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'[OT]: custom plastic enclosures (was: [EE]: Your t'
2002\05\09@025132 by Picmicro List

picon face
Hello Byron and the List,

Even though the topic has been beaten to death, I still find it interesting
to read other people's opinions on PCB design and production.  :)

However, I think that unless Byron's market consists of people like himself,
the boards will eventually need an enclosure.  I have a few questions for
the experts:

1. How would one go about designing a custom plastic enclosure?  What
software is available for that purpose?  Is creating a 3D model of the
enclosure worth the effort, or should I let the manufacturer do the work?

2. Obviously, when creating the enclosure, I should keep in mind the size,
shape, and locations of display and controls.  But shouldn't the
characteristics of enclosure be kept in mind when designing the PCB? How is
this done?

3. Does it make sense to use custom enclosures in a project like Byron's,
100 units or less?  Or would it make more sense to use standard customized
enclosures?

Thank you for your time!

Vitaliy

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2002\05\09@040335 by Alan B. Pearce

face picon face
>Is creating a 3D model of the enclosure worth the
>effort, or should I let the manufacturer do the work?

Well depending on how you want to go about it, if you wish to start by
having a an enclosure to visualise with, carve a wooden block to the desired
internal dimensions. The get a piece of perspex or similar suitable plastic
sheet and put it in the wife's oven with just enough heat to soften the
plastic. It is then possible to use a household vacuum cleaner to vacuum
mould the sheet over the mould.

Sure there are some bits I have left out, like how to get a suitable seal to
get the plastic to suck down, but it is do-able for one-off type
prototyping. Should give you something to play with, and can then take along
to an enclosure manufacturer to say "how much to make this" or "do you have
anything about this size and shape" type questions.

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2002\05\09@094943 by Spehro Pefhany

picon face
At 11:47 PM 5/8/02 -0700, you wrote:

>1. How would one go about designing a custom plastic enclosure?  What
>software is available for that purpose?

You can use a pencil and drafting board- but I'd recommend
Rhino, Solidworks, probably Pro/Engineer (the latter is rather pricey).
You can also work with the traditional clay models, which might be better
for some purposes (if it has to fit a hand, for example). The programs
work with 3-D meshes using Non-Uniform Rational B-Splines (NURBS) that can
be used to create 3-D CNC toolpaths for making hard tooling without
digitizing. The SPE (Society of Plastics Engineers) has a very good
book on designing parts that are to be molded (wall thicknesses, snap
and hinge designs, minimizing undercuts and resulting expensive slide or
unscrewing mechanisms, cavity-count trade-offs, runner designs, core
cooling requirements, selecting plastic materials, and so on).

>  Is creating a 3D model of the
>enclosure worth the effort, or should I let the manufacturer do the work?

If you have a 3-D (computer) model, creating a 3-D solid prototype is a
very good idea before tooling is made. There are companies that specialize
in this (for a price) or for simple ones you can do it yourself. Typically
the finish and material type of the prototypes won't match that of a
molding, but the part can be sanded and painted and sanded again to
simulate the appearance of a given class of mold finish.

>2. Obviously, when creating the enclosure, I should keep in mind the size,
>shape, and locations of display and controls.  But shouldn't the
>characteristics of enclosure be kept in mind when designing the PCB? How is
>this done?

Iteratively? More experience, or pre-existing examples -> fewer iterations.

>3. Does it make sense to use custom enclosures in a project like Byron's,
>100 units or less?  Or would it make more sense to use standard customized
>enclosures?

Injection molding is seldom economical for production of less than ten(s)
(of) thousand(s) of pieces due to the high cost of mold design and mold
making.
Shopping around for the mold, using MUD or mold inserts rather than a full
mold base, and simplifying the part can change the qty by 3:1 maybe but
for 100 pieces it would most likely be over $50-$300 per unit up-front
cost, so only high value applications could afford it. Mold designers,
mold makers, and flow analysis people are quite well-paid in the developed
countries, and without skilled people, it WILL be visible in the product,
and the apparent value to the customer will be reduced, perhaps
drastically for fussier customers.

On the plus side, running the mold can be economical for moderate-sized lots,
and the quality and consistency can be very good for a well-made tool.

(Casting and other low-volume methods produce crude parts that require much
rework and have inferior characteristics. Low volume prototype molds produce
low shot counts for a mold cost 1/2 to 1/3 a production mold, but the
quality is not as good, and the parts will deteriorate near the end of
the life (which may only be 100 to 1000 pieces)). Often standard enclosures
can be modified by machining or by changing an interchangeable part in the
mold so that would probably be the best solution for very small quantities.
Your product will tend to look like all the others that can't afford a
custom enclosure, but beggars can't be choosers.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2002\05\09@100359 by SM Ling

picon face
Sound interesting !  Really keen to try this.  Any good URL to share?

Cheers, Ling SM
> Well depending on how you want to go about it, if you wish to start by
> having a an enclosure to visualise with, carve a wooden block to the
desired
> internal dimensions. The get a piece of perspex or similar suitable
plastic
> sheet and put it in the wife's oven with just enough heat to soften the
> plastic. It is then possible to use a household vacuum cleaner to vacuum
> mould the sheet over the mould.
>
> Sure there are some bits I have left out, like how to get a suitable seal
to
> get the plastic to suck down, but it is do-able for one-off type
> prototyping. Should give you something to play with, and can then take
along
> to an enclosure manufacturer to say "how much to make this" or "do you
have
> anything about this size and shape" type questions.

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2002\05\09@101451 by Bond, Peter

flavicon
face
> Sound interesting !  Really keen to try this.  Any good URL to share?

http://www.studiocreations.com/stormtrooper/vacuumforming/

A pretty good low-cost intro to vac forming.

Peter
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