Plastic cam with spring as follower

[katelyncrist]

Hello. I am a first time poster, and, as such, I am hoping this is the correct forum to post this in. If not, would someone please kindly show me to one that is more appropriate?

I am working on eliminating metal springs from a design for a linear peristaltic pump for medical purposes. The current design involves two plastic cams, each with two followers attached to them. Springs are placed over the followers to press against a metal covering that goes over the cams, so that there is enough force to operate the pump correctly. What I'd like to do to remove the springs is to make the cam followers themselves the spring. Currently, the cam and followers are one injection molded piece. This aspect needs to remain the same, just with the followers as plastic springs instead of straight rods. Is this something that can be injection molded? Has anyone ever seen something like this done before, and if so are there any pictures I could look at to help with my design? Also, what kind of material would be best for something like this?

I am a recent graduate and am new to my company, so I am trying to do all that I can to impress my superiors. Any help at all would be greatly appreciated! Thank you.

 

[Pud]

Plastics are pretty rubbish at being springs due to a phenomenon called "creep".
Acetal (polyoxymethylene) is about the best for creep in it's unfilled state. Still nowhere near metals though...
H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[MikeHalloran]

Plastic springs can work in a low-cycle situation, but they are not easy to design, and they are not at their best in repetitive situations, or where they are left in a strained state. ... like in peri pumps.

You got trouble enough just getting a linear peri pump to work. Steel springs are cheap and dependable. By all means, check the design of the ones you have, and improve that if you can, but you don't want to be explaining to the FDA why you decided to use plastic springs in a medical device.

Here's a hint: If you cite that you did anything, ahem, 'creative', in order to save money, that's the end of the discussion with FDA; all that remains to decide is who gets to write the recall letter.



Mike Halloran
Pembroke Pines, FL, USA

 

[katelyncrist]

Thank you so much for the quick reply. What about a composite resin? So far this company seems to be the best for this type of spring, but is there a possibility of having it made in the custom design I was talking about?

 

[MikeHalloran]

Plastic springs are almost always made as a tapered cantilever leaf, integral to some other part in order to save assembly costs. The are almost always made in neat resin, because fillers and reinforcing fibers don't help the spring's fatigue properties, and the distribution of the fillers/fibers is not controllable in any real sense in injection molding, so the properties you can achieve in a real part are never close to what you get in a test coupon, or with pultruding.

As for that vendor, I have been buying Lee springs for a very long time. I trust their metal springs to behave pretty much as described, and their stock metal springs to be damn close to optimal for any properly designed application.

However that product is new to me, and I suspect to Lee also.

I might order a few hundred just to mess around with (did I mention that Ultem is expensive? ), but I'd be very hesitant to put it in a medical product until it's got fifty years of history behind it.

Is there some compelling factor that would cause you to prefer plastic coil springs over metal?


Mike Halloran
Pembroke Pines, FL, USA

 

[katelyncrist]

The problem is that the company is currently using an X-ray process to ensure that all of the springs are in place. This is taking time and money to do it.

I was tasked to develop a few ways to "poka-yoke" the design so that the springs (which are essential to the proper function of the pump) cannot be left out or forgotten. This was my best idea, apart from simple process controls like checklists and kitting the springs, which don't guarantee the springs actually made it into the part. However, with the high cost it looks like it will take to produce such a part from a plastic, the X-ray is starting to look like less of an issue. Here's to hoping my boss sees it the same way!

Thank you for all of your help.

 

[MikeHalloran]

All you need is some design for inspectability, or faster inspection methods.

As in:

It might take an analytical balance to get the resolution to detect a single missing spring, but try weighing the subassembly.

Add inspection ports so you can see the springs in the assembly.

Arrange it so that some portion of every part reaches the outer perimeter of the assembly, as in buying coil springs with a tangential 'tail' at one end, like a torsion spring's tang, that projects through a hole. Similarly for molded leaf springs; you can even use contrasting colors.

Use an automated spring dispenser at the assembly station; set it up to count the springs per assembly as they are dispensed.

Use an automated dispenser to fill a 'pallet', visually inspect that the pallet is full, and use the pallet to drop in all the springs at once.

Use an automated spring dispenser to feed a robot that places the springs; robots don't get tired or forget a step in their jobs.

You can surely think of more...




Mike Halloran
Pembroke Pines, FL, USA

 

[ornerynorsk]

Can a digital photo of a certain stage of the assembly process be added as verification documentation? It could be fairly fast, cheap, and easy to use. Most likely better on all counts than x-ray imaging, without sacrificing product integrity or reliability of a known design.

Just my 2 cents.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[MacGyverS2000]

Or what about some form of metal detector? Hall effect sensor? X-raying every batch seems like a rough way to prove the existence of springs in a product... wonder who initially came up with that idea...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[cloa]

And the OP decides he still doesn't want metallic spring- ceramic springs are still way better than plastic though way more costly and less impact resistant.

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