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 03:07:39 GMT
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Viewed:
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9528 times
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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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Message has 1 Reply:
 | | Re: Variations in dark blue color.
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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 (...) (17 years ago, 16-Feb-08, to lugnet.color, FTX)
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Message is in Reply To:
| | Re: Variations in dark blue color.
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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 (...) (17 years ago, 16-Feb-08, to lugnet.color, FTX)
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