Letdown ratios

Figure 8.18 Cross-sectional views of extrudate samples at a letdown ratio of 100 to 1 of a white pigmented ABS resin with a black color concentrate for a melting-mixing experiment. The samples were produced at screw speeds ranging from 30 to 150 rpm. The large round white regions for the cross sections at screw speeds of 90 rpm and higher are solid particles discharged with the extrudate...

The mixing requirement presented here is extremely difficult and noncommercial due to the very high letdown ratio used and the use of TlOj in the main resin. For experiments where natural ABS resin (no added Ti02) was used at a 100 1 letdown ratio with the black color concentrate, the very small white streaks at screw speeds... 

A black-colored concentrate was added at a letdown ratio of 220 1 to a white-tinted ABS resin. Labels indicate the axial distances in screw diameters from the start of the screw... 

Figure 8.30 Cross-sectional views [53] of the extrudate samples using a conventional screw with segmented tip for the testing of different types of dynamic mixers a) conventional screw with no mixer, b) Twente Mixing Ring, and c) Barr Fluxion ring mixer. The extruder was operated at a screw speed of 80 rpm and a letdown ratio of 220 1 of a white pigmented ABS resin to a black color concentrate...

Figure 14.10 Cross-sectional views for a Maddock solidification experiment for the ET screw at a letdown ratio of 220 1 white-pigmented ABS resin to a black color concentrate [29]. The A and B channels are labeled along with the axial positions in diameters. The views were for a screw speed of 66 rpm and a rate of 70 kg/h using a 63.5 mm diameter extruder...

A Maddock solidification experiment was performed using ABS resin at a screw speed of 60 rpm [30]. Like the solidification experiment for the ET screw of Section 14.2.2, the feedstock resin contained 2% Ti02 compounded into the pellets, and a black-colored masterbatch was added at a letdown ratio of about 220 1. The cross-sectional views of the resin solidified in the channels are shown by Eig. 14.15. The white pellets and unmixed resin are visible in the views as white regions. The regions that are well mixed are an evenly colored dark or black color. 

Theoretically, the higher the concentration of colorant in a concentrate the lower the dosage or higher letdown ratio needed to obtain properly colored parts. With... 

Liquid colors can be used in injection molding and extrusion. Thin films usually require large amounts of color and so are not usually made with liquid color. Thick films and sheet are suitable applications for Liquid. Usually, 1-3% is the practical limit for a letdown ratio, but liquids have been used up to 8% in special circumstances. For letdown ratios of 1 % and less, liquid color is often the preferred method of coloring or introducing additives. 

Increasing or decreasing letdown ratio can be done very quickly online by adjusting pump rate. 

They are good for letdown ratios as low as 0.1%, possibly lower with care. 

Wax concentrates may be in the form of prills, bars, flakes, or granules. Prills are very small spherical pellets, usually made from a pigment blend and wax. They may be blended, like dry colors, or dispensed by a metering device at the throat of the extruder. If they are blended, it is best to use a powder resin, such as polyvinyl chloride (PVC), so they do not sift to the bottom of the container. The other physical forms are applied in different ways some are designed to be metered in at the hopper, the rest to be blended in before reaching the hopper. The carrier may lubricate, so letdown ratios are limited. 

Figure 20.4 gives an indication of why given coloring methods are effective at different letdown ratios. Liquid color coats each pellet, particularly if an intermixer has been used. No further time or energy is needed to distribute the material to the pellet level. The limitation in use of liquid color is at high letdown ratios. Too much carrier can lubricate too much. Also, if there is an excess of liquid color, it can separate out to some extent, particularly if mixed resin and color are allowed to sit in a stopped extruder. 

Newest "super-concentrates" provide highest level of melt distribution, greatest letdown ratio, supplier press release. Teknor Apex Company 2008. 

Figure 5.37 Cross-sectional slices for a conventional screw at a letdown ratio of 220 1. For each slice, the barrel surface is along the top, the screw root is along the bottom, and the...

Figure 5.38 Cross-sectional views of extrudate samples at a letdown ratio of 100 1 of a white pigmented ABS resin with a black color concentrate for an ET screw.

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