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

>On Mon, 22 Feb 1999, Nyrath the nearly wise wrote:
>
>> Michael Llaneza wrote:
>> > 
>> > IMHO the best thing GW ever did
>> > was the plastic Epic-scale infantry. Plastic is expensive to set up
a
>> > production run for, but dirt cheap over a long production run. 
>> 
>>	I was always curious about that.
>>	An uninformed layman would think that metal miniatures
>>	would be more expensive to produce than plastic ones.
>> 
>>	I guess plastic is less forgiving a medium to work with.
>
>i understand that the expense is in the moulds; since lead melts at
quite
>a low temperature, you can use a vulcanised rubber mould, and you can
make
>that yourself with comparatively little effort or expense. however, the
>lead is expensive. now, the plastic used in minis is a thermoset, so
you
>have to heat it to a high temperature to get it to fuse. thus, you need
>steel moulds, which are very pricey. however, the plastic is cheap.

That's almost right.  As several others have pointed out, the vulcanized
rubber moulds are quite inexpensive (relative to metal moulds) - tho'
they
don't last as long.  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.  To get the plastic into all the tiny detail
crevices in a mould (look at a tank kit or a GW troop sprue - lots of
small
detail) requires enormous pressure - both to maintain plastic flow and
to
expel all the trapped air in the mould.  The pressures are commonly over
a
thousand PSI, hence the need for steel tooling (moulds).  These are very
expensive, 'cause it is difficult to cut steel accurately - well, not so
much difficult as really time consuming, and requiring specialized metal
working equipment.  For commercial injection moulded plastic parts (say,
the front face of a PC or the case of the monitor you are looking at),
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.  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.

As I said in an earlier post, injection mould tooling even for a small
and
simple part will be in the tens of thousands of dollars - so you need to
produce a lot of parts to make it worth-while.	The cost of the plastic
is
minimal - your average tank kit will use less than $1.00 worth of
material.
 The cost is in machine time (for the moulding equipment - which is also
expensive, by the way) and amortizing the tool cost over the production
run.  

Having said all that, I have a pretty good idea what it actually costs
GW
to produce one of their big tank models - say the "Leman Russ
Demolisher",
with a full GWscale tank and a blister pack of pewter detail bits.  I
figure it costs them about $1.50 to $2.00 for the plastic parts, maybe
$1.00 to $3.00 for the metal parts, and maybe $4.00 to $5.00 for the
packaging.  And they sell the kit for over $50.00 here in Canada - they
make a HUGE profit margin, even accounting for shipping and distribution
costs.	(These costs may be high, too...)

>
>thus, you use plastic in steel moulds for your large-volume,
low-diversity
>stuff, like basic tanks and troops, and lead for the more exotic and
>diverse stuff.

yep.

>
>of course, everything GZG makes is exotic and diverse, so they only use
>lead :-). well, except for the resins. 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.  As I said above, the cost in injection moulding
is
the equipment and tool costs, not material.  With resin casting, it's
the
resin that's one of the two expensive parts - the other is the low
production rate (you get a lot of injection moulded parts in the time it
takes to do one cast part).  In the end, the only way you could do the
kind
of things people like GZG, GeoHex, Armorcast, etc. do is to use a
process
like resin casting.

Oh, resin is a less expensive than lead by weight, and a lot lighter
too...
 And yes, it isn't much good for small minis.  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.  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.  In
injection moulding, it is the injection pressure that forces out the
air.
With resin casting, you just have gravity working for you - the weight
of
the resin pushing against the air.  Also, it is difficult/impossible to
have proper air venting in a cast resin mould to allow trapped air out -
the material takes so long to set that you might have problems with it
draining through an air vent (or plugging it up after one or two casts).
With spin casting, the material sets almost immediately, and with
injection
moulding there is cooling liquid passing through the moulds to cool the
plastic very quickly.  Injection moulding typically has a cycle time
measured in seconds (except for really big stuff).  Spin casting in
minutes, and casting in hours.

Incedentally, the reason why most cast resin parts are flat and blocky,
or
have smooth curves (I'm thinking of the GZG vehicles and the Armorcast
GW
40k scale models) is that these are less prone to trapping air bubbles
under comples detail AND they are much easier to demould.  Complex
detail
on cast parts would result in more abuse on the mould each time you pull
a
part out.

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

Adrian