In injection molding, it is often mentioned that product has internal stress, residual stress and its impact on product quality. What is internal stress and residual stress in injection molding? What is relationship between them? Let's sort it out today, especially relationship between internal stress and residual stress. Everyone should also express their correct views.

When an object is deformed due to external factors (force, temperature change, etc.), internal forces that interact with each other in object are generated between parts to resist effect of such external factors and try to restore object from position after deformation to position before deformation. Internal force per unit area at a certain point in cross section under investigation is called stress. Stress perpendicular to cross section is called normal stress or normal stress.

Internal stress refers to stress inside material caused by external loads or temperature changes. This stress is caused by internal structure of material or processing method during manufacturing process.
Internal stress is stress retained inside an object when there is no external force acting on structure.
When there is no external force, stress retained inside an elastic object is called internal stress. Its characteristic is that a balanced force system is formed inside object, that is, it complies with static conditions.

According to nature and scope, it can be divided into macro stress, micro stress and ultra-micro stress.
According to cause, it can be divided into thermal stress and tissue stress.
According to existence time, it can be divided into instantaneous stress and residual stress.
According to direction of action, it can be divided into longitudinal stress and transverse stress.

Internal stress may cause deformation, crack extension and rupture of materials.

(1) Orientation stress
During molding process, molecular chain of plastic material undergoes drastic changes due to high pressure and high shear force. Molecules are frozen before they completely return to their disordered and relaxed natural state, resulting in residual orientation stress. This is especially obvious in PC materials. Other materials such as PC/ABS and PSU also have same problem. Occurrence of this situation is closely related to their molecular chain structure.
Shear orientation stress represents stress caused by shear flow during plastic processing, which is affected by flow rate and viscosity of plastic. At the moment of filling, filling volume decreases, injection rate increases when flow rate is fixed, plastic is cold and has a higher viscosity, so shear stress at last filling position is higher.
Plastics may crack and have high residual stress. Locations of occurrence: Gate position is prone to extrusion orientation stress due to fast injection rate or long holding time; wall thickness changes sharply (especially from thick to thin) and thin wall position will produce extrusion orientation stress due to strong shear force; material flow filling is unbalanced and causes local extrusion and extrusion orientation stress due to overfilling.
(2) Shrinkage stress
In process from melting to cooling, molecular chain has uneven cooling temperature due to differences in product wall thickness or cooling water channels, resulting in different shrinkage at different temperature positions.
At different shrinkage rates, residual stress will be generated between interfaces due to tensile shear. Locations of occurrence: Mainly occurs in products with uneven wall thickness. At locations where the wall thickness changes sharply, different shrinkage orientations are likely to occur due to uneven heat dissipation.
(3) Shear stress
Slower flowing liquid layer will block movement of faster flowing liquid layer, and external force that causes relative movement between liquid layers is called shear force. Force required to be applied per unit liquid layer area is called shear stress τ, with unit of N•m-2, i.e. Pa.

 

Product structure: Presence of sharp corners can easily lead to stress concentration at that location. Stress cracking will occur when subjected to external impact or solvent induction. Uneven wall thickness distribution can also lead to stress. In areas where wall thickness changes, shear velocity will change due to thickness change, which will lead to stress.
Mold structure: Improper setting of gate size and position can also lead to unbalanced material flow filling, and local locations may be overfilled, resulting in large extrusion shear stress, causing stress similar to that caused by excessive holding pressure.

(1) Heat treatment of object (for workpieces such as metal materials and polymer materials).
(2) Elimination under natural conditions (i.e. natural aging to eliminate internal stress).

Residual stress is stress that still exists inside an object after various external factors (external force, temperature change, processing process, etc.) that generate stress are removed. It is called residual stress. Generally speaking, residual stress refers to stress that exists in an object when it is not affected by external factors in order to keep object in self-phase equilibrium.

 

Schematic diagram of residual stress

Causes of residual stress can be summarized into three categories:
First category is uneven plastic deformation;
Second category is uneven temperature change;
Third category is uneven phase change.

1) Effect on yield limit
If material has tensile residual stress, it is equivalent to raising coordinate origin of stress-strain curve, which is equivalent to lowering tensile yield limit of material. If material has compressive residual stress, tensile yield limit is increased, while compressive yield limit is reduced.
2) Effect on fatigue life
When a component subjected to alternating stress has compressive residual stress, fatigue strength of component will be improved, while when there is tensile residual stress, its fatigue strength will be reduced.
3) Influence on component deformation
Influence of residual stress on component deformation includes two aspects: one is deformation capacity of component against static and dynamic loads; the other is the recovery capacity of deformation after load removal.

Injection speed: Increasing injection speed can reduce degree of molecular chain orientation, which is conducive to reducing residual stress.
Injection pressure: If injection pressure is too high, it is easy to cause excessive local pressure and generate stress; however, if injection pressure is too low, set injection speed cannot be achieved, and shearing will increase due to cooling of material flow, resulting in an increase in molecular chain orientation stress, and there will also be a large residual stress.
Holding pressure and time: Excessive holding pressure and too long time will increase molecular orientation at the gate and produce a large residual stress.
Mold temperature: If mold temperature is too low, stress cannot be released in time and remains.
Melt temperature: Increasing molding temperature will reduce viscosity of plastic material and reduce orientation stress of molecular chain, thereby reducing residual stress.
Above molding conditions restrict each other in terms of stress influence, so adjustment of residual stress during molding needs to be comprehensive.