Page 95 - Engineering Plastics Handbook

Processing 69

where L = melt flow length, cm (in)
H = part thickness, cm (in)
Injection-molded polycarbonate sheet for car windows requires injection
2
molding thin-wall large panes up to 11 ft with minimum molded-in stress.
They can be injection-compression-molded, with overmolded frames, and
removed by a press-mounted six-axis articulating robot [11].
The complexity of injection molding equipment is illustrated with the
choice of nonreturn valves which prevent flow of melt back to the screw.
There are two basic types: ball check, and ring check, and their variations.
Valve selection extends further, to include general-purpose three-piece,
four-piece, and five-piece valves; free-flow valves; castle valves; smear
valves; and spring (poppet) valves [12].

Mold Design
Mold design affects process productivity and product quality. Agood exam-
ple is improvement of a product’s mechanical properties, such as impact
strength, by locating gates and weld lines away from high-impact areas
[5]. The effect of mold design on product and process quality begins with
the runners. Fluent Inc. FIDAP fluid dynamics software, a computational
fluid dynamics (CFD) package, is used for analysis of melt flow through
runners[13]; runner design contributes to product quality and process eco-
nomics [14, 15]. Runner diameter contributes to smooth flow through the
gates and into the mold cavity, and the runner diameter directly connected
to the mold cavity is typically the same as the wall thickness. The diam-
eter of upstream runners depends on the number of branches. To calcu-
late the diameter of upstream branches [3], use

D = n 1/3 D
br
Where D = diameter of upstream branches, mm (in)
br
n = number of runner branches
D = diameter of runner connected to cavity, mm (in)

Thermoforming
Principal thermoforming factors contributing to finished-product quality
are the use of right temperature settings, and vacuum or pressure uni-
formity and gradients. Heating can be uniformly or unevenly distributed,
according to the part’s configuration and wall thicknesses, by convection,
radiation, or conduction. The temperature profile is determined by the
part’s configuration. Thin-gage and heavy-gage thermoforming includes
vacuum and pressure forming, predraw stretching, slip forming, and twin
sheet forming. The stretch ratio and depth-of-draw ratio also influence