> 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.

> 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.

> 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.

> 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.

> 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.

> 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.

You know in the days of LPs, you could pay extra to get the high quality 
cuts that were guaranteed to be in the first batch from the set of tools. 
The ones that were made near the end of the life of the tool were lower 
quality.

> Presumably for prototyping they tend to use all the "3D printers" we keep 
> hearing about? (Do they use soft tools to check that the tool design works 
> right?)

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.

> 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.

> 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 (if the tolerance was much 
bigger then either the two halves would keep falling apart with very little 
force, of it would be too stiff to rotate easily).

> How about something that's supposed to snap together? And while we're on 
> the subject, how about something like a USB plug? (Presumably that needs 
> to be a snug fit, but not "too" tight.)

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

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.

> 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.

>> http://img1.tradeget.com/sophiaxiafei/FR5WPIWW1progressive_die_zy1.jpg
>
> What the *hell* is it a tool _for_ though?

In that case it is just two lumps of metal that are pressed together very 
hard with a thin sheet of metal between them.  When they are pulled apart 
again the sheet of metal has magically been cut and bent in clever ways to 
the shape that the designer desired :-)

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.

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