> On Mon, 22 Feb 1999, Nyrath the nearly wise wrote:
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,
yep.
> of course, everything GZG makes is exotic and diverse, so they only use
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.
About a year ago, I remember seeing something about "precious metal clay". It
was a clay like substance that contained some form of silver or gold. The clay
could be shaped to what ever the sculptor wanted and then baked in an oven for
a short period. The sculpture would shrink a certain percent, but would then
be solid silver or gold (I'm not sure what purity, though).
It would be really nice to have a steel, aluminum, or pewter equivalent of
this. Has anyone else heard of anything like this?
Thanks, Nathan
> -----Original Message-----
they
> don't last as long. Yes the plastic used in injection moulding is a
The
> high temperatures are needed to get the plastic to "plasticize" ie
In
> injection moulding, it is the injection pressure that forces out the
> Actually I don't think its either. Centrifugal force is a "fictional"
Well, that's what I meant...:) This "fictional" force is the same one that
holds water in a bucket as you swing it around over your head... I
know the physics of it, you know the physics of it - we're talking about
the same thing... but instead of saying "we accelerate the metal up to x speed
in the spinning mould, and it's inertial tendancy is to move in a straight
line in the direction of the acceleration vector but can't 'cause the mould is
in the way so it moves along the route of least resistance further into the
crevices in the mould forcing the air out 'cause the metal is more dense,
yadda yadda yadda" we use the shorthand term "centrifugal force". Yes there
isn't really a force there. Everybody has swung a bucket of water around their
head, though, and knows exactly what we mean.
Except physicists, who just have to make things complicated...:)
> causing a rejected part. With spin casting, air is forced out of the
> Adrian
Actually I don't think its either. Centrifugal force is a "fictional" force.
Centripidal refers to the "center seeking" acceleration of an object moving in
a curved path(I think). I would say the air is forced out because the metal or
whatever, wants to move in a straight line, but the mould prevents it, so it
does the next best thing and flows into the mould until it can't go any
farther. Anything less dense than it is pushed out of wherever it happened to
be flowing.
Hot damn, that dynamics class was actually useful......of course I could be
wrong.
> know the physics of it, you know the physics of it - we're talking
Wow...this is way off topic now, but point taken. Although I did not know that
you knew the physics of it.....:)
Even worse than complicated is making things so PRECISE. Ack...you left off
the units! How can you expect to solve the problem correctly without the
units!
> On Tue, 23 Feb 1999, Adrian Johnson wrote:
> >On Mon, 22 Feb 1999, Nyrath the nearly wise wrote:
The
> high temperatures are needed to get the plastic to "plasticize" ie
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,
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
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
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
sounds ideal. resins all round!
> And yes, it isn't much good for small minis.
curses!
> You get a high reject rate
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
why can't you spin-cast resin?
> Hope this is informative - I wouldn't have rambled on so much, but
absolutely; minis are our lifeblood, so to speak. we take interest in the
tools we use: minis, the list, statistics, and so on.
Tom
> At 2:46 PM -0800 2/23/99, Christopher K Smith wrote:
heck, with the units I could usually derive an equation I was misremembering.
Gotta have the units...
> It would be really nice to have a steel, aluminum, or pewter equivalent
Devcon makes steel, stainless, titanium, and bronze putties, probably others I
don't recall. Also Flexane liquid or putty urethane, which can be used
for molds. Look up "plastics" in your yellow pages--ideally call AIN
Plastics, the company I work for--or call ITW-Devcon (number should be
in 800# directory assistance) and ask for the closest distributor. Roughly
$30-40 per pound kit, if I recall correctly, but I don't myself sell
much of it so "rough" it the main word there.
> Thomas Anderson wrote:
> ha! i read my mail though a selectric teletype hooked up to a PDP-8!
Having used a Selectric typewriter (salvaged from a Mainframe site, with
> oh, ah, erm, i was using 'fuse' in the sense my physics teacher taught
of course...:)
> the
metalcast.com
> says:
Stereolithography is used for making patterns (masters) in the blow moulding
industry (blow moulding is the process that is used to make plastic soda
bottles, shampoo bottles, etc. and a huge variety of consumer
and industrial stuff - mostly hollow items like that). Blow moulding is
a lower pressure process, and uses aluminum tooling. They use the stereolith
model as a pattern to make a wax copy of the part, then use a lost-wax
process with sand to cast a rough aluminum mould. This is then machine
finished. This isn't used much, however, because the resulting mould is
not as high-quality as a machined mould. Good for quick stuff and for
prototyping, though. I worked as a designer for an automotive parts
manufacturer, and we had a whole stereolighography shop set up to do that kind
of thing. Fascinating process.
Where you see stereolithography used in the injection moulding industry is
for, as your EDM quote mentioned, "rapid prototyping". The resins used in that
process are unbelievably expensive (I think a gallon can goes for about
$20,000), and are not very strong (quite brittle, actually). My first decent
summer job while I was in design school was working at a stereolithography
rapid prototyping company. We used RTV rubber
(industrial stuff - much tougher than what is commonly available to
hobbyists) to make casts of the prototype part, and then used various
urethane-type casting resins to create plastic copies which exhibited
most of the strength and durability characteristics of an injection moulded
part. Some of the rubber moulds we did were quite complex - the last
one I
did before I left the company was a five-part mould - all kinds of
inserts in the middle, etc. Neat stuff.
Piece of trivia for the list - Steve Jackson Games was investigating
making cast metal Ogre models from Stereolithography masters, and the company
I
worked for did some of the masters - so there were a bunch of little
Ogres around the place. That project didn't go anywhere, to the best of my
knowledge, though.
EDM (Electrical Discharge Machining) is a wild thing to watch (I had a tour at
an EDM tool shop). They use that process to manufacture production
moulds - though if you use softer metal (ie not steel) you can make
prototype moulds for injection moulding relatively quickly. Very wet,
though - they use water as a cooling and conducting medium, so there is
water all over the place when the machine is working.
> For
Yes. that's exactly what they use - one end of the pantograph has a
cutting tool, the other a stylus they run over the master. I wanted to mention
it, but forgot what it was called. Thanks.
> You get a high reject rate
The real answer to this is to not have air in the mould or disolved in the
resin to start with. When I worked in the rapid prototyping company, we used a
vacuum chamber to degas all our RTV and casting resin before pouring either.
Cut down on trapped air bubbles by over 90%. If you want to do it really well,
though, you do the pouring IN a vacuum chamber which has been degassed. That
way there is NO air in the mould (well, not enough to make a difference,
anyway) as you pour and therefore no trapped air bubbles. You need a big
vacuum chamber, though, with remote manipulators or something to protect your
hands if you stick them in to do the work with. That would basically eliminate
the air problem. It wouldn't solve the "resin is brittle and arms break off
minis easily" problem though.
Lead/pewter is the best, 'cause it will just bend if you drop it...
> With spin casting, air is forced out of the mould
Because it takes at least several hours to set. You'd lose the efficiency of
spin casting if you had to keep the thing going for five hours just to get one
set of parts. Also, the resin is liquid at first, and the moulds are 2 piece.
If you spun it for that long, you would expel the liquid resin out the sides
of the mold long before it had a chance to set up.
Having said that, I have some casting resin at home that I use to make the odd
bit here and there... It sets up in about 45 seconds, and you can demould in
about 2 or 3 minutes. Great stuff, though you have to work
FAST. The resulting parts are sandable, drillable, cut-able, etc, and
are not particularly brittle. You have to be careful to look for trapped air
(I don't have a vacuum chamber at home...), but otherwise it is great stuff
(well, it is a nasty gooey chemical that shouldn't touch your skin, so I wear
latex gloves, but...). For those who are inclined, it is available in
North America as "Alumilite" Casting Resin - and it is rather pricy ($40
for a 1 litre kit) - though for the stuff I make (small bits, mostly) it
lasts a while. This isn't something you could do large vehicles with, I
think, 'cause of the cost - but then again, I don't know what the
company
would sell it for in large quantities. You can slow the set-up time
down, by sticking the 2 parts into a refrigerator for a couple of hours.
Most of these casting resins react exothermally, and if you heat them, they'll
react faster. When we had rush jobs at the RP shop, we would stick
our moulds full of resin in a hot-air oven and cook them - they'd set up
and go hard in 1/4 the time - the results were often not quite as good,
though. More chance for trapped air.
Anyway - once again I hope somebody finds this interesting/useful. I've
done quite a bit of work with making RTV moulds and casting on my own, and
if anybody has any questions, feel free to drop me a line off-list
(unless
you think "advanced" modelling has a place here too - I guess it
does?!).