Page 254 - Engineering Plastics Handbook

216 Engineering Plastics

60% of the barrel capacity of the machine. However, shots that are 20
to 75% of machine capacity have been successfully molded when tem-
peratures were precisely maintained and all processing conditions were
very closely controlled.
When processing the resin at, or near, the upper limit of the recom-
mended melt temperature range, the shot size should approach 70 to
80% of the machine’s barrel capacity. If the cylinder temperature exceeds
the upper limit of the suggested melt range, thermal degradation of the
resin and loss of physical properties may result.

Ram speed. When one is selecting the injection speed, careful consid-
eration must be given to adequate mold venting, resin melt temperature
and injection pressure, along with the potential for jetting.
Injection pressure. The actual injection pressure will depend on variables
such as melt temperature, mold temperature, part geometry, wall thick-
ness, flow length, and other mold and equipment considerations. Generally,
the lowest pressures that provide the desired properties, appearance, and
molding cycle are preferred. Holding pressures from 60 to 80% of the injec-
tion pressure are usually adequate for normal requirements.
Cushion. The use of a small cushion, 1/8 in (3.18 mm), often reduces
material residence time in the barrel and helps accommodate machine
variations.
Cycle time. When one is adjusting cycle times, it is typically best to use
a fast injection speed and a minimum holding time to achieve gate
freeze-off and a short cooling time.

Purging. Polystyrene and reground cast acrylic are generally effective
purging materials for all modified PPE resins. Purging is typically be done
within the melt temperature range for that particular grade of resin. It is
important to have proper ventilation during the purging procedures.

Regrind. If the application permits the use of regrind, reground sprues,
runners, and nondegraded parts may be added to the fresh pellets up
to a level of 25%. Typically grinder screen sizes are 5/16 to 3/8 in (7.9 to
9.5 mm). It is important to keep the ground parts clean and to avoid con-
tamination from other materials. Regrind utilization may produce some
effect on color. Actual regrind usage should be verified for each individ-
ual application.

Processing PPE/PA Resins
PPE/PA alloys generally have higher melt viscosities than polyamides
and require higher processing temperatures. Processing PPE/PA resins