crack under stress of ABS injection parts

Radial cracks often appear in injection molded parts of (acrylonitrile / butadiene / styrene) copolymer (ABS), which leads to the scrapping of parts. When analyzing the reasons, people often only consider the influence of molding process and ignore the influence of service environment.

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

(acrylonitrile / butadiene / styrene) copolymer (ABS) resin is blended and modified to form a variety of different brands. Its molding methods include injection, extrusion and blister, among which injection molding is the main molding processing method. Injection molding mainly has the advantages of molding complex and precise parts, easy automation and simple operation, but it also has the disadvantages of various quality problems.

The quality of ABS injection molded parts is divided into internal quality and external quality. The internal quality includes the material structure, shape, density, strength, stress, etc;

The external quality refers to the surface quality of the workpiece. The common ones are insufficient injection (not full), obvious parting line (material running), depression (pit collapse or shrinkage mark), discoloration (decomposition mark), dark mark (black mark), fusion mark (compound mark), silver wire (water mark), peeling (peeling), flow mark (water ripple), jet flow (serpentine pattern), deformation (warpage and distortion), poor smoothness (scratch, scratch), cracking (crack) and matte (not bright), bubbles (empty or hollow), whitening (with white print), etc.

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

1. Cause analysis of stress cracking of ABS injection molded parts

Stress classification and generation process after the polymer is stressed, the internal force in balance with the external force will be generated. The internal force per unit area is called stress. According to the causes of formation, the stress can be divided into internal stress and external stress. The internal stress includes two types: active stress and induced stress. The main dynamic stress is related to the external force (injection pressure, holding pressure, etc.) Phase equilibrium internal force, so it is also called forming stress.

The forming 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 and wall thickness of the part. If the forming stress is too large, it is easy to cause forming defects such as stress cracking and melt fracture. There are many reasons for the formation of induced stress, such as temperature difference or uneven shrinkage in the plastic melt or injection molded part Internal force caused by uniform; internal force caused by the difference between mold cavity pressure and external pressure during part demoulding;

The internal force caused by the flow orientation of plastic melt. Obviously, the induced stress can not be balanced with the external force, and it is easy to remain in the cooled part and become residual stress, which affects the quality of the part. The external stress mainly refers to the strain stress caused by the external force in the use of injection molded parts. For plastic structural parts, it is often connected with metal fasteners in use In order to achieve fastening and firmness, so that the workpiece is greatly sheared and extruded, the internal force of the workpiece must be balanced with the external force.

The influence of stress on the quality of parts during the injection process. Theoretically, if the polymer can solidify at an extremely slow cooling rate under the pressure holding pressure after injection and filling, the polymer macromolecules will have sufficient time to deform and rearrange in the mold cavity, so that the deformation can gradually balance with the injection pressure and pressure holding pressure, and there is no residual stress in the parts after demoulding, Stable size and shape.

However, in actual production, the above method is almost impossible due to the requirements of productivity. Even if slow cooling measures are adopted in production, the obtained cooling rate is still very violent for the deformation and rearrangement of macromolecules.

Therefore, when the polymer after mold filling is cooled and solidified under the pressure holding pressure, the macromolecules can only be simply stacked together according to the shape of the mold cavity, and there is no time to arrange in a stable state. Therefore, the deformation does not adapt to the action of injection pressure and pressure holding pressure, and there will still be large residual stress in the part after mold removal.

Macromolecules will continue to deform and rearrange with the extension of time, so as to adapt to the stress action results during forming (eliminate residual stress). Workpieces with large residual stress often embrittle and crack under the action of small external force or solvent, that is, stress cracking.

Stress cracking is one of the common quality problems of injection molded parts, especially in the northern region where the climate temperature difference changes greatly. Cracks mostly appear in the parts with concentrated stress such as gate, edge and weld line.

In addition, due to the effect of stress, parts often have defects such as deformation, warpage and distortion. Internal stress can generally be reduced to a lower limit by taking corresponding measures from the molding process. External stress is often easy to be ignored, so that the cracking of injection molded parts is completely attributed to the stress generated in the molding process, so that the quality problem can not be fundamentally solved.

2. Analysis of factors affecting the stress of ABS injection molded parts

The factors affecting the stress of ABS injection molded parts mainly include the quality of resin, molding conditions, the rationality of part and mold design, the service environment and process of parts, etc. the quality of resin has a great impact on the stress of parts. There are many volatiles and wide molecular weight distribution, so the stress of parts is large.

The influencing factors of molding conditions mainly include moisture in the material, barrel temperature, injection pressure, pressure holding time, mold temperature, etc. ABS resin must be dried before molding. The higher the degree of drying, the more obvious it is to reduce the internal stress. Increasing the barrel temperature can reduce the melt viscosity, relieve the molecular orientation and reduce the stress, but too high barrel temperature is easy to decompose the resin and increase the product size Stress, so the barrel temperature should be appropriate.

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

The part and die structure mainly include part thickness, corner transition, feeding mode, etc. for example, the position of gate and cooling pipe will have a great impact on the forming quality of the part. Increasing the part wall thickness will reduce the molecular orientation stress, but increase the shrinkage stress. The arc transition at the corner of the part can avoid stress concentration.

The service environment of the parts mainly includes the stress condition, whether it is in contact with the solvent, etc. when the parts are combined with metal during assembly, the assembly torque shall be controlled. Excessive torque is easy to cause large stress at the combination of ABS injection molded parts. The solvent or solvent gas environment that is easy to cause stress cracking of ABS injection molded parts shall be avoided.