Plastic injection molding process mainly includes four stages: filling, pressure holding, cooling, and demolding. These four stages directly determine molding quality of product, and these four stages are a complete continuous process.

 

Filling is the first step in the entire injection molding cycle, from time mold is closed and injection is started until mold cavity is filled to approximately 95%. Theoretically, the shorter filling time, the higher molding efficiency, but in practice, molding time or injection speed are restricted by many conditions.
The shorter charging time, the higher molding efficiency, but in practice, molding time or injection speed are restricted by many conditions.
During high-speed filling, shear rate is high, and viscosity of plastic decreases due to effect of shear thinning, which reduces the overall flow resistance; local viscous heating effect will also reduce thickness of solidification. Therefore, during flow control phase, filling behavior often depends on size of volume to be filled. That is, in flow control stage, due to high-speed filling, shear thinning effect of melt is often large, and cooling effect of thin wall is not obvious, so effect of velocity prevails.
Low speed filling, heat conduction control. When filling at low speed, shear rate is lower, local viscosity is higher, and flow resistance is higher. Due to slow replenishment rate and slow flow of thermoplastic, heat conduction effect is more obvious, and heat is quickly taken away by cold film wall. Combined with a smaller amount of viscous heating, solidified layer is thicker, which further increases flow resistance at thinner wall.
Due to fountain flow, plastic polymer chains in front of flow wave are aligned almost parallel to flow wave front. Therefore, when the two strands of plastic melts meet, polymer chains on contact surface are parallel to each other; in addition, properties of the two strands of melts are different (residence time in film cavity is different, temperature and pressure are also different), resulting in intersection of melts. Microscopically, structural strength is extremely poor. When parts are placed at an appropriate angle under light and observed with naked eye, it can be found that there are obvious bonding lines, which is formation mechanism of weld line. Weld lines not only affect appearance of plastic parts, but also cause stress concentration due to loosening of microstructure, which reduces strength of part and causes fracture.
Generally speaking, strength of weld line produced in high temperature area is excellent, so polymer chain has better mobility under high temperature conditions, and can penetrate each other. In addition, temperature of the two melts in high temperature area is relatively close, thermal properties of melts are almost same, which increases strength of welding area. On the contrary, in low temperature area, welding strength is poor.

Function of holding pressure stage is to continuously apply pressure to compact melt and increase density of plastic to compensate for shrinkage behavior of plastic. During pressure holding process, back pressure is high because mold cavity is already filled with plastic.
In process of holding pressure and compaction, screw of injection molding machine can only move forward slowly and slightly, and flow rate of plastic is also relatively slow. Flow at this time is called holding pressure flow. In pressure holding stage, plastic is cooled and solidified faster by mold wall, and melt viscosity increases rapidly, so resistance in mold cavity is very large.
In later stage of pressure-holding, material density continues to increase, and plastic is gradually formed. Pressure-holding stage continues until gate is cured and sealed. At this time, cavity pressure in pressure-holding stage reaches the highest value.
During packing stage, plastic exhibits partially compressible properties due to relatively high pressure. In areas with higher pressure, plastic is denser and denser; in areas with lower pressure, plastic is looser and has a lower density, thus causing density distribution to change with time.
During pressure-holding process, flow rate of plastic is low, and flow no longer plays a leading role; pressure is main factor affecting pressure-holding process.
During pressure-holding process, plastic has filled mold cavity, and gradually solidified melt is used as medium for transmitting pressure.
Pressure in mold cavity is transmitted to surface of film wall through plastic, and there is a tendency to open mold. Therefore, an appropriate clamping force is required for mold clamping. Under normal circumstances, mold expansion force will slightly open mold, which is helpful for exhaust of mold;
However, if mold expansion force is too large, it is easy to cause burrs, overflow of molded product, and even open mold. Therefore, when selecting an injection molding machine, an injection molding machine with sufficient clamping force should be selected to prevent mold expansion and effectively maintain pressure.

 

In injection molding molds, design of cooling system is very important. This is because molded plastic product can only be cooled and solidified to a certain rigidity, and plastic product can be prevented from being deformed by external force after demolding.
Since cooling time accounts for about 70-80% of the entire molding cycle, a well-designed cooling system can greatly shorten molding time, improve injection productivity, and reduce costs. Improperly designed cooling system will prolong molding time and increase cost; uneven cold cutting will further cause warpage of plastic products.
According to experiment, heat entering mold from melt is generally dissipated in two parts, 5% of which is transferred to atmosphere by radiation and convection, and 95% of which is conducted from melt to mold. Due to cooling water pipe in mold, heat is transferred from plastic in mold cavity to cooling water pipe through mold frame through heat conduction, then taken away by cooling liquid through thermal convection. A small amount of heat that is not taken away by cooling water continues to be conducted in mold, and is lost in air after contacting outside world.
The first phase of injection molding consists of mold clamping time, filling time, pressure holding time, cooling time and demolding time. Among them, cooling time accounts for the largest proportion, which is about 70-80%. Therefore, cooling time will directly affect length of molding cycle and output of plastic products.
In demolding stage, temperature of plastic product should be cooled to a temperature lower than thermal deformation temperature of plastic product to prevent plastic product from loosening due to residual stress or warping and deformation caused by external force of demolding.

Demoulding is last link in an injection molding cycle. Although product has been cooled and formed, demolding still has a very important impact on quality of product. Improper demolding method may cause uneven stress on product during demolding, and cause defects such as product deformation during ejection.
There are two main ways of demoulding: ejector demoulding and stripping plate demoulding. When designing a mold, an appropriate demoulding method should be selected according to structural characteristics of product to ensure product quality.
For mold with ejector ejector, ejector pins should be set as uniform as possible, location should be selected at place with the largest ejection resistance, the highest strength and stiffness of plastic parts, so as to avoid deformation and damage of plastic parts.
Stripper is generally used for demoulding of deep cavity thin-walled containers and transparent products that do not allow traces of push rods.