The Process and Equipment
Because most engineering thermoplastic parts are fabricated by injection
molding, it is important for the designer to understand the molding
process, its capabilities and its limitations. The basic process is very
simple. Thermoplastic supplied in pellet form are dried when necessary, melted, injected into
a mold under pressure and allowed to cool. The mold is then opened, the
parts removed, the mold closed and the cycle is repeated.
The Molding Machine
Melting the plastic and injecting it into the mold are the functions of
the plastifying and injection system. The rate of injection and the
pressure achieved in the mold are controlled by the machine hydraulic
Mold design is critical to the quality and
economics of the injection molded part. Part appearance, strength,
toughness, size, shape, and cost are all dependent on the quality of the
mold. Key considerations for Engineering Thermoplastics are:
-Proper design for strength to withstand the high pressure involved.
-Correct materials of construction, especially when reinforced resins are used.
-Properly designed flow paths to convey the resin to the correct location in the part.
-Proper venting of air ahead of the resin entering the mold.
-Carefully designed heat transfer to control the cooling and solidification of the moldings.
-Easy and uniform ejection of the molded parts.
When designing the part, consideration should be
given to the effect of gate location and thickness variations upon flow,
shrinkage, warpage, cooling, venting, etc. Your DuPont representative
will be glad to assist with processing information or mold design
suggestions. The overall molding cycle can be as short as two seconds or
as long as several minutes, with one part to several dozen ejected each
time the mold opens. The cycle time can be limited by the heat transfer
capabilities of the mold, except when machine dry cycle or plastifying
capabilities are limiting.