Vacuum‐assisted resin transfer molding using tackified fiber preforms

A low cost composite fabrication process—tackified SCRIMP—is described for fabricating aerospace‐grade composites based on tackification and vacuum‐assisted resin transfer molding (VARTM). Tackification based on a commercial tackifier (FT 500 from 3M) was used to make the net‐shape fiber preform. It was found that tackifier concentration and application conditions play important roles in governing the moldability of tackified fiber preforms. An epoxy resin (PR 500 from 3M) was used in the VARTM process‐SCRIMP at high temperatures. Experimental results show that composites with high fiber content (> 60% by volume) can be manufactured at low cost using tackification. Effects of tackification methods on composite dimension control, void content and mechanical properties were investigated and compared in both RTM and SCRIMP.     

Resin transfer molding of BMIs and polymides

Bismaleimdes (BMI) and experimental polyimides were resin transfer molded into carbon fiber fabrics using a custom-built injection mold placed within a vacuum hot press. This represents the first time that the latter type of materials has been resin transfer molded. This is a critical stage in developing materials and processing methods for future aerospace applications, such as the High Speed Civil Transport (HSCT).     

Resin transfer molding of natural fiber reinforced polybenzoxazine composities

Kenaf fiber is incorporated in a polybenzoxazine (PBZX) resin matrix to form a unidirectionally reinforced composite containing 20 wt% fiber by a resin transfer molding technique. Two types of benzoxazine monomer are synthesized and used as resin mixtures: Benzozazines based on bisphenol‐A/aniline (BA‐a) and phenol/aniline (Ph‐a). The effects of varying BA‐a:Ph‐a ratio in the resin mixture and curing conditions on mechanical properties of pure PBZX resin and kenaf/PBZX composites are studies. The Flexural strength of the pure PBZX resin increases with increasing ratio of BA‐a:Ph‐a, curing temperature and curing time, but the impact strength increases only slightly. PBZX resin has lower water absorption and higher flexural modulus, when compared with unsaturated polyester (UPE) resin. PBZX composites with 20 wt% fiber content have lower flexural and impact strengths, but higher moduli compared with UPE composites with the same fiber content.     

Resin transfer molding (RTM) with toughened cyanate ester resin systems

This investigation explored the feasibility of recently developed toughened cyanate ester networks as candidate materials for high performance composite matrix applications. The resin investigated was a bisphenol-A cyanate ester toughened with hydroxy functionalized phenolphthalein based amorphous poly(arylene ether sulfone). A series of four toughened cyanate ester resins were generated by varying the concentration and the molecular weight of the toughener. The thermoplastic modified toughened networks exhibited improvement in the fracture toughness over the base cyanate ester networks without significant reductions in mechanical properties or glass transition temperature. Carbon fabric composite panels were manufactured by liquid molding processes (resin transfer molding and resin film infusion) with the untoughened and toughened cyanate ester resin systems. The panels were subjected to physical, impact damage, and fracture toughness tests. The results of physical testing indicate consistently uniform quality, and the maximum void content was found to be less than 2%. The toughened cyanate ester composites exhibited significantly improved impact damage resistance and tolerance compared with hot-melt epoxy systems. A marked increase in the mode II composite fracture toughness was observed with an increase in the concentration and the molecular weight of the toughener.     

Numerical simulation of unsaturated flow in woven fiber preforms during the resin transfer molding process

In this paper, the unsaturated flow encountered in the woven or stitched fiber mats used in RTM is simulated using an adaptation of the Finite Element Method/Control Volume (FEM/CV) technique. The movement of resin through such fiber mats is modeled as flow through dual scale porous media and the mass balance in such media creates a sink term in the equation of continuity of the macroscopic flows. Combining this equation with Darcy's law leads to a non-homogeneous non-linear elliptic partial differential equation for pressure that is solved iteratively. First the simulation is used to study simple flows encountered during the characterization of preforms, such as the constant injection pressure 1-D flow and the constant flow rate radial injection flow. Previously observed experimental results of relatively flatter pressure histories for the latter type of flows in wove fiber mats are replicated, both numerically and analytically, by the pressure equation with the sink term. A quantity called pore volume ratio is shown to play an important role in such flows. Finally, the unsaturated flow in a typical RTM mold, packed with woven fiber mats, is simulated numerically, and inlet pressures, fill times, and mat saturation are studied.     

Resin transfer molding process optimization using numerical simulation and design of experiments approach

The art of resin transfer molding (RTM) process optimization requires a clear understanding of how the process performance is affected by variations in some important process parameters. In this paper, maximum pressure and mold filling time of the RTM process are considered as characteristics of the process performance to evaluate the process design. The five process parameters taken into consideration are flow rate, fiber volume fraction, number of gates, gate location, and number of vents. An integrated methodology was proposed to investigate the effects of process prameters on maximum pressure and mold filling time and to find the optimum processing conditions. The method combines numerical simulation and design of experiments (DOE) approach and is applied to process design for a cylindrical composite part. Using RTM simulation, a series of numerical experiments were conducted to predict maximum pressure and mold filling time of the RTM process. A half‐fractional factorial design was conducted to identify the significant factors in the RTM process. Furthermore, the empirical models and sensitivity coefficients for maximum pressure and mold filling time were developed. Comparatively close agreements were found among the empirical approximations, numerical simulations, and actual experiments. These results were further utilized to find the optimal processing conditions for the example part.     

Resin infusion of triaxially braided preforms with through‐the‐thickness reinforcement

Vacuum assisted resin transfer molding (VARTM) has shown potential to significantly reduce the manufacturing cost of high‐performance aerospace composite structures. In this investigation, high fiber volume fraction, triaxially braided preforms with through‐the‐thickness stitching were successfully resin infiltrated by the VARTM process. The preforms, resin infiltrated with three different resin systems, produced cured composites that were fully wet‐out and void free. A three‐dimensional finite element model was used to simulate resin infusion into the preforms. The predicted flow patterns agreed well with the flow patterns observed during the infiltration process. The total infiltration times calculated using the model compared well with the measured times.     

Durability of powder-coated hot-dip galvanized steel

In this study, the quality and durability of powder-coated hot-dip galvanized products have been investigated. Standard steel coupons hot-dip galvanized in conventional and delta (i.e., high temperature) conditions were coated with powder paint systems or a high quality solvent-based system and then subjected to a wide range of test methods representing mild or highly aggressive exposure conditions. Additional variables in the project were the silicon content of the steel and the treatment prior to painting. The use of delta-galvanized steel as a base for painting offers a number of advantages such as much greater hardness and improved paint adhesion. Maximum corrosion protection is observed if the galvanized base is treated with chromate and sequentially powder coated with an epoxy primer followed by a polyester topcoat. Weert Groep. P.O. Box 129, The Netherlands. University of Cincinnati, Dept. of Material Science and Engineering, Cincinnati, OH 45221-0012.     

Flow analysis of rubber transfer molding

An experimental study has been carried out on rubber I transfer molding. It reveals that the filling is frequently limited more by the resistance of flow across the transfer pot than by resistance of flow through the sprue holes into the cavities. A mathematical model has been derived, which predicts semi-quantitatively the molding behavior observed. The mode1 predicts that fill time is proportional to the ratio of compound viscosity divided by molding pressure raised to about the fourth power. For the common cases where most of the fill time is from the resistance to the transverse flow on the top of the sprue plate, the fill time is proportional to about the fifth power of the ratio of transverse distance divided by the charge thickness. Experimental results showed that preheating and mastication of the compound reduced transfer time substantially. The charge pattern did not seem to have a major influence on transfer time.     

Thermal dispersion in resin transfer molding

This investigation focuses on the effects of thermal dispersion in resin transfer molding (RTM). A set of volume-average balance equations suitable for modeling mold filling in RTM is described and implemented in a numerical mold filling simulation. The energy equation is based on the assumption of local thermal equilibrium and includes a dispersion term. Thermal dispersion is an enhanced transport of heat due to local fluctuations in the fluid velocity and temperature away from their average values. Nonisothermal mold filling experiments are performed on a center-gated disk mold to investigate and quantify dispersion effects. Good agreement is found between the experimentally measured and numerically predicted temperatures, and a function for the transverse dispersion coefficient in a random glass fiber mat is determined. The results indicate that thermal dispersion is important in RTM processes and must be included in simulations to obtain accurate predictions.     

Void transport in resin transfer molding

The transport of single drops through a hexagonal cylinder array is used to study the void movement and deformation in a resin transfer molding process. A transparent flow cell is used to visualize the transport of voids through a porous media model. Experiments are conducted with nearly inviscid water drops and viscous glycerol drops with drop sizes ranging from 0.4 to 80 μl, and with both a Newtonian fluid and Boger fluid with average resin velocities ranging from 0.011 to 0.140 cm/s. Two critical capillary numbers, which determine the breakup (Ca_b) and mobilization (Ca*) of drops, are measured to better understand the flow dynamics of voids. As demonstrated by the experiments, there is a critical drop size, below or above which a quite different flow behavior is observed. Such a transition is analyzed with consideration of the geometry characters in the flow field. Results expand the former studies in this area to a significantly larger range of drop sizes and capillary numbers. Particle Tracking Velocimetry is also used to quantify the local velocity, shear stress, extensional stress and energy dissipation in the flow field. Polym. Compos. 25:417–432, 2004. © 2004 Society of Plastics Engineers.     

Heat transfer in blow molding operations

A one-dimensional model of unsteady state heat conduction has been applied to the cooling and solidification of a polymer in a blow molding process. The approach includes a temperature-dependent specific heat term to account for latent heat effects during the phase change. The model is used to predict temperature profiles in a thick-walled component of high-density polyethylene. These profiles lead to a clearer understanding of the heat transfer process. It is further shown that these temperature distributions can be used to study the influence of the major process variables upon the cooling of the molded component.     

Flow and heat transfer simulation of injection molding with microstructures

Injection molding has been used for mass production of polymer products with microstructures. Conventional Hele‐Shaw 2.5D midplane simulation is unable to describe the flow pattern correctly. It tends to over‐predict the effects of microstructures on global flow patterns. For the unidirectional flow, an x‐z planar based on the general momentum equation is able to achieve better accuracy and to retrieve more detailed flow and heat transfer information around the microstructures. A hybrid numerical technique is developed, which can significantly reduce the nodes and computation time, and yet provide good flow simulation around the microstructures. The mold‐melt heat transfer coefficient and injection speed are shown to be very important factors in determining the filling depth in microstructures. A decrease of the heat transfer coefficient and the occurrence of wall‐slip are likely in microchannels. Polym. Eng. Sci. 44:1866–1876, 2004. © 2004 Society of Plastics Engineers.     

Process parameters estimation for structural reaction injection molding and resin transfer molding

Some design strategies for structural reaction injection molding (S-RIM) and resin transfer molding (RTM) are presented. Our approach makes use of moldability diagrams to define the parameters necessary to meet the process requirements. Moldability diagrams are presented for the filling and curing steps. The criterion for selecting the amount of fiber reinforcement, injection time, catalyst level, and process temperatures in order to optimize properties and demold time is described.     

Mold filling analysis in resin transfer molding

This study is a comparison of independently designed mold flow experiments performed at The Dow Chemical Company with simulations from a computer code developed at The Ohio State University. The experiments used in the validation study included isothermal 1-dimensional flow with line gating and end venting, isothermal 2-dimensional flow with converging flow and center venting, and two different resin systems. The simulation results were compared with experimental pressure and temperature readings and fill times. It was found that simulated fill times could be predicted within experimental error and pressure distributions could be predicted with the application of a scaling factor.     

Modeling of heat transfer in rotational molding

Rotational molding is a process for manufacturing hollow or open‐sided plastic products using a rotating mold subjected to heating and then cooling. The process is attractive for the production of stress‐free objects at a competitive cost. In this article, a modified model for heat transfer in rotational molding is proposed, which assumes that the heat transfer at the mold‐powder interface is because of convection, whereas the powder particles are heated up by conduction. Heat transfer through the mold–air contact is also included. A source‐based formulation is used for modeling the layer‐by‐layer nonisothermal deposition of plastic. The reduced heat transfer due to warpage is calculated by using a modified heat transfer coefficient. Good overall agreement is found between the cycle times as predicted by the model and the experimental data. The model is then used for calculating the cycle time for particulate composites, based on their effective properties. A reduction in the cycle time is observed in the case of reinforced composites. This is attributed to the increase in thermal conductivity of the particulate composites and the reduced mass fraction of the polymer. Numerical calculations of the cycle time for the glass‐bead reinforced composites are found to be in good agreement with the experimental results. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Reactive mold filling in resin transfer molding processes with edge effects

Reactive mold filling is one of the important stages in resin transfer molding processes, in which resin curing and edge effects are important characteristics. On the basis of previous work, volume‐averaging momentum equations involving viscous and inertia terms were adopted to describe the resin flow in fiber preform, and modified governing equations derived from the Navier–Stokes equations are introduced to describe the resin flow in the edge channel. A dual‐Arrhenius viscosity model is newly introduced to describe the chemorheological behavior of a modified bismaleimide resin. The influence of the curing reaction and processing parameters on the resin flow patterns was investigated. The results indicate that, under constant‐flow velocity conditions, the curing reaction caused an obvious increase in the injection pressure and its influencing degree was greater with increasing resin temperature or preform permeability. Both a small change in the resin viscosity and the alteration of the injection flow velocity hardly affected the resin flow front. However, the variation of the preform permeability caused an obvious shape change in the resin flow front. The simulated results were in agreement with the experimental results. This study was helpful for optimizing the reactive mold‐filling conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

直接电加热滚塑工艺的传热分析

通过实验测量了直接电加热的滚塑模具在加热阶段的表面温度和模内温度以及所消耗的电能。然后根据实验数据对该滚塑工艺的加热阶段进行了传热分析,计算了有效热能和无效热能,并提出了评估该滚塑工艺的3个指标参数——热能利用率、加热每单位质量粉料所消耗的电能、加热每单位质量粉料所需的时间。结果表明,该滚塑模具的表面温度具有一定的不均匀性,不同位置处的最大温差为8 ℃。3种实验情形下的最高热能利用率为37.5 %,另外在相同的模内加热温度下,热能利用率随模内粉料质量的增大而减小。     

Resin transfer molding (RTM) process of a high performance epoxy resin. I: Kinetic studies of cure reaction

The cure kinetics of a high performance PR500 epoxy resin in the temperature range of 160–197°C for the resin transfer molding (RTM) process have been investigated. The thermal analysis of the curing kinetics of PR500 resin was carried out by differential scanning calorimetry (DSC), with the ultimate heat of reaction measured in the dynamic mode and the rate of cure reaction and the degree of cure being determined under isothermal conditions. A modified Kamal's kinetic model was adapted to describe the autocatalytic and diffusion‐controlled curing behavior of the resin. A reasonable agreement between the experimental data and the kinetic model has been obtained over the whole processing temperature range, including the mold filling and the final curing stages of the RTM process.