An approach towards the reduction of sink marks in sheet molding compound

Sink marks are shallow depressions normally observed above reinforcing ribs in molded Sheet Molding Compound (SMC) parts. In this paper, the effect of mold geometry, particularly the rib entrance shape, on the flow pattern of molding compound and the resulting sink marks in molded parts is presented. Flat plate specimens with a single reinforcing rib in the center were used in this work. Rib entrance shape was varied and its effect on both sink depth and fiber orientation measured. A reduction in sink depth from 0.0007 in. to less than 0.0001 in. was observed when comparing rounded and protruding rib entrances, respectively. The effect of inducing unequal flow rates from the two sides of the rib was also investigated and found to give a reduction in sink depth of about one-third. A computer simulation of the flow during molding was, used to compare observed flow patterns with simple theoretical predictions. The SMC was modeled as a highly viscous Newtonian fluid and finite difference methods were used to solve the Navier-Stokes equations. Extension of this modeling procedure to more complex geometries will aid in the design of nearly sink free molds.     

A numerical simulation of short fiber orientation in compression molding

A computer simulation has been developed to predict the orientation of fibers in a thin, flat part that is compression molded from sheet molding compound. The simulation combines a finite element/control volume simulation of the mold filling flow, a second order tensor representation of the fiber orientation state and a finite element calculation for the transient orientation problem. Sample results and comparison with experiments are presented. Predictions compare favorably with experiments on SMC (sheet molding compound) plaques and a model suspension of nylon fibers and silicon oil.     

Kinematics of flow in sheet molding compounds

Experiments utilizing charges constructed of black and white sheet molding compound (SMC) reveal the basic kinematic mechanisms controlling the flow of the fiber-filled compound in compression molding. The experimental results show that SMC deforms in uniform extension within individual charge layers, with slip occurring at the mold surface and, for slower closing speeds, also between the layers of SMC. When the mold closes rapidly, the charge extends uniformly through its thickness, with all slip concentrated at the mold surface.     

Rheological characterization of unsaturated polyester resin sheet molding compound

This paper presents a theoretical and experimental analysis of the rheological behavior of sheet molding compound (SMC). The work analyses the squeeze flow in a parallel plate plastometer of SMC discs which contain 25 percent of fiber glass by weight. This method of flow characterization gives a good insight into the basic rheological behavior of SMC for the compression molding process when producing flat parts. The theoretical analysis applies to thickened and matured SMC at room temperature. The analysis treats SMC as a viscoelastic material having an equation of state with viscous, elastic and yield elements. The time variation of compressive force when squeezing SMC discs between two parallel plates (one fixed and one mobile) has been derived from the equation of state. The values of the viscous, elastic and yield parameters were determined by using a least squares method of curve fitting to the experimental results. There are two aspects to the reported experimental work. One aspect is concerned with showing that the three element model for the equation of state provides a realistic mathematical basis for characterizing the rheological behavior of SMC at room temperature. The other shows how the parallel plate plastometer can be used to give data which characterize SMC flow behavior under conditions similar to those of the actual compression molding process.     

Compression molding of sheet molding compounds in plate-rib type geometry

The flow of fiber-reinforced composite materials in a plate-rib type mold geometry during compression molding was investigated using a series of sheet molding compounds (SMC). Material anisotropy in relation to the amount and the length of reinforcing fibers was analyzed. The influence of the interfacial friction between SMC charge and the mold surface on the flow and sink mark formation was also examined. The results were explained qualitatively by computer simulation.     

Monitoring cure in sheet molding compound processing

During the sheet molding compound (SMC) compression molding process, a premeasured polymer charge is placed between the heated halves of a mold which are then brought together to squeeze the polymer and fill the mold, after which pressure is maintained while the part cures. The cure stage constitutes the larger part of the molding cycle and thus affords the largest potential for cycle time reduction. In general, cure times in SMC processing are set longer than necessary, since the inherent material and process variation make it difficult to predict cure times with more than 10 to 20% accuracy. Accurate methods to detect the end of cure would be very beneficial and would permit opening the mold as soon as the material has cured, avoiding unnecessary waste of time. In this paper, several techniques that show promise for monitoring the state of cure are reviewed and experimental results given. Their relative advantages and accuracies are compared. In particular, the use of linear variable displacement transducers, pressure transducers, and thermocouples is discussed. We also show how the measurements compare to theoretical predictions of the state of cure.     

Numerical simulation of non-isothermal SMC (sheet molding compound) molding

Design of molding tools and molding cycles for sheet molding compounds (SMC) is often expensive and time consuming. Computer simulation of the compression molding process is a desirable approach for reducing actual experimental runs. The focus of this work is to develop a computer model that can simulate the most important features of SMC compression molding, including material flow, heat transfer, and curing. A control volume/finite element approach was used to obtain the pressure and velocity fields and to compute the flow progression during compression mold filling. The energy equation and a kinetic model were solved simultaneously for the temperature and conversion profiles differential scanning calorimetry (DSC) was used to experimentally measure the polymer zation kinetics. A rheometrics dynamic analyzer (RDA) was used to measure the rheological changes of the compound. A series of molding experiments was conducted to record the flow front location and material temperature. The results were compared to simulated flow front and temperature profiles.     

Applications of a fiber orientation prediction algorithm for compression molded parts with multiple charges

An application of a finite element simulation of mold filling and predication of fiber orientation in fiber filled compression molded parts is presented. Three-dimensional thin-walled geometries are considered. Following a simulation of the filling process, a set of transort equations are solved to predict the locally planar orientation of short fiber composites. The final orientation states throughout the part provide the necessary information to obtain a locally orthotropic mechanical model of the composite. A sheet molding compound part with a multiple charge pattern is used to illustrate the generality of the algorithms developed for compression flow, fiber orientation, and property predications. Derivations of the orthotropic mechanical properties obtained from the fiber orientation results are outlined.     

Processability study of in‐mold coating for sheet molding compound compression molded parts

In‐mold coating (IMC) is applied to compression molded sheet molding compound (SMC) exterior automotive or truck body panels as an environmentally friendly primer to make the part conductive for subsequent electrostatic painting operations. The coating is a thermosetting liquid that when injected onto the surface of the part cures and bonds to provide a smooth conductive surface. In order to identify the processability of IMC for SMC, it is essential to predict the time available for flow, that is the time before the viscosity starts to increase as well as the time when the coating has enough structural integrity so that the mold can be opened without damaging the part surface (mold opening time). In the present work, we study cure behavior of IMC based on differential scanning calorimetry and rheological experiments and show its relevance to both flow and mold opening time for the IMC process during SMC compression molding. POLYM. ENG. SCI., 59:1688–1694 2019. © 2019 Society of Plastics Engineers     

Injection molding of unsaturated polyester compounds

A bulk-molding compound made of unsaturated polyester resin, glass fiber, calcium carbonate fillers, and low profile additives is studied. The viscosity of the compound in the absence of cure reaction is measured by capillary rheometry. The compound exhibits a shear-thinning behavior. Injection molding in a rectangular plaque equipped with pressure transducers shows that the crosslinking reaction can begin during mold filling for low flow rate or high mold temperature. Fiber orientation in the plaque is complex as the reinforcement appears under two aspects, bundles or filaments. Their lengths and orientations are different. A layered structure throughout the thickness is observed at the mold entrance, whereas the orientation becomes progressively unidirectional in the plaque. Two fiber-free layers near the the mold walls are observed. A numerical simulation of mold filling assuming inelastic non-Newtonian kinetic dependent behavior is presented. The results agree well with pressure measurements. A simplified decoupled fiber motion calculation is finally proposed. A qualitative explanation of the basic phenomena which induce fiber orientation is presented.     

Prediction of fiber orientation in a rotating compressing and expanding mold

In the rotating/compressing/expanding mold (RCEM), one mold wall can expand, compress, and rotate during injection molding, thus offering opportunities to control the thermomechanical history of a polymer and its microstructure. A computer simulation of flow and fiber orientation in RCEM was developed. The predictive model extends the generalized Hele‐Shaw formulation to account for compression/expansion and rotation of the mold wall, and uses the Folgar–Tucker model for fiber orientation predictions. A 20% GF polypropylene was molded under various molding conditions. The predicted fiber orientation distributions were compared with experiments. The model compares favorably with experiments, provided that the fiber orientation equation is modified by a strain‐reduction factor that slows the transient development of fiber alignment. The effect of fountain flow on orientation must also be included to correctly predict fiber orientation near the mold walls, mainly for the case of stationary and linear motions of the mold surface. Compression or expansion of the mold has only a small effect on fiber orientation, but rotation of the mold dramatically changes the orientation, causing fibers to align in the tangential direction across the entire thickness of the molding. This rotation action perturbs the fountain flow and becomes the dominant factor affecting fiber alignment across the entire cavity thickness. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

注塑成型FRP的力学性能预测的数值模拟

纤维增强复合材料的力学性能和热物理性能强烈地依赖于纤维的取向状态，在注射成型过程中，纤维最终的取向状态依赖于充填过程的速度场，因此最终的产品性质依赖于成型的详细过程。研究发现，注塑成型制品的结构呈层状分布，层的数目依赖于模具几何和成型条件，不过大多数的结构在成型表面为沿流动方向取向，而在中心层为横向排列，有时在制件表面还有一层薄的介于二者之间排列的取向层。本文主要给出在两个简单模型中的纤维取向预测的理论和数值方法，这两个模型分别为：中心浇口圆盘和边浇口长条。     

Fiber orientation in divergent/convergent flows in expansion and compression injection molding

This study compares the fiber orientation patterns of short glass fiber‐reinforced polypropylene developed in conventional and nonconventional injection molding, the latter using a special mold, RCEM (rotation, compression, and expansion mold). This mold allows for a wide variety of operating modes during mold filling, which leads to a great versatility in obtaining different fiber orientation patterns. By incorporating through‐thickness convergent and divergent flows during the filling stage (compression and expansion modes, respectively), the fiber orientation can be tailored. These linear dynamics can be superimposed with a simultaneous rotational movement of one of the mold surfaces. These combined actions induce a high fiber orientation transversely to the radial flow direction, this effect being more pronounced in the expansion mode. POLYM. COMPOS. 27:539–551, 2006. © 2006 Society of Plastics Engineers     

Mechanical and microscopic investigation of whisker-reinforced silicone rubber

Franklin Fiber whisker-reinforced silicone rubber composites were molded using both transfer and compression molding. Processing parameters, such as sprue design, milling procedure, and mold temperature were varied. The fiber orientation in the composites is anisotropic and independent of sprue design and mold temperature, but dependent on milling procedure. Fiber attrition is significant for all samples. The Young's modulus is affected by fiber orientation. The experimental modulus values are higher than those predicted by the Halpin-Tsai equations for short fibers.     

Formation mechanism of high-pressure water penetration induced fiber orientation in overflow water-assisted injection molded short glass fiber-reinforced polypropylene

Recently, there has been growing interest in water-assisted injection molding (WAIM) not only for its advantages over gas-assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water-assisted injection molding (OWAIM) parts of short glass fiber-reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short-shot water-assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.     

Experimental and analytical procedures for flow dynamics of sheet molding compound (SMC) in compression molding

A package of procedures have been developed to collect and analyze the response of dynamic variables such as pressure, temperature, and mold separation during the compression molding of Sheet Molding Compound (SMC). From the dynamic responses, the molding process was found to consist of two regions: the flow and the subsequent curing reaction region. With an R-25 formulation and a mold closing rate of 30 mm/s, these two regions are well separated and the average flow time is not significantly affected by the maturation time for the material up to 30 days. Several mechanical parameters were estimated based on relatively simple flow models. The relationship between the press force, mold separation, and mold closing rate is found to be sensitive to the restrictions of the flow.     

Investigation of fiber orientation states in injection‐compression molded short‐fiber‐reinforced thermoplastics

Injection‐compression molding (ICM) has received increased attention because of its advantages over conventional injection molding (CIM). This article aims to investigate the effects of five dominating ICM processing parameters on fiber orientation in short‐fiber‐reinforced polypropylene (SFR‐PP) parts. A five‐layer structure of fiber orientation is found across the thickness under most conditions in ICM parts. This is quite different from the fiber orientation patterns in CIM parts. The fibers orient orderly along the flow direction in the shell region, whereas most fibers arrange randomly in the skin and the core regions. Additionally, the fiber orientation changes in the width direction, with most fibers arranging orderly along the flow direction at positions near the mold cavity wall. The results also show that the compression force, compression distance, and compression speed play important roles in determining the fiber states. Thicker shell regions, in which most fibers orient remarkably along the flow direction, can be obtained under larger compression force or compression speed. Moreover, the delay time has an obvious effect on the fiber orientation at positions far from the gate. However, the effect of compression time is found to be negligible. POLYM. COMPOS., 31:1899–1908, 2010. © 2010 Society of Plastics Engineers.     

Mathematical modeling of the in-mold coating process

Fiber glass reinforced polyester parts compression molded from sheet molding compound (SMC) are prone to such surface inconsistencies as porosity and sinks. Even though it appears that some of these defects could be eliminated by techniques such as vacuum molding, the resulting surface, with current technology, is not yet consistently up to automotive standards for exterior body panels. In-mold coating (IMC) of SMC, is designed to fill porosity, reduce sinks, and furnish a primer-like coating, thus upgrading the part surface to automotive standards. As a consequence, IMC is generally an integral part of the molding cycle when producing compression molded SMC exterior automotive body panels. Most commonly, in-mold coating is injected after opening the press slightly so as to separate the mold cavity and the exterior surface of the part to make room for the coating. A second approach is to let the hydraulic pressure of the injected IMC open the mold. Here, we present a mathematical model of the process and show application in predicting injection pressures, fill times, and filling patterns. A comparison with experimental results is also presented. Cycle times required for IMC injection methods is also discussed.