h a l f b a k e r yAssume a hemispherical cow.
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molds for plastic injection molding cost up to 5 thousand dollars each.
Which is bloodly expensive from an investment standpoint of a product.
What if you can readjust the mold via moving an array of metal pins. While somehow keeping the pins close together to prevent leakage.
Baked
http://etheses.nott...rew_Pin_Tooling.pdf
Rapid Manufacturing of Vacuum Forming Components Utilising Reconfigurable Screw-Pin Tooling [AntiQuark, Feb 19 2011]
[link]
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A rather shapely convex bun [+} |
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Hah. //somehow keeping the pins close together to prevent leakage.// |
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(For those of you who haven't ever done injection molding, be aware that what [Akimbomidget] has proposed is full-on industrial grade magic, complete with a vacuum wand and a pointy hat.) |
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Maybe, if you keep the plastic extra viscous (barely melted), and the pressures low... 'course, you can't push viscous plastic at low pressure... (although I always liked the smell of shear-burned polycarbonate. Reminded me of marshmallows, for some weird reason.) |
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How about a thin (inexpensive!) metal liner, supported by the pins? That would seal, but I'm afraid you'd have problems at the edges. And the heat transfer would be as difficult as a fly rod in a VW... |
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What kind of machine you got, anyway? One of those 1.5 ton hand-cranked Boy key-ring makers? Pins would never hope to support the pressure on one of those multi-hundred ton clamp force jobs, like a big Cinci or Netstal. |
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Could such a pin array be used for vacuum forming of sheet plastic? The strength and sealing requirements would be much less. |
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This is a nice idea, but I think it would not work. |
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First, the products would be "pixellated". In some cases,
this would not matter, but it would prevent you making
anything very precise, even if the pins were 0.1mm across
(which means a 10 x 10cm object would need a mould with
1 million pins). It would also prevent you making objects
with a nice finish, where this matters. |
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Second, you'd be limited to objects that don't re-curve (ie,
you can't make overhangs, except in very simple cases).
Again, many moulding machines do this, and even simple
two-part moulds often produce shallow recurving lips (for
example, to allow parts to be clipped together), which can
be released thanks to the flexibility of the set plastic. |
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Third, the mould would be very fragile - one inadvertent
knock to the pins, and they'd have to be replaced. |
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Fourth, I'm not sure how easy it would be to release the
object after moulding, given the textured nature of the
pin surface, even if no plastic squeezed in between the
pins. |
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Fifth, commercial moulds are often fitted with many
complex features including water cooling channels and
retractable pegs etc, both
to allow more complex products to be made, and also to
speed the moulding cycle. You couldn't do this with a pin
array. |
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ON the other hand, I am now looking at one of those pin-
matrix toys (carrying an impression of a hamster, as it
happens). A slightly finer version which could be locked
and used for vacuum sheet forming, as per
[spidermother]'s suggestion, would make an interesting
toy or hobby device. |
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The pins, I imagine, would be hexagonal. |
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If the pins were fine enough you could even get a
decent reverse surface on vacuum forming. That
being said, it might work for one or two parts-
prototypes or one offs. But in quantity, wear and
de-molding difficulties would offset the
advantages. Also you are going to be very limited
in the detail of features, since you're clean side
will be the un-molded side. And deep drafts are
going to be just about impossible. |
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It would also be extremely expensive (what's
supporting the pins, a small motor each? a stepper
multi part solenoid?) or extremely difficult/time
consuming to reconfigure. |
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Since you can get a decent low volume mold
(wood or polyurethane foam usually) for ~500-
1000, and a good high volume mold for 5000, I
doubt this very expensive tooling will pay for
itself. Full up injection molds are usually
noticeable more expensive than 5k though, which
is why you only go to them if you've got the
volume to support it. 10k for a mold isn't all that
much if you're expecting to produce 100 million
units. |
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And if you actually read through anti-quarks] link,
you find they use the initial pin formed part as a
near net shape, which they then coat with a filler
material and machine into shape for a mold.
Considering you can machine a mold from raw stock
for less effort and barely more cost, it's not worth it. |
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[Max] Why don't you use your hamster directly as the vacuum forming mould? |
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Because, if hamsters behave anthing like cats, once you've pumped all the air out they're never quite the same afterwards. |
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It was a stuffed hamster. Derrr! |
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