Internal stress in injection molded products

Definition and essence of injection molding internal stress

During the injection molding cycle, when the plastic melt undergoes stages such as filling, holding pressure, and cooling, due to the directional arrangement of polymer chains and inconsistent shrinkage rates in various parts, a residual force will be "locked" inside the plastic part, which is the internal stress of injection molding. From a microscopic perspective, its essence is the non-equilibrium state forcibly formed by polymer macromolecules during processing, which cannot quickly recover to its natural relaxation state after cooling and solidification, and ultimately stored inside plastic products in the form of reversible high elastic deformation.

Core classification and distribution characteristics of internal stress

Stress caused by flow orientation

When the melt flows at high speed and shrinks in the mold cavity, long-chain molecules will stretch and align along the direction of the material flow. As the temperature decreases, this elongated state is fixed, forming orientation stress. In terms of spatial distribution, it exhibits a parabolic pattern that gradually weakens from the surface to the inside, and is most pronounced around the inlet. From a side view, it is often accompanied by whitening or glowing phenomena.

Stress caused by temperature difference shrinkage

When cooling and shaping plastic parts, if the cooling rates of each part are not synchronized, shrinkage differences will occur, which will induce cooling stress. Taking thick walled products as an example, the surface layer first contacts the cold mold wall and solidifies and shrinks, but the core is still in a molten state; At this point, the core layer will restrain the contraction action of the surface layer, ultimately causing the surface to bear tensile force while the central area bears compressive force. 

The distribution of this stress is also parabolic and gradually increases from the outside to the inside. In addition, if there are metal components embedded in the product, due to the significant difference in thermal expansion coefficients between metal and plastic, it is highly likely to generate uneven internal stresses near the joint surface.

In addition to the above two types, there are also stresses caused by uneven crystallinity, differences in molecular configuration, or demolding forces, but the proportion is relatively small. Usually, the area with the strongest shear force is where stress is most likely to pile up, such as at the gate position, at the end of the weld, at sharp corners, in areas of sudden wall thickness changes, and around metal inserts.

Key factors determining the magnitude of internal stress

The molecular structure of the material itself

Firstly, the stronger the rigidity of the main chain (such as PC, PPO, PPS containing benzene rings), the higher the viscosity of the melt, the limited mobility of the molecular chain, the difficulty in deformation recovery, and the natural increase in residual stress. Secondly, the denser the polar groups (such as carbonyl, ester, cyanide), the stronger the intermolecular pulling force, which hinders the rebound of elastic deformation and also pushes up the internal stress level. In addition, the steric hindrance effect of side groups cannot be ignored. The larger the volume of substituents (such as the large phenyl group in polystyrene), the easier it is to block the free movement of large molecular chains, leading to the accumulation of more stress inside the product.

Setting of Injection Molding Process Conditions

In terms of parameter control, the longer the holding time, the stronger the continuous shear effect on the melt, and the amount of frozen elastic deformation increases accordingly. The orientation stress will sharply increase with the extension of holding time and the increase of adhesive replenishment. 

Meanwhile, the temperature of the mold, the speed of cooling, the magnitude of injection pressure, and the residual pressure at the moment of mold opening all have a direct impact on internal stress. It is particularly important to note that during mold opening, the residual pressure inside the mold cavity should be reduced to a level similar to atmospheric pressure as much as possible to prevent additional demolding stress during ejection.

Practical strategies for eliminating and preventing internal stress

Annealing heat treatment process

Releasing orientation stress through heat treatment (i.e. annealing) is currently recognized as the most effective method in the industry. The scientific principle is to provide thermal energy to polymers, promoting their transition from unstable forced freezing state to thermodynamic equilibrium state, achieving thermal relaxation of high elastic deformation, thereby significantly weakening or even eliminating internal stress.

Temperature control: It is generally recommended to set the baking temperature within the range of 10 to 20 ℃ higher than the actual operating temperature of the product, or within a safe range of 5 to 10 ℃ lower than the thermal deformation temperature of the material.

Duration control: For parts with regular wall thickness, insulation for 1 to 2 hours is generally sufficient. If the product is thicker, the baking time should be appropriately extended proportionally.

Scenarios where annealing must be performed: materials with high glass transition temperature and rigid main chain; Structures with large wall thickness or containing metal inserts; Components with large temperature differences in the working environment or extremely strict requirements for dimensional accuracy; High internal stress products that are difficult to naturally dissipate stress on their own, as well as parts that have undergone secondary mechanical processing.

Mold optimization and raw material control

The coordination between mold optimization and raw material control needs to fundamentally prevent stress cracking, and multiple measures need to be taken. In injection mold design, it is necessary to ensure that the force on the ejection system is balanced, reserve sufficient demolding slope, avoid sharp corner design as much as possible, and reduce the frequency of use of metal inserts. At the raw material preparation stage, it is necessary to strictly control the addition ratio of recycled materials and ensure that the resin is thoroughly dried before production to reduce the risk of internal stress from the source.