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.