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Three-stage Injection Molding Process

The complexity of injection molding process

① Complex geometric properties of molds and parts

Injection molded parts are often thin-walled structures with complex shapes. The thin wall structure and high-speed injection speed together lead to high flow rate and high shear rate. And these and the complex viscosity characteristics of the material will cause a huge change in the viscosity of the material during the filling process.

The mold has two main roles in the injection molding process:

  1. Form the shape of the part to be produced
  2. Make the mold cool down as soon as possible

Generally speaking, an injection mold is a complex structure with a complex core and ejection system. This complex structure affects the location of the cooling channels, which in turn cause changes in the mold temperature. These changes affect the viscosity of the material and ultimately the flow characteristics of the material.

② Special material properties

Polymer materials for injection molding can be divided into semi-crystalline and amorphous polymers. Both types of polymers have complex thermorheological behavior, which is closely related to the molding process.

Thermoplastics generally exhibit viscoelasticity.

In addition, it also has the properties of shear thinning and supercharged heating. Their thermal properties vary with temperature and can also be affected by the state of stress. For semi-crystalline polymers, the properties of the material depend on the history of processing and the rate of temperature change.

③ Process stability

In the injection molding production process, the process conditions for processing qualified parts may be unstable. These instabilities depend on the aforementioned inherent nonlinearity of the material and the complexity of the part geometry. It is worth noting that a slight deviation in process conditions will have a huge impact on product quality.

In summary, the process of mold filling, pressure holding and cooling, the solidification of the melt involves several heat transfer modes, phase transitions and boundary conditions that vary with time. In addition, the nature of the material and the geometry of the part can further complicate the process.

Three-stage Injection Molding Process

Ordinary two-stage molding process

In order to solve complex injection molding problems and to stabilize production, the injection molding process should:

  • Minimize the difference between each die
  • Process stability
  • Consistent viscosity when the material is molded
  • The switching position of each mold and the viscosity of the material are consistent when switching

To maintain this stability, speed control is usually used during the injection phase. The faster the speed, the lower the viscosity of the material. When the injection reaches 95%~99%, switch to holding pressure, and use pressure control at this time.

When the injection reaches 95%~99%, the end of the mold cavity is not completely filled with plastic, but the plastic in the mold cavity starts to cool and shrink at the same time. That indicates the V/P switching is very unstable.

Three-stage molding process

Like the two-stage molding process, it also needs to maintain the stability of the process. In addition, the characteristics of the three-stage molding process are: the switching point is slightly advanced, and the injection is completed and enters the feeding stage until the cavity is filled to 99%. The holding pressure is only to resist the pressure in the cavity until the gate is closed.

In this way, a new process is formed:

injection phase (speed control), feeding phase (speed control) and pressure holding phase (pressure control).

The feeding stage covers the unstable switching action, making the molding process more stable.

The advantages of the three-stage process are:

  • More stable feeding phase
  • Better control of plastic filling, eliminating quality issues caused by stress andoverfilling
  • More suitable for cavity pressure control