Date of Award

8-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Automotive Engineering

Committee Member

Srikanth Pilla, Committee Chair

Committee Member

Gang Li

Committee Member

David Schmueser

Committee Member

Huijuan Zhao

Abstract

Multi-material design is one of the most attractive methods for automakers to reduce production cost while achieving lightweighting to meet stringent regulations and fuel efficiency concerns. Lightweighting, parts consolidation, reduction in assembly time and cost, and diverse functionalities are some advantages to the use of multi-material design in the automotive industry. However, the current technology of multi-material manufacturing faces some drawbacks, such as high cycle time, the necessity of various tooling and machinery systems, tight tolerance requirements, and extended planning effort on the production line. In this study, a technique named the Hybrid Single Shot (HSS), which is similar to Polymer Injection Forming (PIF), is used to manufacture CF/Epoxy-Thermoplastic components in a single operation. Unlike the PIF method, a carbon fiber /epoxy prepreg sheet is used as an insert material instead of sheet metal. In this technique, an injected polymer melt behaves like a forming medium to form the inserted thermoset sheet, in a single operation. Molten polymer not only forms but also bonds with the thermoset sheet using the high temperature of the polymer, in one process. CF/Epoxy sheet with injected thermoplastic is a hybrid structure that combines high mechanical properties of thermoset composite with the toughness and complex geometries of injected thermoplastic into a single component.

A feasibility study was conducted for developing an integrated technology for the manufacturing of thermoset CF/Epoxy prepreg sheet with an injection of polypropylene to overcome the high cycle time and production cost associated with the manufacturing of such hybrids. Several sample parts were manufactured to demonstrate the effect of the process parameters on the process performance and the appearance of the final hybrid component. Although the results were promising, it showed some practical challenges such as excessive penetration, inadequate deformation, and warpage.

Various process and design parameters are applied to the hybrid single shot process to circumvent these challenges. For example, a lower injection speed rate and the injection temperature are applied to increase the viscosity to prevent the penetration of polymeric melt through the thermoset sheet.

Also, to evaluate the impact of polymer injection on the degree of cure of the prepreg sheet, Differential Scanning Calorimetry (DSC) analysis is conducted at a different pre-heat time before and after injection. The results showed that an increase in pre-heating time and injection temperature significantly enhanced the curing of the prepreg sheet after injection. Further, the mechanical properties of the hybrid part will be examined to identify the effect of individual properties of CF/ Epoxy and PP on the final component.

Another contribution of this study is that it avoids many difficulties that conventional TS/TP joining techniques face. Specifically, these traditional joining methods, namely mechanical fastening, adhesive bonding, and welding, are time-consuming and labor-intensive. Also, mechanical fastening causes delamination and possible galvanic corrosion while adhesive bonding requires extensive surface preparation. Despite the time and weight advantages, welding techniques tend to create local delamination due to high local temperature. The hybrid single shot method is a promising alternative to overcome all the challenges that conventional methods face. A lap shear test is conducted to address the bonding conditions between polypropylene and CF/Epoxy prepreg.

The experimental results presented in the previous chapters have revealed that the final geometry of the hybrid part is highly dependent on the preheating conditions and pressure field applied on the prepreg sheet during the injection phase. The pressure distribution is then a function of selected polymer, process settings, and most importantly of the geometry of the flow channel. To model the forming of the prepreg sheet due to this non-uniform pressure field, it is essential to couple all the physical events occurring inside the cavity. Therefore, the last contribution of this study is to have a better understanding on the effect of interaction injection, forming and curing on the final geometry of prepreg sheet, a quick yet accurate simulation of the HSS process. This simulation includes the consideration of the non-uniform pressure distribution of the melt flow and the prepreg sheet deformation behavior based on a new experimentally calibrated numerical approach.

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