When the time comes to jump into CAD and start the process of modelling components it’s important to plan your modelling and assembly structures. Following an efficient modelling method allows any members of the team who join later in the project to pick up where the previous designer left off. At HD we find that correctly referencing data within a sketch to planes rather than other parts, from existing geometry like faces etc, allows CAD models to be easily adjusted without the model collapsing. Without this structure CAD models can break down into a multitude of rebuild errors after trying to make even the smallest of adjustments! Trying to create features as efficiently possible, within just one sketch, also helps keep the feature tree short and simple. It’s then not littered with different Cuts, Extrudes, and of course every CAD engineer’s get of jail free card… Fillets and Chamfers!
The accurate replication of component design is a key part in the puzzle of medical device characterisation. Time spent evaluating and appreciating the way components have been designed for manufacture is time well spent. Designing parts for plastic injection moulding can be a minefield of feature issues such as undercuts, thick/thin sections and areas requiring more draft! To name just a few. (Draft angle is a slant applied to features of an injection moulded part. The angle, which is positioned to run towards the direction of a mould’s pull and parting line, helps with releasing a part from the mould tool). Running a draft angle and thickness analysis tool within your chosen 3D design package is a fantastic way of identifying problem areas which can be rectified in preparation for sending components off for tooling and moulding. However, we would always recommend having a conversation with your chosen moulders during this phase of the project as each moulder has their own preferences when it comes to these typical “sticking points” especially when it comes to draft angle. Consideration of injection points, ejection and mould flow analysis are something they can advise on too. A close working relationship with your chosen moulders makes the design for manufacture process a lot easier and minimises rework of models to suit the moulding processes. At the end of CAD modelling we will often produce a quick set of low-cost 3D printed prototypes to check the basic CAD models work as intended and identify any unforeseen issues.
Two documents which are key when carrying out a characterisation exercise are an Interaction Matrix and an Essential functions document. Completing an Interactions Matrix forces the user to cross reference and analyse which type of interaction every part has with each other. From “No part interaction” to “Clash or interference” this matrix gives the user a visual indication of how the parts interact and more importantly identify critical interactions that need to be engineered in detail.
Using the data gathered from an Interaction Matrix, an Essential Functions document can be started. The purpose of this document is to identify which component features need to be closely engineered during tolerance analysis. These critical features are identified by running through the user steps of using the device from start to finish, detailing every action, function and interaction. Breaking down the use of the device into this step by step granular level means each component function / feature interaction can be evaluated and can be established whether they are an essential function or not. From there, each essential function is sorted into 2 categories “Assembly Critical” and “Safety Critical”. Once the critical functions and features have been identified, the process of planning and completing tolerance analysis can commence!
In our next blog post we will delve into the process of tolerance analysis on a suite of components from a medical device, as well as manufacture of prototypes, and the final stop on the journey of medical device characterisation – the creation of a manufacturing data pack!
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