A culprit in shrinkage and warpage problems
Residual stress is a process-induced stress, frozen in a molded part. 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 fiber-filled materials, those process conditions that promote uniform mechanical properties will reduce thermal-induced residual stress. China mold supplier
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 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.
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. China mold supplier
- 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:
- 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 uniform temperature is suddenly sandwiched by cold 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 Figure 2 below.
FIGURE 2. The development of residual thermal stress in a “free-quenching” part due to variations in cooling across the molded part and the material’s response to the temperature history.