Aimed at (acrylonitrile/butadiene/styrene) copolymer (ABS) injection molded parts, radial cracks often appear in use, which causes problem of parts scrap. When analyzing reasons, people often only consider influence of molding process and ignore influence of use environment. Through experiment, it was found that cracks in the use of ABS injection molded parts were caused by release of external stress caused by acetic acid, paint thinner, etc., correct operation method for design, manufacturing, assembly and use of ABS injection molded parts is proposed, which provides a scientific basis for 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. Its molding methods include injection, extrusion, blister, etc., among which injection molding is main molding processing method. Injection molding mainly has advantages of being able to form complex and precise parts, easy to realize automation, simple operation, etc., but there are also shortcomings of various quality problems in injection molded parts.
Quality of ABS injection molded parts is divided into two aspects: internal quality and external quality. Internal quality includes internal material organization structure of part, density, strength, stress, etc. of part; external quality is surface quality of part, common ones are undershot (not filled), obvious parting line (running material), depression (collapsed or sink mark), discoloration (decomposition pattern), dark line (black mark) , weld mark (composite pattern), silver wire (water pattern), peeling (skinning), flow mark (water ripple), jet flow (snake pattern), deformation (warping, twisting), poor finish (scratches, scratches), cracks (cracks), dull (not bright), bubbles (hollow or hollow), whitening (white marks), etc.
There are many factors that affect quality of ABS injection molded parts, among which stress cracking is one of common fatal defects, which seriously hinders application of ABS injection molded parts.

1. 1 Stress classification and generation process. After polymer is stressed, internal force will be balanced with external force, internal force per unit area is called stress. According to cause of formation, stress can be divided into internal stress and external stress. Internal stress includes two types: 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.
Size of forming stress depends on many factors such as macromolecular structure of polymer, rigidity of chain segment, rheological properties of melt, complexity of shape of part and size of wall thickness. If forming stress value is too large, it is easy to cause forming defects such as stress cracking and melt fracture.
There are many reasons for formation of induced stress, such as internal force caused by internal temperature difference or uneven shrinkage of plastic melt or injection molded part; internal force caused by difference between cavity pressure and external pressure when part is demolded; plastic melt is due to flow orientation Internal force caused by etc. Obviously, induced stress is generally unable to balance with external force, it is easy to remain inside cooled part and become a residual stress, which will affect quality of part. External stress mainly refers to strain force generated by external force in 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 parts are subjected to greater shear and extrusion, internal forces that are balanced with external forces will inevitably be generated inside parts.
Effect of stress on quality of part during injection process. Theoretically, when polymer is injected and filled, if it can be solidified at an extremely slow cooling rate under action of holding pressure, polymer macromolecule will be in mold cavity. There is sufficient time for deformation and rearrangement, so that amount of deformation can be gradually balanced with effects of injection pressure and holding pressure, there is no residual stress in part after demolding, size and shape are stable.
However, in actual production, due to requirement of productivity, above method is almost impossible. Even if slow cooling measures are used in production, resulting cooling rate is still very severe for deformation and rearrangement of macromolecules.
Therefore, when filled polymer is cooled and solidified under action of holding pressure, macromolecules can only be stacked together in a simple manner according to shape of mold cavity, and there is no time to arrange stable state. Therefore, amount of deformation is not compatible with effects of injection pressure and holding pressure, and there will still be a large residual stress in part after demolding.
Macromolecules will continue to deform and rearrange over time to adapt to results of stress during molding (eliminate residual stress). Parts with large residual stresses often embrittle and crack under action of small external force or solvent, that is, stress cracking.
Stress cracking is one of quality problems that often occur in injection molded parts, especially in northern regions where climate temperature difference changes greatly. Stress cracking phenomenon is more prominent. Cracks mostly appear in parts where stress is concentrated, such as gates, edges, and weld marks of parts.
In addition, due to effect of stress, defects such as deformation, warpage, and twisting often appear in parts. Internal stress can generally be reduced to a lower limit by taking corresponding measures from molding process. External stress is often easily ignored by people, so that cracking of injection molded part is completely attributed to stress generated during molding process, so that quality problem cannot be fundamentally solved.

Main factors that affect stress of ABS injection molded parts are quality of resin, molding conditions, rationality of part and mold design, use environment and process of parts. Quality of resin has a great influence on stress of part. With more volatiles and wide molecular weight distribution, stress of parts will be greater.
Influencing factors of molding conditions mainly include moisture in material, barrel temperature, injection pressure, pressure holding time, mold temperature and so on. ABS resin must be dried before molding. The higher degree of drying, the more obvious reduction of internal stress.
Increasing barrel temperature can reduce melt viscosity, which is beneficial to relieve molecular orientation and reduce stress. However, too high barrel temperature will easily decompose resin and increase stress of parts. Therefore, barrel temperature should be appropriate. Increasing injection pressure or extending holding time will increase molecular orientation stress, but it will help reduce shrinkage stress. An increase in mold temperature will reduce stress, but it will prolong molding cycle and increase possibility of resin decomposition.
Part and mold structure mainly includes part thickness, corner transition, feeding method, etc. Such as location of gate and location of cooling pipe will have a greater impact on molding quality of part. Increasing wall thickness of part will reduce molecular orientation stress, but will increase shrinkage stress. Corners of workpiece are transitioned by circular arcs to avoid stress concentration.
Use environment of part mainly includes stress, whether it is in contact with solvents, etc. When parts are assembled with metal, assembly torque should be controlled. Excessive torque is easy to cause greater stress in assembly of ABS injection parts. Avoid contact with solvents or solvent gas environments that are prone to stress cracking of ABS injection molded parts.