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 01:19:28 GMT
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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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Message has 2 Replies:
 | | 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 (...) (16 years ago, 16-Feb-08, to lugnet.color, FTX)
| | | Re: Variations in dark blue color.
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| (...) Excellent lesson. This kind of material would have been welcome in this group a couple of years ago. Better late than never. Thanks for the info. Doug (16 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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| (...) 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. (...) (16 years ago, 15-Feb-08, to lugnet.color)
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