For (acrylonitrile/butadiene/styrene) copolymer (ABS) injection parts often appear radial cracks in use, resulting in the problem of scrap parts. When people analyze the reasons, they 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 produced in the use of ABS injection parts are caused by the release of external stress caused by acetic acid, paint thinner, etc., and the correct operation methods for the design, manufacture, assembly and use of ABS injection parts are put forward, which provides a scientific basis for the safe use of ABS injection parts.

(acrylonitrile/butadiene/styrene) copolymer (ABS) resin after blending modification, formed a variety of different grades, its molding methods are injection, extrusion, blister, etc., of which injection molding is the main molding processing method. Injection molding mainly has the advantages of forming complex, precision parts, easy to realize automation, simple operation, etc., but there are also disadvantages of injection parts that will have a variety of quality problems. The quality of ABS injection parts is divided into two aspects: internal quality and external quality. The internal quality includes the internal structure of the material, the density, strength and stress of the parts; External quality is the surface quality of the parts, the common underinjection (not filled), the parting line is obvious (run), sag (collapse or shrinkage mark), discoloration (decomposition mark), dark (black mark), weld mark (compound mark), silver wire (water mark), peeling (skin), flow mark (water ripple), jet flow (snake line), deformation (warping, twisting), poor finish (scratch, scratch, scratch, etc.) Scratches), cracks (cracks), matte (not bright), bubbles (hollow or hollow), whitening (white marks) and so on. There are many factors that affect the quality problem of ABS injection parts, among which stress cracking is one of the common fatal defects, which seriously hinders the application of ABS injection parts.

1, ABS injection parts stress cracking cause analysis

1. 1 Stress classification and production process After the polymer is stressed, the internal force will be produced and the external force is balanced, the internal force on the unit area is called stress. According to the formation of the cause of stress can be divided into internal stress and external stress. Internal stress includes two types: active stress and induced stress. Active stress is the internal force that is balanced with external force (injection molding pressure, pressure holding pressure, etc.), so it is also called molding stress.

The size of the forming stress depends on the polymer macromolecular structure, the rigidity of the chain segment, the rheological properties of the melt and the complexity of the shape of the part and the size of the wall thickness and many other factors. The molding stress value is too large, it is easy to make the molding defects such as stress cracking and melt cracking of the parts. There are many reasons for the formation of induced stress, such as internal force caused by temperature difference or non-uniform shrinkage of plastic melt or injection molding parts; The internal force caused by the difference between the cavity pressure and the external pressure when the mold is demoulded;

Internal force caused by the flow orientation of plastic melt, etc. Obviously, the induced stress can not be balanced with external force in general, and it is easy to retain in the cooled parts of the internal, as a residual stress, thus affecting the quality of the parts. External stress mainly refers to the stress caused by the action of external force in the use of injection parts. For plastic structural parts, the use is often connected with metal fasteners, in order to achieve fastness and firmness, so that the parts are subjected to greater shear and extrusion, the internal force of the parts will inevitably produce balance with the external force.

Stress in the injection process of the impact on the quality of the parts theoretically speaking, when the polymer injection molding, such as can be in the holding pressure under the action of extremely slow cooling rate curing, then polymer macromolecules in the cavity will have sufficient time for deformation and rearrangement, so that the amount of deformation and injection pressure and pressure to achieve balance, after the release of molded parts There is no residual stress, 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 is used in production, the resulting cooling rate is still very severe for the deformation and rearrangement of macromolecules.

Therefore, when the polymer after mold filling is cooled and cured under the action of holding pressure, the macromolecules can only be piled together simply according to the shape of the mold cavity, and there is no time for the arrangement tending to a stable state. Therefore, the deformation is not compatible with the injection pressure and the role of the pressure holding, and there will still be a large residual stress in the workpiece after demoulding. Large molecules will also continue to deform and rearrange with the extension of time, in order to adapt to the results of the stress action during molding (eliminate residual stress). Parts with large residual stress often brittle cracking under little external force or solvent action, that is, stress cracking.

Stress cracking is one of the quality problems that often appear in injection molding parts, especially in the northern region where the climate temperature difference is larger, the stress cracking phenomenon is more prominent. More cracks appear in the parts of the gate, edge, weld mark and other stress concentration. In addition, due to the role of stress, the parts often appear deformation, warping, twisting and other defects. Internal stress from the molding process to take the corresponding measures, generally can be reduced to a lower limit. External stress is often easily ignored by people, so that the cracking of injection parts is completely attributed to the stress generated in the molding process, so that the quality problem can not be fundamentally solved.

2, the factors affecting the stress of ABS injection parts

The factors affecting the stress of ABS injection parts are mainly the quality of resin, molding conditions, the rationality of the parts and mold design, the use of the environment and process of the parts. The quality of the resin has a great influence on the stress of the parts. More volatile matter, the molecular weight distribution is wide, the stress of the workpiece is large.

The main influencing factors of molding conditions are moisture in the material, barrel temperature, injection pressure, pressure holding time, mold temperature and so on. ABS resin must be dried before forming, 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 conducive to relieving the molecular orientation and reducing the stress, but too high the barrel temperature is easy to decompose the resin and increase the stress of the parts, so the barrel temperature should be appropriate. Increasing the injection pressure or extending the pressure holding time will increase the molecular orientation stress, but it is conducive to reducing the shrinkage stress. The increase of mold temperature will reduce the stress, but it will prolong the molding cycle and increase the possibility of resin decomposition.

The structure of the parts and the mold mainly includes the thickness of the parts, the corner transition, the feeding method and so on. Such as the location of the gate and the location of the cooling pipe will have a greater impact on the molding quality of the parts. Increasing the wall thickness of the parts will reduce the molecular orientation stress, but increase the shrinkage stress. The corner of the part is used for arc transition, which can avoid stress concentration.

parts of the use of the environment mainly includes the force situation, whether contact with solvents, etc. In the assembly of parts and metal combination, assembly torque should be controlled, too much torque is easy to make ABS injection parts produce greater stress at the combination. The solvent or solvent gas environment that is easy to cause stress cracking of ABS injection parts should be avoided.

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