Methods for optimising the life of polymer rapid tooling (PRT) inserts for injection moulding

Context: Injection moulds manufactured using the polymer rapid tooling (PRT) process have previously been used to mould general commodity resins with relatively low melting temperatures (<150°C), but lack the robustness of conventional metal moulds. Whereas conventional metal moulds can withstand the pressure and temperature applied during the moulding process, PRT moulds tend to fail abruptly during the moulding process. Purpose: To determine the feasibility of using PRT moulds for high-temperature applications and develop a possible methodology for setting the injection moulding process parameters to possibly avoid PRT mould failure at startup. Further, to understand the failure modes and underlying mechanisms leading to failure and establish a relationship between the process parameters, failure mechanisms and PRT mould life. Methodology: Experimental investigations involved designing a sequence of PRT moulds and manufacturing them using two different PRT materials (Digital ABS and Visijet M3-X). These PRT moulds were utilised to perform a series of injection moulding experiments using an engineering thermoplastic (Lexan 943-A). Different moulding process parameters were used to study the effect of process parameters on tool life. The moulded parts and failed PRT moulds were examined under optical microscopy to detect different failures. Validation experiments using real production parts from the aerospace and electronic enclosure industry were also performed. Multiphysics finite element analysis (FEA) simulations involving heat transfer and structural mechanics were performed. Mouldflow simulations were conducted to obtain the temperature and pressure distribution of the part and these data fields were imported into a transient thermal analysis to perform heat transfer analysis and obtain the temperature distribution of the PRT mould. The temperature distribution was imported into a static structural analysis to determine the interference pressure as a result of the temperature distribution (shrinkage of the part and expansion of the PRT mould). A sliding analysis was also performed to simulate the ejection process. Findings: It was found that with modifications to the conventional tool design process and process parameter setting methodologies, PRT moulds could also be used to mould engineering thermoplastics with higher melting temperatures (>280°C). A novel methodology was developed for setting the IM process parameters for PRT moulds. The proposed process-setting methodology helped to avoid start-up failures. During the moulding process, six different failure modes were identified on PRT moulds: bending failure, shear failure, edge failure, avulsion, surface deterioration and surface scalding. The relationship between the root cause of failure and process parameters was also determined. The cooling stage of the IM process was found to be a critical stage, causing the majority of observed failures. Originality: A guideline is presented for PRT mould design, together with a methodology for setting injection moulding process parameters. This has the potential to prevent PRT mould failures at startup and possibly extend the operating life of PRT moulds. Raised feature failure was identified as the most common failure mode, and the generally accepted bending and shear failure explanation for raised feature failure of PRT moulds was disconfirmed. A failure hypothesis was proposed, validated and accepted, suggesting that interference pressure (shrinkage of part and expansion of PRT mould) developed during the cooling stage leads to increased frictional resistance during the ejection process, resulting in tensile failure of the raised features

Categorization of Failures in Polymer Rapid Tools Used for Injection Molding

Background&mdash;Polymer rapid tooling (PRT) inserts for injection molding (IM) are a cost-effective method for prototyping and low-volume manufacturing. However, PRT inserts lack the robustness of steel inserts, leading to progressive deterioration and failure. This causes quality issues and reduced part numbers. Approach&mdash;Case studies were performed on PRT inserts, and different failures were observed over the life of the tool. Parts molded from the tool were examined to further understand the failures, and root causes were identified. Findings&mdash;Critical parameters affecting the tool life, and the effect of these parameters on different areas of tool are identified. A categorization of the different failure modes and the underlying mechanisms are presented. The main failure modes are: surface deterioration; surface scalding; avulsion; shear failure; bending failure; edge failure. The failure modes influence each other, and they may be connected in cascade sequences. Originality&mdash;The original contributions of this work are the identification of the failure modes and their relationships with the root causes. Suggestions are given for prolonging tool life via design practices and molding parameters

Reprodutibilidade de blocos moldantes de moldes híbridos em moldação por injeção de plásticos

Nos últimos anos a indústria de moldes e plásticos sofreu uma evolução significativa quer a nível de processos quer a nível de produtos, devido ao aumento da exigência e das necessidades dos clientes. Até há poucos anos, as ferramentas (moldes) eram construídas exclusivamente a partir de processos convencionais (fresagem, furação, eletroerosão), no entanto, esta realidade sofreu uma profunda alteração fruto da redução de custos e prazos, havendo, por isso, necessidade de procurar tecnologias alternativas que respondam a este aumento de exigência, permitindo um time-to-market consideravelmente inferior. O termo Rapid Tooling (RT) representa de uma forma geral este conjunto de técnicas. O conceito de molde híbrido surge com o progresso das técnicas de fabrico, permitindo combinar os processos de maquinação convencionais com as tecnologias de RT. Os moldes híbridos são ferramentas em que a sua estrutura é fabricada através de técnicas e materiais convencionais, enquanto as zonas moldantes são obtidas através de processos de RT. O objetivo deste trabalho consiste em avaliar a reprodutibilidade de blocos moldantes de moldes híbridos na moldação por injeção. Para isso, serão produzidos blocos moldantes bucha com recurso a processos/materiais alternativos aos convencionais (aço e alumínio). O vazamento de resina epoxy com 60% de carga de alumínio e o FDM serão os processos utilizados na obtenção dos blocos moldantes bucha. Numa fase posterior, estas buchas serão utilizadas na injeção de peças plásticas com materiais amorfos e semicristalinos. Tendo em conta os resultados obtidos, procurar-se-á estabelecer uma relação entre a vida útil da zona moldante com os materiais utilizados (quer na sua construção, quer na injeção) e com as variáveis de processamento usadas. Isto é, será feita uma avaliação da reprodutibilidade de cada ferramenta, ou seja, a sua capacidade de reproduzir peças dentro de tolerâncias admissíveis e com boa qualidade. As conclusões obtidas neste trabalho permitem verificar que a utilização de resina epoxy com carga de alumínio na construção de blocos moldantes de moldes híbridos é uma solução alternativa para a obtenção de pequenas/médias séries de moldações. Apesar dos defeitos superficiais em dois dos blocos utilizados e da consequente perda de reprodutibilidade mais cedo do que previsto, a produção de mais de 2000 peças de plástico sem variabilidade geométrica e dimensional nestes blocos e injetando materiais distintos constitui um claro indicador de que este processo é viável para uma média série. Por outro lado, os blocos construídos em FDM apenas possibilitaram a obtenção de um baixo número de peças protótipo, dada a reduzida temperatura de transição vítrea (Tg) dos materiais utilizados

Beitrag zur Entwicklung 3D-gedruckter Formeinsätze (FFF) für das Spritzgießen von Kunststoffbauteilen

Ziel der vorliegenden Dissertation ist es, das Anwendungspotenzial 3D-gedruckter Kunststoffformeinsätze für das Spritzgießen von technischen Kunststoffbauteilen zu optimieren. Spritzgegossene Formteile werden konventionell in einem Stahlwerkzeug geformt. Dieses Fertigungsverfahren ist insbesondere für die Produktion hoher Stückzahlen lukrativ. Allein für die Herstellung von Prototypen oder Stückzahlen in geringerer Auflage, rentiert sich dieses Verfahren jedoch nicht beziehungsweise nur in speziellen Anwendungsfällen. Der Grund dafür liegt zum einen in den relativ hohen Herstellkosten des Stahlwerkzeuges und zum anderen in der notwendigen Vorlaufzeit zur Werkzeugerstellung. Zur Optimierung sind im Rahmen dieser Dissertation Untersuchungen dazu angestellt, inwieweit sich die formgebenden Werkzeugeinsätze aus Stahl durch 3D-gedruckte Formeinsätze aus Kunststoff substituieren lassen. Als Formeinsatz wird hierbei die minimal notwendige Formfläche bezeichnet, die ausgehend von einer planen Werkzeugoberfläche erforderlich ist, um ein dreidimensionales Formteil herzustellen. Mit Ausnahme dieser signifikanten Modifizierung verbleibt der Aufbau des untersuchten Spritzgießwerkzeuges rein konventionell. Industriell etabliert und repräsentativ für den aktuellen Stand der Technik sind kunststoffbasierte Werkzeugeinsätze (Formeinsätze oder Inserts) aus duroplastischen Photopolymerharzen, hergestellt im Multijet-Verfahren. Mit diesen werden überwiegend Formteile aus Standardpolymeren produziert. Als Injektionsmaterialien dienen unter anderem Polypropylen, Polyethylen oder Acrylnitril-Butadien-Styrol. Die hergestellte Stückzahl ist auf durchschnittlich 50 Stück begrenzt. Darüber hinaus gehen aus dem Stand der Wissenschaft Untersuchungen mit thermoplastischen Kunststoffformeinsätzen aus Polyethylen, Polyamid und Polycarbonat sowohl mit als auch ohne Füllstoffanteilen hervor, welche durch selektives Lasersintern hergestellt werden. Neben Standardpolymeren existieren hierzu auch Nachweise über technische Spritzgießmaterialien, wie beispielweise Polyamid. Die damit nachgewiesene Stückzahl liegt bei rund 5 Formteilen. Durch das Multijet-Verfahren oder das selektive Lasersintern lassen sich besonders hohe Fertigungsgenauigkeiten und eine hohe Oberflächengüte erreichen, welche den mechanischen Belastungen des Spritzgießprozesses unter bestimmten Voraussetzungen nachweislich standhalten. Allerdings sind diese Verfahren im Ver-gleich zu anderen 3D-Druckverfahren vergleichsweise kostenintensiv und hinsichtlich der zur Verfügung stehenden Materialvielfalt begrenzt. Um den Kostendruck zu reduzieren und die Fertigungsrestriktionen hinsichtlich verwendeter Druck- und Spritzgießmaterialien zu erweitern, wird im Rahmen dieses Promotionsprojektes das Anwendungspotenzial des Fused Filament Fabrication weitergehend untersucht. Zu diesem Zweck werden die durch dieses Verfahren 3D-gedruckten Kunststoffformeinsätze im Spritzgießprozess verwendet, durch Spritzgießen verschiedener teilkristalliner Thermoplaste erprobt und im Vergleich zur konventionellen Stahlform evaluiert. Durch dieses Vorgehen soll ein Beitrag zur spezifischen Anwendung 3D-gedruckter Formeinsätze für das Spritzgießen von technischen Kunststoffen geleistet werden.The aspiration of this dissertation is to investigate the application potential of 3D-printed polymer tool inserts for the injection molding of technical plastic components. Injection molded parts are formed conventionally in a steel tool. This manufacturing process is particularly lucrative to produce large quantities. Therefore, this process is not worthwhile to produce just prototypes or small quantities, or only in special applications. The reason for this lies on the one hand in the relatively high manufacturing costs of the steel tool and on the other hand in the necessary lead time for tool manufacture. In order to optimize this current situation, this dissertation investigates the extent to which the steel mold inserts that give the shape can be substituted by 3D-printed plastic mold inserts. The mold insert is the minimum required mold surface, which is required starting from a planar tool surface in order to produce a three-dimensional molded part. Except for this significant modification, the construction of the examined injection mold remains purely conventional. Plastic-based tool inserts (mold inserts or inserts) made of duroplastic photopolymer resins, manufactured using the multijet process, are industrially established and representative of the current state of the art. These are mainly used to produce molded parts from standard polymers. Polypropylene, polyethylene or acrylonitrile butadiene styrene are used as injection materials. The number of pieces produced is limited to around 50 parts. In addition, investigations with polymer tool inserts made of polyethylene, polyamide and polycarbonate, both with and without filler components, which are manufactured using selective laser sintering, are based on the state of the art. In addition to standard polymers, there is also evidence of technical injection molding materials such as polyamide. The number of pieces produced is limited to around 5 parts. The multijet process or selective laser sintering enables particularly high levels of accuracy and a high surface quality to be achieved, which have been proven to withstand the mechanical loads of the injection molding process under certain conditions. However, compared to other 3D printing processes, these processes are comparatively expensive and limited in terms of the variety of materials available. In order to reduce the cost pressure and expand the production restrictions regarding the printing and injection molding materials used, the application potential of fused filament fabrication is being further investigated as part of this doctoral project. For this purpose, the polymer tool inserts 3D printed using this process are used in the injection molding process, tested by injection molding various semi-crystalline thermoplastics and evaluated in comparison to conventional steel cavities. This approach is intended to contribute to the specific application of 3D-printed mold inserts for the injection molding of engineering plastics