Causes, effects, detection and reduction methods of residual stress in injection molded parts

Time：2024-08-30 08:29:21 / Popularity： / Source：

I. What is residual stress in injection molded parts

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

Schematic diagram of residual stress

II. Causes of residual stress

Causes of residual stress can be summarized into three categories:
The first category is uneven plastic deformation.
The second category is uneven temperature change.
The third category is uneven phase change.

III. Influence of residual stress on injection molded parts

1. Mechanical properties

Residual stress will cause strength, toughness and fatigue resistance of injection molded parts to decrease. This is because residual stress can cause changes in internal structure of injection molded parts, such as lattice distortion, hole formation, etc., thereby reducing mechanical properties of material.

2. Dimensional stability

Residual stress can cause dimensional changes in injection molded parts during use, such as shape deformation, warping, etc. This is because residual stress can cause stress release and redistribution inside injection molded parts, resulting in changes in shape of injection molded parts.

3. Thermal stability

Residual stress can affect thermal stability of injection molded parts. In a high temperature environment, residual stress can cause thermal expansion of injection molded parts, resulting in dimensional changes and shape deformation. At the same time, residual stress can also accelerate aging process of injection molded parts and reduce their service life.

4. Surface quality

Residual stress can affect surface quality of injection molded parts. During molding process of injection molded parts, residual stress can cause cracks, defects and other problems on the surface of injection molded parts, thereby reducing appearance quality of injection molded parts.

IV. Detection method of residual stress of injection molded parts

1. Solvent immersion method

(1) Soak PC/ABS parts with screw slots or columns in glacial acetic acid for 30 seconds or longer. Take them out and check appearance. If there are fine and dense cracks, it means that there is internal stress here. The more cracks, the greater internal stress.
(2) Mix isopropyl alcohol and acetone in a ratio of 3:1, add red dye, and uniform coloring indicates that internal stress is small. Stress of parts that are not easy to color is large, which needs attention.

2. Moldflow mold flow analysis simulation method

Use mold flow analysis software to analyze pressure distribution when speed/pressure is switched. The higher pressure, the greater stress involved.

3. Quarter ellipse stress analysis method

Use a quarter ellipse fixture, place test piece in the center as much as possible, and evenly apply solvent on test piece. It is best to make 3-5 groups of test pieces of same raw material. After coating, store them statically (environmental requirements are 23℃, 50%RH), observe cracks of test piece, the more cracks, the greater residual stress.

4. Screw hole test method

Use screws of appropriate size to conduct tapping test. Some commonly used industrial butter can be applied on screws. If necessary, tapped screws can be placed in a high temperature environment to accelerate stress release. Presence of cracks indicates that participating stress is large.

V. Methods to reduce residual stress of injection molded parts

1. Optimize injection molding process parameters

By adjusting injection molding process parameters, such as injection molding temperature, injection molding pressure, injection molding speed, etc., cooling and curing process of injection molded parts can be changed, thereby reducing generation of residual stress.

2. Reasonable design of injection molded part structure

Reasonable design of structure of injection molded parts, such as adding reinforcing ribs, changing wall thickness distribution, etc., can change stress distribution of injection molded parts, thereby reducing magnitude of residual stress.

3. Use appropriate materials

Choosing appropriate injection molding materials, such as changing molecular structure of material, adding antioxidants, etc., can change thermal expansion and contraction properties of injection molded parts, thereby reducing generation of residual stress.

4. Appropriate post-processing methods

Appropriate post-processing of injection molded parts, such as heat treatment and annealing, can change internal structure and stress distribution of injection molded parts, thereby reducing existence of residual stress.

VI. Conclusion

Residual stress is an inevitable problem in molding process of injection molded parts, has an important impact on performance and life of injection molded parts. By reasonably controlling and eliminating residual stress, mechanical properties, dimensional stability, thermal stability and surface quality of injection molded parts can be improved. Therefore, in design and production process of injection molded parts, problem of residual stress should be taken seriously, and corresponding measures should be taken to deal with it.

Gud Mould Industry Co., Ltd