scott wrote:
>> I just notice that plastic objects always seem to be made of *the* 
>> thinnest plastic possible without breaking, that's all.
> 
> Well yeh, someone will have designed the thickness to withstand certain 
> forces, there's absolutely no point in making something thicker than it 
> really needs to be.  To you and me, saving 0.1p per plastic part seems 
> insignificant, but if you are selling 2 million products per month, and 
> each one contains 10 plastic parts, that adds up to quite a lot in the end.

Plenty of plastic items seem to not really be thick enough to withstand 
even "reasonable" levels of force. Still, I guess the designers 
considered that reasonable for the price.

And yes, I remember going to Maplin and ordering a big of 7400s. And I 
distinctly remember they weren't 21p each, they were something likt 
20.153p each. I remember thinking that was really weird - until I 


>> Does it vary significantly depending on which type of plastic you want?
> 
> Yes, some plastics are much stronger than others, have different thermal 
> properties (both at high and low temperatures), you can even add tiny 
> bits of metal or carbon to make them stronger, there are all sorts of 
> possiblities.

One would expect that - logically - the more sturdy plastics would be 
more expensive. But is that actually true? Or is it more driven by how 
difficult the pastic is to manufacture?

>> Do you buy the plastic with dye already added, or do you add that when 
>> you come to use it?
> 
> Dunno, we just use plain white, I guess the parts supplier orders the 
> plastic from the plastics supplier in the colour you want.

I suppose it depends what you're making. Some pastics are painted rather 
than having dye inside them. I suppose if you make lots of different 
coloured items, you add the dye yourself, otherwise you just order the 
colour you want...

>> Does it make any difference what kind of finish you want on the item? 
>> (Matt, shiny, textured, etc.)
> 
> Yes, but the surface finish is determined by the mould, not the 
> plastic.  If you put a rough surface on the metal mould you will get a 
> rough plastic finish, put an ultra smooth finish and you get a shiny 
> plastic part.

I'm presuming a mould with a fake-leather texture to it has gotta be 
pretty expensive to make...

>> Right. So tens of thousands of pounds just for the tool itself? Damn, 
>> I don't even wanna *know* what the machine to work it costs... o_O
> 
> But then that machine probably lasts for many years.

Heh. Not a tool for the curios amature. ;-)

>> Mind you, I guess if *you* had molten liquid poured onto you under 
>> extreme pressure thousands of times a day, you'd wear out pretty fast 
>> too... ;-)
> 
> Exactly :-)  AIUI it's the sharp edges in the mould which start to wear 
> first, and depending on how important those sharp edges are in your 
> design I guess it can be the difference between you only being able to 
> get 100k parts or 1000k parts from the tool.

Yeah, that figures.

I wonder - if you want a really smooth object, how long does that last? ;-)

[Of course, it's fashionable for electronic items to have sexy curves on 
the outside, but usually inside they're all sharp edges for bolting 
circuit boards to, "fins" for extra strength, etc.]

> 3D printers are useful for seeing what things look like, but if you want 
> to make more than a few pieces, or actually test them for strength and 
> tolerances etc then they're quite useless.  Usually if you are designing 
> something with volumes in the millions then you are going to want to 
> make at least a few thousand parts to test before committing to mass 
> production.

Yeah, I guess if you want to check the bits fit together, a 3D model is 
fine, but if you want to see how strong it is, you need to use the real 
material that the final product will use.

Reading Wikipedia, it appears that "most" plastic items are 
thermoplastics rather than thermosetting. Is that true? Does that mean 
there's a possibility of reusing your prototypes once you're done with them?

>> Right. So something like a few tens of thousands at least? (I guess 
>> like anything, it also depends on how fast you want to have the item...)
> 
> Yeh, if you want say 20k parts, then they can probably design and make 
> you some tools for that purpose, but don't expect the per-part cost to 
> be as cheap as if you went and said you wanted 200k parts per month for 
> the next 4 years.

It's all economies of scale. Evidently. ;-)

>> What is the tolerance on a typical hinge? (E.g., the hings on a CD case.)
> 
> +/- 0.1 mm is pretty easy to achieve in a single shot plastic injection, 
> that sounds about right for a CD hinge to work.

That sounds really damned tiny! o_O

OTOH, if it's easy to achieve, then why not?

(I would have thought the metal in the mould would expand and contract 
too much due to the heat, but clearly the designers know more about this 
kind of thing than I do...)

> A quick google reveals that chapter 6 of the USB spec specifies that :-) 
> The outline dimensions of the plug are +/-0.1mm, but some of the pin 
> spacings and inner dimensions are down to +/- 0.05

Wow. OK, that's pretty small...

Suddenly I'm wondering what the tolerances are for those *tiny* little 
screws they have on things like spectacles. ;-) Man, the thread on those 
is tiny!

> There are tables that specify tolerances of holes and things that fit in 
> holes, and what they should be for "very tight fits", "lose fits" etc.

You might even know this one... What is the total travel on the buttons 
of a mobile phone? I mean, you can "feel" them click, but visibly they 
hardly seem to move at all! (But then, I hypothesize that the inside of 
a mobile phone is a very cramped place.)

>> Plus of course anything that says "server" on it is instantly 10x the 
>> price anyway. ;-) But yeah - how the hell *do* you smooth all those 
>> edges? :-.
> 
> Clever tool design.
> 
> Tools can be quite complex, incorporating cutting edges, springs and 
> levers etc to do all sorts of fancy folds and things as the two halves 
> are brought together.

Damn. So after some guy designs what the final thing is even meant to 
look like, some other dude has to figure out what seperate bits to make 
and how to slot them together, and then yet another guy has to figure 
out how the **** to build something to make stuff that shape! o_O

You have to design a whole product to construct your product!

Heh. Damn... I wonder how they cut the mould to exactly the right shape? 
Now that crap must have some *crazy* tolerances on it...

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