to the friction index and the ejection force measured during the pressing cycle. The quality (surface defects, mechanical strength) of the green pellets can also be affected by the amount of lubricant. Thus the quantity and the thickness of the die wall lubricant must be optimized in order to assure an efficient mixed lubrication mode corresponding to the better lubrication mode in our study case.     

Modelling and numerical investigation of the residual stress state in a green metal powder body

Die pressing of metal powder results in a green body. After release from the die, the green body must have enough strength to be handled, to endure transport to a sintering furnace and heating to the sintering temperature. Drilling, turning and milling, which are common operations in the green state, require a green body of high strength, with no defects and excellent mechanical properties. A plane strain finite element model is used to analyse pressing of metal powder into a rectangular bar. The powder behavior is described by a “cap” model, which is implemented as a user material subroutine in the non-linear finite element program LS-DYNA. To improve modelling of strength in the green state a new non-linear density dependent failure envelope has been used. The model is adjusted to the properties of a water atomised metal powder from Höganäs AB. To resolve the severe stress gradient at the side surface of the green body, the smallest element size was chosen to be 65 μm. The aim of this work is to numerically capture and understand the development of the residual axial stress in particular at the side surface. The influence of kinematics, friction, compacting pressure and die taper are studied. Results from the numerical study show that the thickness of the compressive stress region close to the side surface of the green body varies between 50 μm and 600 μm along the surface. Compacting pressure, “upper punch hold down” and die taper geometry all have a significant influence on the residual stress state while die wall friction has only a small influence. The numerical results are in agreement with results from X-ray and neutron diffraction measurements.     

Ring Compression Test for High‐Temperature Glass Using the Generalized Navier Law

Glass‐forming processes such as high viscosity extrusion and Precision Glass Molding take place at temperatures where slip between the glass and mold/die surfaces is known to occur. Characterization of the frictional forces that accompany interface slip are essential in computational simulations of these processes for prediction of pressing time or force, distortion of the part and possible wear of expensive mold surfaces. In this study, the generalized Navier law, where the interface shear stress, τ, is related to the relative sliding speed of the glass on the mold surface, v_s, by τ = kv_s^e, is used in simulations of the ring compression test to produce friction calibration curves. Contrary to the nonlinear form of the Navier law, the linear form, just like the Coulomb friction model, leads to calibration curves that are loading rate and material behavior independent. This independence is achieved by normalizing the Navier friction coefficient with the viscosity. Friction calibration curves for the normalized coefficient are characterized for the full range of interface conditions from no‐slip to no friction. The results showed an approximate one‐to‐one correspondence between the normalized Navier coefficient and the Coulomb friction coefficient for the full range of axial deformation in the ring compression test.     

Effect of die temperature on the flow of polymer melts. Part II: Extrudate swell

The effect of die wall temperature on the extrudate swell of polymer melts flowing through dies with single and dual circular channels was studied. Extrudate swell was measured at constant flow rates using an Instron capillary rheometer with a modified die section. It was found that under isothermal conditions, extrudate swell plotted against the average wall shear stress gave rise to a temperature independent correlation for polystyrene. Under non-isothermal conditions, such a correlation did not exist, which might be due to the change of wall shear stress in the axial direction. The extrudate swell in the non-isothermal cases can be better correlated with the wall shear stress at die exit. For the two-hole die, changes of die wall temperature varied both the flow rate ratio and the extru date swell ratio. The latter is, however, much less sensitive to the die wall temperature than the former.     

What is the role of “pressure” in the use of capillary and slit flows to determine the shear‐rate dependent viscosity of a viscoelastic fluid?

The purpose of this review article is to clear the confusion created by some investigators, who erroneously thought that the pressure transducers mounted on the wall of a capillary or slit die measured a quantity that could meaningfully be called “pressure,” accurately stated “indeterminate isotropic contribution to the total stress,” and then reported on the effect of “pressure” on the shear‐rate dependent viscosity of a viscoelastic fluid. On the other hand, reference to such a quantity is not needed to calculate the wall shear stress and thus shear viscosity in fully developed flow of incompressible, viscoelastic polymer melts in a capillary or slit die; instead only information on the gradient of the total wall normal stress is needed. Further, it is pointed out that much of the literature discussing “pressure shift factor” to describe the effect of “pressure” on the viscosity of polymer melts in flow through a capillary or slit die is based on an erroneous belief that there exists a physically meaningful isotropic “pressure” that can be measured. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Rheological behavior of coir‐fiber‐filled polypropylene composites at constant shear stress

The rheological behavior of coir‐fiber‐filled polypropylene (PP) composite has been studied at constant shear stress. The shear stress versus shear rate relationship for the composite follows power law model of viscous flow. Unlike similar studies in the literature, the viscosity is treated as a stress‐independent parameter, which increases with the increase of fiber loading; but decreases with the rise of temperature. The SEM reveals that the fibers are loosely bound to the polymer matrix and the outer surface of the composite is rough and irregular, making it susceptible to high friction with the wall of the flow channel. With analogy to nth order chemical reaction, new formula has been derived for the activation energy of viscous flow, which is found to increase with the increase in the fiber content. The die‐swell ratio decreases with the increase of fiber loading, but increases with the rise of temperature. The elastics parameters of the composite such as the recoverable shear strain, the first normal stress difference, and the elastic strain induced by the stored energy in the capillary reservoir have been estimated based on the die‐swell data. POLYM. COMPOS., 36:51–61, 2015. © 2014 Society of Plastics Engineers     

新型耐火材料试条模具的研究

半干法压制成型是最常见的耐火材料制品成型工艺之一。在压制过程中,由于物料脊性、散松、没有足够的水分和结合剂,必须施以较大的压力,借助于压力的作用使坯料的颗粒重新分布,在机械结合力、静电引力以及摩擦力的作用下,坯料颗粒紧密结合,排出空气,在模具的型腔内形成预定尺寸和形状并具有一定强度的制品。 通常在其它工艺条件(如物料品种、颗粒级配、水分含量和结合剂等)相同时,压制压力增高,坯体的气孔率下降,密度增大,强度随之提高。显然,压力与气孔率、密度之间存在密切的联系。     

Flowability parameters for chopped switchgrass, wheat straw and corn stover

A direct shear cell to measure the shear strength and flow properties of chopped switchgrass, wheat straw, and corn stover was designed, fabricated, and tested. Yield loci (r² = 0.99) determined at preconsolidation pressures of 3.80 kPa and 5.02 kPa indicated that chopped biomass followed Mohr-Coulomb failure. Normal stress significantly affected the displacement required for shear failure, as well as the friction coefficient values for all three chopped biomass types. Displacement at shear failure ranged from 30 to 80 mm, and depended on preconsolidation pressure, normal stress, and particle size. Friction coefficient was inversely related to normal stress, and was highest for chopped corn stover. Also, chopped corn stover exhibited the highest angle of internal friction, unconfined yield strength, major consolidation strength, and cohesive strength, all of which indicated increased challenges in handling chopped corn stover. The measured angle of internal friction and cohesive strength indicated that chopped biomass cannot be handled by gravity alone. The measured angle of internal friction and cohesive strength were 43° and 0.75 kPa for chopped switchgrass; 44° and 0.49 kPa for chopped wheat straw; and 48° and 0.82 kPa for chopped corn stover. Unconfined yield strength and major consolidation strength used for characterization of bulk flow materials and design of hopper dimensions were 3.4 and 10.4 kPa for chopped switchgrass; 2.3 and 9.6 kPa for chopped wheat straw and 4.2 and 11.8 kPa for chopped corn stover. These results are useful for the development of efficient handling, storage, and transportation systems for biomass in biorefineries.     

Flaring dies to suppress die drool

Die lip build‐up is the unwanted material accumulation on extrusion die lips. Here, flared dies are shown experimentally to suppress die lip build‐up. A semiempirical method for flared die design is also provided. Nonlinear viscoelastic constitutive equations are used to calculate the wall shear stress and first normal stress difference in flared dies. By incorporating melt memory, a promising design method for die flaring is presented. The stress history upstream of the die exit governs the die design. The upstream gap is selected to maximize undershoot of the first normal stress difference N₁ at the die wall caused by flaring. The flare length, on the other hand, is selected to ensure a steady N₁ at the die lips.     

Total applied shear in capillary extrusion

Viscoelastic properties of polymer fluids are single-valued functions of shear stress or shear rate only at high total applied shear. These parameters may vary with applied shear under milder shear histories. The mean total shear in capillary extrusion is shown to be a function of orifice geometry. Apparent flow curves can be measured at various total shear values by changing the length/radius ratio of the capillary. The true shear stress and true shear rate at the orifice wall correspond to infinite total shear conditions. The true flow curve and elastic parameters like die swell are not measured at equivalent total shear unless the capillary is extremely long.     

Measurement of static stresses on the wall of a cylindrical container for particulate solids

Measurements of the stresses acting on the base and wall of a cylindrical container were made in a 100-mm dia. steel tube containing a granular material before downward flow of the charge had commenced. The load cells used were designed for this purpose and have high sensivity and thermal stability, permitting the measurement of very small forces.

The results show that the ratio of the normal stresses on a horizontal and a vertical plane at any point in the container is substantially constant and has a value that can be predicted by a formula derived in the paper.

Drawing the Mohr circles for stress conditions in the material in contact with the wall suggests that friction at the wall is fully mobilised, the coefficient of wall friction varying slightly with depth.

The normal stress on the base does not increase as an exponential function of the bed depth and does not appear to approach a limiting value.

A small shear stress on the base of the container was detected, but friction was very far from being fully mobilised.

The results showed that deformation of the base of the container due to the weight of the charge may cause a large increase in the normal stress on the wall.     

Evaluation of the shear characteristics of steel–asphalt interface by a direct shear test method

Shear characteristics of steel–asphalt interface under the influences of temperature, normal stress level and tack coat material were investigated. The direct shear tests were conducted on composite specimens with epoxy asphalt (EA) and polymer modified asphalt (PMA) tack coat materials at temperatures of 25 and 60 °C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. Results show that the failure modes include adhesive failure at the primer-tack coat interface and material failure of asphalt concrete. Steel–asphalt interface shows strain softening behavior until it reaches the sliding state. The shear strength and the shear reaction modulus increase with decreasing temperature and increasing normal stress levels. The specimens with EA tack coat provides much higher interface shear strengths than those with PMA tack coat at 25 and 60 °C. In addition, the failure envelopes of the shear strength and residual shear strength were obtained for combinations of tack coat materials and temperature conditions based on the Coulomb failure law.     

The synergistic effect of boron nitride particle and die on the capillary extrusion processability of mLDPE

The synergistic effects of boron nitride (BN) powder and die on the rheology and processability of metallocene‐catalyzed low density polyethylene (mLDPE) were investigated. The processability in the extrusion process is closely related to the interfacial properties between the polymer melts and the die wall. BN powder was added to mLDPE to reduce the friction coefficient and surface energy. Adding 0.5 wt% BN powder to mLDPE was very effective in improving the processability and the extrudate appearance. To study the effect of die surface property, three different dies were applied in capillary extrusion. One was conventional tungsten carbide (TC) die, and the others were hot‐pressed BN (hpBN) die and hot‐pressed BN composite (hpBNC) die. The applications of these BN dies were quite effective in delaying surface melt fracture (sharkskin) and postponing gross melt fracture to higher shear rate compared to the TC die. These improvements result from the fact that BN dies reduce the wall shear stress significantly and promote slip. The synergistic effect of processability could be obtained when both BN powder and hpBN die were used together.     

Experiments and flow analysis of a micropelletizing die

A study has been performed to examine the process of micropelletization on four different polyethylenes with melt index values between 1 and 5 g/10 min. The experiments were done on a 50‐mm 30:1 L/D extruder with an underwater micropelletizer attached. The average micropellet size that was produced ranged from 0.4 to 0.5 mm in diameter depending on whether a plastomer or high‐density grade was selected. The dimensions of the pellets were influenced strongly by the occurrence of die‐hole freeze‐off. Minor sharkskin was observed on the surface of the micropellets, a result of the high stresses experienced in the pelletizer die. A non‐isothermal, axisymmetric flow model was used to assist in the analysis by comparing the observed results to the predicted shear stresses in the die. The calculations revealed that extremely high shear rates were present in the die holes, resulting in a significant degree of wall slip. The measured rheological properties of the micropellets did not show any change in comparison to their virgin resins, likely because of the presence of wall slippage and the short residence time of the polymer in the die holes. Polym. Eng. Sci. 44:1391–1402, 2004. © 2004 Society of Plastics Engineers.     

Interfacial Bonding and Friction in Silicon Carbide [Filament]-Reinforced Ceramic- and Glass-Matrix Composites

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC-filament-reinforced reaction-bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross-ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite-thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear-lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (T _d ), and sliding friction stress (T _f ), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear-lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ₀). The cross-ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.     

Characterization of shear stress at the tool‐part interface during autoclave processing of prepreg composites

In this article, shear stress between an aluminum tool and a carbon fiber‐epoxy prepreg is characterized during cure using polymeric release agent and release film at the tool‐part interface. The effects of surface roughness, release materials, pull‐out speed, temperature, and normal force (autoclave pressure) on the shear stress are investigated using a customized friction rig. Results show that the interfacial shear stress decreases as the temperature increases and it increases as the normal force increases when using either the release film or the release agent. Additionally, changes in surface roughness from 1.35 to 0.18 μm decrease the shear stress 10–27% while the use of release agent shows a decrease between 23% and 51% in the shear stress. Furthermore, strong adhesion between the tool and the part is observed when using release agent and pull‐out speeds of 0.05 mm/min (static/dynamic friction ratio of 5.29 ± 0.19). Using the experimental data, a mathematical approach based on the Coulomb's friction model is proposed to predict the friction force at the tool‐part interface. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Industrial Quantitative Control of the Pressing Behavior of Spray-Dried MnZn-Ferrite Granules

In the first stage of a broad factory stabilization program, the compaction properties of spray-dried granulated powders are investigated. Certain compaction process parameters are identified and include (i) the slide coefficient between powder material and die wall during compaction, which mainly affects the pressing tool life time, (ii) the ratio of the pressure drop over the compact to the axial mechanical strength of the compact, basically related to the chance of chipping and crack occurrence upon ejection, and (iii) the homogeneity of the density profile along the compact, basically related to the chance of crack development and product deformation during sintering. The effect of important factors, such as binder and lubricant content, granulate storage humidity, and compaction temperature, on those parameters is investigated. Based on factory data, specification values for the previously mentioned parameters are derived and used to control the compaction quality of the industrially spray-dried powders with satisfactory results.     

平纹编织C/SiC复合材料的失效判据

通过Arcan夹具拉伸实验,研究了平纹编织C/SiC复合材料的唯象强度特性,根据实验结果提出了一种椭圆形失效判据,可以反映拉压强度的差异,以及压缩和剪切同时作用时裂纹面闭合引起最大容许剪应力的回升.结果表明：椭圆形判据和实验结果在第一象限吻合良好,说明椭圆判据在第一象限内对平纹编织C/SiC是适用的.用线弹性有限元方法分析了Arcan拉伸实验中试样的应力状态和分布,并讨论了夹具刚度的影响.分析表明：试样缺口区域剪切应力分布均匀,大小接近平均剪应力;正应力在边缘有应力集中,中央应力约为平均正应力的77%.由于纤维增强陶瓷基复合材料存在微弱缺口敏感性,Arcan圆盘测出的强度略低于均匀拉伸结果.采用系数对测出的强度进行了修正.