What are the important stages of the injection molding process?

Table of Contents

The injection molding process of plastic parts mainly includes 4 stages:

Filling, holding, pressure, cooling, demolding

These 4 stages directly determine the molding quality of the product, and these 4 stages are a complete continuous injection process.

Filling stage

The filling is the first step in the entire process injection molding cycle, from the time the mold closes to the start of injection molding 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.

High-speed filling

When filling at high speed, the shear rate is high, and the viscosity of the molten 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, during the flow control phase, the filling behavior often depends on the volume to be filled. That is to say, in the flow control stage, due to the high-speed filling, the shear thinning effect of the melt is often great, but the cooling effect of the thin wall is not obvious, so the effect of the rate prevails.

Low-speed filling

Heat conduction controls Low shear rates, high local viscosities, and high flow resistance at low filling rates.

Due to the slow replenishment rate and slow flow of hot plastic, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold injection mold wall.

Coupled with a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner wall.

Due to the flow of the fountain, the plastic polymer chains in front of the flow wave line up almost parallel to the flow front. Therefore, when two plastic melts meet, the polymer chains on the contact surface are parallel to each other;

In addition, the properties of the two melts are different (the residence time in the mold cavity is different, and the temperature and pressure are also different), resulting in the fusion area of ​​the melts. Microscopically, the structural strength is poor.

Put the parts at an appropriate angle under the light and observe with the naked eye, and it can be found that there are obvious joint lines, which is the formation mechanism of the weld line.

Weld lines not only affect the appearance of plastic parts but also easily cause stress concentration due to the loose microstructure, which reduces the strength of this part and causes a fracture.

Generally speaking, the strength of the weld line produced by welding in the high-temperature zone is better, because, at high temperatures, the polymer chains have better mobility and can penetrate and entangle with each other.

In addition, the temperatures of the two melts in the high-temperature zone are relatively close, and the melt the thermal properties are almost the same, which increases the strength of the welded area; on the contrary, in the low-temperature area, the welded strength is poor.

Pressure-holding stage

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

During the pressure-holding process, since the mold cavity has been filled with plastic, the back pressure is high.

During the pressure-holding and compacting process, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also relatively slow. The flow at this time is called pressure-holding flow.

Since the plastic is cooled and solidified by the injection mold wall during the pressure-holding stage, the viscosity of the melt increases rapidly, 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 lasts until the gate is solidified and sealed. At this time, the cavity pressure in the pressure-holding stage reaches the highest value.

During the packing phase, due to the relatively high pressure, the plastic exhibits partially compressible properties.

In areas of higher pressure, the plastic is denser and has a higher density; in areas of lower pressure, the plastic is looser and has a lower density, causing the density distribution to change with location and time.

During the injection pressure-holding process, the plastic flow rate is extremely low, and flow no longer plays a leading role; pressure is the main factor affecting the pressure-holding process.

During the pressure-holding process, the plastic has filled the mold cavity, and the gradually solidified melt acts as a medium for transmitting pressure.

The pressure in the mold cavity is transmitted to the surface of the mold wall using plastic, which tends to spread the mold, so proper mold clamping force is required for mold clamping.

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, overflow on the molded product, and even open the 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.

Cooling stage

In injection molding molds, 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 the deformation of the plastic product due to external force can be avoided after demoulding.

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 the cost; uneven cooling will further cause the warping of plastic products.

According to the experiment, the heat that enters the injection mold from the melt is roughly distributed in two parts, 5% of which is transferred to the atmosphere through radiation and convection, and the remaining 95% is conducted from the melt to the mold.

Due to the role of cooling water pipes in the mold of plastic products, the heat is transferred from the plastic in the mold cavity to the cooling water pipes through heat conduction through the mold frame and then 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 is dissipated in the air after contacting the outside world.

The molding cycle of injection molding consists of clamping time, filling time, pressure holding 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 and the output of plastic products.

In the demoulding stage, the temperature of the plastic product should be cooled to a temperature lower than the thermal deformation temperature of the plastic product to prevent the plastic product from being loosened due to residual stress or warping and deformation caused by external force from the demoulding.

Factors that affect the cooling rate of the product are:

Design of plastic products. Mainly the wall thickness of plastic products. The thicker the product, the longer the cooling time.

Generally speaking, the cooling time is proportional to the square of the thickness of the plastic products or proportional to the 1.6th power of the maximum flow channel diameter. That is, the thickness of the plastic product is doubled, and the cooling time is increased by 4 times.

Mold material and its cooling method. Mold material, including mold core, mold cavities material, and mold base material has 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 clamping unit time, and the shorter the cooling time.

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

The coolant flow. The greater the cooling water flow rate (generally, it is better to achieve turbulent flow), the better the effect of cooling water taking away heat by heat convection.

The nature of the coolant. The viscosity and heat transfer coefficient of the coolant will also affect the heat transfer effect 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.

Plastic selection: Plastic refers to the measure of how quickly the plastic conducts heat from a hot place to a cold place.

The higher the thermal conductivity coefficient of the plastic, the better the heat conduction effect, or the lower the specific heat of the plastic, the temperature is easy to change, so the heat is easy to dissipate, the heat conduction effect is better, and the required cooling time is shorter. the

Processing parameter setting. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, and the longer the required cooling time.

Design rules for the cooling system:

The designed cooling channel should ensure a uniform and rapid cooling effect. the

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.

When designing the cooling system, the injection mold designer must determine the following design parameters according to the wall thickness and volume of the plastic parts.

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 flow rate and heat transfer properties.

Demoulding stage

Demolding is the last step in an injection molding cycle. Although the product has been cold-set and molded, molding still has a very important impact on the quality of the product.

Improper molding methods may cause uneven force on the product during demoulding, and defects such as product deformation when ejected.

There are two main ways of demoulding: ejector pin de-molding and stripping plate demoulding. When designing the mold, the appropriate injection molding method should be selected according to the structural characteristics of the product to ensure product quality.

For molds that use ejector pins for demoulding, the setting of the ejector pins should be as uniform as possible, and the position should be selected at the place where the demoulding resistance is the largest and the strength and rigidity of the plastic parts are the largest, to avoid deformation and damage of the plastic parts.

The stripper plate is generally used for demolding of deep-cavity thin-walled containers and transparent products that do not allow traces of push rods. This mechanism is characterized by large and uniform demoulding force, smooth movement, and no obvious leftover traces.


Plastic injection molding is a process that can be used to make a variety of products. The steps in the injection molding process are very important in ensuring the quality of the final product.

By understanding these steps, you can produce high-quality products that meet the needs of your customers. Which stage of the injection molding process do you find most challenging? Please tell us in the email!

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