This is an overview of colorants..their properties, usage
and potential defects.Plastics material colorants can sometimes cause
nearly as many issues as the materials they are mixed with. It is
important to have a solid understanding what potential molding
issues to look for when issues such as color swirls, contamination
or other potential colorant related issues arrise. Having a solid
knowledge of the characteristics that are relevant can sometimes save
alot of headache. This is an overview of colorants..their
properties, usage and potential defects.
We must first define colorant. Colorants are materials that are almost
always made with the the same base as the natural materials they are
mixed with. There are several colorant types, the primary categories
being pellet, liquid and powder forms. Pellet being the concentrate most
commonly used, this article will primarily be aimed towards this method
of developing a colorized material.
Pelletized colorants are often called concentrates due to their purpose.
Initially the colors themselves start as pigments. These pigments of
various colors are often blended together to become a particular color.
For instance...a blue pigment can be combined with a yellow pigment
to develop a shade of green. This color may require a lighter or darker
pigment to become a particular shade of green. When the desired shade has
been achieved, this concentrate is blended with a natural powdered base,
such as nylon or polypropylene. This completed blend is then
extruded and pelletized to be used with natural pellets to mold a specific
color.
Each colorant has a specific ratio. These ratios are often established by
the extrusion facility, but these are generally recommendations and there
are times when less or more than the recommended ratio is used for cost
savings, or better color content. The important thing to remember here is
the cost of using too much colorant...as well as the cost of scrap generated
by lack of colorant. Best practice dictates that the manufacturer's mixtures
be followed. Quite often, this ratio is affixed to the label on the barrel,
as well as documented on the Material Safety Data Sheet. The general formula
is a virgin-to-concentrate number (in ex: "25:1" is twenty-five pounds of virgin
with one pound of color added to it).
Newer age equipment (such as Maguire blenders) do a fantastic job of producing
consistent blends. Some of the older equipment, however can be a bit more
tricky. The digital equipment is harder to establish just what the proper
setpoint should be when first setting up a job. The best method here is to use
the manufacturer's color chip in comparison. Readings are taken from parts
(NOTE: make sure the press was properly purged and the barrel has had time to
heat soak while running several cycles prior to color testing) and compared to
the color of the color chip using a McBeth Spectrometer, or similar piece ofcolor
analyzing equipment. This analysis will help you determine if the colorant
content is to dark or too light. Once a good read has been established, blender
setpoints should be recorded for use in future runs.
Now we'll focus on the key issues regarding color concentrates. I will point out
the most common first, then give some rare but equally as aggravating situations
to watch for:
One of the first keys to success in molding with colorant is tracking lots. This
is often overlooked, but treat colorant with the same precautions as your
materials. Some examples of what can happen during an extrusion lot change are
as noted:
-Contaminated pellets(poor screw cleaning at extrusion line changeover)
-pellet longs (pellet sizes inconsistent due to end/ beginning of run conditions,
or poorly adjusted pelletizers)
-change in pellet size(pellet size changed in diameter or length at changeover
-wrong base material used
-lamination(poor screw clean at changeover while changing base materials)
-change in color (poor blend or improper pigment configuration)
-viscosity changes(too high or low heats, base powder lot change)
Knowing how and when to inspect your concentrates for these conditions is crucial
to profitable use of colorant in comparison to precolored materials. Failure to
consider these potential factors can lead to an improper diagnosis of a change
in molding conditions. One pokayoke that your company should consider is requiring
the concentrate supplier to provide a color chip molded from every lot with every new
shipment.These chips should be verified for proper color content and visual aesthetics.
Colorant usage can have a profitability factor... but only as long as they are used
and analyzed properly.
|
