Recently, phenomenon of product breakage has caused batch returns and caused great losses. In order to solve this problem, relevant information sharing here.
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 molecular chain orientation under action of external force. When polymer chain is forced to transition from a natural stable state to another orientation state, and finally frozen in mold, cooled plastic part will produce residual stress. At the same time, due to large temperature difference in cold mold, molten material quickly changes from a viscous fluid state to a glass state, oriented macromolecules are frozen before they can return to original stable state, which also leaves a part of 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 the gate is higher than other parts, especially when main runner 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, residual stress will also be generated. Since residual stress is a major cause of cracks and ruptures in plastic parts, it is possible to prevent cracks and ruptures in plastic parts 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 minimal pressure loss and higher injection pressure can be used, and forward gate can be changed to multiple pin point gates or side gates, gate can be reduced diameter. When designing side gate, a tab gate can be used in which broken part can be removed after molding.
For example, polycarbonate, polyvinyl chloride, polyphenylene ether and other raw materials have poor melt flow properties and need to be injection molded under high pressure conditions. Cracks are prone to occur at the gate. If lugs or side gates are used, cracks generated in lugs can be removed after molding. In addition, reasonable use of ring-shaped ribs around gate can also reduce cracks at the gate.
In terms of process operation, reducing injection pressure to reduce residual stress is the easiest way, because injection pressure is proportional to residual stress. If cracks produced on the surface of plastic part are black around, it means that injection pressure is too high or amount of material added is too small, injection pressure should be appropriately reduced or amount of material supplied should be increased. When molding under conditions of low material temperature and mold temperature, in order to fill cavity, a higher 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 mold in mold should be controlled so that oriented molecular chain has a longer recovery time.
In addition, under premise of ensuring insufficient replenishment, preventing shrinkage and depression of plastic parts, holding time can be appropriately shortened, because too long holding time can easily cause residual stress and cracks.

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, and ejection angle of mold is insufficient. Too much resistance will cause stress concentration due to external forces, causing cracks and ruptures on the surface of plastic part.
Under normal circumstances, such failures always occur around ejector pin. After such failure, ejector device should be carefully checked and adjusted. Ejector rod is set at the position with the greatest demolding resistance, such as protrusions, ribs, etc.
If number of ejector pins cannot be expanded due to limitation of pushing area, method of using small area and multiple ejector pins can be adopted.
If demolding angle of mold cavity is not enough, surface of plastic part will be scratched to form a wrinkle pattern. When selecting demolding slope, shrinkage of molding material and structure setting of ejector system must be considered. Generally, demolding slope should be greater than 0.85%, demolding slope for small plastic parts is 0.1~0.5% , and draft angle of large plastic parts can reach 2.5%.

Because coefficient of thermal expansion 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 overall shrinkage of plastic part, and resulting tensile stress will be very large. A large amount of residual stress will gather around insert and cause cracks on the surface of plastic part. In this way, metal inserts should be preheated, especially when cracks on the surface of plastic parts occur just after start of machine, most of which are caused by low temperature of inserts.
In addition, in the selection of insert materials, materials with expansion coefficients close to characteristics of resin should be used as much as possible. For example, use of light metal materials such as zinc and aluminum to make inserts is better than steel.
When selecting molding materials, high-molecular-weight resins should 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, cellulose acetate plastics, thickness of plastic around insert should be at least half diameter of insert; for polystyrene, metal inserts are generally not suitable.

Different raw materials have different sensitivity to residual stress. Generally, non-crystalline resins are more likely to produce residual stress and cause cracks than crystalline resins. For water-absorbent resins and resins with more recycled materials, because water-absorbent resins will decompose and become brittle after heating, smaller residual stress will cause embrittlement, while resin with higher content of recycled material contains more impurities, higher content of volatile matter, lower strength of the 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, suitable molding materials should be selected according to specific conditions.
In process of operation, release agent is also a foreign body for melt. If amount is not appropriate, it will cause cracks, so amount should be minimized.
In addition, when plastic injection machine needs to change types of raw materials due to production, remaining material in hopper loader and dryer must be cleaned, and remaining material in barrel must be discharged.

Sharp corners and gaps in the structure of plastic part are most prone to stress concentration, causing cracks and breaks on the surface of plastic part. Therefore, outer and inner corners of plastic body structure should be made as arcs with the largest radius as much as possible. Experiments show that the best 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 part, for parts that must be designed with sharp corners and sharp edges, a minimum transition radius of 0.5mm is still used to make a small arc, which can extend life of mold.

In injection molding process, due to repeated action of injection pressure on mold, fatigue cracks will occur at the edges of cavity with sharp corners, especially near cooling holes.
When mold is in contact with nozzle, bottom of mold is squeezed. If positioning ring hole of mold is larger or bottom wall is thin, fatigue cracks will also occur on the surface of mold cavity.
When cracks on the surface of mold cavity reflect on the surface of plastic part, cracks on the surface of plastic part always appear continuously in same part with same shape. After this kind of crack appears, immediately check whether there is same crack on the surface of cavity corresponding to crack. If crack is caused by re-imaging effect, mold should be repaired by mechanical processing.