Page 32 - Plastics News September 2019

FEATURES

But getting to that result would not be easy. The engineers includes a large-diameter channel as a structural element.
wanted to use many of the same door components—seals, ,W ZDV GLIÀFXOW WR GHWHUPLQH WKH QXPEHU DQG ORFDWLRQ RI
windows, locking systems, handles—as in the original metal WKH KRW UXQQHU QR]]OHV DQG KRZ WKH ÁRZ SDWKV VKRXOG
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need stiffness and other mechanical properties similar to from 2.7 mm in most of the part to 30 mm in the channel
the metal structure. Designers decided to include a large- sections. That’s mainly because the pressure needed to
diameter channel around the outside of the door in order ÀOO WKH ODUJH GLDPHWHU VHFWLRQV LV VLJQLÀFDQWO\ OHVV WKDQ
to meet these requirements. This channel would form LV UHTXLUHG WR ÀOO WKH WKLQQHU DUHDV 7KLV ODUJH SUHVVXUH
the upper and GHOWD PHDQW WKDW DV PHOW ÁRZV LQWR WKH FKDQQHO GXULQJ
side frame for WKH LQMHFWLRQ SKDVH ÀOOLQJ VWRSV DW DOO RWKHU JDWH SRLQWV RU
the window and does not start at all. Therefore the location of the gates
also connect DQG VHTXHQFH RI ÀOOLQJ KDG WR EH FDUHIXOO\ RUJDQL]HG WR
the upper and DFKLHYH SURSHU ÀOOLQJ DQG DYRLG ZHOG OLQHV
low portions of In addition the large-diameter structural channel itself
the door.
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In order to at one end and water at the other, pushing material back
keep weight to the barrel at the injection site. Also, the water had to
to a minimum, drain by gravity out of the channel after doing its work,
Renault designers planned to use gas assist to hollow out so the water injector had to be located at a low point in
this critical channel. However, the results of initial molding the tool. In the end, the mold would be mounted in the
trials were disappointing, since both costs and cycle machine at an angle (rather than square to the platens)
times exceeded project targets by a substantial margin. to achieve the correct orientation for gravity draining.
So they approached PME Fluidtec in Germany (pme- 5HQDXOW HQJLQHHUV FRQGXFWHG FRPSXWHU DLGHG PROG ÀOOLQJ
ÁXLGWHF GH HQ WR VHH LI ZDWHU LQMHFWLRQ WHFKQRORJ\ :,7 studies with the original mold design, anticipating gas
could achieve better results. PME Fluidtec specializes in injection, but there was not much time to re-study the
supplying technology for both gas and water injection, new tool design. Consequently, the automaker had to rely
as well as a Combined Injection Technology (CIT) that on the experience of PME Fluidtech technicians and a
marries the two. certain amount of trial and error to get the design right.

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more commonly
accepted in One of the reasons that Renault decided to switch from
Europe and Asia JDV DVVLVW WR ZDWHU ZDV WKH JUHDWHU ÁH[LELOLW\ DQG VWDELOLW\
than in the U.S. of water, which is due to its incompressibility. Thus,
today. It requires pressure and volume can be managed and controlled
different mold- more effectively
design features during the cycle,
than gas according to PME
injection for )OXLGWHF :,7 DOVR
several reasons. makes it possible
for the channel
)LUVW JDV LQMHFWLRQ XVHV DQ RYHUÁRZ FKDPEHU WR UHFHLYH to end at the
the resin displaced by gas. PME Fluidtec, on the other
hand, planned to use a technique that uses water to place where it
drive displaced material through hot-runner valves is required. The
“back-to-barrel.”This decision necessitated redesigning water pushes
the existing mold, with special attention to the hot- material back into the barrel until the hollowed-out area
runner system. Hot-runner design normally would be extends as far as it needs to and no farther. PME Fluidtec
straightforward for a typical injection molded part, says the water volume and pressure can be controlled so
but there were no established principles for a part that precisely that the process is perfectly repeatable. The
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Plastics News September 2019 32