Re: [DSII] Genre - and details about casting/moulding

On Tue, 23 Feb 1999, Adrian Johnson wrote:

> >On Mon, 22 Feb 1999, Nyrath the nearly wise wrote:
> >
> >> Michael Llaneza wrote:
> >> > 
> >> > Plastic is expensive to set up a
> >> > production run for, but dirt cheap over a long production run. 
> >> 
> >>	I was always curious about that.
> >
> >i understand that the expense is in the moulds
> 
> Yes the plastic used in injection moulding is a
> thermoset, but the high temperature is not needed to get it to fuse. 
The
> high temperatures are needed to get the plastic to "plasticize" ie
become
> "plastic" ie gooey/runny.

oh, ah, erm, i was using 'fuse' in the sense my physics teacher taught
me,
meaning 'melt' (as in 'latent heat of fusion'). honest.

> For commercial injection moulded plastic parts (say,
> the front face of a PC or the case of the monitor you are looking at),

ha! i read my mail though a selectric teletype hooked up to a PDP-8!
actually, that's a lie. but it would be cool.

> the
> cutting tools are driven by 3D CAD data.  It takes a lot of time to
get the
> CAD data correct, and to then develop mould plans from that data.

is stereolithography used for this? that would seem a natural (if
expensive) way to make masters. of course, that doesn't help in making a
steel mould. oh well. how about electro-discharge machining?
metalcast.com
says:

"Do you think that stereolithography is the only 'buck rogers' rapid
prototyping method out there? Think again! EDM *disintegrates metal with
electricity*. Thousands of minature lightning bol tscarve the metal into
a
wide range of three dimensional surface contours."

of course, this is bound to be pricey. pricey, but cool.

> For
> models like GW tanks - picture the Eldar Falcon Grav Tank which is all
> curvy - they do not create 3D CAD data - they produce a master model
about
> 3 times normal size and then use that as a guide for doing the mould
> cutting.  This is all very time consuming and costly.

i've heard people refer to the use of pantographs in this context, like
those drawing reducers you get as kids toys.

> > i assume that resin is cheaper than
> >lead, but that for some reason it's no good for small minis. can
anyone
> >explain this?
> 
> Resin casting is a "low tech" process, as far as these things go.  It
does
> not require a large overhead investment in complex machinery, the
power to
> run that machinery, specialized factory space with reinforced
flooring, etc
> etc etc.  Moulds can be rubber, which is inexpensive.  It is very
suitable
> for small industries who do not do large production runs - like gaming
> miniature producers.	In the end, on a per-piece basis you spend a lot
more
> on material than you would with an injection moulded tank kit, but you
save
> in initial investments.

sounds ideal. resins all round!

> And yes, it isn't much good for small minis.

curses!

> You get a high reject rate
> with small figures because they require small moulds.  Small moulds
have
> tight air passeges (ie the arm or leg of a foot trooper in 25mm), and
there
> is an extremely high likelyhood that you will trap air in the mould,
> causing a rejected part.

so air trapping is the problem? hmm. that's just crying out for a
technological solution. maybe if you used open-cell foam moulds, so you
could suck the air out, but which the resin wouldn't leak through. if
the
holes were small enough, surface tension would keep it in. plus, the
smaller the holes, the less effect they have on the moulded surface. is
there a polymer chemist / materials engineer in the house?

> With spin casting, air is forced out of the mould
> by centrifugal (centripidal?? I always forget which is which) force as
the
> mould spins and the molten metal is forced into the mould cavities.

why can't you spin-cast resin?

> Hope this is informative - I wouldn't have rambled on so much, but
people
> do seem genuinely interested.

absolutely; minis are our lifeblood, so to speak. we take interest in
the
tools we use: minis, the list, statistics, and so on.

Tom