Hamid G. Kia

Thermal Expansion of Sheet Molding Compound Materials

The coefficient of linear thermal expansion (CLTE) of composites is an important property that has to be well understood. In this study, using a mathematical expression, graphic relationships were established for predicting the general trend of CLTE variations with temperature as affected by the materials properties. Subsequently, the CLTE of different types of sheet molding compound (SMC) materials was measured at various temperatures in the direction parallel to the cosmetic surface. It was observed that the CLTE variations with temperature go through a maximum. The experimental results were then interpreted with the help of the developed theoretical relationships. More specifically, the effect of low profile additive on the CLTE of SMC was studied, and it was observed that increasing the concentration of the low profile additive, a high CLTE ingredient, reduces the overall CLTE, and at the same time, shifts the position of the maximum to lower temperatures. As to the effect of glass content, it was observed that changing the glass content from 27% to 37% does not affect the position of the maximum in the CLTE-temperature curves noticeably, although this variation reduces the overall CLTE significantly. These observations were all consistent with the theoretical predictions. The information generated in this work enables one to control the CLTE of composites as required by specific applications. An example is the use of SMC in the production of inner panels for deck lids and hoods, in which a controlled CLTE is essential in order to avoid bond line readout.

Compensating Thermoset Composite Panel Deformation using Corrective Molding

Open mold composite (OMC) body panels, comprising three thermosetting layers, warp significantly after removal from the mold. Due to an asymmetric layup of these laminates, a residual stress develops due to chemical shrinkage of the gel coat resin during cure. The resulting bending moment distorts the panel. This work first develops a finite element model (FEM) to calculate panel deformation based on the chemical shrinkage and mechanical properties of the laminated panel. The FEM is then validated by a direct comparison with the measured distortion of door-outer panels produced using a conventional panel-shaped mold. This FEM is then integrated into an algorithm to calculate a mold shape that corrects for this distortion. Starting with a mold duplicating the desired panel shape, the initial panel deformation is calculated using the FEM. Trial molds are then generated by partial inversion of the deformed panels deviation from the desired shape. The algorithm is iterated until it converges to a corrective mesh that deforms to the mesh of the desired shape of the panel. A corrective mold for a door-outer panel is fabricated and the effectiveness of the corrective mold is evaluated by measuring the distortion of the door-outer panels it produces. Compared to the panels produced by the conventional mold, the corrective mold reduces panel distortion by 45%.

Cure Simulation of Thermoset Composite Panels

The curing of an open mold composite consisting of three laminated thermoset layers is simulated using a one-dimensional finite element model (FEM). This curing simulation addresses the reaction kinetics of each of the thermoset polyester resins as well as the thermal transport across the composite to calculate a time-dependent cure of each layer. Curing of each of the resins is modeled as an autocatalytic reaction based on the isothermal reaction exotherms at several temperatures. These reaction models depend sensitively on processing parameters. In particular, the models are specific to each polyester formulation, pre-promoter, and concentration of methyl ethyl ketone peroxide initiator. A nonlinear, transient, one-dimensional FEM coupling the reaction exotherm with the thermal properties of the composite is being implemented in Abaqus®. The measured temperature profiles of the composite layers during cure are semi-quantitatively reproduced by the model providing the degree of cure for each composite layer. Using the calculated degree of cure, the time evolution of the modulus of each layer can be estimated. While this simulation serves as a useful guide to the relationship between process parameters and laminate properties, it is not sufficiently accurate to provide process control for parameter optimization.

The Effect of Resin Formulation on the Surface Appearance of Glass Fiber Reinforced Polymers

This paper is concerned with the effect of resin formulation on the magnitude of fiber readout for polymers reinforced with continuous glass fiber mats. The observed fiber readout is the deformation of the surface with a pattern similar to the underlying fibers. In our previous studies, it was established that one of the main factors responsible for fiber readout is the coefficient of linear thermal expansion (CLTE) of the resin in the direction normal to the cosmetic surface. In the current study, the differential thermal contraction (Δ∈) between the resin and the glass fibers, a factor directly relatable to the magnitude of fiber readout, has been investigated. This direct relationship was examined by comparing the computed values for the Δ∈ with the measured magnitude of fiber readout for rein forced polyurethanes (filled with milled glass or hollow glass spheres) and polyesters (filled with calcium carbonate). For all experiments, the predicted values for (Δ∈) were found to be consistent with the experimental measurements for fiber readout.

Modeling Surface Deformation of Glass Fiber Reinforced Composites

Surface waviness in glass fiber reinforced composites is predominately due to dif ferences in the thermal expansion properties of the resin and the reinforcement. A model has been developed which simulates the surface deformation of glass fiber reinforced composites as a function of factors such as molding temperature, thickness of glass fibers, and the thermal expansion properties of the resin and the glass. Application of a top layer helps to conceal fiber readout which is a type of surface waviness. Based on the predic tions of the model, increasing the top layer thickness is more effective than increasing the top layer stiffness.

Powder priming of SMC. Part II: Failure mechanism

Current sheet molding compound (SMC) panels, when coated with powder primer, show primer popping in the bake oven due to the degassing of the substrate moisture. In the absence of moisture, however, it was found that some SMCs showed severe popping, while other SMC materials did not. These results suggested that, contrary to general opinion, the moisture content in SMC might not be the only cause for popping. This report covers the results of a systematic study that was carried out to identify the factors that contribute to powder primer popping. Several potential variables that could affect popping, such as volatiles in the substrate, thermal conductivity of the substrate, static charges, and powder bake profile were studied. The experiments showed that the poly(vinyl acetate) in the low profile additive (LPA) is the main cause. More specifically, the micro void formation of this low profile additive that eliminates SMC shrinkage, and enables a smooth surface finish, also causes primer popping. Based on the experimental results, it was concluded that the air permeation into these micro voids is the reason behind the popping of the moisture free SMC substrates. This understanding of the failure mechanism, paves the way for the development of low moisture absorbing SMC materials that can be powder primed in our assembly plants, and could remove the barriers for the usage of SMC on GM vehicles.

Powder Priming of SMC. Part I: Assessment of the Current Technologies

A cosmetic defect called ‘popping’ on the surface of sheet molding compound (SMC) body panels that are coated with powder primers has become a major hurdle in expanding the usage of SMC in automotive applications. General Motors R&D, therefore, initiated a project to study the mechanism of popping and to explore different solutions to this cosmetic problem. The current report covers the benchmarking phase of this project, in which a variety of SMC materials and conductive primers/sealers were evaluated for their ability to produce a pop free surface. In this work, among other factors, the effects of molding pressure and the panel moisture content on the degree of popping have been studied. The experimental results showed that popping increased with the increase in moisture content and with the decrease in molding pressure. The extent of popping, however, varied with the type of SMC and the conductive primer. We found that none of the SMC materials or conductive primers were able to eliminate the pops completely at high moisture levels. More importantly, it was concluded that moisture is not the only cause of the popping and there are other factors that contribute. The results of the current work will be used in the next phase of the investigation, which is focused on identifying solutions to this cosmetic problem.

Theoretical Modeling of Bond-line Read-out in Adhesive Joined SMC Automotive Body Panels

A visible deformation on the exterior surface of adhesive-joined automotive body panels along the adhesive application line is called bond-line read-out (BLRO). A 2D finite element model with isotropic linear elasticity and plain strain approximation for a plate/hat design was developed to investigate the physical phenomenon responsible for the formation of BLRO. It was found that a combination of process induced compressive stress on the outer side and tensile stress on the inner side of the exterior panel deforms it into a convex outward shape. The results of this work enables researchers to develop new strategies for reducing BLRO in adhesively bonded exterior panels.

Strategies to Reduce Bond-line Read-out in Adhesive Joined SMC Automotive Body Panels

Parameter sensitivity analysis on a 2D finite element model of adhesive-joined SMC hat/plate structure was performed to evaluate the relative impact of design, application, and material parameters on the bond-line read-out (BLRO) defect. Based on the sensitivity analysis and the ease of implementation, a list of strategies in the order of their effectiveness in reducing the BLRO defect is proposed. Choosing a low modulus, low cure temperature, and low shrinkage adhesive, raising the stiffness of the exterior SMC panel, and lowering the stiffness of the interior SMC panel in the regions around the bond-line were found to reduce the BLRO defect.

Development of Low Moisture Absorbing SMC

In this work, the role of SMC constituent components in controlling the moisture absorption of the final molded panels is discussed. A variety of polyester resins, low profile additives, and fillers were exposed to high humidity, defined as 90% relative humidity at 40°C, for extended hours. It was concluded that calcium carbonate, which is the most common filler for SMC, at a 0.12 wt% moisture absorption level, is the second lowest in moisture absorption. The comparison of the calculated and measured moisture absorption numbers indicated that rule of mixtures can be used to accurately predict the behavior of SMC based on the polymeric matrix. However, the polymeric matrix moisture uptake has to be measured experimentally and cannot be predicted using the mixing rule for its constituent ingredients due to the complexity of the interactions in the mixture. It was also identified that MgO, which is the most commonly used thickening agent, has a significant impact on moisture uptake of SMC. Based on the generated information, two low moisture absorbing SMC formulations were developed and their properties were measured.