Subject:
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Re: Variations in dark blue color.
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Newsgroups:
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lugnet.color
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Date:
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Sat, 16 Feb 2008 03:55:16 GMT
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Viewed:
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10704 times
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In lugnet.color, John Patterson wrote:
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In lugnet.color, Dave Sterling wrote:
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In lugnet.color, John Patterson wrote:
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In lugnet.color, Scott Lyttle wrote:
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In lugnet.color, John Patterson wrote:
Scott
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Thanks for the explanitation. I still would think that if the supplier of
the pellets starts off with a clear plastic and then adds color, it would
be the same for Flextronics starting with a clear plastic and adding color
to make the pellets. Perhaps I am a little dense, but when the pellet
supplier added the color they were the same as Lego or Flexrtonics adding
color. They all start with clear plastic and add color.
-SNIP-
John P
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John:
Not quite. Flextronics is a molder, while the suppliers they would buy
precolored pellets from are compounders. The difference is all in the
method and intensity of mixing. When a compounder colors a resin like ABS,
they do start with a natural. The pellets are pre-blended with the pigments
and/or dyes along with stabilizers, lubes, and other additives. The
preblend is then put through an extruder. The extruder consists of a metal
screw wrapped by a heated barrel. The preblend is fed in at one end and
comes out the other end in molten strands which are typically cooled in a
water bath. The design of the screw along with the processing conditions
(temperature, screw RPM) determines how much mixing occurs. This is the
heart of compounding technology. Most precolors are done on twin-screw
extruders because they offer the best mixing capabilities.
In contrast when a molder colors resin “at the press”, they are adding the
colorant in some way (liquid color, masterbatch/concentrate, etc.) at the
hopper of the injection molding machine. Mixing is less effective in
injection molding machines because they are primarily designed to melt and
convey materials rather than mix them. The L/d (length/diameter) ratio of
the screw along with the profile of the screw play a part in how much mixing
occurs. Injection molding machines typically have low L/d ratios and
low-compression or general purpose screws which means that minimal mixing
occurs in a molding machine. You can improve mixing by adjusting some
processing parameters, but in the end, it will never be as efficient as a
twin-screw compounding extruder.
Here’s a very basic overview
of compounding.
In my experience, the control over color consistency diminishes the farther
you take the colorant away from the compounder. Precolor is usually the
most consistent, followed by masterbatches/concentrates, followed by liquid
color or “at the press” blending. Granted, I’m probably a little biased as
I work for a specialty compounder. :-) Some molders are better than others
at maintaining consistency and liquid color can be done well. It just takes
someone dedicated to learning the process and all the intricacies.
Also, some machine companies are also starting to come out with
“compounding/injection molding machines” which basically combine an extruder
with a molding machine. These new machines will allow molders to compound
their own materials right at the press. The downside is that they will have
to hire engineers who are experienced in materials development to put
together the formulations for them. :-)
-Dave
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Thanks, it is starting to clear things up for me. Would a solution be to
make the bricks from one batch, or at least put bricks into the box from the
same mixture? I am somewhat familiar with dye lot variations as I worked in
the needlecraft industry for a number of years and it was critical to match
dye lots. Stitchers were always told to buy enough thread from the same dye
lot. The thread manufactures made it easier in that the box of thread sent to
the stores with 10 balls of yarn or 12 skeins of yarns always came from the
same lot. I see that it is a lot harder in the plastic industry. Around the
time Lego changed the greys, the yarn suppliers changed their grey dye also.
There was such an uproar that they had to go back to the original dye lot and
the wholesale price of all the colors of the yarn from the manufacture went
up. John P
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John:
That might work if you were using precolored pellets...but honestly...if you
were using precolor, you wouldn’t see the variation. :-) With both
concentrates and liquid color, there can easily be shot-to-shot variation if
mixing/dispersion is not optimized and/or controlled.
Imagine grabbing a handful of snow. Now take a drop of red food coloring and
drop it on that snow. Now you have a small speck that is really red surrounded
by a bunch of white. This equates to a bunch of natural pellets surrounding a
concentrate red pellet. Now mix the snow up. As you start to mix, the red
spreads out. But it spreads slowly and requires a lot of mixing to get to
uniform color dispersion. Herein lies the problem. You have to put a lot of
work into mixing to get even color.
Now, as a materials supplier, we can add things to the concentrate to aid
dispersion of the color. I can’t comment on any specifics...but the technology
is out there. Most color concentrates can be fairly evenly dispersed and many
companies use this technology. One other thing to keep in mind though is that
certain pigments are harder to disperse due to their chemical makeup/structure.
Organic based pigments can be particularly tricky to disperse. Perhaps some of
the inconsistency comes from the pigment type? I’m not sure, but it’s another
piece of the puzzle to look at. I can almost guarantee that LEGO is looking at
this though. I firmly believe that the inconsistency you saw is just “start-up
jitters” with a new color and new color technology.
Interestingly enough (speaking of new colors and technology), I’ve never heard
anyone make mention of the flow-lines that were present in some of the metallic
colors produced recently. And I’ll quell any complaining right now by telling
everyone that the flow lines in metallics cannot be eliminated...period. :-)
We’ve been trying for years. The only thing you can do is try and gate the part
to hide the flow lines.
Anyway, I’m babbling now. Hope this information is useful.
Best Regards,
Dave
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Message has 2 Replies:
Message is in Reply To:
 | | Re: Variations in dark blue color.
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| (...) Thanks, it is starting to clear things up for me. Would a solution be to make the bricks from one batch, or at least put bricks into the box from the same mixture? I am somewhat familiar with dye lot variations as I worked in the needlecraft (...) (17 years ago, 16-Feb-08, to lugnet.color, FTX)
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