Residual stress is too high. When residual stress in plastic part is higher than elastic limit of resin, cracks and ruptures will occur on the surface of plastic part.
During injection molding, molecular arrangement of polymer melt will produce orientation of molecular chain under action of external force. When polymer chain is forced to transition from a natural stable state to another orientation state, it is finally frozen in mold. When inside, cooled plastic parts will generate residual stress.
At the same time, due to large temperature difference in cold mold, molten material quickly changes from a viscous flow state to a glass state, and oriented macromolecules are frozen before returning to initial stable state, which also causes some residual internal stress on the surface of plastic part.

 

In general, cracks and ruptures caused by residual stress are most likely to occur near gate, because molding pressure at gate is higher than that of other parts, especially when sprue is a direct gate.
In addition, when wall thickness of plastic part is uneven and cooling rate of melt is inconsistent, due to different shrinkage of thick and thin parts, the former is stretched by the latter, and residual stress will also be generated. Since residual stress is a major cause of cracks and ruptures in plastic parts, cracks and ruptures in plastic parts can be prevented by reducing residual stress. Main method to reduce residual stress is to improve structure of gating system and adjust molding conditions of plastic parts.
In terms of mold design and production, direct gates with the smallest pressure loss and higher injection pressure can be used, forward gate can be changed to multiple pin point gates or side gates, and gate size can be reduced diameter. When designing side gate, it can be in the form of a tab gate in which cracked part can be removed after molding.
For example, raw materials such as polycarbonate, polyvinyl chloride, and polyphenylene ether have poor melt flow properties and require injection molding under high pressure conditions. Cracks are easily generated at gate. After cracks generated in lug portion are removed. In addition, rational use of annular reinforcing ribs around gate can also reduce cracks at gate.

 

In terms of process operation, reducing residual stress by reducing injection pressure is the easiest way, because injection pressure is proportional to residual stress. If cracks on the surface of plastic parts turn black, it means that injection pressure is too high or feeding amount is too small, and injection pressure should be appropriately reduced or feeding amount should be increased. When molding under conditions of low material temperature and mold temperature, in order to fill cavity, a high injection pressure must be used, resulting in a large amount of residual stress in plastic part.
In this regard, temperature of barrel and mold should be appropriately increased, temperature difference between melt and mold should be reduced, cooling time and speed of parison in mold should be controlled, so that oriented molecular chain has a longer recovery time.
In addition, under premise of ensuring insufficient replenishment, no shrinkage and depression of plastic parts, holding time can be appropriately shortened, because holding time is too long, it is easy to generate residual stress and cause cracks.
External forces lead to residual stress concentrations.
Before plastic part is demolded, if cross-sectional area of ejector mechanism is too small or number of ejector pins is not enough, ejector pin is set in an unreasonable position or installation is inclined, balance is poor, ejection slope of mold is insufficient, ejection resistance is too large, stress concentration will be caused by external force, surface of plastic part will be cracked and broken.
Under normal circumstances, this kind of failure always occurs around ejector rod. After such a fault occurs, ejector device should be carefully checked and adjusted. Ejector rod is set at part with the largest demoulding resistance, such as protrusion and reinforcing rib.
If number of ejector pins set cannot be expanded due to limitation of ejection area, method of using multiple ejector pins in a small area can be adopted.
If demolding slope of mold cavity is not enough, surface of plastic part will also be scratched to form a wrinkle pattern. When selecting demolding slope, shrinkage rate of molding material and structure of ejection system must be considered. In general, demolding slope should be greater than 0.85%, demolding slope of small plastic parts is 0.1~0.5%, and demolding slope of large plastic parts can reach 2.5%.
There is a difference in thermal expansion coefficient of molding raw material and metal insert
Because thermal expansion coefficient of thermoplastics is 9 to 11 times larger than that of steel, and 6 times larger than that of aluminum. Therefore, metal insert in plastic part will hinder the overall shrinkage of plastic part, resulting in a large tensile stress, and a large amount of residual stress will accumulate around insert, causing cracks on the surface of plastic part. In this way, metal insert should be preheated, especially when crack on the surface of plastic part occurs at the beginning of machine, most of which are caused by low temperature of insert.
In addition, in selection of insert materials, materials with expansion coefficients close to resin characteristics should be used as much as possible. For example, use of light metal materials such as zinc and aluminum for inserts is preferable to steel.
When selecting molding raw materials, high molecular weight resins should also be used as much as possible. If low molecular weight molding materials must be used, thickness of plastic around insert should be designed to be thicker. For polyethylene, polycarbonate, polyamide, acetic acid For cellulosic plastics, thickness of plastic around insert should be at least half diameter of insert; for polystyrene, metal inserts are generally not appropriate.

 

Different raw materials have different sensitivities to residual stress. Generally, amorphous resins are more prone to residual stress than crystalline resins and cause cracks; for water-absorbent resins and resins with more recycled materials, water-absorbent resin will decompose and become brittle after heating. Smaller residual stress will cause brittle cracking, and resin with higher content of recycled material has more impurities, higher content of volatile matter, lower strength of material, and is also prone to stress cracking.
Practice has shown that low-viscosity loose resins are not prone to cracks. Therefore, in production process, appropriate molding raw materials should be selected according to specific conditions.
In process of operation, release agent is also a foreign matter for molten material. If amount is not appropriate, it will also cause cracks, so amount of release agent should be minimized.
In addition, when plastic injection machine needs to change raw material variety due to production, residual material in hopper feeder and dryer must be cleaned up, and residual material in barrel must be removed.

 

Stress concentration is most likely to occur at sharp corners and gaps in structure of plastic part, resulting in cracks and ruptures on the surface of plastic part. Therefore, outer and inner corners of plastic body structure should be made into arcs with the largest radius as far as possible. Experiments show that optimal transition arc radius is ratio of arc radius to wall thickness at the corner of 1:1.7, that is, arc radius at the corner is 0.6 times wall thickness.
When designing shape and structure of plastic parts, minimum transition radius of 0.5mm should be used to make a small arc for parts that must be designed into sharp corners and sharp edges, which can prolong life of mold.

During injection molding process, due to repeated injection pressure of mold, fatigue cracks will occur on the edges with sharp and acute angles in cavity, especially near cooling holes.
When mold is in contact with nozzle, bottom of mold is squeezed. If positioning ring hole of mold is large or bottom wall is thin, fatigue cracks will also occur on the surface of mold cavity.
When cracks on the surface of mold cavity are reflected on the surface of plastic part, cracks on the surface of plastic part always appear continuously in same part in same shape. After such a crack occurs, cavity surface corresponding to crack should be checked immediately for same crack. If crack is caused by re-image, mold should be repaired by machining.