When I need a flexible part how do I choose which process to use?
Flexible friend or foe? It would be very easy to become overwhelmed and unsure when faced with the myriad of information available on all the different processes available. Your requirement will be unique to you and various factors including time, functionality and hardness will need to be taken into account.
The most critical is 'fitness for purpose', get this wrong and it is all for nothing! This provides us with a simple selection process based on:
Aesthetics - the RP processes work in layers and usually in proprietary materials. Dressing layers out of a flexible part is not viable, thus if we are looking for a 'step free' part, possibly colour matched, our options are between silicone tooling and PU or tooling and injection moulding. There is a large cost and leadtime difference between these solutions so the answer usually depends upon the maturity of the design. Small runs of PU parts, say 1 - 40 can frequently be achieved in around 10 - 15 days, injection moulding from 12 - 30 days depending on complexity. The properties of PU will not match those of injected grades like Santoprene, but it remains a very valuable prototyping option.
Functionality - clearly the ultimate is the injected moulded production material and this can sometimes be viable via a prototype tool, timescales permitting. Usually we are forced to compromise and this gives us two options, silicone tooling and PU parts or laser sintered (SLS) materials such as Duraform Flex or TPE210S. PU has the advantage that it can meet aesthetic requirements, whilst SLS has the speed advantage. Basic colouring of SLS can be achieved through infiltration, red, green, blue etc. Properties such as elongation and hardness can be varied by selection of PU grade and infiltrant in the case of SLS.
Environment - if the part is to be exposed to specific fluids, this must be considered as some materials will dissolve, others soften. There are too many combinations to cover here so please contact Plunkett Associates.
Porosity - the laser sintering option must be used with caution depending on whether gas or liquid sealing is required. Thin wall sections on uninfiltrated sintered parts can demonstrate porosity. Infiltration will prevent this and no problems have been recorded with Objet, PU or moulded parts when processed correctly.
Hardness - in the prototyping stage there is frequently a great deal of uncertainty about the exact hardness required for an application. Thus processes that offer the ability to bridge the predicted hardness have an advantage. Silicone tooling and PU parts, Polyjet and SLS with TPE210S can all achieve this. PU is typically available from around 35 - 90 Shore A, SLS from 40 - 70 Shore A and Objet from 27 Shore A upwards.
Time - the quickest solution is Objet, parts are produced directly in a flexible material, simultaneously in a range of hardnesses, typical leadtimes 3 - 4 days. SLS parts typically 6 - 7 days depending on material and infiltration requirements. PU parts typically 10-15 days.