It mainly includes four stages, such as filling – holding pressure – cooling – demolding, etc. These four stages directly determine the molding quality of the products, and these four stages are a complete and continuous process.
Filling is the first step in the whole injection molding cycle, and the time is counted from the time the mold is closed to the time the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.
High speed filling: high shear rate at high speed filling, plastic due to shear thinning effect and the presence of viscosity decline, so that the overall flow resistance to reduce; local viscous heating effect will also make the thickness of the curing layer thinner. Therefore, in the flow control phase, the filling behavior often depends on the volume size to be filled. That is, in the flow control phase, due to high-speed filling, the shear thinning effect of the melt is often large, while the cooling effect of thin walls is not obvious, so the utility of the rate prevails. l
Low-speed filling: heat transfer control low-speed filling, the shear rate is lower, the local viscosity is higher, and the resistance to flow is greater. Due to the slower rate of thermoplastic replenishment, the flow is slower, so that the heat transfer effect is more obvious, heat quickly for the cold mold wall away. Together with a smaller amount of viscous heating phenomenon, the thickness of the curing layer is thicker, and further increase the flow resistance at the thinner part of the wall. l
Generally speaking, in the high-temperature area of the fusion of the fusion trace strength is better, because the high-temperature situation, the polymer chain activity is better, can penetrate each other winding, in addition to the high temperature area of the two melt temperature is closer, the thermal properties of the melt is almost the same, increasing the strength of the fusion area; on the contrary, in the low-temperature area, the fusion strength is poor.
The role of the holding pressure stage is to continuously apply pressure to compact the melt and increase the density of the plastic (densification) to compensate for the shrinkage behavior of the plastic. During the holding pressure process, the back pressure is higher because the mold cavity is already filled with plastic. In the process of holding pressure compaction, the injection molding machine screw can only slowly move forward for a small movement, and the flow rate of plastic is also slower, which is called holding pressure flow. As the plastic is cooled and cured by the mold wall, the viscosity of the melt increases quickly, so the resistance in the mold cavity is great. In the later stage of holding pressure, the material density continues to increase, and the molded part is gradually formed. The holding pressure phase should continue until the gate is cured and sealed, at which time the cavity pressure in the holding pressure phase reaches the highest value.
In the holding phase, the plastic is partially compressible because the pressure is quite high. In the higher pressure area, the plastic is denser and the density is higher; in the lower pressure area, the plastic is looser and the density is lower, thus causing the density distribution to change with position and time. The plastic flow rate is very low in the holding process, and the flow no longer plays a dominant role; the pressure is the main factor affecting the holding process.
In the injection molding mold, the design of the cooling system is very important. This is because the molded plastic products can only be cooled and cured to a certain rigidity, after demolding, to avoid deformation of plastic products due to external forces. Since cooling time accounts for about 70% to 80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity and reduce costs. Improperly designed cooling system will make the molding time longer and increase the cost; uneven cooling will further cause warping and deformation of plastic products.
Demolding is the last part of the injection molding cycle. Although the products have been cold-set, demolding still has an important impact on the quality of the products. Improper demolding may lead to uneven force during demolding and deformation when ejecting the products. There are two main ways of demoulding: top bar demoulding and stripping plate demoulding. When designing the mold, we should choose the suitable demoulding method according to the structural characteristics of the product to ensure the product quality.