The combination of being fast turnaround and high-quality makes the vacuum casting process a valuable choice for design engineers during the prototyping stage, as most 3D printing techniques cannot replicate injection moulded parts as accurately as vac casting can.

But what exactly is vacuum casting, how does it work and how can it benefit the product and medical device prototyping process? Design Development Engineer, Phil, explains…

What is vacuum casting?

The vacuum casting process is very similar to the plastic injection moulding process in that liquid plastic is poured into a mould to cast the final part. One of the big differences with vacuum casting comes from the mould halves themselves. A vac cast mould would be made from silicone rubber, so would be considered a “soft tool”. Whereas a plastic injection mould would be CNC machined from a block of steel and considered a “hard tool”. Vacuum casting produces high-quality parts that are comparable to injection moulded components. Due to the high quality and accuracy of vac cast models, this makes the process a great manufacturing choice to produce parts for fit and function testing, visual representation marketing purposes or even just a small run of high-quality parts.

 

How vacuum casting works:

The vacuum casting manufacturing process can be split into 3 sections: the master model, the silicone mould, and finally the cast part. The master model is basically a one-off model that is produced and is exactly the same as the final cast part. The master model is usually 3D printed in SLA as SLA is great for applying finishes. Alternatively, it is CNC machined, as tight tolerances can be held from a CNC’d master model.

To produce the mould, the master model is suspended in an enclosure whilst liquid silicone is poured in, encasing the master model. Once the silicone has dried, the mould is carefully and accurately split into two halves so that it can easily be reassembled. It is possible to make multiple moulds from one master model, as normally the silicone mould will only be able to produce 20-30 parts (depending on the complexity of the part geometry) before becoming too worn. The final step of the process is casting the part. The mould is filled with a liquid material whilst under a vacuum, usually a type of urethane. The liquid urethane is then drawn into the mould under the pressure of the vacuum until an air bubble free part is cast. Once the cast part is dried, the two halves of the mould are split apart, the cast part is removed, then any required finishing is carried out on the cast part. The two halves of the silicone mould are then reassembled together, ready for more parts to be cast.

 

The benefits of vacuum casting for prototyping products and medical devices:

Opting to go for a set of vacuum casted parts in the prototype stage is hugely beneficial to us as design engineers. The parts don’t have to be designed with draft in mind as the halves of the silicone mould are so flexible, they can be easily peeled off the part. This is always handy to know! Especially if you just need some quick prototype parts to test out the fits of a clip design etc, without having to worry too much about DFM tweaks. Undercuts can also be replicated using mould inserts, just in the same way they would be used in injection moulding. Most manufacturers offering the vac casting process offer a range of both rigid and flexible materials for the final parts, as well as double-shot over moulding being available too. This opens up a whole new world of great looking prototypes, that really are just like the real thing!

The best thing about vac casting is the high production quality short-run parts on a quick turnaround. The combination of being fast turnaround and high-quality makes the vacuum casting process a valuable choice for design engineers during the prototyping stage, as most 3D printing techniques cannot replicate injection moulded parts as accurately as vac casting. I would heavily recommend using vacuum casting as a process to produce fantastic prototypes. It’ll give you the confidence in your design to push the button on final injection mould tooling!

If you would like to learn more about how Haughton Design can help with your product or medical device development project(s), please reach out to enquiries@haughtondesign.co.uk

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