Reasons for the Stress Cracking of ABS Injection Molded Parts

Radial cracks often appear in injection molded parts of (acrylonitrile / butadiene / styrene) copolymer ABS during use, which causes the problem of scrap. When analyzing the reasons, people often only consider the influence of the molding process and ignore the influence of the use environment.

Through experiments, it is found that the cracks in the ABS injection molded parts are caused by the release of external stress of acetic acid, paint thinner, etc.. And the correct operation methods for the design, manufacturing, assembly and use of ABS injection molded parts are proposed. And i provides a scientific basis for the safe use of ABS injection molded parts.

(Acrylonitrile / butadiene / styrene) copolymer (ABS) resin has been blended and modified to form a variety of different grades. The molding methods include injection, extrusion, blister, etc., of which injection molding is the main molding processing method. Injection molding mainly has the following advantages:  

  • Complicated and precise parts can be formed
  • Easy to realize automation
  • Simple operation, etc.

But there are also disadvantages of various quality problems in injection molded parts.

The quality of ABS injection molded parts is divided into two aspects: internal quality and external quality.

  • Internal quality includes the material organization structure inside the part, the density, strength, stress, etc. of the part.
  • The external quality is the surface quality of the part. The common ones are undershot (not filled), obvious parting line (running material), depression (collapsed or sink mark), discoloration (decomposition pattern), dark line (black mark) , weld marks (composite pattern), silver wire (water pattern), peeling (skinning), flow mark (water ripple), jet flow (snaking pattern), deformation (warping, twisting), poor finish (scratches, wounds), cracks, dull (not bright), bubbles (hollow), whitening (white marks), etc.

There are many factors that affect the quality of ABS injection molded parts, among which stress cracking is one of the common fatal defects. And it seriously hinders the application of ABS injection molded parts.

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

Stress classification and generation process

After the polymer is stressed, it will create one internal force which will be balanced with the external force. The internal force per unit area is called the stress. According to the cause of formation, the stress can be divided into internal stress and external stress. Internal stress includes active stress and induced stress. Active stress is an internal force balanced with external forces (injection pressure, holding pressure, etc.). So it is also called molding stress.

The size of the molding stress depends on many factors such as the macromolecular structure of the polymer, the rigidity of the chain segment, the rheological properties of the melt, the complexity of the shape of the part and the size of the wall thickness. If the molding stress value is too large, it is easy to cause forming defects like stress cracking and melt fracture. There are many reasons for the formation of induced stress, such as:

  • The internal force caused by the internal temperature difference or uneven shrinkage of the plastic melt or the injection molded part.
  • The internal force caused by the difference between the cavity pressure and the external pressure when the part is demolded.
  • The internal force caused by the flow orientation of the plastic melt, etc.

Obviously, the induced stress is generally unable to balance with the external force. It is easy to remain inside the cooled part and become a residual stress, which will affect the quality of the part.

External stress mainly refers to the strain force generated by the external force in the use of injection molded parts.

For plastic structural parts, they are often connected with metal fixing parts in use. In order to achieve tightening and firmness, so that the parts are subjected to greater shear and extrusion, internal forces that are balanced with external forces will inevitably be generated inside the parts.

The effect of stress on the quality of the part during the injection process. Theoretically, after the polymer is injected into the mold, if it can be solidified at an extremely slow cooling rate under the action of the holding pressure, the polymer macromolecules have sufficient time to deform and rearrange in the mold cavity, so that the amount of deformation can be gradually balanced with the effects of injection pressure and holding pressure, there is no residual stress in the part after demolding, and the size and shape are stable.

However, in actual production, due to the requirement of productivity, the above method is almost impossible. Even if slow cooling measures are used in production, the resulting cooling rate is still very severe for the deformation and rearrangement of macromolecules.

Therefore, when the filled polymer is cooled and solidified under the action of the holding pressure, the macromolecules can only be stacked together in a simple manner according to the shape of the mold cavity. There is no time to arrange the stable state. Therefore, the amount of deformation is not compatible with the effects of injection pressure and holding pressure. And there will still be a large residual stress in the part after demolding.

Macromolecules will continue to deform and rearrange over time to adapt to the result of stress during molding (eliminate residual stress). Parts with large residual stress will often become embrittlement and crack under the action of small external force or solvent, that is, stress cracking.

Stress cracking is one of the quality problems that often occur in injection molded parts, especially in the northern regions where the climate temperature difference changes greatly. The stress cracking phenomenon is more prominent. Cracks mostly appear in parts where the stress is concentrated, such as gates, edges, and weld marks of the parts.

In addition, due to the effect of stress, the parts often have defects such as deformation, warpage, and twisting. The internal stress can generally be reduced to a lower limit by taking corresponding measures from the molding process. The external stress is often easily ignored by people. Because of that reason, the cracking of the injection molded part is completely attributed to the stress generated during the molding process. So the quality problem cannot be fundamentally solved.

Analysis of factors affecting the stress of ABS injection molded parts

The main factors that affect the stress of ABS injection molded parts are: 

  • quality of the resin
  • molding conditions
  • rationality of the parts and mold design
  • use environment and process of the parts

The Resin Quality

The quality of the resin has a great influence on the stress of the part. With more volatiles and wide molecular weight distribution, the stress of the parts will be greater.

Molding Conditions

The influencing factors of molding conditions mainly include moisture in the material, barrel temperature, injection pressure, pressure holding time, mold temperature and so on. ABS resin must be dried before molding. The higher the degree of drying, the more obvious the reduction of internal stress. Increasing the barrel temperature can reduce the melt viscosity, which is beneficial to relieve molecular orientation and reduce stress. However, too high barrel temperature will easily decompose the resin and increase the stress of the parts. Therefore, the barrel temperature should be appropriate.

Increasing the injection pressure or extending the holding time will increase the molecular orientation stress. But it will help reduce the shrinkage stress. An increase in mold temperature will reduce stress, but it will prolong the molding cycle and increase the possibility of resin decomposition.

Parts and Mold Design

The part and mold structure mainly includes part thickness, corner transition, feeding method, etc. The location of the gate and the cooling runner will have a greater impact on the molding quality of the part. Increasing the wall thickness of the part will reduce the molecular orientation stress, but will increase the shrinkage stress. The corners of the workpiece are transitioned by circular arcs to avoid stress concentration.

Use Environment

The use environment of the part mainly includes the stress, whether it is in contact with solvents, etc. When the parts are assembled with metal, the assembly torque should be controlled. Excessive torque is easy to cause greater stress in the assembly of ABS injection parts. Avoid contact with solvents or solvent gas environments that are prone to stress cracking of ABS injection molded parts.

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