In design of plastic parts, generation of residual stress has a significant impact on performance and life of parts. Residual stress is internal stress generated during molding process of plastic parts. It mainly includes two types: one is internal stress (flow residual stress) that still exists after external load is removed, the other is internal stress (thermal residual stress) caused by uneven cooling and solidification of part. Therefore, when wall thickness of plastic part is uneven, or cooling rate of molten material is inconsistent, residual stress will occur due to different shrinkage of thick and thin parts. In addition, rigidity and polarity of molecular chain will also affect generation of residual stress. For example, the greater rigidity of molecular chain, the higher melt viscosity, poor mobility of polymer molecular chain, and poor recovery from reversible high elastic deformation, so residual internal stress is easily generated.
In order to reduce stress on parts, designers generally adopt following methods: "Keep wall thickness consistent", "Eliminate sharp corners", and "Correctly design core pulling function". In addition, selecting appropriate materials, optimizing mold design, and adjusting processing parameters are also key factors in controlling stress.

 

Figure 1: Light refraction and rupture caused by stress

First, maintaining consistent wall thickness is critical to reducing stress on part. Achieving this goal is no easy task due to complexity of today's product geometries. Changes in thickness will cause molecular alignment orientation stress, compressive stress and cooling stress. Alignment orientation stress means that during filling stage, molecular chain is similar to a rubber band, will be aligned and stretched under influence of flow field. When a polymer chain is stretched too far or too fast, or when it is stretched and held under a constant load, it breaks. Compressive stress will be generated during holding phase, while uneven cooling during cooling phase will generate cooling stress.

Second, eliminating sharp corners helps reduce stress on part. During molding process, polymer is subjected to stresses from filling, packing and cooling. Sharp corners are another area that can lead to improper material handling, which can increase stress on plastic and result in a less functional part. This is because when a load is applied, sharp corners become the weakest link in chain, causing part to fail in that area. In addition, because sharp corner geometry tends to have a sudden change in area, it is easy to produce shear rate imbalance, which further increases possibility of stress generation.

Correctly designing core structural functions of products is also an important method to reduce stress on parts, such as core pulling, chamfering, etc. Core-pulling refers to through-holes, blind holes, or any element around which material must flow. To properly design core pull features, avoid corners or sharp edges and place gates in correct locations. This way, material can flow in a direction parallel to feature rather than perpendicular to it.

A factor in mold design that has a huge impact on stress within molded part is design of gates and cooling channels. Gate size, location, and number of molds must be designed appropriately, otherwise shear heat and shear stress on material may exceed material's recommended range. At the same time, layout of cooling water path will also greatly affect heat dissipation capacity of plastic parts. If heat is not evenly distributed to part, shrinkage will vary significantly, resulting in a product with varying degrees of stress and a high likelihood of warping.

Additionally, choosing right material is critical to controlling stress. Each material has a recommended shear rate limit. Exceeding recommended values will result in excessive shear heating and potential shear stress. Also, shear rate limitations of various additives mixed into polymer, such as color, UV stabilizers, thermal stabilizers, and lubricants, need to be considered.

Finally, adjustment of machining process parameters can help control and reduce inherent stress. First, make sure actual melt temperature of resin is within range recommended by material manufacturer. Second, set correct firing rate for injection stage to avoid excessively high shear rates that can cause a sharp increase in melt temperature and possibly degrade polymer or additives. In addition, holding pressure and time are also relevant factors. Finally, consider mold cooling and its effect on stresses, such as through constant flow rates in water pipes to achieve even distribution of part temperature during filling.

 

Figure 2: Stress distribution under same process and different materials

When designing plastic products, influence of residual stress should be fully considered to prevent or reduce problems such as warping, deformation, and even breakage that may occur in injection products during later use, to improve performance and life of parts.