Stiffening element Adhesion principles

[elinah34]

hello,

we have 2 plastic parts that should be bonded to each other with structural adhesive as described in the illustrative picture below:

I have to point out that all of the photos below aren't the exact design itself but an illustration I creadte very fast.
one of the concerns regarding this design was the adhesion interface that might be subjected to peeling due to bending as described in the illustrative picture below:

one of the solutions was adding another bonded part as described in the illustrative picture below:

The suggested manufacturing process is first bonding the added part while it has extra material outside and afterwards mutual machining.
The added part has to be with a small thickness, so the whole part might be able to be assembled in its destined place.
The mutual machining is for assuring the high precision that is critical.
Here is an illustrative description of the manufacturing process:

Now we are getting to the reason I am writing here - we had some argues about the end condition of the added bonded part.

1. one of the concepts is finishing with "zero" thickness that is optional due to the fact we have mutual machining.

The motivation was to get the best gradual stiffness transition, but some said the edge will be detached when finishing the machining process.

2. Another option was to have an end with a thickness of about 0.5 mm for not having a "zero" thickness end.

3. The last suggestion (that can't be accomplished since the added part isn't splitted but whole, but I still want to hear what you think) is having a kind of a face to face end with one of the parts:

I would like to hear what is your opinion.
As for now the leading option is he first one, and I am intrested to know if there is any risk in it.
if you have any guide book which deals in such issues - I will be happy to get a link.

Thanks!

 

[drawoh]

elinah34,

Are these parts solid? If you have problems with adhesive peeling, why can't the blue piece be made thicker and more rigid? Is there a good reason to have two pieces?

Is the added part a ring, or an added on strip? If it is a ring, it must be very precise, and expensive. IF you wrap the thing in a strip, it will be thin and it won't have to be precise.

--
JHG

 

[elinah34]

Yes, these parts are solids.
The blue part can't be changed, but don't let the pictures mislead you - the proportions in the illustration isn't perfect and in reality the blue part is quite stiff, but peeling can still happen due to bending, no?

The added part is a precise ring.

 

[drawoh]

elinah34,

Are these parts machined? Plastic is hard to machine accurately. If the added part is thin, accuracy is even more difficult. Do the contact faces have to be flat?

--
JHG

 

[desertfox]

Hi elinah34

Why can’t the whole thing be moulded as one part?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[elinah34]

The plastic parts are indeed machined. We actually have some experience with machining plastic, and we get nice results - profile in a boundary of 0.1 mm.
The contact faces are flat today, and we aren't allowed to change it.

 

[elinah34]

desertfox, these are different kind of materials.

 

[SnTMan]

I'd think you'd want to avoid the zero thickness option. Is is possible to increase the overlap between the two, for a larger bond area?

EDIT: What's the tolerance on zero thickness?

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[elinah34]

SnTMan, interesting comment - why do you think I should avoid the zero thickness option? This is the one that has a gradual stiffness transition.
I can enlarge the area only a little bit.
The tolerance is +/- 0.1 mm

 

[SnTMan]

The tolerance is +/- 0.1 mm

If I interpret correctly you may undercut the base part by 0.1 mm. I wouldn't.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Pud]

What's the size of this assembly?


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[SWComposites]

Worrying about tapering the lower end of the pink strap is a waste of time. The bond will fail between the pink and blue parts.

 

[mfgenggear]

I believe a better design would have been to nest the blue and yellow parts. even if it is required to machine both parts prior to assembly.
then bond the two parts. I don't do this type of work, however to my knowledge bonding to different plastics can be tricky.
and depending on the type of plastic. I would also look at plastic weld.

 

[3DDave]

If this is to add a shear path to the existing tensile path as the primary then that doesn't look as helpful as expected for the effort of doing so. (Lotta work for not much result) Is it the case these parts have wall thicknesses that are far less than the proposed additional piece?

The "zero thickness" part isn't where it would fail - partly it cannot be zero thickness as shown as the adhesive has some thickness, so it will taper out over the tapered layer of adhesive. How durable it is depends on how comparable in hardness the adhesive is to the base materials. If a dig can start then leaving the reinforcement thick will act as a (weak) stop to prevent progressing.

The bigger problem is that the joint is unbalanced with so much adhesion area on one side and very little on the other. That's a lot of extra material on the yellow part that will still be attached after the blue part is long gone.

If those parts are solid, then the additional section is a waste of time unless they cannot be adhered to each other.

 

[elinah34]

Thanks for the comment.
By the way, the added part is bonded to both parts and not only to the yellow one.
I understand and agree with your comments - but taking in account that no change to both primary parts is possible, leaving it like this is preferable?