THE EFFECT OF MOULD MATERIALS IN THE PERFORMANCE OF PRODUCTS MOULDED BY RIM

The growing of national and international competition, forces the companies to opt for new technologies, aiming at obtaining competitive, higher quality and precision products with reduced lead time to market. Rapid Prototyping and Tooling (RPT) are important technologies used for short series of products with tight specifications. Simultaneously with these techniques, it is possible to use Reaction Injection Moulding (RIM). Due to the low pressures involved, RIM allows for the production of high quality prototypes moulded in moulds made from various materials. The use of RIM together with RPT allows for the production of high quality and complex prototypes at even lower cost. RIM products are used by the automobile and medical industries. In this study the characterization of the performance of products produced by RIM is done, using moulds in several materials (aluminum, silicone and resin with graphite) and polyurethane (PUR) reactants in various percentages. The use of materials with different thermal properties affects the PUR curing process, which influence the morphology and mechanical properties of the mouldings. Introduction The reaction injection moulding (RIM) is a process for the rapid production of complex plastic parts mostly in polyurethane systems (PUR) [1]. In RIM, the mixture and the chemical reaction of the liquid isocyanate and polyol components originate the polyurethane. The mixture of the two components is done just before injection into the mould. The polyol determines the physical characteristics of the PUR including density, impact strength, flexural modulus and colour of the moulded part [2]. Inside the mould, the liquid undergoes an exothermic chemical reaction, which forms the polyurethane polymer in the mould. The polymerisation or curing occurs in the mould, at relatively low temperature and pressure [3]. The shot and cycle times vary, depending on the part size and the PUR system used [4]. With the RIM process less expensive moulds, less energy, and lower-tonnage presses are required in comparison with thermoplastic processing [5]. These characteristics provide design, economic and processing flexibility [6]. When RIM is associated with RPT technologies for making the moulds these display lower structural rigidity and thermal conductivity [7]. The use of moulds in materials different from steel, for example, PUR, epoxy resin with fillers or silicone, leads to the cost, and allows for rapid geometry modifications whenever required. Furthermore, these moulds are sufficiently strong to support the injection pressures during the process. However the low thermal conductivity of this material will originate an increase of temperature in cavity of mould and consequently affect curing process. The choice of material for a RIM tool depends on several factors, such as: number of cavities, required superficial quality, production run and the need to accomplish subsequent alterations [8]. The adequate selection of the manufacturing process for making moulds depends on the complexity of the part and its tolerance, the number of parts to produce and the material to be used [9].For the fabrication of the moulds, aluminum, resin with graphite and polyurethanes with deferent density were used. Because of their hardness and mechanical strength, special aluminum alloys, either cast of forged, are suitable for prototype moulds [8]. Aluminum moulds show advantages in terms of weight, heat transfer and low production cost. Aluminum moulds can produce between 5 000 and 50 000 plastic parts [9]. RPT subtractive processes can be used to produce polyurethane moulds for RIM. PUR block materials have good machinability, their manufacturing time is short and the cost is low. The major drawback is the low thermal conductivity leading to longer moulding cycles. The epoxy resins possess important characteristics, such as, dimensional stability even at high temperatures, they are easy to process and they possess a low contraction .The epoxy moulds are normally used in prototype series. Their main limitations are brittleness, the surface finish and low heat transfer [9].When epoxy moulds are used, the chemistry of the PUR systems must be adjusted to slow down the polymerization reaction [5]. In this study the influence of the thermal conductivity of the mould in the cure process and in the mechanical properties was studied. A moulded part was produced in moulds made from several materials: aluminum, two PUR and graphite filled epoxy. Experimental Mould materials. The following materials were used to manufacture and respective thermal conductivity are represented in the table 1. Mould material Lab 850 Prolab 65 Resin with graphite Alumec 89 Thermal conductivity(W/mK) 0.2 0.2 100 165 Table 1: material for moulds and respective thermal conductivity RIM. For the production of the mouldings a resin RIM876900 (from Axson) and RIM machine (Dopag EconoMIx Compact) as used. The following processing conditions were used for all the moulds: Tabela 2: processing conditions Mouldings and test specimens. The mouldings for the experimental work are shown in fig.1. They are square in shape and the thickness changes from 6 mm in the rim and 4 mm in the central section. The moulding features a hole in the centre for obstructing the flow and allowing the study of weld lines. In this figure it is possible to see the locations where from the test samples (Fig. 1) were cut. Pressure Temperature of the reagents Temperature of the mould 6 bar 25 oC 40oC