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

1. Filling stage

Filling is the first step in the entire injection molding cycle, calculated from the moment the mold is closed, until the mold cavity is filled to approximately 95%. In theory, the shorter the filling time, the higher the molding efficiency. However, in practice, the molding time or injection speed are subject to many conditions.

High speed filling. When filling at high speeds, the shear rate is high, and the viscosity of the plastic decreases due to the effect of shear thinning, resulting in a decrease in overall flow resistance; The local viscous heating effect can also cause the thickness of the curing layer to become thinner. Therefore, in the flow control stage, the filling behavior often depends on the size of the volume to be filled. In the flow control stage, due to high-speed filling, the shear thinning effect of the melt is often significant, while the cooling effect of the thin wall is not significant, so the effectiveness of the rate dominates.

Low speed filling. When low speed filling is controlled by heat conduction, the shear rate is lower, the local viscosity is higher, and the flow resistance is greater. Due to the slow replenishment rate and flow of thermoplastic, the heat conduction effect is more obvious, and the heat is quickly carried away by the cold mold wall. Combined with a small amount of viscous heating phenomenon, the thickness of the curing layer is thicker, which further increases the flow resistance at the thinner part of the wall.

Due to the flow of the fountain, the plastic polymer chains in front of the flow wave are aligned almost parallel to the flow wave front. Therefore, when two strands of plastic melt adhesive intersect, the polymer chains on the contact surface are parallel to each other; In addition, the properties of the two strands of adhesive are different (with different retention times in the mold cavity, as well as different temperatures and pressures), resulting in poor microstructural strength in the intersection area of the adhesive. By placing the parts at an appropriate angle under light and observing with the naked eye, it can be observed that there are obvious joint lines, which is the mechanism of the formation of fusion marks. The fusion mark not only affects the appearance of the plastic part, but also causes stress concentration due to the loose microstructure, resulting in a decrease in the strength of the part and fracture.

Generally speaking, the strength of fusion marks generated in high temperature regions is better because under high temperature conditions, the activity of polymer chains is better and can penetrate and entangle with each other. In addition, the temperature of the two melts in high temperature regions is relatively close, and the thermal properties of the melt are almost the same, increasing the strength of the fusion zone; On the contrary, in low-temperature areas, the welding strength is poor.

2. Pressure maintaining stage

The function of the pressure maintaining stage is to continuously apply pressure, compact the melt, increase the density (densification) of the plastic, and compensate for the shrinkage behavior of the plastic. During the pressure maintaining process, due to the fact that the mold cavity is already filled with plastic, the back pressure is relatively high. During the pressure maintaining compaction process, the screw of the injection molding machine can only move slightly forward slowly, and the flow rate of plastic is also relatively slow. At this time, the flow is called pressure maintaining flow. Due to the accelerated solidification of plastic due to the cooling of the mold wall during the pressure maintaining stage, the viscosity of the melt also increases rapidly, resulting in significant resistance in the mold cavity. In the later stage of pressure maintaining, the material density continues to increase and the plastic part gradually forms. The pressure maintaining stage should continue until the gate is cured and sealed, at which point the mold cavity pressure during the pressure maintaining stage reaches its highest value.

During the pressure holding stage, due to the relatively high pressure, the plastic exhibits partially compressible properties. In areas with high pressure, plastic is relatively dense and has a higher density; In areas with low pressure, plastic is relatively loose and has a lower density, resulting in changes in density distribution with location and time. During the pressure maintaining process, the plastic flow rate is extremely low, and the flow no longer plays a dominant role; Pressure is the main factor affecting the pressure holding process. During the pressure maintaining process, the plastic has already filled the mold cavity, and the gradually solidified melt serves as the medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall through plastic, which tends to prop up the mold. Therefore, appropriate locking forces are needed to lock the mold. Under normal circumstances, the expanding force will slightly open the mold, which is helpful for the exhaust of the mold; But if the expansion force is too large, it can easily cause burrs, overflow of the formed product, and even open the mold. Therefore, when choosing an injection molding machine, one should choose one with sufficient locking force to prevent mold expansion and effectively maintain pressure.

3. Cooling stage

The design of the cooling system is very important in injection molding molds. This is because formed plastic products can only be prevented from deformation due to external forces after being cooled and solidified to a certain degree of rigidity after demolding. Due to the cooling time accounting for about 70% to 80% of the entire molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity, and reduce costs. Improperly designed cooling systems can prolong the molding time and increase costs; Uneven cooling can further cause warping and deformation of plastic products.

According to the experiment, the heat entering the mold from the melt is generally dissipated in two parts, with 5% being transmitted to the atmosphere through radiation and convection, and the remaining 95% being transmitted from the melt to the mold. Plastic products in the mold, due to the role of cooling water pipes, heat is transferred from the plastic in the mold cavity through thermal conduction through the mold base to the cooling water pipes, and then carried away by the cooling liquid through thermal convection. A small amount of heat that has not been taken away by the cooling water continues to be transmitted in the mold, and after coming into contact with the outside world, it disperses into the air.

The molding cycle of injection molding consists of mold closing time, filling time, holding time, cooling time, and demolding time. The cooling time accounts for the largest proportion, approximately 70% to 80%. Therefore, the cooling time will directly affect the length of plastic product molding cycle and the size of production. During the demolding 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 relaxation caused by residual stress or warping and deformation caused by external demolding forces.

The factors that affect the cooling rate of products include:

In terms of plastic product design. Mainly the wall thickness of plastic products. The thicker 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 to the 1.6 th power of the maximum channel diameter. Double the thickness of plastic products and increase the cooling time by four times.

Mold materials and their cooling methods. Mold materials, including mold cores, cavity materials, and mold base materials, have a significant impact on the cooling rate. The higher the thermal conductivity coefficient of the mold material, the better the effect of transferring heat from the plastic per unit time, and the shorter the cooling time.

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

Coolant flow rate. The larger the flow rate of cooling water (usually to achieve turbulent flow), the better the effect of cooling water taking away heat through thermal convection.

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

Plastic selection. Plastic refers to a measure of the speed at which plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity coefficient of plastic, the better the thermal conductivity effect, or the lower the specific heat of plastic, which is prone to temperature changes, making it easy for heat to dissipate, resulting in better thermal conductivity and shorter cooling time required.

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

Design rules for cooling systems:

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

The purpose of designing a cooling system is to maintain appropriate and efficient cooling of the mold. Cooling holes should be of standard size to facilitate machining and assembly.

When designing a cooling system, the mold designer must determine the following design parameters based on the wall thickness and volume of the plastic part - the position and size of the cooling holes, the length of the holes, the types of holes, the configuration and connection of the holes, as well as the flow rate and heat transfer properties of the coolant.

4. Demolding stage

Demolding is the last step in an injection molding cycle. Although the product has been cold formed, demolding still has a significant impact on the quality of the product. Improper demolding methods may lead to uneven stress during demolding and defects such as product deformation during ejection. There are two main ways of demolding: top rod demolding and stripper demolding. When designing molds, appropriate demolding methods should be selected based on the structural characteristics of the product to ensure product quality.

For molds that use top rods for demolding, the setting of the top rods should be as uniform as possible, and the position should be selected in the area with the highest demolding resistance and the highest strength and stiffness of the plastic part to avoid deformation and damage to the plastic part.

The stripper plate is generally used for demolding deep cavity thin-walled containers and transparent products that do not allow push rod marks. The characteristics of this mechanism are high and uniform demolding force, smooth movement, and no obvious left traces.