> This leaves me wondering why so many pastical items seem to be 
> specifically shaped to be hollow so they use less plastic. I mean, if 
> plastic is so cheap, why bother with more elaborate designs to try to 
> conserve it?

I suspect the more elaborate designs are to save weight or allow space for 
other components.  Of course the plastic isn't completely free, so if there 
is nothing else to lose you may as well try to use as little as possible.

> Is this stuff priced in pounds of Kg or per tonne?

Definitely talking of the order of pounds per ton.

> Now, is that just the mould, or the entire machine? (I would have expected 
> the mould itself to be a replacable component, while the rest of the 
> machine isn't...)

That's just the "tool", the bit that actually defines the shape and where 
the plastic enters and flows, it fixes into a much bigger machine that 
actually moves the two tool sides together and supplies the plastic under 
great pressure.

> Really? I hadn't realised they wear so fast.

Yep, and that's the "hard" tools that are designed to last as long as 
possible.  During prototyping you often use "soft" tools that only last for 
a thousand shots or so (but are cheaper to make).

> By "large enough" we're presumably talking about tens of millions of 
> units?

Not necessarily, just large enough that you use up most of the lifetime of 
the tool, otherwise things get very expensive (and you would use a cheaper 
method of production than making hard tools).

> Yeah, figures. So a plastic cup is going to be pretty cheap, but a 
> complicated jig for holding a structure together isn't.

You got it, additionally if there are lots of sections that need to have 
accurate tolerances (eg because they attach to some other part) then it gets 
expensive.

> Perhaps I should clarify: I'm talking about a "keyboard" of the musical 
> kind. Somebody might attempt to put it on a stand that only supports the 
> ends of the unit, and it's over a meter wide. (Even so, all it contains 
> inside is a circuit board. No batteries or power supply or anything heavy, 
> just the key action.)

Ah ok, well yeh maybe they do, or they just use thicker plastic?

> Have you ever noticed that cheap PC cases always have jagid edges on their 

> smooth? (And the steel seems about 4x thicker, BTW.)

Yes, the tooling for nice smooth edges requires more steps (metal tools 
usually have several steps because it's impossible to bend and cut 
everything correctly in just one stamp) = more cost.

> I have no idea what steel costs per Kg, but presumably a lot more than 
> plastic!

I don't think it's vastly different, again it's usually the tooling and 
manufacturing costs that far outweigh the raw material costs.

> Also, presumably thick steel takes more effort to bend and cut...

Have you seen the size and general beefyness of the machines and tools that 
actually do the bending and cutting :-)

http://www.pdf-inc.com/img/press3.jpg

And an example of a tool:

http://img1.tradeget.com/sophiaxiafei/FR5WPIWW1progressive_die_zy1.jpg

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