When I saw the snap design I had to check if it was a joke or there was any hidden camera in my office. The designed snap should be assembled by the end user of the product! I mean a person not used to assembly plastic parts. The snap do not has a guidance to help the assembly, instead of a chamfer it had a fillet and the snap was too thick and created sink marks on the visible area of the part.
The snap design was modified to something close to the second picture. A good chamfer, with the same degrees of the other part, good to guide and assembly the part. The second important improvement was to reduce the thickness of the base of the snap in order to minimize the risk of sink marks.
The base thickness reduction is an important tip, but be careful. The snap becomes very weak at that area and it is difficult to fill and pack. Sometimes 1 small gusset at the back of the snap or 2 small ones at the front solve the weakness problems. If the snap needs calculations to get a specific snap force, or you want to check if the snap resin is working on the elastic or plastic area, it is important to take into account that the snap thickness is not constant and that most of the snap deflection will be on the lower part of the snap.
This is the first time I can see such snap and I must say that I am quite doubtful. Does this type of snap work fine? It seems to me that there are more complicated to mold, aren't they? I guess they must be designed in very special cases such as when the main thickness is too thin to prevent sink marks or when making a slot before the snap is not an option. Anyway, I will keep that design in mind, thanks ;)
ReplyDeleteBy the way, many design guides can be found about snaps and there is even on-line calculators but it is hard to find informations about the snap shape itself. Some make it with a fillet, other with a chamfer, ... I don't know if there is a sort better than another, although I usually design snaps the same way. Still, I'm curious to know: based on your experience, what would be the best snap shape? Could you give us some guidelines on this? Thanks in advance
ReplyDeleteHello again!
ReplyDeleteThe snap works, and works nice, I've used this kind of snap in a lot of parts.
They are more flexible than the constant thickness ones, but you can add small ribs / gussets at the front or the rear of the snap (also used in several parts with good results).
They are more difficult to mould, most of the designs I did the rib had around 0.8-1mm of thickness on its base, so it is not easy to fill the top of the snap that uses to have thicker walls. Compacting the snap is not possible, but enough to not to loose the function.
If there is not risk of sink marks or the part does not have aesthetical requirements it is better to use a standard snap design.
On the snap design I'm used to use a shape close to the right picture, to be honest I'm not sure if it is the best shape or not, it is the shape I'm used to use, and that's all.
ReplyDeleteThe BASF snap calculator is nice, but unless the assembly or disassembly forces are a requirement I prefer to design the snaps using "thumb rules" or what we say in Spanish "a ojo".
Hello ALTI-DEPTH,
ReplyDeleteI have some questions about snap-fits again, thought you might help me with it ;)
I have troubles designing a reliable hook/snap assembly. I'm often disappointed with the result as a matter of fact. I don't know where I'm doing wrong although I'm using "thumb rules" to design it and there might be a critical dimension that I miss.
First, I always put a clearance of 0.1mm between plastic parts when they are supposed to be in contact. Perhaps, should I remove that clearance to 0mm when I want a good hook/snap assembly. What is your point of view?
Also, I always leave some clearance between the surfaces of the hook and the snap that will be in contact. In fact, I'm always afraid that the parts do not fit on into another if I don't leave a small gap between them. However, I often find the assembly a bit loose. Perhaps there's no need for that. Could you help me on having a reliable design? Both for a permanent and removable snap-fit.
Lokking forward to reading from you ;)
Hello! I was out... this time for holidays :)
ReplyDeleteTo leave a gap 0.X mm on the snap is a good point, to leave 0.X mm in "all" the gaps is a mistake. Let me explain it in other words.
The snap has to be designed with zero gap in all the contacts except in one of the points, where a small gap is useful to absorb the tolerances. Unfortunatelly "nominal" tools are not possible.
Take care with the painted surfaces! if you have painted or coated surfaces you should leave the paint gaps. If you are making snaps for chrome plated parts, the design of the snap has to be different, the chromed parts are not flexible.
I like to design the snap with a small gap in one of the "contact" points and then adjust the snap after the first Tool Try Out.
I don't know if I explained it well... if you want, you can upload a picture (section cut) or a 3D, and I can take a look on it ad tell you what I think about it.
Regards!
Hello again! Sorry for my late answer, I have been busy these days...
ReplyDeleteI do not know how to upload anything here, I haven't found any link to do so. I will try to draw it with usual characters (I know, I know, it's not the best way but I'm pretty sure you will let me know how to upload a file in your answer so this will do the trick by now).
Let's imagine that we have two plastic parts, one with the snap [..] and one with the hook [xx] in the following usual configuration (I hope my drawing will be readable when posted...) :
__________ __
___ | |xxx| = E
|....\ | |xxx|
|.....\| D|xxx|
|......\ |xxx|
|.......| |xxx|
|.......| |xxx|
|...| |___|xxx| = A
|...| |xxxxxxx|
|...| |xxxxxxx|
|...| |xxxxxxx|
|...|__________ = B
C
Between two plastic parts, I usually leave a clearance of 0.1mm. So I would set B and C to 0.1mm. Is it correct according to you? Now, I don't really consider D as it depends on cases, I usually left enough space (minimum 0.2mm). For A, I don't know whether to put 0 or 0.1mm actually. And I believe that E also depends on the clearance A. What would be your recommended value for A, B, C, D and E?
Hello!
ReplyDeleteTo be honest I'm not able to fully understand your sketch :(
I don't know how to add pictures on the comments, but you can upload the image in http://www.imageshack.us/ and add the image link on your comment.
To talk about gaps, we need to know the main dimensions and tolerances of the parts you are designing. It is not the same to make a bumper of 4 to 5mm of main thickness than to make a micro switch of 0.6mm of main thickness. So please also tell me something about the main part thickness and snap dimensions.
I'm quite surprised about to have 5 different clearances on a snap design... I usually work with 2 functional clearances, and then big gaps in all other clearances.
Actually, I do have 2 functional clearances (A & B in my previous sketch), I just wanted to know what you usually put in other areas in critical cases ;) Anyway, you should find a better picture here: http://img864.imageshack.us/i/79781268.jpg/
ReplyDeleteOn this picture, the cover (in green) has a main thickness of 1.8mm. The cover is inserted into an overmolded assembly made of plastic (in black) and steel (in gray). So the snap is actually fixed and it is the hook on the cover that is deflecting when inserted. There is another part inside which doesn't matter in our case. Tell me what you think.
First of all tell you that the application looks interesting :) In this design case it is not possible to use any snap design software, because your snap is a "not flexible" snap, and it is made of plastic overmoulded on a steel frame... so the behaviour of this snap is not "standard".
ReplyDeleteIt seems a 360º snap, right?
0.1mm dimension --> is this dimension marking the assembly position? I mean is this the end-of-assembly, so what is defining the final placement of the cover? Or it is a "useless" gap.
From my side it looks a too small gap. Any deviation on any of the plastic parts could create a system not able to be snapped (unless your injection tolerances are lower than 0.05mm). If the parts have interference before to snap, they won't be able to snap.
0.111 & 0.124mm dimensions --> Same as on the 0.1mm dimension. Any tolerance deviation could create an interference between the cover and the "case". If there is an interference the snap will not work as supposed to. If the snap is 360º you have an additional problem, you can have a very good tolerance on a section but a bad "roundess" tolerance and to have interferences :( Any interference on these 2 areas will create a "non-snaped" area, what could not be acceptable for your application.
0.1mm & snaping gap --> there is a gap on the snapping area. I mean the gap from the diagonal of the case to the flat (horizontal) surface of the cover. This gap is very important and has to be compatible with the 0.1mm gap. At this moment you have gap in the snap, and gap between the parts (the 0.1mm gap) it means if any part is at nominal dimension the part won't be assembled on the nominal position but 0.1mm moved and the snap won't be in "working" position. The part will rattle.
I really don't know if you can understand anything I wrote... I think my explanations are not clear :(
Suggestion: Add a chamfer (same angle as the snap) on the top of the green part to improve the assembly.
Suggestion 2: maybe it is better to continue this comments by email... easy to make long comments and to add drawings... here you are my email ttravesa@gmail.com.