Enclosure design review 2

[ivan1237]

Hello Engineers!

I'm working on the design of an enclosure for electronic stuff.

Being an electronic engineer, I've check tips and articles on plastic injection to get something realistic and not too expensive to produce. But now I'm getting to a point where i need some help to review my design.

If you check the attached drawing you'll see a flat bottom with many ribs (do I need all of them?). The electronic pcb is holded on the side ribs (+ few points inside). Behind you'll see the holding "ring".

I plan to use 2 screws and some "snap" stuff (in the front) to close the enclosures...
I'm also worring about the tightness of the assembly (shall I add an "o-ring" in the assembly profile?)

But now my question is about the draft angle on the ribs and walls. Where should I take the reference plan from? Would it be the top assembly face? Or the flat bottom face?

Please give me some comments!

Any link to technical documentation (or example) would be welcome too!

Thanks in advance for your replies!

Ivan

 

[patprimmer]

You should talk with a moulder, a toolmaker and a material supplier, preferably all together at some time.

I have no idea if the ribs are excessive or inadequate as I have no idea of the loads and temperature or material or expected life.

I have no idea if you require an "O" ring as I don't know what quality of seal you require.

If you need a tight seal, remember plastics creep with time and some plastics solvent stress crack in the presence of some rubbers.

Regards

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[ivan1237]

Thanks Patprimmer! You're quick!

Here are some more details about the parts:

The part is the base of a display attached on the front tube of a bike (or bicycle).

The load on the part is not much, just as someone pushing on buttons. The temperature range is given by the outdoor condition (-10°C to 40°C).

The enclosure should be tight against water splach (eavy rain).

And I will talk to a moulder and material supplier next week. I'm trying to be there as prepared as possible!

Should the green face be the reference for the draft angle?

Thanks for your interest

Regards

 

[patprimmer]

Draft angle should be relative to line of draw.

Regards

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[MacGyverS2000]

If this is something the size of your fist (give or take), the amount of ribbing is quite excessive. I know just enough about molding to be dangerous, so take that with a grain of salt, but look in any comparable-sized ABS package and you won't see that kind of ribbing. A sufficiently thick case will not collapse under the pressure of a finger press. Consider looking at similar-sized items, such as the TV remote on you coffee table.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Oh yes, the ribs are both excessive and poor design. There should be less, but they should be continuous to avoid stress risers and soft spots at the end of each rib.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

Imho: First thoughts:

You have a very complicated part requiring a very complicated tool - side cores etc. (for complicated read expensive!)

I would remove all but the functional ribs as most of the assembly stifness will be due to box shape when the other half is fixed. Ribs are time-consuming to put in, give sink marks on the visible faces and waste material. They can be put in at a later date if really required. (ribs should be no more than 0.6 x wall thickness for amorphous materials such as ABS) Cored out pillars are cheaper to produce in the tool than ribs.

I would seriously look at the clamp ring arrangement with a view to making the tool "open and shut". To make a separate saddle bracket tool would be cheaper than trying to mould all in one as drawn.

Harry

 

[ivan1237]

Thanks for your comments!

The few ribs needed to hold the pcb in place with the right angle will be kept, the other ribs will removed.

Regarding the bracket, Harry, do you mean it would be cheaper to make 2 parts assembled (e.g. with screw)?
(We plan to produce about 3 to 5000 pieces)

I will post a new design soon!

Thanks again for your help!

 

[MacGyverS2000]

Absolutely! I never even considered the holding ring as part of the case, it just seemed obvious to me that it would be done as a separate piece. It would also open the possibility of two different materials, say ABS for the case and something more springy for the ring (though ABS may still be a fine choice).

If you had shown that design to a molder he would likely have quoted you several dollars per piece rather than several dimes.

You will get a bit of water resistance (splashes) from the piece as you have it, but certainly not water proof. Adding a rounded "lip" to the entire surround may help... some snap-together alarm keyfobs have this, like the entire DEI line (Clifford, Viper, etc.). For the buttons, you'll need to design your rubber membrane carefully with this requirement in mind. That hole on the side is a possible water entry point, particularly if you're using some form of power connector there (notorious for not being "to spec" in placement or size)... a grommet may be necessary.

As Pud suggested, replace those solid pillars with cored ones, as if you're going to put a screw in it... which will also give you a possible screw-down location for the PCB if it's decided later you want some more stability from key presses. The ribs coming off of the pillars do not need to be long at all (about as long as the shortest ones you now have).

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ivan1237]

The side hole will be used to have the cable out. This will be equiped with a rubber-style strain relief...

What do you mean by "rounded lip"? Can you explain it a bit more?

As for the buttons, I'm not yet done with the design and difficulties... I'll be back with more questions soon!!!

Thanks

 

[MacGyverS2000]

The vertical wall that is within and above the green piece... at the top of that wall, place a light (almost) half-round along the outer portion. Place a similar half-round along the inside wall of the mating top piece. There will be enough give in the plastic so they snap past each other and it tends to make for a fairly high water resistance (again, not "proof", just resistant).

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ivan1237]

Ok Mc, I got it but then how does the part go out of the mould?

 

[MacGyverS2000]

There's enough give in the plastic to kick it out... the lip is maybe 0.5mm in radius away from the vertical wall.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

It depends on the plastic. Polypropylene will jump over quite big undercut, but Polycarbonate will only jump over a very small one. Talk to the raw material suppliers about that sort of detail. I am not that familiar with ABS, but I suspect it will jump over a reasonable undercut. It will vary a lot from grade to grade.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivan1237]

Dear Advicers!

After visiting a protoype & tooling company, I'm back with my design modified (see side-cut view attached).

The housing woulod be made of Polycarbonate. The wall are 2mm thick. No "ribs" are needed. The bottom and upper shell fit together within a small straight slot and 2 or 3 screws.

I need advice about the buttons interface. I plan to use "rubber style" soft polymere for the "membrane". It should press the button with a max of 0.2mm course.
Would my design be "water resistant"?
Would it be possible to mount it in?
Any advice how to have this function done?

Thanks again for all your advices and ideas!

With my best regards

Ivan

Ivan, Owner
STEIMER ROBOTICS

 

[MacGyverS2000]

Yeah, I'd call that "water resistant"... assuming the case fit is snug and the button rubber is pliable enough.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Wht polycarbonate.

It is relatively expensive, relatively hard to mould and unpredictable in performance due it's strong tendency to solvent stress crack i the presence of some very common chemicals.

It has good UV, rigidity and toughness under good conditions.

The plasticisers in PVC and some other elastomers can solvent stress crack it.

Many lubricants stress crack it.

ABS seemed a reasonable choice. Polypropylene would also be a reasonable choice unless you want to glue it.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

I'm with Pat on the ABS. Less potential processing errors to give you a crud product. Colours will be cheaper too as lower melt temp. If you need transparent then acrylic or SAN.

Cheers

H

Good luck.

 

[ivan1237]

Thanks for your comment.

We choose PC for the UV and rigidity. Is ABS as good for outdoor condition?
If cheaper and easier to manufacture, then I'll change to ABS of course!

Can we easely glue on ABS?

There will be a glass to protect the LCD, we plan to use PC again. It's the best transparent material, isn't right?

Regarding the button, I've a question about sharpness.
What would be the sharpness value of such "rubber" (very flexible) material?
I've heard about possible sharpness class A 30 to 80. What would be the hard one, 30 or 80??? (I don't know this unit...)

Thanks again for your advices!

Best regards

Ivan, Owner
STEIMER ROBOTICS

 

[patprimmer]

ABS is poor to UV.

ASA is better.

PC is best transparent material? There is no best transparent plastic like there is no best metal. What is best depends on what you want to do with it. Gold makes a poor crankshaft for a high output engine and magnesium makes poor electrical contact breaker points.

Certainly a correctly coated PC window will perform well as a lens on an outdoors instrument. High impact acrylic might also work. Unmodified acrylic will be excellent IF there is no real impact involved

If the lens is to be PC and glued in, then PC makes sense for the body due to compatibility when gluing.

Mar resistant sheet PC has much better overall properties than injection moulded PC components for reasons of the protection from solvents and scratching from the coating and the much higher molecular weight.

This is the first we have heard of a clear lens being included.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.