Some molded parts are not partially broken, but only have fine cracks on the surface. According to difference in their degree and appearance, more serious ones are called cracks, and lighter ones are called microcracks. Cracks are same as microcracks. It looks very similar, but in essence it has two meanings of difference, that is, cracks are not defects like voids, because added stress is arranged in polymer itself in a parallel direction, so if it is heated, it can return without cracks. In this way, cracks and microcracks can be distinguished. Among them, micro-cracks are not only generated after molding; they also occur after being placed or in contact with solvent vapors.
However, ABS and impact-resistant polystyrene do not produce this phenomenon at all, but it is manifested in the form of whitening at the top of ejector rod, which can be eliminated by heating with hot air. There are two reasons for above phenomenon. Whitening refers to stress whitening. From process of stress yield curve and crack whitening in the figure, it does take energy. After whitening, cracks will occur as long as a little energy is added. (Figure A)

 

We consider that stress cracking may be caused by reduction of bonding force of product due to action of solvents, oils and medicines (see below)

 

During molding, draft angle of mold is insufficient, which becomes reverse draft angle, or roughness and polishing of punch of mold cavity are extremely poor, so part is difficult to demould, which makes pushing force too large and produces stress, often causes part to be damaged or whitish. This phenomenon is more likely to occur when main runner is not polished well, so that workpiece sticks to static mold or moving mold, sidewall is convex and concave, rigid demoulding is used.
In short, when this defect occurs, first of all, attention should be paid to polishing of mold, and while increasing draft angle, a mandrel is added near crack of molded part, so that part is not bent and can be demolded reasonably. For polymethyl methacrylate (PMMA) molded parts, most of them are formed by chrome-plated molds because plastic itself is brittle and surface is required to be shiny. However, electroplating has property that flat surface is not easy to be plated, but it is easy to be placed at corner; thus it becomes reverse draft angle, so special attention must be paid.
In addition, because polycarbonate (PC), PVC and other materials are easy to stick to chrome-plated layer of mold, especially in the corners, it becomes an inverted cone, so pay attention. Polycarbonate (PC) is prone to cracks at mold inserts, and in such cases it is better to add reinforced glass fibers to material. Products with residual stress will have cracks (stress cracks mentioned below) due to adhesion of solvents, oils, and chemicals. After a trace amount of oil is applied, many cracking effects will occur.
Demolding slope should be sufficient, distribution of demolding pins should be balanced, product should not be designed with acute angles, thickness difference of product should be avoided as much as possible. Cause can be determined by looking closely at location of crack.

Due to excessive worry about forming shrinkage cavity, result is that too much melt is injected into mold cavity, which causes great strain inside molded part. At this time, shrinkage becomes very small, not only is it easy to crack, but also internal strain is more likely to cause micro-cracks after being placed for a period of time.
To eliminate overfill cracking, increase melt temperature, lower injection pressure, and increase mold temperature, as long as melt is easily injected into mold cavity. According to appearance of molded part and other related reasons, when it must be molded by overfilling, in order to prevent part from micro-cracks, post-processing (such as heat treatment) should be performed after molding, which is effective to eliminate internal strain.

Part is ejected before it is fully hardened, sometimes cracking or whitening around ejector pin. By fully cooling or changing cooling method of mold itself, it can be ejected after it is completely hardened. It is also possible to reduce mold temperature and prolong cooling time.
However, local cooling of some molds is insufficient, and deformation may not be prevented under normal molding conditions. In this case, consideration should be given to changing cooling water path, location of cooling water channel, or adding cooling tip holes, especially considering not water cooling, but air cooling.

During insert molding, stress is significantly concentrated around insert due to shrinkage of plastic. This force holds insert firmly, but plastic around insert tends to crack when stress is too high.
When inserting metal parts during injection molding, stress is most likely to occur, and it is easy to crack after a period of time, which is extremely harmful. This is mainly because thermal expansion coefficients of metal and resin are very different to generate stress, and over time, strength of gradually degraded resin material should be exceeded to generate cracks.
To reduce cracking around insert, it is effective to preheat insert or minimize shrinkage difference, and effect of plasticizing treatment is more obvious (that is, preheating metal insert can alleviate residual stress during molding. Annealing after forming instead of insert heating is also a method).