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Antonio J. Pontes & Miguel P. Queirós & Pedro G. Martinho & Paulo J. Bártolo & António S. Pouzada
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Abstract Hybrid moulds are a novel approach for rapid tooling of injection moulds that combines conventional machining for the mould structure and rapid prototyping techniques for the moulding blocks (core and cavity). In this study, two routes were used for producing the moulding blocks: selective laser sintering of stainless steel-based powder (hard tool) and epoxy resin vacuum casting (soft tool). The experimental work was based on a complex tridimensional commercial part. Themouldings were made in polypropylene,and the processing performancewas monitored online in terms of pressure and temperature at the impression. The performance of the moulding blocks was analysed in terms of thermal and cycle performance and structural integrity. The epoxy tooling route is more adequate for fine detailing than selective laser sintering but is not adequate for parts with extensive ribs or deep bosses. The structural integrity of the less costly epoxy composite can be compromised during ejection, this suggesting the need to evaluate the stress field by simulation at the design stage of the mould.6001
Keywords Hybrid mould . Moulding blocks . Injection moulding . Rapid tooling . SLS . Epoxy casting
1 Introduction
Important sectors of modern industries, as automotive,urban furniture or electronics, are pressed with the need to rapidly introduce new products in the market. This comes along with higher expectations towards product performance and product inpidualization [1]. Also there is a trend of moving from mass production to customised production of small series of products [2]. The demand of high-accuracy plastic injection parts produced in small quantities with increasingly shorter development periods led to integrated design solutions from conception and development to actual production [3, 4]. The alternative methodologies for the design and fabrication of tools for prototypes and short runs of plastic components at a competitive cost have considered the capabilities offered by the recent rapid prototyping and tooling (RPT) techniques [5–8] and the use of new materials in the moulding blocks [6, 8–11]. Building moulds with moulding blocks fabricated resorting to RPT is the basis of the hybrid mould concept (e.g. [12, 13]). The mould-making industry has seen the possibility of developing moulds by resorting to these alternatives for the moulding blocks and retaining the traditional manufacturing expertise for producing the standard mould structures [3, 14]. This paper describes the use of two RPT technologies, epoxy casting and selective laser sintering to produce the moulding blocks (core and cavity) of a hybrid mould for a consumer product. The main objectives are to investigate the issues associated to the use of these RPT approaches to produce moulding blocks and to assess the hybrid mould performance in terms of structural and thermal behaviour.
2 Rapid prototyping and tooling
Rapid tooling, as a technology that adopts rapid prototyping techniques to tool and die making [15], was seen as a key factor to enable companies to meet the demands for rapid product development and to reduce mould making time and cost quoted [16]. Two approaches frame the rapid tooling (Fig. 1): Indirect approach—moulds are produced from rapid prototyping master patterns; and direct approach moulds are produced directly in rapid prototyping systems concept [15, 17]. Tools for short manufacturing runs are known as soft tools, while tools for longer manufacturing runs are known as hard tools. Today, the reference to RPT is becoming common as the manufacturing route that resorts to the use of rapid prototyping as one of the steps to obtain products as components of the tools for use in injection moulding [18]. In this study, two fabrication routes were chosen to produce the mould blocks: epoxy tooling for a soft tool and metal selective laser sintering for a hard tool. In the epoxy tooling process, also known as aluminiumfilled epoxy tooling, composite tooling or vacuum cast resin tooling allows the fabrication of soft tools using epoxy composites with fillers as aluminium powder [6]. The process starts with the production of a pattern that can be obtained by rapid prototyping. The epoxy composite is cast onto this pattern and allowed to cure in a vacuum chamber to obtain each half of the mould. Ejectors and runners are typically milled into the tool to ensure total functionality. Due to the poor thermal behaviour of the epoxy resin, enhanced conformal cooling systems are used to improve the heat transfer rate and thus minimising the cycle time [19, 20]. The cost of this process is typically less than 40% of the conventional tooling cost, the lead-times lying between 2 and 4 weeks [6, 21]. Fine details can easily be reproduced, but flash can occur more often than with conventional tooling, leading to additional work on deflashing the mouldings. The process of selective laser sintering (SLS) is based on the use of a high-power laser to selectively heat powder material just above its melting point [22]. The model is built layer by layer. After all the layers have been defined, the nonbonded powder is brushed away to reveal the solid object formed inside. When metal powders are used, the process is usually referred to as SLSm and has been adapted to produce metal-based prototypes that can be used in rapid tooling applications [10].