Page 32 - Plastics News May 2019

FEATURES

other light colors, is very abrasive and can promote a melt will also negatively impact the integrity of
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the mold had not changed between the two production )LQDOO\ LI UHJULQG LV EHLQJ XVHG WKH ÀEHUV LQ WKH UHJURXQG
runs and no color was being added to the raw material. material are shorter than those in the virgin material,
(The slight yellow color of the good part arose from a even if the process conditions are optimal. The amount
somewhat longer time in the dryer). This points to the of regrind should be carefully controlled to keep the
process. properties from progressively declining as the material is
:KDW 'HFUHDVHV )LEHU /HQJWK" continually recycled. The higher the regrind percentage,
the more second-, third- and fourth-generation reclaim
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of revolutions per minute, recovery time and backpressure. JODVV ÀEHU UHLQIRUFHG 3%7 SRO\HVWHU WKDW ZHUH FRQVLVWHQWO\
Some backpressure is needed to provide a stable volume EULWWOH :H GLVFRYHUHG WKDW SHUFHQW RI WKH VKRW ZDV LQ
of material in front of the screw at the start of injection. the runner, and the runner was being reground and put
But if it is excessive it will increase the mechanical work LPPHGLDWHO\ EDFN LQWR WKH GU\LQJ KRSSHU :LWK YHU\ OLWWOH
on the material to a point where it does more harm than virgin material being added to the mix, it was not long
good. In addition, running too low a temperature in the before at least half the material in the hopper was at least
rear zone of the screw will create additional mechanical
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ZRUN WKDW ZLOO FDXVH PRUH ÀEHU EUHDNDJH DQG DOVR FDXVH more than powder. One last example is worth including.
the screw and barrel to wear much faster. I once saw a There is an increase in the need for polymers that can
SURFHVVRU ZKR ZDV UXQQLQJ D SHUFHQW ÀOOHG 336 JR shield electromagnetic and radio-frequency interference
through three screws in two months because they insisted (0, 5), 7KLV LV DFKLHYHG E\ XVLQJ FRQGXFWLYH ÀOOHUV DQG
on running the rear zone at a temperature lower than the
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FDXVH H[FHVVLYH ÀEHU EUHDNDJH $QG LI WKH VFUHZ HPSOR\V results. But effective shielding relies on an overlapping
a mixing element that is of the high-intensity variety, this
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Vented barrels are an example of this, but mixing elements has been shown that excessive length reduction of the
that produce high levels of shear to achieve a homogeneous
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Plastics News May 2019 32