Mechanical properties of thermoset-metal composite prepared under different process conditions
Abstract
This paper reports a study on the morphology and mechanical behavior of a thermoset-metal composite used in prototype molds as a function of the process conditions. The investigation of the mechanical properties of epoxy-aluminum specimens post-cured using different routines showed that they are related to the self-controlled diffusion characteristic of thermoset polymeric systems. A high-temperature post-cure routine resulted in higher values for the modulus, stiffness and glass transition temperature, Tg, for the specimens. The fracture surfaces analysis showed the presence of defects, which appeared as empty spaces in the epoxy matrix, due to the mixing and casting process. The defect size and the specimen strength showed a direct correlation. The Weibull modulus was 7.95 for the epoxy specimens characterizing low toughness and the presence of defects in the material, as revealed by fractography. The probability of failure increased rapidly to 50% for applied stress greater than 38 MPa
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Introduction
Thermoset resin curing permits the rapid manufacturing of parts and objects by casting. The main applications of thermoset curing are the production of prototypes and parts used in the automobile, electrical, biomedical and aerospace industries. Thermoset curing is also applied in the manufacture of functional prototypes, such as prototype tools [1-7]. The thermal and mechanical properties are of great importance for prototype tools such as molds for the injection of thermoplastics, since these tools are subjected to particular work conditions, with temperature variations and mechanical requirements [7-12].
The mechanical properties of parts or tools manufactured by thermoset curing are dependent on the degree of cure of the resin. Manufacturing parameters, such as mixing, curing, post-curing and resin characteristics (monomers, oligomers and curing agent composition), maximum density of cross-linking, photosensitivity, rate and degree of cure, and post-cure method are very important in relation to the structure and properties of the part or tool [10-17]. Knowledge of the relationship between the thermoset resin structure and properties is useful in the manufacturing process, application and quality control of tools. Prototype molds with molding blocks produced by thermoset polymer resins have low mechanical properties compared to conventional steel tools. Prototype molds produced by casting of a thermoset-metal composite are commonly used to produce a small series of polymeric parts [5, 7 12, 14-17]. During injection molding, the mold is subjected to several types of loading, either static or dynamic. This means that the composite in the molding blocks needs to have good mechanical performance to avoid premature failure of the prototype mold. This paper reports a study on the morphology and mechanical behavior of an epoxy-aluminum composite used in prototype molds as a function of the post-cure process conditions.
Conclusion
The relationship between the resin structure and properties provides useful information for the casting manufacturing, application and quality control of tools. The mechanical properties of epoxy-aluminum specimens post-cured by different routines were found to be related to self-controlled diffusion, which is characteristic of thermoset polymeric systems. A greater improvement in the crosslink density occurred when gradual thermo-curing ramp routines were applied, reaching higher tensile strength values and a lower number of defects due to a more homogenous cure. A high-temperature post-cure routine resulted in higher values for the modulus, stiffness and glass transition temperature, Tg, for the specimens. The fracture surface analysis showed the presence of defects due to the mixing and casting process, which appeared as empty spaces in the epoxy matrix. The defect size and the specimen strength showed a direct correlation. The Weibull modulus was 7.95 for the epoxy specimens, characterizing low toughness and defects in the material, as observed by fractography. For applied stress higher than 38 MPa the failure probability increased rapidly to 50%. The results demonstrate that epoxyaluminum composites show interesting thermal and mechanical properties for tools manufacturing. However, the aluminum content and manufacturing defects can limit their use under critical conditions such as specific geometries and in high stress molding.