Conservative approach to getting tight interlocking fit? 5

[edwardmatos]

Hi all. I am about to commission my first injection moulds, made in China. I trust the company I am contracting to do what they do best, but thought I might ask here for some independent advice, just in case. Many thanks for any help you might be able to offer. The question below is described in general terms, but if you are interested in learning more about the specifics I have included links to detailed information at the bottom of this post.

Two different plastic, injection-moulded parts (one ABS, on PP) have been designed to interlock with a tight fit. Once push-fitted together, the tight-fitting features should provide a resistance to separation. No snap-latch-like features have been used so as to minimise mould complexity (and for aesthetics), so the resistance to separation is provided entirely by frictional forces from the tightly interlocking parts.

I have been told that a somewhat trial-and-error approach might be required, with numerous mould modifications being required before the fit is perfected. Fair enough. However, on the assumption that this will be a process that takes many weeks (even months) it would be very beneficial for us to get a handful of early parts out of the mould that we could use for fundraising/field-trial purposes whilst the mould is being perfected. It would be acceptable for such trial parts to imperfect, but they would have to be "imperfectly-loose", rather than "imperfectly-tight". Is imposing such a restriction on the trial-and-error process likely to cause more problems than it solves? Or is this is a normal request made of mould builders that is easily satisfied?

For further specific details about our parts you can find some illustrations, renderings and prototype photos on our website, and a prototype demonstration video (used for our recent crowd-funding campaign) on youtube. In case it isn't clear already, the interlocking parts described above refer to the white "blocks".

Many thanks for any advice offered.

 

[dgallup]

If you achieve an acceptable press fit initially it will wear and get loose with multiple insertions & removals. Also, mold tools wear and process variations give day to day dimensional changes. You really need to design some springiness into one of you parts to allow for wear & tolerances. There are hundreds of different snap fits and re-useable connections all around you, take a look at how they work and choose one.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Paulkuklych]

I think you have done the right thing: you told them what you wanted (a loose fit initially rather than a tight fit)

The mould builder can then figure out how to do it.

Paul Kuklych

http://www.improve-your-injection-molding.com

 

[MikeHalloran]

Another star for dgallup.

You need to build controlled flexibility into the parts in order to provide interchangeability and compensate for mold wear. For example, dimensionally and otherwise inspect, very closely, the way that Lego bricks attach to one another.



Mike Halloran
Pembroke Pines, FL, USA

 

[edwardmatos]

Thank you all for your valuable help and suggestions!

Edward Matos
AMIMechE

 

[hendersdc]

I may be a little late to the party but this sounds like a good application for crush ribs. They would only be good for the initial joining though and would loose a lot of their retention force if parts went through several mating cycles.

Other potential options are ultrasonic welding or heat staking if a more permanent fastening is desired. The parts and molds would be simple but the joining would require a second process and tooling.

 

[edwardmatos]

Amazingly this is actually what we ended up going ahead with! In theory the part only has to be pushed in once in its life and then should never be pulled out. In practice there might be as much as 10-20 insertions over the years, with some deterioration acceptable.

The other advantage of crush ribs is their ease to be tweaked in the mould. A metal safe adjustment can be made to easily deepen the ravines in the mould, increasing the frictional force of insertion if the first samples show this to be too low.

The tooling process is now underway, so let’s see how it goes. I’ll try to remember to post the results here for reference.

Edward Matos
AMIMechE

 

[edwardmatos]

Sorry, just re-read post above and realised it's not clear whether it was in reference to crush ribs or ultrasonic welding/heatstaking. It's in reference to crush ribs.

Edward Matos
AMIMechE