Causes and solutions of stress cracking of ABS injection parts

- Jun 19, 2019-

For injection parts of acrylonitrile/butadiene/styrene copolymer (ABS), radial cracks often appear in use, which leads to the scrap of products.When analyzing the reasons, people often only consider the influence of molding technology, but ignore the influence of using environment.It is found that the crack in the use of ABS injection parts is caused by the release of external stress caused by acetic acid and paint thins, and the correct operation method of ABS injection parts design, manufacture, assembly and use is put forward, which provides scientific basis for the safe use of ABS injection parts.

(acrylonitrile/butadiene/styrene) copolymer (ABS) resin is modified by blending, forming a variety of different grades, its molding methods include injection, extrusion, blister, etc., of which injection molding is the main molding method.Injection molding mainly has the advantages of molding complex, precise size parts, easy automation and simple operation, but there are also shortcomings of injection parts with various quality problems.ABS injection parts quality is divided into internal quality and external quality.The internal quality includes the material organization and structure of the parts, and the density, strength and stress of the parts.External quality for surface quality of parts, namely common owe note (not fill), parting line is obvious (run), depression (collapse pit or shrink mark), color (decomposition), dark (black), welding scar (grain), silver (water), peeling (peeling), flow mark () water ripple, jet (sinusoidal lines), deformation (warping, distortion), bright and clean degree difference (scratch, scratch), crack (crack), matt (bright), bubble (empty or hollow), bleaching (such as white printing).There are many factors affecting the quality 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. Analysis of stress cracking reasons of ABS injection parts

1. 1 stress classification and production process: when the polymer is stressed, internal forces in equilibrium with external forces will be generated. The internal forces per unit area are called stresses.Stress can be divided into internal stress and external stress according to the cause of formation.Internal stress includes two types: active stress and induced stress.Active stress is the internal force in balance with external force (injection pressure, pressure retaining pressure, etc.), so it is also called molding stress.

The degree of forming stress depends on the macromolecular structure of polymer, the rigidity of chain segment, the rheological properties of melt, the complexity of the shape of the product and the thickness of the wall.Forming stress value is too large, it is easy to produce stress cracking and melt fracture forming defects.Induced stress is caused by many factors, such as internal force caused by temperature difference or uneven shrinkage of plastic melt or injection parts;The internal force caused by the difference between the cavity pressure and the external pressure;

The internal force caused by flow orientation of plastic melt, etc.Obviously, the induced stress is generally unable to balance with external forces, and it is easy to retain in the cooling of the product inside, become residual stress, and thus the quality of the product has an impact.External stress mainly refers to the stress caused by external forces in the use of injection parts.For plastic structural parts, the use is often connected with the metal fixed parts, in order to achieve fastness, fastness, so that the parts by the larger shear, extrusion, the internal parts and external forces are bound to produce a balance of internal force.

Stress on the quality of the product in the process of injecting theoretically, after the polymer injection molding, such as in the holding pressure to very slow cooling rate of solidification, the polymer molecules within the cavity is sufficient time for deformation and rearrangement, thus can make the deformation gradually and the role of the injection pressure and holding pressure balance, no residual stress in parts after demoulding, stable size and shape.In practice, however, this approach is almost impossible because of the need for productivity.Even if cooling measures are adopted in production, the cooling rate obtained is still very severe for the deformation and rearrangement of macromolecules.

Therefore, when the mould-filled polymer cools and solidifies under the action of pressure retaining, the macromolecules can only be simply stacked together according to the shape of the mold cavity, and there is no time for the arrangement tending to a stable state.Therefore, the amount of deformation and injection pressure and pressure holding does not adapt to the role of the parts will still exist after the release of large residual stress.The macromolecules will also continue to deform and rearrange over time, in order to adapt to the stress effects of the molding process (eliminating residual stress).The products with large residual stress often brittle cracking under the action of small external force or solvent, that is, stress cracking.

Stress cracking is one of the quality problems of injection parts, especially in the north where the climate temperature difference varies greatly, stress cracking is more prominent.The crack appears in the gate, edge, weld mark and other places where the stress is concentrated.In addition, due to the role of stress, parts often appear deformation, warping, distortion and other defects.Internal stress from the molding process to take appropriate measures, generally can be reduced to a lower limit.External stress is often easy to be ignored by people, so that the plastic parts of the crack is completely attributed to the molding process of the stress, so that the quality problem can not be fundamentally solved.

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

The main factors affecting the stress of ABS injection parts are the quality of resin, molding conditions, the rationality of parts and mould design, the use environment and process of parts, etc.The quality of resin has a great impact on the stress of the parts.More volatile, wide molecular weight distribution, the stress of the product is large.

The main influencing factors of forming conditions are water content, cylinder temperature, injection pressure, holding time and mold temperature.ABS resin must be dried before molding, the higher the degree of drying, the more obvious the reduction of internal stress.Increase the temperature of the cylinder, can reduce the melt viscosity, is conducive to remove the molecular orientation, reduce the stress, but too high temperature of the cylinder easy to make resin decomposition, but increased the stress of the products, so the temperature of the cylinder should be appropriate.Increasing injection pressure or prolonging pressure holding time can increase molecular orientation stress, but is beneficial to reduce shrinkage stress.The increase of mold temperature will reduce the stress, but will prolong the molding cycle and increase the possibility of resin decomposition.

The structure of parts and die mainly includes parts thickness, corner transition and feeding mode.Such as the location of the gate, the location of the cooling pipe will have a greater impact on the molding quality of the parts.Increasing the wall thickness will reduce the stress of molecular orientation, but increase the stress of shrinkage.Corner with arc transition, can avoid stress concentration.

The working environment of the parts mainly includes stress condition, whether or not to contact solvent and so on.The assembly torque should be controlled when the parts are combined with metal in the assembly. Excessive torque will easily make ABS injection parts produce greater stress in the assembly.Avoid contact with solvent or solvent gas environment which may lead to stress cracking of ABS injection parts.