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TitleInjection Molding
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Page 1

How does plastic flow?
Material behavior

Molten thermoplastic exhibits viscoelastic behavior, which combines flow characteristics of
both viscous liquids and elastic solids. When a viscous liquid flows, the energy that causes
the deformation is dissipated and becomes viscous heat. On the other hand, when an elastic
solid is deformed, the driving energy is stored. For example, the flow of water is a typical
viscous flow, whereas the deformation of a rubber cube falls into the elastic category.

In addition to the two types of material flow behavior, there are two types of deformation: simple shear
and simple extension (elongation), as shown in (a) and (b) below. The flow of molten thermoplastics
during injection molding filling is predominantly shear flow, as shown in (c), in which layers of material
elements "slide" over each other. The extensional flow, however, becomes significant as the material
elements undergo elongation when the melt passes areas of abrupt dimensional change (e.g., a gate
region), as shown in (d).

FIGURE 1. (a) Simple shear flow. (b) Simple extensional flow. (c) Shear flow in cavity filling. (d) Extensional flow in
cavity filling.

In response to an applied stress (force per unit area), molten thermoplastics exhibit viscoelastic behavior,
which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid. In other
words, under certain conditions, molten thermoplastics behave like a liquid, and will continuously
deform while shear stress is applied, as shown below. Upon the removal of the stress, however, the
materials behave somewhat like an elastic solid with partial recovery of the deformation, as shown in (b)
and (c). This viscoelastic behavior stems from the random-coil configuration of polymer molecules in
the molten state, which allows the movement and slippage of molecular chains under the influence of an
applied load. However, the entanglement of the polymer molecular chains also makes the system behave
like an elastic solid upon the application and removal of the external load. Namely, on removal of the
stress, chains will tend to return to the equilibrium random-coil state and thus will be a component of
stress recovery. The recovery is not instantaneous because of the entanglements still present in the

How Does Plastic Flow?

Page 2

FIGURE 2. (a) Ideal viscous liquid deforms continuously under applied stress. (b) Ideal elastic solid deforms
immediately upon the application of stress, but fully recovers when the stress is removed. (c) Molten thermoplastic
deforms continuously under the applied stress (like a viscous liquid), but it also recovers partially from the
deformation upon removal of the applied stress (like an elastic solid).

How Does Plastic Flow?

Page 16

Shrinkage and warpage
Why do they occur?

Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density
of polymer varies from the processing temperature to the ambient temperature (see Specific
volume (pvT diagram)). During injection molding, the variation in shrinkage both globally and
through the cross section of a part creates internal stresses. These so-called residual stresses
(see Residual stress) act on a part with effects similar to externally applied stresses. If the
residual stresses induced during molding are high enough to overcome the structural integrity
of the part, the part will warp upon ejection from the mold or crack with external service load.

The shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured at the
processing temperature and the ambient temperature. Crystalline and semi-crystalline materials are
particularly prone to thermal shrinkage; amorphous materials tend to shrink less. When crystalline
materials are cooled below their transition temperature, the molecules arrange themselves in a more
orderly way, forming crystallites. On the other hand, the microstructure of amorphous materials does not
change with the phase change. This difference leads to crystalline and semi-crystalline materials having
a greater difference in specific volume ( ) between their melt phase and solid (crystalline) phase. This
is illustrated in Figure 1 below. We’d like to point out that the cooling rate also affects the fast-cooling
pvT behavior of crystalline and semi-crystalline materials.

FIGURE 1. The pvT curves for amorphous and crystalline polymers and the specific volume variation ( )
between the processing state (point A) and the state at room temperature and atmospheric pressure (point B).
Note that the specific volume decreases as the pressure increases.

Causes of excessive part shrinkage
Excessive shrinkage, beyond the acceptable level, can be caused by the following factors. The
relationship of shrinkage to several processing parameters and part thickness is schematically plotted in
Figure 2.

low injection pressure
short pack-hold time or cooling time
high melt temperature
high mold temperature

Shrinkage and Warpage

Page 17

low holding pressure.

Problems caused by part shrinkage
Uncompensated volumetric contraction leads to either sink marks or voids in the molding interior.
Controlling part shrinkage is important in part, mold, and process designs, particularly in applications
requiring tight tolerances. Shrinkage that leads to sink marks or voids can be reduced or eliminated by
packing the cavity after filling. Also, the mold design should take shrinkage into account in order to
conform to the part dimension. Part shrinkage predicted by C-MOLD offers a useful guideline for proper
mold design.

FIGURE 2. Processing and design parameters that affect part shrinkage

Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of the
design. Part warpage results from molded-in residual stresses, which, in turn, is caused by differential
shrinkage of material in the molded part. If the shrinkage throughout the part is uniform, the molding
will not deform or warp, it simply becomes smaller. However, achieving low and uniform shrinkage is a
complicated task due to the presence and interaction of many factors such as molecular and fiber
orientations, mold cooling, part and mold designs, and process conditions.

Warpage in molded parts results from differential shrinkage. Variation in shrinkage can be caused by
molecular and fiber orientation, temperature variations within the molded part, and by variable packing,
such as over-packing at gates and under-packing at remote locations, or different pressure levels as
material solidifies across the part thickness. These causes are described more fully below.

Differences in filled and unfilled materials
Non-uniform mold cooling across the part thickness or over the part
Cooling rates that differ because of Part thickness variation
Part geometry asymmetry or curvature

Differences in filled and unfilled materials
Differential shrinkage for filled and unfilled materials is shown in Figure 3 below. When shrinkage is
differential and anisotropic across the part and part thickness, the internal stresses created can lead to
part warpage.

Filled materials
For fiber-filled thermoplastics, reinforcing fibers inhibit shrinkage due to their smaller thermal
contraction and higher modulus. Theref

Shrinkage and Warpage

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FIGURE 2. Fiber distribution parallel to the weld line leads to a weaker bond

The exact strength of the weld line depends on the ability of the flow fronts to weld (or knit) to each
other. The strength of the weld-line area can be from 10 to 90 percent as strong as the pure material
used. With such a wide range possible, the conditions that are favorable to better weld-line quality are
worth examining:

High injection pressure and speed.
High melt and mold-wall temperature.
Formation of the weld lines closer to the gate.
A temperature difference of less that 10ºC between the two emerging melt fronts.

If a weld line forms before the filling is complete and is immediately subject to additional packing
pressure, the weld line will typically be less visible and stronger. For complex part geometry, flow
simulation helps to predict the weld/meld-line position with respect to changes in the tool design, and
to monitor the temperature difference.

FIGURE 3. Changing the weld-line position by modifying the delivery system.

FIGURE 4. Improving he weld-line position by modifying the delivery system.

Weld Lines and Meld Lines

Page 32

Alter the part design

Increase the wall thickness.

Adjust the gate position and dimension or decrease the part thickness ratio.

Alter the mold design

Increase the size of gate and runners.

Place a vent in the area of the weld/meld line.

Change the gate design to eliminate weld/meld lines or to form them closer to the gate at a high temperature and
under high packing pressure.

Adjust the molding conditions

Increase the melt temperature, injection speed, or injection pressure.

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