A culprit in shrinkage and warpage problems

Residual stress is process-induced stress, frozen in molded plastic parts. It can be either flow-induced or thermal-induced. Residual stresses affect a part similarly to externally applied stresses. If they are strong enough to overcome the structural integrity of the part, the part will warp upon ejection, or later crack, when external service load is applied. Residual stresses are the main cause of part shrinkage and warpage. The process conditions and design elements that reduce shear stress during cavity filling will help to reduce flow-induced residual stress. Likewise, those that promote sufficient packing and uniform mold cooling will reduce thermal-induced residual stress. For fibre-filled materials, those process conditions that promote uniform mechanical properties will reduce thermal-induced residual stress.

Flow-induced residual stress

Unstressed, long-chain polymer molecules tend to conform to a random-coil state of equilibrium at temperatures higher than the melt temperature (i.e., in a molten state). During processing the molecules orient in the direction of flow, as the polymer is sheared and elongated. If solidification occurs before the polymer molecules are fully relaxed to their state of equilibrium, molecular orientation is locked within the molded plastic part. This type of frozen-in stressed state is often referred to as flow-induced residual stress. Because of the stretched molecular orientation in the direction of flow, it introduces anisotropic, non-uniform shrinkage and mechanical properties in the directions parallel and perpendicular to the direction of flow.

Deformation issue

Frozen-in molecular orientation

Due to a combination of high shear stress and a high cooling rate adjacent to the mold wall, there is a highly oriented layer frozen immediately below the part surface. This is illustrated in Figure 1. Subsequent exposure of a part with high residual flow stresses (or frozen-in orientation) to high temperature may allow some of the stresses to relieve. This typically results in part shrinkage and warpage. Due to the thermal insulating effect of the frozen layers, polymer melt in the hot core is able to relax to a higher degree, leading to a low molecular orientation zone. China mold supplier

FIGURE 1. The development of residual flow stresses due to frozen-in molecular orientation during the filling and packing stages.
(1) High cooling, shear, and orientation zone

(2) Low cooling, shear, and orientation zone

Reducing flow-induced residual stress

Process conditions that reduce the shear stress in the melt will reduce the level of flow-induced residual stresses. In general, flow-induced residual stress is one order of magnitude smaller than the thermal-induced residual stress.

• higher melt temperature
• higher mold-wall temperature
• longer fill time (lower melt velocity)
• decreased packing pressure
• shorter flow path.

 Thermal-induced residual stress

Thermal-induced residual stress occurs due to the following reasons:

• The material shrinks as the temperature drops from the process settings to the ambient conditions reached when the process is complete.
• The material elements experience different thermal-mechanical histories (e.g., different cooling rates and packing pressures) as the material solidifies from the mold wall to the center.
• Changing pressure, temperature, and molecular and fiber orientation result in variable density and mechanical properties.
• Certain mold constraints prevent the molded part from shrinking in the planar directions.

Free quenching example

Material shrinkage during injection molding can be conveniently demonstrated with a free quenching example, in which a part of the uniform temperature is suddenly sandwiched by cold runner mold walls. During early cooling stages, when the external surface layers cool and start to shrink, the bulk of the polymer at the hot core is still molten and free to contract. However, as the internal core cools, local thermal contraction is constrained by the already-rigid external layers. This results in a typical state of stress distribution with tension in the core balanced by compression in the outer layers, as illustrated in Figure 2 below.

Variable residual stresses arise and the part deforms as layers of different frozen-in specific volume interact with each other

Process-induced vs. in-cavity residual stress

Process-induced residual stress data are much more useful than in-cavity residual stress data for molding simulation. Following are definitions of the two terms, along with an example that illustrates the difference between them.

Process-induced residual stress

After part ejection, the constraints from the mold cavity are released, and the part is free to shrink and deform. After it settles to an equilibrium state, the remaining stress inside the part is called process-induced residual stress, or simply, residual stress. Process-induced residual stress can be flow-induced or thermal-induced, with the latter being the dominant component.

In-cavity residual stress

While the part is still constrained in the mold cavity, the internal stress that accumulates during solidification is referred to as in-cavity residual stress. This in-cavity residual stress is the force that drives post-ejection part shrinkage and warpage.

Example

The shrinkage distribution described in Warpage due to differential shrinkage leads to a thermal-induced residual stress profile for an ejected part, as shown in the lower-left figure below. The stress profile in the upper-left figure is the in-cavity residual stress, in which the molded part remains constrained within the mold prior to ejection. Once the part is ejected and the constrained force from the mold is released, the part will shrink and warp to release the built-in residual stress (generally tensile stress, as shown) and reach an equilibrium state. The equilibrium state means that there is no external force exerting on the part and the tensile and compressive stresses over the part cross-section should balance with each other. The figures on the right side correspond to the case with a non-uniform cooling across the part thickness and, thereby, causing an asymmetric residual-stress distribution.

In-cavity residual stress profile (top) vs. process-induced residual stress profile and part shape after ejection (bottom).

Reducing thermal-induced residual stress

Conditions that lead to sufficient packing and more uniform mold-wall temperatures will reduce the thermal-induced residual stresses. These include:
–  Proper packing pressure and duration
–   Uniform cooling of all surfaces of the part
–  Uniform wall-section thickness