Tolerance analysis is used to understand how small variations in component dimensions and assembly constraints stack up across assemblies, and how the accumulation of differences affects function of the design. At HD, we split up tolerance analysis into two different elements: fits & stacks. With fits we just look at the interaction between two components. For example, how a cap bumps over, and clips on to the rim of a bottle. The ID of the cap and the OD bottle rim would be run through the tolerance analysis to ensure that on the upper and lower tolerance limits of both components, there would still be an interference bump fit. Fine tuning the fits between these two features by adjusting their tolerance limits, exploring how they fit together at both their largest and smallest limits allows us to find the worst-case tolerance requirements. This ensures 100% of the components will assemble together, and function properly – regardless of variation in component manufacture. This is the same practice with the stacks element of tolerance analysis. During this element we look at the accumulative stack up of tolerances of components stacked together in the assembly. At this stage, working closely with the chosen injection moulding manufacturer is key to a successful tolerance analysis. Knowing how fine the chosen manufacturer can go with certain tolerances is a huge help in establishing the upper and lower tolerance limits which can be achieved and maintained during the manufacturing process.
Now we’ve reached the exciting part of the device characterisation process, producing prototypes!
This is a real test to see if all of the work which has been carried out up to this point has paid off, by producing a successful, and most importantly a correctly functioning prototype!
At this stage there are countless different prototype manufacturing options to go for. For first off initial prototypes at HD we would always opt for 3D printing. This lower cost alternative to “soft tooling” injection moulded parts, allows us to have a physical prototype in our hand, in some cases within just a few days. Building up the first refined prototype really produces an appreciation for the overall size of the prototype assembly and specific features of individual components. We can all be susceptible to losing our grip on the real size and scale of parts when working on zoomed in CAD models on big screens for months. Producing a set of prototypes doesn’t just give an appreciation of size, but also the ease and order of assembly. A process which should be given full attention during the component design phase, if the individual components can’t be assembled together, the design is of no use!
Usually we would go through a few different iterations of prototypes as the design develops, getting prototypes 3D printed in increasing levels of accuracy as we near the final design. FDM printing is a great, cheap, quick printing option to start off with, then moving all the way up to Viper high-resolution SLA prints that can hold a tolerance of up to +/- 0.1mm. These high-resolution prints are unbelievably useful when testing out the fit and strength of clip designs etc. before going into soft tooling and eventually validating the chosen design for the final injection moulded device created during production.
To accompany prototyping files, we find it prudent to generate basic 2D CAD drawings of the assembly and each component part. It’s not vital but we find it reduces risk of issues in prototype manufacture, helps control the data released and these drawings can be subsequently developed for the final manufacturing data pack.
The final stop on the journey of medical device characterisation is the creation of a manufacturing data pack. At this stage the design has been decided on, tolerance analysis complete, prototypes produced and tested, it’s now time to produce geometrically toleranced drawings to use in final production. These drawings contain all the component tolerance data, which was produced from the tolerance analysis, Specification of materials, surfaces finish and even moulding information such as split line locations and gate feed points. Once the manufacturing data pack is complete, the process of device characterisation is complete!
That’s the final piece of our 4-part blog. Design Development Consultant, Phil Sampey, has skimmed across some details and focused on the work completed by design engineers. There’s obviously more to consider in the wider project overall but hopefully there’s enough guidance for others considering characterising a medical device. If you need any help with medical device characterisation, please get in touch with the HD team and we will do our best to help.