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

1. Filling stage of injection molding process

(1) Filling is the first step in the entire injection cycle process. The time starts from when the mold is closed and the injection is started until the mold cavity is filled to about 95%.  In theory, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is restricted by many conditions.

(2) High-speed filling.  The shear rate is higher during high-speed filling, and the viscosity of the plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; the local viscous heating effect will also make the thickness of the solidified layer thinner.  Therefore, in the flow control stage, the filling behavior often depends on the volume to be filled.  That is, in the flow control stage, due to high-speed filling, the shear thinning effect of the melt is often great, and the cooling effect of the thin wall is not obvious, so the effect of the speed prevails.

(3) Low-speed filling.  When the heat conduction controls the low-speed filling, the shear rate is lower, the local viscosity is higher, and the flow resistance is higher.  Due to the slower replenishment rate and slower flow of the hot plastic, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall.  Coupled with a smaller amount of viscous heating, the thickness of the cured layer is thicker, which further increases the flow resistance at the thinner part of the wall.

Due to the fountain flow, the plastic polymer chains in front of the flow wave are aligned almost parallel to the flow wave front.  Therefore, when the two strands of plastic melt meet, the polymer chains on the contact surface are parallel to each other; and the properties of the two strands of melt are different (the residence time in the mold cavity is different, and the temperature and pressure are also different).

The microscopic structural strength of the melt junction area is poor.  Place the parts at an appropriate angle under light and observe with the naked eye, you can find that there are obvious joint lines, which is the formation mechanism of weld marks.  The weld mark not only affects the appearance of the plastic part, but also due to the looseness of the microstructure, it is easy to cause stress concentration, which reduces the strength of the part and breaks.

Generally speaking, the strength of the weld line produced in the high temperature zone is better, because the polymer chains are more mobile at high temperature and can penetrate each other. In addition, the temperature of the two melts in the high temperature zone is closer.  The thermal properties are almost the same, which increases the strength of the welding area; on the contrary, in the low temperature area, the welding strength is poor.

2. Pressure holding stage of injection molding process

The function of the holding pressure stage is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic.

During the pressure holding process, the back pressure is relatively high because the mold cavity has been filled with plastic.  In the process of maintaining pressure and compaction, the screw of the injection molding machine can only move forward slowly, and the flow speed of the plastic is relatively slow. The flow at this time is called the pressure maintaining flow.  In the pressure holding stage, the plastic is cooled and solidified by the mold wall faster, and the melt viscosity increases quickly, so the resistance in the mold cavity is very large.

In the later stage of pressure holding, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage should continue until the gate is cured and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.

In the pressure-holding stage, the plastic exhibits partial compressibility due to the relatively high pressure.  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, so the density distribution changes with position and time.

In the process of holding pressure, the plastic flow rate is extremely low, and the flow no longer plays a leading role; pressure is the main factor that affects the process of holding pressure.  In the process of holding pressure, the plastic has filled the mold cavity, and the gradually solidified melt is used as the pressure transmission medium.  The pressure in the mold cavity is transferred to the surface of the mold wall through the plastic, which tends to open the mold, so a proper clamping force is required to lock the mold.

Under normal circumstances, the mold expansion force will slightly open the mold, which is helpful for the exhaust of the mold; but if the mold expansion force is too large, it is easy to cause burrs and flashes of the molded product, or even open the mold.  Therefore, when choosing an injection molding machine, you should choose an injection molding machine with a large enough clamping force to prevent mold expansion and effectively maintain pressure.

3. Cooling stage of injection molding apartment

In the injection molding mold, the design of the cooling system is very important.  This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and then the plastic product can be prevented from being deformed due to external force after being demolded.  Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection molding productivity, and reduce costs.  An improperly designed cooling system will lengthen the molding time and increase costs; uneven cooling will further cause warpage and deformation of plastic products.

According to the experiment, the heat that enters the mold from the melt is generally dissipated in two parts, one of which is 5% transmitted to the atmosphere by radiation and convection, and the remaining 95% is conducted from the melt to the mold.  Due to the action of the cooling water pipe in the mold of the plastic product, the heat is transferred from the plastic in the mold cavity to the cooling water pipe through the mold base through heat conduction, and then is taken away by the cooling liquid through heat convection.  A small amount of heat that is not taken away by the cooling water continues to be conducted in the mold, and escapes into the air when it comes into contact with the outside world.

The molding cycle of injection molding consists of clamping time, filling time, holding pressure time, cooling time and demolding time.  Among them, the cooling time accounts for the largest proportion, about 70% to 80%.  Therefore, the cooling time will directly affect the length of the molding cycle of plastic products and the size of the output.  The temperature of the plastic product in the demolding stage should be cooled to lower than the thermal deformation temperature of the plastic product to prevent the plastic product from relaxing due to residual stress or warping and deformation caused by the external force of demolding.

Factors affecting the cooling rate of products are:

(1) Design of plastic products.  Mainly the wall thickness of plastic products.  The greater the thickness of the product, the longer the cooling time.  Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic product, or proportional to the 1.6th power of the largest runner diameter.  That is, the thickness of the plastic product is doubled, and the cooling time is increased by 4 times.

(2) Mold material and its cooling method.  Mold materials, including mold core, cavity materials and mold base materials have a great influence on the cooling rate.  The higher the thermal conductivity of the mold material, the better the effect of transferring heat from the plastic per unit time, and the shorter the cooling time.

(3) Cooling water pipe configuration method.  The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter and the larger the number, the better the cooling effect and the shorter the cooling time.

(4) Coolant flow rate.  The larger the cooling water flow (generally to achieve turbulent flow), the better the effect of cooling water to take away heat by thermal convection.

(5) The nature of the coolant.  The viscosity and thermal conductivity of the coolant will also affect the thermal conductivity of the mold.  The lower the viscosity of the coolant, the higher the thermal conductivity and the lower the temperature, the better the cooling effect.

(6) Plastic selection.  Plastic refers to the measurement of the speed at which plastic conducts heat from a hot place to a cold place.

The higher the thermal conductivity of the plastic, the better the heat conduction effect, or the lower the specific heat of the plastic, the temperature is prone to change, so the heat is easily dissipated, the heat conduction effect is better, and the required cooling time is shorter.  Processing parameter setting.  The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, and the longer the cooling time required.

Design rules of the cooling system:

(1) The designed cooling channel should ensure uniform and rapid cooling effect.

(2) The purpose of designing the cooling system is to maintain proper and efficient cooling of the mold.  Cooling holes should use standard sizes to facilitate processing and assembly.

(3) When designing the cooling system, the mold designer must determine the following design parameters based on the wall thickness and volume of the plastic part-the location and size of the cooling hole, the length of the hole, the type of hole, the configuration and connection of the hole, and the cooling liquid  Flow rate and heat transfer properties.

4. Demoulding stage

Demoulding is the last link in an injection molding cycle.  Although the product has been cold-formed, demolding still has a very important impact on the quality of the product. Improper demolding method may cause uneven force on the product during demolding, and product deformation during ejection.  There are two main ways of demolding: ejector rod demoulding and stripping board demolding.  When designing the mold, choose the appropriate demoulding method according to the structural characteristics of the product to ensure the quality of the product.

For molds that select ejector pins, the ejector pins should be set as evenly as possible, and the location should be selected where the ejection resistance is the largest and the strength and rigidity of the plastic parts are the largest, so as to avoid deformation and damage of the plastic parts.  The stripper is generally used for the demolding of deep-cavity thin-walled containers and transparent products that do not allow traces of push rods. The characteristics of this mechanism are large and uniform demolding force, smooth movement, and no obvious leftover traces.