Subject:
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Re: Technic's Dead (was: I need Technic)
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Newsgroups:
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lugnet.technic
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Date:
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Wed, 7 Aug 2002 11:24:18 GMT
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Viewed:
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2907 times
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"Ralph Hempel" <rhempel@bmts.com> wrote in message
news:CAEBIOGHPFFJALBLJBEDCEGDDOAA.rhempel@bmts.com...
> Steve,
>
> My Dad was one of the mold design and production managers at Husky • Injection
> Molding way back when. I can tell you that LEGO bricks in general have a
> degree of mold complexity that is very high.
>
> There are a number of technical hurdles, not the least of which is that • the
> parts are sprue-less, which means they don't have any obvious points at
> which the plastic "comes in" to the mold. This also means that the parts
> don't need to have sprues trimmed after production.
Huh?? I'm not sure about that. For as long as I can remember, the 1x1
round plates which have come in a great variety of colors, including
transparent, have had a small sprue piece to which both are attached, it's
only been recently that LEGO have been taking them off before putting them
into a set. I also seem to remember a discussion somewhere that said most
parts have either two or four of them made at one time, I do know for a fact
that some studs or sides in earlier bricks did have a tiny imperfection in
the mold where the plastic was injected.
> On Technic parts, these injection points are typically inside the stud • hole
> where they can't interfere with any close tolerances. LEGO parts also have
> almost invisible seams where the main mold halves join. These seams are
> usually along edges, which is the hardest kind of mold making.
Usually some of the larger parts, not necessarily Technic beams, do have two
or more injection points in them. There was a 6x24 plate, which came in set
6399 Airport Shuttle, which had two studs with mold injection dimples on two
of the LEGO logos, about 8-12 studs apart, if I recall correctly. I think
that was done to make sure the injection-molding process reached the most
cavity space in the mold in the shortest possible time, to make sure the
mold filled up properly.
> A Technic part such as a beam is molded in a two-cavity mold. One
> for the top, and another for the bottom. The holes are accomplished with
> what are called side-pulls. These are cam activated pins that retract so
> that the parts can be removed from the mold.
This certainly sounds right, although I suspect the newer beams might well
be two-piece molds, and would come out fairly quickly. If a mold can
quickly come apart in two pieces, it might make a small difference than if
it had to come apart in four pieces. But I would expect there to be a large
number of factors affecting this, such as mold complexity, etc.
> If you look closely at a technic beam, there are recessed ridges that mate
> with the flanges on a pin. So you need a side pull from each side of the
> mold. More cost, more complexity.
Wouldn't a mold have to be awfully precision-made to make sure that no
flanging from such side-pulls appeared in the finished part? I've looked at
a lot of technic beams recently, and can't see anything at all that would
give away the presence of such side-pulls. I must admit it would be
fascinating to learn how LEGO mold them.
> Take plastic shrinkage and mold life into account, and then realize that
> molds often have anywhere from 4 to 24 cavities, and pretty soon you have
> a big production problem. Incidentally, the part number is often molded
> into a brick followed by a dash and another number. The second number
> often indicates the cavity number of the part. If a part starts going
> bad, production can either fix the mold or seal off a cavity to prevent
> bad shots from getting into the finished goods.
I would expect that they would take the mold out and fix it as soon as the
problem manifests itself. Most of the molds are for sets that require a
particular ratio of parts, nothing like a cavity gone bad to make a run have
to go a lot longer than if it hadn't. Also, sealing off a cavity would
alter the pressure ratio of the remaining cavities, they'd have to adjust
the molding machines to compensate for this. I suppose the modern machines
would automatically compensate, though there is the problem of unevenly
distributed flow. I would suspect, perhaps if it were small runs of a part
not used in a lot of sets, they might just seal one off, and fix it after
production finishes, but with a large run where it's in demand, fixing it
right away would seem the better way to go.
> Oops, that's more than anyone probably wants to know. I'll stop now.
On the contrary, I am quite fascinated by it all! :-)
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Message is in Reply To:
 | | RE: Technic's Dead (was: I need Technic)
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| (...) Steve, My Dad was one of the mold design and production managers at Husky Injection Molding way back when. I can tell you that LEGO bricks in general have a degree of mold complexity that is very high. There are a number of technical hurdles, (...) (22 years ago, 5-Aug-02, to lugnet.technic) !
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