Shrinkage is typically characterized in one of two ways: volumetric or linear.
Volumetric shrinkage results in well-known types of warpage. The "bowl" occurs when the perimeter has more volume, stays hotter, and shrinks more, causing the center area to pop up. The "saddle" occurs when the perimeter freezes but the center continues to shrink, pulling the perimeter in and causing it to buckle and/or twist to maintain its length.
When plastics are injected into a mold, they are subjected to a new set of conditions that affect how they shrink. Specifically, shear and extensional forces act on the polymer during the filling and packing phases. Plastic molecules tend to align themselves in the direction the polymer is flowing. This alignment, or orientation, determines linear shrinkage. Orientation can vary in direction and magnitude, meaning that many polymers shrink more parallel to flow and less perpendicular to flow.
Extensional flowOrientation effects can align molecules in ways that are difficult to predict. In some cases, extensional forces can take over at the center of the part to align molecules perpendicular to flow. Extensional flow is an expanding flow front, or “fountain” in the center of the part that causes chains and additives to become oriented in multiple directions.
Skin laminates have no distinct orientation pattern. Outer laminates inside the frozen layer exhibit high shear rates and are oriented in the direction of flow. Transition laminates have medium shear rates, but no distinct orientation. And inner laminates have lower shear rates and tend to be oriented perpendicular or transverse to flow. The thicker the part, the more influence extensional flow tends to have. Gate type and location can also contribute to this effect.
When the part exhibits both extensional orientation effects in the middle laminates and radial orientation effects in the outer laminates, whichever type of shrinkage is higher will determine the direction of warpage. This also correlates to whether the material is filled (see Figure 2).
Transient flowAnother orientation effect to be aware of is transient flow, or underflow. This refers to a flow front that changes direction during filling, typically due to a filling imbalance. The change in direction creates variations in shrinkage that create residual internal stress in the part. The greater the difference in flow between the parallel and perpendicular directions, the higher the internal stresses will be and the more warpage the part will exhibit. Another way to describe this situation is "anisotropic shrinkage," which describes shrinkage that varies by direction, as opposed to "isotropic shrinkage" that is the same in all directions.
How simulation can helpManaging shrinkage is a complicated task, given the number of factors involved and how each one can affect the others. Simulation software can make this work easier by allowing engineers to address the problem earlier in the product design cycle.
Using simulation tools (such as Autodesk Moldflow) allows you to set up and run analyses to visualize how much shrinkage to expect, given the current part material, design, and expected processing conditions. Results can be scaled for easier interpretation. Then engineers can change the processing conditions or part design and run the simulation again to see how much shrinkage is reduced. Simulation tools also make it faster and easier to consider a wider range of potential solutions, such as changing the material or the size of the mold, all of which is more convenient than dealing with shrinkage after it has already occurred.
It's also important to remember that shrinkage is unavoidable. However, understanding how and why plastic shrinks gives engineers an edge when trying to control its effect on a part and develop an appropriate solution that conforms with your budget and schedule.