Page 96 - Engineering Plastics Handbook

70 Introduction

product quality and process economics. Stretch ratio is the initial film or
sheet gage, divided by wall thickness of the thermoformed product. Depth-
of-draw ratio is the maximum depth of the thermoformed part, divided by
the minimum part width.
Low-volume production of building panels, automotive body panels, and
bumpers is thermoformed from engineering thermoplastics. ABS, poly-
carbonate, polycarbonate/ABS, polyamide, and polyesters are thermo-
formed into in-mold decorating films which are robotically placed into an
injection molding mold. The substrate is back-molded onto the film.

Blow Molding
Engineering thermoplastic blowmolding developments are gaining, such
as the three-dimensional blow-molded automotive coolant ducting, and
higher-MW, higher-melt-strength PPS for parison formation [16].
With more blow molding applications being introduced, the blow mold-
ing engineering thermoplastics capability becomes more important, and
the R* index is a rating for blow molding processing capability. High R*
indicates that melt flows more easily through the extruder and still main-
tains a high enough melt strength during parison formation where there
is no shear. Shear thinning, which occurs during extrusion blow molding,
differs from shear thinning behavior for other melt processes. Shear is gen-
erated only when there is a sliding action between a metal surface such
as the plasticating screw and the melt flow [5].

Assembly
Assembly methods include in-mold assembly [5] and

1. Welding
2. Press-fits
3. Snap-fits
4. Heat staking
5. Cements and adhesives
6. Mechanical fastening

Welding methods include
1. Spin
2. Inertia
3. Ultrasonic