Plastic Enclosure Design 5

[rc3ng]

Do you have suggestions on what process is the most cost efficient to fabricate limited number of plastic boxes of 24"x36"x2"? the box can be made with one face removed (let's say the large face:24"x36")?

I believe any process involving a mold is automatically eliminated since the quantities are limited....

Any suggestions?

Thanks!

 

[KENAT]

Will depend on lots of factors, however I might look at thermoforming - especially vacuum forming - since the molds can be relatively inexpensive, especially for very low volume, simple shapes.

However, for most molding processes some kind of draft will likely be required.

You might want to look at cast urethane and variations there on but the molds can get a bit expensive.

You could also look at fabricating from sheet plastic. Either bonding from multiple pieces or even forming a bit like you would sheet metal

http://www.envplastics.com

is one company I saw at a trade show that does this.



For better answers you may want to let us know what type of 'plastic', why it has to be plastic (could it be sheet metal, composite [such as fiber glass] or some other material) and what you mean by 'limited number'. The best answer for 2 may be different than for 5 may be different for 25...

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[1gibson]

Acrylic (or other) sheet, tape the backside and mill it almost all the way through with a 45* V bit (do not mill through the tape.) Put glue in the grooves, fold up the sides, tape together while it dries. Put the top on using preferred method.

Is limited like 5-10, or 50-100?

 

[rc3ng]

Thank you very much for your reply!

For my application implementing a draft will be possible...I should think of vacuum forming.

I like the sheet metal idea used by envplastics.com ... I'm wondering if the final cost will e more than real sheet metal or not.

I haven't decided on the type of plastic yet, but it should be flame resistant since electronics parts are going to be installed in the box-as if I were making LCD TVs that are going to be mounted on the wall.

To sum up:

Material: Flame Resistant, not very brittle, not very soft (just like the touch of a tube or LCD TV housing) and of course not expensive!
Quantity: About 20, if the project succeeds, my be up to hundred. But 20 is the answer for now.
Why Plastic: I already am using sheet metal. Works perfect but the costs are higher. I'd like to make this enclosure as inexpensive as possible....please tell me if you have any less expensive suggestions!


I appreciate your comments on this.

Thanks!

 

[rc3ng]

@1gibson: Thank you for your reply. It seems like the "sheet metal" approach. I like this approach, does it have a specific name? maybe "line bending"?

By limited quantity I mean about 20 pieces.

 

[KENAT]

Talk to a vac forming shop, I've had different stories from different ones so it's hard to say. 20 may be high for simple wooden tooling but they may be able to take a resin splash of the mold or some such to make it last longer. Also vac forming won't hold super tight tolerances

20 would suit cast urethane but molds for that process typically last only 25 units (some places claim 50) so you'd have to reinvest in tooling several times.

With the volumes you're talking, certainly the 20 mark, I'm not sure even vac forming will make sense compared to sheet metal - and on sheet plastic I'm not sure what if any cost saving there'd be.

(By the way, I ruled out rapid prototyping (FDM, SLS et.) processes based on your size - but I could be missing a trick I suppose.)

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[rc3ng]

Thank you! This is a lot of information I can start with.

Regarding the plastic sheets: I just read in envplastics.com that their method can compete with sheet metal when the sheet metal process involves welding, grinding and painting. I think for my case sheet metal will be cheaper than sheet plastic.




Other than thinking of sheet metal and plastics, is there any other way to make a casing for electronics?


Thanks again

 

[MikeHalloran]

In every instance of which I am aware, the decision to house electronics in plastic for cost reasons later became more expensive because when you get around to controlling EMI/RFI radiation and susceptibility, you end up with either a steel box or a steel lining within the plastic box.

Now the younger sparkys will chime in and say that aluminum is just as good as steel.
Well, it ain't.



Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

Now that it is clear this is a sub-assembly for you (and not your finished product) I have the following suggestion:

This type of stuff is a cakewalk for plastics fab shops, shops that make plastic display cases (trophy cases, sports collectibles, etc.) so it may be worth your while to find a vendor and discuss your requirements. Worst case you will get some practical design advice, and material recommendations.

 

[KENAT]

Mike, care to expand on your Al V Steel comments? I'm familiar with the 'mechanical' difference but are you thinking shielding or some such?

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[MikeHalloran]

Aluminum is wonderful stuff, but it's almost completely transparent to magnetic fields.
Steel is cheaper, even when plated with nickel for corrosion resistance, and is effective against both magnetic and electric fields.
The third alternative that will inevitably come up is plastic coated with metallic particles, by means of metal spraying or paint. The problem with that scheme, in addition to the horrendous expense of conductive paint, is that it's impossible to ground it reliably.
In order to be effective against both E and H fields, the metallic shield surrounding the electronics must be almost completely continous, and grounded. The allowable gap size is dependent on the frequency involved, and is now down to a couple of millimeters, thanks to cheap fast computer chips.
For a really good example of effective contemporary shielding, open the case of a PowerMac G3 or similar. Inside the plastic case you will find a nickel plated steel shell, with teeth lining every edge. Be careful; the steel is thin and the edges are razor sharp.


Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

Thanks Mike, kind of what I was expecting but I was more than open to learning a new trick!

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[rc3ng]

Thanks everyone for the great comments.

 

[HDS]

Does it need to be custom?

Why not an industrial enclosure with a hole in the door for the screen?

 

[rc3ng]

@HDS: Unfortunately I was not able to find any enclosures with dimensions close to what I want

 

[KENAT]

Plus, there's always the risk the commercial enclosure will change, be discontinued, have supply problems...

We face this issue every now and then.

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