Development of a Reactive Polyurethane Foam System for the Direct Sandwich Composite Molding (D-SCM) Process

Abstract

Sandwich structures utilize the geometric stiffening effect by increasing the area moment of inertia. This reduces carbon fiber (CF) material within CF-reinforced plastic (CFRP) components, and thus, the CO2 footprint. A suitable material combination for lightweight design is the use of continuous fiber-reinforced face sheets with a light foam core. CFRP sandwich structures with foam core are manufactured by combining a prefabricated foam core with fiber-reinforced cover layers in a two-step press process. Besides the reduction of the used CFRP material, more efficient manufacturing processes are needed. The aim of this paper is to develop a novel polyurethane foam system to enable the direct sandwich composite molding (D‑SCM) process for the production of CFRP sandwich structures by utilizing the resulting foaming pressure during the reactive polyurethane (PUR) foam system expansion for the impregnation of the CF reinforced face sheets. The developed formulation enables D-SCM structures with 150–250 kg/m3 foam density and 44–47.5% fiber volume content, based on a preliminary evaluation.

The Direct Sandwich Composite Molding (D-SCM) Process: Sandwich Manufacturing and Characterization

“…In addition to the foaming pressure [18], the foam morphology is also influenced by the PUR foam density. During the chemical foaming process, the cell structure must be stabilized by a sufficient amount of polymeric PUR material.…”

“…Further additives (e.g., distilled water, catalysts and nucleating agents) are mixed into the polyol component. The PUR system formulation is adjusted to the D-SCM process requirements and includes 3.1 parts of water and 1.1 parts of catalyst related to the polyol quantity [18].…”

“…Consequently, different resin types for the face sheets (e.g., use of epoxy-based systems) can be used to improve the process flexibility. Additionally, an optimized polyurethane foam system is used for ensuring a high lightweight potential of the structures by decreasing the necessary foam density and increasing the achievable fiber volume content (FVC) of the sandwich face sheets [18].…”

“…The focus of this paper is the realization of a one-step process-the Direct Sandwich Composite Molding (D-SCM) Process-for the manufacturing of CFRP sandwich structures by using an optimized polyurethane foam system [18]. The process strategy is described in Figure 1.…”

“…The process strategy is described in Figure 1. The stack of a wetted textile fabric and a TPU film is introduced in the heated cavity (1), an optimized reactive PUR foam system [18] is applied into the mold (2) and a second wetted fabric stack plus TPU film forms the upper face sheet (3). After closing the mold to the final component thickness (4), the chemical reaction of the foam material results in an expansion pressure on the face sheets (5).…”