General Design Principles for DuPont Engineering Polymers - Module

2—Injection Molding
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 resins
such as Delrin ® acetal resin, Rynite ® PET thermoplastic
polyester resin, Zytel ® nylon resin or Hytrel ®
polyester elastomer, 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.
Figure 2.01 is a schematic of the injection molding
machine.
Figure 2.02 is a schematic cross section of the
plastifying cylinder and mold.
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 system.
Injection pressures range from 35–138 MPa (5,000–
20,000 psi). Melt temperatures used vary from a low
of about 205°C (400°F) for Delrin ® acetal resins to a
high of about 300°C (570°F) for some of the glass
reinforced Zytel ® nylon and Rynite ® PET thermoplastic
polyester resins.
Processing conditions, techniques and materials
of construction for molding DuPont Engineering
thermoplastic resins can be found in the Molding
Figure 2.01 Schematic of the injection molding
machine
Feed Hopper
Mold Melting
Cylinder
5
Guides available for Delrin ® acetal resins, Minlon ®
engineering thermoplastic resins, Rynite ® PET
thermoplastic polyester resins, Zytel ® nylon resins
and Hytrel ® polyester elastomers.
The Mold
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. as discussed in subsequent sections. 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.
Figure 2.02 Schematic cross section of the plastifying
cylinder and mold
Machine
Platen
Mold
Machine
Platen
Plastifying
Cylinder
Feed
Hopper