Processing enhancers for rotational molding of polyethylene

Rotational molding is a zero shear process used to manufacture hollow plastic parts. One disadvantage of this process is long cycle times, which are significantly affected by the sintering rates of thermoplastic powder. The objective of this work was to evaluate low molecular weight additives as sintering enhancers for polyethylene and to validate the results in rotational molding. The following additives were blended with linear low‐density polyethylene: mineral oil, glycerol monostearate and pentaerythritol monooleate. The additives resulted in decreased melt viscosity and/or elasticity at low shear rate. The reduction in melt elasticity was particularly significant. Sintering studies confirmed that the additives resulted in significantly faster coalescence. In uniaxial rotational molding, the decreased melt viscosity and elasticity obtained with mineral oil were observed to result in much faster densification and bubble removal. Part thickness was uniform and there was no warpage. Adding mineral oil to polyethylene reduced the cycle time in uniaxial rotational molding and the peak impact strength was identical to that obtained without any additive. Biaxial rotational molding experiments confirmed that the use of mineral oil resulted in shorter cycle time without sacrificing peak impact strength.     

3D model for powder compact densification in rotational molding

During rotational molding, a loosely packed, low‐density powder compact transforms into a fully densified polymer part. This transformation is a consequence of particles sintering. Powder compact density evolution of the polymer powder is measured experimentally. Obtained results show that the powder densification process consists of two stages, and its mechanism during these two stages is not the same. During the first stage, densification occurs by grains coalescence, and air between the grains escape by open pores between particles. These open pores close in time by particles coalescence progress, and remaining air entrapped in polymer melt becomes air bubbles. Surface tension, viscosity, grains size, and temperature are the controlling parameters during first stage. A three‐dimensional model is proposed for the densification of polymer powder during first stage. Second stage starts after bubble forming. Diffusion is the controlling phenomena during this stage. A diffusion‐based model is used for the second stage of densification. By comparing with the other models, proposed model exhibits several advantages: it is proposed in three‐dimensional and takes into account the nature of layer‐by‐layer powder densification. Model verification by experimental data obtained for densification of two different polymers shows a close agreement between model prediction and experiments. POLYM. ENG. SCI., 52:2033–2040, 2012. © 2012 Society of Plastics Engineers     

Powder‐shape analysis and sintering behavior of high‐density polyethylene powders for rotational molding

The quality of rotational molded products is strongly affected by the sintering behavior of the powders used in the process. In turn, for a given material, the sintering behavior of polymer powders is dependent on the size and the shape of particles obtained in the milling apparatus. The quality of powders for rotational molding is usually determined by means of size distribution, dry flow, and bulk density tests. However, these tests do not provide insight into the relationship between the shape of powders, the milling conditions, and the sintering behavior during the rotational molding cycle. Nevertheless, the application of mathematical tools to powder analysis can significantly improve the efficiency of the grinding process, looking not only at the size but also at the shape of the powder. This can in turn result in a higher reliability of rotational molding and in better performances of the products obtained in processes dominated by the sintering behavior of polymer powders. In this work the grinding process of recycled high‐density polyethylene was analyzed using a quantitative approach to the shape and size of the powders. In particular, shape factors, capable of characterizing powders obtained in different milling conditions, were studied. Finally, the influence of the powders' shape and size on sintering behavior was studied by thermomechanical analysis. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 449–460, 2004     

Principles of rotational molding

Rotational molding is a process by which powdered or liquid plastics are converted into hollow articles. This paper is devoted to the theoretical understanding of the process of rotational molding. There are seven sections: The Art, wherein we describe the process, discuss previous attempts at understanding the process, and mention processes that are similar in principle to rotational molding. Transient Heating of Mold Surface, wherein we show that the criterion for selection of mold materials is the ratio of the thermal diffusivity to the thermal conductivity, and present the heating curve for a mold in a rotational mold machine. Melting of Plastic Powder in a Rotating System, wherein we discuss in detail those physical powder characteristics that are necessary for good flow within the mold cavity. Fluid Flow During Rotational Molding, wherein we discuss the velocity profiles within the melt film, point out that there is very little bulk polymer flow possible within the mold cavity under normal processing conditions, and consider capillary flow forces and surface wetting. Sinter-Melting, wherein we compare the Kuczynski-Neuville empirical sintering model with the Lontz viscoelastic model, conclude that the latter is correct for the sintering of materials such as ABS, and apply the Frenkel glass densification theory to the prediction of void disappearance in sinter-melt polymers. Degradation, wherein we compare our experimental tensile strengths of polystyrene, obtained at varying oven cycle times and oven set point temperatures, with values obtained from degradation models given in the literature. Laboratory Simulation of Rotational Molding, wherein we propose two series of experiments, the first series being carried out without using rotational molding equipment, and the second using rotational molding equipment with molds having relatively simple geometries.     

The rotational molding of a thermotropic liquid crystalline polymer

Thermotropic liquid crystalline polymers (TLCPs) exhibit a number of mechanical and physical properties such as excellent chemical resistance, low permeability, low coefficient of thermal expansion, high tensile strength and modulus, and good impact resistance, which make them desirable as a rotationally molded storage vessel. However, there are no reports in the technical literature of the successful rotational molding of TLCPs. In this article, conditions are identified that lead to the successful rotational molding of a TLCP, Vectra B 950. First, a technique was developed to produce particles suitable for rotational molding because TLCPs cannot be ground into a free‐flowing powder. Second, because the viscosity at low shear rates can be detrimental to the sintering process, coalescence experiments with isolated particles were carried out to determine the thermal and environmental conditions at which sintering should occur. These conditions were then applied to static sintering experiments to determine whether coalescence and densification of the bulk powder would occur. Finally, the powders were successfully rotationally molded into tubular structures in a single axis, lab‐scale device. The density of the molded structure was essentially equivalent to the material density and the tensile strength and modulus were approximately 18 MPa and 2 GPa, respectively. POLYM. ENG. SCI., 45:410–423, 2005. © 2005 Society of Plastics Engineers     

Experimental study on the flow and deposition of powder particles in rotational molding

The nature of powder flow and its effect on particle deposition in rotationally molded parts were considered in this work. Experiments were carried out to observe the effects of various parameters, such as particle characteristics and operating conditions, on the deposition patterns of polyethylene powders and micropellets. The results indicate that the polymeric powders were cohesive enough to prevent size segregation at ambient temperature; however, segregation occurred when particles that had a smooth surface and regular shape were used. During processing, however, a new phenomenon of reverse cohesive segregation was observed. The results showed that the final deposition patterns are controlled primarily by the initial segregation patterns, as well as by the heating rate and rotation speed, which affect the evolution of adhesive forces between particles during heating and melt deposition process. An order‐of‐magnitude analysis was conducted to evaluate the development of cohesive forces between particles, and to estimate their effects on the movement of particles. This study provides a better understanding of the flow characteristics of polymer particles during the rotational molding process, which is very important in the development of techniques for fabricating composites and multilayered products. Polym. Eng. Sci. 45:62–73, 2005. © 2004 Society of Plastics Engineers.     

Bubble removal in rotational molding

Closed form solutions have been obtained for bubble dissolution in typical polymer melts encountered in rotational molding. The solutions are in excellent agreement with experimental data available in the literature. Using these solutions, it is shown that under typical rotational molding conditions the polymer melts may be almost saturated. As a result, bubble shrinkage occurs over long periods. Depending on the degree of saturation, surface tension may contribute substantially to the concentration gradient that drives bubble shrinkage. It is also shown that a pressure increase imposed on a nearly saturated polymer melt leads to a steep concentration gradient at the bubble/melt interface that can cause extremely fast bubble shrinkage. Applied to the rotational molding process, such a pressure increase can result in substantial cycle‐time shortening through elimination (or reduction) of the currently used excessive heating. A further benefit may be that additional resins, which at present cannot be used because of oxidation at sustained high‐temperatures, can become available to the rotational molding industry. Under the under‐saturated conditions created by a pressure increase, the effect of surface tension on the rate of bubble shrinkage is negligible.     

Investigation of melt manipulation phenomena during injection molding via in situ birefringence observation

This study investigates the effects of melt manipulation on the development of molecular orientation during injection molding processing. Vibration‐assisted injection molding (VAIM), a particular method of melt manipulation, is a variation of conventional injection molding in which oscillatory energy is imparted to the polymer melt by vibrating the injection screw axially during the injection and packing stages of the molding cycle. Previous studies have shown that this process positively affects the tensile strength of polystyrene parts, but that the magnitude of the increase is dependent upon the processing parameters. Observation of birefringence patterns in VAIM processed samples show a significant impact on molecular orientation. A specially designed mold and associated image capture system has been developed and is used in this study to record the birefringence patterns of the polymer melt within the cavity during processing. Observation of birefringence shows that orientation develops primarily during post‐vibration packing of the part and not during the vibration phase as previously thought. The observed effects of process parameters such as melt temperature, packing pressure, and vibration duration are discussed. POLYM. ENG. SCI. 46:1691–1697, 2006. © 2006 Society of Plastics Engineers     

Rheological and dynamic mechanical characteristics of rotationally moldable linear low‐density polyethylene fumed silica nanocomposites

In rotational molding process, polymer powders undergo a cycle of heating, melting, cooling, and subsequent solidification in the mold. Resins, like linear low‐density polyethylene (LLDPE), are used in this process on a large scale mainly because of its good mechanical properties and excellent thermal stability. Yet, incorporation of additives is necessary to further improve the visco‐elastic, thermal as well as melt flow properties of the resin. This study investigates the effects of nanocomposites of fumed silica (FS) with rotationally moldable LLDPE. Thermal transitions in the LLDPE‐FS nanocomposites were investigated and correlated with their melt flow characteristics. The effect on melt processing during rotational molding and compounding, were analyzed by melt flow index and torque rheometry studies. A suitable blend of FS in LLDPE has been recommended for rotational molding based on rheological studies and dynamic mechanical analysis. POLYM. COMPOS., 37:2995–3002, 2016. © 2015 Society of Plastics Engineers     

Design and fabrication of an affordable polymer micromixer for medical and biomedical applications

In this article, an affordable split‐and‐recombine polymer static micromixer and its fabrication process are described. The structure of the micromixer was designed to take advantage of the process capabilities of both ultraprecision micromachining and microinjection molding. This arrangement allows a considerably short machining time, high productivity, and flexibility as compared with many of the previous fabrication processes used in manufacturing of micromixers, for example, the cleanroom process. Moreover, in recent years, microinjection molding has become a cost effective mass production method for polymeric components with micro and nano scale features. Specifically, in this study four dominating processing parameters in microinjection molding, including melt temperature, injection velocity, packing pressure, and packing time were selected as variables to investigate their effects on replication quality. Both experimental and numerical results showed that the packing pressure and packing time played an important role on replication, followed by melt temperature while the injection velocity seemed to have no influence on replication. Finally, experiments using two colored water solutions were conducted to evaluate the device performance, and the mixing result demonstrated the successful fabrication and performance of the injection molded polymer micromixer. POLYM. ENG. SCI., 50:1594–1604, 2010. © 2010 Society of Plastics Engineers     

Characterization of polymer powder motion in a spherical mold in biaxial rotation

As rotational molding of thermoplastic parts become more and more complex, understanding the fundamental processing aspects such as powder flow becomes important for good and uniform part quality. In this work, an image analysis technique is used to determine the effect of polymer powder particle size and distribution in a biaxially rotating spherical mold. Experimental parameters such as particle diameter (d), ratio of larger over smaller particles diameters (d_r), and initial powder position inside the mold and camera viewing position were studied. From the results obtained, conditions leading to homogeneity and segregation are discussed. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Real‐time diagnosing polymer processing in injection molding using ultrasound

Ultrasonic diagnosing technique with a new high‐temperature ultrasonic transducer is developed to real‐time diagnose polymer processing and its morphology changes in injection molding processing. Compared with the previous researches, the new technique can provide more and accurate information. In this study, ultrasound diagnosis shows that longitudinal wave can real‐time characterize the data of the injection process and polymer morphology changes, including melt flow arrival time, the part ejection time, filling and packing stages, polymer solidification process, and the morphology changes during polymer crystallization. Shear waves can real‐time diagnose Young's and shear storage modulus, anisotropy property of polymer in injection molding. During our research, real‐time ultrasonic diagnosis shows that the storage modulus along the vertical direction is larger than that of the parallel to the melt flow direction under our setup injection conditions. Scanning electron microscopy and dynamic mechanical analysis measurements present that it is because the crystalline lamellas of HDPE are parallel arrangement and grow in a vertical to melt flow direction owing to injection shear force under a certain injection conditions. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

汽车接插件随形水路设计及工艺优化

根据汽车接插件的模具规格和使用要求,基于Moldflow软件对汽车接插件的成型过程进行数值模拟,设计冷却系统和流道系统,针对冷却系统对热流道区域冷却不足的问题进行优化,增加随形水路。根据Taguchi正交实验分析了熔体温度、模具温度、注塑时间、保压时间、保压压力5个因素对产品框口区域翘曲变形的影响规律,得到最佳工艺参数组合,最后对最佳参数组合进行了模拟验证。实际生产表明,该方案可以缩短产品的开发周期与成型周期,提高生产效率。     

Automated Design for the Runner System of Injection Molds Based On Packing Simulation

Multiple injection cavities are automatically balanced by adjusting runner and gate sizes based on an iterative redesign methodology integrated with computer-aided engineering (CAE) packing simulation. For runner balancing, each cavity must be filled simultaneously at uniform pressure. In addition, the time-pressure history of the polymer melt over the entire molding cycle should be considered. Based on the proposed methodology, a multicavity mold with identical cavities is balanced to minimize entrance pressure differences among various cavities at discrete time steps of the molding cycle. The results have shown more than a 95% reduction of the entrance pressure differences over other related studies, and also have demonstrated increased searching performance over other optimization techniques. A family mold with different cavity volumes and geometries is also balanced to minimize pressure differences at the end of the melt flow path in each cavity on a basis of discrete time steps of the molding cycle. The methodology has shown uniform pressure distributions among the cavities during the entire molding cycle.     

Motion control for uniaxial rotational molding

Motion control parameters of rotational molding can affect process efficiency and product quality. Different motion control schemes will lead to varied powder flow regimes exhibiting different levels of mixing and temperature uniformity. The change in nature of powder flow during a molding cycle suggests that varying the rotational speed could improve the powder mixing and temperature uniformity, therefore potentially reducing processing time and energy consumption. Experiments completed investigating powder flow under uniaxial rotation show that savings of up to 2.5% of the heating cycle time can be achieved. This validates the hypothesis that altering the rotational speed to maintain the ideal powder flow throughout the heating cycle can be utilized to reduce the time taken for all the polymer powder to adhere to the mold wall. The effect of rotational speed on wall thickness uniformity and impact strength were investigated and discussed. Results show a strong influence of rotational speed (and powder flow) on the wall thickness uniformity of the moldings with wall thickness uniformity deviations of up to 50% found (within the 2–35 RPM speed range tested).     

Experimental investigation and molecular dynamics simulations of the effect of processing parameters on the filling quality of injection-molded micropillars

Microinjection molding has been attracting increasing attention and application in fabricating products with functional surface microstructures. The processing parameters, packing pressure, and melt temperature have important effects on the filling quality. In order to study the mechanisms of the packing pressure and melt temperature on the filling quality of micropillars, a simulation model of injection molding of nanopillars was constructed by molecular dynamics software and a series of injection molding experiments of micropillars were carried out in this paper. Subsequently, the mechanisms were analyzed qualitatively. The results showed that the frozen layers were formed at the interface between the polymer melt and mold under the action of heat transfer, which prevented effective filling of the polymer melt. The filling quality of the micropillars could be improved significantly via increasing the melt temperature and the packing pressure, but the mechanisms were different. To be specific, the increase of the packing pressure could make more polymer melts fill into the cavity fully. Thus, the density of the micropillars was increased and the filling quality could be improved. The forming rate of frozen layers could be slowed down by increasing the melt temperature. As a result, the purpose of improving the filling quality was achieved.     

颗粒堆积现象的计算机模拟

颗粒堆积现象的计算机模拟是为能经济地、有铲地分析和优化不同材料如陶瓷、水泥、医药、粉末冶金、聚合物等的性能，并为设计先进材料朝代一种行之有效的手段，近15年来该领域研究的评述表明颗粒堆积现象模拟，尤其在动态过程方面已取得了一些进展。有些计算机模拟软件已应用于模拟如陶瓷和混凝土的颗粒堆积过程，离散元方法也成功地被应用于模拟过程中。另外，为了简化现有的计算机模拟过程，有关专家用一维Monte Carlo方法来取代复杂的三维方法，同时讨论了现有的颗粒堆积过程模拟方法所存在的问题和局限性。     

Characterization of Particle Packing in an Injected Molded Green Body

The particle packing structure of alumina green bodies made by injection molding has been examined with the liquid immersion technique coupled with a polarized light microscope. In this method, the specimen is made transparent by an immersion liquid and is examined in the transmission mode. Fine particles of near-equiaxed alumina powder are found to be aligned, with their slightly elongated axis parallel to the flow direction of the molding process. The presence of very large alumina particles was also noted. Their concentraton was extremely low and can be detected only by the present method. The origins and significance of particle orientaton and large particles in ceramic processing are discussed.     

Rotational molding of plastics: Comparison of simulation and experimental results for an axisymmetric mold

We present a validation study comparing the mold wall temperature estimates from an axisymmetric thermal finite element simulation of the rotational molding process with temperature data obtained during experiments conducted on industrial‐grade rotational molding equipment. The finite element model simulates the heat transfer processes involved in rotational molding through the end of powder deposition, and uses an Arbitrary Lagrangian Eulerian technique to track the growing molten plastic layer. The experiments were conducted with nine different operating conditions on a single axisymmetric mold shape. The simulation results for mold wall temperature agreed well with the experimental data under all of the conditions tested.