Analysis of the causes of stress cracking in ABS injection molded parts

Analysis of the causes of stress cracking in ABS injection molded parts


Stress classification and generation process

After the polymer is stressed, the internal force balanced with the external force will be generated, and the internal force on the unit area is
called stress. According to the formation of stress can be divided into internal stress and external stress. Internal stresses include active stresses
and induced stresses. Active stress is the internal force balanced with the external force (injection pressure, holding pressure, etc.).
Internal stresses are also called molding stresses. The magnitude of the molding stress depends on the macromolecular structure of the polymer, the stiffness of the chain segments, the rheology of the melt, and the internal stress.
The magnitude of the molding stress depends on many factors such as the macromolecular structure of the polymer, the stiffness of the chain segments, the rheological properties of the melt and the complexity of the part shape and wall thickness. Molding stress
If the molding stress value is too large, it is easy for the part to have molding defects such as stress cracking and melt rupture. Stress induced
There are many reasons for the formation of stresses, such as internal forces caused by temperature differences or uneven shrinkage within the plastic melt or injection molded parts.
The internal force caused by the difference between the pressure of the mold cavity and the external pressure when the part is demolded; the internal force caused by the flow of the plastic melt.
The internal force caused by the flow of plastic melt, etc. Obviously, the induced stresses are generally not balanced with external forces and are easily retained
After cooling, they become residual stresses and affect the quality of the part. External Stress
refers to the strain force generated by the external force during the use of injection molded parts. For plastic structural parts, they are
For plastic structural parts, they are often connected with metal fixtures, so that the parts are subjected to large shear and squeeze in order to achieve tightness and security.
In order to achieve a tight and secure connection, so that the parts are subjected to large shear and compression, the internal forces inside the parts must be balanced with the external forces.
Stress in the injection process on the quality of the parts from the theoretical point of view, when the polymer injection filling mold, if the pressure can be maintained at the pressure of the pressure.
Theoretically, when the polymer is injected into the mold, if it can be cured at a very slow cooling rate under pressure-holding pressure, the polymer macromolecules will have sufficient time to develop in the mold cavity.
Theoretically, when the polymer is injected into the mold, if it can be cured at a very slow cooling rate under the action of holding pressure, the polymer molecules will have sufficient time for deformation and rearrangement in the mold cavity, so that the deformation can be gradually balanced with the injection pressure and holding pressure.
This allows the amount of deformation to be gradually balanced with the injection and holding pressures, resulting in a stable size and shape with no residual stress in the part after demolding. However, in
However, in actual production, the above method is almost impossible due to the requirement of productivity. Even with the use of slow cooling measures in production, the resulting parts are not as stable in size and shape.
Even with the use of slow cooling measures, the resulting cooling rate is still very intense for the deformation and rearrangement of the macromolecules.
Therefore, when the polymer is cooled and cured under holding pressure after filling the mold, the macromolecules simply follow the shape of the mold cavity and accumulate together.
The polymer after filling is cooled and cured under pressure-holding, the macromolecules can simply accumulate according to the shape of the mold cavity, and there is no time to arrange them towards a stable state. Therefore, the amount of deformation depends on the injection
Therefore, the amount of deformation is not compatible with the injection pressure and holding pressure, and there will still be large residual stresses in the part after demolding.
The macromolecules will continue to be deformed and rearranged over time in order to match the results of the stresses during molding.
The residual stresses are eliminated. Parts with large residual stresses will often be brittle and crack with little external force or solvent.
Stress cracking is a common cause of stress cracking in injection molded parts. Stress cracking is one of the most common quality problems of injection molded parts.
Especially in the northern region, where the temperature difference between the climate is large, the stress cracking phenomenon is more prominent. Cracks mostly appear
In the parts with more concentrated stress, such as the gate, edge, and fusion mark. In addition, due to the role of stress, the
In addition, due to the effect of stress, the parts are often deformed, warped, twisted and other defects. The internal stress can be reduced to a lower level by taking corresponding measures in the molding process.
Generally, it can be reduced to a low limit. External stress is often overlooked, so that the injection
The cracking of plastic parts is completely attributed to the stress generated during the molding process, so that the quality problem cannot be fundamentally solved.
The quality problem cannot be solved fundamentally.