Page 256 - Engineering Plastics Handbook

218 Engineering Plastics

When one is working with boxes or bags, some general guidelines
include these:
■ Routinely monitor the starting moisture content of the resin or container
several times during a shift. This will help to minimize variability and
point out situations that have resulted in higher than normal moisture
contents.
■ Use the resin in the order it is received. Aged inventory is more sus-
ceptible to picking up moisture.
■ Reseal the package after use to retard exposure to moisture in the envi-
ronment.
■ The number of partial containers in use at any one time in the mold-
ing operation should be minimized.
■ Keep track of the time from when a box is opened to when it is emp-
tied—especially in humid weather, during start-up, shutdown, down-
time, end of the workweek, etc.

General comments on melt temperature, mold temperature, etc., are
similar to those listed earlier for modified PPE

Shot size. It is suggested that the total shot size (all cavities plus runners
and sprues) be 30 to 50% of the machine capacity. Very small shots in a
large barrel machine may create unnecessarily long resin residence times
that may lead to resin degradation. If it is necessary to mold at the high
end of the temperature range, reduced residence time (6 min or less) is usu-
ally required to reduce the possibility of material heat degradation.
Generally the shot size should be between 40 to 50% of the machine capac-
ity for higher-temperature molding situations. Shot sizes in excess of 50%
barrel capacity can lead to difficulties resulting in inconsistent, nonho-
mogeneous plastic melt.

Nozzle requirements. Short, open-bore nozzles with a minimum orifice of
3/16 in (4.76 mm) in diameter are generally useful for molding unfilled
PPE/PA resins. For glass-reinforced grades of PPE/PA an orifice of 5/16 in
(7.81 mm) is suggested. Land lengths should generally be no longer
than 3/16 in (4.76 mm). The nozzle bore should be 0.5 in (12.7 mm) min-
imum. The nozzle opening should typically be kept 1/3 in (0.8 mm) smaller
than the orifice dimension on the sprue bushing. Accurate heat control and
full heater band coverage are important for maintaining part appearance.
Specialty nozzles such as static mixers, shutoff nozzles, and screen
pack filters are not typically suggested, as they generally have sharp cor-
ners and areas of high pressure drop that can cause a variety of molding
problems.