scott wrote:
>> 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.

I just notice that plastic objects always seem to be made of *the* 
thinnest plastic possible without breaking, that's all.

>> Is this stuff priced in pounds of Kg or per tonne?
> 
> Definitely talking of the order of pounds per ton.

Wow, as cheap as that? o_O

Does it vary significantly depending on which type of plastic you want?

(I'm still trying to figure out why the plastic cooking utensils my mum 
bought actually MELT IF YOU GET THEM TOO HOT. I mean seriously, *why* 
would you design a product which can't be used for its intended purpose?)

Do you buy the plastic with dye already added, or do you add that when 
you come to use it?

Does it make any difference what kind of finish you want on the item? 
(Matt, shiny, textured, etc.)

>> 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.

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

>> 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).

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... ;-)

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?)

>> 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).

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...)

>> 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.

What is the tolerance on a typical hinge? (E.g., the hings on a CD 
case.) 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.)

>> Perhaps I should clarify: I'm talking about a "keyboard" of the 
>> musical kind.
> 
> Ah ok, well yeh maybe they do, or they just use thicker plastic?

Actually, thinking about it... It probably *is* metal for EM shielding 
purposes! ;-)

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

>> beautifully 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.

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? :-.

>> 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.

Well I don't know - they like to charge you 10x for PC cases that are 
aluminium rather than steel. Is the metal that much more expensive, or 
is it just a luxury premium?

I'll bet *wood* costs more! ;-)

>> 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

OMGWTFBBQ!! O_O

I wonder what the electricity bill for that is like? :-P

Reminds me of a TV program I saw. Guy wanted to make some new leaf 
springs, so he goes to the metal shop. They have a hydraulic press, and 
we watch it casually snip through 23mm spring steel in a way not unlike 
scissors cut through tissue paper. (!) "Mind your fingers" the operator 
says. o_O (Fortunately, the machine rolls the prescribed length of sheet 
past the cutters and automatically cuts it, so your body parts don't 
have to go anywhere near the vicious thing!)

> And an example of a tool:
> 
> http://img1.tradeget.com/sophiaxiafei/FR5WPIWW1progressive_die_zy1.jpg

What the *hell* is it a tool _for_ though?

Post a reply to this message