Studies on the mouldability of structural foams in hybrid moulds

Abstract

In the context of the research project Hybridmould 21, studies on the mouldability of structural foams using hybrid moulds have been carried out. Hybrid injection moulds are an increasingly considered alternative for prototype series or short production runs of large dimension parts. In this solution for injection moulds the moulding elements (blocks or other inserts) are manufactured in alternative metallic materials or in synthetic materials typically using rapid prototyping techniques. Structural foams, known since the 70s, are moulded by injection moulding without using the high pressures typically used in injection moulding. The formation of the structural foam results from the dispersed gaseous phase, which derives from the expansion of a chemical blowing agent usually compounded in a compatible masterbatch. In this project various thermoplastics and thermosets were used, namely, PP, ABS and PUR, using a hybrid mould instrumented for the monitoring of temperature, pressure and expansion force. The moulding block was manufactured by vacuum casting of an epoxy composite. In this paper are mainly discussed the results obtained on liquid injection moulding polyurethane resins in the hybrid mould. common in SF injection moulding (Kamal et al. 2009). The polymer is blended with a CBA and then a short shot is injected in the mould under controlled temperature and pressure conditions. As soon as injection ends, the blowing agent expands and the foam fills completely the impression (Lanz et al. 2002). The typical thickness of a SF is between 4 and 9 mm, the density reduction is normally 10 to 35% and the pressure in the impression is approximately 4 MPa, an order of magnitude lower than in conventional injection moulding (Malloy 1994). RIM involves the chemical reaction between two or more liquids, allowing that the mixture and polymerization occurs inside the mould (Park and Colton 2003). The RIM process use reagents with low viscosity, which requires low pressure to fill large and complex parts. The quick filling of the impression can result in turbulent flow and formation of air bubbles. However, there are systems with fast reaction that can solidify before the complete filling, causing incomplete parts (Tomori et al. 2004). RIM is more appropriate to produce thick and large parts, with shorter cycle time. For small parts, this process is less competitive (Wohlers and Grimm 2003). The low viscosity of the raw materials and the reduced pressure during processing results in lighter and cheaper moulds, which can be manufactured in alternative materials (e.g. filled epoxy resins). Consequently RIM is usually seen as an alternative for short run and prototype series that become economically viable. The main problems of RIM are the poor surface quality and voiding, and flash in the final parts. Thus, finishing operations are frequently necessary, to a cost in the final part (Park and Colton 2003).